IMPROVEMENT OF LASER-CLADDED SURFACES WITH ADDED … · 2017. 10. 27. · 4 C. Vimalraj, P. Kah, B. Mvola and P. Layus ever, the cladded surface with a nominal amount (i.e. 2 wt.%)

1Improvement of laser-cladded surfaces with added nanomaterials

© 2017 Advanced Study Center Co. Ltd.

Rev. Adv. Mater. Sci. 51 (2017) 1-15

Corresponding author: Paul Kah, e-mail: [email protected]

IMPROVEMENT OF LASER-CLADDED SURFACES WITHADDED NANOMATERIALS

Cyril Vimalraj, Paul Kah, Belinga Mvola and Pavel Layus

Laboratory of Welding Technology, Lappeenranta University of Technology, P.O. Box 20, FI 53851,Lappeenranta, Finland

Received: November 29, 2016

Abstract. Industries like the aerospace, automobile, marine, and other manufacturing sectorsrequire surface modifications on substrates through laser cladding process, because the laserprocess uses only minimum resources to provide better surface quality compared to conventionalprocesses. As industry’s demands have widened, the conventional coating material can nolonger satisfy the growing criteria. New technologies have innovated a solution to the coatingmaterial through the addition of nanomaterials, which satisfies the growing demands and attainssignificant improvements in properties. This study reviews the literature about the effects ofnanomaterial addition to laser cladding materials. The paper mainly focuses on the effects andfactors of nanomaterial addition influencing the microstructure, properties, as well as defectreduction. The review shows that adding nanoparticles to the coating material significantlyimproves the cladded surface quality and the properties compared to the conventional claddingsurface. The cladded surface with nanoparticles has reduced defect formation, increased stablemicrostructural phase formations and improved mechanical, wear resistance and thermal shockresistance properties. The general factors that influence these improvements are the appropriateselection of nanomaterial addition, uniform dispersion of the nanoparticles, characteristics ofthe nanoparticles and the quantity of nanoparticles added to the coating material. However, thenanoparticle-added cladding material may also have degraded surface properties. Many studiesare investigating various other coating materials using different cladding processes to attaingood surface properties and satisfies the demands of industries.

1. INTRODUCTION

Laser cladding is an efficient process of depositingmaterials on substrates compared to other coatingprocesses based on factors, which are detailed inTable 1 [1]. This process provides a protective layeron the substrate, which is mostly used for the re-duction of wear and corrosion. For instance, mostof the engine parts and tools in the aircraft and au-tomobile industries have been coated; moreover,coatings are also used in repairing turbine blades,buildings and exterior usage applications [2]. Re-cently, many products only require surface modifi-cation with upgraded surface properties rather than

processing the whole components such as metaljoining, machining, drilling, forming and rolling. Asthe surface coatings demands in industries are grow-ing for its high strength, requirements have beenrising to improve the cladding materials.

Surface coating requires minimum dilution, inorder to improve the mechanical properties andchemical composition between the substrate andthe coating material [3]. The minimum dilution be-tween the coating material and the substrate re-quires the manipulation of the heat input, speed andother processing parameters, which can been effi-ciently processed by the laser cladding process.Generally, the frequently used cladding or coating

mailto:[email protected]

2 C. Vimalraj, P. Kah, B. Mvola and P. Layus

materials are nickel (Ni)-based, cobalt (Co)-based,and iron (Fe)-based, which show better propertiesand are used for various applications in the indus-tries.

Studies have shown that the addition of a rareelement (RE) to the coating material improves theproperties and surface appearance by its uniquephysical and chemical characteristics [4]. However,adding RE to the coating material has not satisfiedthe industries demands. Other research showed thatthe particle size in the cladding coating materialhas a great influence in the properties and micro-structural formation between the substrate and thecoating material [1]. For instance, Wellman andNicholls [5] showed that the size of the carbide inthe coating material had a significant influence inthe surface property, especially in wear and erosionresistance.

As nanoparticles have unique thermal conduct-ing, mechanical and strengthening properties, dueto their size and surface characteristics [6] [6]. In-cluding nanoparticles to the coating materials in-stead of microparticles will improve the claddedsurface with a stable microstructure, thereby gen-erating better surface properties. Research hasshown that the nanoparticles reinforces the metalmatrix composites through coating, leading to im-proved properties and surface integrity [7-9]. Stud-ies also show that the amount of nanoparticles addedto the coating material have greater influences onthe surface microstructures and properties. There-fore, there is need to acknowledge the effect ofnanomaterials on the coated surfaces and the fac-tors that influence good surface integrity throughthe formation of a stable microstructure and improve-ments in surface properties. The study mainly fo-cuses on microstructural changes and the surface

FEATURES Surface deposition processesLaser cladding Thermal spray Chemical Vapour Physical Vapour

Deposition Deposition

Bonding strength High Moderate Low LowDilution High Nil Nil NilCoating material Metals, Ceramics Metals, Ceramics Metals, Ceramics Metals, CeramicsCoating thickness 50 m to 2 mm 50 m to 0.05 m to 20 m 0.05 m to10 m

several mmRepeatability Moderate to High Moderate High HighHeat-affected zone Low High Very low Very lowControllability Moderate to High Moderate Moderate to High Moderate to HighCost High Moderate High High

Table 1. Comparison of laser cladding process with other surface deposition processes, see [1].

property improvements through the addition ofnanoparticles to the coating materials in Ni-based,Co-based, and Fe-based coating material as wellas the factors that influence these improvements.

