292 DENTAL MATERIALS 24(2008)289-298 dispersed t-Zro2 phase lenticular t-zro, precipitates on cubic faces a. Ziconia-toughenend alumina(ZTA b Mg partially-stabilized zirconia(Mg-PSz) Yttria-stabilized tetragonal zirconia polycrystalline(y-tZP Fig. 1- Microstructural features of the three major categories of transformation-toughened zirconia Compiled with permission from Heuer[11] and Matsui et al.22 ical properties, for example, KIc ranging from 3 to 16MPa m12 8 phase is thought to directly explain the improved mechan with lower values occurring in both un-aged and over-aged ical properties of Mg-PSZ during aging[18]. One commercial ceramic [16]. Specific insights into bulk phase developments Mg-PSZ appears to be available as a dental ceramic( Denzir-M and microstructural control over toughening in Mg-PSZ came Dentronic AB, Sweden) and has received attention during with the advent and application of neutron diffraction; X-ray vitro testing of fixed partial dentures [191 penetration in these materials is extremely limited compared to centimeter depths possible with neutrons. Aging in Mg-PSZ 3.3. Single-phase, polycrystalline t-Zro2 involves the development of a complex microstructure form- ing from essentially a two-phase c+t starting system. These In 1977 it was reported that fine grain ZrO2 (gener changes include (1)tertiary t-phase precipitation,(2)some ally <0.5um) with small concentrations of stabilizing Y203 C-m transformation,(3)limited orthorhombic (o) phase for- could contain up to 98% of the metastable t phase fol- mation, and most critically,(4)growth of an anion-ordered lowing sintering [20]. High strengths coincided with high acancy phase termed delta( a )having the composition tetragonal phase content and low strengths coincided with Mg2ZrsO12[17, 18]. This 8 compound nucleates on the broad high monoclinic phase content [20). Subsequent investigation lenticular tetragonal-cubic phase boundary and grows with revealed that the highest strengths(700 MPa)and toughness onsumption of c-ZrO2. Although the t-8-phase boundary is KIc 6-9 MPa m1)were only found below a critical average coherent, some lattice parameter mismatch exists leaving the grain size(<0.3 um)[21]. This critical grain-size phenomenon t phase increasingly susceptible to transformation as the 8 indicated a strength/toughness mechanism beyond the sim- layer thickens [18].Growth of this Mg-rich 8 phase also appears ple flaw-related grain-size effects generally recognized for to occur with Mg depletion of the t precipitates. It has been polycrystalline ceramics. However, many attributes are still calculated roughly that the stress required for the t-m trans- shared with other polycrystalline materials, including the formation decreases from 470 to 70 MPa with aging, in a linear simplicity of processing; norequirement for"aging"heattreat fashion with &-phase formation [18].Thus, the precipitation of ments, and reciprocally, relative insensitivity to follow-on heat292 dental materials 24 (2008) 289–298 Fig. 1 – Microstructural features of the three major categories of transformation-toughened zirconia. Compiled with permission from Heuer [11] and Matsui et al. [22]. ical properties, for example, KIC ranging from 3 to 16 MPam1/2 with lower values occurring in both un-aged and over-aged ceramic [16]. Specific insights into bulk phase developments and microstructural control over toughening in Mg-PSZ came with the advent and application of neutron diffraction; X-ray penetration in these materials is extremely limited compared to centimeter depths possible with neutrons. Aging in Mg-PSZ involves the development of a complex microstructure form￾ing from essentially a two-phase c + t starting system. These changes include (1) tertiary t-phase precipitation, (2) some c→m transformation, (3) limited orthorhombic (o) phase for￾mation, and most critically, (4) growth of an anion-ordered vacancy phase termed delta (ı) having the composition Mg2Zr5O12 [17,18]. This ı compound nucleates on the broad lenticular tetragonal-cubic phase boundary and grows with consumption of c-ZrO2. Although the t-ı-phase boundary is coherent, some lattice parameter mismatch exists leaving the t phase increasingly susceptible to transformation as the ı layer thickens [18]. Growth of this Mg-rich ı phase also appears to occur with Mg depletion of the t precipitates. It has been calculated roughly that the stress required for the t→m trans￾formation decreases from 470 to 70 MPa with aging, in a linear fashion with ı-phase formation [18]. Thus, the precipitation of ı phase is thought to directly explain the improved mechan￾ical properties of Mg-PSZ during aging [18]. One commercial Mg-PSZ appears to be available as a dental ceramic (Denzir-M, Dentronic AB, Sweden) and has received attention during in vitro testing of fixed partial dentures [19]. 3.3. Single-phase, polycrystalline t-ZrO2 In 1977 it was reported that fine grain ZrO2 (gener￾ally < 0.5m) with small concentrations of stabilizing Y2O3 could contain up to 98% of the metastable t phase fol￾lowing sintering [20]. High strengths coincided with high tetragonal phase content and low strengths coincided with high monoclinic phase content [20]. Subsequent investigation revealed that the highest strengths (∼=700 MPa) and toughness (KIC ∼= 6–9 MPam1/2) were only found below a critical average grain size (<0.3m) [21]. This critical grain-size phenomenon indicated a strength/toughness mechanism beyond the sim￾ple flaw-related grain-size effects generally recognized for polycrystalline ceramics. However, many attributes are still shared with other polycrystalline materials, including the simplicity of processing; no requirement for “aging” heat treat￾ments, and reciprocally, relative insensitivity to follow-on heat