Author's Accepted Manuscript

Comparative Investigation on the Adhesion ofHydroxyapatite coating on Ti-6Al-4V Implant:A Review Paper

E. Mohseni, E. Zalnezhad, A.R. Bushroa

PII: S0143-7496(13)00168-1DOI: http://dx.doi.org/10.1016/j.ijadhadh.2013.09.030Reference: JAAD1419

To appear in: International Journal of Adhesion & Adhesives

Accepted date: 17 July 2013

Cite this article as: E. Mohseni, E. Zalnezhad, A.R. Bushroa, ComparativeInvestigation on the Adhesion of Hydroxyapatite coating on Ti-6Al-4VImplant: A Review Paper, International Journal of Adhesion & Adhesives, http://dx.doi.org/10.1016/j.ijadhadh.2013.09.030

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Comparative Investigation on the Adhesion of Hydroxyapatite coating on Ti-

6Al-4V Implant: A Review Paper

E. Mohsenia, E. Zalnezhadb, A. R. Bushroac*

a,b,c Center of Advanced Manufacturing and Material Processing, Department of Engineering

Design and Manufacture, Faculty of Engineering, University of Malaya, Kuala Lumpur, 50603,

Malaysia

aehsanmohseni2008@gmail.com be.zalnezhad@gmail.com Corresponding author: cbushroa@um.edu.my

Abstract

Hydroxyapatite (HA) has been used in clinical bone graft procedures for the past 25 years. Although a biocompatible material, its poor adhesion strength to substrate makes it unsuitable for major load-bearing devices. Investigations on various deposition techniques of HA coating on Ti-6Al-4V implants have been made over the years, in particular to improve its adhesion strength to the metal alloy and its long-term reliability. This review comprehensively analyzes nine techniques mostly used for deposition of HA onto Ti-6Al-4V alloys. The techniques reviewed are Plasma sprayed deposition, Hot Isostatic Pressing, Thermal Spray, Dip coating, Pulsed Laser deposition (PLD), Electrophoretic deposition (EPD), Sol-Gel, Ion Beam Assisted deposition (IBAD), and Sputtering. The advantages and disadvantages of each method over other techniques are discussed. The adhesion strength and the factors affecting the adhesion of HA coating on Ti-6Al-4V implants are also compared. Keywords: Adhesion; Hydroxyapatite; coating; Ti-6Al-4V implant

1. Introduction

Biological fixation is defined as the process where prosthetic components become firmly

bonded to the host bone by ongrowth or ingrowth without the use of bone cements [1-3]. In the

late 1960s, the concept of biological fixation of load-bearing implants using bioactive

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hydroxyapatite (HA) coatings was proposed as an alternative to cemented fixation.

Hydroxyapatite (HA: Ca10(PO4)6(OH)2), a pure calcium phosphate phase, is a preferred

biomaterial for both dental and orthopedics use due to its favorable osteoconductive and

bioactive properties [4, 5]. HA has a similar chemical composition and crystal structure as the

apatite in the human skeletal system, and is therefore suitable for bone substitution and

reconstruction [6]. Furthermore, HA has shown significant success in implants due to its

favorable in vivo behavior [7, 8] and the presence of HA films prolongs the lifetime of

prostheses [9]. However, HA coatings are susceptible to fatigue failure, making it unsuitable for

load bearing implants [10, 11].

Nevertheless, there is a large demand for implants with excellent mechanical properties.

These implants should possess similar properties to the human bones, such as in the value of its

Young’s modulus, which result in less stress shielding effect [12] and extends its service life.

The implants can made into different shapes such as plates, rods, screws and pins [13].

Historically, titanium-based alloys are the most common material for this purpose since it is

known to be a tolerable metal in the human body [14].

Titanium (Ti) and its alloys are the most commonly used metallic materials for medical

implants in orthopedic and dental applications, due to their low density, high strength, non-

toxicity and excellent corrosion resistance [15]. However, there have been reports on

inflammatory reaction around these implants as a result from the creation of an avascular fibrous

tissue that encapsulated the implants [16, 17]. A coating of hydroxyapatite layer can be deposited

on the metal alloy to assist the osseointegration of these implants with surrounding tissues [16].

The bond strength between the coating layer and the metal substrate is a very critical

factor. Separation of the coating layer from the implant during service in the human body results

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in adverse effects on the implants and the surrounding tissue caused by detached particles [18].

The main reason of using HA coating on metallic substrates is to keep the mechanical properties

of the metal such a load-bearing ability and, at the same time, to take advantage of the coating’s

chemical similarity and biocompatibility with the bone [19].

According to Blind et al., the HA coating allows rapid osteointegration as a result of

bone tissue bonding properties [20]. The first clinical results from HA coatings on titanium

dental implants were promising, showing excellent results, even with poor bone quality.

However, after a long period, mechanical failure would occur at the interface of HA and metallic

substrate [21]. The HA coating dissolves as a result of poor crystallized structure [22, 23],

decrease of adherence with the titanium surface and dramatic late implant failure [23, 24].

Moreover, HA itself has poor mechanical properties, with a bending strength of less than 100

MPa [25]. Thus, it can be concluded that the stability of the HA coating is the most critical

factor to ensure the success of this type of implant. Furthermore, the method used to deposit HA

powder onto the substrate could influence the coating characteristics such its adhesion strength

and reliability.

Several techniques have been used to create the HA coating on metallic implants, such as

plasma spraying process [26], thermal spraying [27], sputter coating [28], pulsed laser ablation

[29], dynamic mixing [30], dip coating [31], sol–gel [32], electrophoretic deposition [33],

biomimetic coating [34], ion-beam-assisted-deposition [35],and hot isostatic pressing [36].

Amongst the techniques listed, plasma spraying is the only process which is approved by the

Food and Drug Administration (FDA), USA for biomedical coatings due to its excellent coating

properties as compared to other coating processes [37]. However, plasma sprayed hydroxyapatite

coatings suffer from poor mechanical properties on tensile strength, wear resistance, hardness,

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toughness and fatigue. Improvements in plasma spraying techniques over the years have

addressed many of these limitations. However, other coating methods are available which can be

used as an alternative to conventional techniques.

Limitations such as high porosity, poor uniformity in thickness, phase impurity, limited

crystallinity, and poor adhesion are common in HA coating. However, low coating adhesion

seems to be the major issue, limiting its extensive use for implants at a commercial scale [38-40].

Hence, improvement of bonding strength between the metallic substrate and ceramic coating is a

general requirement regardless of the techniques used.

This review focuses on adhesion strengths between HA coating and Ti-6Al-4V substrate,

fabricated using various techniques such as plasma sprayed deposition, hot isostatic pressing,

thermal spray, dip coating, pulsed laser deposition (PLD), electrophoretic deposition (EPD), sol-

gel, ion beam assisted deposition (IBAD). Parameters affecting the adhesion of coating and other

factors influencing the enhancement of bonding strength of coating surface and the substrate are

also discussed in detail.

2. Coating techniques

2.1 Plasma sprayed coating technique

Plasma spraying process involves melting of ceramics or metal powders using the heat of

ionized inert gas (plasma). The molten powders are then sprayed onto the surface to be coated,

forming the protective layer which provides a barrier against corrosion, wear or high

temperatures. The technique offers advantages such as low cost and rapid deposition rate [41,

42]. In addition, the risk of thermal degradation of the coating and substrate is much less than

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other high-temperature processes since the gas in the plasma flame is chemically inert and the

target can be kept relatively cool [43]. However, plasma sprayed coatings suffers from poor

adhesion between the coatings and substrates [44], and the process may induce structural

changes in the microstructure of the coating material [45, 46].

2.1.1 Plasma sprayed hydroxyapatite (HA) coatings

Plasma spray was the first method used for the production of calcium phosphate coatings,

such as HA coating, due to its ease of application [26]. Plasma sprayed hydroxyapatite (HA)

coatings are biocompatible and able to bond directly to the bone [38], thus making plasma

spraying a favorable choice amongst the many techniques available for coating HA layers onto

metallic substrates [47]. Recent studies on plasma sprayed HA coatings (HACs) on titanium have

shown encouraging results in orthopedic implant applications. These studies reported that the

new bone could appose directly onto the HA coatings and very good adhesion between the

HACs and the new bone can be obtained [48-51]. The plasma sprayed HA coatings have also

assisted in overall quick bone recovery [52].

Nevertheless, the brittle nature of the HA coating makes it prone to crack and fracture,

non-uniformity in density of coating [53], wear of the coated layer, weak mechanical adhesion to

the substrate [44, 54], and alteration of structure [55].

Overall, plasma sprayed coating did not show significant improved long-life performance, better

mechanical integrity and reliability over uncoated implants [56, 57]. An alternative to plasma

spraying is the pulsed laser deposition (PLD) which enables the stoichiometric transfer of

sintered HA yields to form a thin and adherent bioactive coating on titanium substrate surface

[58].

2.1.2 Adhesion of plasma-sprayed hydroxyapatite (HA) coatings on Ti-6Al-4V

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It is well understood that, the determination of the adhesion between the substrate and

coating is one of the main concerns when using plasma spraying techniques [59]. It’s quite

complicated that how coating adheres to a substrate and by today it is not completely understood.