2. NICKEL-BASED COATINGMATERIAL

Coatings of high-temperature protection have beenwidely used in the aerospace, thermal power sta-tion, chemical and other fields [10,11] for fluctuat-ing service conditions. These high-thermal protec-tion coatings have been most frequently used in gasturbine components [12]. The Ni-based coating isoften used and a recommended coating material forthermal protection applications [10]. As there areradical changes in service conditions, improvementsto the coating materials prior to the substrates arerequired. Nanoparticles have been added to the Ni-based coating material to improve properties as wellas to attain the microstructural stability at varyingclimatic and service conditions.

2.1. Effect of nanoparticle-addedcoating material on defects

Defect formations between coated surface andsubstrate have a significant influence on the proper-ties and strength of the coated surface. TheNiCoCrAlY coating material is prone to the poresformation and crack formations by the alloying ele-ment in the coating material. The NiCoCrAlY-coatedsurface shows crack formation in the interfacial re-gion and its propagation from the interfacial claddedregion to the cladded surface degrades the strengthof the coated surface [13,14].

By the addition of nominal amount (i.e. 2 wt.%)cerium oxide (CeO

2) nanoparticles to the NiCoCrAlY


cladding material produced better surface withoutcracks. However, the high content (3 wt.%) of CeO

2

nanoparticles to the coating material generated poreformation on the cladded surface [14]. Instead ofRE (i.e. CeO

2) as nanomaterial, adding metal oxide

nanoparticles (i.e. Aluminium oxide-Al2O

3) in low

amount (i.e. 0.5% mass fraction) to the NiCoCrAlYcladding material also resulted in pores at the inter-face region. The increased content of Al

2O

3

nanoparticles to a nominal level (i.e. 1%) in the coat-ing material decreased pore formation, as shown inFig. 1 (a and b). The high content (1.5% mass frac-tion) of Al

2O

3 nanoparticles in the cladding again re-

sulted in pores formation as shown in Fig. 1c, [13].The main cause for the crack and pores forma-

tion in cladded surface was because of the differ-ence in the thermal expansion co-efficient betweenthe materials [13]. The nominal quantity ofnanoparticles addition to the cladding material de-creased the difference in thermal expansion betweenthe coating and the substrate, thereby reducing theformations of pores and crack compared to low andhigh quantity of nanoparticles. This thermal expan-sion difference occurred by the segregation of parti-cles in grain boundaries thereby leading to cracks.Thus, either RE nanoparticle or metal oxide

Fig. 1. Defect in interfacial region between the Ni-base super-alloy and Ni-based coating material withaddition of (a) low content nanoparticles, (b) nominal content nanoparticles, (c) high content nanoparticles.Reprinted with permission from Wang et al., © 2009 Transactions of Nonferrous Metals Society of China.

Fig. 2. Cladding interfacial region between the Ni-base super-alloy and Ni-based coating material formed (a)without nanoparticles (b) with CeO

2 nanoparticles. Reprinted with permission from Lepski et al., © 2009

Springer Netherlands.

nanoparticles added to the NiCoCrAlY claddingmaterial with a nominal quantity reduces the defectformation by influencing the thermal expansion ofmaterials.

2.2. Effect of nanoparticle-addedcoating material on themicrostructure

Grain refinement and grain transformation plays animportant role in the formation of stable microstruc-tures on Ni-based cladding surface. Insertingnanoparticles to the surface coating material wouldaid in grain refinement for the formation of stablemicrostructures between the surface and substrate.For instance, the experiment by Wang et al. [14]on a Ni-based alloy substrate, with and without theaddition of CeO

2 nanoparticles in the NiCoCrAlY

coating showed variation in the microstructuresalong the interface region and the surface. Thecladded surface without added nanoparticles had amicrostructure containing equiaxial grains in theclad layer; moreover, the interface region betweenthe substrate and the cladding surface contained adendrite and epitaxial microstructure with non-uni-form distribution, which is shown in Fig. 2a. How-


ever, the cladded surface with a nominal amount(i.e. 2 wt.%) of CeO

2 nanoparticles led to fine

equiaxial grains in the clad layer and the dendritesstructure in the interface region also transformedinto equiaxial grains with a uniform distribution, whichis shown in Fig. 2b. Thus, only equiaxed microstruc-ture were found in the interface region with a nomi-nal amount of nanoparticles, and other than nomi-nal a content of nanoparticles added to the coatingmaterial produced hardly any dendrite microstruc-tures.

Besides using RE materials as nanoparticlesincluded in Ni-based coatings, Al

2O

3 nanoparticles

were added to the NiCoCrAlY coating material.Observation showed that the surface coated with-out Al

2O

3 nanoparticles also formed dendrite struc-

tures perpendicular to the interface surface; how-ever, a coated surface with Al

2O

3 nanoparticle addi-

tion had transformed these structures into fineepitaxial structures. This transformation was due tothe promotion of heterogeneous nucleation that ob-structed the growth of grains. The grain boundariesof the cladded region formed without nanoparticlesaddition were smaller compared to region claddedwith the nanoparticle [13]. The nanoparticles addi-tion (i.e. Al

2O

3 andCeO

2) in the coating material has

led to the segregated into grain boundaries. Moreo-ver, increasing the nano Al

2O

3 particle as well as

nano CeO2 particle content in the Ni-based coating

materials increased segregation at boundaries re-sulting in degradation of properties [13,14].

