SINTERING AND SURFACE SMOOTHING STUDIES ON NICKEL AND NICKEL WITH DISPERSED OXIDE PARTICLES

Sintering studies were performed on nickel powders with and without aluminum oxide second phase particles, at 750(DEGREES)C, in hydrogen environments. The presence of Al(,2)O(,3) in concentrations as low as 2.5 volume per cent was found to strongly retard nickel surface area loss due to sintering. In similar experiments on nickel powders with dispersed Cr(,2)O(,3) particles, reduction of much of the chromia was observed. The remaining particles and the resulting metallic chromium alloyed with the nickel were found to have no measureable effect on the sintering rate.;Surface smoothing results obtained from pure nickel crystals yielded estimates of the surface self-diffusion coefficient on the (100) plane, in the temperature range 750(DEGREES)C to 1016(DEGREES)C. These were found to agree with extrapolations of higher temperature data reported in other investigations.;These experiments were repeated with aluminum oxide particles deposited directly on the nickel crystal surface. The presence of dispersed Al(,2)O(,3) particles was found to retard the surface smoothing kinetics at 885(DEGREES)C. The amplitude decay rate was found to vary inversely with the surface concentration of particles, as predicted by earlier theoretical models.;The mechanism by which aluminum oxide retards nickel sintering and surface smoothing kinetics may involve either diffusion or a reaction at the nickel/aluminum oxide interface. The current experiments did not gain conclusive evidence as to which process was rate limiting.;Surface smoothing studies were performed on nickel single crystals with sinusoidal surface profiles, prepared by photolithography. The process by which the sinusoid amplitude decreases with time is known to involve diffusion mechanisms, similar to sintering processes. Surface impurities were found to have a strong influence on the results. At temperatures of 875(DEGREES)C or lower, these impurities can cause an order of magnitude reduction in the surface smoothing kinetics. The most consistent results were obtained when the nickel surface was cleaned by argon sputtering prior to initiating experiments.