- Email: [email protected]
hydroxyapatite composite coatings
/Ionmalof
ELSEVIER
Materials Processing Technology Journal of Materials Processing Technology 71 (1997) 280-287
Post-spray hot isostatic pressing of plasma sprayed Ti-6AI-4V/hydroxyapatite composite coatings K.A. Khor
a,*,
C.S. Yip a, p. Cheang b
a School of Mechanical and Production Engineering, Nanyang Technological Uni~'ersity, Nanyang Avenue, Singapore 63 9798, Singapore b School of Applied Science, Nanyang Technological UniversiO,, Nanyang At,enue, Singapore 63 9798, Singapore
Received 29 June 1996
Abstract A series of Ti-6A1-4V/hydroxyapatite (HA) composite coating wa~ produced by plasma spraying. Several compositions (20, 33 and 80 wt.% HA) were prepared. Subsequent examination of the coatings showed them to have a high level of porosity. However, some amount of porosity, within a specified size range, may be desirable in biomedical applications to enhance bony tissue ingrowth, although inter-lamella pores in the size range 10-300 nm (100-3000 A,) in the plasma sprayed coatings are detrimental to their mechanical properties, and these small pores should be reduced drastically in order for the coatings to have sound mechanical strength. Hot isostatic pressing (HIP) is applied in this study to reduce the amount of micropores in the plasma sprayed coatings. The influence of HIP temperature on the pore size distribution, microstructure and other physical properties of the composite coatings is investigated. Scanning electron microscopy revealed that the lameUae in the HIPped samples appeared 'compressed' because of the plastic deformation of the Ti-6AI-4V phase. A mercury intrusion porosimeter measured the pore size distribution of the HIPped samples, the results indicating that the majority of the micro-pores, most likely inter-lamella pores, are reduced drastically after HIP. A dynamic mechanical analyser is employed to measure the storage modulus of the composites by a 3-point bend fixture, the results showing that the storage modulus of the 20 and 33 wt.% HA coatings improved with HIP and that there is a corresponding increase with the HIP temperature employed. Other physical properties such as density and microhardness also improved with HIP. Overall, the results demonstrate that HIP can effectively enhance the mechanical properties of the Ti-6AI-4V/HA composite coatings. Tensile adhesive bor, d tests show the interface between the coating and the substrate to be improved. The mode of failure apparently transferred from adhesive failure in the as-sprayed coatings to a predominantly cohesive mode of failure in the HIPed samples and suggests that the influence of HIP is greater in the enhancement of the coating/substrate interface than in inter-lamellae strengthening. © 1997 Elsevier Science S.A. Keywords: Titanium; Hydroxyapatite; Composite; Coating; Plasma spray; Hot isostatic pressing; Porosity; Properties; Microstructure
1. Introduction Recently, there have been several attempts at processing and fabricating hydroxyapatite (HA)-based composites for biomedical applications. [1-4] HA has very attractive bioactive properties that enhance the osseo-conduction and osseo-integration of biological tissues onto artificial implants or prosthesis by appropriate chemical bonding, but its poor mechanical properties are a major setback [5-10]. By forming a composite with either a stronger ceramic such as A1203, * Corresponding author. Fax: [email protected]
+ 65
7911859; e-mail:
0924-0136/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. Pll S0924-0136(97 )00086- I
and ZrO2, or metals such as Ti-6AI-4V, can help to alleviate this hindrance. The present investigation focuses on enhancing the mechanical properties of HA-based coatings through hot isostatic pressing (HIP) of a series of Ti-6AI-4V/ HA composites. The latter can offer the combined bioactive property of HA with the excellent mechanical properties of titanium alloy, for biomedical implants and prosthesis. Recently, a series of Ti-6AI--4V/HA composite coatings was produced by dc plasma spraying [11,12]. Powder feedstock prepared by a novel ceramic slurry mixing technique is fed into a high temperature plasma where the powders are melted and the molten droplets
K.A. Khor et al. ,Jonrnal of Materials Processing Technoh~g)' 71 (1997) 280-287
are propelled at high speeds towards a prepared surface where rapid solidification takes place. The coating is~ thus, built up by the successive solidification of the droplets that spread out as a lamella. The plasma sprayed Ti-6AI-4V/HA coatings are composed of fine HA that is deposited in the form of a lamella, sandwiched between the Ti-6AI-4V phase. Scanning electron microscopy (SEM) examination of the polished cross-sections showed the presence of pores around the unmelted particles, at the Ti-6AI-4V/HA interface and between the lamellae. In medical applications, whilst some amount of porosity is needed for bony tissues to grow into the coating for efficient fixation, the small micropores that exist between the lamellae are detrimental to the mechanical properties of the coating. High-magnification SEM observation shows that the HA-rich regions are mechanically weak with evidence of pull-outs during metallographic polishing. In addition, some HA-rich lamellae have cracks. These cracks need to be ~healed' for the composite coatings to have reasonable mechanical strength during usage. One effective method of improving the adhesive strength and physical properties of the plasma sprayed coatings is to subject the coatings to HIP [13]. The use of HIP technology in the processing of advanced materials such as ceramics (A1203 and yttriaalloyed partially-stabilised zirconia) and high-temperature metal alloys such as TiAI and nickel-based alloys has increased, [14-18], especially in obtaining greater densification after sintering and the elimination of pores after casting the molten metal. In the HIP process, pressure and temperature are applied to the workpiece simultaneously. A maximum tcinperature of ---2000°C and a maximum pressure of 200 MPa can be applied. making this process suitable for both advanced ceramics and high temperature metal alloys. The pressurization medium is a gas (argon, nitrogen and oxygen), the pressure applied being equal in all directions (isostatic). The part to be pressed must have a gas-tight skin, such as a glass, or must be encapsulated. The advamages of HIP are better temperature control, compared to uniaxial hot pressing, and a homogeneous structure and properties. This paper presents the effect of HIP on the microstructure and mechanical properties of plasma sprayed Ti-6AI-4V/HA composite coatings. The
Fig. I. Cross section of plasma sprayed Ti-6AI-4V/HA coatmg.
porosity level,.~ of the as-sprayed and HIPped coatings are measured using a mercury intrusion porosimeter (MIP). A dynamic mechanical analyser (DMA) measured the storage modulus of the coatings. The tensile adhesive bond strength of the coatings on the substrates is evaluated through a tensile adhesion strength test (ASTM C633t.
