Surface and Coatings Technology 166(2003) 153–159

0257-8972/03/$ - see front matter� 2003 Elsevier Science B.V. All rights reserved.PII: S0257-8972Ž02.00855-1

Microstructural characterizations of plasma sprayed hydroxyapatitecoatings

V. Deram , C. Minichiello , R.-N. Vannier , A. Le Maguer , L. Pawlowski *, D. Muranoa a a a a, b

Laboratoire de Cristallochimie et Physicochimie du Solide, UMR CNRS 8012, ENSCL, USTL, BP 108, F-59652 Villeneuve d’Ascq, Francea

Terolab Services SNMC, BP 3, F-94290 Villeneuve-le-Roi, Franceb

Received 16 May 2002; accepted 19 November 2002

Abstract

The plasma sprayed hydroxyapatite(HA) coatings are used on metallic implants to improve their adhesion to bone. Thecoatings microstructure results from a rapid quenching of molten or partly molten particles that impact the surface at high velocity.The powder particles used to spray present coatings were prepared by spray drying technique and their sizes were determinedusing image analysis and laser sizers. The crystal sizes of initial powder and sprayed coatings were compared using Bragg peaksbroadening method. The phase composition of coatings was analyzed by the use of room temperatures and high temperature(upto 900 8C) X-ray diffraction as well as by differential thermal analysis(DTA). The homogeneity of the deposits was evaluatedusing scanning electron microscope(SEM) and electron microprobe analysis(EMPA). The results suggest that the powderparticles melt partially and the HA crystallites in coating are slightly greater than that of the powder. A small quantity of thedecomposition phases was observed by EMPA and confirmed by DTA.� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Plasma sprayed coatings; Hydroxyapatite; Microstructure

1. Introduction

HA has a similar chemical and phase composition toa living bone and is frequently applied as a coating onthe metal orthopedic implants. The bioactive behaviorof this ceramic has proved to accelerate the integrationof a prosthesis to a body. An implant is made typicallyof a titanium alloy and the HA coating covers the partthat will enter into direct contact with the bone. Thetechnique of the coating deposition that is used in theindustrial practice is plasma spraying of powderw1x butother coating methods are being testedw2x. The majorproblem related to this type of processing is a possibledecomposition resulting from an incongruent melting ofhydroxyapatite and a formation of amorphous phaseresulting from a rapid solidificationw3x. The decompo-sition phases such as Ca P O(TP) or Ca (PO ) (TCP)4 2 9 3 4 2

as well as amorphous HA are less bioactive than

*Corresponding author. Tel.:q33-320-336-165; fax:q33-320-336-165.

E-mail address: Lech.Pawlowski@ensc-lille.fr(L. Pawlowski).

crystalline HA and their content in a sprayed coatingshould be kept low. This may be achieved by applicationof coarse powder injected to less energetic plasma thatis able only partly to melt the particles. The coatingformed upon its impact is composed predominantly ofcrystalline HAP but has low integrity and high porosity.This is a method adopted at present by many of theprosthesis manufacturers. Another possibility of crystal-lization of a multiphase deposit offers laser heatingw4,5xor furnace annealingw6x. In the present paper themicrostructure of the commercial HA coatings preparedwith a coarse powder was investigated. The aim of thestudy was a verification of the homogeneity of thedeposits with relation to the coating process.

2. Experimental

2.1. Samples preparation

The powder used to spray was manufactured byTomita (Japan) by a spray drying technique. The sam-ples were sprayed using an F-4 plasma spray torch of

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Fig. 1. SEM image(secondary electrons) of the HA powder used tospray.

Fig. 2. Distribution of diameters of the HA powder obtained using image analysis and laser sizer Malvern and Cilas.

Sulzer Metco(Switzerland) using the spray conditionsoptimized by Terolab Services(France). These condi-tions include an argon and nitrogen mixture applied asplasma working gas and a stand-off equal to 80 mm.The 150mm thick coatings were deposited onto titaniumsubstrates and sand blasted prior to processing.

