Experimental Study on the Kinetics of Sintering of Metal Powder at Constant Temperature. II

The densification rate of various compacts of some metal powders, such as silver, iron, and nickel during sintering, was studied with particular reference to the characteristics of powder and to the temperature. The behaviour of the densification rates of those metal powder compacts at constant temperature was expressed approximately by a simple power relation, at least in a steady-progressive range of sintering process, as well as in the case of sintering of copper powder compact. On the basis of the empirical formula between the fraction of total decrease in volume of pores in a compact and the sintering time, an investigation was made by the same experimental techniques as those in the previous work on the influences of sort of metal and powder, of pressing load or initial density of compact, and of other sintering conditions, upon the rate of densification. From the results obtained, it was explained qualitatively that the rate of densification of powder compact in isothermal sintering process depends most intensively on the geometrical characteristics of powder ; that a little difference in pressing load or initial density of compact does not introduce any effect upon the rate of densification. Temperature dependence of the rate of desification in a range of moderate temperature was confirmed to be approximately exponential to absolute temperature over all these metal powders, if the systematic deviation from this relation at high temperature could be ignored. The heats of activation in this process, obtained from this relation, were of the same order as that for self-diffusion of these metal atoms. Moreover, it was found that the densification processes in the different powder compacts of same metal produced by different processes were governed by different heats of activation which were nearly independent of other sintering conditions. In addition, a general tendency of lowering the activation heat towards high temperature was observed for the same powder compact of same metal. In this paper, however, this result was not stressed because of little reliability of the experimental data for the rate of densification at high temperature