James R. Varner

Importance of load cell sensitivity in determination of the load dependence of hardness in recording microhardness tests

The behaviour of microhardness under varying load was investigated with an apparatus which measured both load and diamond pyramid motion simultaneously. There have been several experiments with this type of apparatus, which are designed to measure the hardness under load of a material. This type of measurement eliminates the effect of elastic recovery after the diamond is removed from the sample. Two types of load-independent hardness have been proposed on the basis of studies performed on this type of apparatus. The first follows the theory of Tate stating that elastic recovery is responsible for the load dependence of hardness. The second, proposed by Froelichet al. states that the load dependence of hardness is due to surface forces. This investigation used an apparatus similar to that of Froehlichet al. The results indicated that the load-independent hardness of Froehlichet al. was an experimental artifice caused by late detection of the surface, leading to underestimation of the penetration and overestimation of the hardness. Hardness measured under load using the apparatus in the present project was found to be load dependent.

Density and microhardness of As–Se glasses and glass fibers

Abstract The density and Vickers microhardness of bulk glasses and glass fibers in the As–Se system have been determined for up to 50% As. Arsenic selenide glasses were batched in an argon atmosphere using high purity arsenic and selenium to produce samples that contained between 5 and 50% As, the balance being Se. The density and hardness of both fibers and bulk glasses increase up to the stoichiometric As 2 Se 3 composition, with the hardness exhibiting a maximum at this composition, in accordance with the average coordination number (〈 r 〉) approach. The microhardness of aged fibers of all compositions is found to be higher than the corresponding bulk glass, indicating different structures. Both the density and microhardness of AsSe 4 are found to increase rapidly with time after drawing. Density and microhardness results are discussed in terms of quench induced stresses which result from the fiber drawing process, and stress relaxation in arsenic selenide glasses.

Structure-hardness relation for high-temperature SO2-dealkalized float glass

Abstract Changes in the surface mechanical properties of float glass that had been heat treated in atmospheres containing SO2 gas were measured. An analysis combining dynamic microhardness and structure investigations indicates that a relation exists between the hardness and surface structure. This relation is used in establishing an understanding of the microhardness variations and the corresponding surface mechanical properties. The treatment parameters, temperature, time and treatment atmosphere conditions, are shown to have an effect on these properties. Their extent is suggested to depend on the formation of a thermal expansion coefficient difference between a dealkalized surface and the interior dealkalization properties and structure alterations. The changes in the mechanical properties are described which connect these effects. The structure-hardness relation of the air side differs from the tin side of float glass.

Replicas as a Technique for Examining Fracture Surfaces of Ceramics

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.

2.45 GHz Microwave Sintering of Silicon Nitride

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.