Many theories describe the mechanism of adhesion, although, there is no single clear

interpretation for all adhesion behaviors [60]. Many factors seem to affect the adhesion: (1) Van

der Waals physical interaction forces mechanical anchorage; (2) mechanical anchorage; (3)

metallurgical processes and (4) chemical interaction [59].

Recent reports on alternative orthopedics implant fixation utilizing plasma sprayed HA

coatings (HACs) on Ti-6Al-4V have shown that the new bone was able to appose directly onto

the HA coatings, which resulted in a very good adhesion between the HACs and the new bone

[48-51]. From the viewpoint of materials science, characteristics of HACs are varied with the

spraying parameters such as phase composition, the microstructure, OH-ion content,

crystallinity, and the ration of calcium to phosphorus for the HACs. Among these parameters,

high bonding strength of HACs can be achieved by high spraying power due to a denser

microstructure caused by the greatest extent of coating melting.

Y.C. Yang, et al. experimented on six plasma sprayed HA on Ti- 6Al- 4V substrates by

varying the cooling conditions and the substrate temperatures [61]. The residual stresses and

bonding strengths were measured by XRD “ sin2φ “ technique and a standard adhesion test

(ASTM C-633). Results of the bonding strength evaluation shows that the HA coating with the

lowest residual stress exhibited a higher bonding strength (9.18±0.72 MPa).

The deposition stress and thermal stress are the two major sources of residual stresses in

plasma sprayed coating. Deposition stresses are produced during the cooling of sprayed particles

after solidification. Thermal stresses are generated from differential thermal contraction during

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the post-fabrication cooling phase after coating [62, 63]. The residual stresses are present near

the interface of metal substrate and coating [64-66], due to the difference of thermal expansion

coefficients between both materials [62, 63]. These stresses may vary with substrate cooling

effects, parameters of spraying [62, 63], and coating thickness [67, 68]. Generally, it is believed

that the increased thickness of coating and the temperature of the specimen during plasma

spraying are the mains reasons for the rise in the residual stress.

In addition, high-powered, dense plasma sprayed HA coatings would have stronger

bonding strength than those sprayed using low power. The result is not solely due to the

difference in adhesive strength of HA coating. The value for bonding strength reflects the

combination of both cohesive (within the coating layers) and adhesive (coating to substrate)

strengths of a coating [61]. In a similar study, Tsui, Y. et al. claimed that the cohesive and

adhesive integrity of the coatings influence the long term performance of HA coated implants

considerably [69]. The adhesive strength is usually evaluated based on surface roughness,

coating properties, residual stress, and the mechanical interlocking between the coating and the

substrates, whereas the cohesive strength is determined by coating properties, such as

microstructure and crystallinity [61].

The bonding strength of HA coatings on metallic substrates can be evaluated using

several techniques such as the standard tensile adhesion test [69], interfacial indentation test [58],

tensile adhesion strength (TAS) [61], and indentation method [63]. However, there are

limitations on these techniques to accurately measure the adhesion strength, such as a probability

of penetration of glue into the coating layer , and a dependence of coating failure to the flaw

distribution at the edge of specimen [69]. However, Z. Mohammadi et al. have demonstrated that

the tensile adhesion strength test measured by the standard adhesion test ISO 13779-4, can be

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used in conjunction with the interface indentation test to predict the effects of different

parameters on the adhesion properties of the HA coating by plasma spraying [70]. In general, the

HA coatings with the densest structure (i.e. lowest porosity, and predominantly amorphous

phase) have a higher tensile adhesion strength than those of lower density [61, 71]. The report by

Z. Mohammadi et al. [70] also showed that the tensile adhesion strength was in the range of ~25

MPa for HA coated on the Ti-6Al-4V.

2.2 Hot isostatic pressing technique

Hot isostatic pressing (HIP) is an enabling technology providing an efficient method for

the densification of ceramic powders which allows production of net-shape ceramics with

superior and consistent properties [72]. HIP is an alternative method of producing an HA coating

on a Ti substrate in which pressurized gas is used to exert the required load at the desired

temperature. This requires a gas-tight metal or glass encapsulation around the porous HA coated

implant [73]. In the HIP process, pressure and temperature are applied to the workpiece

simultaneously [74-77].

In hot isostatic pressing, high-pressure levels can be obtained since there is no

dependency on rigid tools with limited strength (such as graphite tools in uniaxial hot pressing)

to transmit the pressure to the body. Typical operating pressure ranges are 100-320 MPa (15-50

ksi), with temperatures exceeding above 2000 °C conducted in large industrial equipment [72].

The advantages of HIP are better temperature control as compared to uniaxial hot pressing, and a

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resultant homogeneous material structure and properties. The reduced sintering temperature

enables control or even avoidance of grain growth and undesirable reactions. A very high

uniformity of properties as well as freedom from directionality can also, if desired, be obtained

[72]. Some researchers have used HIP treatments to densify plasma sprayed coatings, and results

have shown that HIP is useful in reducing the porosity and improving the physical and

mechanical properties of ceramic coatings [78].

Thus, the most important advantage of the hot isostatic pressing is the ability to control

the size and shape of the product to a very high precision without costly diamond machining

operations. Under ideal conditions no change of shape (just a change of scale) of the body

occurs. It has an inherent ability to produce parts with exceptionally accurate shape, virtually

with no dimensional or shape limitation [72].

2.2.1 Hot isostatic pressing of hydroxyapatite (HA) Coatings

Reports shows that, sort of problems such as porosity and crack appearance are

conducted with existing dc plasma sprayed Ha coating on Ti-6Al-4V [79]. In medical

applications some amount of porosity is needed for bony tissue to grow into the coating for

efficient fixation. In addition, the crack propagation needs to be healed for the composite coating

to have reasonable mechanical strength during usage. In this sense, HIP introduces its profound

advantages by improving the adhesion and physical properties of the plasma sprayed HA

coatings as a post- treatment [79].

2.2.2 Adhesion of hot isostatic pressing of hydroxyapatite (HA) coatings on Ti-6Al-4V

K. Khor et al. [79] investigated the effect of post-sprayed HIP on plasma sprayed HA on

Ti-6Al-4V. Fig.1 shows the bond strengths of HA coated Ti-6Al-4V for the plasma sprayed

samples, and after HIP treatment at different temperatures with respect to the coating thickness.

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In general, it was shown that the bonding strength generally improves after HIP. It is also shown

that the adhesion strength decreases with increasing coating thickness. The enhancement of the

adhesion strength in the 20 wt.% HA coating after HIP is apparent for coating below 160 μm.

However, the result of adhesion strengths for coatings thicker than 160 μm show that HIP may

have adverse effects on the coating strengths.

2.3 Thermal spray coating technique

Thermal spray technology is a group of coating processes that provide functional surfaces

to protect or improve the performance of a substrate or component. Many types and forms of

materials can be thermal sprayed to provide protection from corrosion, wear, and heat; to restore

and repair components; and for a variety of other applications [80]. Thermal spraying of

biomedical coating is a relatively new class of applications for thermal spray coating as

compared with other industrial applications, [81]. Thermal spray processes are grouped into three

major categories: flame spray, electrical arc spray, and plasma arc spray. These energies sources

are used to heat the coating material (in powder, wire, or rod form) to a molten and semi-molten

state. The resultant heated particles are accelerated and propelled towards a prepared surface by

either process gases or atomization jets. A schematic diagram of thermal spray coating is

illustrated in Fig. 2.

2.3.1 Thermal spray deposition of hydroxyapatite (HA) coatings

Thermal spraying of HAP on implant devices can be compared with plasma spray coating

technique, having the advantage of high deposition rate and low cost [82, 83]. Thermal spray

technique has the ability to produce HA layer with thickness from 30 to 200 µm depending on

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the coating condition However films deposited by thermal spraying suffers from poor coating–

substrate adherence and non-uniform crystallinity which reduces the lifetime of implants [84,

85]. In addition, thermal spray requires high sintering temperature which may result in crack

propagation on the surface of the coating [86-90].

2.3.2 Adhesion of thermal spray deposition of hydroxyapatite (HA) coatings on Ti-6Al-4V

J. Hsiung et al. [91] have evaluated the applications and characterizations of biological

coating such as hydroxyapatite on titanium alloy, particularly Ti-6Al-4V, in artificial knee joint

by thermal spray coating technology. The process involves melting of HA powder and guiding

the molten mass via a jet stream of air to form a coating on the substrate, as shown in Fig. 3. The

thermal spray process conditions of the three coating materials are shown in Table 1,

highlighting the important parameters affecting the quality of the coating, such as inert gas

compositions, currents, voltage levels, powder feeding rates, and spraying distances.

The tensile test is commonly used to evaluate the bond strength in accordance to ASTM

C633 standard method [92]. A bonding strength of 33.2 Mpa was obtained by J. Hsiung et al.

[91] for the HA coating on Ti-6Al-4V by thermal spraying technique. In comparison, this result

is not satisfactory when compared to other coatings for the same application such as Al2O3,

ZrO2. In addition, results of microstructure analysis shows that the HA coatings suffers from

spalling, interface separation and high levels of porosity.

Several pre and post-treatments of HA coating were also investigated by J. Hsiung et al. [91].