2. Experimental techniques 2.1. Preparation of 77- 6AI- 4 V/hydroxvapatite coatings A 40 kW plasma spray torch (SG-100, Miller Thermal Inc., USA) was used to prepare the coatings, the plasma forming gases being argon and helium. The feedstock is pre-mixed Ti-6AI-4V/HA aggregates prepared by a ceramic slurry technique [12], this technique effectively 'coats' the Ti-6AI-4V particles with HA. The aggregates are debound at 550-600°C before being plasma sprayed onto Ti-6AI-4V substrates to form the
Table 1 Parameters for the hot isostatic pressing of Ti-6AI-4V/HA coatings (TIHA) Specimen
Temperature (°C)
Pressure (MPa)
Time th)
TIHA ! TIHA 2 TIHA 3
900 950 1000
180 180 180
1 1 1
281
Fig. 2. Observation of cracks in HA-rich iamellae.
ii!!ii!i ••)• • •)•••% •• C?•••• •¸
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iii!iiii!ii!iiiiii:iI iiiiiiiii!:i/!!iiiii!i!iilLi(ii!iiiiiif
........ .........
Fig. 3. Polished cross section of Ti-6AI-4V/HA coatings after HIP.
Fig. 5. Incomplete closure of pores in Ti-6AI-4V/hydroxyapatite coatings after HIP.
composite coatings. The HIP system used is the System 5X from Kobelco, Japan. A molybdenum heating element is used in the present study. Table 1 lists the HIP parameters for the plasma-sprayed coatings.
cquation [19]. Mercury is used because for most solid surfaces it is non-wetting and non-reactive. It has a contact angle of ~ 130° for most solids and requires some applied pressure before it will intrude into the pores. The underlying physical principle can be summarised as:
2.2. Characterisation and evaluation of coatings SEM is performed on a Cambridge Stereoscan $360 equipped with an energy dispersive X-ray analyser (Link AN 10/85S). Density measurement is performed on an Ultrapycnometer from Quantachrome, USA. Porosity measurement is done using a mercury intrusion porosimeter Autopore II from Micromeritics, USA. In this technique mercury is intruded into the coating to measure the pore size. This technique requires pores that are inter-connected, a common he determinaP is based on
Fig. 4. SEM view of particle clusters in a Ti-6AI-4V/hydroxyapatite coating that did not become densified by HIP.
Pr = - 2X cos 0
(1)
where ~, is the surface tension of mercury, 0 is the contact angle between the mercury and the pore wall, and Pr is the applied external pressure, which ranged from several bars to several tens of thousand bars. The variation of the external applied pressure caused changes in the intruded volume, this in turn being related to pore-size distribution. The porosity in thermal-sprayed coatings can be understood from the vantage point of a two-level porosity model [20]. Pores in the size range 1-10 pm arose from incomplete melting of some particles in the plasma flame and coalesce in an awkward manner that results in voids between the lamella and the semi-molten particle, whilst pores in the region of 0.3 ~tm and below are largely inter-lamella or inter-planar pores that are formed as a result of the poor bond rate amongst the lamella as they impinge on each other prior to final solidification. The mechanical properties of the specimen are elucidated using a Perkin Elmer 7-series dynamic mechanical analyser (DMA). Each specimen is subjected to a static stress of 2.26 MPa, subsequently super-imposed by a sinusoidal stress in the frequency range 0-20 Hz of magnitude 2.0 MPe. Tensile bond strength tests were conducted on an Instron 4302 (10 kN load cell) in accordance with ASTM C-633, a hot-cured araldite epoxy glue being used as the adhesive. The tensile adhesion tests were performed with a cross-head speed of 0.5 mm rain-~.
283
K.A. Knot el al. Journal ~/ Malerials Processing T~'clmoloKr 71 (1997) 280~ 287
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Fig. 6. XRD patterns of Ti-6AI-4V/hydroxyapatite coatings in the as-sprayed state and after HIP. (a) 20 wt.% HA; (b) 33 wt.% HA: (c) 80 wt.% HA.
