DENTAL MATERIALS 24(2008)289-298 293 treatments (e.g. porcelain firings in dental usage)and the much broader grain size range, approximately 8-0.25 um[26] opportunity to explore chemistry-based powder fabrication Both grain size and the test temperature in relationship to the and nano-scale microstructures Ms temperature control the size of the transformation zone (t These materials consist primarily of equiaxed grains of or h)associated with a growing crack(to be discussed momen t-zro2 sintered (and sometimes hot isostatically pressed or tarily); with rr directly influencing toughness mechanisms HIP'ed)to 96-99.5% of theoretical density. It has recently For 2 mol% Y-TZP, AK was clearly shown to decrease as tem- been reported that some grain-boundary segregation of Y3+ peratures moved away(higher from the Ms temperature[26] ions occurs during late-stage sintering[22]. It is energetically Transformation zone size of a crack extended at two different favorable for the cubic phase to form at yttria-rich triple junc- temperatures indicated an rr of slum at 295K and =10 um ons and grain boundaries at temperatures between 1300 and at 80K(with Ms presumed to be below 80K)(26] 1500 C[22]. The implications for hydrolytic stability(low tem- Nano-scale transformation-toughened Zro2 is alread erature degradation) by having minor volume fractions of appearing in the literature and in limited commercializa Zro2 segregated at grain boundaries along with the depletion tion. It was reported in 2002 that the trend toward increased of Y3+ from t-Zro2 will be discussed later. phase stability with decreasing particle-size of t-ZrO2 could Properties of these essentially homogeneous ceramics are be overcome by adjusting the yttria dopant concentration[23] primarily a function of grain size, in that grain size controls the Whereas 3 mol% Y2O3 has been found to optimize toughness Ms temperature and the ease of transformation(and hence the in micrometer and sub-micrometer t-ZrO2, dopant concen- toughness effect). The closer the test or service temperature trations and critical grain size for nano-scale material were is to the Ms temperature(but still above it avoiding sponta- identified as 1.0 mol% for 90nm and 1.5 mol% for 110 nm; neous transformation) the less stable are the t grains and the both combinations reaching around 16 MPa m[23. As with higher the available toughness increment. For a given dopant micro-scale zirconias, strong grain size effects(decreasing concentration the toughness increment decreases as grains toughness with decreasing grain size) were exhibited in these become much smaller than the critical size, presumably due to nano-scale ceramics as well[23]. At least one commercial over-stabilization"of the grains, eliminating the t-m trans- nano-scale Ce-TZP containing 20%Al2 O3(Nanozir, Matsushita formation upon introduction of a crack[23] Electrical Works, Ltd. is being examined as a dental ceramic This grain size effect itself is controlled by the type [28 having a reported fracture toughness of approximately dopant and the dopant concentration that are essentially 20 MPa m12(E. Rothbrust, Ivoclar Vivadent, Inc, personal com- determining(1)the degree of tetragonality (i.e. crystal lattice munication). It may be that nano-scale t-zro2 will primarily length ratio of c/a being>1.0)and(2) the thermal expan- appear in a polycrystalline form due to the difficulties for sion anisotropy (c versus a directions) of the unit cells. The intra-granular precipitation and tertiary phase development compositional effect of the dopant can be represented by required in PSZ. materials the resultant unit cell tetragonality (c/a[24, 25]. In general higher tetragonality contributes to a less stable material char- cterized by an increased Ms temperature [24]. Based on4. Mechanism(s)and consequences of tragonality, at similar grain size and dopant concentra- transformation tions yttrium appears to be a stronger stabilizer than cerium and especially titanium [25]. Anisotropic thermal expansie Numerous factors are discussed in the literature as (1)nucle- for the c and a axes, can influence residual strains in t ating and driving the transformation and (2)controlling grains; higher residual stresses can lower the nucleation stress the consequences of transformation. Two main phenomena threshold for the t-m in the presence of crack-tip strain resulting from transformation include (1)increased resistance energy[26]. Linear thermal expansion coefficients()are avail- for growth of both short(<100 um)and long(>0.3mm)cracks obtained by direct measure oflattice parameters from 800 to with crack length (termed R-curve behavior) until generally room temperature. These measures indicate that anisotropy reaching a toughness plateau. These transforming is minimal (aa and ac crossover or, equivalently, aa/ac =1) at step away from the simple Griffith dependence on flaw size 4mol%Y2O3[27] and many have strengths that depend on the stress needed In its most basic form, a transformation-toughening con to trigger transformation rather than being flaw-size ser tribution(ak)has been defined as[26 tive. Quite non-linear behavior is exhibited by the toughest materials bordering on quasi-plasticity with measurable pre (2) failure deformation. Therefore, as will be discussed below, high strength and high toughness do not present in the same where ko is the fracture resistance inherent to the matrix with- material out transformation toughening. In general, the toughness con Driving forces and the role of temperature particularly for tribution from transformation(15 MPam1/2)exceeds that for the t-m transformation can be simply considered within microcracking (2-6 MPam 2) or deflection(-2-4MPamv2) a thermodynamic framework, as reproduced here following the work of Becher and Swain [26]. The total unit volume At a given temperat ransformation-toughening free energy change AFo for the transformation, including an ontribution(△K)for2 TZP decreased from approx applied stress is ain size decreased from 0.5 um; for 12 mol% Ce-TzP this same range occurred △Fo=△FcH+△Ue+△Us-△U1dental materials 24 (2008) 289–298 293 treatments (e.g. porcelain firings in dental usage) and the opportunity to explore chemistry-based powder fabrication