Stability of hydroxyapatite while processing short-fibre reinforced hydroxyapatite ceramics

Stability of hydroxyapatite while processing short-fibre reinforced hydroxyapatite ceramics

ELSEVIER PII: S0142-9612 (97) 00066-5 Biomaterials 18 (1997) 1523-1529 © 1997 Elsevier Science Limited Printed in Great Britain. All rights reser...

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Biomaterials 18 (1997) 1523-1529 © 1997 Elsevier Science Limited Printed in Great Britain. All rights reserved 0142-9612/97/$17.00

Stability of hydroxyapatite while processing short-fibre reinforced hydroxyapatite ceramics M. Knepper*, S. Moricca t and B.K. Milthorpe* *University of New South Wales, Graduate School of Biomedical Engineering, Sydney, NSW 2052, Australia ~Australian Nuclear and Science Technology Organisation, Advanced Materials Division, New Illawarra Rd, Lucas Heights, NSW 2234, Australia

Reinforcement by short fibres has been adapted from modern ceramic processing technologies to achieve an improvement of structural properties of hydroxyapatite. However, the influence of the reinforcement fibres on the thermochemical behaviour of the hydroxyapatite has yet to be clarified comprehensively. Titanium, alumina and 316L-stainless steel, all materials with a proven record as implant materials, were chosen as reinforcement materials. Short fibres of these materials were incorporated in a matrix of hydroxyapatite to toughen the hydroxyapatite. Composites were processed by sintering in air, hot isostatic pressing and a method combining sintering in inert gas atmosphere and hot isostatic pressing. © 1997 Elsevier Science Limited. All rights reserved Keywords: hydroxyapatite, composites, fibre reinforcement, titanium, alumina, 316L-stainless steel

Received 16 December 1996; accepted 21 March 1997

Hydroxyapatite (HAp) is the main mineral component of natural bone. The well-known bioactivity of HAp leads to a high bone ingrowth capability which gives HAp the potential of being one of the most desired bone replacement materials. At this time, applications for synthetic HAp are restricted to areas free of dynamic load bearing, as synthetic HAp is known for its weakness and brittleness, revealed by the material's low fracture toughness value (Kic = 1.11.2MNm-3/2) 1. The only exception where HAp is applied in dynamically loaded situations is the use of HAp as a relatively stable coating material 2-6. The use of solid HAp requires a suitable method of material toughening. A common method of improving the fracture toughness of ceramic materials is toughening the ceramic matrix by the addition of short fibres as a second phase. Manufacturing techniques such as sintering, hot isostatic pressing (HIP) or sintering, then hot isostatic pressing (Sinter-HIP) seem to be suitable for processing reinforced HAp ceramics. The mechanical and structural properties of the fibre-HAp composite will be influenced strongly by the fibre type, the preparation method of fibres and green bodies and by the sintering method. Results of our preliminary studies have shown that hot isostatically pressed fibre reinforced HAp increases the fracture toughness up to a range comparable with bone (Kic= 2-12MN m 3/2 7 ). For 316L-stainless steel fibre reinforced HAp ceramics of 96% density, we

have obtained fracture toughness values K~c = 11 MN m -3/28. However, in the case of applying these high temperature/high pressure processes, the material's sensitivity to thermal treatment also has to be taken into account. High temperatures in conjunction with impurities accelerate the formation of non-desired tricalcium-phosphate and tetra-calcium-phosphate9-11. This chemical reaction is of a non-reversible nature. Impurities may even decrease this temperature and fibres have to be regarded as desired impurities. Figure 1 illustrates the decomposition behaviour of hydroxyapatite. The structural properties of the fibre reinforced HAp are affected by the complete manufacturing process. Thus, each manufacturing stage (powder production, green body production, sintering) is crucial for the material s performance. MATERIALS AND METHODS

Hydroxyapatite Bioactive synthetic HAp is commercially available in different forms depending on the materials application, e.g. as fine powder (particle size < 20 #m} or as spray dried agglomerates (particle size up to 165/~m). HAp powder 'for bioceramics' of the type MERCK 2196 (Merck AG, Darmstadt, Germany), with a particle size in the range 2-20pro, was used for the studies. A chemical analysis of the powder (Table 1) was provided by the supplier. Figure 2a shows the scanning electron microscopy (SEM) image of the powder as received.

