DENTAL MATERIALS 24(2008)289-298 8. Low temperature degradation of 3Y-TZP 8.1. Cubic phase and accelerated aging 2.31%; t-m approximately 4.5%. Sintered structures trans 1. Toward improved reliability: ( 1)flaw control and (2) flaw tolerance forming from t to m on cooling from sintering temperatures (approximately 1300-1500C)undergo spallation with por- tions crumbling into multi-grained powders Two research paths aimed at increasing the structural relia Beginning about 1972, the ceramic engineering commu- bility of ceramics have been pursued over the past 30 years. nity was discovering that alloying with lower valance oxides The first involved efforts to minimize the number and size such as Cao, Mgo, La203, and Y203, disfavored the strained of critical flaws based on the well-accepted Griffith flaw- m phase at room temperature and favored more symmetric ize/strength relationship for linearly elastic brittle fracture c"and t lattice structures(with'indicating metastability)[6] These cand t' phases are analogous to those in pure zirco- (1) nia but have dopant ions substituted on Zrt sites and have a fraction of oxygen sites vacant to retain charge neutrality where of is the fracture strength, c the critical flaw radius, 14]. The amount of dopant required for full cubic stabiliza- Kic critical stress intensity in mode I opening and Y is the tion is substantial; 8 mo1% in the case of the dopant Y2O3 crack shape factor. This research, part of a discipline known with one oxygen vacancy created for every two yttrium ions as ceramics processing, continues to investigate a multitude [7]. Partial stabilization of tetragonal zirconia can occur at of steps including powder fabrication (to control chemi- dopant concentrations of 2-5mo1% depending on grain size, 1 homogeneity particle size, size uniformity, etc ) particle to be discussed below. These metastable c and I phases have dispersion in processing media, powder consolidation and prolonged stability at room temperature given that cation and ho mobility in zirconia is quite low and that the oxygen vacan sities), sintering control, and"flaw kind"finish machining cies are locally ordered 17]. Recent consistent, but apparently (or finishing to net-shape avoiding the need for machining) independent work, attributes tetragonal metastability solely [1]. Second were efforts controlling ceramic microstructures to the presence of the oxygen vacancies that allow both anion to increase their resistance toward crack propagation, and cation relaxations to occur dependent on their vacancy to increase toughness. Counter to the Griffith relationship, proximity 14, 7,8. Overall, three mechanisms are discussed for increased strength and increased toughness do not gener ally correlate in transformation-toughened ceramics as will Y203 and CeO2: ()dopants inducing oxygen vacancies that are beelaborated on later. However, suchhigh toughness ceramics generally trivalent (e.g. Gd", Fe3+, Ga3+, and Y3); (i)tetrava- of lower strength are appealing for structural use due to their lent dopants being undersized or oversized with respect to the damage tolerance. Table 1 lists five major ceramic toughen ing mechanisms along with engineering material examples 2 One of these mechanisms, transformation toughening along with microcracking and deflection mechanisms are the Table 1-Ceramic toughening mechanisms [2] toughening mechanisms now prominent in zirconia-based or Mechanism Highest toughnes Example zirconia-containing ceramics and are the topic of this paper. aterials Transformation 2. Zirconia polymorphs: temperature dependence and transformation strains Microcracking Al O3/ZrO? SinG/sic Pure zirconia is monoclinic (m) at room temperature and pressure. With increasing temperature the material trans Metal dispersion AlO/Al forms to tetragonal (t), by approximately 1170 C and then Al2O3/Ni to a cubic (c) fluorite structure starting about 2370C with melting by 2716 C[3, 4]. These lattice transformations are Whiskers/platelets SigNa/Sic martensitic, characterized by(1)being diffusionless (i.e. involv SigN4/SigN Al2 O3/Sic ing only coordinated shifts in lattice positions versus transport of atoms),(2) occurring thermally implying the need for a Fibers CAS/SiC temperature change over a range rather than at a specific temperature and,(3) involving a shape deformation [S]. This Al2O3/SiC C/SiC transformation range is bounded by the martensitic start(Ms) and martensitic finish temperatures. Volume changes on cool Al2O3/Al2O ing associated with these transformations are substantial enough to make the pure material unsuitable for applica- a Calcium aluminum silicate glass ceramic. tions requiring an intact solid structure: c- t approximately Lithium aluminum silicate glass ceramic290 dental materials 24 (2008) 289–298 8. Low temperature degradation of 3Y-TZP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296 8.1. Cubic phase and accelerated aging. