Formation and reactivity of borides, carbides and silicides. I. Review and introduction

Borides, carbides and silicides are reviewed with special reference to newer production methods and fabrication techniques. Crystal structures and types of bonding in binary and ternary compounds are classified and discussed. The scope and limitations of the Pauling‐Rundle theory, molecular orbital treatment and the Ubbelohde‐Samsonov theory are examined critically and appropriate experimental evidence is summarised. Information so far available on the sintering of borides, carbides and silicides is summarised in relation to their chemical reactivity. The sintering is influenced by additives or impurities such as oxides formed by partial hydrolysis and oxidation. Resistance to oxidation is increased by sintering and hot pressing the refractories, but since the affinity of the metals is exclusively higher for oxygen, exchange reactions diminish the quality of the materials. Boride and carbide coatings generally have poor resistance in air or oxygen, but some silicides are more suitable. The kinetics and products of oxidation of borides, carbides and silicides so far studied depend mainly on the intrinsic reactivity of the material and the available surface at which oxidation can occur.