DENTAL MATERIALS 24(2008)289-298 oxide cations(e.g. Ti++, Ge++, Ce4+); or, (ii) dopants resulting 3.1. Dispersion-toughened ceramics In minor, stabilization mechanism involves matrix constraint of The simplest material utilizes the dispersion of zirco- t-zrO2 grains held within non-transforming materials nia particles in another matrix and appears to be the The"absorption of energy"during the room temper- least widely published and commercially important. These ature t-m transformation in partially stabilized zirconia dispersion-toughened materials, such as Zro2-toughened alu (Cao-ZrO2; described in more detail below) was recognized mina(Al2O3)or ZrO2-toughend mullite(3Al2O3 2SiO2)have as a strengthening mechanism in 1975 [10]. In the 1975 been termed ZTA and ZTM [11]. In contrast with the other two publication reference was made to similarities between trans- classes, stability of the t phase to room temperature does not forming zirconia and austenite-martensite strengthening primarily involve the use of dopants but is controlled instead of TRIP steels(transformation-induced plasticity)[10]. TRIP by particle size, particle morphology and location (intra-or steels contain retained austenite when they have carbon con- intergranular, see Fig. 1a). In ZTA, for example, particles above centrations in excess of 1 wt % Some mechanical properties, a critical size will transform to monoclinic symmetry upon in particular toughness and ductility, rely on the diffusionless cooling to room temperature [12]. Since this t-m transfor- transformation of this austenite into high-carbon martensite mation is known to be martensitic, a useful way to describe nduced by stress and strain. Three features, in particular, particle size effects has been to examine their influence on vere seen shared with strengthened steels leading Garvie et the martensitic start(Ms) temperature; essentially all t-phase al. [10] to term these new zirconia compositions"ceramic stabilization can be viewed as decreasing the Ms to below steel":(1)three allotropes,(2)martensitic transformations, room temperature. Such investigation has suggested that the and ()metastable phases. Stabilized zirconia and steels also particle size effect is likely due to difficulties in nucleating share similarities in elastic moduli and coefficients of thermal the transformation, although considerations have also been expansion Garvie et al. further predicted that a vast range of given to the possible effects of surface and strain energy and ceramic materials with different properties would be devel- chemical free energy driving forces[12. Within dental mate oped analogously to iron systems [10] rials the sole commercial example of a dispersion-toughened ceramic is In-Ceram Zirconia (Vita Zahnfabrik) which is an interpenetrating composite of 30% glass and 70% polycrys 3. Three distinct zirconia ceramics: talline ceramic consisting of Al2O3 ZrO2 in a vol. ratio of terminology, processing and microstructures As foreseen by Garvie wide latitude was found in the applica- 3. 2. Partially stabilized zirconia tion of the zirconia t-m transformation in ceramics, leading to development of three different materials each having an These materials are the most widely studied,commer- associated terminology [10]. These three classes are listed cially important, microstructurally complex, and in the case in Table 2 along with some dental examples; the first two of Mg-doped some of the toughest of the transformation of these are at least two-phase materials with t-ZrO2 as the toughened ceramics. In these ceramics t-ZrO2 intra-granular minor phase(dispersed and precipitated respectively) and precipitates exist within a matrix of stabilized c-zrO2. Sta- the last is essentially a single-phase t-zrO2. The origin and bilization involves dopant addition, such as with Cao, Mgo, details of stabilization of the t phase differs among these three La2O3, and Y203, in concentrations lower than that required toughened microstructures; photomicrographs of representa- for full c-Zro2 stabilization. Full stabilization is purpose- tive materials are presented in Fig. 1. The three materials share fully not achieved in these materials, hence the historical that stabilization of t occurs and that toughness involves the derivation for the term "partially stabilized zirconia"or martensitic t- m transformation PSZ, to which the relevant dopant is often appended: Ca PSZ, Mg-PSZ, Y-PSZ, etc. [11]. Precipitates are fully coherent with the cubic lattice, forming on a nanometer scale with lenticular morphology(approximately 200nm diameter and Table 2-Forms of transformation-toughened zirconia 75nm thick) parallel to the three cubic axes (refer 1. Zirconia(dispersed phase)toughened ceramics; e.g, ZTA Fig. 1b)[12, 13]. Following sintering or solution annealing in (alumina), ZTM(mullite) the cubic solid solution single-phase field(approximately Dental example: >1850C), precipitates are nucleated and grown at lower In-Ceram ita Zahnfabrik) temperatures(approximately 1100C)within the two-phase 2. Partially stabilized zirconia(PSZ; e.g. Ca-PSZ, Mg-PSZ, Y-psz) tetragonal solid solution plus cubic solid solution phase fiel Lenticular (ens shaped) tetragonal precipitates in a cubic matrix a process termed"aging"[12, 13]. Aging optimization(time- Denzir-MDentror 3. Tetragonal zirconia polycrystals (TZP; e.g. Y-TZP, Ce-TZP and phase stability [14]. Metastability can be lost when tetrag Nominally 98% tetragonal, fine grain size onal precipitates are too small(they will not transform) and Dental examples. when precipitates grow too large spontaneous transformation DC Zirkon(DCS Precident, Schreuder Co can occur to m with twinning and microcracking [14] Complex decomposition and tertiary precipitation pro- Lava(3M ESPE cesses have also been reported to occur with aging of Mg-PS In-Ceram YZ(Vita Zahnfabrik) [15] along with the development of quite some range of physdental materials 24 (2008) 289–298 291 oxide cations (e.g. Ti4+, Ge4+, Ce4+); or, (iii) dopants resulting in charge-compensations (YNbO4, YTaO4) [9]. Another, more minor, stabilization mechanism involves matrix constraint of t-ZrO2 grains held within non-transforming materials. The “absorption of energy” during the room temper￾ature t→m transformation in partially stabilized zirconia (CaO–ZrO2; described in more detail below) was recognized as a strengthening mechanism in 1975 [10]. In the 1975 publication reference was made to similarities between trans￾forming zirconia and austenite–martensite strengthening of TRIP steels (transformation-induced plasticity) [10]. TRIP steels contain retained austenite when they have carbon con￾centrations in excess of 1 wt.