In both cladded regions with additions of RE andAl

2O

3 nanoparticles, the cladded and the interface

Fig. 3. Uniformity of Ni/WC composite coating (a) without nanoparticles, (b) with 0.5% La2O

3, (c) with 1%

La2O

3 and (d) with 2% La

2O

3. Reprinted with permission from Sun et al., © 2005 Surface and Coatings

Technology.

region without added nanomaterial had phases-Ni, -NiAl, ’ -Ni

3Al, and -Cr (Chromium- Cr). How-

ever, the phase -Cr was not found in the clad re-gion formed with added nanoparticles in both CeO

2

and Al2O

3 surfaces. This disappearance was due to

an increase in the solid solubility of the Cr element.As Ce is a RE material, the adhesion of Ce reducesthe grain growth and results in the solubility of theCr element. In the case of Al

2O

3 nanoparticle-added

coating, Al2O

3 nanoparticles obstructing the disper-

sion of the element and Cr produced a solid reac-tion with the Ni substrate [13,14].

As composite materials provide better proper-ties and strength compared to the metal oxide andRE nanoparticles, the composite materials in thecoating process produced good properties andstrength on the substrate and the surface [9]. Inaddition, the nanoparticles included in the compos-ite material will provide far better properties com-pared to the commercial composite coating materi-als. For instance, Farahmand et al. [15] experi-mented on laser cladding with coating materialssuch as the Ni/WC (tungsten carbide) compositeon mild steel substrates and found three differentmicrostructure formations: fine bone structure, in-ter-dendrite structure and equiaxed structure. Thedendrite structure initiated growth due to the ther-mal properties of the WC particles. Moreover, thesaturated solid solution present in the microstruc-ture exhibited significant amounts of C (carbon) andW (tungsten).

By the addition of WC nanoparticles to the Ni/WC coating material improved the distribution of WC


particles, which resulted in a similar microstructure.Yet, the microstructure was fine compared to thecoating without added nanoparticles. However, thesignificant addition of WC nanoparticles with a lowwelding temperature resulted in cluster formation,which thereby formed blocky carbide in the coat-ing. Instead of adding WC nanoparticles, the addi-tion of nanoparticle La

2O

3 (lanthanum oxide) into the

Ni/WC coating material showed an improvement inthe homogeneity of the cladded surface with an in-crease in the nanoparticles, which is shown in Fig.3. The addition of La

2O

3 resulted in a microstructure

with only dendrite and inter-dendrite. The dendritestructure size and inter-dendrite spacing as well assolidified re-melted carbides decreased, due to theLa

2O

3 addition that increased latent heat, thereby

reducing the melting temperature and increasingsolidification time. Moreover, there were O-rich re-gions and Cr-rich regions found in the microstruc-ture compared to the Ni/WC coating. The new phaseand region formation was due to the RE additioninto the Ni/WC composite material [16].

Thus, the addition of nanomaterials in the Ni-based cladding material shows transformation in thetransition region microstructure between thesubstrate and coating. Moreover, phase formationdisappeared from the microstructure. The compos-ite material coating with added carbide nanoparticlesexhibited a fine microstructure and blocky carbideformation, and added RE nanoparticles showed ahomogeneous microstructure and the formation ofnew phases. Thus, the refinement of the microstruc-ture and microstructural transformation occurred bythe nominal content of nanomaterials, uniform dis-persion of nanomaterials and appropriate selectionof nanomaterials.

Fig. 4. Surface hardness of laser cladded Ni-based coating on Ni super-alloy with different nanoparticleaddition. Reprinted with permission from Zhong et al., © 2010 Proceedings of the Institution of MechanicalEngineers, Part C: Journal of Mechanical Engineering Science.

2.3. Effect of nanoparticle-addedcoating material on surfaceproperties

As the Ni-based alloy cladding material providesgrain refinement and stable microstructural forma-tions through nanomaterial addition, the nanomaterialwill yield high wear and thermal shock resistancefor fluctuating service conditions. Besides the wearand thermal shock resistance properties, other prop-erties also influence the service condition.

2.3.1. Hardness property

The hardness characteristic of the surface reflectsthe surface strength and the bonding between thesubstrate and cladding, and, thus, the nanoparticlesadded to the cladding surface show improved hard-ness. For example, Wang et al. [14] observed thatadding CeO

2 nanoparticles to the Ni-based coating

had uniform hardness distribution on the surfacecompared to a surface coated with the Ni-basedmaterial without nanoparticles. Moreover, themicrohardness value increased by the addition ofnanoparticles; however, with a medium amount (i.e.2 wt.%) of CeO

2 nanoparticles, the cladded surface

showed the highest hardness value compared toother cladded surfaces.

Previous research has shown that carbide inclu-sion particles in fabrication processes result in highhardness values compared to other particles [17,18].Thus, Wang et al. [19] compared Al

2O

3

nanoparticles, SiC (silicon carbide) nanoparticles,and CeO

2 nanoparticles added to the NiCoCrAlY

coating on a Ni alloy surface. The study showedthat the highest hardness of the surface was at-


tained by the coating with SiC nanoparticles, fol-lowed by the coating with Al

2O

3, and the lowest hard-

ness values were measured by the coating with CeO2

particles, which is shown in Fig. 4. This improve-ment in surface hardness was due to the greaterabsorption of the laser beam by Al

2O

3 and SiC

nanoparticles than by CeO2 particles. A previous

study showed that the hardness depends on thenanoparticle size and the reason for hardness im-provement is that the Al

2O

3 nanoparticles are in mi-

cro-aggregate form and the SiC nanoparticles existin single particles; however, the CeO

2 nanoparticles

existed in both single particles and micro-aggre-gates leading to more disintegration of the laser beam[20].

