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 toFig. 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