Correspondence to Dr M. Knepper. 1523

Biomaterials 1997, Vol. 18 No. 23


Short-fibre reinforced hydroxyapatite ceramics: M. Knepper et al. I

C3P + C4P CaO + C4P


~ 1 1 ...... ~ ~ 1


Calcium Oxide



~ !








C4P +



: :~ ~~ :• ~ ~

i| ~! ! !

: ~ | ~ I~1!

.!, .....



C3P +


~ 1 I ~ 1 ~ , ~:, ~ i ~ l~l ~ i I

CaO + HAp Calcium Oxide + Hydroxyapatite

C3P + C2P

: I I





~'~1 :~ ~




% P20s v

Phase equilibrium diagram of different calcium phosphates.

Figure 1

Table 1

Analysis of the HAp-powder 12

Analysis (complexometric)


Impurities Non-soluble in HCI Chloride (CI)

Fluoride (F) Sulphate (SO4) Heavy metals (As, Pb) Arsenic (As) Iron (Fe)

<0.05% <0.05% <0.005% <0.25% <0.003% <0.0002% <0.04%

Particle size

<20/~m 2-20 #m >20 #m

~20% ~75% <5%

Water content


Alumina Among the ceramics used in orthopaedics and orthodontics, alumina is the most chemically inactive. After its early evaluation alumina was soon regarded as the prototype of so-called bioinert materials 13. For this study commercially available alumina fibres of the type ALMAX (Mitsui Boeki Ltd., Osaka, Japan) were used. The fibres shown in Figure 2b are of 10pm diameter with a length of 0.5 ± 0.2 mm.

Titanium The outstanding biocompatibility of titanium and titanium alloys was recognized by early medical researchers. Of the many titanium alloys that have been found to be suitable for medical applications, pure titanium and Ti-AI-V 6-4 alloy have become Biomaterials 1997, Vol. 18 No. 23

most widely used. The clinical success of titanium alloy is owing to their outstanding mechanical properties, corrosion resistance and superior biocompatitbilty14. Pure titanium fibres were supplied by Bekeart Fibre Technologies B.V. (Zwevegem, Belgium). The titanium fibres were produced by bundled wire drawing before being chopped to segments of 1.01.5mm. The fibre diameter was 50#m. Figure 2c shows the SEM image of the fibres as used.

316L-stainless steel Iron-based alloys currently form one of the predominant groups of metallic materials for biomedical applications. For implantation in case of multi-component weight bearing situations, type 316L stainless steel (Fe33CrsNi6Mo), an austenitic alloy combining extremely high corrosion resistance with high mechanical strength, is most frequently selected 15. 316L stainless steel wire was supplied by Knight Precision Wires (UK). The chopped 316oL fibres were of 50/~m diameter and of 1.0-0.15 mm length. Figure 2d shows the SEM image of the stainless steel fibres. Table 2 summarizes and compares the main mechanical properties of all materials used. It becomes obvious that the mechanical properties differ widely. A composite material created of HAp and the fibre materials is expected to unify the advantages of both the matrix and the fibre material by combining mechanical strength and bioactivity. Thus, the new materia] can be expected to overcome the structural weakness of pure HAp.

Materials processing Composite powders of a volumetric matrix/fibre ratio of 9"0:10 and 80:20 for HAp-316L, HAp-A1203 and HAp-Ti

Short-fibre reinforced hydroxyapatite ceramics: M. Knepper et al.




Figure 2

Table 2


Raw materials: a, hydroxyapatite; b, alumina; ¢, titanium; ~1, 316L-stainless steel.

Properties of matrix and fibre materials 17'18

Matrix Fibres

HAp Alumina Titanium 316L


K~c (MPa.v'~)

E-modulus (GPa)

40-300 270-500 300-750 500-750

0.6-1.0 3q~ 40-70 80-150

80-120 380-410 110 196

isostatic pressing. HIP temperatures were 900 and 1000°C applying a gas pressure of 100MPa. Samples used with Sinter HIP were sintered in an inert atmosphere for a certain time before the HIP-pressure was applied. Figure 6a shows the pressure-time diagram of the applied HIP process and Figure 6b shows the parameter set for Sinter-HIP.

