Fretting wear behaviors of thermal sprayed hydroxyapatite (HA) coating under unlubricated conditions

WEAR ELSEVIER

Wear 21711998) |32-139

Fretting wear behaviors of thermal sprayed hydroxyapatite (HA) coating under unlubricated conditions Yongqing Fu *, Andrew W. Batchelor, Ying Wang, K.A. Khor M, teri, l.~Lab. School q/'Mech.nical and Production Eug#wering. Nany.ng Techn,h..gical University. Singapore. ri3~7tlS. .~'il~gttpore

Received 10 June 1997:accepted2 January 1998

Abstract Fretting damage (fretting wear and frening fatigue) is one of the m¢glem plagues lbr the orthopedic implants because the oscillatory mieromovement oRen occurs at the interface between the implant and bone, In this investigation, the frening wear behaviors of plasma sprayed hydmxyapatite (HA) bioceramic coatings on titanium alloy substrate were investigated under unlubricated conditions as a function of number of o~illatory cycles, normal load and amplitude. Coefficient of friction, wear volume and wear mechanisms were studied. For the different test conditions in this study, the fretting regime is gross-slip. Results showed that the frening wear mechanisms of plasma sprayed bioeeramic coatings were mainly delamination and abrasive wear, whereas the wear mechanisms of the substrate Ti a|loys were oxidation, delamination as well as abrasive wear. Under unlubricated condition, the fretting wear resistance of bioceramic coatings was not as good as that of Ti-rAI4V substrate due to the porous, lamellar and loose structure of HA coating. © 1998 Elsevier Science S.A. All rights reserved. Ko'words: Hydmxyapatitecoating:Frettingwear: Unlubricalion:Frettingmechanism:Coeflicicntof friction

1, Introduction The performance of biomaterials used for implants is controlled by their biofunctionality and biocompatibility [ I ]. The sophistication of implants and prostheses has placed an increasing demand on the materials used in recent years. The greater the complexity oftbe function of an implant, the more stringent are the requirement of the constructional materials. The success or failure of any device is often as much dependent on the materials choice as it is on the configurational and functional design 121. Titanium alloys have been used as orthopedic implants because of their good ductility and formability, high modulus of elasticity and low density [ 3 I. Their bioactivity and biocompatibility with osseous tissues, however, are not as good as certain forms of calcium phosphate ceramics, tot e~ample, hydroxyapatite (HA, Caltd PO.s h,(OH)2 ), but the.~ ceramics are mechanically weak [ 4 ]. An effective method to solve the above problems is to use these ceramics as a coating on the surface of metallic materials, such as titanium alloys and stainless steels [ 5,6 I. HA coatings have been applied on various substrates by a wide range of surface deposition techniques such as plasma spraying, high velocity oxy-fuel (HVOF) spraying, ion beam sputtering, pul.~d laser ablation, dectrophomtic deposition,

* C~,rrCslamdingauthor. uo43-1648/98/sIg.tN) c, 1998 l-lsxwicrscience S.A. All right~ rescr,.'¢d. Pll S0043-1648( 98 )on 142-2

ff magnetron sputtering, sol-gel and conventional ceramic processes that involves pressing and sintcring [7-12l. Amongst these surfacing processes, thermal spray techniques offer the attractive prospect of economy and efficient deposition of HA. Thermal sprayed calcium phosphate coatings have been actively studied and positive results of encouraging bone growth have shown from either in vitro or in vivo testing l l 3 - 1 8 ] . However. few investigations on wear resistant behaviors of HA coating have been performed. Fretting damage ( fretting wear and fretting fatigue) is one of the modem plagues for orthopedic implants [19,201. Shearing micromovements may often appear at the interface between the implant and bone due to the large differences in elastic modulus of the two materials in contact, lnsuflicient initial lixing ( prosthesis design problem), or the movement of the limb which sustains a large number of stress reversals in the course of one day can also cause micromovement 121,221. The oscillatory micromovements at the contact induce fretting wear and sometimes, fatigue cracks, causing the early failure of joint prosthesis. There have been some investigations on how to prevent fretting damage of orthopedic implants made from titaniun, alloys and stainless steels 123.24 ]. The study of the fretting behavior of plasma sprayed bioceramic coating, however, is very limited and much effort is still needed to explore the influence of oscillatory micro-

Y. Fu el aL / Wear217 ( Iq~Sj 132-139

133

movement on the failure of the joint prosthesis made from bioceramic coating systems. The aim of this study is to investigate the fretting mechanisms and behaviors of HA coating in order to provide some basic information for the use of these kinds of coating systems.

