Ba0.5+x/2Zr2P3−xSixO12 or BaZPS compounds were synthesized by the sintering of powders formed by a solid-state reaction. The cell parameters of Ba0.5Zr2P3O12 and Ba0.5875Zr2P2.825Si0.175O12 were determined from X-ray diffraction (XRD) data based on the (#148) space group with hexagonal setting. The cell parameters were found to increase with increasing Si content in BaZPS.

In situ observations of crack propagation in an applied moment double cantilever beam specimen were used previously to obtain the R-curve behavior of ceramic composites. To predict the R-curve using constitutive models, knowledge of the crack profile is required to derive the bridging stress distribution along the crack length and to analyze the toughening effect. To predict the crack profile in an applied moment double cantilever beam test, both the deformation of the crack surface due to the bending moment and the movement of the crack surface due to the rigid body motion of the loading fixture need to be considered. The analytical solution for the crack profile is derived in the present study. The predicted crack profiles agree well with experimental measurements.

A simple melt-infiltration processing route has been developed for the fabrication of TiC/Ni3Al ceramic/intermetallic composites, which involves a combination of infiltration and subsequent liquid phase sintering. For Ni3Al contents from 8 to 25 vol. %, densities in excess of 98% of theoretical are readily obtained when processing at 1450 °C. TiC and Ni3Al are the only phases detected in the densified materials. Ni3Al ductility is retained after processing, leading to the possibility of ductile phase toughened TiC composites for elevated temperature applications (up to ∼1100 °C).

The performance of reinforced ceramics, particularly the toughness and creep resistance, is often determined by the nature of the interface between the reinforcement and the ceramic matrix. Specially-designed experiments to investigate the role of the interfacial characteristics on toughening mechanisms and crack propagation in reinforced (silicon carbide whisker reinforced alumina) and self-reinforced (silicon nitride) ceramic composites will be described. In the whisker-reinforced composites, the interfacial topography and chemistry were of primary importance, whereas in the silicon nitride materials the formation of interfacial phases and glassy-phase chemistry influenced the interfacial debonding process. The composite interfaces were characterized by high resolution electron microscopy and high spatial resolution microchemical analysis, including energy-dispersive X-ray and electron energy loss spectroscopy. Results from energy-filtered images from ceramic interfaces will also be shown.

Intergranular glass phases can have a significant influence on the fracture resistance (R-curve behavior) of silicon nitride ceramics and appears to be related to the debonding of the β-Si3N4/oxynitride-glass interfaces. Applying the results from β-Si3N4-whisker/oxynitride-glass model systems, self-reinforced silicon nitrides with different sintering additive ratios were investigated. Silicon nitrides sintered with a lower Al2O3:Y2O3 additive ratio exhibited higher steady-state fracture toughness together with a steeply-rising R-curve. Analytical electron microscopy studies suggested that the different fracture behavior is related to the Al content in the SiAlON growth band on the elongated grains, which could result in differences in interfacial bonding structures between the grains and the intergranular glass.

Rising fracture resistance with crack extension (R-curve response) can lead to improvements in the mechanical reliability of ceramics. To understand how microstructures influence the R-curve behavior, direct observations of crack interactions with microstructural features were conducted on SiC whisker-reinforced alumina. The contribution of the dominant toughening mechanisms to the R-curve behavior of these composites is discussed using experimental and theoretical studies.

Neutron powder diffraction techniques have been used to characterize the pseudo-macro (PM) residual stresses in ZrO2(CeO2)/Al2O3 ceramic composites as a function of ZrO2(CeO2) volume fraction and fabrication procedures. The diffraction data were analyzed using the Rietveld structure refinement technique. From the refinement, we found that the CeO2 stabilized tetragonal ZrO2 particles were in tension and the Al2O3 matrix was in compression. Different sintering time had little impact on the PM stresses. On the other hand, the magnitude of the PM stresses in both ZrO2 and Al2O3 decreased linearly with the increase of their volume fractions.

The introduction of elongated silicon nitride grains during densification in the presence of a liquid phase can impart considerable improvement to the fracture toughness. This toughening is not universally attained but depends on the activation of intergranular rather than transgranular fracture. This is reminiscent of the requirement of interfacial debonding in whiskerreinforced ceramics. In fact, additional observations such as bridging in the crack wake by elongated grains and pullout of some of these grains further suggest that the crack wake mechanisms that contribute to the toughening of whisker-reinforced ceramics can also operate in silicon nitrides containing elongated grains. Various investigators have found that, consistent with crack wake mechanisms, the fracture toughness of silicon nitrides increases with increase in the diameter of the larger elongated grains. However, little is known about the effects of the grain boundary phase(s) and their properties on the interfacial debonding/intergranular fracture in such silicon nitrides. This is critical as observations show that crack propagation in some systems exhibiting larger elongated grains occurs transgranularly and no toughening occurs.

