Self Lubrication on the Atomic Scale : Design, Synthesis and Evaluation of Coatings

• Main Author: Lindquist, Mattias
• Format: Doctoral Thesis
• Language: English
• Published: Uppsala universitet, Institutionen för teknikvetenskaper 2008
• Subjects: Materials science; Tribology; low friction; PVD; sputtering; nanocomposite; Materialvetenskap
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-8443; http://nbn-resolving.de/urn:isbn:978-91-554-7088-3

In this thesis a new design concept of tribologically active coatings aimed for low friction applications, have been explored. Materials modeled by ab initio DFT calculations were realized through deposition of carbide and nanocomposite coatings by DC-magnetron sputtering. The design concept employs destabilization of a carbide material by alloying with a weak carbide-forming element, which refines the structure into a nanocomposite. The destabilization creates a driving force for superficial ejection of carbon in a tribological contact, forming a lubricious graphitic carbon layer. The otherwise hard material limits the real contact area and the transformed layer accounts for low shear resistance. Hence, the ideal situation for low friction is provided by formation of an easily sheared thin surface layer on a hard material. TiAlC was chosen as a model system for the theoretical modeling as well as for the depositions. The elemental composition, microstructure and mechanical properties of the coatings were characterized to relate the inherent properties to the experimentally achieved tribological response. As predicted by theory, TiAlC coatings were shown to provide self-lubrication on the atomic scale by giving low friction through a tribologically induced surface restructuring. It was shown possible to reduce the friction coefficient from 0.35 for TiC to 0.05 by addition of Al. Alloying with Al also proved to be a potent method in tailoring residual stresses from high and often detrimental levels to acceptable levels, with no significant reduction in either hardness or Young’s modulus. The effect of adding Al into TiC on the oxidation resistance was also explored. The critical temperature for onset of oxidation proved to increase with the Al-content from about 350°C for TiC to about 450°C for TiAlC with about 7 at% Al. A further increase in Al content did not change the onset temperature further but reduced the oxidation rate.