北京化工大学：《材料导论》课程阅读材料（无极非金属）Zirconia_as_a_ceramic_biomaterial

变 Biomaterials ELSEVIER Biomaterials20()1-25 Review Zirconia as a ceramic biomaterial C.Piconi".*G.Maccaurob Reccived 19 March 1997:accepted 18 December 1997 Abstract Zirconia ceramics have severa advantages over other ceramic materials,due to the transformation toughening mechanism ;Mechanical properties Stability Biocompatibility Wear,Radioactivity 1.Introduction Thename te meohe Aumbe same using zirconia as a ceramic biomaterial. Zargon (golden in colour)which in turn comes from the The R&D on zirconia as a biomaterial was started in d)and Gun (Colour).Zirco in the reaction product obtained after heating some of zirconia to manufacture ball heads for Total Hip gems,and was used for a long time blended with rare or ce In the early stages of the development,several solid ceramics are used to manufacture parts operating in solutions (ZrO2-MgO,ZrO2-CaO,ZrOz-Y2O3)were aggressive environments,like extrusion dyes,valves tested for biomedical applications(Table 1).But in the for com on engines,low co ollowing years the rese d to be more foundries.Zirconia blades are used to cut Kevlar,mag netic tapes,cigarette filters (because of their reduced ia ocramics uable as solide e almost all the manufactu arers that are introducing into the market zirconia ball heads (Table 2).More than 30000 TZP ball heads has been implanted [4],and only ailures were reported [5]up to now Pabiod by eveed

Biomaterials 20 (1999) 1 —25 Review Zirconia as a ceramic biomaterial C. Piconi!,*, G. Maccauro" !ENEA, New Technologies Dpt., New Materials Div., Roma, Italia "Institute of Orthopaedics, Universita` Cattolica del S. Cuore, Roma, Italia Received 19 March 1997; accepted 18 December 1997 Abstract Zirconia ceramics have several advantages over other ceramic materials, due to the transformation toughening mechanisms operating in their microstructure that can give to components made out of them, very interesting mechanical properties. The research on the use of zirconia ceramics as biomaterials started about twenty years ago, and now zirconia (Y-YZP) is in clinical use in THR, but developments are in progress for application in other medical devices. Recent developments have concentrated on the chemistry of precursors, in forming and sintering processes, and on surface finish of components. Today’s main applications of zirconia ceramics is in THR ball heads. This review takes into account the main results achieved up to now, and is focused on the role that microstructural characteristics play on the TZP ceramics behaviour in ball heads, namely mechanical properties and their stability, wear of the UHMWPE paired to TZP, and their influence on biocompatibility. ( 1998 Published by Elsevier Science Ltd. All rights reserved Keywords: Zirconia; Mechanical properties; Stability; Biocompatibility; Wear; Radioactivity 1. Introduction Zircon has been known as a gem from ancient times. The name of the metal, zirconium, comes from the Arabic Zargon (golden in colour) which in turn comes from the two Persian words Zar (Gold) and Gun (Colour). Zirco￾nia, the metal dioxide (ZrO2 ), was identified as such in 1789 by the German chemist Martin Heinrich Klaproth in the reaction product obtained after heating some gems, and was used for a long time blended with rare earth oxides as pigment for ceramics. Although low-quality zirconia is used as an abrasive in huge quantities, tough, wear resistant, refractory zirconia ceramics are used to manufacture parts operating in aggressive environments, like extrusion dyes, valves and port liners for combustion engines, low corrosion, ther￾mal shock resistant refractory liners or valve parts in foundries. Zirconia blades are used to cut Kevlar, mag￾netic tapes, cigarette filters (because of their reduced wear). High temperature ionic conductivity makes zirco￾nia ceramics suitable as solid electrolytes in fuel cells and in oxygen sensors. Good chemical and dimensional * Correspondence address: ENEA-INN-NUMA, CR Casaccia 049, Via Anguillarese 301, 00060 Roma, Italy. Fax:#39 6 3048 4928; e-mail: piconi@infosl.casaccia.enea.it stability, mechanical strength and toughness, coupled with a Young’s modulus in the same order of magnitude of stainless steel alloys was the origin of the interest in using zirconia as a ceramic biomaterial. The R&D on zirconia as a biomaterial was started in the late sixties. The first paper concerning biomedical application of