K.A. Khor et al./Journal of Materials Processing Technology 71 (1997) 280-287
284
Table 2 Summary of MIP results of HIPped Ti-6AI-4V/hydroxyapatite coatings Samples
TIHA 1 TIHA 2 TIHA 3
Average pore diameter (.am)
Overall porosity (",4,,)
As-sprayed
HIPped at 1000°C
As-sprayed
HIPped at 1000°C
0.1676 0.0542 0.3971
0.7873 0. ! 291 0.5713
18.74 22.46 31.69
17.14 13.91 21.07
3. Results and discussion
3.1. Microstructure and phase composition of Ti6Al4V/hydroxyapatite composite coatings after HIP Fig. 1 shows the polished cross-section of Ti-6AI4V/HA coating, revealing a typical lamella structure consisting of alternating layers of Ti-6AI-4V and HA. Some unmelted particles are also observed, High-magnification observation shows that the HA-rich regions are mechanically weak and evidence of pull-outs during polishing can be observed quite vividly. In addition, some HA-rich lamellae have cracks (Fig. 2), these cracks needing to be 'healed' for the composite coatings to have reasonable mechanical strength during usage. SEM observation of polished cross-sections of the coatings shows that the lamellar structure is preserved after HIP (Fig. 3). However, the HA-rich portion appeared 'compressed'. This arises as a result of plastic deformation of the Ti-6AI-4V phase during HIP that cause the ductile metal to yield plastically and close up the pores between the lamellae. In addition, the individual lamellae appear to be more linear compared to the assprayed lamellae, similar observation being made on HIPped Ni,based coatings [21]. There are, however, limitations to what HIP can do in the coatings, and particular defects in the as-sprayed coatings cannot be eliminated effectively, these being unmelted particle clusters (Fig. 4) and incomplete closure of pores (Fig. 5). Part of the reason lies in that the coatings were not encapsulated in a steel capsule during HIP. This was done deliberately to ensure that the surface pores in the coatings would not be fully closed up, since these pores are likely to facilitate tissue ingrowth during implantation of the prosthesis. Thus, the intrusion of the pressurising gases would likely minimise the efficacy of HIP in the closure of all pores. The XRD patterns of the Ti-6AI-4V/hydroxyapatite coatings in the as-sprayed condition and after HIP are shown in Fig. 6. There is a marked increase in the amount of crystalline phases in the coatings following HIP. In the 80 wt.% HA coating, amorphous calcium phosphate is converted to the crystalline HA that is bioactive and beneficial to the osl~o-integration af the
hip implant. Formation of amorphous calcium phosphate during plasma spraying is attributed to the loss of hydroxyl groups from the crystalline HA [22].
3.2. Porosity of Ti6Al4 V/hydroxyapatite composite coat#1gs t~ter HIP There is apparently less porosity in the HIPped coatings. MIP measurements showed that the porosity of the as-sprayed 20 wt.% HA composite was ~ 19%, this value being reduced to ~ 17% for the sample HIPped at 1000°C. Although the porosity reduction was relatively minor the interesting aspect of HIP is the drastic reduction of small pores of less than 0.3 ~tm, the average pore diameter being observed to increase from 0.1676 lam in the as-sprayed coating to 0.787 ~tm in the sample HIPped at 1000°C. The effect of HIP on the coatings, as shown by SEM observations, is such that the ductile phase (Ti-6AI-4V) undergoes plastic deformation during HIP and subsequently closes up many of the micropores between the lamellae. With the elimination of the micropores, the average pore size of the coating is increased. The improvement in the physical properties can be observed succinctly in the increase in the microhardness values of the respective coatings and in density increase, the microhardness increasing by ~ 25% and the density improving by ~ 8% [12]. The 80 wt.% HA coatings showed that HIP effectively reduced the amount of large ( > 50 ~tm) pores as well as the fine pores ( < 0.1 ~tm). Table 2 summarises the porosity results of the HIPped coatings. The main mechanisms responsible for the reduction in the porosity would be the direct effects of high isostatic pressure and increased temperature. However, there are different dominant mechanisms for the 20 and 80 wt.% HA coatings. In the 20 wt.% HA coating, plastic yielding, brought about by the high pressures, caused the Ti lamellae and particles to deform plastically, closing up most of the pores, whilst increased temperature together with the stress field applied caused interlamellar diffusion such that material diffused from areas between particles to the voids, enabling the lamellae to move closer together and, consequently, to close up the pores. Although the
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K.,4. Khor el al. Join'rod q f MateriaLs Processing Techmd,,,,,y 71 (I 997) 280-287
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same mechanism would be expected to exert its influence in the 86 wt.% HA coating, the amount of Ti phase is nonetheless smaller. Still, the plastic yielding of the 20 wt.% Ti-6A1-4V phase in the 80 wt.°/,, HA coating would have been the key instrument in the reduction of the pores, particularly the small pores. What is perhaps more dominant in the case of the 80 wt.% HA coating is the break down of the HA regions that have weak inter-lameilae or inter-particle bonds during HIP. This is evident from the significant reduction of the large pores > 50 lam after HIP. The pores, in particular the large pores, appear to collapse during HIP, which leads subsequently to particle fragmentation and the formation of 'new' inter-particle contacts. Similar observation was made in a previous study on the HIP of a plasma-sprayed zirconia coating stabilised by Y203 [23].
3.3. Dynamic mechanical properties of Ti6Al4V/hydroxyapatite coathtgs after HIP Both the 20 and 33 wt.% HA coatings exhibit improvements in storage modulus after the HIP treatment, the results showing that the 20 wt.% HA reinforced composite possesses a greater modulus than the 33 wt.% HA coatings (Fig. 7). The storage modulus of the 20 wt.% HA coatings improved from 60 GPa (as-sprayed) to ,,-80 GPa (HIPped at !000°C). The reduction of pores < 0.1 lam is significant in plasma sprayed coatings because these are likely to be the inter-lamella pores that formed as a result of incomplete bonding between the ~uccessive lamellae during the spray process [7],, their presence serving to weaken
the whole coating, resulting in poor mechanical properties. The 33 wt.% HA coatings show dramatic response to HIP at 950°C where the modulus improved by 20 GPa (Fig. 8). This indicates that with a greater percentage of the ceramic reinforcement, a greater HIP temperature may be required to restore the coating defects. probably by the plastic deformation of the Ti-6AI-4V phase under HIP.
3.4. Tensile adhesire strength of coatings a./i'er HIP Figs. 9 and 10 show the bond strength of the Ti 6A1-4V/hydroxyapatite coatings (as-sprayed and HIPped) with respect to the coating thickness, from which it can be seen that the tensile adhesive strength decreases with increasing coating thickness. The results also demonstrate that the tensile adhesive strength generally improves after HIP. The enhancement of the adhesion strength in the 20 wt.% HA coating after HIP is obvious for coatings below 160 ~tm, coatings thicker than 160 ~tm showing that HIP may have adverse effects on the coating strength. Observation of the fractured surface of the as-sprayed coatings test sampies revealed adhesive failure, i.e. that is failure of the coatings occurred at the coating/substrate interface. However, the HIPped coatings showed a different mode of failure. SEM revealed that the failure of the HIPped coatings is predominantly cohesive in nature (Fig. I1). Failure of the HIPped coatings during the tensile test occurred within the coatings and not at the coating/substrate interface, which may indicate that tim HIP treatment enhanced the coating/substrate interface more than it did the inter-lamella contacts.