Microstructural Investigations and Decomposition Behaviour composites were prepared. HAp powder was prepared without the addition of fibres as standard material, too. In order to manufacture a solid composite material characterized by an optimal fibre dispersion in the matrix, a dispersion method which avoids fibre clumping and allows dispersion throughout the matrix material was applied 16 to manufacture a composite powder consisting of HAp/fibre mixture as shown in

Figure 3. Green bodies of the composite material were produced by choosing either uniaxial pressing followed by cold isostatic pressing (Figure 4). Green body pellets were manufactured according to the procedure described in the literature 16. The green bodies were then either treated by sintering, HIP or Sinter-HIP. Sintering was performed in a graphite furnace. Heating and cooling rates were 150°C h -1, soaking time was I h in air atmosphere and pressureless. The soak temperatures were 900, 1000, 1100 and 1200°C. Hot isostatic pressing was performed using AIP 630H (American Isostatic Pressing Inc., USA). The composite green bodies (d = 19mm, h = 30mm) were hot isostatically pressed either encapsulated in steel or in pyrex glass. Figure 5 illustrates the process of hot

For the microstructural analysis of the interfaces matrix/fibre segments of sample disks were embedded in epoxy under vacuum. After hardening the samples were prepared by grinding and polishing. SEM investigations using a Cambridge $360 (Cambridge Instruments, UK) scanning electron microscope were conducted to evaluate the microstructure of the composite bodies. Other segments of the samples were polished and evaluated by X-ray diffraction (XRD) using a Siemens 5000 (Siemens AG, Germany) diffractometer. The results of the XRD investigations were compared with the JCPDS-standards of the pure raw materials. In this manner phases occurring in the composites could be detected in order to identify the decomposing behaviour of the HAp matrix.


Microstructure of the composites The SEM studies revealed that the density of the green bodies is highly dependent on the sintering method Biomateriats 1997, VoL 18 No, 23


Short-fibre reinforced hydroxyapatite ceramics: M. Knepper et al.

t Figure ~

HAp-composite ~ow6er pro6uction.

Uniaxial Pressing (CP)

Cold lsostatic Pressing (CIP)

Figure 4 Uniaxial pressing and cold isostatic pressing for

applied. Figure 7 compares micrographs of crosssections of hydroxyapatite/stainless steel 80/20 samples processed by sintering (Figure 7a) and by HIP (Figure 7b). The sample processed by sintering shows a low porosity (dark areas in between the matrix represents epoxy). The relative density Pr~ ( : P/Pth . . . . fical) was measured as 56%. The hydroxyapatite matrix is stabilized by the a mesh formed of the 316L-fibres and so the whole composite is prevented from collapsing. In contrast, the sample produced by HIP seems to be fully dense Prel : 97%) and the fibres are completely embedded in the HAp-matrix. Figure 8 compares micrographs of cross-sections of hydroxyapatite/titanium 80/20 samples processed by SinteroHIP and HIP. It is apparent that the density of the samples processed by HIP (Figure 8b) (Pre~ = 95%) is higher than the density of the samples processed by Sinter-HIP (pre~= 63%) (Figure 8a). Highest density can be achieved by applying the HIP technology. Sinter-HIP allows to process samples showing a much higher density compared with samples processed by sintering. This was confirmed by SEM microstructural investigations which also revealed that the microstructure of HAp-fibre composites depends on the process. The relative density of the HAp composites is lower compared with that of pure HAp sintered at the same temperature and soaking time. Differences in the shrinkage behaviour between pure HAp and fibre reinforced HAp, leading to a lower density in HAp reinforced sintered ceramics, have been noted by Ruys et al. 19 for samples sintered in air only. In general, density values of fibre-reinforced HAp processed by Sinter-HIP can be expected to be higher 1000 ).~-T-~-temp. -



HAp-composite green body preparation. stage 1 green

stage 4

stage 5

removal of encapsulation

denslfied solid ceramic

npcrature 2 2 0 0



of the hydroxyapatite matrix

The raw materials were evaluated prior to investigating the composites. Figure 9 shows the XRD-patterns for pure hydroxyapatite MERCK 2196 (Table 2), as received and after being calcined at 1200°C, for titanium, alumina and 316L-stainless steel. The results reveal that all raw materials were of high purity and without any indication of detectable impurities. In the case of a hydroxyapatite control which was calcined at 1200°C no decomposition caused by the calcination could be observed.


~ " ~














120 100

E~ :



B 600' ~





20 0

0 50 100150200250300350 time [rain] b

Pressure-time diagram of: a, the HIP-process; b, the sinter-HIP-process.

Biomaterials 1997, Vol. 18 No. 23


than those for fibre reinforced HAp processed by sintering only.