2. Experimental methods 2. I. Preparation and analysis o f bioceramic coating

Calcined HA powders ( Kyoritsu, Japan ) were spheroidized by combustion flame spraying ( Miller FP73). The combustion gas mixture was oxygen and acetylene. The flame-spheroidized powders were sieved using a sonic sieve shaker (Fritsch, Germany). The particle size range of HA powder used for plasma spraying was 20--125 t~to. Ti-6AI--4V plate with a thickness of 3 ram was u ~ d as the substrate. Prior to plasma spraying, the substrate surfaces were sand blasted with silicon carbide grits and ultrasonically cleaned in acetone. A 40 kW plasma torch (Miller Thermal, USA) with a computerized clo~ loop controlled rotor powder feed hopper was used to spray HA coatings on the substrates. The spraying conditions are shown in Table I+ The total thickness of the bioceramic coating was about 200 + 20 pm. The first layer on Ti--6AI--4V subs(rate is deposited ~ ;th HA particles in the size range of 20--45 tzm which can prove,,,, high adhesive strength, whereas the top layer is deposited with large HA particles in the size range of 75- ! 25/zm which can provide good biocompatibility 125 I. The mid-layer was sprayed with HA particles in the size range of 45-75/~m. A Cambridge 360 scanning electron microscopy and an optical microscopy ( LEITZ DMR. Switzerland) was used to study the surface morphology and micros(me(urn of the coatings. A Vickers hardness tester ( Matsuzawa DMH- 1. Japan) was used to analyze the rnicrohardness of the coating. 2.2. Fretting test

Fretting wear tests were conducted using a ball-on-flat fretting apparatus which is schematically illustrated in Fig. I. The upper specimen made of hardened stainless steel AISI 410 (0. I 1% C, 0.44% Si, 0.34% Mn, 0.39% Ni, 12.41 c~ Cr. 0.06% Mo, O.029c~ S andO.019% P. HV 550) withadiameter of 25 mm was fixed on a lever. The flat specimen of the material investigated was mounted on a table and reciprocated at a given frequency and displacement driven by an

Fig. I. Schematicillustrationof the experimentalaplr,ratus, electromagnetic exciter. A lubricant was u ~ d between the lower specimen and the suppoaing table to decrease the friction and wear. Contact between two.+qx+cimenswas achieved by pressing the upper specimen against the flat.surface of the lower specimen under a normal load -',s is shown in Fig. I. The oscillating movement was generated by a compact eletro-magnetic exciter, B&K 4809 {[gruel & Kjar, Denmark), with a force rating of 45 N, A function generator ( Wavetek. Germany) was used to control the displacement via a power amplifier B&K 2706. The amplitude of oscillatory movement was measured by a laser .sensor (Winglor. Germany) with a toiaimum resolution of 2 t~m, and (hertangential friction force F~, resulting from the oscillating movemeat, was measured with a piezoelectric force transducer. The signals of amplitude and friction fmce were .sent to a computer via an amplifier and the data were used to obtain the fretting loops ( friction force vs. displacement hysteresis loops ) and the coefficient of friction (COF). Dry fretting tests were performed at room teraperature (25°(?) with a humidity of 80% RH. The frequency was fixed at 5 Hz, and the loads were 5 and I0 N. The oscillatory amplitudes were 50, 100 and_'200 pro, respectively. After the fretting tests, all specimens were ultrasonically cleaned in acetone to remove the loosely bound debris on the wear surface, and the wear ,scar were analyzed by .scanning electron microcopy (SEMI and EDAX system to determine the extent of material transfer. The wear volume was determined with a Talysurf 5 laser profilometer.

