Synthesis and Evaluation of TaC:C Low-Friction Coatings

Sammanfattning: In the large family of carbon-based coatings there are members capable of providing a rare and very desirable combination of high wear resistance and low friction. The field of application for this kind of coatings is steadily increasing which, naturally, give raise to new questions and demands.Coatings of this type can be produced in a number of different ways, but the parameter influencing their properties most is the chemical composition. To facilitate investigations of the compositional impact on the synthesis and properties of coatings, a way to alloy magnetron-sputtered thin films was developed in this thesis. It does not involve the use of reactive gases or additional material sources; instead metallic foils are attached onto the magnetron target surface and thus sputtered alongside the target material. This co-sputtering route was later used to synthesize carbon coatings alloyed with Ta, Zr, W and Al in various amounts and configurations.It was shown that the co-sputtering method could be used to alloy coatings in a very simple and straightforward manner, with excellent possibilities of controlling the amount of alloying elements. The process temperature could be kept as low as 70 °C.Carbon coatings alloyed with transition metals (MeC:C) displayed polycrystalline nanocomposite structures with 5 nm metal carbide crystallites in a matrix of near-amorphous carbon. Alloying with Ta resulted in a radical 80 % reduction in friction coefficient during dry sliding in air, from 0.22 for pure carbon to 0.04 for TaC:C. This was found to be due to facilitated graphitization and the formation of lubricating surface oxides. Al additions to TaC:C resulted in a transformation of the TaC phase to a metastable, previously unreported Ta1-?Al?C. The coefficient of friction remained unaltered, but the oxidation rate compared to TaC:C was significantly reduced due to the formation of AlTaO4 instead of Ta2O5.