Treatment conditions include high pressure cleaning, ultrasonic cleaning and cryogenic

treatments. [92]. Table 2 shows the result of the bond strength test using ASTM C633 [93],

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indicating the bond strengths of samples cleaned with high pressure air are lower as compared

with those ultrasonically cleaned, and the bond strengths with cryogenic treatments are better

than those without cryogenic treatments. The result shows that the inclusion of ultrasonic

cleaning and cryogenic treatments can effectively improve the coating properties.

2.4 Dip coating technique

Dip coating involves the deposition of a wet liquid film by withdrawal of a substrate from

a liquid coating medium. The complete process of film formation involves several stages, as

shown in Fig 3. The process starts by immersion of the substrate in the solution of the coating

material. When the substrate is withdrawn from the coating fluid, a coherent liquid film is

entrained on the surface of the substrate. A thin layer of coating is formed upon evaporations of

solvents and any accompanying chemical reactions in the liquid film. Normally an additional

post-treatment such as curing or sintering is required to obtain the final coating. Dip coating

technique is similar to sol-gel coating technique, although the process is significantly faster in

which a complete transition can be achieved within a few seconds if volatile solvents are used

[94]. Dip coating is fairly popular in the industry and in laboratory applications due to its low

cost, simple processing steps and high coating quality.

2.4.1 Dip coating of hydroxyapatite (HA) coatings

HA can be homogenously coated onto metal substrates to obtain coating thickness in the

range of 0.05 to 0.5 mm. The surface uniformity of HA can be controlled well using this

technique, as can be seen in the Fig. 4. In addition, the processing time for dip coating can be

very short, even for substrate with complex shapes. The coating layer is deposited on the surface

of the substrate without decomposition or reaction with the metal substrate. However, this

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technique requires high sintering post-treatments which may induce crack formations on the

surface of the substrate [95].

2.4.2 Adhesion of dip coating of hydroxyapatite (HA) coatings on Ti-6Al-4V

B. Mavis et al. [96] had developed several compositions of the liquid coating medium for

the dip coating of HA on Ti-6Al-4V substrates, using chemically precipitated hydroxyapatite

precursor powders. To evaluate the adhesion strength, two steel cylinders 5 mm in diameter were

attached to both sides (coated and uncoated after the coating layer was ground off) of the dipped

strips by a thin layer of glue. The adhesive strengths were determined by measuring the tensile

stress needed to separate the cylinders from the strips [97]. It is reported that, the HA coatings

obtained were highly porous, with bonding strengths of more than 30 MPa.

2.5 Pulsed Laser Deposited Coating Technique

Laser processing is a rapid and clean process which can be used for surface modification

and controlled micro-structuring of materials. In biomedical applications, laser has been used to

modify the surface texture of materials to improve its bio-functionality [98-102]. Pulsed laser

deposition (PLD) technique can be used to grow ceramic thin films. By using appropriate laser,

thin films such as semiconductor films [103], cuprate superconductor films [103, 104], and

ferroelectric films [105] can be deposited onto substrates. PLD process involves using high

power laser energy to vaporize the bulk coating material from a target. The vaporized material is

ejected from the target and condenses on the substrate. Repeated laser pulses will result in the

deposition of the thin film as a coating on the substrate [106].

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The formation of thin film by PLS can be separated into the following three stages [103,

107, 108]:

1- Laser radiation interaction with the target.

2- Dynamic ablation of the materials.

3- Deposition of the ablation materials with the substrate, nucleation and growth of a thin

film on the substrate surface.

One of the main advantages of PLD technique is the ability to retain the stoichiometry of

the target in the deposition films [107]. This is due to the high ablation rate which causes all

compounds or elements to evaporate at the same time [106]. Conversely, limitations of PLD

include the splashing of the particulates deposition on the film. Some methods have been

developed to decrease splashing problem since it is a major issues of the PLD [109]. One method

is to apply a mechanical particle filter that includes a velocity selector acting as a high-velocity

pass filter to eliminate slow-moving particulate. The second method is using a smooth, high-

density target which can be obtained by polishing the target surface before each coating run. The

third method is by applying a lower deposition rate or low energy density. Furthermore, the

deposited films have only a small area of structural and thickness uniformity, due to the angular

distribution of the ablation plume. Several methods have been proposed to scale up the PLD

process for large area thin films, such as laser beam rasterizing across a rotating target. [106].

High quality hydroxyapatite thin films deposited by the PLD was first reported in 1992

[105, 110] and since then the process have been improved significantly to obtained well adhered

and highly crystalline HA thin films under certain conditions [29, 111-113].

2.5.1 Pulsed laser deposited hydroxyapatite (HA) coatings

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Preparing hydroxyapatite thin films by pulsed laser deposition allows accurate control of

hydroxyapatite growth parameters at low deposition temperatures and the ability to produce

highly crystalline HA coatings[16, 112]. In-vitro evaluations shows that these HA coatings are

stable and osteoinductive [114, 115]. Nanostructured hydroxyapatite layer having unique

biological properties can be obtained by selection of suitable parameters for the deposition

process [16].

2.5.2 Adhesion of pulsed laser deposited hydroxyapatite (HA) coatings on Ti-6Al-4V

Adhesion strength of HA coating on metals depends on the microstructure of the

substrate, the surface chemistry and the PLD process parameters such as laser power density and

substrate temperature. [58, 116-118]. Various surface modification techniques have been used to

improve the metal-ceramic interface such as nitridation, surface oxidation and ion implantation

[119-123].

Blind, et al. reported that adhesion of pulsed laser deposited HA films on titanium alloy

is due to the existence of an oxide, specifically titanium dioxide, at the interface between the

substrate and the coating layer [20]. Another report suggests that there may be some effects of

epitaxy between the oxide and coating [124]. Fernandez- Pradas et al. [54] commented whether

the presence of a titanium oxide interface would favour adhesion of the HA coating to the Ti-

6Al-4V substrate is still a cause for debate. Some authors consider that such a layer favours

adhesion [125, 126]. Other studies have attributed the weak adhesion in the first calcium

phosphate coatings deposited by PLD at high temperatures to the formation of a titanium oxide

layer during the process of pressure stabilisation [105]. A study of the adhesion strength in

coatings deposited by ion bombardment on passivated and non-passivated substrates, suggest

that this oxide layer should be as thin as possible [127].

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C. Koch et al. [16] investigated pulsed laser deposition of hydroxyapatite on Ti-6Al-4V

for medical and dental applications. A pull-off testing method was used to determine the coating-

to-substrate adhesion strength. Garcia-sanz et al. had also examined hydroxyapatite films

prepared using pulsed laser deposition using a pull-off test based upon a modified ASTM C-633

procedure [128]. The measured tensile strength of hydroxyapatite grown at 480 °C was 58 MPa

and failure was observed at the coating–substrate interface. Wang et al. obtained tensile bonding

strength values within the range of 30 MPa and 40 MPa for hydroxyapatite coatings grown on

Ti-6Al-4V in an argon–water atmosphere at 500–600 °C [129]. Zeng et al. determined the bond

strength values for hydroxyapatite films grown using 3rd harmonic YAG:Nd lasers (λ=355 nm),

and 4th harmonic YAG:Nd lasers (λ=266 nm) on Ti-6Al-4V substrates in an argon–water

atmosphere at 500–520 °C [116]. Films grown on unpolished titanium substrates had tensile

strength values of 30 MPa while films grown on polished titanium possess lower tensile

strength values of 20 MPa.

In a study to enhance the bonding strength of HA , Nelea et al. [110] utilized a TiN

interfacial layer between the Ti-6Al-4V substrate and HA coating. The study reported that the

adhesion was improved due to better bonding of HA to TiN, which is a ceramic, and then to the

surface of metallic substrate. Man et al. [40] and Cui et al. [130] described the utilization of a

pre-treatment process which included etching and laser surface nitriding on titanium to produce a

TiN dendritic scaffold network structure. This coralline-like structure provides additional surface

area for interlocking of the coating material.

Man et al. [119] reported the influence of pre-treatments on the adhesion of the HA

coating to the substrate. Five types of pre-treatments, shown in Fig. 5 were: (i) mirror finished

specimen, (ii) 60 grit grinded SiC paper (specimen 2), (iii) 320 grit grinded SiC paper (specimen

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3), (iv) mirror finish with 1-μm diamond paste (specimen 1), and (v) 10 s etching with Knoll

solution after polishing (specimen 4). The surface roughness of the specimens were determined

using a profilometer (Taylor Hobson Surtronic 25) and the adhesion strengths between HA

coatings and the substrates were evaluated in accordance to ASTM C-633 [131]. The maximum

adhesion strength obtained was ~ 16 MPa for specimen 5 (nitrided+ etching).

Figure 5 shows the adhesion strength of deposited HA on different pre-treated specimens

and surface roughness. Generally, an increase in surface roughness increases the adhesion

strength. Based on these results, it can be concluded that significant enhancement in the adhesion

strength of pulsed laser deposited HA on Ti-6Al-4V can be obtained by laser surface nitriding

and subsequent etching [119].

A related study has concluded that a controlled surface microstructure can be obtained by

using few laser pulses without affecting the bulk mechanical property of titanium substrate [132].

Figure 6 plots the average surface roughness values, measured after laser treatment and after HA

coating versus their initial roughness.