3. Results and discussions

Table I Pl-',maspraying¢ondititm~,

3. I. Aficrostructure and microhardness o f HA cmzting

Main arc gas I ArgonI Auxiliarygas (HeLium) Arc curt'ca! Arc vollage Sprayingdistance

11.28MPa 11.28MPa 8(10-831)A 31)--35V 8- IIIcm

The external morphology of the as-sprayed HA coating observed by SEM is shown in Fig. 2. The as-sprayed coating surface is characterized by undulating surfaces comprised of unmelted, partially melted and fully melted splats. Tim sur-

134

E bi~ el cd. I Wear 217 (1~981 132-139

with a lot of pores can be observed. These pores, normally spherical with different sizes, arise from the existence of entrapped gases, unmelted particles and cracking ! 261. Fig. 4 shows the cross-section fracture surface morphology of HA coating indicating the lamellar structure. There are some elongated pores existed between two splats as can be ob~rved from Fig. 4. For a single splat, the columnar structure can be observed clearly. The unmelted panicles can also be observed in Fig. 4. The pores, cracks, unmelted panicles and lamellar structure that exist in plasma sprayed HA coatings are detrimental to the physical and mechanical properties of HA coating. The microhardness of the HA coating is 280 HV, a little lower than ~hat of the substrate, Ti-6AI-4V, 320 HV. Fig. 2. External morphology of HA coating showing the microl~res, cracks and unmehed particle~.

3.2. Fretting wear and friction behavior under unlubricated condition 3.2. I. FrenhJg regime and the coefficient offriction Fig. 5a shows the friction force vs. fretting amplitudecurve for HA coating at an amplitude of I00/,tm and a normal load of I0 N. Fig. 5b shows the friction force vs. fretting amplitude curve for HA coating at an amplitude of 200/~m and a non'hal loud of 5 N. The graphical relationship between friction force and amplitude shows the typical gross slip characteristic, i.e., a parallel-epipedic shape, and the friction force remains constant during a very large part of the sliding amplitude, which corresponds to complete sliding in the contact area. Usually 9,0.,

Fig. 3. C~,~-,,~-~ion oi" HA bioceramic coating showing the existence of ix~re.,i.

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lace also contains u large number of ubiquitous micropores and micr(x:racks originating from the spray process and the thermal residual stress. Fig. 3 .,,how.,, the SEM micrograph of the cros.~-.~ctioned HA coating, where a lamellar structure

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I:ig. 5. I a ) Freuing rcL:inle fur ItA coating at the amplitude t)l" 21)0 pm and 5 N. ( h I Fretling regime fur liP, coaling al the amplitude of IIX) pm and II) N.

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the gross slip regime determines the occurrence of fretting wear (not fretting fatigue). The variations of coefficient of friction (COF) for bioceramic coatings and titanium alloys as a functiou nf elapsed oscillatory cycles are shown in Fig. 6. h can be seen that for bioceramic coatings, the COF value is stable at a constant value of about 0.7-0.8. While for titanium alloys, the COF rises sharply at the beginning, then is stable at a constant value around 0.6-0.7.

135

3.2.2. Mechanisms of fretting wear qf Ha coatbJg The surface of as-sprayed bioccramic coating was very rough as shown in Figs. 2 and 7a. At the beginning of the fretting movements, only a few large asperities came into contact with the countcffacc. Due to the high pressure applied on the specimen, plastic deformation would occur at the tips of these asperities. Becau~ of the relative movement of the two counterfaces, some of the asperities were fractured and formed the debris, while the others could move back and tonh to take up some tangential movement by elastic defo~marion. Many debris formed e ~ a p e d from the contact areas in the surrounding hollows or holes in the coating. With the increasing number of o.~iilatory cycles, a large amount of debris was formed, and some of the asperities of the coating were gradually worn flat as shown in Fig. 7a and b. Dclamination and abrasive wear were the main wear mechanisms of the fretting damage of HA coating under unlubricared conditions. This probably can be attributed to the lamellar structure o f HA coating which facilitates the generation of debris. As can be observed from Figs, ( 2 ) - ( 4 ) , the microcracks that existed in the coating are easy to propagate along the lamellar or the boundary of splats where there the elongated lxwes existed. Eventually. the cracks propagate to surface, causing [he genera(ion of wear debris. Exislgnce of Lhe pores will promole the delamination proccs~s, Because

Fig. 7. 3-D fi'elting wear morphl~logy of HA coaling determined by Talysurf layer pn~lih)mctcr. [ a ) Freuinp cycl,'s: 5.4 × liP: I h ) [Yelling cycl~.~; 2.16 × IriS:

{c ) frettingcycles: 1.314× 10".

