A review is presented of experimental observations of the mechanism and crystallography of the martensitic tetragonal to monoclinic transformation occurring both in dispersed tetragonal ZrO2 particles in partially stabilized zirconia and in polycrystalline tetragonal zirconia. Preliinary results of determination of the orientation relationship and habit plane for the stress-activated transformation in a CeO2 –stabilized TZP ceramic are reported and compared with predictions of the crystallographic theory for the transformation. This orientation relationship is such that (100)m//(100)t and [001]m//[001]m, and for this variant of the orientation relationship the habit plane is approximately ( 301 )t. These results are in good agreement with theoretical predictions. Progress in the application of the formal theory of martensitic transformations to the transformations in both types of system is examined critically and implications for theories of transformation toughening are discussed.

Attention is also given to tetragonal orthorhombic and orthorhombic monoclinic transformations occurring in Zro2 particles in thin foil specimens of partially stabilized zirconia. Formation of a metastable orthorhombic phase appears a possible, but not essential, intermediate stage in the tetragonal to monoclinic transition. However, present evidence strongly suggests that the orthorhombic structure only occurs in those particles experiencing the relaxed matrix constraints typical of thin foil specimens.

Tetragonal to rhombohedral phase transformation was studied by X-ray diffraction technique on the ground surfaces of tetragonal zirconia polycrystals (Y-TZP) and partially stabilized zirconia (Y-PSZ) with 2.0 to 5.0 mol% Y2 O3 contents prepared by hot isostatic pressing. The rhombohedral phase increased with increase of Y2 O3 content from 2.0 to 5.0 mol%, and also with the increase of HIPing temperature from 1400 to 1600°C. The stability of the phase was also studied with regard to the surface finish and annealing. The subsequent heat treatment of the samples was found to promote the reverse rhombohedral to tetragonal transformation.

The phase composition of Y-TZP surfaces has been shown to vary greatly depending on the thermo-mechanical history of the surface. The orientation of these different phases in the surface region is not always random. There is speculation that the alignment of the tetragonal phase before fracturing may play a part in increasing the toughness of these materials. This article deals with an X-ray diffraction analysis of various Y-TZP surfaces with special emphasis on the texture of the different phases. Surfaces which have been ground (and polished), fractured, and aged (200°C) have been examined. In all cases, the monoclinic component that was formed was strongly oriented. The tetragonal phase may or may not be oriented depending on surface treatment. Annealing above the monoclinic-tetragonal transition temperature had little effect on the tetragonal orientation in most cases. Samples fractured at 1000°C have no unusual orientation on the fracture faces.

The theory of internal variables is used in order to develop multiaxial constitutive laws for ceramics undergoing martensitic stress-assisted transformation, such as partially stabilized zirconia or A12O3-ZrO2. The internal variable is identified with the volume concentration of transformed particles, and we assume that transformation occurs so that the change in potential energy due to the transformation is maximized. When the rate of transformation depends on the applied stresses only through the corresponding change in potential energy, it is shown that the inelastic strain rates are along the normal of a stress function in stress space. The constitutive law depends on all three stress invariants. We further discuss specific stress environments such as crack tip fields, the special case of homogeneous transforming particle distribution, and conditions under which normality is not obeyed.

The stress induced transformation in Y2 O3 containing tetra gonal zirconia polycrystals (Y-TZP) by a three-point bending test was studied by Raman microprobe. Transformation zone extends to as large as 100μm in the starting side of the fracture and the transformation zone size becomes smaller in the direction from the starting to the ending side of the fracture. The transformation zone size has no correlation with fracture toughness and Y2O3 content. It is suggested that the transformation plastici y may operate in the initial stage of the fracture. The obtained results have remarkable contrast with the case of the fracture of a pre-indented specimen. The transformation behavior of defect-dominating fracture is also discussed.

Previously, the pressure, temperature and strain rate sensitivities of transformation plasticity have been investigated for monotonic loading of Mg- PSZ. Research in this area has been extended to fully reversed cyclic loading of the type used in plastic strain control fatigue. Cyclic deformation experiments were performed to permit investigation of constitutive behavior under stable deformation conditions at microstrain levels. It was found that cyclic microstrains over a range of temperatures and strain rates were associated with reversible transformation plasticity in the strongly thermally-activated regime. These results are compared to the constitutive relations of transformation plasticity which have been previously developed to explain macrostrain observations.

Recent experimental work on the yield and flow behavior of both single crystal and polycrystalline transformation toughened zirconia is presented. In addition, related work by other researchers is reviewed. The resulting picture is used to assess the relative plastic deformation contributions of phase transformations, dislocation activity, and grain boundary sliding. Particular emphasis is placed on the effects of stabilizer chemistry, grain size, temperature, strain rate, and state of stress. The results are shown to reflect the strong role of shear stresses in selecting, and controlling the operation of, each deformation mode.

