Adhesion of films of MoS2 to oxidized metal surfaces

Abstract

The adhesion of MoS2 (a layer‐lattice compound used as a solid lubricant), burnished by a standard procedure onto various metal substrates, has been investigated. The degree of adhesion, as determined by microscopic examination of the burnished surfaces, has been related to differences in states of surface oxygen observed by x‐ray photoelectron spectroscopy and Auger electron spectroscopy. Experimental data confirm our previously reported conclusion that the substrate metal–sulfur bond strength is of primary importance in MoS2 adhesion. However, the substrate metal atoms must be accessible to sulfur in order for these bonds to form. On some metals, e.g., copper, it was found that surface oxygen is not present as an oxide but exists in a precursor state less tightly bound than an oxide. The formation of substrate metal–sulfur bonds by displacement of oxygen is facilitated by the existence of this precursor state. On metals with more stable surface oxides, e.g., titanium, oxygen lattice vacancies are present that again expose surface metal atoms to sulfur. Experiments in which MoS2 films were burnished onto both oxidized metals and metal–oxide single crystals are reported here. All results are consistently interpreted in terms of the nature of the oxygen initially present on the metal surface.