Abstract
A survey has been made of the sintering behaviour of chromic oxide compacts in which, in general, a powdered reducing agent has been incorporated prior to cold pressing and sintering in vacuo. Most of the work has involved the addition, in widely varying amounts, of carbon or chromium as reductant. When a certain amount of reducing agent has been added, the sintering operation converts the compact to a low--- porosity, high-strength body. The strength was estimated principally by Rockwell Hardness, and also by tests for crushing strength and Vickers Hardness. During sintering, shrinkage occurs to an extent greater than 10% linear. The mechanism of this has been investigated, principally by metallography and electrical measurements, but also including chemical analysis, X-ray diffraction and hot-crushing tests. It has been found that chromic oxide particles, after exposure to reducing conditions at high temperatures, develop a surface film, and it is inferred that shrinkage takes place during the formation of this film, The film does not consist of chromic oxide of O/Cr ratio different from normal, though it Is probable that sub-stoichiometric oxide is formed as an intermediate stage; in constitution the film is of cubic type and is likely to be Cr[3]O[4] or Cr or a mixture of the two. A contributory cause of high shrinkage and sound, crack-free bodies appears to be the high vapour pressure which exists during sintering and operates in a manner jet to be established. In this connection, experiments analogous to those on chromic oxide have been carried out on mixtures based on other oxides, namely SiO[2], MgO, Al[2]O[3], and some oxides of Mn, Fe, Co, Nb and Ta. The evidence, though limited, is consistent with the vapour pressure hypothesis.