0 50 100150200250300350 time [rain]

Figure 6

{~ {


Figure 5 The principle of hot isostatic pressing.

4O ~


II,'1 ~ssure 200 MPa



stage 3

hot isostatie pressing

:~sph~re (At, Nz):



stage 2


Jl l



"~ 200


Short-fibre reinforced hydroxyapatite ceramics: M. Knepper et al.



b Figure 7

b HAp/316L 80/20 processed by: a, sintering; b,


The composites were evaluated by XRD after being processed using the different methods. The main attention was drawn to the formation of tri-calciumphosphate within the composite materials, as the presence of TCP indicateg a non-reversible phase transformation and dehydration of the HAp. Figure 10 illustrates the outcome of the XRD investigations of HAp/316L 80/20 composites manufactured by (a) sintering in air and (b) hot isostatic pressing. Obviously sintering in air enhances transformation of HAp to TCP. In contrast, the HAp matrix is thermally stable while being hot isostatically pressed at the same temperature. Table 3 summarizes the results for all composites evaluated by XRD and indicates the presence of TCP qualitatively. TCP present in the matrix indicates an occurrence of dehydration while being processed. This dehydration process is shown related to the process parameters. In summary, the results of the XRD investigations reveal that the HAp transformation to TCP is highly dependent on (i) the type of fibre material; (ii) the matrix/fibre ratio, and (iii) the atmosphere and gas pressure while processing. For the fibre materials investigated the tendency

Figure 8 b, HIP.

HAp/titanium 80/20 processed by: a, Sinter-HIP;

of increasing decomposing temperature is in the order: Td. . . .


(HAp/316L) < Td. . . .



< Tdecompose(HAp/A1203) < Td. . . . pose(HAp) The results strongly suggest that applying pressure while processing these composites suppresses the HAp transformation to TCP. Thus processes applying pressure and an inert atmosphere (as HIP and Sintero HIP) not only offer the advantage of higher bulk density but also a TCP-free HAp composite. The results are in accordance with a previous study conducted by Ruys et a111. which looked at powders of various types. However, titanium was not considered as a reinforcement material. Decomposition of HAp is known to be effected by a wide variety of conditions. These include inter-alea temperature 2°, production history21, reaction with substrate materials and the presence of particulate reinforcement1°. Krajewski et 0]22. have indicated that thermal history of prepared HAp powders can influence their dehydration behaviour. For plasma spraying on titanium and titanium alloys there have been studies that noted a considerable reaction that Biomaterials 1997, Vol. 18 No. 23


Short-fibre reinforced hydroxyapatite ceramics: M. Knepper et al. •

,....,.'"'t .

~ : Z ~ e ~ e . tJ : 8-'0~2 | C



~mS. L i b r a "

~T'~i .:


~L. ~ 2~h~










• : ~

. " ~ ? ' ~ s . . :





i .~



N .O~dl)




~,J..~, - . ~.,.- . l.b, . d / ! ~ ¢ . J . ~ . - 1 ~ . , . J t J *4 :



i~-~i~S I RI~09 C~,'O'"~'M ~

J ~ 4 U ~ l d ~ d L | ~ , ~ ~U~

,.,., .... .,,,__,__~l]~,~





7~. L i n e , "



1.~8 C~(a|~t

~,lJ.~.~,lnJ..-,.hL,,.,~,.,.~il ~ d ~ , ~ ! _ " ~_~__._o~..J,.,~. . . . . . .

!~1,. ~ - ~


S~l~12 ! r l t~.t,,¢~i~

~SJII~ ~ CPF~Hi ~ r ~ i ~

x : 2 ~ [~n K~I

V ~




Figure 9 XRD patterns of the raw materials: a, HAp as received; b, alumina fibres; ¢, titanium fibres; d, 316L stainless steel fibres.

produces a calcium-titanium-phosphate layer in the interface between substrate material and HApcoating23.24. However, none of these studies have reported the effects of macroscopic fibres on thermal decomposition. It is well known that surface area to volume ratio has a considerable effect on reaction rates and the conditions found during sintering or HIP are very different to those found during plasma spraying. Therefore there is no a priori reason to expect significant thermal decomposition from the presence of fibres during sintering or HIP. The finding of a difference in decomposion of the HAp between sintering and HIP for processing these composites is significant in terms of controlling the composition of the final product. SUMMARY AND CONCLUSIONS Composite materials containing hydroxyapatite as matrix phase and fibres of alumina, 316L-stainless steel or titanium as reinforcement phase were processed using three different methods: sintering in air, HIP and Sinter-HIP. Density can be varied by choosing an appropriate ceramic process. More importantly, employing the appropriate sintering technology enables the composite to be formed without detectable decomposition of the hydroxyapatite. Also, the nature of the reinforcement fibre determines the tendency of the matrix phase to decompose. However, applying pressure during the sintering process suppresses the decomposition of HAp in the composites process at elevated temperatures. Biomaterials 1997, Vol. 18 No. 23

