Papers by Author: James R. Varner

Abstract: This paper discusses reasons for using replicas of fracture surfaces of ceramics, describes several replicating methods and materials, and presents examples that compare replicas with actual fracture surfaces. Advantages of using replicas include being able (1) to examine large pieces without having to cut them down to size, (2) to provide convenient archiving of fracture surfaces, and (3) to eliminate sub-surface scattering of light. Many times, replicas provide clearer views of untreated (uncoated) fracture surfaces than can be obtained by direct observation. Replication using cellulose acetate tape and two types of silicone rubber (filled and unfilled) are described. Both methods are fast, relatively inexpensive, and non-destructive. Images of replicas of fracture surfaces of a glass and several ceramic materials (including electrical porcelain and several polycrystalline ceramics) are compared with images of actual fracture surfaces.

Abstract: Microwave sintered Si3N4-MgO system that contains 2, 4 and 10 wt% of ZrO2 as secondary particulates were investigated with respect to phase transformation and microstructure development. The experimental results of microwave sintered samples were compared with conventional methods. Complete α to β phase transformation was observed in the case of microwave sintered samples due to the volumetric nature of microwave heating. High temperature X-ray diffraction (HTXRD) analysis was performed to study in-situ the oxidation behavior of Si3N4 specimens. Si3N4 specimens with 10 wt % ZrO2 were exposed to air at temperature between 25°C and 900°C for up to 24 hours. Microwave sintered sample were structurally stable in air 25°C and 900°C for up to 24 hours of testing.