zirconia was published in 1969 by Helmer and Driskell [1], while the first paper concerning the use of zirconia to manufacture ball heads for Total Hip Replacements (THR), which is the current main applica￾tion of this ceramic biomaterial, was introduced by Christel et al. [2]. In the early stages of the development, several solid solutions (ZrO2 —MgO, ZrO2 —CaO, ZrO2 —Y2 O3 ) were tested for biomedical applications (Table 1). But in the following years the research efforts appeared to be more focused on zirconia—yttria ceramics, characterised by fine grained microstructures known as Tetragonal Zirconia Polycrystals (TZP). Nowadays, TZP ceramics, whose minimal require￾ments as implants for surgery are now described by the standard ISO 13356 [3], are the materials selected by almost all the manufacturers that are introducing into the market zirconia ball heads (Table 2). More than 300 000 TZP ball heads has been implanted [4], and only two failures were reported [5] up to now. 0142-9612/98/$—See front matter ( 1998 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 6 1 2 ( 9 8 ) 0 0 0 1 0 - 6

2 C.Piconi.G.Maccauro/Biomaterials 20(1999)1-25 Property Units Alumina Mg-PSZ TZP Chemical 9%A0, 450-700 1200 engrth 2000 re toughn css K 7-10 1x10- ardness 1200 1200 with monoclinic and tetragonal zirconia precipitates as Producers of zirconia ball heads for THR the minor phase.These precipitates may exist at grain Producer Country boundaries or within the cubic matrix grains.In 1972 Garvie anc holson [8]sh wed that the mechanica Astrome USA of the Ceramte Germany matrix.The development of zirconia as an engineering material was marked by Garvie et al.[9],who in their paper owed how to make the best of tion in PS Z improving mecn observed that tetrago nal metastable p dispersed within the cubic matrix were able to be trans forme into the monoclinic phase v e m e by a cra 2.Microstructural properties associated with ex nsion due to the tion acts in opposition to the stress field that promotes Zirconia is a well-known polymorph that occurs in hree forms:monoclinic(M),cubic(C)and tetragonal(T) es Because the propa ransforms into tetragonal and then into cubic phase at due to the volume nsion.A schematic representation 2370C.During cooling.a T-M transformation takes of this phenomenon is given in Fig.1.The development ol suc tetrago I metastable may ob ing plac on ol s of Mgo to ZrO imat matrix of a tetra nate cracks in pure zirconia ceramics that,after sintering gonal metastable phase.during controlled cooling and in the range 1500-1700C,break into pieces at roo ageing. s1n92 that n and coworkers [6 also be ob ined in the ZrC aoe时adin trm ed a ure with a tetra of Cao onal phase only.called TZP.This result was reported The addition of'stabilising'oxides,like CaO.Mgo. first by Rieth et al[]and by Gupta et al.[11] TZP materials,containing approximately 2-3 %m0 (PSZt d by tetragonal ra fraction of T-phase retained at room temperature is

Table 1 Characteristics of some ceramics for biomedical applications Property Units Alumina Mg—PSZ TZP Chemical composition 99.9% Al2 O3 ZrO2 ZrO2 #MgO #8%10 mol% MgO #3 mol% Y2 O3 Density g cm~3 *3.97 5.74—6 '6 Porosity % (0.1 — (0.1 Bending strength MPa '500 450—700 900—1200 Compression strengrth MPa 4100 2000 2000 Young modulus GPa 380 200 210 Fracture toughness KIC MPa m~1 4 7—15 7!10 Thermal expansion coeff. K~1 8]10~6 7—10]10~6 11]10~6 Thermal conductivity W mK~1 30 2 2 Hardness HV 0.1 2200 1200 1200 Table 2 Producers of zirconia ball heads for THR Producer Country Astromet USA Ceraver France Ceramtec Germany Norton France Kyocera Japan Metoxit Switzerland Morgan Matroc United Kingdom NGK Japan SCT France Xylon USA 2. Microstructural properties Zirconia is a well-known polymorph that occurs in three forms: monoclinic (M), cubic (C) and tetragonal (T). Pure zirconia is monoclinic at room temperature. This phase is stable up to 1170°C. Above this temperature it transforms into tetragonal and then into cubic phase at 2370°C. During cooling, a T—M transformation takes place in a temperature range of about 100°C below 1070°C. The phase transformation taking place while cooling is associated with a volume expansion of approx￾imately 3—4%. Stresses generated by the expansion origi￾nate cracks in pure zirconia ceramics that, after sintering in the range 1500—1700°C, break into pieces at room temperature. It was in 1929 that Ruff and coworkers [6] showed the feasibility of the