286
K.A. K/mr et al./Journal of Materials Processing Technology 71 (1997j 280-287
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Fig. 8. DMA results of HIPped Ti-6AI-4V/33 wt.% HA coatings.
4. Conclusions
The HIP of plasma-sprayed composite coatings of Ti-6AI-4V/HA improved the physical properties, such as microhardness and the density, of the coatings even though the increase in porosity was rather modest. This was attributed to the drastic reduction in the small micro-pores <0.3 ~tm that lie mainly between the lamellae, thus disrupting the intcr-lamellae contact. SEM examination or" the plasma-sprayed coatings revealed the microstructure to have retained its typical lamella structure. However, the HA-rich portion appeared 'compressed' as a result of the plastic yielding brought about by the• ductile Ti-6AI-4V phase. Results from the dynamic mechanical analyser (DMA) showed that the storage modulus of the 20 and
33 wt.% HA coatings improved with HIP and that there is a corresponding increase with the HIP temperature employed. HIP treatment at 950°C elicits a significant response from the 33 wt.% HA coating. Other physical properties, such as density and microhardness, also improved with HIP. Tensile adhesive bond tests show the interface between the coating and the substrate to be improved. The enhancement of the adhesion strength in the 20 wt.% HA coating after HIP is obvious only for coatings below 160 lam. The mode of failure apparently transferred from adhesive failure in the as-sprayed coatings to a predominantly cohesive mode of failure in the HIPped samples, indicating that 20
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thickness(urn) Fig. 9. Tensile bond test results of plasma sprayed Ti-6AI- -4V/20 wt?/. , hydroxyapatite coatings (as-sprayed and HIPped).
Fig. 10. Tensile bond test results of plasma sprayed Ti-6AI-4V/33 wt.% hydroxyapatite coatings (as-sprayed and HIPped).
K.A. Kkor et al. ,Journal of Material.s Proc,'.s.~ing ]i'clmoh,~:l" 71 !1997) 280 2 8 7
287
References
Fig. ! 1. SEM view of a fractured Ti- 6AI surface after the bond test.
4V/hydroxyapatite coating
the HIP treatment improved the coatmg/substrate interface more than it did the inter-lamella contacts within the coatings.
Acknowledgements Financial support lor the plasma spray system from NTU RP 56/92 is gratefully acknowledged. The hol isostatic press system was financed by a research grant from Singapore Ministry of Finance.
[11 M. Takagi, M. Mochida, N. Uchida, K. Saito. K. Uew.ats~, J. Mater. Sd. Mater. Med. 3 {1992) 199 203. [2] J.H. Chern Lin, M.L. Liu. C.P.{}. Ju. J. Mater. Sci. Mayer. Med. 5 {1994t 279 283. [3] J. 15. S. Forberg. L. Hemlansson. Bioma~erials 12 (1991) 438 440. [4] tt. ,li, P.M. Marquis, Biomaterials 13 ! l l i {19~12) 7 4 4 7 4 8 . [5] M. Jarcho, C I m Orthop. 157 ! 1 9 g l ) 2 5 9 278. [6] R.G.T. Geesink.. K. de (}root, C.P.A.T. Klein, J. Bone Joi~'ll Surg. 70B tl98gl 17 22. [7] C.P.A.T. Klein, P. Parka. I . G ( ' . Wolke, J.M.A. de BlicckHogervorst, K. de Grool, Biomaterial.,, 15 {1994) 146 150. [81 P. Ducheync. J.M. Cucklcr, Clin. Orthop. 276 (1992) I¢b2. 114. [9] A. Moroni, V.L. Ct0a, E.L. Egger, k. Trinchese, E.Y.S. Chao, Biomaterials 15 {19941 9 2 6 930. [10] R.G.T. Oeesmk, CIm. Orti~op. 2(',1 !199H) 39 5S. [11] K.A. Khor, C.S. Yip, P. ( h e a n g , J Tl~ermal Spray Tech. 6 {19971 1{)9 115. [12] C.,~. Yip, K.A. Khor. N.L. Lol, and P. ('heai~g, J. Mats. Proc. T,:'ch., lin press). [13] H. Kuribayashi. K. StlgUlltllll~.t. ~'. Mi}amoto. M. Koizumi, Am Ceram. Soc. Btdl. 65 (9) {1986~ 1306 1.tl0. [141 M. Nakamura, Y. Kaieda, Powder Me:. 31 i l 9 8 8 1 2 0 1 211. [15] I,.M. Sheppard, Am. Ceram. Soc. Bull. 71 119921 311 322. [i6] D.W. Shin, K.K. Orr, H. Sdmberl, J. Am. Ceram. Soc. 73 (1990) 1181 1188. [17] F.F. Lange, J. Am. Ceram. Soc. 66 {0l {1983) 390. [18] M.J. Blackburn and R,A. Sprague, Superalloxs Source Book, ASM, 1984. pp. 2 6 9 276. [19] E.W. Washburn, Proc. Nati Acad. Sci. 7 {1921) 115 116. [20] R. McPherson, Thin Solid Films 83 (1981) 297 381. [21] K.A. Khor, N.L. Lob, J. Thelmal Spray Tech. ASM Int. 3 Ill {1994) 57 6 2 . [22] S.R. Radin, P. Ducheyne, J. Mater. Sci. Mater. Med. 3 {19921 33. [23] K.A. Khor. N.I,. Loh, m T.S. Sudarshan. W. Reitz, J.J. Stiglich {Eds.}, Surface Moditication Technologies IX, TMS. Warrendale, PA, USA, 1996, pp. 515 531).
