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The mechanism of deposition of calcium phosphatecoatings from solution onto magnesium alloy AZ31

J. E. Gray-Munro, M. StrongDepartment of Chemistry and Biochemistry, Laurentian University, Sudbury, Ontario, Canada P3E 2C6

Received 24 September 2007; revised 31 January 2008; accepted 12 February 2008Published online 28 May 2008 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.a.32107

Abstract: In recent years, magnesium alloys have beenproposed as a new class of metallic bioabsorbable implantmaterial. Unfortunately, hydrogen gas evolution and anincrease in alkalinity are both byproducts of the degrada-tion process. This necessitates the development of magne-sium alloys with controlled degradation rates. The develop-ment of biocompatible coatings that can delay the onset ofcorrosion is essential for improving the lifetime and per-formance of these materials in vivo. Calcium phosphatecoatings have been shown to improve the biocompatibilityof metallic implants for orthopedic applications. In this arti-

cle, we report a solution chemistry technique for depositingcalcium phosphate coatings on magnesium alloy surfaces.Our kinetic studies indicate that the deposition of the coat-ing is related to the anodic dissolution of the substrate.Characterization of the coating by XPS, SEM/EDS, andXRD reveal that the coating produced is a poorly crystallinecalcium magnesium hydroxyapatite material. � 2008 WileyPeriodicals, Inc. J Biomed Mater Res 90A: 339–350, 2009

Key words: calcium phosphate coating; magnesium;hydroxyapatite; surface analysis

INTRODUCTION

Bioabsorbable materials are desirable for a widevariety of clinical applications including fracture fix-ation, orthopedic surgery, endovascular stents, andcontrolled drug delivery systems.1–3 The use of bio-absorbable materials in orthopedics is of particularinterest, because in many instances, the implantserves no function as soon as healing and union ofthe tissue has occurred.2 These implants are typicallyfabricated from bioinert materials such as titaniumor stainless steel and they are only truly necessaryfor a 3- to 6-month period, but in order to removethem, the patient would have to undergo a secondsurgery.1 This is both costly and can pose a serioushealth risk for the patient. Another possible courseof action is to simply leave the implant in place.This also has disadvantages including corrosion ofthe implant in vivo, stress-protection weakening of

the surrounding bone, and development of allergicreactions to metals.1–3 The use of a bioabsorbableimplant would effectively avoid all of these prob-lems. The implant would provide the necessary sta-bility in the initial stages of healing and would grad-ually be replaced with bone as it degraded.1–3

Unfortunately, the biobsorbable polymeric materi-als that are currently in use have been shown tohave less than ideal biocompatibility.1,3 The develop-ment of magnesium alloy implant materials is anovel approach to overcome the current drawbacksassociated with both traditional metallic implantmaterials and polymeric bioabsorbable materials.Magnesium and its alloys have a number of advan-tages over traditional metallic biomaterials, such astheir high strength to weight ratio, inherent biocom-patibility, and mechanical properties that are similarto bone.4 Unfortunately, magnesium also hasextremely poor corrosion resistance, particularly inthe presence of chloride ions5–7; this has limited itsuse as a metallic biomaterial for permanent implants.Magnesium-based metals corrode according to thefollowing general reaction scheme:

Mg0 ! Mg2þ þ 2e�

2H2Oþ 2e� ! H2 " þ 2OH�

However, this susceptibility to corrosion can beseen as a distinct advantage when considering

Correspondence to: J. E. Gray-Munro; e-mail: [email protected] grant sponsor: Natural Sciences and Engineer-

ing Research CouncilContract grant sponsor: Canada Foundation for Innova-

tionContract grant sponsor: Laurentian University Research

Fund

� 2008 Wiley Periodicals, Inc.

potential materials for biodegradable implants. Fur-thermore, it has been demonstrated that magnesiumions have a very positive effect on both cellular ad-hesion and cellular differentiation of human-derivedbone cells on an alumina substrate.8 It is thereforepossible that the release of magnesium ions duringthe biodegradation of the alloy will result inimproved cell/surface interactions compared to tra-ditional polymeric biodegradable materials. Prelimi-nary cytotoxicity tests have also indicated that mag-nesium had no inhibitory effects on marrow cellgrowth9 and that these materials have good biocom-patibility.10–13 It has even been demonstrated thatmagnesium alloys may have antibacterial proper-ties.14 This would be an added feature for the pre-vention of bacterial biofilm formation on the surfaceof these implants.

