Paper 3: Influence of the Atmosphere on the Endurance of Some Solid Lubricants Compared at Constant Layer Thickness

Abstract

Friction and endurance of five powdered solid lubricants were studied on a pin and ring machine. Simple methods were developed to produce layers of constant thickness (1 μm). The ease of rubbing-in varies widely with structure and with particle size. Ease of layer formation is completely unrelated to endurance: NbSe₂ is rubbed in readily, but is rubbed off directly on loading.

Layers of constant thickness from MoS₂, WS₂, and WSe₂ powders behave roughly similarly, but MoS₂ has a much longer life. The lubricant films are gradually destroyed by oxidation and therefore endurance increases in the series oxygen, air, and nitrogen (with 100 p.p.m. oxygen). The process of lubricant degradation is the same for the synthetic crystalline powders as found previously for MoS₂: blistering of the run-in film surface on release of loading. Oxygen accelerates this process, leading finally to descaling and the onset of seizure. The endurance of MoSe₂ goes through a pronounced maximum in oxygen–nitrogen mixtures. It is suggested that oxygen plays a dual role, first by enhancing film formation, and second by causing oxidative embrittlement of the lubricant.