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297 1. Toward improved reliability: (1) flaw control and (2) flaw tolerance Two research paths aimed at increasing the structural relia￾bility of ceramics have been pursued over the past 30 years. The first involved efforts to minimize the number and size of critical flaws based on the well-accepted Griffith flaw￾size/strength relationship for linearly elastic brittle fracture: f = KIC Y √c (1) where f is the fracture strength, c the critical flaw radius, KIC critical stress intensity in mode I opening and Y is the crack shape factor. This research, part of a discipline known as ceramics processing, continues to investigate a multitude of steps including powder fabrication (to control chemi￾cal homogeneity, particle size, size uniformity, etc.), particle dispersion in processing media, powder consolidation and packing (creating high and homogeneous greenware den￾sities), sintering control, and “flaw kind” finish machining (or finishing to net-shape avoiding the need for machining) [1]. Second were efforts controlling ceramic microstructures to increase their resistance toward crack propagation, i.e. to increase toughness. Counter to the Griffith relationship, increased strength and increased toughness do not gener￾ally correlate in transformation-toughened ceramics as will be elaborated on later. However, such high toughness ceramics of lower strength are appealing for structural use due to their damage tolerance. Table 1 lists five major ceramic toughen￾ing mechanisms along with engineering material examples [2]. One of these mechanisms, transformation toughening along with microcracking and deflection mechanisms are the toughening mechanisms now prominent in zirconia-based or zirconia-containing ceramics and are the topic of this paper. 2. Zirconia polymorphs: temperature dependence and transformation strains Pure zirconia is monoclinic (m) at room temperature and pressure. With increasing temperature the material trans￾forms to tetragonal (t), by approximately 1170 ◦C and then to a cubic (c) fluorite structure starting about 2370 ◦C with melting by 2716 ◦C [3,4]. These lattice transformations are martensitic, characterized by (1) being diffusionless (i.e. involv￾ing only coordinated shifts in lattice positions versus transport of atoms), (2) occurring athermally implying the need for a temperature change over a range rather than at a specific temperature and, (3) involving a shape deformation [5]. This transformation range is bounded by the martensitic start (Ms) and martensitic finish temperatures. Volume changes on cool￾ing associated with these transformations are substantial enough to make the pure material unsuitable for applica￾tions requiring an intact solid structure: c→t approximately 2.31%; t→m approximately 4.5%. Sintered structures trans￾forming from t to m on cooling from sintering temperatures (approximately 1300–1500 ◦C) undergo spallation with por￾tions crumbling into multi-grained powders. Beginning about 1972, the ceramic engineering commu￾nity was discovering that alloying with lower valance oxides, such as CaO, MgO, La2O3, and Y2O3, disfavored the strained m phase at room temperature and favored more symmetric c* and t* lattice structures (with * indicating metastability) [6]. These c* and t* phases are analogous to those in pure zirco￾nia but have dopant ions substituted on Zr4+ sites and have a fraction of oxygen sites vacant to retain charge neutrality [4]. The amount of dopant required for full cubic stabiliza￾tion is substantial; 8mol% in the case of the dopant Y2O3 with one oxygen vacancy created for every two yttrium ions [7]. Partial stabilization of tetragonal zirconia can occur at dopant concentrations of 2–5mol% depending on grain size, to be discussed below. These metastable c* and t* phases have prolonged stability at room temperature given that cation mobility in zirconia is quite low and that the oxygen vacan￾cies are locally ordered [7]. Recent consistent, but apparently independent work, attributes tetragonal metastability solely to the presence of the oxygen vacancies that allow both anion and cation relaxations to occur dependent on their vacancy proximity [4,7,8]. Overall, three mechanisms are discussed for stabilization of t-ZrO2 with the most common dopants being Y2O3 and CeO2: (i) dopants inducing oxygen vacancies that are generally trivalent (e.g. Gd3+, Fe3+, Ga3+, and Y3+); (ii) tetrava￾lent dopants being undersized or oversized with respect to the Table 1 – Ceramic toughening mechanisms [2] Mechanism Highest toughness (MPam1/2) Example materials Transformation ∼=20 ZrO2 (MgO) HfO2 Microcracking ∼=10 Al2O3/ZrO2 Si3N4/SiC SiC/TiB2 Metal dispersion ∼=25 Al2O3/Al Al2O3/Ni WC/Co Whiskers/platelets ∼=15 Si3N4/SiC Si3N4/Si3N4 Al2O3/SiC Fibers ≥30 CASa/SiC LASb/SiC Al2O3/SiC SiC/SiC SiC/C Al2O3/Al2O3 a Calcium aluminum silicate glass ceramic. b Lithium aluminum silicate glass ceramic.