To date, there are relatively few publicationsdescribing magnesium alloys as a biomaterial. Pre-liminary in vivo animal studies have demonstratedthat a magnesium alloy implanted in the femura ofguinea pigs was completely replaced by new boneformation within 18 weeks7,15 and that magnesium-based alloys may be a promising material for cardio-vascular applications.10,16–17 Bone remodeling aroundopen-porous magnesium alloy scaffolds has alsobeen observed.12

Unfortunately, hydrogen gas evolution and anincrease in alkalinity are both byproducts of the cor-rosion reaction. However, a study by Witte et al.showed that subcutaneous gas bubbles were formedin the first 2–3 weeks of magnesium alloy implanta-tion (in guinea pigs) and then disappeared.15 Thegas bubbles were readily removed by puncturingwith a syringe, and no adverse effects were observedin the guinea pigs.15 This demonstrates that if thecorrosion rate can be controlled, the hydrogen gasgenerated should not be a problem. Therefore, thedevelopment of biocompatible coatings that candelay the onset of corrosion is essential to the suc-cessful development of magnesium alloy implants. Ithas been demonstrated that surface treatment by al-kali heat treatment,18 anodizing,19 and titanium ionimplantation20 may have a positive impact on thecorrosion rate of magnesium alloy materials in simu-lated biological fluid (SBF).

The main mineral component of bone is calcium-deficient carbonate hydroxyapatite.21 Calcium phos-phate coatings have been shown to improve the bio-compatibility of metallic implants and to increasebone growth at the site of implantation.22,23 In particu-lar, biomimetically deposited coatings have receivedrecent attention because of their excellent biocompat-ibility and ability to promote osseointegration.24–27

The biomimetic approach is a solution techniquethat involves immersing the substrate material inSBF at low (ambient to physiological) temperatures.

This technique has been successfully used to applycalcium phosphate coatings onto various sub-strates.25 This type of approach is advantageouswhen compared to the more widely used plasmaspraying for a number of reasons:

1. It is a low-temperature process that does notalter the substrate significantly and can be usedto coprecipitate biological molecules such asproteins and growth factors.24

2. Bone-like apatite crystals having high bioactiv-ity and good resorption characteristics can bedeposited.25

3. Implants with complex or porous geometriescan be uniformly coated.

The composition of the coating bath and the chem-istry of the implant surface have both been shown toplay a key role in the nucleation and growth of cal-cium phosphate coatings from SBF. The Ca/P ratio,presence of inorganic ions such as Mg2þ, surfacepretreatment, and coating post-treatment have allbeen shown to have a significant influence on thedeposition rate and type of calcium phosphate phasedeposited on implant materials.28–34

Although the precipitation of calcium phosphateson magnesium alloys has been observed during bothin vitro and in vivo corrosion studies of magnesiumalloys,4,15,18,35 there are no reports of the develop-ment of calcium phosphate coatings for these materi-als. In this article, an aqueous phase coating processfor depositing calcium phosphate coatings on mag-nesium alloy AZ31 has been reported. The mecha-nism of deposition has also been explored. AZ31was chosen for this study for three reasons: (1) itcontains a relatively small amount of potentiallytoxic aluminium, (2) its mechanical properties havebeen shown to be a good match to bone,4 and (3) ithas been shown to degrade in vivo at a similar rateas AZ91 and WE43.15 This makes it an optimumchoice because AZ91 contains more aluminium, andWE43 contains several rare earth elements that areknown to have moderate toxicity.

EXPERIMENTAL DETAILS

Pretreatment of magnesium alloy substrates

Magnesium aluminium zinc foil (2 cm 3 1 cm coupons;Alfa Aesar) with a composition of 96%Mg:3%Al:1%Zn byweight were ultrasonically cleaned in trichloroethylene(Baker analyzed reagent grade) for 30 min at room temper-ature in a Bransonic 220 sonicator and subsequently rinsedwith distilled water. The samples were then ultrasonicallycleaned in a sodium carbonate monohydrate solution (25g/L) for 30 min at 508C in a Transsonic Digital S sonicator.The samples were then rinsed thoroughly with distilled

340 GRAY-MUNRO AND STRONG

Journal of Biomedical Materials Research Part A

water. Subsequent pretreatment procedures include alkalineaging in a 200 g/L sodium hydroxide solution (Aldrich)followed by heat treatment for 24 h at 1408C. Alkalineaging was used to increase the surface concentration ofhydroxyl groups. This has been shown to be effective ininducing calcium phosphate deposition from aqueous sol-utions on other materials such as titanium and stainlesssteel.31 The overall pretreatment scheme is illustrated inFigure 1.

Deposition of calcium phosphate coatings

Calcium phosphate solutions were prepared in a 1.67:1calcium: phosphate ratio (the stoichiometric ratio of hy-droxyapatite) unless otherwise specified. The compositionof the coating bath was 3.0 mM CaCl2 and 1.8 mMNa2HPO4. These concentrations were chosen in order toobtain a saturated solution. The pH of the solution wasadjusted to 5 with a dilute solution of hydrochloric acid.This pH was chosen because our preliminary results indi-cated that the best calcium phosphate coatings wereobtained under mildly acidic conditions. The magnesiumalloy coupons were immersed in 250 mL of the calciumphosphate bath for a predetermined period of time from afew minutes to 1 week. Upon removal from the coatingbath, the samples were immediately air-dried.