The constitutive behavior of transformation plasticity in single phase fine grain Ce-TZP and polyphase coarse grain Mg-PSZ was studied. A material independent pressure sensitivity and a microstructure dependent strain rate sensitivity and temperature sensitivity were found to be associated with the transformation yield stress. Autocatalysis of various extent operates in all cases, forming broad macroscopic shear bands in TZP and fine crystallographic slip bands which terminate at grain boundaries in PSZ. Transformation plasticity is apparently athermal near the Mb temperature in TZP, but not in PSZ except in brittle fracture. A strong crystallographic texture of the transformed phase develops during deformation. These results are analyzed in terms of a shear-dilatant yield criterion and a rate equation which account for stress assistance in martensitic transformation. An elastic-plastic fracture mechanical model is developed to estimate both the shear and dilatation contributions to the transformation plastic work in the crack tip plastic zone. On the basis of approximately equal shear and dilatation contributions to transformation plasticity, the model predicts a transformation zone height which is four times that of the previous model, and a toughness increment which is two times that of the previous model. These predictions are found in good agreement with the reported toughness-zone size relationship. The effect of temperature and chemical stability on toughness is also rationalized.

The flexural strength and the single edge notch beam fracture toughness of undoped ZrO2 crystals, grown by the skull melting technique, were examined from room temperature to 1400°C. On heating the toughness increased with test temperature to a maximum of 4.0 MPajm at 1225°C then gradually decreased to 2.6 MPa/m. Upon cooling after a 20 minute hold at 1250°C an increase in toughness to 5 MPa/m was observed at 1200°C; upon cooling to lower temperatures Kic gradually diminished. The loaddeflection curves for the flexural strength tests showed marked nonlinearity before failure for samples tested on cooling. The temperature dependence of the apparent yield stress suggests that initial yielding occurs by slip above 1200°C but that from 1200°C to 1050°C the observed yielding is due to stress induced tetragonal to monoclinic transformation.

The influence of temperature between R.T. and liquid nitrogen (−196°C) on the strength and toughness of three grades of Mg-PSZ and a 3 mol% Y-TZP material have been investigated. The toughness of the Mg-PSZ materials passes through a maximum at the Ms temperature, whereas the strength increases monotonically with decreasing temperature regardless of Ms. The Y-TZP material exhibits increasing toughness with decreasing temperature but the strength passes through a maximum at -80°C. The results are discussed in terms of transformation and R-curve limited strength of transformation toughened ceramics. For Mg-PSZ materials it is suggested that the critical stress to initiate a non reversible transformation on the tensile surface is responsible for microcracking and ensueing R-curve development.

Certain coarse-grained Mg-PSZ's exhibit marked R-curve behavior, e.g., the crack resistance increases from ∼5 to ∼15 MPam1/2 with the first 500 microns of crack extension. These high toughness materials also show unusually-good flaw tolerance.

Crack propagation and crack path development in Mg-PSZ were investigated during stable crack growth using a modified DCB specimen geometry. Crack extension was monitored with a traveling microscope equipped with a video recording system. PSZ's with various heat-treatments were tested to determine the effect of microstructure on R-curve behavior.

Cracks were found to propagate discontinuously and to arrest. Smaller microcracks would develop around the arrested crack, and link together to form the macrocrack which led to eventual failure.

There are various types of microstructure in the TZP materials and their composite materials. The fine-grained microstructure is well-known as the basic microstructural type of TZP materials. Y-TZP and Ce-TZP belong to this group. The large-grained microstructure can exist in the TZP consisting of the tetragonal phase with low metastability, that is, Y-TZP doped with TiO2 and Ce-TZP containing a high CeO2 content. The composite material between Y-TZP and β-lanthanum alumina possessed a unique microstructure including elongated grains of β-Al2O3 type structure. This study provides a summary of the mechanical properties of these TZP and β-Al2O3 type structure composite materials, and points out how the mechanical behavior depends on the microstructural features.

Transformation toughened zirconia (TTZ) is a material being considered for use in advanced heat engines. However, at elevated temperatures TTZ materials undergo a phase transformation fron tetragonal to the monoclinic with an associated volume increase of approximately 5%.This transformation results in a loss of strength and fracture toughness. Six commercially available Japanese TTZ materials and one experimental domestic grade were examined for the extent and effect of this phasetransformation after exposure to elevated temperatures (1000 to 1200°C) for times of 100 and 500 hours. Tests completed to date show that all the TTZ materials examined transform and lose strength, but to various degrees. Strength losses after heat treatment at 1000°C for 100 and 500 hours, ranged from a high of 60% to as little as 7%. Additional heattreatments of 500 hours at 1100 and 1200°C were carried out on TTZ's whichhad strength losses of 15% or less after exposure to 1000°C.

