DEVELOPMENT AND APPLICATION OF PULSED-AIR-ARC DEPOSITION

Pulsed-air-arc deposition (PAAD) is a process of depositing coatings using a chain of high current short-duration pulsed electrical arcs to melt and evaporate material from a source anode and to transport it to the workpiece which is held in close proximity. The workpiece serves as the cathode, and the electrical discharge action at its surface removes surface contaminants so that an adhesive coating forms. The short distance between the source electrode and the workpiece, together with the high pressure of the plasma jet emitted from the source anode, excludes air from the vicinity of the deposition and minimize oxidation. The required equipment is very compact, having a volume similar to a lunch box, and can be readily transported to the work site for field service. The source electrode can be manually held and, since the process can work in atmospheric air, no process chamber or vacuum system is required. PAAD has been used to apply hard carbide coatings manually to metal and wood cutting tools, increasing their lifetime by factors of 2-6. The lifetime of punches has similarly been increased by factors of 2-7. Machinery parts have been reconditioned and hardened, and anticorrosion and heat-resistant coatings have been applied to mechanical parts. The maximum coating thickness which can be applied, about 100 mum, is limited by residual tensile stress (RTS), which ultimately causes surface damage and material loss. The RTS increases with increasing deposition time, until a maximum is reached, after which surface damage and mass loss occur. Ductile and brittle materials exhibit different surface damage patterns and have different post-RTS maximum behaviours. The maximum coating thickness can be increased by a factor of 2 by periodically interrupting the deposition process and annealing. A compressive stress externally applied to the source electrode decreases the erosion rate, and conversely tensile stresses increase the erosion rate. An external tensile stress applied to the workpiece decreases the maximum coating thickness and conversely an externally applied compressive stress can increase the maximum coating thickness by a factor of 2.