W. Roger Cannon

Creep of ceramics: Part 1 Mechanical characteristics

Many experiments have been undertaken to investigate the creep behaviour of ceramics. This review tabulates the available data in terms of the shapes of the creep curves and the dependence of the steady-state creep rate on stress, grain size and temperature. Numerous theoretical mechanisms are available for intragranular and intergranular deformation processes, and the predictions of these mechanisms are summarized for comparison with the experimental data.

Creep of ceramics: Part 2 An examination of flow mechanisms

This review analyses a wide range of experimental data on the creep of ceramic materials and reveals many similarities with the creep of metals. It is demonstrated that there are two important differences in the creep behaviour of ceramics: (1) there is an enhanced role of diffusion creep, and (2) in the power-law regime, ceramics divide into two categories with stress exponents of ∼ 5 and ∼ 3, respectively. It is concluded that the behaviour with an exponent of ∼ 5 represents fully ductile behaviour as in f cc metals, whereas the behaviour with an exponent of ∼ 3 is due to dislocation climb from Bardeen-Herring sources under conditions where there is either a lack of five independent slip systems or, if five independent slip systems are available, a lack of interpenetration of these systems.

Thermal decomposition behaviour of poly(propylene carbonate)

Abstract Thermal analysis techniques including thermal gravimetric analysis (TGA), differential thermal analysis (DTA), and TG-FTIR were employed to investigate the thermal decomposition behaviours of poly(propylene carbonate) as a neat polymer and in the presence of AlN powder. It was found that the decomposition of poly(propylene carbonate) followed a depolymerisation mechanism and left no detectable residues. The combustion reaction in air enhanced the decomposition of the PPC binder in air only slightly compared to acrylic and PVB binders. In the presence of AlN powder, the PPC binder left primarily “gas phase mediated char” on the powder surface.

Rheology and Component Interactions in Tape Casting Slurries

Rheology and the component interactions which affect rheology were studied for a tape casting composition similar to commercial systems. Viscosity measurements at different shear rates were compared to measured tape properties to determine if high or low shear rate rheological behavior controls tape characteristics. Relative viscosity was measured to assess the contribution of each component to the stability of the dispersion.

2-D particle shape averaging and comparison using Fourier descriptors

Fourier transforms mathematically represent complex contours to a high degree of accuracy. Normalization of Fourier coefficients with respect to contour size, starting point from which the contour is traced, and its orientation, make the coefficients invariant to these contour characteristics that do not represent shape. The mean of the normalized Fourier coefficients and the error can be used to reconstruct the average shape and its deviations. Shape averaging of the 2-D contours using normalized Fourier coefficients was implemented on various ceramic particle systems. The averaged shape provides a convenient mode of shape comparison amongst various powder systems.

Measurement of Particle Orientation in Tape Cast Ceramic Microstructures

Anisotropic microstructures in ceramic green bodies result in nonuniform dimensional changes during sintering. Quantitative characterization of particle orientation together with processing/orientation relationships in these ceramic components are essential to control the dimensional variations during sintering. In this report, we discuss some practical aspects of measuring edge orientation in microstructures obtained by Field Emission Scanning Electron Microcscopy. Edges in the scanning direction were found to be less sensitive to the edge detection operator. Various edge sharpening algorithms were used to improve the edge contrast and hence achieve unbiased edge detection. Edges were detected using the Laplacian of Gaussian operator. Edge orientation polargrams were obtained by computing the intensities of detected edges with a 5×5 Prewitts operator convolved with a Gaussian. The computed edge orientation polargrams from the edge sharpened images were found to be internally consistent. Quantitative measure of particle orientation in tape cast ceramic microstructures was used for the first time to explain nonuniform sintering shrinkages.

Dilute Aqueous Dispersions of Zro 2 And Al 2 O 3

Colloidal interactions in a heteroparticulate mixture of zirconia and alumina in water were studied for use in a transformation toughened alumina composite. The microelectrophoresis technique was used to measure the mobility of three zirconia powders and an alumina powder. The electro-phoretic mobility and particle size data were used to calculate total potential energy curves. The maximum height of the total potential energy barrier was used to predict the stability of a zirconia/alumina mixture. Theoretical predictions were compared to experimental results obtained from sedimentation and rheology measurements carried out as a function of pH of the dispersion. For a 5 v/o aqueous zirconia/alumina system stable dispersions were made at pH 3 and pH 5.

Interactions between Al2O3 and ZrO2 powder in a concentrated suspension

Abstract The electrophoretic mobility of Al 2 O 3 and two types of ZrO 2 powder was measured and correlated with viscosity measurements. Slip casting was performed at several total powder concentrations on the slips dispersed at pH 3 where the electrophoretic mobility of both Al 2 O 3 and ZrO 2 was high. ZrO 2 stratified in slips containing less than 20 vol.% solids but did not at higher concentration.

Alumina/zirconia composites produced from commercial alumina and synthesised hydrous zirconia

Abstract Zirconia hydrate was precipitated from zirconyl chloride onto commercial alumina powder to be used for transformation-toughened alumina composites. Two preparation routes were considered: (1) as-prepared powder was slip cast and sintered directly, or alternatively, (2) the powder was calcined at 500°C and subsequently dry pressed. The second route produced material of a higher strength and density (ZrO 1 > 7·5% vol.). X-ray line broadening and small angle scattering indicated that much of the zirconia prepared by the former route did not coarsen during sintering at 1600°C as long as the ZrO 2 content remained below about 7·5%, although at higher ZrO 2 contents, it did coarsen. This was thought to account for the much lower strengths achieved by the first preparation route.

Edge Sharpening for Unbiased Edge Detection in Field Emission Scanning Electron Microscope Images

: We report here a specific type of edge strength anisotropy observed in field emission scanning electron microscope (FESEM) images. The images show weaker edge gradients in the scanning direction and hence these edges frequently go undetected. Direct application of edge detection algorithms to images with nondistinct edges, such as powder particles, show strong bias to edges perpendicular to the scanning direction. Edge orientation polarograms obtained from these images always show strong fictitious particle orientation in the scanning direction. In this work, we discuss an edge-sharpening algorithm that corrects for this bias and results in relatively more accurate and consistent edge orientation information.