DENTAL MATERIALS 24(2008)289-298 297 less appealing characteristic of zirconia: its susceptibility to further establishing that the enrichment of the cubic grains in low temperature degradation(LTD). This phenomenon was Y+ ions corresponded to a depletion in yttrium in the neigh first reported by Kobayashi in Ref. [40]. It was shown that boring tetragonal grains which became less stable and acted a slow t-m transformation from the metastable tetragonal as nucleation sites for the(t-m)transformation. phase to the more stable monoclinic phase occurs in surface n excellent review by Chevalier, evaluating the future grains in a humid environment at relatively low temperatures of zirconia as a biomaterial has recently been published (150-400C) The zirconia studied in this work was a polycrys- [51]. With zirconia becoming increasingly popular as a den talline zirconia stabilized by 4.5-6 mol% yttria. These results tal restorative material, under various forms and with a wide prompted a series of investigations on low temperature degra- range of processing conditions(Table 2), it seems wise to keep dation of zirconia stabilized with various amounts of Y2O3 in mind that some forms of zirconia are susceptible to aging that helped define the characteristics of the transformation and that processing conditions can play a critical role in the 41-47] low temperature degradation of zirconia. Classically, LTD initiates at the surface of polycrystalline zirconia and later progresses toward the bulk of the mate- rEFERENCES rial. The transformation of one grain is accompanied by an increase in volume that causes stresses on the surrounding grains and microcracking Water penetration then exacerbates the process of surface degradation and the transf [1] Messing GL, Hirano S, Gauckler L Ceramic processing science. J Am Ceram Soc 2006: 89 (6): 1769-70 progresses from neighbor to neighbor. The growth of the trans- [2] Evans AG. Perspectives on the development of formation zone results in severe microcracking, grain pullout high-toughness ceramics. J Am Ceram Soc and finally surface roughening, which ultimately leads to 1990;73(2):187-206 strength degradation. Any factor that would be detriment 3 Subbarao EC. Zirconia-an overview In: Heuer AH, Hobbs LW, to the stability of tetragonal zirconia is susceptible to promot editors. Science and technology of zirconia. Columbus, OH: low temperature degradation. Amongst these factors are the The American Ceramic Society: 1981. p. 1-24 grain size[48, the amount of stabilizer [9] and the presence of 4 Goff JP, Hayes W, Hull S, Hutchings MT, Clausen KN Defect structure of yttria-stabilized zirconia and its influence on residual stresses[49). he ionic conductivity at elevated temperatures. Phys Rev B One consequence of the above mentioned observations 1999:59(22):14202-19 was a 1997 posting by the Food and Drug Administration [5]Evans AG, Heuer AH. Review-transformation toughening in (http://www.fda.gov/cdrh/steamst.htmlcautioningagain ceramics:martensitic transformations in crack-tip stress fields. J Am Ceram Soc 1980; 63(5-6): 241-8 steam sterilization of zirconia femoral heads for total hip pros-(6] Garvie RG, Nicholson PS Structure and thermomechanical theses and specifying thatit could cause phase transformation and roughening of the material, later leading to increased roperties of partially stabilized zirconia in the Cao-zr02 Fabris S, Paxton A, Finnis MW. A stabilization mechanism of zirconia based on oxygen vacancies only. Acta Mater 8.1. Cubic phase and accelerated aging 8 Foschini CR, Filho T, Juiz SA, Souza AG, oliveira More recently, in 2001, some series of Y-TZP femoral heads t al. On the stabilizing behavior of zirconia: a co experimental and tical study. J Mater Sci were recalled due to spontaneous fractures. These inci- 200439:1935-41 dents were traced back to a manufacturing issue that led to 19) Hannink RH], Kelly PM, Muddle BC.Transformation accelerated tetragonal to monoclinic transformation in the toughening in zirconia-containing ceramics. J Am Ceram thecatastrophicfailures(http://www.prozyr.com/pages.Uk/[10GarvieRc,hAnninkRh,PascoeRt.Ceramicsteel?nature Biomedical/Committee. htm). These