stabilisation of C-phase to room temperature by adding to zirconia small amounts of CaO. The addition of ‘stabilising’ oxides, like CaO, MgO, CeO2 , Y2 O3 , to pure zirconia allows to generate multi￾phase materials known as Partially Stabilized Zirconia (PSZ) whose microstructure at room temperature gener￾ally consists [7] of cubic zirconia as the major phase, with monoclinic and tetragonal zirconia precipitates as the minor phase. These precipitates may exist at grain boundaries or within the cubic matrix grains. In 1972 Garvie and Nicholson [8] showed that the mechanical strength of PSZ was improved by an homogeneous and fine distribution of the monoclinic phase within the cubic matrix. The development of zirconia as an engineering material was marked by Garvie et al. [9], who in their paper ‘Ceramic Steel?’ showed how to make the best of T—M phase transformation in PSZ improving mechan￾ical strength and toughness of zirconia ceramics. They observed that tetragonal metastable precipitates finely dispersed within the cubic matrix were able to be trans￾formed into the monoclinic phase when the constraint exerted on them by the matrix was relieved, i.e. by a crack advancing in the material. In that case, the stress field associated with expansion due to the phase transforma￾tion acts in opposition to the stress field that promotes the propagation of the crack. An enhancement in tough￾ness is obtained, because the energy associated with crack propagation is dissipated both in the T—M trans￾formation and in overcoming the compression stresses due to the volume expansion. A schematic representation of this phenomenon is given in Fig. 1. The development of such tetragonal metastable precipitates may be ob￾tained by the addition of some 8% mol of MgO to ZrO2 . This allows the formation a fully cubic microstructure at 1800°C, and the nucleation within the matrix of a tetra￾gonal metastable phase, during controlled cooling and ageing. PSZ can also be obtained in the ZrO2 —Y2 O3 system (Fig. 2). However in this system it is also possible to obtain ceramics formed at room temperature with a tetra￾gonal phase only, called TZP. This result was reported first by Rieth et al. [10], and by Gupta et al. [11]. TZP materials, containing approximately 2—3% mol Y2 O3 , are completely constituted by tetragonal grains with sizes of the order of hundreds of nanometers. The fraction of T-phase retained at room temperature is 2 C. Piconi, G. Maccauro / Biomaterials 20 (1999) 1—25

C.Piconi.G.Maccauro/Biomaterials 20(1999)1-25 process zone 88 O ● ● 0 crack 0kTi ● 6oo 02 Y:O content (mol%) untransformed transformed transforming particle particle particle Fig.1.Repr of stre and in c ing the matrix transforming toughned roonia seramics.Mat Sci Tech Tetragonal PhaseVolume Fraction 2500 2000 50 sz TZP 500 Y:O content (mol%) mas+Cas s vs.ytt system M 92120 2 Y:O content (mol%) onia part of zirconia-yttria phase diagram.Commer ant to onal grains.A critical grain size exists.linked to the yttria concentration,above which spontaneous T-M transt characteristic of transformatiot dependent on the size of grains,on the yttria content,on toughened zirconia ceramics is the formation of compres sive layers on their surface [13].Surface tetragonal grains an transform to

Fig. 1. Representation of stress-induced transformation toughening process. Energy of the advancing crack is dissipated in phase trans￾formation and in overcoming the matrix constraint by transforming grains (Reprint with permission from Butler EP, Transformation toughned zirconia ceramics. Mat Sci Tech 1985;1:417—32.). Fig. 2. High zirconia part of zirconia—yttria phase diagram. Commer￾cial PSZ and ZTP composition and processing temperatures are in￾dicated by shaded regions (Reprint with permission from Scott HG, Phase relationship in zirconia—yttria systems. J Mater Sci 1975; 10:1527—35.). Fig. 3. Retention of tetragonal phase. Critical grain size against Yttria content in tetragonal zirconia (Reprint whith permission from Lange FF, Transformation toughenining, Part 3—Experimental observations in the ZrO2 —Y2 O3 system. J Mater Sci 1982;17:240—6.). Fig. 4. Fracture toughness vs. yttria content (Reprint whith permission from Lange FF, Transformation toughenining, Part 3—Experimental observations in the ZrO2 —Y2 O3 system. J Mater Sci 1982;17:240—6.). dependent on the size of grains, on the yttria content, on the grade of constraint exerted on them by the matrix. Mechanical