ELSEVIER
Materials Processing Technology Journal of Materials Processing Technology 71 (1997) 280-287
Post-spray hot isostatic pressing of plasma sprayed Ti-6AI-4V/hydroxyapatite composite coatings K.A. Khor
a,*,
C.S. Yip a, p. Cheang b
a School of Mechanical and Production Engineering, Nanyang Technological Uni~'ersity, Nanyang Avenue, Singapore 63 9798, Singapore b School of Applied Science, Nanyang Technological UniversiO,, Nanyang At,enue, Singapore 63 9798, Singapore
Received 29 June 1996
Abstract A series of Ti-6A1-4V/hydroxyapatite (HA) composite coating wa~ produced by plasma spraying. Several compositions (20, 33 and 80 wt.% HA) were prepared. Subsequent examination of the coatings showed them to have a high level of porosity. However, some amount of porosity, within a specified size range, may be desirable in biomedical applications to enhance bony tissue ingrowth, although inter-lamella pores in the size range 10-300 nm (100-3000 A,) in the plasma sprayed coatings are detrimental to their mechanical properties, and these small pores should be reduced drastically in order for the coatings to have sound mechanical strength. Hot isostatic pressing (HIP) is applied in this study to reduce the amount of micropores in the plasma sprayed coatings. The influence of HIP temperature on the pore size distribution, microstructure and other physical properties of the composite coatings is investigated. Scanning electron microscopy revealed that the lameUae in the HIPped samples appeared 'compressed' because of the plastic deformation of the Ti-6AI-4V phase. A mercury intrusion porosimeter measured the pore size distribution of the HIPped samples, the results indicating that the majority of the micro-pores, most likely inter-lamella pores, are reduced drastically after HIP. A dynamic mechanical analyser is employed to measure the storage modulus of the composites by a 3-point bend fixture, the results showing that the storage modulus of the 20 and 33 wt.% HA coatings improved with HIP and that there is a corresponding increase with the HIP temperature employed. Other physical properties such as density and microhardness also improved with HIP. Overall, the results demonstrate that HIP can effectively enhance the mechanical properties of the Ti-6AI-4V/HA composite coatings. Tensile adhesive bor, d tests show the interface between the coating and the substrate to be improved. The mode of failure apparently transferred from adhesive failure in the as-sprayed coatings to a predominantly cohesive mode of failure in the HIPed samples and suggests that the influence of HIP is greater in the enhancement of the coating/substrate interface than in inter-lamellae strengthening. © 1997 Elsevier Science S.A. Keywords: Titanium; Hydroxyapatite; Composite; Coating; Plasma spray; Hot isostatic pressing; Porosity; Properties; Microstructure
1. Introduction Recently, there have been several attempts at processing and fabricating hydroxyapatite (HA)-based composites for biomedical applications. [1-4] HA has very attractive bioactive properties that enhance the osseo-conduction and osseo-integration of biological tissues onto artificial implants or prosthesis by appropriate chemical bonding, but its poor mechanical properties are a major setback [5-10]. By forming a composite with either a stronger ceramic such as A1203, * Corresponding author. Fax: [email protected]
+ 65
7911859; e-mail:
0924-0136/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. Pll S0924-0136(97 )00086- I
and ZrO2, or metals such as Ti-6AI-4V, can help to alleviate this hindrance. The present investigation focuses on enhancing the mechanical properties of HA-based coatings through hot isostatic pressing (HIP) of a series of Ti-6AI-4V/ HA composites. The latter can offer the combined bioactive property of HA with the excellent mechanical properties of titanium alloy, for biomedical implants and prosthesis. Recently, a series of Ti-6AI--4V/HA composite coatings was produced by dc plasma spraying [11,12]. Powder feedstock prepared by a novel ceramic slurry mixing technique is fed into a high temperature plasma where the powders are melted and the molten droplets
K.A. Khor et al. ,Jonrnal of Materials Processing Technoh~g)' 71 (1997) 280-287
are propelled at high speeds towards a prepared surface where rapid solidification takes place. The coating is~ thus, built up by the successive solidification of the droplets that spread out as a lamella. The plasma sprayed Ti-6AI-4V/HA coatings are composed of fine HA that is deposited in the form of a lamella, sandwiched between the Ti-6AI-4V phase. Scanning electron microscopy (SEM) examination of the polished cross-sections showed the presence of pores around the unmelted particles, at the Ti-6AI-4V/HA interface and between the lamellae. In medical applications, whilst some amount of porosity is needed for bony tissues to grow into the coating for efficient fixation, the small micropores that exist between the lamellae are detrimental to the mechanical properties of the coating. High-magnification SEM observation shows that the HA-rich regions are mechanically weak with evidence of pull-outs during metallographic polishing. In addition, some HA-rich lamellae have cracks. These cracks need to be ~healed' for the composite coatings to have reasonable mechanical strength during usage. One effective method of improving the adhesive strength and physical properties of the plasma sprayed coatings is to subject the coatings to HIP [13]. The use of HIP technology in the processing of advanced materials such as ceramics (A1203 and yttriaalloyed partially-stabilised zirconia) and high-temperature metal alloys such as TiAI and nickel-based alloys has increased, [14-18], especially in obtaining greater densification after sintering and the elimination of pores after casting the molten metal. In the HIP process, pressure and temperature are applied to the workpiece simultaneously. A maximum tcinperature of ---2000°C and a maximum pressure of 200 MPa can be applied. making this process suitable for both advanced ceramics and high temperature metal alloys. The pressurization medium is a gas (argon, nitrogen and oxygen), the pressure applied being equal in all directions (isostatic). The part to be pressed must have a gas-tight skin, such as a glass, or must be encapsulated. The advamages of HIP are better temperature control, compared to uniaxial hot pressing, and a homogeneous structure and properties. This paper presents the effect of HIP on the microstructure and mechanical properties of plasma sprayed Ti-6AI-4V/HA composite coatings. The
Fig. I. Cross section of plasma sprayed Ti-6AI-4V/HA coatmg.
porosity level,.~ of the as-sprayed and HIPped coatings are measured using a mercury intrusion porosimeter (MIP). A dynamic mechanical analyser (DMA) measured the storage modulus of the coatings. The tensile adhesive bond strength of the coatings on the substrates is evaluated through a tensile adhesion strength test (ASTM C633t.