events had a noticeable 1975;258:703-4 negative impact on the use of zirconia as an implant bioma- [11 Heuer AH. Transformation toughening in ZrOz-containing ceramics.JAm Ceram Soc 1987: 70(10) 689-98 terial, triggering a considerable amount of research work in [12 Heuer AH, Claussen N, Riven WM, Ruhle M Stability of order to elucidate the possible origin of the failures (50, 51] tetragonal Zro2 particles in ceramic matrices. J Am Ceram Although anticipated from the phase diagram established Soc1982:65(12):642-50 earlier by Scott[52], recent work has demonstrated that both [13] Porter DL, Heuer AH. Mechanisms of toughening parti bic and tetragonal phases co-exist in 3Y-TZP [22, 50, 53 .Mat sui et al. reported that the amount of cubic zirconia in 3Y-TZP [4 Montross CS. Comparison of bulk properties of Mg-PSZ with sintered at 1300oC was 12.7 mass% and reached 18.6 mass% 1993;76(8):1993-7 when the sintering temperature was 1500C. These values [15] Hughan RR, Hannink RH). Precipitation during controlled were determined by X-ray diffraction and Rietveld calcula- ions [22]. In addition, the distribution of the Y3+ ions inside Ceo ing of magnesia-partially stabilized zirconia.JAm geneous while r fter sintering at 1300 C was nearly homo- [16] Steffen AA, Dauskardt RH, Ritchie RO Cyclic fatigue life and the zirconia ns appeared to concentrate within the rack-growth behavior of microstructurally small cracks in larger cubic grains after sintering at 1500 C. It was also shown magnesia-partially stabilized zirconia ceramics. JAm Ceram Soc1991;74(6):125968 that cubic phase regions started to form from grain bound- [171 Hannink RHJ Microstructural development of the aries and triple junctions where the y=+ ions had segregated sub-eutectoid aged Mgo-zro2 alloys. J Mater Sci 22 These results were confirmed by Chevalier et al. 50 1983;18:457-70dental materials 24 (2008) 289–298 297 less appealing characteristic of zirconia: its susceptibility to low temperature degradation (LTD). This phenomenon was first reported by Kobayashi in Ref. [40]. It was shown that a slow t→m transformation from the metastable tetragonal phase to the more stable monoclinic phase occurs in surface grains in a humid environment at relatively low temperatures (150–400 ◦C). The zirconia studied in this work was a polycrys￾talline zirconia stabilized by 4.5–6mol% yttria. These results prompted a series of investigations on low temperature degra￾dation of zirconia stabilized with various amounts of Y2O3 that helped define the characteristics of the transformation [41–47]. Classically, LTD initiates at the surface of polycrystalline zirconia and later progresses toward the bulk of the mate￾rial. The transformation of one grain is accompanied by an increase in volume that causes stresses on the surrounding grains and microcracking.Water penetration then exacerbates the process of surface degradation and the transformation progresses from neighbor to neighbor. The growth of the trans￾formation zone results in severe microcracking, grain pullout and finally surface roughening, which ultimately leads to strength degradation. Any factor that would be detrimental to the stability of tetragonal zirconia is susceptible to promote low temperature degradation. Amongst these factors are the grain size [48], the amount of stabilizer [9] and the presence of residual stresses [49]. One consequence of the above mentioned observations was a 1997 posting by the Food and Drug Administration (http://www.fda.gov/cdrh/steamst.html) cautioning against steam sterilization of zirconia femoral heads for total hip pros￾theses and specifying that it could cause phase transformation and roughening of the material, later leading to increased wear on the acetabular component. 