The study shows that apart from thenanoparticles influencing the improvement of hard-ness, there are also other factors such as micro-structures and cladding process parameters thathave an effect on the surface properties. For exam-ple, cladding with the Ni/WC composite as a coat-ing material on a mild steel substrate showed thatthe hardness increased by the increasing laser clad-ding speed, which was due to the fine dendrite struc-ture formation by the high solidification rate. Thecoating with a nominal amount (i.e. 5 wt.%) of WCnanoparticles produced the highest hardness com-pared to the coating with the La

2O

3 nanoparticle and

without nanoparticles. This highest hardness valuewas the result of the uniform distribution of WC par-ticles and their dissolution impedance. Moreover,adding WC nanoparticles above the nominal level(i.e. 10 wt.%) led to porosity as well as increases inthe carbon concentration, thereby showing hardnessdegradation. The cladding surface with a nominallevel (i.e. 1 wt.%) of La

2O

3 nanoparticles improved

Fig. 5. Cracks formation after 100 thermal shock cycles from room temperature to 1050 °C on Ni-basedcladded surface (a) without nanoparticles (b) with nanoparticles. Reprinted with permission from Wellmanet al., © 2004 Surface and Coatings Technology.

in hardness but not to the value of the cladding sur-face with WC nanoparticles. The improvement wasdue to the grain refinement and new intermetalliccompound formations. However, the La

2O

3

nanoparticles above the nominal level (i.e. 2 wt.%)produced spongy phases, which decreased thehardness [15].

Therefore, adding a suitable amount of allnanoparticles in the cladding material and dispers-ing the nanomaterial on the cladded surface withappropriate laser parameters improved the hardness.Above the nominal level, however, nanoparticlesdegraded the hardness property as well as resultedin the formation of discontinuities.

2.3.2. Wear resistance and thermalshock resistance

The general material requirements for industries inservice at fluctuating climatic conditions are highwear resistance and thermal shock resistance. Asthe Ni-based coating materials are best suited forcladding, adding nanoparticles will provide betterresistance to the cladding surface. For the case ofthermal shock resistance, the cladding surface with-out nanoparticles exhibited extended spalling; how-ever, with a nanoparticle-added coating only unitspalling was observed with low thermal cycles.Moreover, by increasing thermal cycles, the coat-ing without nanoparticles had large cracks, suchas propagating cracks and internal cracks, asshown in Fig. 5a. In the case of a coating with addedCeO

2 nanoparticles, propagating cracks decreased

and no formation of internal cracks was observed,as shown in Fig. 5b, [14]. This improvement wasdue to the fact that the RE increases the anti-spallingcapacity of the coating [21].


Wear resistance on a cladding surface withoutnanoparticles showed deep penetration and a largewear scar. However, a cladded surface with SiCnanoparticles had smaller width compared to theAl

2O

3 as well as the CeO

2 nanoparticle-added sur-

face [19]. The depth wear for the cladded surfacewith and without nanoparticle addition was similar.The reason for the high wear rate in the coating withCeO

2 and Al

2O

3 nanoparticles compared to the SiC

nanoparticle coating was the coating hardness.Moreover, the Al

2O

3 nanoparticles produced higher

coating hardness compared to the surface with CeO2

nanoparticles, which resulted in the greater reduc-tion of the wear rate in the Al

2O

3 than the CeO

2 added

surface. Thus, wear resistance and thermal shockresistance were improved by nanoparticle additioncompared to the cladded surface with no addednanoparticles.

3. COBALT-BASED COATINGMATERIAL

Co-based materials have wide usage in various in-dustries such as coating on bearing, tool and die’sfor its wear resistance and the Co having uniqueresistance properties even on fluctuating service andclimatic conditions [22]. Industrial demands haveincreased for the improvement of product life by re-ducing wear rate on various applications. However,the Co-based coatings coating could not satisfy theincreasing demands for high properties. In addition,the crack and other defect formations were one ofthe barriers for attaining the high-level properties [23].Studies have validated that the addition of appropri-ate reduced particle size (i.e. nanoparticles ornanocrystalline) to the Co-based cladding materialwould provide significant improvements in the prop-erties with the stable microstructure along the in-terface region as well as on the surface [24,25].

3.1. Effects of nanoparticle-addedcoating material on defects

Defects and the surface appearance have a greatinfluence on the properties prior to the service con-dition. The Co-based coating resulted in inevitablecracks or discontinuity formations particularly in thecase of multi-coatings, which resulted in the degra-dation of properties [23]. Likewise, Li et al. [26] ob-served crystallising cracks by the Co surface coat-ing with no added nanoparticles on low-carbon steel,which reduced the coating bonding as well as theproperties. This cracking occurred due to the mi-cro-segregation of the dendrite structure and impu-

rities leading to thin layer formation, which was tornby thermal stress during solidification [27]. Crackformation decreased, however, in the coated sur-face with added CeO

2 nanoparticles in the Co-based

alloy, which was due to the Ce in CeO2 which re-

duced dendrite growth and formed into homogene-ous fine dendrite [26]. The reduction of cracks wasalso observed in the Ni-based alloy by the coatingof a Co-based alloy with added Al

2O

3 nanoparticles

[25].The surface without nanoparticle addition had a

coarse surface appearance on low-carbon steel,whereas the coating surface with nanoparticles ex-hibited a smooth and defect-free surface. The in-creasing amount of CeO

2 nanoparticles in the Co-

based coating material improved the surface qual-ity [26]. Thus, cracks can be reduced and the sur-face quality improved by the addition of the rare el-ement oxide and metal oxide nanoparticles to Co-based coating materials.