2. Experimental techniques 2.1. Preparation of 77- 6AI- 4 V/hydroxvapatite coatings A 40 kW plasma spray torch (SG-100, Miller Thermal Inc., USA) was used to prepare the coatings, the plasma forming gases being argon and helium. The feedstock is pre-mixed Ti-6AI-4V/HA aggregates prepared by a ceramic slurry technique [12], this technique effectively 'coats' the Ti-6AI-4V particles with HA. The aggregates are debound at 550-600°C before being plasma sprayed onto Ti-6AI-4V substrates to form the
Table 1 Parameters for the hot isostatic pressing of Ti-6AI-4V/HA coatings (TIHA) Specimen
Temperature (°C)
Pressure (MPa)
Time th)
TIHA ! TIHA 2 TIHA 3
900 950 1000
180 180 180
1 1 1
281
Fig. 2. Observation of cracks in HA-rich iamellae.
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Fig. 3. Polished cross section of Ti-6AI-4V/HA coatings after HIP.
Fig. 5. Incomplete closure of pores in Ti-6AI-4V/hydroxyapatite coatings after HIP.
composite coatings. The HIP system used is the System 5X from Kobelco, Japan. A molybdenum heating element is used in the present study. Table 1 lists the HIP parameters for the plasma-sprayed coatings.
cquation [19]. Mercury is used because for most solid surfaces it is non-wetting and non-reactive. It has a contact angle of ~ 130° for most solids and requires some applied pressure before it will intrude into the pores. The underlying physical principle can be summarised as:
2.2. Characterisation and evaluation of coatings SEM is performed on a Cambridge Stereoscan $360 equipped with an energy dispersive X-ray analyser (Link AN 10/85S). Density measurement is performed on an Ultrapycnometer from Quantachrome, USA. Porosity measurement is done using a mercury intrusion porosimeter Autopore II from Micromeritics, USA. In this technique mercury is intruded into the coating to measure the pore size. This technique requires pores that are inter-connected, a common he determinaP is based on
Fig. 4. SEM view of particle clusters in a Ti-6AI-4V/hydroxyapatite coating that did not become densified by HIP.
Pr = - 2X cos 0
(1)
where ~, is the surface tension of mercury, 0 is the contact angle between the mercury and the pore wall, and Pr is the applied external pressure, which ranged from several bars to several tens of thousand bars. The variation of the external applied pressure caused changes in the intruded volume, this in turn being related to pore-size distribution. The porosity in thermal-sprayed coatings can be understood from the vantage point of a two-level porosity model [20]. Pores in the size range 1-10 pm arose from incomplete melting of some particles in the plasma flame and coalesce in an awkward manner that results in voids between the lamella and the semi-molten particle, whilst pores in the region of 0.3 ~tm and below are largely inter-lamella or inter-planar pores that are formed as a result of the poor bond rate amongst the lamella as they impinge on each other prior to final solidification. The mechanical properties of the specimen are elucidated using a Perkin Elmer 7-series dynamic mechanical analyser (DMA). Each specimen is subjected to a static stress of 2.26 MPa, subsequently super-imposed by a sinusoidal stress in the frequency range 0-20 Hz of magnitude 2.0 MPe. Tensile bond strength tests were conducted on an Instron 4302 (10 kN load cell) in accordance with ASTM C-633, a hot-cured araldite epoxy glue being used as the adhesive. The tensile adhesion tests were performed with a cross-head speed of 0.5 mm rain-~.
283
K.A. Knot el al. Journal ~/ Malerials Processing T~'clmoloKr 71 (1997) 280~ 287
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K.A. Khor et al./Journal of Materials Processing Technology 71 (1997) 280-287
284
Table 2 Summary of MIP results of HIPped Ti-6AI-4V/hydroxyapatite coatings Samples
TIHA 1 TIHA 2 TIHA 3
Average pore diameter (.am)
Overall porosity (",4,,)
As-sprayed
HIPped at 1000°C
As-sprayed
HIPped at 1000°C
0.1676 0.0542 0.3971
0.7873 0. ! 291 0.5713
18.74 22.46 31.69
17.14 13.91 21.07
3. Results and discussion
3.1. Microstructure and phase composition of Ti6Al4V/hydroxyapatite composite coatings after HIP Fig. 1 shows the polished cross-section of Ti-6AI4V/HA coating, revealing a typical lamella structure consisting of alternating layers of Ti-6AI-4V and HA. Some unmelted particles are also observed, High-magnification observation shows that the HA-rich regions are mechanically weak and evidence of pull-outs during polishing can be observed quite vividly. In addition, some HA-rich lamellae have cracks (Fig. 2), these cracks needing to be 'healed' for the composite coatings to have reasonable mechanical strength during usage. SEM observation of polished cross-sections of the coatings shows that the lamellar structure is preserved after HIP (Fig. 3). However, the HA-rich portion appeared 'compressed'. This arises as a result of plastic deformation of the Ti-6AI-4V phase during HIP that cause the ductile metal to yield plastically and close up the pores between the lamellae. In addition, the individual lamellae appear to be more linear compared to the assprayed lamellae, similar observation being made on HIPped Ni,based coatings [21]. There are, however, limitations to what HIP can do in the coatings, and particular defects in the as-sprayed coatings cannot be eliminated effectively, these being unmelted particle clusters (Fig. 4) and incomplete closure of pores (Fig. 5). Part of the reason lies in that the coatings were not encapsulated in a steel capsule during HIP. This was done deliberately to ensure that the surface pores in the coatings would not be fully closed up, since these pores are likely to facilitate tissue ingrowth during implantation of the prosthesis. Thus, the intrusion of the pressurising gases would likely minimise the efficacy of HIP in the closure of all pores. The XRD patterns of the Ti-6AI-4V/hydroxyapatite coatings in the as-sprayed condition and after HIP are shown in Fig. 6. There is a marked increase in the amount of crystalline phases in the coatings following HIP. In the 80 wt.% HA coating, amorphous calcium phosphate is converted to the crystalline HA that is bioactive and beneficial to the osl~o-integration af the
hip implant. Formation of amorphous calcium phosphate during plasma spraying is attributed to the loss of hydroxyl groups from the crystalline HA [22].