] 36

K Fu e! aL / W~'ur217 (1998,1132-139

of the small amplitude of fretting amplitude, the debris which formed and was confined in the fretting region, eventually became compacted into thin hardened layer in the contact region, as can be seen from Fig. 8a. Transfer laye~ from the steel counterface ~nay also have formed the compacted layer. After a certain running time, the brittle compacted layer could not accommodate the imposed displacement and contact stress any more. so it cracked as shown in Fig. 8b. Because of the porous and lamellar structure of as-sprayed HA coating. these cracks could not propagate deeply into the substrate. and they may have propagated through the boundary of splats and finally sheared to the surface, yielding wear debris, Some

flakes of the compacted debris eventually began to detach, as shown in Fig. 8c. Sometimes delamination could occur around the unmelted particles in HA coating formed during plasma spraying (shown in Fig. 8d) because of the weak bond of these particles with the coating. After a long running time. when more and more debris were formed and the asperities on the surface of the coating became flat, a 'halo' of surface damage: was developed, as is shown in Fig. 6c. Within the annular region, cracking of the compacted layer and delamination are much more evident. At this stag,~, ploughing became significant becau~ the debris formed could not escape from contact area and acted as abra-

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Fig. K. (a ) The compacted layeron tl~,.~urlaccof H A coating {5 x IW cycles), qb) Cracking of the compacted layer.~c ) Typica| dclaminatkm morphology o f H A coalin~. ( d ) Dclumioa[ion lhr[>uk:h ~)mc unmelted panicles. (c) Debris forma|km and sc~rc m.',rk -',t'[cr a 5.rag running ( I x I0").

Y, Fit el al. / Wear 217 ( I ~ b 132-130

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Fig. 9. (a) Fmuingstcrxe morphelo~ ~ i ~ trarLsferlayer (the dark~ areal. (b) Dot mappingof F¢elementon the frclting~at, sires between two surfaces. Score marks on the surface of HA coating are shown in Fig. Be. Them debris played a dual role in controlling wear and friction: the formation of the compacted layer and the abrasive action before their compaction and after the delamination of the compacted layer+ The wear process is thus governed by th+ plasticity and frac+ tare of them debris and the possibility of their ejection from the contact area [ 27 I- The debris were spherical in shape with a typical diameter of mveral microns (see Fig. 8el, On the fretting scar, significant amounts of transferred iron oxide layer can he obmrved from the reddish brown color on the wear scar of the HA coatings. Fig. 9a shows that them steel film transfer layer occurred and consisted of small islands. Fig. 9b shows the dot-mapping of Fe element on the fretted wear of HA coating. The transfer of iron from the counterface is apparent and uniform on the whole fretted area. Analysis on stainless steel connterface a l ~ indicates the transfer of HA coating. In brief, fretting wear mechanism.~ of HA coatings under the above fretting conditions were mainly delamination and abrasive wear. Examination of the scar on the untreated titanium alloys revealed typical fretting damage of metals [ 281. At the beginning of the oscillatory movement, mechanical action disrupted oxide films on the surface and the oxide rapidly formed in the presence of the atmosphere. Fretting debris could he formed by ploughing (see Fig. iOa) or by delamination (see

Fi$. I0. (a) Debris formation by ploughing at Thehnginnlne~of fretting of "l'i alloy. (hi Debris formalion by delaminalion mechanism.(¢) Transfer

layeron the surfaceof ffgucdTi alloys. Fig, lOb). then they were trapped, compacted, oxidized, broken-up, crushed forming new debris in the contact area. After a long period of fretting, hecau~ of the hard titanium oxide formed on the surface, a fully developed transfer layer of ferrite and titanium oxide was formed a~ shown in Fig. IOc. In short, ploughing, oxidation and delamination were the main c a u l s of fretting wear of titanium alloys. ,¢.2.3. Fretting w e a r resislance ¢~f HA coating a n d T i sub,~irate

