Effect of Stoichiometry on the Plastic Deformation Behaviour of Cobalt Monoxide Scales

Abstract

ON the basis of both theoretical considerations and experimental evidence it is usually recognized that oxide scales formed on metals are under stress. It is reasonable to assume that these scales will deform plastically, at least to some extent, at the elevated temperatures relevant to many thermal oxidation studies. In fact, few results of mechanical property measurements of scales formed by thermal oxidation have been reported, and most of these have been concerned with strength measurements on copper oxides¹ and iron oxides^2,3, although some creep data for iron oxides are available⁴. The mechanical properties of nickel oxide scales have recently been assessed⁵ and probably the most interesting observation was that NiO deformed plastically at 850° C (0.51 T_m), in contrast with the early observations of Tylecote², who reported no significant plasticity even at 1,100° C. It was also shown that the behaviour of the creep deformation was strongly dependent on impurity level, the secondary linear creep rate being considerably greater for high purity oxide than for an oxide derived from relatively low purity nickel.