3.2. Porosity of Ti6Al4 V/hydroxyapatite composite coat#1gs t~ter HIP There is apparently less porosity in the HIPped coatings. MIP measurements showed that the porosity of the as-sprayed 20 wt.% HA composite was ~ 19%, this value being reduced to ~ 17% for the sample HIPped at 1000°C. Although the porosity reduction was relatively minor the interesting aspect of HIP is the drastic reduction of small pores of less than 0.3 ~tm, the average pore diameter being observed to increase from 0.1676 lam in the as-sprayed coating to 0.787 ~tm in the sample HIPped at 1000°C. The effect of HIP on the coatings, as shown by SEM observations, is such that the ductile phase (Ti-6AI-4V) undergoes plastic deformation during HIP and subsequently closes up many of the micropores between the lamellae. With the elimination of the micropores, the average pore size of the coating is increased. The improvement in the physical properties can be observed succinctly in the increase in the microhardness values of the respective coatings and in density increase, the microhardness increasing by ~ 25% and the density improving by ~ 8% [12]. The 80 wt.% HA coatings showed that HIP effectively reduced the amount of large ( > 50 ~tm) pores as well as the fine pores ( < 0.1 ~tm). Table 2 summarises the porosity results of the HIPped coatings. The main mechanisms responsible for the reduction in the porosity would be the direct effects of high isostatic pressure and increased temperature. However, there are different dominant mechanisms for the 20 and 80 wt.% HA coatings. In the 20 wt.% HA coating, plastic yielding, brought about by the high pressures, caused the Ti lamellae and particles to deform plastically, closing up most of the pores, whilst increased temperature together with the stress field applied caused interlamellar diffusion such that material diffused from areas between particles to the voids, enabling the lamellae to move closer together and, consequently, to close up the pores. Although the
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K.,4. Khor el al. Join'rod q f MateriaLs Processing Techmd,,,,,y 71 (I 997) 280-287
285
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same mechanism would be expected to exert its influence in the 86 wt.% HA coating, the amount of Ti phase is nonetheless smaller. Still, the plastic yielding of the 20 wt.% Ti-6A1-4V phase in the 80 wt.°/,, HA coating would have been the key instrument in the reduction of the pores, particularly the small pores. What is perhaps more dominant in the case of the 80 wt.% HA coating is the break down of the HA regions that have weak inter-lameilae or inter-particle bonds during HIP. This is evident from the significant reduction of the large pores > 50 lam after HIP. The pores, in particular the large pores, appear to collapse during HIP, which leads subsequently to particle fragmentation and the formation of 'new' inter-particle contacts. Similar observation was made in a previous study on the HIP of a plasma-sprayed zirconia coating stabilised by Y203 [23].
3.3. Dynamic mechanical properties of Ti6Al4V/hydroxyapatite coathtgs after HIP Both the 20 and 33 wt.% HA coatings exhibit improvements in storage modulus after the HIP treatment, the results showing that the 20 wt.% HA reinforced composite possesses a greater modulus than the 33 wt.% HA coatings (Fig. 7). The storage modulus of the 20 wt.% HA coatings improved from 60 GPa (as-sprayed) to ,,-80 GPa (HIPped at !000°C). The reduction of pores < 0.1 lam is significant in plasma sprayed coatings because these are likely to be the inter-lamella pores that formed as a result of incomplete bonding between the ~uccessive lamellae during the spray process [7],, their presence serving to weaken
the whole coating, resulting in poor mechanical properties. The 33 wt.% HA coatings show dramatic response to HIP at 950°C where the modulus improved by 20 GPa (Fig. 8). This indicates that with a greater percentage of the ceramic reinforcement, a greater HIP temperature may be required to restore the coating defects. probably by the plastic deformation of the Ti-6AI-4V phase under HIP.
3.4. Tensile adhesire strength of coatings a./i'er HIP Figs. 9 and 10 show the bond strength of the Ti 6A1-4V/hydroxyapatite coatings (as-sprayed and HIPped) with respect to the coating thickness, from which it can be seen that the tensile adhesive strength decreases with increasing coating thickness. The results also demonstrate that the tensile adhesive strength generally improves after HIP. The enhancement of the adhesion strength in the 20 wt.% HA coating after HIP is obvious for coatings below 160 ~tm, coatings thicker than 160 ~tm showing that HIP may have adverse effects on the coating strength. Observation of the fractured surface of the as-sprayed coatings test sampies revealed adhesive failure, i.e. that is failure of the coatings occurred at the coating/substrate interface. However, the HIPped coatings showed a different mode of failure. SEM revealed that the failure of the HIPped coatings is predominantly cohesive in nature (Fig. I1). Failure of the HIPped coatings during the tensile test occurred within the coatings and not at the coating/substrate interface, which may indicate that tim HIP treatment enhanced the coating/substrate interface more than it did the inter-lamella contacts.
