Ammonia reacts on heating with metals to give nitrides. These include oxides, sulfides, nitrides, and halides. They include oxides, sulfides, nitrides and phosphides. The hardness is achieved by the formation of nitrides. Reaction with ammonia affords sulfur nitrides related to SN. The diffusion layer contains nitrides and carbides. He has made contributions to the coordination chemistry of transition metal nitrides, phosphides, and carbides. These materials consist of few-atoms-thick layers of transition metal carbides, nitrides, or carbonitrides. Heteroepitaxial growth of InN with other nitrides (GaN, AlN) has proved to be difficult. The formation of surface nitrides makes for example chromium catalysts ineffective. The chalcogen is assigned positive oxidation states for the halides, nitrides, and oxides. The main group elements also form some molecular nitrides. Broadly speaking, UHTCs are borides, carbides, nitrides, and oxides of early transition metals. Exceptions include non-oxide inorganic materials, such as nitrides, borides and carbides. Certain carbides and nitrides confer wear-resistance.