Composite coatings with dry lubrication ability on light metal substrates

Abstract

Due to the increasing importance of lightweight engineering and design driven by ecological and economical reasons, advanced composite coatings are needed to improve the surface properties of machine elements and system components made of light metals and their alloys, like magnesium and aluminum. Light metals in general exhibit very poor tribological properties in unlubricated condition resulting in severe seizing and wear. The application of thin solid lubricant coatings by PVD and CVD processes improves the tribological behavior significantly, but in many cases these coatings fail under high surface loading due to the low Young's modulus of the light metal substrate and the mechanical incompatibility of thin solid film and substrate. In addition, the damage tolerance of the coatings is unsatisfying. This study focuses on the evaluation of composite coatings consisting of a thermally sprayed ceramic or metallurgical primary layer and a polymer or CVD/PVD deposited secondary layer. The thermally sprayed coating protects the light metal substrate by providing compressive strength, hardness and wear resistance. The polymer or CVD/PVD deposited top coating in appropriate composition on the other hand causes a low friction coefficient. For this study special polymer coatings containing microscale PTFE particles as solid lubricants as well as cathode sputtered MoS 2 coatings and pure carbon containing coatings (a-C:H) applied by plasma assisted CVD were used. The tribological performance of the composite coatings is evaluated in high cycle pin-on-disk testing.