properties of TZP ceramics (Figs. 3 and 4) depend on such parameters. It is very important to consider the metastable nature of the tetragonal grains. A critical grain size exists, linked to the yttria concentration, above which spontaneous T—M transformation of grains takes place, whereas this transformation would be inhibited in a too fine grained structure [12]. An interesting characteristic of transformation toughened zirconia ceramics is the formation of compres￾sive layers on their surface [13]. Surface tetragonal grains are not constrained by the matrix, and can transform to monoclinic spontaneously or due to abrasive processes C. Piconi, G. Maccauro / Biomaterials 20 (1999) 1—25 3

4 C.Piconi.G.Maccauro/Biomaterials 20(1999)1-25 that can induce compressive stresses at a depth of several a loss of stability of the tetragonal phase moreo microns under the surface. mullite (3AlO.2Sio.)pockets were detected in the The surface phase transition and the consequent sur aluminosilicate glass,which leads to a loss of stability of face hardening may have a relevant role in improving the prop in T-M surface transformation may originate surface cracking.followed by ejection of grains from the surface with catastrophic effects on mech- anica 3.Mechanical properties our and joi cially Mg- as ceran There is no doubt that zirco ni vourable results.But R&D on this material for biomedi- ical properties better than other ceramic hiomaterials ie cal applications appears to have to be stopped in the alumina [18].as shown in Table 1.Comparison among fac Young's moduli,strength and hardness of some bio- materials,including ceramics,are shown in Table 3 by a residu with in they 3.1.Results of compression tests on TZP ball heads UHMWPE sockets that are currently coupled with zir- ive Load (UCL)of ball heads is into acp ME-PSZ at higher )720 6-5 stand The initation ment of the metastable tetragonal precipitates,that empera It can be erved fro all heads UCL t that using erancan mm can wi Difficulties in obtaining Ms-PSZ free of heads UCL depends on design and material character SiO2,Al2O3 and other impurities [14].increase in SiO istics both of the ball head and of the metallic spigot:the contents due to the wear ing media during powde angle mismatch betw een bore and taper,and the surface ma yhave contrib TZP the magr an d position ma materials.In ceramics containing MgO.magnesi nalysis on different design [20-221 of ceramic hall cates like enstatite (MgSiO3)and forsterite (Mg2SiO) heads has shown that two main stress concentrations are may form at grain boundaries [14],lowering the Mgo ocalized in the inner surface of the ceramic bore,one d prom oting the bending fress)at the top of the cavity,an one (hoop aland its stabilit anichal prope d -me ape The mago Nevertheless.Mg-PSZ ball heads were used in the USA he metal-ceramic contact area.the roug ness of [16]and Australia.Also TZP precursors can contair he surfaces and the friction coefficient of the two which i as a liquid phase fo rming ac ests per ormed on TZP ball heads [23]show that to served that in the und aries scavenge yttrium ions from TZP grains,leading to of mem ormdi Property Units Ti 6Al 4V 316SS CoCr Alloy TZP Alumina Young's modulus 210 380

Table 3 Properties of some materials for biomedical applications Property Units Ti 6Al 4V 316 SS CoCr Alloy TZP Alumina Young’s modulus GPa 110 200 230 210 380 Strength MPa 800 650 700 900—1200 '500 Hardness HV 100 190 300 1200 2200 that can induce compressive stresses at a depth of several microns under the surface. The surface phase transition and the consequent sur￾face hardening may have a relevant role in improving the mechanical and wear properties of zirconia parts, the thickness of the transformed layer being one of the limit conditions. Progresses in T—M surface transformation may originate surface cracking, followed by ejection of grains from the surface with catastrophic effects on mech￾anical behaviour and joint wear. Several PSZ were tested as ceramic biomaterials, espe￾cially Mg—PSZ, which was extensively tested with fa￾vourable results. But R&D on this material for biomedi￾cal applications appears to have to be stopped in the early 1990s. Several reasons can account for this fact: Mg—PSZ are characterised by a residual porosity as is normal in materials with grain sizes in the range 30—40 lm. This can influence negatively the wear rate of UHMWPE sockets that are currently coupled with zir￾conia