3.2. Effects of nanoparticle-addedcoating material onmicrostructural formation

As the added RE nanoparticles provided a defect-free surface coating, they are expected to have aneffect on grain refinement for stable microstructureformations in Co-based surface coating materials.The rare earth element influences the microstruc-ture through new phase formations as well as trans-formation in crystal structures. For instance, Li etal. [26] examined low-carbon steel with a Co-basedalloy cladding surface with no added nanoparticlesand found dendrite formation in perpendicular direc-tion to the interface; moreover, the structures weremulti-orientated and had only two phases (-Co andCr

23C

6). However, the coating with CeO

2

nanoparticles produced equiaxed crystal structures,which was due to the CeO

2 nanoparticles that acted

as a nucleation site by their melting temperature.Moreover, the -Co and Cr

23C

6 phases and two other

new phases (-Co and CeCo2) were found in the

coating with nanoparticles. CeCo2 formed while the

decomposition of CeO2

nanoparticles and -Coformed due to the low thermal conductivity of CeO

2.

No new phases in the microstructure wereformed, but instead there were changes in the micro-structural phase by the addition of nanoparticles.This was observed in the experiment by Li et al.[28] on coating a Ni-based alloy substrate with Y

2O

3

nanoparticles added in the Co coating material,which produced a phase change in the primaryphases (-Co and -Co) with the addition of


nanoparticles; however, the original phases i.e. coat-ing with no added nanoparticles were -Co andCr

23C

6. The phase change was due to the low ther-

mal conductivity of Y2O

3 (yttrium oxide)

nanoparticles. The coating formed with no addednanoparticles exhibited a columnar dendrite micro-structure while rapid solidification, which is shownin Fig. 6a. However, the coating with the addition ofnanoparticles showed a reduced dendritic structurewith different orientation and an equiaxed grain struc-ture, which is shown in Fig. 6b. Moreover, the changewas more intense, when the amount of nanoparticleswas increased. The reduction of the grain size re-sulted in a change from dendrite to equiaxed struc-ture. This change was due to the Cr

23C

6 phase and

Y2O

3 nanoparticles that act as a heterogeneous

nucleation site accompanied with dispersion [29].Besides the RE nanoparticle addition to the Co-

based coating material, metal oxide (i.e. Al2O

3)

nanoparticles were also added to the Co-based coat-ing material on a Ni-based alloy substrate, whichresulted in fine microstructure formations. Moreo-ver, increasing the content of Al

2O

3 nanoparticles

increased the fine microstructures. Attaining 1%(mass fraction) of Al

2O

3 nanoparticles and an in-

crease in laser power transformed the dendritemicrostructure into equiaxed grain structures, whichalso occurred with Y

2O

3 nanoparticle addition in the

Co-based coating material. The reason for the for-mation of equiaxed grains were the fine grains atthe surface while solidification, which settled at thebottom by gravity and resulted in the reduction ofdendrite grain growth. In addition, the phases with ahigh melting temperature and a significant amountof dispersed nanoparticles also resulted in equiaxedgrain formations [25].

Thus, grain refinement and transformation in themicrostructural phases occurred due to the addi-tion of nanomaterials (i.e. CeO

2, Y

2O

3, and Al

2O

3),

Fig. 6. Microstructure of Co-based coating on Ni super alloys (a) without nanoparticles and (b) with 1.0%nano- Y

2O

3 particles. Reprinted with permission from Gleiter, © 2000 Acta Materialia.

and the nanoparticle size had an influence on thedistribution to the grain boundaries. If the particlesare aggregated along the grain boundaries, it re-sults in unstable microstructural formation.

3.3. Effects of nanoparticle-addedcoating material on properties

Due to the addition of RE materials, the grains wererefined and stable microstructural phases wereformed, which provided improvement in the proper-ties. Li et al. [26] observed that the hardness of thecoating increased with an increasing content ofCeO

2 nanoparticles. However, a nominal content (i.e.

1.5 wt.%) of CeO2 nanoparticles in the coating

showed higher hardness compared to the low (i.e.0.5 wt.%) and high content (i.e. 2.0 wt.%) of CeO

2

nanoparticles in the coating. This highest hardnesswas due to the uniform distribution of thenanoparticles. Moreover, wear resistance was alsohighest in the coating with a nominal content ofCeO

2 nanoparticles compared to the other

Fig. 7. Wear resistance vs CeO2 nanoparticle con-

tent in the Co-based cladding material. Reprintedwith permission from Xu et al., © 2003 Chinese Jour-nal of Mechanical Engineering.


Fig. 8. (a) Defect free forging surface without nanoparticle addition to the Fe-based coating material and (b)Micro-pores formation by the Y

2O

3 nanoparticle addition to the Fe-based coating material. Reprinted with

permission from Li et al., © 2005 Rare Metal Materials and Engineering.

nanoparticle contents in the coatings, shown in Fig.7. The decrease in hardness with the high contentCeO

2 nanoparticles coating was due to the accu-

mulation of the particles, which led to an increasein wear.