286
K.A. K/mr et al./Journal of Materials Processing Technology 71 (1997j 280-287
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Fig. 8. DMA results of HIPped Ti-6AI-4V/33 wt.% HA coatings.
4. Conclusions
The HIP of plasma-sprayed composite coatings of Ti-6AI-4V/HA improved the physical properties, such as microhardness and the density, of the coatings even though the increase in porosity was rather modest. This was attributed to the drastic reduction in the small micro-pores <0.3 ~tm that lie mainly between the lamellae, thus disrupting the intcr-lamellae contact. SEM examination or" the plasma-sprayed coatings revealed the microstructure to have retained its typical lamella structure. However, the HA-rich portion appeared 'compressed' as a result of the plastic yielding brought about by the• ductile Ti-6AI-4V phase. Results from the dynamic mechanical analyser (DMA) showed that the storage modulus of the 20 and
33 wt.% HA coatings improved with HIP and that there is a corresponding increase with the HIP temperature employed. HIP treatment at 950°C elicits a significant response from the 33 wt.% HA coating. Other physical properties, such as density and microhardness, also improved with HIP. Tensile adhesive bond tests show the interface between the coating and the substrate to be improved. The enhancement of the adhesion strength in the 20 wt.% HA coating after HIP is obvious only for coatings below 160 lam. The mode of failure apparently transferred from adhesive failure in the as-sprayed coatings to a predominantly cohesive mode of failure in the HIPped samples, indicating that 20
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thickness(urn) Fig. 9. Tensile bond test results of plasma sprayed Ti-6AI- -4V/20 wt?/. , hydroxyapatite coatings (as-sprayed and HIPped).
Fig. 10. Tensile bond test results of plasma sprayed Ti-6AI-4V/33 wt.% hydroxyapatite coatings (as-sprayed and HIPped).
K.A. Kkor et al. ,Journal of Material.s Proc,'.s.~ing ]i'clmoh,~:l" 71 !1997) 280 2 8 7
287
References
Fig. ! 1. SEM view of a fractured Ti- 6AI surface after the bond test.
4V/hydroxyapatite coating
the HIP treatment improved the coatmg/substrate interface more than it did the inter-lamella contacts within the coatings.
Acknowledgements Financial support lor the plasma spray system from NTU RP 56/92 is gratefully acknowledged. The hol isostatic press system was financed by a research grant from Singapore Ministry of Finance.
[11 M. Takagi, M. Mochida, N. Uchida, K. Saito. K. Uew.ats~, J. Mater. Sd. Mater. Med. 3 {1992) 199 203. [2] J.H. Chern Lin, M.L. Liu. C.P.{}. Ju. J. Mater. Sci. Mayer. Med. 5 {1994t 279 283. [3] J. 15. S. Forberg. L. Hemlansson. Bioma~erials 12 (1991) 438 440. [4] tt. ,li, P.M. Marquis, Biomaterials 13 ! l l i {19~12) 7 4 4 7 4 8 . [5] M. Jarcho, C I m Orthop. 157 ! 1 9 g l ) 2 5 9 278. [6] R.G.T. Geesink.. K. de (}root, C.P.A.T. Klein, J. Bone Joi~'ll Surg. 70B tl98gl 17 22. [7] C.P.A.T. Klein, P. Parka. I . G ( ' . Wolke, J.M.A. de BlicckHogervorst, K. de Grool, Biomaterial.,, 15 {1994) 146 150. [81 P. Ducheync. J.M. Cucklcr, Clin. Orthop. 276 (1992) I¢b2. 114. [9] A. Moroni, V.L. Ct0a, E.L. Egger, k. Trinchese, E.Y.S. Chao, Biomaterials 15 {19941 9 2 6 930. [10] R.G.T. Oeesmk, CIm. Orti~op. 2(',1 !199H) 39 5S. [11] K.A. Khor, C.S. Yip, P. ( h e a n g , J Tl~ermal Spray Tech. 6 {19971 1{)9 115. [12] C.,~. Yip, K.A. Khor. N.L. Lol, and P. ('heai~g, J. Mats. Proc. T,:'ch., lin press). [13] H. Kuribayashi. K. StlgUlltllll~.t. ~'. Mi}amoto. M. Koizumi, Am Ceram. Soc. Btdl. 65 (9) {1986~ 1306 1.tl0. [141 M. Nakamura, Y. Kaieda, Powder Me:. 31 i l 9 8 8 1 2 0 1 211. [15] I,.M. Sheppard, Am. Ceram. Soc. Bull. 71 119921 311 322. [i6] D.W. Shin, K.K. Orr, H. Sdmberl, J. Am. Ceram. Soc. 73 (1990) 1181 1188. [17] F.F. Lange, J. Am. Ceram. Soc. 66 {0l {1983) 390. [18] M.J. Blackburn and R,A. Sprague, Superalloxs Source Book, ASM, 1984. pp. 2 6 9 276. [19] E.W. Washburn, Proc. Nati Acad. Sci. 7 {1921) 115 116. [20] R. McPherson, Thin Solid Films 83 (1981) 297 381. [21] K.A. Khor, N.L. Lob, J. Thelmal Spray Tech. ASM Int. 3 Ill {1994) 57 6 2 . [22] S.R. Radin, P. Ducheyne, J. Mater. Sci. Mater. Med. 3 {19921 33. [23] K.A. Khor. N.I,. Loh, m T.S. Sudarshan. W. Reitz, J.J. Stiglich {Eds.}, Surface Moditication Technologies IX, TMS. Warrendale, PA, USA, 1996, pp. 515 531).