ball heads. Also technological aspects may have been taken into account. Mg—PSZ sinter at higher tem￾peratures than TZP (1800°C vs. 1400°C), implying the need of special furnaces. The precipitation and develop￾ment of the metastable tetragonal precipitates, that occurs during cooling, requires a strict control of the cooling cycle in terms of temperature and time, especially in the ageing step that takes place at about 1100°C, during which the precipitation of T-phase occurs. Difficulties in obtaining Mg—PSZ precursors free of SiO2 , Al2 O3 and other impurities [14], increase in SiO2 contents due to the wear of milling media during powder processing before firing [15] may have contributed to shift the interest of ball head manufacturers towards TZP materials. In ceramics containing MgO, magnesia sili￾cates like enstatite (MgSiO3 ) and forsterite (Mg2 SiO4 ) may form at grain boundaries [14], lowering the MgO contents in the grains and promoting the formation of the monoclinic phase, reducing the mechanichal proper￾ties of the material and its stability in a wet environment. Nevertheless, Mg—PSZ ball heads were used in the USA [16] and Australia. Also TZP precursors can contain silica, which is sometimes used as a liquid phase forming additive to achieve full density at temperatures lower than 1500°C limiting grain growth. Lin et al. [17] ob￾served that aluminosilicate glasses in the grain bound￾aries scavenge yttrium ions from TZP grains, leading to a loss of stability of the tetragonal phase. Moreover, mullite (3Al2 O3 )2SiO2 ) pockets were detected in the aluminosilicate glass, which leads to a loss of stability of the material in a wet environment. The use of such additives is hence to be avoided in TZP as ceramic biomaterials. 3. Mechanical properties There is no doubt that zirconia ceramics have mechan￾ical properties better than other ceramic biomaterials, i.e. alumina [18], as shown in Table 1. Comparison among Young’s moduli, strength and hardness of some bio￾materials, including ceramics, are shown in Table 3. 3.1. Results of compression tests on TZP ball heads Ultimate Compressive Load (UCL) of ball heads is tested following the ISO 7206-5 standard [19]. The test procedure consists of the application of static loads to the ball head inserted in a metallic spigot until fracture, and it may be considered a useful tool to compare different designs. It can be observed from UCL tests that using TZP ceramics, ball heads of 022.22 mm can withstand static loads ranging several times the physiologic ones. Ball heads UCL depends on design and material character￾istics both of the ball head and of the metallic spigot: the angle mismatch between bore and taper, and the surface roughness controls the magnitude and position of max￾imum stress in the ceramic ball head. Finite elements analysis on different designs [20—22] of ceramic ball heads has shown that two main stress concentrations are localized in the inner surface of the ceramic bore, one (bending stress) at the top of the cavity, and one (hoop stress) at the ceramic—metal taper interface. The magni￾tude of such stresses is dependent on the position, the metal—ceramic contact area, the roughness of the surfaces and the friction coefficient of the two counterfaces. Tests performed on TZP ball heads [23] show that to minimize the concentration of stresses it is necessary to maintain a gap *2 mm between the spigot and the top of the conical cavity, and maximize the extension of the 4 C. Piconi, G. Maccauro / Biomaterials 20 (1999) 1—25

C Piconi G.Maccauro/Biomaterials 20 (1999)1-25 contact area,taki 3.2.Stability of the tetragonal phase peeynenedii0hecoaicaheore Mechanical r perties of zirconia relate to its fine In standard prostheses design,taper to ceramic bore coupling is made with a tolerance of some 0 to -5'on structure during the lifetime of TZP components is the e taper and o to on the conical bore [24].An key point to attain the expected of tay tha nowr extended to the complete taper surface due to the metallic taper strain following loading.Drouin and Cales [23]. monoclinic phase.This behaviour is well known in the Da ore observing a decrease in bovin the pnc of r were summarised by There is ex erimental evidence r25.261 that UCL of Swab [331 in the followi zirconia ball heads is 2-2.5 times higher than the UCL of (1)The most critical temperature range is 200-300C alumina bal heads of thes (2)The t al.