The sintering of zinc oxide powder compacts has been investigated at constant rates of heating of 0.5° to 15°C/min. For samples with the same initial relative density (0.50), the temperature derivative of the densification strain versus density fits within a single, relatively narrow band. At low temperatures the densification rate as a function of temperature increases almost linearly with the heating rate. The data, covering a wide density range of 0.5 to 0.98, are consistent with an analysis that accounts for the coarsening (defined as an increase in the mean pore separation) in terms of two classes of microstructural coarsening processes: those associated with densifying and with nondensifying mechanisms.

MgO and Al2O3 were sintered by two types of processes: a conventional isothermal sintering and a two-step sintering consisting of a n initial low-temperature precoarsening treatment before conventional isothermal sintering. The final microstructure from two-step sintering can be more uniform and finer than that of compacts sintered conventionally. A narrow-size-distribution a l u m i n a powder was sintered under constant-heating-rate conditions, with and without a precoarsening treatment, and the results were compared. The differences between two-step and conventional processing were clarified by experiments on precoarsened and as-received ZnO powders. These compacts were precoarsened at 450°C for 90 h with virtually no increase in the overall density. The resulting grain size was 1.7 times the starting one, but the standard deviation of the precoarsened powder size distribution was smaller than that of the asreceived powder. Precoarsened compacts sintered to nearly full density showed improved homogeneity. The sintering stress of the precoarsened ZnO was approximately 0.8 that of the as-received one. A computational model has been used with two components of coarsening to describe the differences in pore spacing evolution between the precoarsened and the as-received system. The benefit of two-step sintering is attributed to the increase in uniformity resulting from precoarsening. The increased uniformity decreases sintering damage and allows the system to stay in the open porosity state longer, delaying or inhibiting additional coarsening (grain growth) during the final stage of densification. Twostep sintering is especially useful for nonuniform powder systems with a wide size distribution and is a simple and convenient method of making more uniform ceramic bodies without resorting to specialized powders or complicated heat schedules. [

The effect of green density on both the densification rate and the creep rate was measured simultaneously during sintering by loading dilatometry. The experiments were performed on zinc oxide powder compacts with five different green densities covering a range of 0.39 to 0.73 of theoretical. The samples were heated at a constant rate of 4°C/min up to 1100°C in air. The densification rate at any temperature increases significantly with decreasing green density. The data for the densification rate and creep rate as a function of density show two quite distinct regimes of behavior; the rates were strongly dependent on density below 0.80, while above this value they were weakly dependent on density. The ratio of the densification rate to the creep rate was almost independent of temperature but increased almost linearly with increasing green density. The representation of the data in terms of models for sintering and creep is discussed.

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The effects of a pore size distribution and of the pore coordination number on the sintering stress is examined using a simple model. The sintering stress is found to be proportional to the mean of the pore sizes weighted according to the Voronoi cell pertaining to each pore, rather than to the simple pore size average. Large heteropores are shown to have little effect on the sintering stress. Decreases in pore coordination number of such pores, resulting from grai~ growth, are found to affect the sintering stress little, but can significantly increase the stress intensification factor. The near-constancy of the sintering stress, observed experimentally for many powders over a wide range of sintered densities, does not directly follow from the simple model. It is argued that this constancy results from pore shrinkage, due to densification, which is compensated by pore growth due to coarsening.

This study identiJies factors that affect slider performance andEXPERIMENTAL SETUP reporb the effect of the nature of slider materials on wear behavior. Three factors relating to wear behavior are discussed: slider shape, wear data reproducibility and patterns of wear behavior. First, the perjormance of various sliders shows a strong correlation to the slider crown shape. Second, wear behavior is more reproducible when unlubricated disks replace lubricated disks. With lubricated thin-film disks, slider failure is reasonably reproducible, but when the faihtre occurs it is erratic. Although unlubricated disks show more reproducible wear data, the wear life of the disks is roughly an order of magnitude shorter than lubricated disks. It is important fo recognize these fators before evaluating material differences in slider performance. Finally, multi-phase slider materials have more variation in failure patterns than single-phase slider materials; the difference is attributed to more paws present in the multi-phase material.