Monitoring of the solution chemistry duringcoating deposition

The concentrations of magnesium, calcium, and phos-phorus in the coating bath were monitored as a functionof coating deposition time. This was achieved by takingsmall aliquots from the coating bath hourly for the first 3h of immersion in the bath and subsequently at 24-h timeintervals. The samples were then analyzed quantitativelyfor magnesium and calcium by flame atomic absorptionspectroscopy and by high-performance liquid chromatog-raphy for phosphate.

Aqueous magnesium and calcium concentrationsby flame atomic absorption spectroscopy

For the first 3 h of immersion, 200-lL aliquots weretaken and diluted up to 25 mL using 2% v/v nitric acid.The remaining aliquots were taken in 24-h time intervals(50 lL diluted up to 25mL using 2%v/v nitric acid). Thesamples were immediately placed in a refrigerator to pre-vent volume changes. All aqueous magnesium and cal-cium determinations were done using a Perkin-Elmer 5000flame atomic spectrophotometer. For magnesium determi-nations, a wavelength of 285.5 nm was used with a currentof 5 mA, for calcium these parameters were 422.7 nm and5 mA, respectively. Optimal flame conditions wereachieved using a C2H2/air ratio of 30/40, the scan timewas 0.5 s.

Determination of aqueous phosphateconcentrations By HPLC

Aliquots for these determinations were taken in a simi-lar manner as for the magnesium and calcium experimentswith distilled water as a solvent. The aliquots were diluted50-fold and 10-fold, respectively, for the aliquots takenwithin the first 3 h and subsequent samples. All analyseswere taken using a Dionex DX 500 chromatography sys-tem using 12 mM Na2CO3 to 5 mM NaHCO3 as an eluentwith a flow rate of 1.00 mL/min. The current and conduc-tivity were set at 100 mA and 100 lS, respectively. Thepump pressure was maintained below 3000 psi for allanalyses.

Coating characterization

The composition of the calcium phosphate coatingobtained was characterized by X-ray diffraction (XRD),energy-dispersive spectroscopy (EDS), X-ray photoelectronspectroscopy (XPS), and atttenuated total reflection–Fouriertransform infrared spectroscopy (ATR-FTIR). The morphol-ogy of the coatings was examined by scanning electron mi-croscopy (SEM).

XRD analysis

The coating was removed from the substrate and wasinvestigated using a Debye-Scherrer camera to record dataon an image plate. Subsequently, the recorded diffractionpattern was integrated, cumulating the recorded intensitieson a 2u-scale. The 2u resolution is on the order of �0.18.The higher order diffraction data is not reported due tothe limitations of the technique and the poor crystallinityof the sample (the higher order peaks were not welldefined). The phase(s) were identified using CRYSTAL-LOGRAPHICA Search/Match. The experimental condi-tions were 40 kV, 30 mA, and collimator 0.5 mm. The ex-posure time was incrementally increased, and the bestresult was achieved after 12 h of exposure.

Figure 1. Pretreatment scheme.

DEPOSITION OF CALCIUM PHOSPHATE COATINGS ON MAGNESIUM ALLOYS 341


XPS analysis

XPS spectra were recorded with a Kratos Axis Ultra X-ray photoeletron spectrometer. The XPS was performed invacuum at 1 3 1029 Torr with a take off angle of 908. Thesurvey scans were recorded over a spot size of 1000 lmbetween 0 and 1000 eV, and the high-resolution spectrawas recorded over an area of 300 3 700 lm with a passenergy of 20 eV. All high-resolution spectra were chargedcorrected to the C��C in the C(1s) spectrum at 285.0 eV.

FTIR analysis

FTIR spectra were collected using a Bruker Optics Ten-sor 27 spectrometer. The resolution of the spectrometerwas 4 cm21. Each spectrum is the result of 1000 scans. Thespectrometer was equipped with a MCT detector that wasliquid nitrogen cooled. The spectra were corrected for CO2

and H2O with the atmospheric compensation function ofthe software.

SEM/EDS analysis

Samples to be analyzed were sputter-coated with a thinfilm of carbon to render the sample conductive. All sam-ples were analyzed in the Zeiss EVO50 SEM, EDS spec-trometer. The vacuum in the chamber was 1.14 3 1026

mbar. The beam current of the electron gun was set at 1.0nA. A spot size of 520 nm was used with a working dis-tance of 8.5–9 mm. The collection time for the EDS was setfor 45 s.

RESULTS AND DISCUSSION

Time course of the reaction for calciumphosphate deposition

The kinetics of deposition of the calcium phos-phate coating on the magnesium alloy surface hasbeen studied by monitoring both the changes in so-lution chemistry of the coating bath and changes inthe surface chemistry of the coating itself as a func-tion of immersion time in the bath.