Fig. l !a and b show the fretting wear volume of HA coating and titanium alloys at the amplitudes of 50, I00 and 200/Lm at a function oi" fretting cycles under the normal load of 5

] 38

Y. Fu et al. / Wear 217 (1998) 132-13~

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surface of the as-sprayed coating, there was greater chance that wear debris escaped from the contact areas into the adjacent hollows or depressions, instead of ploughing the wearing surfaces. After sustained fretting ( i × 10~ cycles or more), the asperities on the surface of the coatings were worn flat and the fretting volume of HA coating became much larger than that of titanium (as can be seen from Fig. I la and b). The higher the load and the larger the slip amplitude, the more significant the wear loss of HA coating. The porous and lamellar structure of the as-sprayed coatings appeared to promote the delamination wear and more and more debris generated could then plough the two counterfaces. For the titanium alloy, the formation of a compact layer of titanium oxide, which was much harder than the titanium alloy, protected the titanium surface and led to metal transfer from the steel ball. As a result, the fretting wear rate of titanium alloy was deduced. From above results, it is found that the long term fretting wear resistance of HA coating under dry conditions is not good because of the lamellar and porous structure and poor adhesion strength and cohesion strength of HA coatings, Further investigation will be concentrated on the fretting behavior of HA coatings under the lubrication of body fluids and how to improve the fretting wear resistance of HA coatings.

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Fretting Cycles Fig. I t. (a) Fretting wear volume of HA coating and Ti alloys al different stip amplimdc~ under the normal load ol 5 N. ,' b ) Frcuin 8 wear v(dumc of HA coaling and Ti alloy.~ at different slip amplitudes under Ih¢ normal load of I() N.

and 10 N, respectively. It is found that with the increase in slip amplitude and normal load, the wear volume of HA coatings incrca~s significantly and the results are the same with tho~ in Rel:s. [ 29,30]. Fretting wear volumes of the titanium alloys are often a little less than tho~ of as-prayed HA coatings at the initial fretting period roughly within about I X l0 ~ cycles. The first renan could be the different fretting wear mechanisms. In the ea,~ of titanium alloys, fretting action could produce local adhesion of the contacted surfaces followed by high strain fatigue and rapid oxidation, and the resultant fracture produced hard abrasive debris which led to pitting and extensive surface damage. For as-sprayod coating, there is not much evidence of adhesion. Another reason may be the rough surface of the as-sprayed bioceramie coating. Rough surfaces have a higher plasticity index than smooth surfaces [ 31 1, so some plastic deformation would occur at the tips of the asperities. Work hardening was likely to prevent these deformed asperities from being completely flattened so that the sharper asperities on a rough surface would be able to accumulate more of the tangential movement by elastic deformation 1321. During fretting, a lot of debris accumulated on the contact surface and caused .~vere ploughing, but on the rough

4.

Conclusion

From a study of the fretting wear of titanium and hydroxyapatite (HA) coated titanium alloy, the following conclusions can be obtained. ( I ) The fretting regime for all test conditions was gross slip. The coefficient of friction for HA coatings was stable :~t a constant value of about 0.7-0.8, whereas for the titanium alloy, the long-term value of coefficient of friction is around 0.6-0.7. (2) Fretting wear mechanisms of HA coatings under dry sliding conditions were mainly delamination and abrasive wear, whereas the subslrate Ti alloys were oxidation, delamination as well as abrasive wear. ( 3 ) The fretting wear resistance of bioceramic coatings was a little higher than that of the titanium alloy substrate heft)re 5 × I0 ~ fretting cycles had elapsed due to the difference in wear mechanism and surface roughness. However, with the increase of the fretting cycles beyond 5 × IOt', wear of HA coatings become severe because of crack initiation from pores insnde the coating and increased ploughing by debris, At long numbers of fretting cycles, the wear resistance of HA coatings was significantly lower than that of titanium alloy. Acknowledgements The author would like to acknowledge the support of the School of Mechanical and Production Engineering. Nanyang T e c h n o l o g i c a l University.

)t. Fu e t a L / W e u r 2 1 7 ( 1 9 ~ 8 ) 132~13~

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