Figure 7 compares the adhesion strengths of HA coating on substrates treated with 500–

18,000 laser pulses with those of untreated, polished titanium. The adhesion of HA to the

substrate is examined in accordance to ISO 20502:2005(E) [133] using a micro-scratch tester

(micro-combi tester; CSM Instrument Switzerland) equipped with a diamond Rockwell tip of

100 μm [132]. It was found that in all cases, the laser treated substrates would have higher

bonding strengths, which imply that the surface roughness directly influences the adhesion

strength. Varying the laser pulses would affect the surface morphology. Figure 6 shows that the

roughness increases with the increase in the number of laser pulses, which starts from ~ 0.4 μm

at 500 laser pulses/min up to ~ 1 μm at 12000 pulses/min. However, there is a significant

18  

decrease in the roughness value for laser pulses in the range of 12000 to 18000 laser pulses/min.

Low rate of laser pulses ( 500, 1,000, and 3,000 pulse/min) would only etch the surface and may

not be able to control the surface roughness. The surface roughness is under control only after ~

3000 pulses/min. A surface with controlled structure/pattern is obtained using 18,000 pulses

[132].

The polished surface of specimen does not have much adhesion strength to the coating.

However, once the surface is treated with laser, the surface roughness increases which results in

increased adhesion (from 0 to 1000 pulses/min) due to the initial material removal from the

surface. However, at this stage, certain regions are unaffected by the laser and a control over the

adhesion at this stage is not predictable. Once the laser pulses reaches ~3000 pulses/min, the

surface attains a certain level of smoothness since the large, number of pulses would completely

remove the original top surface to uncover a fresh coating surface. Therefore, the morphology

and adhesion can be controlled by the number of laser pulses (3000 to 18000) [132]. Figure 7

shows the trend of adhesion strength versus number of laser pulse, showing that the adhesion

strength would gradually increase until 1000 pulse/min, then decreases in between 1000 to 3000

pulse/min, and increases again past 3000 pulse/min. The highest adhesion strength obtained was

10.87 N and 11.21 N at 2,000 and 18,000 laser pulses respectively, while untreated substrate

showed a lower adhesion strength value of 4.57 N [132].

HA coatings by PLD exhibit good biocompatible and mechanical properties making it

suitable for medical implants. PLD HA coatings, on titanium alloy such as Ti-6Al-4V, resulted

in higher adhesion between the coating and substrate and have only minor undesirable phase

under optimal conditions [54, 106].

2.6 Electrophoretic deposition coating technique

19  

Electrophoretic deposition (EPD) is a process in which particles in a suspension is coated

onto an electrode under the effect of an electric field [134]. The colloidal particles suspended in a

liquid medium migrate under the influence of an electric field (electrophoresis) and are then

deposited onto an electrode. Electrophoretic deposition (EPD) is particularly advantageous for

ceramic film and coatings as well as laminar ceramic composites applications [134-137].

Furthermore, the method used low-cost equipment, easy to set-up, and is able to coat complex

shapes and patterns. A high degree of control on the coating results can be achieved by

regulating the deposition conditions and the ceramic powder size and shape [138]. EPD is a

cheaper method than chemical vapor deposition, sol gel deposition, and sputtering for producing

films of a wide range of thickness, from less than 1 mm to more than 100 µm thick [139].

However, limitations of the technique includes low adhesion strength, and cracking on the coated

surface due to post-process shrinkage.

EPD has shown its potential use in biomedical applications in recent years [140-142].

The interest in electrophoresis for biomedical applications [143-147] stems from a variety of

reasons such as the possibility of stoichiometric deposition, high purity material to a degree not

easily achievable by other processing techniques and the possibility of forming coatings and

bodies of complex shape [140]. Considering all advantages and disadvantages of this technique,

electrophoretic deposition is one of the favorable coating techniques which can be utilized for

hydroxyapatite coating.

2.6.1 Electrophoretic deposition hydroxyapatite (HA) coatings

20  

There is a growing interest in processing of HA powders using EPD technique, owing to

its uniformity and good sinterability of the deposits, possibility of impregnation of porous

substrates, and composite consolidation [142, 148]. However, reports on the use of EPD for

depositing HA on titanium substrate are thus far, relatively limited. Nie et al. [149] and Soares

et al. [150] have used EPD to deposit HA on Ti-6Al-4V substrates and have obtained uniform

thin coating with good mechanical strength. Stoch et al. [146] have also coated HA on titanium

implants with intermediate layer of silica. EPD process of HA is a colloidal process where HA

powders are deposited directly from a stable colloid suspension by using a DC electric field [25].

Electrophoretic deposition of HA can be processed at room temperature or lower, which

avoids problems related to formation of amorphous phases. The nature of the bond is more

metallurgical rather than mechanical, thus HA coatings using EPD are expected to have

improved adhesion strength as compared to thermal sprayed techniques. However, a major

drawback is the presence of porosities which may later on leads to corrosion and delamination of

the titanium caused by penetration of body fluids into the substrate. Post-treatment high

temperature sintering can be utilized to minimize the porosity by increasing the coating density.

Unfortunately, cracks in the coating can form during high temperature sintering due to the

difference in the thermal expansion coefficients and large reduction of the pore volume between

the titanium and HA [151].

For nanostructured materials, the mismatch in thermal expansion coefficient is not a

significant problem [152]. In nano-ceramics, the thermal expansion coefficient is fairly matched

with the metal alloy because the large quantity of atoms located at the grain boundary improves

mobility [152-154]. However, the success of electrophoretic deposited HA has been limited to

conventional materials in the range of micron-sized grains [134, 140, 154]. Limitations on the

21  

mechanical properties of the micron size HA are poor fracture toughness, adhesion, and

compressive strengths. There is a need for the HA coating and the substrate to have sufficient

interfacial bond strength since the coating would endure high interfacial stresses during in vivo

service.

2.6.2 Adhesion of electrophoretic deposited hydroxyapatite (HA) coatings on Ti-6Al-4V

Zhang, et al. [151] have developed a unique room temperature EPD process to deposit

nanostructured HA coating having adhesion strength of 50-60 MPa, which is 2-3 times better

than thermal-sprayed HA coating. The interfacial bond strength was measured in accordance to

ASTM Standard F 1501-95 using a tensile tester [151]. The corrosion resistance of this

nanostructured HA is 50 to 100 times higher than conventional HA coating. Fig. 8 shows the

corrosion resistance results for both EPD coatings and thermal sprayed coatings, where the

corrosion current of n-HA coating is 50-100 times smaller than the thermal sprayed coating in

simulated human body fluid at room temperature.

High quality HA nano-coating can be produced using EPD technique. The adhesion

stress obtained was 60 MPa, measured using a direct-pull-tests, which exceeds the 50 MPa

requirements of the food and drug administration (FDA) [155]. A 2 months in vitro testing also

showed that the bonding strength of the EPD n-HA coating on the titanium alloy was able to be

maintained in the range of 50-60 MPa, which is significantly better than plasma sprayed HA

coatings [151].

Ma, et al. [139] reported that HA particles were successfully deposited onto a titanium

substrate via a single electrophoretic deposition. Good adhesion between the coating and

substrate was verified by scanning electron miscopy examination and shear strength tests,

22  

following methods outlined by Wei et al. [148] and ASTM standard F1044-87. The shear stress

of the HA coating after sintering at 1000 ˚C was 3.34 MPa, indicating a good adhesion of the

coating has been obtained. Figures 9 and 10 show SEM micrographs of the cross-section and the

surface deposit of the 1000 ˚C sintered HA coating, respectively. It can be seen that a layer of

HA coating as thick as 400 µm has adhered well into titanium substrate and no delamination or

crack was observed at both the interface and the surface. The deposition was found to be uniform

with the coating thickness maintained consistently along the surface of the sample. No

observable crack, which is one of the common problems of EPD, was detected. It is believed that

the good deposition result is due to the stable and dispersed HA suspension used for the

deposition [148].

Studies on EPD coating of HA on titanium alloys show that particle size is an important

factor for the process as the mobility of the charged particles is proportional to the size of the

particles [156]. Ferrari et al. [157] have also reported that the charges, hence the conductivity of

the suspension, play an essential role and has an optimum value for the process. Nevertheless,

the colloidal stability of the suspension could also be a main factor to obtain good coating

uniformity and bonding strength in the EPD process [142].

Like many similar techniques for coatings involving ceramics, EPD coating of HA

requires a densification stage involving the sintering of the coated implants. This requirement

poses a dilemma, especially since high sintering temperature is sometime necessary. Low

sintering temperatures results in weak bond with low-density coatings whereas high sintering

temperatures can lead to the degradation of the HA and the metal substrate (oxidation and

impaired mechanical properties) as a result of the metal substrate catalyzing decomposition of

the HA to anhydrous calcium phosphates [158, 159].

23  

A high sintering temperature may also lead to phase transformation and grain growth of

the metal substrate, causing significant decrease in mechanical properties. It has been

demonstrated that the mechanical properties of these titanium alloys degrade significantly when

heated above 1050˚C [138]. Therefore, it is recommended to keep the densification temperatures

below 1000˚C to minimize degradation of the HA and the metal substrate.

The sintering phase for EPD implants improves densification and the bonding of the

coating. However, HA may decompose in the process [160]. An interlayer can be used in

between the HA and the metal substrate to moderate the problem of HA decomposition. Nie et

al. deposited a dense layer of titanium dioxide (TiO₂) as the inner layer between HA top layer

and titanium alloy substrate to achieve a very good combination of mechanical integrity,

chemical stability and bioactivity [149].