Solution chemistry

The approximate rates of change for the concentra-tions of each of the relavent species in the early andlate stages of deposition are listed in Table I. Thereaction has been divided into early and late stagesto illustrate that the reaction slows as the substratebecomes coated with the calcium phosphate film. Inthe first 12–24 h, the rate of calcium deposition/mag-nesium dissolution is an order of magnitude higherthan the rates observed at later deposition times.The fact that the respective rates seem to follow eachother may indicate that the anodic dissolution ofmagnesium is the catalyst for calcium phosphatedeposition.

The change in the concentration of Ca2þ ions,PO4

32 ions, and Mg2þ ions in the coating bath duringdeposition is shown in Figure 2. A steep drop in thecalcium concentration in the bath is observed in thefirst 12 h of deposition. This steep deposition rate isfollowed by a moderate deposition rate up to about96 h that is an order of magnitude less than the ini-tial rate. Finally, the concentration of calcium ions inthe bath levels off. A steep decrease in the aqueousphosphate concentration is also observed in the first24 h of deposition. This is followed by a very slowdecrease in phosphate concentration up to about 96h, after which no further change in phosphate con-centration is observed. A rise in the concentration ofmagnesium ions in the bath is also observed (thereis no magnesium present in the coating bath ini-tially). This increase in magnesium concentration hasthe steepest slope in the first 12 h of deposition. Thissteep rise is followed by a moderate increase in mag-nesium concentration for the rest of the coating pro-cess. This increase in magnesium ion concentrationis also coupled with a sharp rise in solution pH in

TABLE IApproximate Rates of Change for the Concentrationsof Relevant Ions in Solution in the Early and Late

Stages of Deposition

IonicSpecies

Approximate Rate of Change (M/min)

Early Stagesof Deposition

Later Stagesof Deposition

Ca2þ 22.0 3 1026 21.0 3 1027

PO432 28.0 3 1027 26.0 3 1029

Mg2þ 1.0 3 1026 2.0 3 1027

Figure 2. Solution concentration of relevant ions andchange in pH during coating deposition.



the first 12 h of deposition, with a subsequent mod-erate rise in pH until the end of the coating process.

The increase in magnesium ion concentration andpH in the bath can both be attributed to anodic dis-solution of the alloy in the coating bath. It is wellknown that magnesium metal is unstable in aqueoussolution at pH values less than 11.36 Our coatingbath has an initial pH value of 5. This is necessaryin order to prevent homogeneous nucleation andgrowth of calcium phosphate crystals in solution. Athigher pH, the solutions are unstable and calciumphosphate crystals precipitate rapidly from solution,eliminating the potential for heterogeneous nuclea-tion of calcium phosphate coatings on the magne-sium substrates. Upon immersion in the coatingbath, an increase in magnesium ions, pH and bub-bling are all observed. This can be explained withthe following half reactions.

Mg0 ! Mg2þ þ 2e� ð1Þ

2H2Oþ 2e� ! H2 " þ 2OH� ð2Þ

The first reaction is the anodic dissolution of themagnesium metal according to reaction 1, and this isthe source of the observed increase in magnesium.The second reaction is the reduction of water to givehydrogen gas, the observed bubbles, and hydroxylions which are responsible for the observed rise in pH.

This rise in pH also has an effect on the phosphatespecies in the coating bath. The following equilib-rium [Eq. (3)] represents the species present as afunction of pH. The acid dissociation constants forthe three equilibria are pK1 5 2.14, pK2 5 6.87 andpK3 5 12.32.37

H3PO4 $ H2PO�4 $ HPO2�

4 $ PO3�4 ð3Þ

This means that at our initial pH of 5 the dominantform of phosphate in the coating bath is H2PO4

2, asthe pH rises to seven the dominant species is mixedH2PO4

2 and HPO42 and at a pH of 9 (the final pH of

the bath) the dominant species in solution is HPO42.

The pH that we have measured is the bulk pH of thesolution, however, because of the reduction of waterat the magnesium coupon/coating bath interface, itis likely that the local pH at the interface is substan-tially higher than the measured pH of the bulk. Thisopens up the possibility of the PO4

32 phosphate spe-cies dominating at this interface.

The initial steep decrease in calcium and phos-phate ion concentration in the coating bath indicatesthat the calcium phosphate coating begins to formimmediately upon immersion in the coating bath.No homogeneous (solution) precipitation of calciumphosphate is observed in the early stage of deposi-tion. The decrease in Ca2þ and PO4

32 concentration

in the early stages of deposition must therefore bedue to heterogeneous nucleation and growth of cal-cium phosphate on the magnesium surface. As thepH of the coating bath rises to approximately pH 57.5 or after 48 h, calcium phosphate crystals areobserved in the bulk of the solution.