and nano-scale microstructures. These materials consist primarily of equiaxed grains of t-ZrO2 sintered (and sometimes hot isostatically pressed or HIP’ed) to 96–99.5% of theoretical density. It has recently been reported that some grain-boundary segregation of Y3+ ions occurs during late-stage sintering [22]. It is energetically favorable for the cubic phase to form at yttria-rich triple junc￾tions and grain boundaries at temperatures between 1300 and 1500 ◦C [22]. The implications for hydrolytic stability (low tem￾perature degradation) by having minor volume fractions of c-ZrO2 segregated at grain boundaries along with the depletion of Y3+ from t-ZrO2 will be discussed later. Properties of these essentially homogeneous ceramics are primarily a function of grain size, in that grain size controls the Ms temperature and the ease of transformation (and hence the toughness effect). The closer the test or service temperature is to the Ms temperature (but still above it avoiding sponta￾neous transformation) the less stable are the t grains and the higher the available toughness increment. For a given dopant concentration the toughness increment decreases as grains become much smaller than the critical size, presumably due to “over-stabilization” of the grains, eliminating the t→m trans￾formation upon introduction of a crack [23]. This grain size effect itself is controlled by the type of dopant and the dopant concentration that are essentially determining (1) the degree of tetragonality (i.e. crystal lattice length ratio of c/a being > 1.0) and (2) the thermal expan￾sion anisotropy (c versus a directions) of the unit cells. The compositional effect of the dopant can be represented by the resultant unit cell tetragonality (c/a) [24,25]. In general higher tetragonality contributes to a less stable material char￾acterized by an increased Ms temperature [24]. Based on tetragonality, at similar grain size and dopant concentra￾tions yttrium appears to be a stronger stabilizer than cerium and especially titanium [25]. Anisotropic thermal expansion, for the c and a axes, can influence residual strains in t grains; higher residual stresses can lower the nucleation stress threshold for the t→m in the presence of crack-tip strain energy [26]. Linear thermal expansion coefficients (˛) are avail￾able for yttria-doped ZrO2 over a limited concentration range, obtained by direct measure of lattice parameters from 800 ◦C to room temperature. These measures indicate that anisotropy is minimal (˛a and ˛c crossover or, equivalently, ˛a/˛c ∼= 1) at 4.5mol% Y2O3 [27]. In its most basic form, a transformation-toughening con￾tribution (KT) has been defined as [26]: Kc = Ko + KT (2) where Ko is the fracture resistance inherent to the matrix with￾out transformation toughening. In general, the toughness con￾tribution from transformation (≈15 MPa m1/2) exceeds that for microcracking (≈2–6 MPam1/2) or deflection (≈2–4 MPam1/2) mechanisms [9]. At a given temperature, the transformation-toughening contribution (KT) for 2mol% Y-TZP decreased from approx￾imately 12–2.5 MPam1/2 as grain size decreased from 2 to 0.5m; for 12mol% Ce-TZP this same range occurred over a much broader grain size range, approximately 8–0.25m [26]. Both grain size and the test temperature in relationship to the Ms temperature control the size of the transformation zone (rT or h) associated with a growing crack (to be discussed momen￾tarily); with rT directly influencing toughness mechanisms. For 2mol% Y-TZP, KT was clearly shown to decrease as tem￾peratures moved away (higher) from the Ms temperature [26]. Transformation zone size of a crack extended at two different temperatures indicated an rT of ≤1m at 295 ◦K and ∼=10m at 80 ◦K (with Ms presumed to be below 80 ◦K) [26]. Nano-scale transformation-toughened ZrO2 is already appearing in the literature and in limited commercializa￾tion. It was reported in 2002 that the trend toward increased phase stability with decreasing particle-size of t-ZrO2 could be overcome by adjusting the yttria dopant concentration [23]. Whereas 3mol% Y2O3 has been found to optimize toughness in micrometer and sub-micrometer t-ZrO2, dopant concen￾trations and critical grain size for nano-scale material were identified as 1.0mol% for 90 nm and 1.5mol% for 110 nm; both combinations reaching around 16 MPam1/2 [23]. As with micro-scale zirconias, strong grain size effects (decreasing toughness with decreasing grain size) were exhibited in these nano-scale ceramics as well [23]. At least one commercial nano-scale Ce-TZP containing 20% Al2O3 (Nanozir, Matsushita Electrical Works, Ltd.) is being examined as a dental ceramic [28]; having a reported fracture toughness of approximately 20 MPam1/2 (F. Rothbrust, Ivoclar Vivadent, Inc., personal com￾munication). It may be that nano-scale t-ZrO2 will primarily appear in a polycrystalline form due to the difficulties for intra-granular precipitation and tertiary phase development required in PSZ materials. 4. Mechanism(s) and consequences of transformation Numerous factors are discussed in the literature as (1) nucle￾ating and driving the transformation and (2) controlling the consequences of transformation. Two main phenomena resulting from transformation include (1) increased resistance for growth of both short (≤100m) and long (≥0.3mm) cracks and, for many ceramics, (2) toughness continuing to increase with crack length (termed R-curve behavior) until generally reaching a toughness plateau. These transforming ceramics step away from the simple Griffith dependence on flaw size and many have strengths that depend on the stress needed to trigger transformation rather than being flaw-size sensi￾tive. Quite non-linear behavior is exhibited by the toughest materials bordering on quasi-plasticity with measurable pre￾failure deformation. Therefore, as will be discussed below, high strength and high toughness do not present in the same material. Driving forces and the role of temperature particularly for the t→m transformation can be simply considered within a thermodynamic framework, as reproduced here following the work of Becher and Swain [26]. The total unit volume free energy change FO for the transformation, including an applied stress is: FO = FCH + Ue + US − UI (3)