Kumar and Wang [161] investigated the coating of TiO₂ powders on Ti-6Al-4V

substrates as the first layer, followed by the HA- TiO₂ composite layers of different weight

ratios, coated onto the TiO₂ layer. Wei et al. [138] studied on the adhesion strength of HA

coating in which HA powders are used as both inner and outer layer. Hence, no change occurred

in the structure of coating layers. Sintering was also applied after the deposition of every single

layer. In the HA coating on TiO₂ deposited substrate, the decomposition of HA is decreased; and

generally adhesion of coating, which is tested according to ASTM F1044-99, was enhanced with

the reduction of voltage value used for TiO₂ coating [160]. Table 3 shows the result of adhesion

strengths of HA coated samples with and without TiO2 inner layer deposited using different

voltages.

2.7 Sol-Gel derived coating technique

24  

The Sol-gel method is one of the simplest technique to manufacture thin films which can

produce almost any single or multicomponent oxide coating on glass or metals [162, 163]. Sol-

gel derived coating can be used for optical, electronic, magnetic or coating with chemical

functions [164]. Sol-gel derived ceramic films are widely used as a protective layer against

corrosion and oxidation of stainless steel [165], Ag [166], and Al [167] substrates. The sol-gel

process involves the formation of solid materials, mainly inorganic non-metallic materials from

solution. This can be a solution of monomeric, oligomeric, polymeric or colloidal precursors

[168].

The sol-gel process, shown in Fig. 11 [169], consist of: (i) producing a homogeneous

solution of purified precursors in an organic solvent which can be mixed with the reagent used in

the next step or water; (ii) shaping the solution to the ‘sol’ form by using treatment with a

suitable reagent, e.g. water for oxide ceramics; (iii) changing the sol to a ‘gel’ by

polycondensation; (iv) converting the gel to the finally preferred shape like thin film, fiber, and

(v) finally converting( sintering) the shaped gel to the desired ceramic material at temperatures

(~500˚C) much lower than those required in the conventional procedure of the melting the oxides

together [168, 170-172].

Olding et al. [172], reports that sol-gel techniques has considerable advantages such as :

1. Ability to produce thin bond-coating to provide excellent adhesion between the metallic

substrate and the top coat.

2. Corrosion resistance performance due to ability to form thick coating.

3. Ability to shape materials even complex geometries in the gel state.

4. Production of high purity samples.

5. Low temperature sintering, usually in the range of 200 to 600 °C [173].

25  

6. A simple, economic and effective method to produce high quality coatings.

However, the sol-gel technique has disadvantages such as high permeability, low wear-

resistance, and difficult porosity control, which has limited its utilization in the industry. For

crack-free coating, the maximum thickness of the coating is only 0.5 μm [172]. Furthermore,

trapped organics during the thermal process would result in coating failure. Recent advancement

in high substrate sensitive sol-gel also suffers from thermal expansion mismatch. Nevertheless,

there is a wide room for improvement in the technique and further investigation should be done

to improve this highly potential method for biomaterial coating.

2.7.1 Sol-Gel derived hydroxyapatite (HA) coatings

The sol-gel is a low temperature process, thus does not suffer from the implications of

structural instability of hydroxyapatite at elevated temperatures [174-177]. A major processing

stage involves solution chemistry, whereby a sol is produced from suitable alkoxides or salts to

yield a hydroxyapatite composition upon heating [178].

Gross, et al. [178] described that the production of sol-gel hydroxyapatite coatings on

titanium substrates using alkoxide precursors requires more control on firing temperature and the

aging time. X-ray diffraction of the coatings heated to various temperatures, as illustrated in Fig.

12, indicated that the titanium substrate would start to oxidize at temperatures starting at 800˚C.

Thus for sol-gel hydroxyapatite coating, it is suggested that the processing temperature should be

around 800˚C to reduce possible phase transformation in the metallic substrate as well as the

occurrence of oxidation. [178]. Nanograined hydroxyapatite coating with an average grain size

of 50 nm was achieved using this technique. Figure 13 shows a scanning electron micrograph of

26  

a coating fired to 800˚C for 10 min. Densification of the coating can then be obtained with a

longer duration of firing at 800˚C.

Fabrication of sol-gel deposited HA on implants HA[173, 179, 180] requires extremely

stringent processing parameters, particularly for the thermal processing phase such as the

duration and calcining temperature, chemical compositions of the precursor , types of substrate,

and number of HA- coated layers. Major issues include the crystalline phases, adhesion strength

and biocompatibility of the resulted coatings.

2.7.2 Adhesion of Sol-Gel deposited hydroxyapatite (HA) coatings on Ti-6Al-4V

Tests have shown that pure HA suffers relatively high dissolution rate in simulated body

fluid that would affects its long-term stability. High dissolution may lead to disintegration of the

coatings and hinder the fixation of implant to the host tissue [181, 182]. To address this issue,

Zhang, et al. [183] incorporated fluorine ion, which exists in human bone and enamel, into HA

crystal structures. Mixing of fluorine into HA, or fluoridation, decreases the solubility of HA

while still maintaining its biocompatibility [184].

Zhang, et al. [183] have successfully deposited dense, crack-free fluoridated

hydroxyapatite (FHA, Ca10(PO4)6 (OH)2−xFx) coatings ( 1.5 μm) through sol–gel dip coating on

Ti-6Al-4V substrates. Scratch testing has shown an increase of over 35% in the adhesion

strengths of the coating to Ti-alloy. The increase in adhesion is more prominent for high

annealing temperatures. This increase is most likely due to the formation of chemical bonding at

the interface and the incorporation of fluorine in HA which provided relief of thermal mismatch.

Figure 14 illustrates the coefficient of friction in terms of relative voltage as a function of

normal load while scratching (a) pure HA coating; (b) fluoridate HA (FHA6) coating on Ti-6Al-

27  

4V. At the beginning of the scratch and because of the “soft” nature of the coating, coefficient of

friction increases as load increases. The fluctuation in the diagram, before point 1, is caused by

the surface roughness. After point 1, the indenter would start to advance into the coating,

resulting in a sharp increase in friction coefficient. The indenter would completely peel off the

coating and scratches the substrate as the load increases to point 2, or 370 mN for pure HA

(shown in curve a), which results in a sudden increase in friction at about 470 mN for FHA6.

Comparison of curves (a) and (b) in Fig. 14 shows that curve “b” appears to have less

fluctuation before the indenter completely digs in and the adhesion of coating and substrate is

better since there is a slower gradient rise after the indenter digs in. A sharp increase of friction

would indicate a brittle peeling-off of the coating from the substrate surface. Since curve “b”

lacks the sharp change in friction, it is thus a more ductile interface and subsequently have better

coating-substrate bonding than those of curve “a” (pure HA) [185].

Figure 15 shows the “upper critical load”,Lc, of all FHA coatings as a function of firing

temperature and fluorine. Both firing temperatures and fluorine content seems to have a

significant effect on the adhesion strength of the coating. Increasing firing temperature or

fluorine concentration results in a dramatic raise of the critical load. For coatings with the same

amount of fluorine content, higher adhesion is due to higher annealing temperatures. Similarly, at

the same firing temperature, adhesion strength increases with fluorine content.

Zhang, et al. [186], in similar studies [183], reported that FHA is a potential

replacement for pure HA coating on metallic implants due to FHA’s significant biocompatibility

and resistance to biodegradation [184, 187]. Ding, et al. [188] identified two critical aspects as

the main contributors for long-term stability of the ceramic-coated implants: high adhesion

strength of substrate to coating and low solubility of the coating. Incorporation of fluoride ions

28  

into HA lattice structure results in reduction of HA solubility. However, reports on adhesion

improvements, especially on adhesion studies after in vitro dissolution test have yet to be studied

extensively. In vitro dissolution tests can be used to investigate the influence of dissolution

behavior on the adhesion. Zhang, et al. [186], evaluated the adhesion of FHA coated on Ti-6Al-

4V using sol-gel technique before and after dissolution tests. The dissolution tests were

conducted by soaking FHA coatings in a Tris-buffered physiological saline solution (TPS)

(0.9%NaCl, pH7.4) at a fixed temperature of 37 °C for a duration of 3 weeks (Fig. 16). It worth

to mention that the “P “value in the Fig. 16 is one-way ANOVA test was conducted to assess the

statistical significance of the adhesion and toughness results.

Figure 16 shows the nominal adhesion strength between the coating and the Ti-6Al-4V

substrate. “Adhesion failure” and “cohesion failure” cannot be recognized by “nominal”.

Without fluoridation (sample F0), the adhesion strength is about 19 MPa. Fluoridated samples

(F1 and F2) show significant increase in adhesion strength to about 26–27 MPa. Zhang, et al.

[186] concluded that, the strength range starts from about 19 MPa for pure hydroxyapatite (x=0)

up to about 26 MPa for x=1. However, after 21 days of soaking the coating in Tris-buffered

physiological saline solution, the adhesion strength increases to about 30 MPa for pure HA and

to over 40 MPa for FHA.