For all the solution species monitored, there is alarge rate of change in the initial stages of depositionfollowed by a more moderate rate of change after12–24 h. The decrease in the rate of magnesium andhydroxyl ion production is due to the formation ofthe coating itself. As the film thickness and uniform-ity increases, it acts to passivate the surface from fur-ther corrosion to a small extent. However, even atlong coating times the magnesium corrosion contin-ues, albeit at a slower rate. The calcium phosphatelayer slows the diffusion of water and ions to thesurface of the sample thus reducing the corrosionrate. However, the poorly crystalline nature of thecoating (contributes to dissolution of the coating inthe bath) and the presence of small defects (cracksor pores) means that aqueous ions can still diffuse inand out of the coating. The anodic dissolution of themagnesium substrate thus continues.

The change in the rate of consumption of calciumand phosphate ions is likely due to a combination oftwo main factors. The first factor is the depletion ofreactants as the reaction proceeds. The initial coatingbath is supersaturated with calcium and phosphate.

As the reaction proceeds, the coating bath becomesmore dilute in these two ions, and therefore the sol-ubility of the calcium phosphate increases resultingin less deposition. Furthermore, the decrease in thecorrosion rate due to passivation of the surface bythe coating results in a lower pH at the interface andan increase in the solubility of the calcium phos-phate salts.

The second factor that may contribute to an appa-rent decrease in the rate of calcium and phosphateconsumption is the dissolution of the coating in thecoating bath. In particular, a net dissolution of amor-phous hydroxyapatite phases in SBF has been shownto occur, particularly in the early stages of immer-sion in these solutions.38,39

In the final stages of deposition, the concentrationsof calcium and phosphate ions both level off indicat-ing no further deposition of calcium phosphate,most likely due to depletion of these ions to a levelwell below supersaturation.

Surface chemistry

SEM studies

SEM images showing the morphology of the coat-ings as a function of immersion time in the coating



bath are shown in Figure 3. It is immediatelyobvious from these images that as the immersiontime in the bath increases from 3 h [Fig. 3(a)] to 24 h[Fig. 3(b)] and finally to 96 h [Fig. 3(c)] the filmthickness also increases. Chemical analysis by EDSconfirms this assertion, since the calcium/magne-sium ratio increases from 0.087 at 3 h to 1.49 after 24h and finally to 19.31 after 96 h in the coating bath,indicating an increase in the amount of calcium atthe surface as a function of deposition time. Thisincrease in calcium at the surface correspondsdirectly with the observed decrease in the concentra-tion of calcium ions in the coating bath solution. Fig-ure 3(a) shows the SEM image of the magnesium

alloy surface after a relatively short immersion timein the bath. This image shows spherical deposits ofcalcium phosphate that seem to be localized aroundsmall cracks at the surface of the substrate. Theobserved cracks are due to corrosion of the substrate.As discussed previously, the anodic dissolution ofmagnesium is coupled with the reduction of water.This produces hydroxyl ions at the liquid/solidinterface leading to a localized high pH and subse-quent precipitation of calcium phosphate at the mag-nesium alloy surface. As the immersion timeincreases, the calcium phosphate spheres grow andbegin to precipitate on all areas of the magnesiumalloy surface until they coalesce, and a continuouscalcium phosphate film is achieved as seen in Figure3(b). The visible cracks in the calcium phosphatefilm are likely due to dehydration of the film. Ateven longer immersion times, additional layers ofcalcium phosphate deposit on top of the initiallyformed calcium phosphate layer [Fig. 3(c)].

One important point to note is that the presence ofmagnesium is observed by EDS even at long coatingtimes and for relatively thick coatings. This suggeststhat magnesium is present in the coating; however,the sampling depth for EDS is on the order of 3–4 lm. Therefore, our EDS results cannot conclusivelydetermine if the magnesium is present in the coatingitself or part of the substrate.

XPS studies

XPS is a surface sensitive technique that probesthe first 5–10 nm of the sample surface. From ourSEM results, it is obvious that by a 24-h immersiontime the coating thickness has exceeded the sam-pling depth of XPS.

Figure 4 shows survey scans for a magnesiumcoupon before and after immersion in the calciumphosphate bath for 120 h. Even at the longest coatingtimes, magnesium is still detected on the surface ofthe coated substrates. This confirms that the magne-sium must be incorporated into the calcium phos-phate coating since the coating thickness for thesesamples is much greater than 5–10 nm.