In conclusion, hot isostatic pressing is a favoured method of processing HAp-fibre reinforced ceramics as it leads to composites of high density and free of detectable decomposition products, when using high purity starting materials. These studies demonstrate that a metal fibre reinforcement of HAp is possible in order to achieve a product with desired properties.

ACKNOWLEDEGMENTS This work was supported by the German Research Foundation. The authors would like to thank Dr R. Specht of E. Merck AG, Darmstadt, Germany, for the supply of hydroxyapatite powder and Mr B. de Loose of Bekaert Fibre Technologies BV, Zweregem, Belgium for the supply of titanium fibres. The authors gratefully acknowledge the invaluable contributions of Mr D. Koch of the Materials Science Institute Aachen, Germany, regarding the preparation of the samples and of Mr Christian Ragoss regarding the green body production.



Li, J. a n d H e r m a n s o n , L., Interceram. 1990, 39(2), 1 3 15. De Groot, K. et aL, J. Biomat. Res., 1987, 21, 1375-1381. Lugscheider E., W e b e r T. a n d Knepper, M., In Proceedings 2nd National Thermal Spray Conference, Cincinnati, 1988, 337-343. Klein, C. P. A. T. eta]., J. Biomed. Mater. Res., 1991, 25, 53-65.

S h o r t - f i b r e r e i n f o r c e d h y d r o x y a p a t i t e c e r a m i c s : M. Knepper et al.

Table 3


Decomposing of the HAp phase in fibre reinforced HAp composites Tmax, Process


900°C sinter

1000°C sinter



A10 A20 Sl0 $20 Til0 T20

,/ n.a. TCP TCP #' n.a.

n.a.: not available;

1100°C sinter

1200°C sinter

900°C HIP

1000°C HIP

900°C sinterHIP





J n.a. TCP TCP TCP n.a.

TCP n.a. TCP TCP TCP n.a.

/ n.a.


,/ n.a. TCP TCP #" n.a.

,/ n.a. n.a. n.a. n.a. n.a.

n.a. /

TCP: Tri-Calcium-Phosphate;

10. 11.

12. 13. 25,~

g'D~ ~

~ : 2~h~ C a ~ I ) fl~)~UL~PD~ = ~ 1 ~ 1 ~ 1~ N ~ I I

V :

~ .~

24S. L | n ~ r







~.m ru~,~,~



18. •

~.~ x : ~"Ute't= ~ ; ~g~. ~.fv"Je~" ~'-~d.~Z | ~=~(~O4 ~=~;r,~,f.~ R~.~u~¢=~, ~,~1



b Figure 10 XRD patterns of HAp/316 stainless composites: a, 80/20 sintered in air; b, HIP.

5. 6.

7. 8.

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21. 22. 23.

Symposium on Molecular Engineered Biomaterials, 9.

n.a. n.a. n.a. ,/

#': HAp composites.



,/ ,/ ,/ n.a. ~"


Hastings and K.E. Tanner. Butterworth-Heinemann, Oxford, 1991. Klein, C.P.A.T., Wolke, J. G.C. and De Groat, K. An Introduction to Bioceramics, eds L.L. Hench and J. Wilson. World Scientific, Singapore, 1993. Krajewski, A., Gelotti, G., Ravaglioli, A. and Toriyama, M., Cryst. Res. Technol., 1996, 31(5) 843-852. Krajewski, A., Gelotti, G., Ravaglioli, A. and Pincastelli, A., Cryst. Res. Technol., 1995, 30(6) 637-646. Brantley, W.A., Tufekci, E., Mitchell, J.C., Foreman, D.W. and McGlumphy, E.A., Cells Mater., 1995, 5(1) 73-82. Weng, J., Wolke, J. G. C., Zhang, X.D. and De Groat, K., J. Mater. Sci. Lett., 1995, 15, 333-335.

Biomaterials 1997, Vol. 18 No. 23