2.2. Samples characterizations

The image analysis of powder shape was carried outwith the aid of the software Matrox Inspector developedby Matrox Electronic System(Canada). The powdergranulometry was evaluated with a laser sizer HR 850B of Cilas (France). The powder was kept in a solutionof water with a NaPO dispersion agent. The solution3

was submitted to ultrasonical agitation prior to measure-

ment. Another measurement was made with a Malvernlaser sizer of Malvern Instruments(UK). The powderwas kept in pure water. No ultrasonic agitation wasapplied in that case. The DTA investigations were madeusing L62 DTA of Linseis(Germany) and the roomtemperature XRD data were collected using SiemensD5000 diffractometer using CuKa radiation. The crys-tallite sizes were determined by a Bragg peaks broad-ening. LaB standard was used to improve the6

diffractometer resolution. The Williamson and Hall plots(bcosu vs. sinu, whereb is a full width at half maximumof the Bragg peaks) enabled it to extract the contributionof residual stresses in the broadeningw7x. The hightemperature experiments were made using a HKT 1200device of Anton Paar(Austria) in the temperature range25–900 8C and a heating rate of 0.18Cys. The SEMinvestigations were made using a Leo 982 microscopeworking under low acceleration voltage. The samples(powder and coatings) for EMPA experiments wereembedded in epoxy resin and polished metallographi-cally. The microprobe was a Camebax-Microbeamdevice (type 1SI-2S2) of Cameca(France) workingwith the wavelength dispersion spectrometer(WDS).The following crystals were used in the experiments:

● PC2, for Ka of O;● TAP, for Ka of P and Al;● PET, for Ka of Ca.

The weight concentrations of elements were calculat-ed using PAP method described elsewherew8x. At theexperiments, the set-up was calibrated with the use ofCa F(PO ) and Al O monocrystals.5 4 3 2 3

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Fig. 3. XRD diagram of initial powder(a) and sprayed coating(b). The zoomed area indicates two small peaks of TP phase.

Fig. 4. Williamson and Hall plots corresponding to powder(emptycircles) and sprayed coating(full circles).

3. Results

3.1. Powder characterizations

The morphology of the powder(Fig. 1) is character-ized by a porous internal structure of particles as wellas the large holes inside them. These features indicateclearly the spray drying method of the manufacturing.This process does not seem to be followed by any kindof high temperature densification. The particles have amainly spherical shape. The microscopic image of theparticles was submitted to an analysis that enabled it toobtain a size distribution(Fig. 2). The areas correspond-ing to the particles projection on a plan were assimilatedto that of circles which enabled the calculation of thediameters. The frequency distribution of these diametersshows that HA particles are smaller than 100mm. Theparticle size 30–50mm are frequently represented in atotal population. Another, smaller, local maximum ofthe particles is approximately 70mm. The results coin-cide relatively well with those of Cilas laser sizer. TheMalvern sizer shows a very different distribution with asingle maximum approximately 100mm that decreasesmonotonically to the sizes greater than 200mm. Thisresult can be interpreted by an agglomeration of spraydried powder particles. In fact, prior to the measurementsusing Malvern sizer, no ultrasonic agitation was applied,thus the agglomerates could have been measured.The DTA diagram of the powder does not indicate

any phase transformation up to a temperature of 14508C. The XRD diagram of the powder(Fig. 3a) showsonly crystalline HA(ASTM 86–0740). The Bragg peaks

analysis enabled it to find only a small grain sizebroadening with a mean crystallite size of approximatelyds0.08mm (Fig. 4). This size corresponds roughly tothe dimensions of the crystallites inside the HA precur-sors applied to spray drying of the powder. The hightemperature X-ray diffraction of the powder was carriedout in the temperatures ranging from room temperatureup to 900 8C. A better crystallization of the powder,evidenced by thinner Bragg peaks, were visible startingfrom Ts50 8C but no other change was found in thehigh temperature diagrams.

3.2. Coating characterization

The surface of sprayed coating showed the followingfeatures:

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Fig. 5. SEM image(secondary electrons) of HA coating surface: powder precursors resulting from an unmolten particle disintegration uponimpact(a) and partly molten particle(b).

Fig. 6. SEM image(secondary electrons) of HA coating polished cross section(a) and corresponding EMPA observations of distributions of Ca-atoms(b) and P-atoms(c).

● Precursors of a powder particle that have not beenmolten in plasma flame and disintegrated upon impact(Fig. 5a);

● Partly molten particles having porous internal struc-ture (Fig. 5b).

The DTA diagram indicates a very small endothermicpeak at approximately 13908C. The peak correspondsa transformation of the phases of decomposition. In factat a relatively close temperature, for the compositionslightly richer in CaO than that of HA, the

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Fig. 7. Profile of the elements atomic concentrations along the line of analysis.

phase diagram shows the following phase transformationw3x:

(C1360

TPqHA ™ CaOqHA (1)

The XRD diagram(Fig. 3b) confirms this finding,because the small peaks visible at approximately 2us30–318 may correspond to the TP phase(ASTM 70–1379). All other peaks can be attributed to the crystallineHA similar to that in the initial powder. However, thesmall shifts in Bragg peaks position in coating wasnoticed with regard to that of the starting powder. Theleast squares refinement enables to find the followingvalues in that hexagonal system:

● Powder: as0.94196(9) nm, cs0.68813(7) nm;● Coating: as0.9400(2) nm, cs0.6904(2) nm.