%. Some mechanical properties, in particular toughness and ductility, rely on the diffusionless transformation of this austenite into high-carbon martensite induced by stress and strain. Three features, in particular, were seen shared with strengthened steels leading Garvie et al. [10] to term these new zirconia compositions “ceramic steel”: (1) three allotropes, (2) martensitic transformations, and (3) metastable phases. Stabilized zirconia and steels also share similarities in elastic moduli and coefficients of thermal expansion. Garvie et al. further predicted that a vast range of ceramic materials with different properties would be devel￾oped analogously to iron systems [10]. 3. Three distinct zirconia ceramics: terminology, processing and microstructures As foreseen by Garvie wide latitude was found in the applica￾tion of the zirconia t→m transformation in ceramics, leading to development of three different materials each having an associated terminology [10]. These three classes are listed in Table 2 along with some dental examples; the first two of these are at least two-phase materials with t-ZrO2 as the minor phase (dispersed and precipitated respectively) and the last is essentially a single-phase t-ZrO2. The origin and details of stabilization of the t phase differs among these three toughened microstructures; photomicrographs of representa￾tive materials are presented in Fig. 1. The three materials share that stabilization of t occurs and that toughness involves the martensitic t→m transformation. Table 2 – Forms of transformation-toughened zirconia 1. Zirconia (dispersed phase) toughened ceramics; e.g., ZTA (alumina), ZTM (mullite) • Dental example: In-Ceram zirconia (Vita Zahnfabrik) 2. Partially stabilized zirconia (PSZ; e.g. Ca-PSZ, Mg-PSZ, Y-PSZ) • Lenticular (lens shaped) tetragonal precipitates in a cubic matrix • Dental example: Denzir-M (Dentronic AB) 3. Tetragonal zirconia polycrystals (TZP; e.g. Y-TZP, Ce-TZP • Nominally 98% tetragonal, fine grain size • Dental examples: DC Zirkon (DCS Precident, Schreuder & Co) Cercon (Dentsply Prosthetics) Lava (3M ESPE) In-Ceram YZ (Vita Zahnfabrik) 3.1. Dispersion-toughened ceramics The simplest material utilizes the dispersion of zirco￾nia particles in another matrix and appears to be the least widely published and commercially important. These dispersion-toughened materials, such as ZrO2-toughened alu￾mina (Al2O3) or ZrO2-toughend mullite (3Al2O3·2SiO2) have been termed ZTA and ZTM [11]. In contrast with the other two classes, stability of the t* phase to room temperature does not primarily involve the use of dopants but is controlled instead by particle size, particle morphology and location (intra- or intergranular; see Fig. 1a). In ZTA, for example, particles above a critical size will transform to monoclinic symmetry upon cooling to room temperature [12]. Since this t→m transfor￾mation is known to be martensitic, a useful way to describe particle size effects has been to examine their influence on the martensitic start (Ms) temperature; essentially all t-phase stabilization can be viewed as decreasing the Ms to below room temperature. Such investigation has suggested that the particle size effect is likely due to difficulties in nucleating the transformation, although considerations have also been given to the possible effects of surface and strain energy and chemical free energy driving forces [12]. Within dental mate￾rials the sole commercial example of a dispersion-toughened ceramic is In-Ceram Zirconia (Vita Zahnfabrik) which is an interpenetrating composite of 30% glass and 70% polycrys￾talline ceramic consisting of Al2O3:ZrO2 in a vol.% ratio of approximately 70:30. 3.2. Partially stabilized zirconia These materials are the most widely studied, commer￾cially important, microstructurally complex, and in the case of Mg-doped some of the toughest of the transformation￾toughened ceramics. In these ceramics t-ZrO2 intra-granular precipitates exist within a matrix of stabilized c-ZrO2. Sta￾bilization involves dopant addition, such as with CaO, MgO, La2O3, and Y2O3, in concentrations lower than that required for full c-ZrO2 stabilization. Full stabilization is purpose￾fully not achieved in these materials, hence the historical derivation for the term “partially stabilized zirconia” or PSZ, to which the relevant dopant is often appended: Ca￾PSZ, Mg-PSZ, Y-PSZ, etc. [11]. Precipitates are fully coherent with the cubic lattice, forming on a nanometer scale with lenticular morphology (approximately 200 nm diameter and 75 nm thick) parallel to the three cubic axes (refer to Fig. 1b) [12,13]. Following sintering or solution annealing in the cubic solid solution single-phase field (approximately >1850 ◦C), precipitates are nucleated and grown at lower temperatures (approximately 1100 ◦C) within the two-phase tetragonal solid solution plus cubic solid solution phase field; a process termed “aging” [12,13]. Aging optimization (time￾temperature-transformation) involves both precipitate size and phase stability [14]. Metastability can be lost when tetrag￾onal precipitates are too small (they will not transform) and when precipitates grow too large spontaneous transformation can occur to m with twinning and microcracking [14]. Complex decomposition and tertiary precipitation pro￾cesses have also been reported to occur with aging of Mg-PSZ [15] along with the development of quite some range of phys-