Thus, the Co-based coating material with a nomi-nal content of added nanoparticles showed improve-ment in properties; moreover, other than the nomi-nal content of nanoparticles added resulted in re-duced strength. The degradation of properties wasdue to the accumulation of nanoparticles, which alsoled to unstable microstructural formation.

4.IRON-BASED COATING MATERIAL

The coatings in forging dies require high thermalshock and wear resistance due to the forming ofshape by hot metal ingots. Coatings are degradedby mechanical fatigue due to high loads and hightemperatures [30,31]. Therefore, forging dies requirea surface coating with low friction wear and hightemperature resistance which has been achievedby the Fe-based coating with added nanoparticlesthrough laser processing. The insertion ofnanoparticles in the Fe-based coating alloy mate-rial showed improvements in the properties andmicrostructure [32,33].

4.1. Effects of nanoparticle-addedcoating on defect formation

As the forging dies are used in fluctuating tempera-tures and friction, stable microstructure formationwill provide resistance to enhance the properties andreduce defects on the surface. Behrens et al. [33]experimented forging dies as a substrate with theaddition of TiC and WC nanoparticles in the toolsteel coating material (AISI H10) and showed that adefect-free surface coating was produced by a

nanoparticle coating. The addition of Y2O

3

nanoparticles to the tool steel coating material, how-ever, resulted in micro-pores, shown in Fig. 8.

Besides the forging surface, the Fe alloy as asubstrate coated with the Fe-based alloy coatingmaterial with added Al

2O

3 nanoparticles also resulted

in micro-pore formation. Even though the amount ofAl

2O

3 nanoparticles in the Fe-based coating mate-

rial was varied, the coating exhibited same micro-porous formation [32]. Hence the studies showedthat an appropriate addition of nanoparticles accord-ing to the substrate and with proper cladding pa-rameters should produce a defect-free surface coat-ing. Moreover, the quantity of nanoparticles addedto the surface coating material does not have aneffect on defects.

4.2. Effects of nanoparticle-addedcoating on microstructuralformation

The microstructural phase formation determines theproperties of the surface coating. The surface coat-ing used in industry mainly requires wear resist-ance and thermal resistance with a stable micro-structure, which does not lead to the formation ofdiscontinuities while in service. Adding nanoparticlesto the coating material improves grain refinementand microstructural change, thereby resulting in theimprovement of surface coating properties. For in-stance, the experiment by Behrens et al. [33] onforging dies as a substrate without any additions tothe Fe-based coating material resulted in an inter-face region with the grain size of 50 m and a puremartensitic microstructure due to the rapid coolingrate of above 100 K/s. The coating material addedwith WC nanoparticles resulted in a surface with aslightly decreased grain size of 45 m with a


martensitic microstructure; in addition, the WC pre-cipitated at the coating microstructure, whereas thesurface coating with added TiC nanoparticles re-duced the size of grains to the minimum of about10 m. Moreover, precipitation was noted with anaddition of TiC nanoparticles at the grain bounda-ries and the transformation of the microstructureoccurred from martensite to austenite. Thus, anappropriate addition of nanoparticles improved sta-ble microstructural formation in accordance withproperty requirements [33].

If nanoparticles are inappropriately added to thecoating material without considering the substratematerial, the surface coating can result in deterio-rated properties, unstable microstructural formationsand defects. The Y

2O

3 nanoparticle coating on the

substrate increased the grain size (i.e. 85 m) abovethat of the coating material with no addednanoparticles. Moreover, adding Y

2O

3 to the coat-

ing material resulted in defect formations; however,the Y

2O

3 nanoparticles melted completely and dis-

persed compared to other added nanoparticles [33].Thus, RE nanoparticles (i.e. Y

2O

3) in the cladding

surface have a negative effect on the microstructureand defect formation, whereas adding carbide (i.e.

Fig. 9. Microstructure of Fe-based alloy coating material with addition of Al2O

3 nanoparticles (a) cladded

region, (b) interfacial region and (c) Fe alloy substrate. Reprinted with permission from Zhang et al., © 2006Journal of Anhuni University of Technology.

WC & TiC) in the cladding surface produces grainrefinement and a defect-free cladded surface.

The amount of nanoparticles added to Fe-basedcoating material alloys does not have an impact onmicrostructural formations. For instance, the studyon the Fe-based alloy coating material with an ad-dition of varying amounts (i.e. 4, 8, and 12 wt.%) ofAl

2O

3 nanoparticles coated on the Fe alloy substrate

showed similar microstructural formation on thesurface coated, and also there were three differentzone formations while cladding. Firstly, the claddedzone contained three microstructural phases, black,grey, and white, shown in Fig. 9a. The uniformlydistributed black region is the martensite phase,the grey region is the pearlite phase and the whiteregion is the ferrite phase [32].

Secondly, the interfacial region only containedthe pearlite and ferrite phases, when the martensitephases were reduced by low heat, shown in Fig.9b. Finally, the low heat by the laser had no influ-ence on the Fe alloy substrate, shown in Fig. 9c,[32].