Comparing the sol–gel and thermal spraying methods for the same FHA coatings on Ti-

6Al-4V, Gu, et al. [189] described that after soaking, the adhesion strengths of thermal sprayed

specimens tends to decline, with reductions up to 75%. For example the adhesion strength had

decreased from 27 MPa before soaking down to 19 MPa after soaking in synthetic body fluid

(SBF) for 2 weeks. The reduction in adhesion strength of thermal spray deposited HA coatings

is probably due to the presence of cracks in the coating [83].

29  

Cheng, et al. [190] used a pull-out method and scanning scratch technique to evaluate the

bonding strength of FHA coatings on Ti-6Al-4V. Figure 17 shows the result of measurements

by pull-out strength, showing the strength is about 11 MPa for pure HA coating (FHA0), with

considering of F content, the strength intensifies up to about 22 MPa, and then decreases to

around 17–18 MPa. Coating peeling-off value is about 390mN for pure HA. In contrast, the

coating peeling- off increases with increasing F content, 447 mN for FHA1, 450 mN for FHA2,

449 mN for FHA3 and 478mN for FHA4. The result of the study confirms that the presence of F

in FHA coatings has improved the adhesion strength [190].

2.8 Ion beam assisted deposition technique

Surface modification techniques based on the bombardment method have been used since

the mid-1970s, and many have been developed and are now widely used for surface engineering

of materials such as ceramics, bioceramics, and metals. Examples of such methods are ion beam

deposition, ion beam mixing and ion beam assisted deposition (IBAD) [191-195].

IBAD is a vacuum deposition process based on the combination of ion beam

bombardment and physical vapor deposition. The major characteristic of IBAD is the

bombardment with a specific energy ion beam during coating deposition. Many parameters can

affect the composition, mechanical properties, chemical properties, and structural properties of

the deposited coating in the IBAD process. The most important processing parameters in IBAD

are evaporation rate or sputtering rate, coating materials, ion species, ion beam current density

and ion energy [196].

IBAD has the ability to prepare bio-coatings with considerably higher adhesive strength

as compared to traditional coating methods. The high adhesive strength is the result of

interaction between the substrate and coating atoms, assisted by ion bombardment. This results

30  

in an atomic intermixed zone in the substrate-coating interface [196]. IBAD process is highly

reliable, reproducible and is conducted at low substrate temperature, without unfavorably

affecting the bulk substrate characteristics. Furthermore, the process has superior control over

coating microstructure and chemical composition [197].

2.8.1 Ion beam assisted deposition of hydroxyapatite (HA) coatings

As it mentioned earlier, there are several methods to make HA coating on Ti-6Al-4V,

among which plasma spraying is the most frequently used [198, 199]. However, long-term

clinical follow-up has demonstrated that there are significant deficiencies in the plasma-sprayed

HA coatings. The limited cohesive strength of the coatings and the limited strength of the

coating-metal substrate interface are the main problem with plasma-sprayed coting technique.

Moreover, heat treatments in plasma-sprayed HA coatings results in cracks in the coating layer

because of thermal expansion mismatch between the metal substrate and coated layer. This leads

to a severe decreasing in bond strength [200-203]. In order to produce more permanent bone-

bonding calcium phosphate coatings, ion beam assisted deposition (IBAD) is introduced as an

alternative technique for plasma spraying technique. Previous studies shows that implants coated

with HA by the IBAD method demonstrate a very good adhesion to the substrate [204].

2.8.2 Adhesion of ion beam assisted deposition of hydroxyapatite (HA) coatings on Ti-6Al-4V

In the IBAD process, a wide atomic intermixed zone between the coatinsg material and

the substrate can be created, assisted by the bombardment with energetic ions during deposition.

This creates a strong adhesion of the coating to the substrate [205, 206]. Ohtsuka et al. first used

50 keV Ca+ implantation into Ti, followed by Ca+ IBAD to deposit HA coating on Ti substrate

and has obtained higher adhesive strength than conventional methods [204]. It has been

demonstrated that Ca+ implantation alone into Ti was unable to provide the bioactive surface.

31  

Cui, et al. [207] proposed using Ar+ IBAD to form highly adhesive hydroxyapatite

coatings on titanium alloy. The coatings prepare by IBAD was compared to those formed by ion

beam sputtering deposition (IBSD) of calcium phosphate coatings. Scratch test is used to

investigate the adhesive strength of the IBSD and IBAD coatings on the substrates. Figure 18

shows the typical Fz- Fy curves of scratch test results for the specimens prepared by IBSD and

IBAD. Markers “A” and “B” indicate the points of the first occurrence of coating detachment

from the substrate. Fz and Fy , as the normal and tangential forces respectively, are affecting the

diamond indenter during the test. A load speed of 2000 gf /min was chosen for the tests. The

results have shown that the critical loads were 660 gf for IBSD and 1050 gf for IBAD samples.

Generally, it was seen that the adhesive strength of the coatings prepared by IBAD technique is

almost twice that of the IBSD coatings.

It has been shown that the adhesion strengths of coatings prepared by IBSD and plasma

sprayed technique are generally similar [127]. Thus, it can be deduced from the comparative

results between IBSD and IBAD that the adhesive strength of IBAD coatings would be

reasonably higher than that of plasma sprayed depositions. The main benefit of IBAD is the

improved adhesion strength due to the wide atomic intermixed zone at the interface of the

coating and substrate [204, 206]. Thus, the issue of low adhesion strength, which exists in

plasma sprayed coatings can be significantly eliminated by using the IBAD technique [207].

Choi, et al. [35] have used an Ar ion beam in the coating of HA on Ti-6Al-4V deposited

by IBAD technique. Figure 19 illustrates the bonding strength as a function of the ion beam

current, before and after the heat treatment. Increasing the current would increase the ion

bombardment and broadens the atomic intermixed zone during the deposition. This results in the

increase of adhesion strength between the substrate and coating layer [207].

32  

Several studies have shown that heat treatments would decrease the bond strength [200-

203]. Figure 20 shows the SEM micrographs of the coating layer before and after heat

treatments. The morphologies were found to be relatively similar regardless of the current level.

Before heat treatment, the layer was rather featureless, as shown in Fig. 20 (A). The lines at the

interface are Wallner lines frequently observed when hard coating layers are detached from a

metal substrate [208]. However, after heat treatment, the layer became severely cracked, as

shown in Fig. 20 (B). This is probably due to the thermal expansion mismatch between the

coating and the substrate [209]. These cracks are the main reason for the reduction in bond

strengths. The micrograph also reveals that the metal surface was slightly oxidized, presumably

by OH in the coating layer [209]. Overall significant improvement in the bond strength is

resulted by Choi, et al. [35] using an Ar ion beam while deposition.

Hamdi and Ide-Ektessabi [197] have proposed the deposition of hydroxyapatite layer

using a combination of technique of IBAD and simultaneous vapor deposition (SVD), namely

ion-beam-assisted simultaneous vapor deposition (IBASVD). Figure 21 illustrates the result of

coating detachments for two sets of IBASVD samples as function of different annealing

temperatures. Both types of samples resulted in similar curve patterns with the minimum

detachment forces recorded at 700 ˚C annealing temperature and the maximum adhesion strength

at 1200 ˚C. In all cases the adhesion strength for the 260 µA/cm2 sample was higher than the 180

µA/cm2 sample. In general, the recorded data for both samples are extremely higher than the

maximum adhesion strength obtainable by the SVD samples, which was less than 100 mN [210].

It is suggested that the increase in adhesion strength was the result of the formation of a mixed

layer between the substrate and the HA film , consisting of a gradient fill of Ca, P and the

element of the substrate [207, 211]. Hamdi and Ide-Ektessabi [197] described that the energetic

33  

ions assisted the reactions between the migrated atoms and the substrate atoms to generate an

intermixed layer, which have specific properties different from the deposited films and the

substrate. It was also understood that high current density of ion beam resulted in a wider atomic

intermixed zone, which consequently improved the overall adhesion strength.

2.9 Sputter coating technique

Sputter deposition is a physical vapor deposition (PVD) method of depositing thin films

by sputtering. This involves ejecting material from a source, known as a "target", onto a

"substrate" such as a silicon wafer. It was reported that initial sputtering using multi-component

ceramic targets such as superconducting oxides, HA and other CaP materials would produce

coatings whose chemistries were different upon deposition than the bulk target [212, 213].

Sputtering utilizes a gas plasma (argon, neon, krypton or xenon) to remove material from a

negatively charged target which is then deposited as a thin film coating onto the substrate.

Studies have shown successful deposition of thin HA layers on titanium substrates using RF

magnetron sputtering [214].

2.9.1 Sputter coating of hydroxyapatite (HA) coatings

Sputtering techniques have been used to deposit homogeneous thin films coatings of high

adhesion strength with thicknesses ranging from 0.5 to 3 μm. However, sputter coated HA films

on metals were found to be of low crystallinity [214-216]. The low crystallinity increases the rate

of dissolution of the coating in the living body. Post-treatment thermal process can be used to

crystallize the film, hence reducing the possibility of dissolution. However, conventional thermal

treatment in the electric furnace increases the likely formation of cracks and may degrade the HA

films.

