Precoarsening to Improve Microstructure and Sintering of Powder Compacts

Abstract

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. [

Mechanisms of Microstructure Control in Conventional Sintering

“…A direct consequence of the rate-controlled sintering method is fast firing (Harmer & Brook, 1981), which can produce dense materials with small grain size, minimizing the time of exposure at temperatures where grain growth is fast compared with densification (Chu et al, 1991). This is possible because, generally, coalescence mechanisms (eg, surface diffusion and vapor transport) prevail over densification mechanisms (eg, volumetric diffusion by grain boundary diffusion) at low temperatures.…”

“…De Jonghe et al (Chu et al, 1991;Lin & DeJonghe, 1997a, 1997b found that pre-heat treatment (50 to 100 hours) at low temperatures (800 ºC), in which little or no densification occurs, can improve densification and microstructure of a high purity alumina with and without MgO addition. A consequence of these pre-treatments was reducing the densification rate in the initial stages of sintering.…”

“…However, benefits of evolving a more homogeneous microstructure are evidenced in the final stages of sintering, allowing a final microstructure refinement. According to DeJonghe et al (Chu et al, 1991;Lin & DeJonghe, 1997a, 1997b, the pre-treatment leads to more compaction due to the strong increase in the neck formation among particles, promotes the elimination of fine particles, probably through the ripening process of Ostvald and produces a narrower distribution in pore size. These factors make decrease the density fluctuation during sintering, thus favoring the achievement of more uniform microstructures.…”

“…The rate-controlled sintering (Brook, 1982, German, 1996 is one of the ways in which the relationship between densification rate and grain growth rate is determined to identify the sintering temperature at which densification rate is maximized (Chu et al, 1991). Ragulya and Skoroklod (Ragulya & Skoroklod, 1995) studied the rate-controlled sintering of ultra fine nickel powders obtaining sintered samples with high densities (~ 99% TD) and grain size smaller than 100nm.…”

“…One hypothesis to the heating curve control can be achieved by improving the narrowing of grain size distribution in a pre-densification sintering stage followed by a final densification stage namely at a maximum densification rate temperature (Chu et al, 1991;Lin & DeJonghe, 1997a, 1997b. In a thermodynamics point-of-view, another hypothesis is regarded to control the heating schedule at temperature ranging the active grain boundary diffusion.…”