Knowledge about the nature of interfaces between SiC whiskers and alumina matrices is critical to understanding the mechanical behavior of these types of composites. The fracture toughness, in particular, is strongly influenced by the bond characteristics of this interface. Previous results have shown that oxidizing the whiskers in air prior to composite fabrication by vacuum hot-pressing results in composites which have lower toughness compared to those made with whiskers in the as-received condition. This investigation focusses on the effects of oxidizing and reducing surface treatments on the whiskers themselves and on the whisker-matrix interface. High resolution electron microscopy (HREM) was used to characterize the morphology and the amount of amorphous phase at the whisker-matrix interface, as well as the crystallography and topography of the interface. The present results indicate that the characteristics of the whisker/matrix interface may not be directly related to the initial whisker surface chemistry. Surprisingly, the thickest thermally-grown oxide layer resulted in the thinnest amorphous film at the SiC/Al2O3 interface.

Intermetallic alloys with the nominal compositions Al–24.5Nb, Al–25Nb, and Al–25.5Nb (at.%) were fabricated by hot-pressing of pre-alloyed powders in graphite dies in vacuum. The hot-pressed disks contained substantial porosity even at processing temperatures of 0.95 Tm, where Tm is the absolute melting point. In addition to some copper and silicon contamination, significant concentrations of oxygen and carbon were measured in Al–25.5Nb. Alpha alumina precipitates as well as niobium-enriched precipitates were identified. The fracture toughness of pre-cracked bend specimens with the nominal composition Al–25.5Nb was found to be 2.5 ± 0.5 MPa m1/2. Possibilities for improving this value are discussed.

The addition of SiC whiskers to Al2O3 causes significant improvement in mechanical properties, including fracture toughness, thermal shock resistance, and creep resistance. The creep response of a whisker-reinforced alumina composite has been measured using four-point flexural loading at temperatures of 1200 and 1300C. Composites were fabricated by hot-pressing a blend of alumina powder with 33 volume percent SiC whiskers. The creep data showed a stress-dependent stress exponent equal to 1 at low stress levels and ranging from 4–6 at higher stresses. The applied stress at which the transition occurred was temperature dependent and ranged from 50–125 MPa. Mechanisms of creep deformation were determined from TEM observations of specimens prepared from interrupted creep tests. Voids were observed at grain boundary-interface junctions in tensile regions and whiskers within the composite were sometimes oxidized where voids had formed. TEM observations from specific stages of steady state creep reached under different applied loads are presented, and the relative contributions of different deformation mechanisms are discussed.

Whisker reinforced ceramics offer the potential for increased fracture strength and toughness [2]. However, residual strain due to the thermal expansion mismatch between Al2O3 and SiC may affect mechanical properties of such composites. Crack tip interaction with the whisker/matrix may lead to changes in debonding behavior or influence other toughening mechanisms. The strain field in the Al2O3 matrix surrounding SiC whiskers was analyzed with a High Voltage Transmission Electron Microscope (HVEM). Strain contrast oscillations indicating the presence of residual stress in the specimen were observed in a Al2O3-5 vol % SiC composite having ≃15 μ grain size.The strain field was found to have both radial (perpendicular to whisker axis) and axial (parallel to whisker axis) components. A strain field was also present near the end faces of SiC whiskers. In situ thermal annealing to 573, 873, and 1173 K showed a decrease in the residual strain while in situ cooling to ≃77 K revealed little change in the strain. These results show that residual stresses in the compacts result from differences in thermal expansion and elastic constants of the matrix and whisker materials. Dynamic in situ fracture experiments performed in an HVEM on the Al2O3-5 vol % SiC having ≃1 μm as well as on Al2O3-20 vol % SiC having ≃1 μm grain size revealed that fracture resistance is due to a number of mechanisms including debonding near the whisker matrix interface, crack deflection, pinning, and bridging by SiC whiskers. Formation of secondary fractures and rocracks near and in front of propogating crack tips was also observed in the larger grain size composite.

Aqueous colloidal routes for processing binary suspensions containing Al 2 O 3 and ZrO2 were designed and tested in order to achieve homogeneous microstructures. Effects of particle size and size ratio of each component, pH, and electrolyte concentration of composite suspensions on sedimentation, green density, and ZrO2 distribution in sintered microstructures were examined. The pH conditions for inhibiting differential sedimentation without impairing green density were optimized. Overall suspension and coagulation behavior for these composite systems were explained using the DLVO approach. Optimum balance of colloidal and gravitational forces occurred when the secondary minimum heterocoagulation was maximized.

An Analytical Electron Microscopy investigation of TiB2 hot-pressed and pressureless sintered with Ni revealed the presence of Ni3B and tau intergranular phase, respectively. Convergent Beam Electron Diffraction (CBED) was used for crystal structure determination and compositions were determined by quantitative x-ray Energy Dispersive Spectroscopy (EDS) and Electron Energy Loss Spectroscopy (EELS). The phase analyses were compared with phase diagram data. An evaluation was also made of TiB2 hot pressed with Ni3Al. Quantitative EDS and EELS microanalysis indicated a Ni,Al type boride tau (Cr23C6 type) intergranular phase.