2.9.2 Adhesion of sputter coating of hydroxyapatite (HA) coatings on Ti-6Al-4V

34  

K. Ozeki et al. [217] compared the thermal treatments of the HA coated on titanium alloy

substrate prepared by sputter coating with those prepared by plasma spraying technique. The

substrates were sandblasted using Al2O3 (125-180 µm) abrasive before coating. The specimens

were post-treated with a hydrothermal process for 24 hours. The film thickness obtained for

sputter coating was 1.2 µm while the thickness for plasma spraying was 60-100 µm.

Figure 22 shows the shear strength results of the sputter coating, the plasma sprayed

coatings and the non-coated columns over a period of time. The sputter coating showed the

highest bonding strength overall with recorded strengths of 3.3 ± 0.2, 5.7 ± 0.5, and 8.6 ± 1.6

MPa after two, four, and 12 weeks, respectively. The plasma sprayed coatings resulted in

strength values of 1.9 ± 0.25, 4.0 ± 0.3, and 6.6 ± 0.7 MPa, respectively, for the same period of

time. The strength values of the non-coated columns were 0.4 ± 0.3 and 1.1 ± 0.3 MPa after four

and 12 weeks, respectively. The strength of the sputter coating exceeded that of the plasma

sprayed coating by more than 70, 40, and 30% after a period of two, four and twelve weeks,

respectively. K. De Groot et al. reported that coating thicknesses above 100 μm were associated

with fatigue failure under tensile loading [218]. According to S. Hasegawa et al., thin plasma

sprayed coatings are bound more strongly than thick coatings [219].

S.J. Ding et al. [220] investigated on a series of thin (<10 μm), single layered HA/Ti

coatings deposited on Ti-6Al-4V substrate using an RF magnetron-assisted sputtering system.

For the experiments, six HA/Ti targets with different compositions (95HA/5Ti, 90HA/10Ti,

85HA/15Ti, 75HA/25Ti, 50HA/50Ti, and 25HA/75Ti) were prepared. Generally it was found

that the coating with higher Ti contents resulted higher adhesion strengths. The highest adhesion

strength (of the 25HA/ 75Ti coating), evaluated using a Sebastian adhesion test system

(Sebastian Five, Quad Group, Spokane, WA) [127] was even higher than 80 MPa, which

35  

exceeded the maximum value achievable using the bonding resin in the pull-out test. Table 4,

reports the adhesion strength and their corresponding failure point for different compositions and

Fig 23 shows the adhesion strength for each composition.

The high adhesion strength of sputtered monolithic HA coating is higher than most

plasma sprayed HA coatings [221, 222], and is believed to be attributed to the sputter cleaning

and ion bombarding processes. The sputter cleaning process would remove contaminants and

adsorbed gas molecules from the surface of the substrate to produce a clean, highly active

surface [223]. The ion bombarding process during sputtering would enhance atomic diffusion

and mixing near the interface region [207, 224]. Mechanical interlocking effect may have

contributed to the higher average adhesion strength of coating sputtered on the rougher surface

(Ra = 0.7 mm) as compared to the lower value obtained for the smoother surface (Ra = 0.06

mm). However this effect was not as significant for sputtering with Ti-containing targets.

Results from S.J. Ding et al. [220] have shown that all coatings had adhesion strengths

between 60 and 80 MPa. Furthermore if the sputtering uses a target comprising of more than 15

vol % Ti, the resulting coating adhesion strength and hardness were significantly higher than

those of monolithic HA coating.

3. Discussion

There have been numerous studies on coatings of hydroxyapatite (HA) onto Ti-6Al-4V

because of its significant utilization in orthopedic prostheses and implants. Table 5 summarizes

the previous discussion on the various techniques for coating of HA on Ti-6Al-4V, with

comparison on their advantages and disadvantages.

Plasma spraying is the most frequently investigated method to coat HA onto Ti-6Al-4V

specimen, [198, 199]. Plasma spray is the first method used for HA coating, owing to its ease of

36  

application [26]. Moreover, the determination of the adhesion between the coating and the

substrate has been always a main concern when using plasma spraying technique [59]. High

spraying power results in high adhesion strength of HACs due to significant melting of the

coating material which forms dense microstructure. However, the high-temperature process can

lead to phase transformation and grain growth of the metal substrate which may cause significant

decrease in the mechanical properties of the metal.

Results of the study [61] has established the relationship between residual stress and

bonding strength especially for plasma sprayed hydroxyapatite coatings. This stress in the

coating is influenced by the spraying parameter, coating thickness [67, 68], and substrate cooling

effect (i.e. temperature of substrate) [62, 63]. Generally, the residual stresses increase with the

increase in the thickness of coating and the temperature of the specimen during plasma spraying.

Moreover, high-power sprayed HA coatings generally possess higher adhesion strength than

those sprayed with lower power. In some cases, the adhesion of the plasma sprayed HA can be

significantly improved by a subsequent hot isostatic pressing operation.

The adhesion strength is a reflection of the combination of cohesive (within the coating

layers themselves) and adhesive (coating to substrate) strengths of a coating [61]. The cohesive

strength is obtained by coating properties, such as the microstructure and crystallinity, but the

adhesive strength is mostly influenced by coating properties, such as surface roughness, residual

stress, and the mechanical interlocking between substrate and HACs [61].

Overall, it was found that plasma sprayed coating has not improved the service-life

performance of uncoated implants. In addition, there are issues with poor reliability and

mechanical integrity [56, 57]. The pulsed laser deposition (PLD) is a better alternative than the

plasma spray technique because the PLD transfers sintered HA stoichiometrically to deposit a

37  

thin adherent coating onto titanium substrate surface [58]. The substrate temperature is lower in

PLD as compared to plasma spray and different calcium phosphate compositions can be

deposited by changing the parameters of deposition [112, 114, 225]. In addition, undesirable

phases of HA coatings by PLD are reduced under optimal conditions and generally have better

coating to substrate adhesion [54, 106].

TiO2 and TiN layers can be used as an interfacial layer between coating and the metal

substrate as reported in studies related to the adhesion of crystalline PLD HA thin films on Ti-

6Al-4V substrates [20]. Some authors consider that this interfacial layer favours adhesion due to

better bonding of HA to TiN which is then,directly bonded to the substrate [125, 126]. These

layers can be created using pre-treatment processes, such as laser surface nitriding and etching

on titanium, which have been reported to improve the bonding strength of the coating. Thus,

laser surface nitriding and subsequent etching of the substrate is an effective pre-treatment

method for improving the adhesion strength of HA coated onto Ti-6Al-4V by PLD [119].

EPD is a technique which is gaining attention due to its ability to economically produce

films of a wide range of thicknesses as compared to conventional methods such as thermal

spraying, sol gel deposition, and sputtering [139]. Moreover, EPD of HA has ability to be

processed at room temperature, reducing the possibility of formation of the amorphous phase in

HA. The good uniformity and bonding strength results is mostly due to the colloidal stability of

the suspension [142]. The EPD technique can also produce nanostructured HA coating having

bond strength 2-3 times better than thermal sprayed HA coating.

Similar to PLD, studies have shown that an intermediate layer, such as silica or TiO2,

improves the adhesion strength of coating fabricated using EPD [146]. Dense titanium dioxide

(TiO2) films possess a very good combination of bioactivity, chemical stability and mechanical

38  

integrity [149]. A TiO2 inner layer would also reduce the decomposition of HA and increases the

and overall adhesion strength of coating [160].

The sol-gel technique is a simple technique which can create single or multicomponent

oxide coating on glass or metals [162, 163]. However, there is a coating thickness limit of 0.5 μm

[172]. Fluoridation of HA can enhance the coating’s resistance to biodegradation while still

maintaining good biocompatibility [184, 187]. An increase in fluoridation ratio would increase

the adhesion strength by about 40%. The strength range for FHA is about 26 MPa which is

higher than the value of the bonding strength of 19 MPa for pure hydroxyapatite. The fracture

toughness increases about 200 to 300% and the scratch test results in adhesion improvement of

35 % for fluoridated HA coatings as compared to pure hydroxyapatite coating [183, 186, 190].

The enhancement in adhesion strength is believed to be caused by the formation chemical

bonding at the interface and the relief of thermal mismatch resulting from the incorporation of

fluorine (F) into the HA structure.

Dip coating can be generally compared with sol-gel coating technique. The technique is

simple, economical and is able to generate high coating quality. Dip coating process is rapid,

where the complete transition can be completed within a few seconds or less if volatile solvents

are used.

IBAD technique can deposit highly adhesive HA coating on Ti-6Al-4V due to atomic

interactions between the substrate and coating materials, assisted by ion bombardment [196]. The

main advantage of IBAD compare to other methods, such as IBSD or plasma spraying, is that

there is a wide atomic intermixed zone at the coating-substrate interface which significantly

improves the adhesive strength of the coating. Heat treatment of IBAD coated samples reduces

39  

the adhesion strength, due formation of cracks in the layer and the thermal expansion mismatch

between the coated layer and the metal substrate [200-203].

Figure 24 shows adhesion strength values of HA coatings on Ti-6Al-4V coated using

various techniques. The sputtering technique has the highest adhesion of coating to the substrate

compares to other methods which can be attributed to the sputter cleaning and ion bombardment

processes.

