PROCESSES INVOLVED IN SINTERING

Abstract

1.-INTRODUCTION THE considerable advances in powder metallurgy during the past two decades call for a knowledge of, and an ability to interpret, the physicochemical processes that control sintering. This need has stimulated a large amount of experimental and theoretical research in an attempt to establish the laws governing sintering, but this objective has not yet been achieved. Our knowledge of sintering has undoubtedly been extended and clarified, but on certain points there are gaps to be filled and in some cases conflicting evidence to be reconciled. This is easily understood in view of the complexity of the phenomenon, dependent as it is upon a large number of variables not easily separable from one another. To appreciate the truth of this statement, it is only necessary to consult the bibliography. Before embarking upon a discussion of so controversial a subject, it is therefore essential first to arrive at a precise and clear definition of the term sintering: progressive transition, without melting, from a state consisting of an agglomeration of metallic particles in simple juxtaposition to a massive state practically free from porosity. The theory of sintering has as its object to explain by what mechanism and under the influence of what forces this transition takes place. To arrive at a theory, it is necessary to select a physical property characteristic of the phenomenon, and to study its variations with temperature and time. From the interpretation of the curves so obtained, the nature of the forces brought into play may be deduced. Among the properties so used we may cite: