The influence of deposition parameters on the structure of Al, Zr and W coatings deposited by closed-field unbalanced magnetron sputtering

The technique of closed-field unbalanced magnetron sputtering (CFUBMS) is now established as a versatile technique for the high rate deposition of high quality metal, alloy or ceramic coatings. A key factor is the ability to transport high ion currents to the substrate. This can enhance the formation of fully dense ''high temperature'' structures at relatively low values of homologous temperature. To characterise the CFUBMS system in more detail, an extensive study has been made of the influence of a number of deposition parameters on the resulting structure and properties of Al, Zr, and W coatings. This paper, however, concentrates on structural aspects of the coatings. The deposition parameters varied were substrate bias (over the range -30 to -300 V), target current (2-8 A), coating pressure (6.7 x 10(-2) to 0.4 Pa) and substrate-to-target separation (80-150 mm). For each metal, experiments were designed using the Taguchi Method. The coating structures were characterised by examination in the SEM. Substrate temperature and deposition rate were also measured and the current-voltage characteristics at the substrate were investigated. From this information, ion current densities (0.8-8 mA/cm(2)) and the ion-to-atom ratios incident at the substrate (0.6-6) were estimated. A number of trends in these data were identified using Taguchi analysis techniques. In all cases, dense structures with good coating-to-substrate adhesion were obtained. Indeed, for certain Zr coatings, the adhesion exceeded the cohesive strength of the substrate material. For the W coatings, ''Thornton Zone II'' structures were obtained at homologous temperatures as low as 0.12.