Effects of heating history on sintering properties of nickel powder compacts

Abstract

The effects of heating history on the sinterability of nickel powder compacts as a model material are examined from a view point of driving force and flow resistance, that is, the sintering stress and viscosity. Sinter-compression tests were conducted at different constant temperatures for cylindrical specimens, which underwent one-step or two-step heating. In the two-step heating, the specimens were heated up to a higher temperature and cooled down to the test temperatures. After the tests, the microstructure of each specimen was examined and compared with the change in sintering stress and viscosity. For the whole range of test temperatures, viscosity was increased by the two-step heating, which can be explained by the effects of time-hardening. On the other hand, for the lower range of test temperatures, the sintering stress was decreased by the two-step heating. This may be a result of the formation of large pores due to inhomogeneous shrinkage in the unstable state of the powder with small contact in the initial stage of sintering.Key words : Sintering stress, Viscosity, Two step sintering, Sinter compression test, Pore IntroductionSintering as a manufacturing process is used to fabricate many things including ceramic parts, metallic machine components of complex shape, and various functional devices with complicated structures. In order to make the sintering process more effective, however, issues such as shrinkage accompanied with densification, and microstructural evolution in elevated temperatures, should be known and controlled to promote high dimensional accuracy as well as good material properties. The shrinkage as well as the microstructure of sintered materials is sometimes significantly affected by the heating history. In the two-step sintering of nanocrystalline ceramic powder, for example, grain growth is suppressed and densification is promoted, where the triple-point dihedral angle of grain or grain boundary/pore junctions is considered to be changed into a stable state by pre-firing (Chen and Wang, 2000;Wang et al., 2006). During the second step of two-step sintering, the mobility of the junctions may be lower than the grain boundary diffusion rate. The effects of the change in grain boundary/pore junctions on the driving force for densification has not yet been revealed, therefore, the effects of two-step sintering should be clarified from the view point of driving force as well as diffusion rate.Sinter-compression (forging) tests can be used to determine the sintering stress and the viscosity of powder compacts (Venkatachari and Raj, 1986;Rahaman et al., 1986; Zuo et al., 2003a), composites (Bordia andRaj, 1988;Salamone et al., 2003), glass (Ducamp and Raj, 1989), and metal/ceramic mixtures (Shinagawa, 2008(Shinagawa, , 2009. In these studies, microstructural evolutions, such as, grain growth, were considered, but no experiments have been done for the effects of heating history. Even with non-nanocrystalline powders, some effects regarding the sintering stress and viscosity may be o...