These values indicate the small compression stressesin a-direction associated with the tensile stresses in c-direction. The corresponding Williamson and Hall plotindicate a small slope value of 0.0053 that might beinterpreted as the microstrain effect of approximately0.27% broadening associated to the grain size. The meancrystallite size was found to be approximately ds0.17mm. This value is approximately double that in thepowder which can be explained by a growth of thecrystals at the coating process.The cross-section of the sprayed coating(Fig. 6a)

indicates an important porosity and the corresponding2D distribution of Ca and P atoms seems to be homog-enous(Fig. 6b,c). The WDS quantitative analyses weremade with steps of 2mm along a straight line perpen-dicular to the substrate starting from the interface andfinishing inside the HA coating. The concentration in

at.% of P, Ca and Al are shown in Fig. 7. Theconcentrations of P and Ca atoms follow the parallelpaths. It confirms that no selective evaporation ofelements took place at plasma spraying. There is asignificant concentration of Al at the interface of coatingwith the substrate resulting from the preliminary blastingwith the use of alumina grit. The small particles ofalumina should have been incrusted in the titaniumsubstrate surface. The profile of atomic ratio CayP alongthe same line of analysis is shown in Fig. 8. The ratiois not constant but varies in the range from 1.5 to 2.6.The CayP ratio of 1.66 is frequently represented and itis characteristic for hydroxyapatite. The value of CayPs1.5 is characteristic to the decomposition phaseCa (PO ) and CayPs2 to Ca P O . The possibility of3 4 2 4 2 9

the presence of the latter was already mentioned at XRDand DTA measurements. Finally, the ratios greater that2 can be attributed to CaO.

4. Discussion

The powder used to spray coating was prepared by aspray drying technique. Each powder particle is anagglomerate of small precursors kept together with anorganic binderw9x. The size of precursors is typicallynot greater than 1mm while the entire particle mayreach a size of 120mm (Figs. 1 and 2). After injectionin the plasma flame the precursors inside any particlemay have sintered in a solid or in a liquid state. Thecalculations carried out recently suggest that at the timeof interaction between plasma and particle, the solidstate sintering is only a minor process and the majorone is the liquid state sinteringw10x. The solid statesintering could have contributed in a small increase ofthe crystal size at the processing(from 0.08 mm in

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Fig. 8. Profile of the atomic ratio CayP along the line of analysis.

Fig. 9. Possible phase distribution inside a lamella obtained by plasmaspraying of HA powder particle.

powder to 0.17mm in coating). As the plasma generatedin present operational conditions has a low enthalpy, theparticles are molten only partly. At contact with thesubstrate the liquid solidifies and the solid part ofparticle disintegrates under impact(Fig. 5a,b). As thesolidification is very rapid the liquid that has thechemical composition of HA is transformed in anamorphous phase of similar composition or in one ofthe phases of decomposition(Fig. 9). In fact, the presentDTA and XRD measurements(Fig. 3) indicate thepossibility of formation of TP phase and the previousDTA study on the HA coating prepared in the sameconditions indicated the possibility of TCP phase for-mationw11x. However, the quantity of the decompositionphase is, in both cases, very small and their presence ishardly visible on the XRD diagram(Fig. 3b). The XRDtechnique is not well adapted to the analyzed phasespresented in small concentrations with regard to crystal-line HA. The WDS technique seems to be better off tovisualize such phases in a microscopic scale as it showsthe profile of the atomic ratio CayP (Fig. 8). Again, thequantity of amorphous phase was also to small too bedetected on the XRD method. The other techniques thatcan be possibly applied to detect the minor quantitiesof amorphous phase include microprobe Raman andcathodoluminescence emissionw12x.

5. Conclusions

The microstructural analysis of the commercial HAcoating obtained by plasma spraying revealed the majorphase was the crystalline HA. The small quantity of thedecomposition phase, being probably Ca P O , was4 2 9

found using X-ray diffraction and differential thermalanalysis. The wavelength dispersion spectroscopy quan-titative analysis enabled to visualize these phases in amicroscopic scale. The HA crystallites grow hardly at

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the processing and their size remains smaller than 0.2mm.

Acknowledgments

The authors wish to express their gratitude to Dr D.Le Maguer for the EMPA investigations and Dr E.Meillot for laser sizer granulometry determination. DrFatah and Dr Pierlot are acknowledged for discussionabout powder granulometry measurements. Mr C. Boy-aval has done the SEM analyses, Mrs N. Bouremma-Djelal has carried out the DTA measurements and MrsL. Burylo–the X-ray diffraction.

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