Hot dynamic consolidation of hard ceramics

Diamond and cubic boron nitride powders were shock compacted at high temperature (873 and 973 K) by using a planar impact system at 1.2 and 2.0 km s^–1. Silicon, graphite or a mixture of titanium and carbon powders were added to enhance the bonding of these superhard materials. Hot-consolidated specimens exhibited fewer surface cracks as compared with the specimens shock consolidated at room temperature. Diamond compacts having microhardness values over 55 GPa were obtained by subjecting porous mixtures of diamond crystals (4-8 m) plus 15 wt% graphite (325 mesh) to an impact velocity of 1.2 km s^–1 at 873 K. Well-consolidated c-BN samples, with microhardnesses (starting powders with 10–20 and 40–50 (m) over 53 GPa were obtained.     

Multiple cracks in transformation toughened ceramics

The problem of a collinear array of cracks in a transformation toughened ceramic under monotonically increasing load is analyzed by the theory of complex potentials and dislocation formalism. The analysis is applicable to both a homogeneous material containing multiple cracks and a brittle matrix ceramic reinforced by distributed transformable particulates. The detrimental effect of transformation at the onset of crack growth is generally found to be amplified by crack tip interactions. For the particulate reinforced ceramic the maximum relative strengthening is achieved for moderate volume fractions of large particles.     

Hot isostatic consolidation and transformation of sigma phase powders

Abstract

Superferritic stainless steel compositions containing 38%Cr and at least 2% each of Ni and Mo can be converted to sigma phase by heat treatment and then to powder by mechanical attrition. This powdered sigma phase has been experimentally processed by powder metallurgical techniques to produce green compacts which were consolidated in a hot isostatic press. Depending on composition, several of the powdered alloys were converted back to ferrite during hipping. The mechanism of the consolidation process, the nature of the microstructures produced, and the prognosis for its industrial exploitation are examined.     

Martensitic transformation in zirconia containing ceramics and its applications

An introduction to tetragonal (t) → monoclinic (m) martensitic transformation in zirconia containing ceramics, especially tetragonal zirconia polycrystalline (TZP) was presented. Thermodynamics, crystallographics and kinetics of t → m martensitic transformation in TZP were emphasized. Transformation toughening and shape memory effect (SME) associated with t → m martensitic transformation in the TZP were reviewed. Perspective of future challenges was briefly mentioned at the end.     

Transformation criterion of phase transformation toughening ceramics with misoriented microcracks

Phase transformation criterion is the key to investigating the toughness of phase transformation ceramics. In this paper, the modified equivalent inclusion theory by the authors is employed to study the interaction between microcracking and transformation in ceramics. The transformation criterion is derived. The influence of microcracks and transformation particles on the critical transformation load is discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

A micromechanics theory for the transformation toughening of two-phase ceramics

Summary Based on the principle of thermodynamic equilibrium, the condition of stress-induced phase transformation in a two-phase ceramic is established. The development makes use of the change of potential energy that was calculated with a mean-field approach. In this process the elastic heterogeneity of the constituent phases, and the shape and volume concentration of the randomly oriented metastable ellipsoidal inclusions, are fully accounted for. Both the transformation heightH of the process zone with a steadily growing crack and the fracture toughness increment K of the transforming system are derived. The derived theory is then used to address the effect of inclusion shape and elastic inhomogeneity on the transformation toughening of two-phase ceramics. By considering the metastable ellipsoidal inclusions as phase 1 and the stable matrix as phase 0, it is found that, when ₁/₀>1, flat-like discs always provide a larger transformation-height while spherical ones provide the smallest, and vice versa. As the ratio of ₁/₀ increases, the size of the process zone also increases. For the toughness increment, the results indicate that thin-disc inclusions are again the most effective toughening medium. It is further found that Poisson's ratio of the constituent phases also has a significant effect; the combination ofv ₁0.5 for the inclusions andv ₁0 for the matrix has the best enhancement for fracture toughness. But whenv ₁, the toughness increment K all approaches to an asymptotic value regardless of the values of Poisson's ratios. Some explicit solutions of toughness change for several distinctive shapes of inclusions are also derived for the first time.     

The size effect of the martensitic transformation in ZrO2-containing ceramics

The relationship between the starting temperature of the martensitic transformation, M _s, and the grain size of the parent phase, d, in ZrO₂-containing ceramics was investigated. The experimental results showed that in tetragonal zirconia polycrystals doped with CeO₂ (8 mol%) and Y₂O₃ (0.25 mol%) (8Ce, 0.25Y-TZP), the M _s temperature displays a linear relationship with d ^–1/2, its slope being negative. A new explanation for this phenomenon, the so-called the size effect, has been presented, in which the grain size of the parent phase affects the M _s temperature through the strength of the parent phase. Thermodynamic calculation of the relationship between M _s and d gives a result consistent with the experimental ones.     

Relation of transformation temperature to the fracture toughness of transformation-toughened ceramics

The stress induced tetragonal to monoclinic ZrO₂ martensitic transformation contribution to fracture toughness is described in terms of the required external strain energy and the thermo-dynamic stability of the constrained tetragonal phase. The strain energy, derived from an externally applied stress acting on the main crack, required to achieve transformation toughening is shown to be a function of the term (T - M _s) whereT is the test temperature andM _s is the martensite start temperature for the case ofT > M _s. Thus for a givenT (T > M _s), the transformation toughening component increases asM _s approachesT and for a fixedM _s, the toughness decreases asT increases. Experimental data for partially stabilized zirconia ceramics confirm these results and show that increasing tetragonal precipitate size is the primary feature which affects an increase inM _s. In the case ofT M _s, autotransformation occurs, resulting in decreasing toughness with decrease inT due to a continuous loss in the tetragonal phase content. A temperature region is thus obtained over which transformation toughening exhibits a maximum in its contribution. The temperatures over which this occurs then is shown to be dependent on theM _s temperature of the material.     

