Fundamentals studies of a magnetically steered vacuum arc.

In recent years demand from production industry for high performance cutting tools, aero and automobile engine parts has prompted research into both existing and novel methods of laying down hard, low friction coatings . A key process for the production of such coatings has been Physical Vapour Deposition (PVD) which has proved to be a consistent and reliable tool for industry. For this technique to continue to be improved and more advanced coatings to be produced, research at the fundamental level is required. This thesis describes research investigating the behaviour of the steered arc cathode spot and methods of improving existing steered arc coating technology.The majority of existing steered arc systems use either permanent magnets or a combination of permanent and electromagnets to steer the arc. Described here is a novel system which employs a pair of electromagnetic coils of cylindrical geometry which enable the arc to be positioned on a circular orbit through a range of continuously variable radii. In addition to this the coils are capable of controlling the transverse and normal magnetic field profiles independently of the steering radius selected. This enables the behaviour of the arc spot to be investigated under a range of magnetic field conditions thus allowing the comparison of measured arc behaviour with a new model of arc motion. Care has described the motion of the arc spot as a biased random walk and has derived an analytical solution to describe the time dependent, probability density function for the arc position in two dimensions. Two distributions are proposed (one in each dimension); the first describes the probability density for the arc position in the direction of driven motion, the second the probability density in the direction of arc confinement. The shape of these distributions is dependent upon the transverse and normal components of the applied magnetic field.A series of experiments are described here that measure the shape of these distributions as a function of either magnetic field, cathode material or both. In the case of the distribution of the arc position in the direction of confinement the width of the distribution is measured as a function of normal and transverse field components. In the direction of steered motion, distributions of arc orbital transit times are measured for a number of cathode materials: analysis of these distributions allow the determination of a mean macroscopic spot velocity and the spot diffusion coefficient. In both cases comparison with Care's model reveals good agreement between experiment and theory to the limits of the experimental apparatus. A further experiment was also conducted to test the prediction that the diffusion coefficient (measured in the direction of steered motion) is independent of the applied field. In this case results are inconclusive and further work is recommended.The mean spot velocity and diffusion coefficients for four materials were measured; titanium, zirconium, aluminium and 316 stainless steel. The results for aluminium and stainless steel compared favourably with some measurements performed by other workers, whilst those for titanium and zirconium are new results with no data available for comparison.