Thus, the study showed that the selection ofappropriate nanoparticles to the Fe-based coatingmaterial, in accordance with the substrate, had a


Fig. 10. Microhardness of the Fe-based alloy cladded surface with Al2O

3 nanoparticle addition. Reprinted

with permission from Schulz et al., © 2003 Aerospace Science and Technology.

significant influence in microstructural formations.However, the quantity of the nanoparticles added tothe Fe-based coating had only a small effect on themicrostructure.

4.3. Effects of nanoparticle-addedcoating material on surfaceproperties

The coating requirement of the wear resistance,hardness and thermal resistance properties neces-sitates a stable microstructure. The nanoparticle-added coating attains these requirements and hasno influence on other properties. Consequently, add-ing nanoparticles to the coating influences micro-structural formation, which fulfils the properties.

4.3.1. Hardness and tensile properties

The martensite phase in the microstructure improvesthe hardness property of the coating. The forgingdies with added WC nanoparticles in the tool steelcoating material exhibited increased hardness bythe increasing content of WC nanoparticles. TheY

2O

3 and TiC nanoparticle content in the cladded

surface, however, reduced the hardness propertiesdue to the greater amount of ferritic content in themicrostructure. Hence, the proper nanomaterialaddition corresponding to the substrate improvedthe hardness property [33]. Moreover, the quantityof nanoparticles added to the Fe-based coatingmaterial had no effect on the hardness property. Thiswas demonstrated by the experiment by Yu et al.[32] on the Fe alloy substrate coated with the Al

2O

3

nanoparticle-added Fe-based alloy coating material.

The study showed that the low (i.e. 4 wt.%), me-dium (i.e. 8 wt.%) and high (i.e. 12 wt.%) amount ofAl

2O

3 nanoparticles added produced similar hard-

ness properties. Moreover, the hardness value washigh in the cladded zone, which was due to themartensite microstructure. Hardness decreasedrapidly at the interfacial region by the pearlite micro-structure formation; in addition, the hardness valuehad no effect on the substrate, which was due tothe low heat input by the laser. The hardness rangeis shown in Fig. 10, [32].

In the case of tensile property, the addition ofWC nanoparticles to the Fe-based coating materialon the forging die resulted in improvement in tensilestrength; however, elongation was reduced. The ten-sile strength and elongation decreased by the Y

2O

3

nanoparticle-added coating as well as the TiCnanoparticle-added coating, which was also due tothe ferritic content.

The studies thus revealed that the appropriateselection of nanoparticles considering the Fe-coat-ing material and the substrate improves the hard-ness and tensile properties by influencing the micro-structures. Moreover, the amount of nanoparticlesadded to the coating material had little effect on theproperties.

4.3.2. Wear resistance

The cladded surface with the WC nanoparticlesshowed the lowest wear rate compared to surfaceswith TiC nanoparticles and without nanoparticles inthermal fluctuating conditions, shown in Fig. 11. Thecladded surface with TiC nanoparticles had a higherwear rate than the surface without nanoparticles.


When increasing the thermal cycles, the wear rateof the cladded surface with WC nanoparticles low-ered, whereas that of the surface with TiCnanoparticles increased [33]. This degradation inthe properties could be due to microstructural for-mations by the high ferrite content.Thus, an inappropriate selection of nanoparticles inthe cladding material will result in the degradationof properties due to undesirable microstructure for-mation. In addition, carbide in the nanoparticles pro-vided defect-free microstructures and surfaces.

5. FACTORS AND EFFECTS BYNANOMATERIAL ADDITION TOCOATING MATERIALS ON THESURFACE

The overview effect and factor of nanoparticle addi-tion to various coating material are shown in Table2.

6. DISCUSSION AND CONCLUSIONS

In the competitive world, the manufacturing indus-tries are focusing on more and more efficient and

Fig. 11. Total wear vs forging cycle for different nanoparticle addition in the Fe-based coating surface.Reprinted with permission from Liu et al., © 2003 Chinese Journal of Nonferrous Metals.

economical practices of production both qualitativelyand quantitatively. This has led them to improve theirprocesses and the materials used. Various prod-ucts processed in industry only require surfacemodifications with high properties produced throughcladding. Laser cladding is one of the best proc-esses for coating substrates due to the minimumdilution between the substrate and the coating ma-terial, as well as the better manipulation of processparameters. The products processed through lasercoating cannot, however, satisfy the increasing de-mands. This issue has been solved by the additionof nanomaterials to the coating material, which ful-fils the demands and improves the material strengthand properties.

Adding nanoparticles to the cladding materialreduces weld defects, such as crystallisation cracksand micro-pores caused by the heterogeneousmicrostructure and non-uniform thermal expansion.The addition of appropriate nanoparticles in properamounts to the coating not only reduces defects,but also improves the properties.

Nanoparticles in the cladding material also im-prove the microstructure by reducing brittle phase



formation and by producing uniform stable micro-structures and grain refinement. Furthermore, theformation of new phases increases.Adding nanoparticles also improves the mechani-cal properties: the hardness and tensile propertiesimprove mainly due to the quantity of nanoparticlesadded and their dispersion.Nanoparticles moreover improve wear resistance andthermal shock resistance: the wear rate is reducedthrough an increase in the nanoparticles added andtheir uniform dispersion.These improvements are mostly influenced by theappropriate selection of nanomaterial added in ac-cordance with the coating material and substrate,the uniform dispersion of the nanoparticles to thegrain boundaries, the characteristics of thenanoparticles and the quantity of nanoparticlesadded to the coating material. By considering thesefactors, the nanoparticle coating material can pro-vide high surface properties. Moreover, thenanoparticle added cladding material can be usedin industry for the mass production of products, whichrequires economical surface enhancement technolo-gies. Still, research is being conducted to achievehigh surface properties with other coating materialsusing various cladding processes.

