Dual sourced pulsed plasmas for the deposition of high performance, low friction, hard wearing films

Freeman, John Alan (2012) Dual sourced pulsed plasmas for the deposition of high performance, low friction, hard wearing films. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

The work described in this thesis formed part of a larger collaborative project between Manchester Metropolitan University, the University of Liverpool, and our industry partner Teer Coatings Ltd., which was entitled “Dual source pulsed plasmas for the production of ultra-high performance coatings.” Closed field unbalanced magnetron sputtering (CFUBMS) has become a mainstay of sputter deposition techniques. However improvements and new approaches are always being sought. This project has focused on the application of one variation, pulsed substrate bias (PSB) deposition, to the reactive sputter deposition of chromium nitride. CrxN is of great interest to industry, as it offers similar wear and corrosion resistance to the better known TiN, but with greater thermal stability. Pulsed substrate biasing is a relatively recent technology. It potentially allows scope for improvement of coating structure, and hence tribology, through greater ion bombardment of the coating during deposition. The initial aims of this project were: To characterise the pulsed CFUBMS system during reactive deposition of CrxN; to gain an understanding of the plasma behaviour and processes during deposition; to understand the influence of the pulsed plasma over coating microstructures, and relate relevant changes in these microstructures to changes in coating tribology. With the resultant data the project then turned to identifying the strengths and weaknesses of PSB deposition, and finding means to enhance coating performance using the technique. Plasma studies were largely undertaken at the University of Liverpool, using optical spectrometry, CCD camera imaging, and Langmuir probe measurements. Based on these finding 5 coatings were deposited at MMU, and subjected to structural and tribological tests such as scratch adhesion testing, nanoindentation, thrust washer wear testing, surface profilometry, and optical microscopy. Selected coatings were deposited onto cutting tools (twist drills) and tested to failure in a simulated industrial environment. Coating microstructure was investigated at MMU by SEM and EDS. Selected coatings were investigated by XRD at the University of Sofia, and by TEM at Oxford University. The pulsed CFUBMS system is shown to be adequately stable to reactively sputter CrxN. PSB deposition is shown to increase ion bombardment at the substrate. Enhanced ion bombardment is in turn related to the growth of a denser coating, with a more ordered crystallographic structure with greater surface hardness. However the PSB approach is also shown to increase compressive stress within the coating, and potentially damage effective adhesion. As a consequence we have sought to combine the DC bias and PSB methods, as well as changes in coating stoichiometry, in a series of layered coatings. The best performing of these layered coating architectures is significantly harder wearing than standard CrN coatings, and approaches the wear resistance of CrTiAlN coatings in drill tests. Pulsed substrate bias deposition is a potentially powerful addition to the portfolio of sputter coating techniques. Here it has been successfully applied to enhance the microstructure, tribology, and wear resistance of CrxN coatings.