Investigation into the effects of chromium cross-contamination in niobium sputter deposited coatings using a multi-target PVD system.
The optimisation of the tribological properties of engineering components has led to the development of a number of surface coating techniques. One such technique is Physical Vapour Deposition (PVD) which has proved to be a consistently reliable method for the production of thin coatings. The PVD coating system used in this study was the Hauzer HTC 1000-4 ABS which utilises four unbalanced magnetrons in closed-field geometry.A typical coating process uses Arc evaporation to clean the substrates prior to deposition and can involve the use of a target for this stage that is not required during the coating process. During deposition to prevent unused targets becoming coated a negative potential is applied to the cathodes. As a result, due to the lack of individual target shutters, the possibility of crosscontamination between targets of different metals therefore arises.This thesis describes research into the existence and possible effects of Cr cross-contamination on the physical properties of Nb coatings on M2 High Speed Steel (HSS) and 304 bright annealed stainless steel substrates.During this investigation coatings were co-sputter deposited using a main Nb target power of 8kW and Cr target voltages of 0-350V in increments of 25/50V, (the potential range commonly used to prevent unwanted deposition on unused targets), at a temperature of ~400°C.The subsequent coatings were characterised using Glow Discharge Optical Emission Spectroscopy, GDOES to determine depth profiles of chemical composition, and X-Ray Diffraction, XRD to determine the structure and to give an indication of the intrinsic stress and texture of the coatings. Other testing methods included Rockwell indentation, Universal hardness, and corrosion tests.GDOES indicated levels of Cr at or below the limit of calibration at target voltages below 200V. Above this potential an increase in the atomic % of Cr was found at the coating/substrate interface which increased as the potential was increased. This was attributed to the initially clean Cr target's higher sputter yield. XRD indicated body centred cubic structure for all coatings, a pronounced minimum in the intrinsic stress in the coatings when using a Cr target voltage of 150V. A reduction in the (110) texture of the coatings was initially also observed for this target voltage, however, subsequent repeat coatings failed to show the same behaviour. This reproducibility failure may be due to changes in the set-up of the vacuum chamber between processes. Rockwell tests indicated good adhesion. Ultramicrohardness tests were inconclusive, probably due to the thickness of the coatings which were approximately 400nm for all samples. Corrosion tests showed improved resistance to alkaline attack for the samples with increasing atomic % Cr with the best corrosion results observed for the coating deposited using a Cr target voltage of 350V.
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Sheffield Hallam University (United Kingdom).