Figure 5 shows a graph of the Ca/Mg ratio as afunction of immersion time. The correspondingchanges in Mg/C ratio are shown in Figure 6(c).There is an initial increase in the Ca/Mg ratiocoupled with a decrease in the Mg/C ratio up to a6-h immersion time in the bath. These two observa-tions can both be attributed to an increase in filmthickness up to this point. As the immersion timeincreases beyond 6 h there is a decrease in the sur-face Ca/Mg ratio and a very slight increase in theMg/C ratio. This can be attributed to a combinationof two factors. The first is that the film thickness has

Figure 3. Scanning electron microscopy images of cal-cium phosphate coatings on magnesium AZ31 substratesat various deposition times. (a) 3 h, (b) 24 h, (c) 96 h.



exceeded the sampling depth of the techniquebeyond 6 h of immersion in the coating bath. Thiswould be expected to result in a leveling off of theCa/Mg ratio and a drop to zero for the Mg/C ratio.The observed decrease in the Ca/Mg ratio and slightincrease in Mg/C ratio implies an increase in surfacemagnesium concentration however we know that

the substrate is completely covered with the calciumphosphate film at the longer immersion times (Fig.3) so we are no longer probing the sample substrate.This observation can therefore be attributed to acombination of two factors: (1) incorporation of mag-nesium ions, from the corrosion of the alloy, into thecoating and (2) magnesium ions adsorbing to thesurface from solution.

Figure 4. XPS survey scans of calcium phosphate coating on magnesium AZ31 substrates before and after immersion inthe coating bath for 5 days. (a) Before immersion in the bath and (b) after immersion in the bath for 5 days.

Figure 5. XPS Ca/Mg ratio as a function of coating depo-sition time.

Figure 6. XPS ratios as a function of coating depositiontime. (a) Zn/C, (b) Al/C, (c) Mg/C.



Figure 6 also shows the Zn/C and Al/C XPSratios for the coated samples as a function of immer-sion time in the calcium phosphate bath. The carbonin the XPS spectra is adventitious carbon from theatmosphere. It is commonly used as an internal ref-erence since it is present on all samples. This figurehas two interesting trends. The first is that the sur-face concentration of aluminum steadily decreases,until at 6 h of immersion time it is no longerdetected [Fig. 6(a)]. Since aluminum is mainly pres-ent in the substrate, this confirms that by an immer-sion time of 6 h in the coating bath the substrate iscovered by a coating greater than 5–10 nm in thick-ness. The second interesting observation is that thesurface concentration of zinc increases up to animmersion time of 6 h and then drops to zero as theimmersion time is increased [Fig. 6(b)]. Althoughthe magnesium alloy substrates contains �1% zinc,the XPS survey scan for the control sample indicatesthat the concentration of zinc at the surface of thealloy is below the detection limit of the technique.This is likely due to the presence of a passive oxide/hydroxide layer that forms at the surface of thesealloys in air or highly basic solutions. Upon immer-sion in the coating bath, the passive layer is dis-rupted and corrosion occurs. Bulk magnesium alloyAZ31 has a two-phase microstructure that consists ofa magnesium-rich matrix (a phase) and an aluminium-rich intermetallic species, Mg17Al12 (b phase) that isprecipitated along the grain boundaries.40 The zincin the alloy is known to be dissolved primarilywithin the b phase of the material.40 Localized corro-sion of these alloys has been shown to initiate at theinterface between these two phases due to galvaniccoupling.36 Under these conditions, the magnesium-rich phase acts as an anode while the aluminium-rich phase acts as the cathode. The increase in theconcentration of zinc is a result of an initial localizedcorrosion at these sites. As the anodic dissolution ofthe magnesium proceeds, the reduction of wateroccurs near the b phase of the material resulting in alocal high pH at the interface of the a and b phasesof the material. It is therefore likely that calciumphosphate deposition begins in these regions. In fact,our SEM image for the lower deposition time of 3 h[Fig. 3(a)] does show an increase in calcium phos-phate at cracks caused by the corrosion process.

By an immersion time of 24 h, the substrate iscompletely covered with calcium phosphate, and theoverall corrosion rate has slowed due to the presenceof the calcium phosphate coating and the increasedpH of the coating bath. At this point, no zinc isdetected on the surface of the coating. In the case ofthe magnesium ions in solution, we observed aslight increase in surface magnesium at higherimmersion times which we attributed to its incorpo-ration into the coating and adsorption to the surface.

We do not see this same effect with zinc. Thisimplies that the zinc that is released into solutiondue to the corrosion of the substrate is incorporatedback into the calcium phosphate coating in the earlystages of deposition rather than remaining in theaqueous phase. The fact that there is no evidence ofeither aluminium or zinc adsorption to the surface ofthese coatings at higher immersion times eventhough the corrosion must continue, as evidenced bythe continued increase in the amount of magnesiumin the bath (Fig. 2), may indicate that at longerimmersion times the b-phase becomes passivatedwhile the magnesium-rich a-phase continues to cor-rode.

Coating characterization

In order to confirm the composition and phase ofthe calcium phosphate coatings the films were char-acterized by a combination of high-resolution XPS,infrared spectroscopy, and XRD. The XPS spectrawere obtained on the as-coated magnesium alloycoupons. In order to perform the infrared and XRDanalysis, the coating was carefully removed from thesubstrate prior to analysis.