[2 rve s ar nd density,and an 1 toughne UCL of alumina ball he ads of 02 s due to the but itmust be remarked that the failure by these authors are far below the ones of currently manu racking of the material factured ball heads -M transition arts on the surface and progress es variations in spigo to the material bulk and/or increase in conc mismatch of the an and in the tanet inser tion of stabilising oxide reduce the transformation tion depth all play an important role on the results rate. obtained by this test.A summary of results obtained by (6)T-M transformation is enhanced in water or in va- several authors [23, ]is reported in Table4 ed to on the incou s T-M published [164]. zirconium hydroxides [31,32,34]or yttrium hydroxides Taper material Taper type Ball head Neck length Remarks (mm) 30 28 35 mm (S) +35mm( mm (S Ti6Al4V 10/12 200 28 0 3.5 mm (L) 540107%54 28 12/14 HV:o:310 Ti6l4VA 288 0 +3mm四

Table 4 UCL of TZP ball heads on different tapers Ref. Taper material Taper type Taper roughness Ball head Neck length UCL Remarks (lm) diameter (kn) (mm) !3.5 mm (S) 110 30 28 0 105 #3.5 mm (L) 85 !3.5 mm (S) 140 22 Ti6Al4V 10/12 200 28 0 130 #3.5 mm (L) 110 3 22.22 0 78 30 22.22 0 98 27 Ti6l4VA 8/10 Not specified 22.22 Not specified 45 10/12 34 Ti alloy 4.6 80 HV10 : 352 28 Ti alloy 48.4 93 HV10 : 320 CoCr alloy 12/14 2.7 28 L 44 HV10 : 435 CoCr alloy 2.7 47 HV10 : 644 NiCrMo alloy 60.3 108 HV10 : 310 32 0 122$16 29 Ti6l4VA 4° Not specified 28 0 97$11 28 #3 mm (L) 84$6 28 !3 mm (S) 133$13 contact area, taking care of the rise of hoop stresses in the rim portion of the ball head when the taper is not com￾pletely inserted into the conical bore. In standard prostheses design, taper to ceramic bore coupling is made with a tolerance of some 0 to !5@ on the taper and 0 to #5@ on the conical bore [24]. Angle mismatch is selected in such a way that contact takes place first in the upper part of the ceramic bore, and is extended to the complete taper surface due to the metallic taper strain following loading. Drouin and Cale´ s [23], reported that the angle mismatch (10@) can be more than doubled in TZP ball heads before observing a decrease in ball heads UCL. There is experimental evidence [25, 26] that UCL of zirconia ball heads is 2—2.5 times higher than the UCL of alumina ball heads of the same diameter and neck length. Also Tateishi et al. [27, 28] observed UCL of TZP ball heads of 022.2 mm on Ti6Al4V spigots almost double the UCL of alumina ball heads of 028 mm on CoCr spigots, but it must be remarked that the failure loads reported by these authors are far below the ones of currently manu￾factured ball heads. It is clear that variations in the spigot material and roughness, in the roughness of the ceramic bore, in the mismatch of the bore/taper angle, and in the taper inser￾tion depth all play an important role on the results obtained by this test. A summary of results , obtained by several authors [23, 28—30] is reported in Table 4. A com￾prehensive summary of the main parameters of the taper influencing the head-Trummion assembly was recently published [164]. 3.2. Stability of the tetragonal phase Mechanical properties of zirconia relate to its fine grained, metastable microstructure. The stability of this structure during the lifetime of TZP components is the key point to attain the expected performances. Mechanical property degradation in zirconia, known as ‘ageing’, is due to the progressive spontaneous trans￾formation of the metastable tetragonal phase into the monoclinic phase. This behaviour is well known in the temperature range above 200°C in the presence of water vapour [31, 32]. The main steps of TZP ageing were summarised by Swab [33] in the following way: (1) The most critical temperature range is 200—300°C. (2) The effects of ageing are the reduction in stength, toughness and density, and an increase in monoclinic phase content. (3) Degradation of mechanical properties is due to the T—M transition, taking place with micro and macro￾cracking of the material. (4) T—M transition starts on the surface and progress es into the material bulk. (5) Reduction in grain size and/or increase in concentra￾tion of stabilising oxide reduce the transformation rate. (6) T—M transformation is enhanced in water or in va￾pour. The models proposed to explain the spontaneous T—M transformation in TZP are based on the formation of zirconium hydroxides [31, 32, 34] or yttrium hydroxides C. Piconi, G. Maccauro / Biomaterials 20 (1999) 1—25 5