| Description |
The sintering of nanosized particles (or nanosintering) is an approach to the manufacture of bulk nanocrystalline materials. The goal of nanosintering is to achieve fully densified parts with grain size less than 100 nm. However, in practice, it is very difficult to reach. Due to the extremely small size and the high surface to volume ratio of nanosized powders, nanosintering exhibits a number of different phenomena compared to the sintering of coarse powders. For example, it is generally found that the sintering temperatures of nanosized particles are drastically lower than those of their micron or submicron sized counterparts, and grain growth during heating up is considerably rapider for nanosized powders in comparison with micronsized powders. In order to obtain a comprehensive understanding about these different phenomena during nanosintering, this study, by using tungsten as the example material, aims to examine size dependence of the sintering behavior and further explore the characteristics of densification and grain growth of nanosized powders, especially during initial and intermediate stages of sintering. The nanosized tungsten powder was produced by high energy mechanical milling. It is demonstrated that the sinterability of nanosized tungsten powder, compared with that of coarser powder, is significantly enhanced at lower sintering temperatures, and the enhancement of sintering at low temperatures for nanosized powders can be rationalized by Herring scaling law. The characteristics of densification and grain growth during nanosintering are examined by both nonisothermal heating up and isothermal holding experiments. The experimental results show linear densification behavior during the initial stage of sintering at low temperatures when density is less than ~50% relative density. Grain growth also exhibits a linear behavior during initial and intermediate stages of sintering. The mechanisms for linear densification and linear grain growth during early stage of sintering of nanosized tungsten powder are discussed based on kinetic analysis of experimental data. The evaluation results show surface diffusion is the mass transport mechanism for linear densification and linear grain growth. On the basis of the understanding of the densification and grain growth mechanisms, the general principles for inhibiting grain growth during nanosintering are proposed. |
