This study investigates arc instability phenomena associated with hollow-cathode plasma torches under low pressure. Stable and unstable regimes are evaluated based on Fourier analysis of gun voltage. For comparison, arc fluctuations with a solid-cathode torch are studied as well. Different internal hardware configurations are used during spraying to identify and assess important factors in the design of hollow and solid cathode guns. This work also characterizes a number of parameters that would exploit plume elongation and accelerated gas velocities under low-pressure conditions.

The hybrid spray process that combines arc-spray with HVOF/plasma jet, has recently demonstrated its effectiveness in the deposition of functionally gradient coatings. This approach aims at exploiting the attributes of both the arc-spray technique combined with those of the HVOF/APS technique. This paper presents high speed visualization and plume characterization of an Arc/HVOF hybrid spray gun as well as a twin-wire arc spray gun. The physics of atomization in hybrid spray process is examined using high-speed camera. A DPV-2000 particle diagnostics sensor is used to measure particle velocity, temperature, size and distribution. The influence of feed material, arc-spray parameters, and the jet parameters on the particle characteristics is presented. The differences in the in-flight characteristics between the hybrid and the twin wire arc process have been discussed aided by the observed atomization phenomena with the high speed camera.

A simulated environment erosion test platform consisting of a control chamber, a modified HVOF gun, and a programmable logic control module is presented. This robust equipment can vary angles of impingement from 0-90 degrees, erodent velocities up to 300 m/s, and temperatures exceeding 2000°C. The erodent velocity, flow rates and temperatures are directly measured employing a high speed camera, and a DPV/CPS 2000 particle flight diagnostic sensor in contrast to estimates typically reported in the literature. It is demonstrated that, although the gas jet type erosion test apparatuses are frequently used in laboratories, quantitative parameter characterization is essential for setting up a test protocol. Both room and high temperature tests have been performed on this test bed. It is believed that the quantitative characterization of test parameters would greatly assist the designer in choosing and developing the appropriate erosion resistant coatings for their application.

This paper presents process development efforts aimed at exploiting the benefits of both the electric arc-spray technique and the High Velocity Oxygen Fuel (HVOF) technique. This so-called hybrid process offers the benefits of using wire stock and high deposition rates of an electric arc-spray gun combined with the higher coating densities obtained using a HVOF gun. Various configurations starting from a single wire arc to 4-wire arc in combination with HVOF have been investigated. Experiments with single material wires to multi-material wire combinations have also been investigated. The feed rates of individual electric arc wire pairs and HVOF powder/wire are controlled independently allowing the creation of functional gradient materials (FGM). This gun has been used to create alloys as well as patterned deposits of different materials. The ability to create in-situ alloys and patterned deposits with excellent quality has been demonstrated.