REFERENCES

[1] E. Toyserkani, A. Khajepour and S. Corbin,Laser Cladding (CRC Press, London, NewYork, 2004).

[2] D. Lepski and F. Brückner, In: The Theory ofLaser Materials Processing, ed. by J. Dowden(Springer, Netherlands, 2009), p. 235.

[3] S. Sun, Y. C. Durandet and M. Brandt //Surface and Coatings Technology 194 (2005)225.

[4] M. Zhong and W. Liu // Journal of MechanicalEngineering Science 224 (2010) 1041.

[5] R. G. Wellman and J. R. Nicholls // Surfaceand Coatings Technology 177-178 (2004) 80.

[6] H. Gleiter // Acta Materialia 48 (2000) 1.[7] B. Xu, S. Liu and X. Liang // Chinese Journal

of Mechanical Engineering 39 (2003) 21.[8] M. Li, Y. He and G. Sun // Rare Metal

Materials and Engineering 34 (2005) 248.[9] H. Zhang and Y. He // Journal of Anhuni

University of Technology 23 (2006) 21.[10] U. Schulz, C. Leyens, K. Fritscher,

M. Peters, B. Saruhan-Brings, O. Lavigne,J. M. Dorvaux, M. Poulain, R. Mévrel andM. Caliez // Aerospace Science andTechnology 7 (2003) 73.

[11] C. Liu, F. Lin and X. Jiang // Chinese Journalof Nonferrous Metals 17 (2007) 1.

[12] I. Gurrappa and A. Sambasiva Rao // Surfaceand Coatings Technology 201 (2006) 3016.

[13] H.-Y. Wang, D.-w. Zuo, Y.-l. Sun, F. Xu andD. Zhang // Transactions of NonferrousMetals Society of China 19 (2009) 586.

[14] H. Wang, D. Zuo, X. Li, K. Chen andH. Mingmin // Journal of Rare Earths 28(2010) 246.

[15] P. Farahmand , S. Liu, Z. Zhang and R.Kovacevic // Ceramics International 40 (2014)15421.

[16] K. Wang, Q. Zhang, M. Sun and X. Wei //Journal of Materials Processing Technology139 (2003) 448.

[17] . Asadi, M. K. B. Besharati Givi, K. Abrinia,M. Taherishargh and R. Salekrostam //Journal of Materials Engineering andPerformance 20 (2011) 1554.

[18] M. Fattahi, M. Mohammady, N. Sajjadi,M. Honarmand, Y. Fattahi and S. Akhavan //Journal of Materials Processing Technology217 (2015) 21.

[19] H. Wang, D. Zuo, M. Wang, G. Sun, H. Miaoand Y. Sun // Transactions of NonferrousMetals Society of China (English Edition) 21(2011) 1322.

[20] H. Wang, D. Zuo, X. Chen, S. Yu and Y. Gu //Chinese Journal of Mechanical Engineering(English Edition) 23 (2010) 297.

[21] B. Ruiliang, Y. Huijun, C. Chen and Q. Dong// Surface Review and Letters 13 (2006) 509.

[22] G. Freeman, In: ASM Handbook: PowderMetal Technologies and Applications-Volume 7, ed. by A. I. H. Committee (ASMInternational, 1998), p. 179.

[23] M. L. Hu, C. S. Xie, B. L. Zu and A. H. Wang// Transactions of Materials and HeatTreatment 22 (2001) 23.

[24] X. Li, M. L. Curry, G. Wei, J. Zhang, J. A.Barnard, S. C. Street and M. L. Weaver //Surface and Coatings Technology 177-178(2004) 504.

[25] M. Li, Y. He, X. Yuan and S. Zhang //Materials and Design 27 (2006) 1114.

[26] M. Li, S. Zhang, H. Li, Y. He, J. Yoon andT. Cho // Journal of Materials ProcessingTechnology 202 (2008) 107.

[27] Y. He, Z. Huang, H. Qi, D. Wang, Z. Li andW. Gao // Materials Letters 45 (2000) 79.

[28] M. Li , Y. He and X. Yuan // Applied SurfaceScience 252 (2006) 2882.


[29] Y. Zhou, Z. Hu and W. Jie, SolidificationTechnology (Mechanical Industry Press,Beijing, 1998).

[30] J. Sjöström and J. B. Bergström //Scandinavian Journal of Metallurgy 34 (2005)221.

[31] H. Paschke, M. Weber, G. Braeuer,T. Yilkiran, B.-A. Behrens and H. Brand //

Archives of Civil and Mechanical Engineering12 (2012) 407.

[32] S. Yu, Y. Liu, L. Ren and W. Li //Metallurgical and Materials Transactions A:Physical Metallurgy and Materials Science 37(2006) 3639.

[33] B. A. Behrens, T. Yilkiran, S. Ocylok,A. Weisheit and I. Kelbassa // ProductionEngineering 8 (2014) 645.

Documents

IMPROVEMENT OF LASER-CLADDED SURFACES WITH ADDED … · 2017. 10. 27. · 4 C. Vimalraj, P. Kah, B. Mvola and P. Layus ever, the cladded surface with a nominal amount (i.e. 2 wt.%)