W. J. McDonough

Laser Processing of Ceramics

Results of experiments exploring the feasibility of using a laser heat source for sealing plasma sprayed coatings, toughening ceramic bars, and making ceramic alloy powders are reported. Chemical modification of the coated surfaces in conjunction with surface melting showed some promise of controlling craze type cracking. Precipitates which should be beneficial to the toughness of zirconia coatings were developed as a result of laser treatment. Fine, complex microstructures were developed in the surface of ceramic alloy bars and in spherical particles. Strengthening of some bars was indicated, but it was limited by the shape and character of the underlying material. Molten droplets of selected ceramic compositions typically solidified into spheres of the order of 40–200 μm for the range of rotational speeds used in the study. Spheres typically contained porosity which enhances their crushability to give powders for possible future body fabrications.

Oxidation weight gain and strength degradation of Si3 N4 with various additives

Oxidation behaviour and strength degradation, due to long-term high temperature exposure in air, of Si3N4 with MgO, ZrO2 or Y2O3, as densification aids have been studied. It was found that the weight change and strength degradation in the specimens depend largely on the kind of densification aid. Strength degradation, which generally occurred and was greatest at higher additive levels, was related to the generation of pits in the specimen surface. Possible mechanisms for pit formation are discussed.

Partially Stabilized ZrO2 As A Possible Ir Dome Material

There is increasing interest in IR window materials with transmissionn limits of > 5 μm, increased mechanical toughness, and higher temperature capability. ZrO2, which transmits to 7-8 μm is a possible candidate material for such needs. While its IR cut-off is at somewhat shorter wavelengths than other candidates (e.g. Y203 with a cut-off of - 9 μm), ZrO2 offers the potential of significant toughening via a two phase, i.e. partially stabilized, structure. Such structures, and the relevant toughening mechanisms are reviewed, and the resultant mechanical properties discussed, particularly for the Zr02-Y203 system. For example, some materials in this system can give strengths of about 1.5 GPa (200,000 psi) at 22°C and nearly 0.7 GPa (100,000 psi) at 1500°C for laboratory test specimens. Preliminary optical measurements, including some outlining optical scattering effects of the second (precipitate) phase, required for mechanical toughening are presented. These suggest that useful transmission in the range of interest is achievable with these toughened ZrO2 materials.

Partially Stabilized ZrO 2 As A Possible Ir Dome Material

There is increasing interest in IR window materials with transmissionn limits of > 5 μm, increased mechanical toughness, and higher temperature capability. ZrO2, which transmits to 7-8 μm is a possible candidate material for such needs. While its IR cut-off is at somewhat shorter wavelengths than other candidates (e.g. Y203 with a cut-off of - 9 μm), ZrO2 offers the potential of significant toughening via a two phase, i.e. partially stabilized, structure. Such structures, and the relevant toughening mechanisms are reviewed, and the resultant mechanical properties discussed, particularly for the Zr02-Y203 system. For example, some materials in this system can give strengths of about 1.5 GPa (200,000 psi) at 22°C and nearly 0.7 GPa (100,000 psi) at 1500°C for laboratory test specimens. Preliminary optical measurements, including some outlining optical scattering effects of the second (precipitate) phase, required for mechanical toughening are presented. These suggest that useful transmission in the range of interest is achievable with these toughened ZrO2 materials.

Partially Stabilized ZrO[sub]2[/sub] As A Possible Ir Dome Material

There is increasing interest in IR window materials with transmissionn limits of > 5 μm, increased mechanical toughness, and higher temperature capability. ZrO2, which transmits to 7-8 μm is a possible candidate material for such needs. While its IR cut-off is at somewhat shorter wavelengths than other candidates (e.g. Y203 with a cut-off of - 9 μm), ZrO2 offers the potential of significant toughening via a two phase, i.e. partially stabilized, structure. Such structures, and the relevant toughening mechanisms are reviewed, and the resultant mechanical properties discussed, particularly for the Zr02-Y203 system. For example, some materials in this system can give strengths of about 1.5 GPa (200,000 psi) at 22°C and nearly 0.7 GPa (100,000 psi) at 1500°C for laboratory test specimens. Preliminary optical measurements, including some outlining optical scattering effects of the second (precipitate) phase, required for mechanical toughening are presented. These suggest that useful transmission in the range of interest is achievable with these toughened ZrO2 materials.

Analysis of the Pressure Sintering Kinetics of Ceramic Oxides

An analysis of the kinetics of final stage pressure sintering for Al2O3 shows that more than one mechanism is normally operative. In this analysis, the effective stress (σeff) at constant temperature on the sintering particles is taken as the applied pressure (PA) divided by the relative density ( ρ), as recently suggested by Coble. A plot of the measured instantaneous true strain-rate (i. e., densification-rate) versus (1/ρ) for a given experiment results in a constant slope over the final ∼20% of densification. This constant slope is the steady-state strain-rate (έs) for that experiment and is related to the applied pressure (PA) by analogy with the power law creep relationship. This same relationship

Intrinsic volume changes of self-propagating synthesis

\dot \varepsilon _s = AP_A ^n

Effects of self-propagating synthesis reactant compact character on ignition, propagation, and microstructure

where A is a constant, is then used to analyze the overall kinetics by a log-log plot of the various (έs) values determined by individual (PA) values from a number of hot pressings. The value of the slope (n) relates to the different kinetic mechanisms: n=1 implies Newtonian flow, a constant n⁠1 implies non-Newtonian flow, and a variable n implies that more than one mechanism is operative (i. e., multi-component kinetics)