High-resolution XPS

Both the SEM/EDS and XPS survey scans indicatethat substrate ions are incorporated into the calciumphosphate coating itself during deposition. Ourhigh-resolution XPS spectra (Table II) indicate thatthere is an increase to higher binding energy forboth the Mg 2p and Al 2p peaks after immersion inthe coating bath compared to the bare substrate. Ini-tially, the Mg 2p peak occurs at a binding energy of49.8 eV. This is consistent with the presence ofMg(OH)2

41 and is not surprising since our pretreat-ment procedure includes soaking the magnesiumalloy coupons in a sodium hydroxide solution topromote the formation of surface hydroxyl groups.An increase in surface hydroxyl groups has beenpreviously shown to promote calcium phosphatenucleation and growth on titanium substrates.31 Af-

TABLE IIXPS Binding Energies for the Selected Elements Before

and After Immersion in the Coating Bath

Peak

Binding Energy in Coating Bath (eV)

Before Immersion After Immersion

Mg 2p 49.8 50.5Al 2p 74.5 75.1Ca 2p N/A 347.6P 2p N/A 133.4



ter immersion in the coating bath, the Mg 2p peakshifts by 0.7 eV to higher binding energy. The Al 2ppeak on the uncoated substrate occurs at a bindingenergy of 74.5 eV. After immersion in the coatingbath, this peak shifts by 0.6 eV to higher bindingenergy. The binding energy prior to immersion inthe coating bath is consistent with aluminium ox-ide/hydroxide compounds.42 The shift to higherbinding energy for both of these elements supportsour assumption that these ions become incorporatedinto the calcium phosphate coating since both theMg 2p peak of magnesium phosphate and the Al 2ppeak of aluminium phosphate have been shown tohave higher binding energies than the correspondingoxide/hydroxides.42,43

High-resolution XPS spectra for the Ca 2p and P2p peaks of the calcium phosphate coating areshown in Figure 7. The Ca 2p peak [Fig. 7(a)] hasbeen deconvoluted into three peaks, consisting of themagnesium auger peak at 352.54 eV, the Ca 2p1/2

peak at 351.20 eV, and the Ca 2p3/2 peak at 347.65.The P 2p peak [Fig. 7(b)] has been deconvoluted intotwo peaks: the P 2p1/2 peak at 134.21 eV and the P2p3/2 peak at 133.37 eV. The binding energies ofthese peaks are consistent with the reported bindingenergies for Ca 2p and P 2p in hydroxyapatite.44

However, it is not possible to differentiate betweenhydroxyapatite and other calcium phosphate phaseson the basis of XPS binding energies alone, since thebinding energies for these elements do not shift sig-nificantly for other calcium phosphate compounds.44

Infrared analysis

Figure 8 shows a representative infrared spectrumof the calcium phosphate coating obtained. The mainpeaks of interest are annotated on the spectrum.Overall, this spectrum is consistent with the infraredspectrum for hydroxyapatite.45–47 The sharp peak at3695 cm21 is the OH stretching vibration forMg(OH)2

45. The compound is present at the surfaceof the sample substrate and is introduced into thecoating sample during the removal of the coatingfrom the substrate. Additionally, there is a broadpeak centered at 3424 cm21. This peak has twoobvious shoulders; one at 3551 cm21 and the otherat �3300 cm21. The higher wavenumber shoulder at3551 cm21 is indicative of the OH stretch for hy-droxyapatite45 while the lower wavenumbershoulder at 3300 cm21 is consistent with the OHstretch for CaZn2(PO4)2(2H2O) or scholzite.48 Thepresence of this compound will be confirmed in theXRD pattern in the next section. The main peak at3424 cm21 and the peak at 1631 cm21 are due to theOH stretch and bending mode for intercalated water,respectively.45 The two peaks at 1452 and 1493 cm21

are due to the presence of carbonate ions in the hy-droxyapatite structure. It is well known that carbon-ate, generated from reaction of atmospheric CO2

with aqueous solutions, readily substitutes for phos-phate ions in the crystal structure of hydroxyapatite.Finally, a P��OH band is observed at 869 cm21 aswell as three bands at 562, 603, and 1047 cm21

Figure 7. High-resolution XPS spectra for the final calcium phosphate coating. (a) Ca 2p, (b) P 2p.