Experimental investigation of the transformation zone of tetragonal zirconia polycrystalline ceramics

Stress-induced martensitic transformation plastic zones of ceria-stabilized tetragonal zirconia polycrystalline ceramics, under some typical loading conditions, were studied by Moiré interferometry. The full-field fringe patterns, including u-fields and v-fields, were acquired. According to these patterns, the transformation zone shape and transformation plasticity distributions of the specimens were obtained. The experimental results show that the stress-induced transformation at room temperature is concentrated in some narrow bands and the transformation plasticity is not uniform within the transformation zone. Experiments also reveal that the transformation zone with a characteristic elongated shape ahead of the notch, in a single-edge notch bending specimen, is obviously different from that resulting from some constitutive relation of transformation. This work provides a significant experimental foundation for establishing the theoretical models of transformation toughening.     

The stiffness and strength of transformation toughening ceramics with misoriented microcracks

Theoretical studies on misoriented transformation particles and microcracks in transformation toughening ceramics are presented using the Eshelby equivalent inclusion method. The stress field, stiffness and strength were calculated. Experiments were done by the three-point bend method using Al₂O₃/ZrO₂ ceramics and the stiffness and strength were also measured. Comparison between theoretical and test results confirmed the important role of microcracks.     

The effect of anelasticity and phase transformation on crack growth in Y-TZP ceramics

Crack growth behavior has been investigated under monotonic and cyclic loadings for Y-TZP that produces remarkable anelastic strain. Monotonic loading testing was carried out under the condition of various stress rates (8 × 10², 8 × 10^–1 and 8 × 10^–4 MPa/s) and temperatures (RT and 373 K). Resistance of crack propagation was observed at the lowest stress rate at elevated temperature. Cyclic fatigue crack growth rate was examined under the condition of different frequencies and stress waveforms. Crack growth rate clearly depended on stress waveform, which was explicable by exhaustion and restoration of anelasticity at the crack tip region. Experimental results make it clear that anelasticity works as strong resistance against crack growth. In this study, the effect of environment-induced tetragonal to monoclinic phase transformation on fracture strength was also investigated for pre-cracked sample. Aged (transformed) samples have shown extreme crack closure and considerable improvement in strength.     

Role of entropy of fusion in phase transformation and self-diffusion

The present paper discusses the role of the entropy of fusion vis-à-vis the phase transformation characteristics and the self-diffusion behavior of crystalline matrices. The data correlating the entropy of fusion with these properties in metals, alkali halides and some other inorganic compounds are presented and analyzed. It is shown that the occurrence of a solid-solid state phase transformation decreases the magnitude of the entropy of fusion. In addition, the self-diffusion rates within any class of solids scale inversely with the entropy of fusion. The functional relationship of the entropy of fusion vis-a-vis the compressibility and the volume expansion coefficient is also discussed. The conclusion is that the entropy of fusion is not just a physical parameter describing the energy changes associated with the melting. It is, in fact, related in a substantial manner to the bulk properties of the solid and controls the phase transition characteristics and self-diffusion behavior within any group or class of solids in a uniform and consistent manner.     

Role of metal additives in mechanochemical phase transformation of zirconia

Reasons for a phase transition in zirconia subjected to intensive mechanical treatment in planetary mill have been considered. If steel balls and vials are used for milling the comminution is accompanied by oxidation of wear metal particles and successive mechanochemical reaction with ZrO₂. Aluminum has been shown to behave similarly and, being deliberately added to zirconia powder, to form solid solutions. Foreign metal cations introduced into the lattice stabilize a modification with higher crystal symmetry thus increasing the threshold size above which the monoclinic modification is stable. An increase of surface energy contribution to the Gibbs’ energy of zirconia plays an important role in phase transformation at the initial stages of mechanical treatment, while henceforth thermodynamic stability is more and more determined by the stabilizing effect of the impurity cations. Under the conditions that rule out contamination of ZrO₂ with wear material or other metal additives, dynamic equilibrium sets in between the direct transition to the tetragonal phase and the reverse transition to the monoclinic form.     

Effects of composition on the phase transformation behavior of anatase TiO2 in photocatalytic ceramics

The present work focuses on the phase transformation mechanism of TiO₂ nanoparticles in photocatalytic ceramics. TiO₂ nanoparticles were mixed with fused quartz, glaze No.1, and glaze No.2 respectively and heat-treated. The phase transformation behavior of anatase TiO₂ was analyzed by XRD. The results show that the phase transformation behavior of anatase TiO₂ in photocatalytic ceramics is highly dependent on the glaze compositions rather than the crystal size effect, which is significantly different with TiO₂ alone. The phase transformation from anatase to rutile always starts near the softening temperature and finishes below the sphere temperature of the glaze. The higher flux contents in glaze the lower phase transformation temperature. The significant retarding effects of silica and phosphate on the anatase-to-rutile phase transformation appear negligible when modified TiO₂ are applied in glaze. The eutectic liquid is essential for the phase transformation behavior of anatase TiO₂ in photocatalytic ceramics.