8.1. Cubic phase and accelerated aging More recently, in 2001, some series of Y-TZP femoral heads were recalled due to spontaneous fractures. These inci￾dents were traced back to a manufacturing issue that led to accelerated tetragonal to monoclinic transformation in the central area of the femoral heads that likely played a role in the catastrophic failures (http://www.prozyr.com/PAGES UK/ Biomedical/Committee.htm). These events had a noticeable negative impact on the use of zirconia as an implant bioma￾terial, triggering a considerable amount of research work in order to elucidate the possible origin of the failures [50,51]. Although anticipated from the phase diagram established earlier by Scott [52], recent work has demonstrated that both cubic and tetragonal phases co-exist in 3Y-TZP [22,50,53]. Mat￾sui et al. reported that the amount of cubic zirconia in 3Y-TZP sintered at 1300 ◦C was 12.7 mass% and reached 18.6 mass% when the sintering temperature was 1500 ◦C. These values were determined by X-ray diffraction and Rietveld calcula￾tions [22]. In addition, the distribution of the Y3+ ions inside the zirconia grains after sintering at 1300 ◦C was nearly homo￾geneous while Y3+ ions appeared to concentrate within the larger cubic grains after sintering at 1500 ◦C. It was also shown that cubic phase regions started to form from grain bound￾aries and triple junctions where the Y3+ ions had segregated [22]. These results were confirmed by Chevalier et al. [50], further establishing that the enrichment of the cubic grains in Y3+ ions corresponded to a depletion in yttrium in the neigh￾boring tetragonal grains which became less stable and acted as nucleation sites for the (t→m) transformation. An excellent review by Chevalier, evaluating the future of zirconia as a biomaterial has recently been published [51]. With zirconia becoming increasingly popular as a den￾tal restorative material, under various forms and with a wide range of processing conditions (Table 2), it seems wise to keep in mind that some forms of zirconia are susceptible to aging and that processing conditions can play a critical role in the low temperature degradation of zirconia. references [1] Messing GL, Hirano S, Gauckler L. Ceramic processing science. J Am Ceram Soc 2006;89(6):1769–70. [2] Evans AG. Perspectives on the development of high-toughness ceramics. J Am Ceram Soc 1990;73(2):187–206. [3] Subbarao EC. Zirconia-an overview. In: Heuer AH, Hobbs LW, editors. Science and technology of zirconia. Columbus, OH: The American Ceramic Society; 1981. p. 1–24. [4] Goff JP, Hayes W, Hull S, Hutchings MT, Clausen KN. Defect structure of yttria-stabilized zirconia and its influence on the ionic conductivity at elevated temperatures. Phys Rev B 1999;59(22):14202–19. [5] Evans AG, Heuer AH. Review—transformation toughening in ceramics: martensitic transformations in crack-tip stress fields. J Am Ceram Soc 1980;63(5–6):241–8. [6] Garvie RG, Nicholson PS. Structure and thermomechanical properties of partially stabilized zirconia in the CaO–ZrO2 system. J Am Ceram Soc 1972;55(3):152–7. [7] Fabris S, Paxton A, Finnis MW. A stabilization mechanism of zirconia based on oxygen vacancies only. Acta Mater 2002;50:5171–8. [8] Foschini CR, Filho T, Juiz SA, Souza AG, Oliveira JBL, Longo E, et al. On the stabilizing behavior of zirconia: a combined experimental and theoretical study. J Mater Sci 2004;39:1935–41. [9] Hannink RHJ, Kelly PM, Muddle BC. Transformation toughening in zirconia-containing ceramics. J Am Ceram Soc 2000;83(3):461–87. [10] Garvie RC, Hannink RH, Pascoe RT. Ceramic steel? Nature 1975;258:703–4. [11] Heuer AH. Transformation toughening in ZrO2-containing ceramics. J Am Ceram Soc 1987;70(10):689–98. [12] Heuer AH, Claussen N, Kriven WM, Ruhle M. Stability of ¨ tetragonal ZrO2 particles in ceramic matrices. J Am Ceram Soc 1982;65(12):642–50. [13] Porter DL, Heuer AH. Mechanisms of toughening partially stabilized zirconia (PSZ). J Am Ceram Soc 1977;60(3–4):183–4. [14] Montross CS. Comparison of bulk properties of Mg-PSZ with temperature-time contour diagrams. J Am Ceram Soc 1993;76(8):1993–7. [15] Hughan RR, Hannink RHJ. Precipitation during controlled cooling of magnesia-partially stabilized zirconia. J Am Ceram Soc 1986;69(7):556–63. [16] Steffen AA, Dauskardt RH, Ritchie RO. Cyclic fatigue life and crack-growth behavior of microstructurally small cracks in magnesia-partially stabilized zirconia ceramics. J Am Ceram Soc 1991;74(6):1259–68. [17] Hannink RHJ. Microstructural development of the sub-eutectoid aged MgO-ZrO2 alloys. J Mater Sci 1983;18:457–70