Although the major thrust of this program is to examine the effects of high temperatures on TTZ materials, a preliminary examination of the effects of low temperatures on the properties is also being done. A small number of specimens are undergoing treatments at 200°C and ∼.8 MPa water vapor pressure for 50 hours. Early indications are that strength greatly reduced after this treatment.

Yttria-doped tetragonal zirconia polycrystals (Y-TZP) containing 2 and 3 mol% of Y2O3 uniformly alloyed by 0-15 mol% of CeO2 or diffusing CeO2 on the surface were fabricated, and the mechanical properties and thermal stability of the sintered bodies were evaluated by annealing in humidity conditions at 50–600° C. The tetragonal-to-monoclinic phase transformation proceeded at 100–500°C in air, and accompanied microcracks. The phase transformation proceeded rapidly on the surface, but slowly inside the body. The bending strength of the annealed specimens depended on the depth of the transformation layer thickness, but not on the degree of the phase transformation on the surface. Alloying CeO2 was useful to improve the thermal stability of Y-TZP, but noticeably decreased the fracture strength. Diffusing CeO2 on the suface of Y-TZP seemed to be useful to improve the thermal stability without loss of the fracture strength.

A1 2 03–15 vol. % ZrO2 bar shaped composite specimens were fabricated by pressing three layers. The two outer layers consisted of Al2O3 and unstabilized ZrO2 (primarily in the monoclinic polymorph), and the inner layer consisted of Al2O3 and partially stabilized zirconia in the tetragonal polymorph. The transformation of ZrO2 from tetragonal to monoclinic, upon cooling from sintering temperature, led to the establishment of residual compressive stresses in the outer layers. Flexural tests at room temperature showed that residual stresses contributed to strength increasing from 450 to 825 MPa. The existence of these stresses was verified by measuring apparent fracture toughness, as well as using strain gages. Strength and toughness data were obtained at 500, 750, and 1000°C. X-ray diffraction was used to explain the elevated temperature data by monitoring the monoclinic to tetragonal transformation upon heating to 1000°C.

Zirconia dispersed mullite ceramics were fabricated through hot-pressinq at a pressure of 25 MPa from the amorphous material of 2Owt%ZrO2-8Owt%(3Al2O3 2SiO2) obtained by rapid quenching of the melts. The densification was initiated at ∼950°C and accelerated by crystallization of mullite and t-ZrO2 to yield almost pore free samples about 1050°C. This sample seemed to contain Si02-rich glassy phase in addition to A12O3-rich mullite and t-ZrO2 The further heat-treatments with/without pressing brought about the reaction between the Si02-rich glassy phase and the A1203-rich mullite phase to yield nearly stoichiometric mullite. and growth of the mullite grains and the t-ZrO2 particles up to 1 u m in size at 1600°C. The TEM observation revealed a duplex microstructure of twinned m-ZrO2 at the triple points and fine t-ZrO2 in the center of the mullite grains. The fracture toughness of the specimen increased with hot-pressing temperatures up to 2.2 MPam1/2 in the samples at 1600°C.

Results of recent micromechanics analyses of the reinforcing influence of frictionally bonded fibers in ceramic composites are summnarized. Direct measurements of the fiber/matrix interface properties are also discussed.

When considering the strength of a fibre reinforced ceramic composite, it is often assumed that the fibres retain their full strength of several GPa after cracking of the weaker matrix. The strength of the composite after matrix cracking is then calculated by the rule of mixtures as the product of fibre volume fraction and fibre strength. This paper demonstrates that such a calculation is not consistent with the principles of fracture mechanics for an isolated fibre embedded in an elastic matrix of the same elastic modulus, because the strength of the fibre is much reduced by the stress concentration arising from the matrix crack. Experimental measurements of the strength of a glass fibre embedded in a brittle matrix support the theory. The case of a fibre in a matrix of different elastic modulus is also considered, together with the proDlem of cracking along the fibre-matrix interface.

The influence of toughness on the Weibull modulus of ceramics is discussed. It is shown that an increase in toughness does not give a higher Weibull modulus if the ceramic is truly brittle, following the Griffith criterion of fracture. However, R curve behaviour, such as that shown by tetragonal zirconia, by Dugdale material, or by fibrous ceramic composites, leads to an improved Weibull modulus. Theoretical argument and experimental results support these conclusions.

Indentation/strength data on a series of Ca-doped aluminas are presented, revealing considerable microstructural influence and increasing fracture resistance during crack propagation. The source of the increasing resistance is considered in terms of a restraining zone of grain-localized ligamentary bridges behind the crack tip. A simple fracture mechanics model is developed to characterize the effect of the restraining ligaments, which also allows the steady state toughness, applicable to large cracks, and the intrinsic strength, applicable to small cracks, to be predicted. The segregation of Ca to the grain boundaries is shown to correlate with decreases in grain boundary toughness and increases in toughness in the large crack limit. These latter two combined effects lead to decreases in intrinsic strength with increasing steady state toughness.