which are due to the PO432 stretching vibrations.45

Figure 8(b) shows the 1047 cm21 phosphate bandregion of the spectrum in more detail. Upon closerinspection it is clear that this band has three compo-nents; the main peak centered at 1040 cm21 and twoshoulders at 940 and 1100 cm21. The weak shoulderat 940 cm21 is the m1 symmetric P��O stretchingmode.47 This mode is IR-inactive in the free phos-phate ion, but appears as a weak band due to areduction in symmetry of the phosphate ion uponincorporation into the crystal lattice.47 The m3 anti-symmetric P��O stretching mode is triply degeneratein the free phosphate ion, but this mode is resolvedinto at least two distinct peaks in crystalline hy-droxyapatite.47 It has been shown that as the crystal-linity of the hydroxyapatite increases these two m3bands become increasingly resolved.47 In our infra-red spectrum, the two m3 bands are observed at 1040and 1100 cm21 However, they are poorly resolved,which indicates that the coating is only poorly crys-talline.47 This is not surprising since our XPS resultshave demonstrated that a significant amount of mag-nesium ions are incorporated into the coating itself,and magnesium ions are known to inhibit crystalgrowth in hydroxyapatite.49 These two observationscoupled together suggest that the hydroxyapatite

coating formed is a calcium-deficient/magnesium-rich hydroxyapatite material.

XRD results

The XRD pattern for the coating is shown in Fig-ure 9. This pattern indicates that there are likelytwo different compounds present in the sample,hydroxyapatite (Ca10(PO4)6(OH)2) and scholzite(CaZn2(PO4)2�2H2O). The broad diffraction peaks

Figure 9. X-ray diffraction pattern for the final calciumphosphate coating.

Figure 8. Infrared spectrum for the final calcium phosphate coating. (a) Full spectrum and (b) phosphate band only.



indicate that both of these compounds are poorlycrystalline, however, the intensity of the scholzitepeaks suggest that this compound has improvedcrystallinity compared to the hydroxyapatite. OurXPS data indicate that the concentration of zinc inthe coating is �1% therefore the increased intensityof the scholzite peaks must be due to the crystallin-ity of the compound and not an abundance of thematerial. The XRD pattern obtained is not typical ofcrystalline hydroxyapatite. However, the peaks wehave attributed to hydroxyapatite very closely matchpublished XRD patterns for calcium-deficient hy-droxyapatite.49 Our XPS results show that the Ca/Pratio in our coatings is �1:1, this is much lower thanthe predicted Ca/P ratio of 1.67:1 for hydroxyapatite.This result indicates that our hydroxyapatite coatingis calcium-deficient. We also know that there is asignificant amount of magnesium incorporated intothe coating which makes a calcium magnesium hy-droxyapatite the most likely phase present. In fact, ithas been previously observed that the substitution ofmagnesium ions into the crystal structure of hy-droxyapatite significantly decreases the crystallinityof the material.50

Proposed mechanism of deposition

The mechanism of apatite deposition on the mag-nesium alloy substrate can be summarized by thefollowing three reactions:

Step 1 � Mg ! Mg2þ þ 2e� and

2H2O þ 2e� ! H2 " þ 2OH�

Step 2 � H2PO�4 ! HPO2�

4 ! PO3�4

Step 3 � ð10� xÞCa2þ þ xMg2þ þ 6PO3�4

þ 2OH� ! ðCað10�xÞMgxÞðPO4Þ6ðOHÞ2

Upon immersion of the substrate in the coatingbath, the anodic dissolution of magnesium begins.This is evidenced by the increase in the concentra-tion of magnesium in the bath and the observedincrease in pH. Our XPS results reveal that there isan increase in the surface concentration of zinc inthe early stages of deposition. The XRD spectrumconfirms the presence of a small amount of calciumzinc phosphate (scholzite) in the coating material.Coupled together, these two results imply that theinitial corrosion of the magnesium alloy substrateoccurs at the interface between the a and b phasesof the material, since zinc is associated primarilywith the aluminium-rich b phase. It is likely that theanodic dissolution at these sites catalyzes the hetero-geneous nucleation and growth of the calcium phos-phate coating due to a local rise in pH.

Although the XRD and infrared results confirmthat the coating is primarily hydroxyapatite, our XPSresults reveal that the Ca/P ratio is less than that ofstoichiometric HA. Furthermore, the coating ispoorly crystalline and has been shown to containsignificant amounts of magnesium. These resultssuggest that the magnesium ions are readily incorpo-rated into the crystal lattice of the hydroxyapatiteduring the deposition of the coating. The composi-tion of the final coating is primarily a magnesium-rich/calcium-deficient hydroxyapatite.

CONCLUSIONS

In this article, a solution chemistry technique forthe deposition of calcium phosphate coatings onmagnesium alloys has been reported. Kinetic studiesof the deposition process have shown that the heter-ogeneous nucleation and growth of the calciumphosphate coating is catalyzed by anodic dissolutionof the magnesium alloy substrate in the early stagesof deposition. Characterization of the coatingrevealed that the primary phase formed is a poorlycrystalline calcium magnesium hydroxyapatite mate-rial.

We thank Mr. Mark Biesinger of Surface Science West-ern for technical assistance with our X-ray photoelectronspectroscopy studies.

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The mechanism of deposition of calcium phosphate coatings ... › assets › files › Department... · of action is to simply leave the implant in place. This also has disadvantages