4. Conclusion

Adhesion strength of HA on Ti-6Al-4V substrate has been reviewed in detail. Nine

common techniques of deposition such as plasma sprayed deposition, hot isostatic pressing,

thermal spray, dip coating, pulsed laser deposition (PLD), electrophoretic deposition (EPD), sol-

gel, ion beam assisted deposition (IBAD), and sputtering were evaluated and discussion were

made on the coating parameters affecting the adhesion strength of the coating. Advantages and

disadvantages of each method were discussed and a quantitative comparison was made on the

different techniques of HA coating on Ti-6Al-4V substrate. Based on this review, the best

adhesion of HA coating to substrate is obtained by sputtering deposition technique while the

worse bonding strength was obtained by PLD at 1000 laser pulses. Using an interfacial layer

(such as TiO2 or TiN) as the initial coating layer on the substrate followed by HA coating layer

can enhance the bonding strength. Pretreatments such as nitriding, followed by etching, can

enhance the adhesion strength in PLD. Moreover, post-treatments also have similar effects on

other techniques such as IBAD and thermal spray.

Acknowledgement

40  

The authors would like to acknowledge the University of Malaya for providing the necessary

facilities and resources for this research. This research was fully founded by the Ministry of

Higher Education, Malaysia with the high impact research grant number of

um.c/625/1/HIR/MOHE/ENG/27.

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Table 1 Thermal spray condition of HA powders [92].

Parameters Argon

(l/min)

Helium

(l/min)

Current Voltage Powder

Rate

(g/min)

Spray

Distance

(mm)

Surface

Speed

(m/min)

Travers

Speed

(mm)

Cooling

Setting 41 60 700 52 30 115 75 8 yes

Table 2 Bond strength test results with different pretreatment and cryogenic treatment [92].

Coating Bonding Strength ( MPa)

Without Cryogenic

Treatment

With Cryogenic Treatment

Ultrasonic High

Pressure Air

Ultrasonic High

Pressure Air

HA 26.56 18.91 36.65 29.30

53  

Table 3 Adhesion strengths of HA coated samples with and without TiO2 inner layer

deposited using different voltages [160].

Samples

(substrate + inner layer + Ha)

Shear Strength

(MPa)

Ti-6Al-4V + ----- + HA 13.8 ( s=1.8)

Ti-6Al-4V + TiO2 (50 V)+ HA 11.9 ( s=1.8)

Ti-6Al-4V + TiO2 (20 V)+ HA 13.1 ( s=1.8)

Ti-6Al-4V + TiO2 (10 V)+ HA 21.0 ( s=1.8)

Note. S: standard deviation.

Table 4 Adhesion Strength and Failure Mode of Coatings [220].

Coating Code

Adhesion Strength (MPa) Failure Mode (Ra= 0.06 µm) (Ra= 0.06 µm) (Ra= 0.7 µm)

HA 59.9 ±12.4 (41) 71.8±14.7 (25) R/C, C/S

95HA/5Ti 59.5 ± 6.5 (20) 60.7±5.8 (23) R/C, C/S

90HA/10Ti 58.4 ± 6.2 (18) 54.5±6.1 (12) R/C, C/S

85HA/15Ti 64.8 ± 6.2 (17) 69.5 ± 10.3 (19) R/C, C/S

75HA/25Ti 64.0 ± 6.9 (32) 65.3 ± 6.5 (50) R/C, C/S

50HA/50Ti 75.1 ± 5.5 (22) 72.9 ± 5.4 (28) R/C

54  

25HA/75Ti 81.9 ± 5.2 (19) 79.8 ± 6.3 (17) R/C

Ti 79.5 ± 9.1 (15) 85.1 ± 5.1 (34) R/C

Table 5: Different techniques to deposit HA coating.

Technique Thickness Advantages Disadvantages

Plasma Spraying < 20 µm rapid deposition ; sufficiently low cost; fast bone healing, less risk for coating degradation

Poor adhesion, alternation of HA structure due to coating process; non- uniformity in coating density; extreme high temperature up to 1200 ºc, phase transformation and grain grow of substance due to high temperature procedure; increase in residual stress; unable to produce complete crystalline HA coating

Thermal Spraying

30- 200 µm High deposition rates; low cost;

Line of sight technique; high temperatures induce decomposition; rapid cooling produces amorphous coatings; lack of uniformity; crack appearance; low porosity; coating spalling and interface separation between the coating and the substrate

Sputter Coating 0.5- 3 µm Uniform coating thickness on flat substrates; dense coating; homogenous coating; high adhesion

Line of sight technique; expensive time consuming; produces amorphous coatings; low crystallite which accelerates the dissolution of the film in the body

Pulsed Laser Deposition

0.05- 5 mm Coating with crystalline and amorphous; coating with dense and porous; ability to produce wide range of multilayer coating from different materials; ability to produce high crystalline HA coating; ability to restore complex stoichiometry; high degree of control on deposition parameters

Line of sight technique; splashing or particle deposition; need surface pretreatment; lack of uniformity

Dip Coating < 1 µm Inexpensive; coatings applied quickly; can coat complex substrates; high surface uniformity; good speed of coating;

Requires high sintering temperatures; thermal expansion mismatch; crack appearance

Sol-gel 0.1- 2.0 µm Can coat complex shapes; Low processing temperatures; relatively cheap as coatings are very thin; simple deposition method; high purity; high corrosion resistant; fairly good adhesion

Some processes require controlled atmosphere processing; expensive raw materials; not suitable for industrial scale; high permeability; low wear resistance; hard to control the porosity;

55  

Electrophoretic Deposition

0.1- 2.0 mm Uniform coating thickness; rapid deposition rates; can coat complex substrates; simple setup, low cost, high degree of control on coating morphology and thickness, good mechanical strength; high adhesion for n-HA

Difficult to produce crack-free coatings; requires high sintering temperatures; HA decomposition during sintering stage

Hot Isostatic Pressing

0.2- 2.0 mm Produces dense coatings; produce net-shape ceramics; good temperature control; homogeneous structure; high uniformity; high precision; no dimensional or shape limitation

Cannot coat complex substrates; high temperature required; thermal expansion mismatch; elastic property differences; expensive; removal/interaction of encapsulation material

Ion Beam Assisted Deposition

<0.03 µm Low temperature process; high reproducibility and reliability; high adhesion; wide atomic intermix zone are coating-to-substrate interface

Crack appearance on the coated surface

Figure 1 Tensile bond strength result of plasma sprayed Ti-6Al-4V/ 20 wt.%

hydroxyapatite coating (as sprayed and HIPed) [79].

56  

Figure 2 A schematic diagram of thermal spray coating [82].

Figure 3 Fundamental stages of dip coating (the finer arrows indicate

the flow of air) [94].

57  

Figure 4 SEM micrographs from cross-sectional view of HA coatings (via SOL 2) on Ti-

6Al-4Vsubstrates after heating at 840°C [96].

58  

Figure 5 Comparison of adhesion strength for HA on substrates with different pre-treatments

[119].

Figure 6 Average surface roughness of titanium substrates treated with different laser pulses and

HA coating compared with control sample [132].

59  

Figure 7 Failure values obtained by scratch test (Lc1, Lc2 and Lc3) for the HA coatings on

different irradiated and non-irradiated titanium substrate [132].

Figure 8 Electro-polarization corrosion curves for both EPD n-HA coating and HA thermal

sprayed coating [151].

60  

Figure 9 Cross section SEM micrograph of the EPD deposited under the identified optimum

suspension condition [140].

Figure 10 SEM micrograph of the uncrack deposit surface [140].

100µm

61  

Figure 11 Steps in the sol-gel process for ceramic materials [169].

Figure 12 X-ray diffraction of sol-gel coatings preferred to 500˚C on titanium substrates and

then fired at various temperatures [179].

62  

Figure 13 A scanning electron micrograph of a coating fired to 800˚C for 10 min, the field of

view is 250 nm × by 250 nm [179].

Figure 14 Coefficient of friction in terms of relative voltage as a function of normal load while

scratching (a) pure HA coating; (b) fluoridate HA (FHA6) coating on Ti-6Al-4V [174].

63  

Figure 15 Adhesion strength of pure HA and fluoridated HA coatings on Ti-6Al-4V substrates

as indicated by upper critical load in scratch test. Firing temperatures are indicated [174].

Figure 16 Pull-out adhesion strength of FHA coating before and after soaking in TPS solutions.

* indicates a significant increase of adhesion strength with respect to F0 (as prepared coatings);

64  

** indicate a significant increase of adhesion strength with respect to F0 (after soaking in TPS

for 21 days) [186].

Figure 17 Pull-out strength of coatings with different F content [190].

65  

Figure 18 Fz-Fy curve of scratch test from specimen prepared by (a) IBSD and (b) IBAD [207].

Figure 19 Layer-metal substrate bond strengths, before and after heat treatment, as a function of

ion beam current [207].

66  

Figure 20 SEM micrographs of the coating layer (A) before and (B) after the heat treatment

[208].

67  

Figure 21 Adhesion strength of the IBASVD samples at different elevated temperatures [197].

68  

Figure 22 Bone bonding strengths of sputtered films [217].

69  

Figure 14 Adhesion strength of coatings [220].

70  

Figure 24 Quantative comparison of different coating techniques.

Plas

ma

Spra

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PLD

(100

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Las

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PLD

(Nitr

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EPD

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EPD

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Sol-G

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The

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The

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Dip

Coa

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Hot

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Pre

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Pla

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Sput

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