The paper presents an analysis of possible improvements in HVOF torches and associated processes. It is shown that increasing the efficiency of combustion modules, widening the available range of operating pressures as well as optimizing the powder injection and nozzle expansion lead to significant improvements in comparison with the currently utilized HVOF processes using liquid fuel and radial powder injection. The analysis is confirmed by experimental data obtained using an advanced HVOF torch designed to use ethyl alcohol as a fuel. The new torch operates at a higher thermal efficiency of a combustion module, able to generate products of combustion with better capability to transfer heat to particles and more dissociated molecules. High thermal efficiency also results in a wider operating window and better control of a desirable balance between particles’ kinetic and thermal energy, which depends on the spraying material and a particular coating’s requirements. Shock wave generators inside the nozzle/barrel are also included in the design to allow an improvement in powder injection and an increase in the heat exchange between products of combustion and particles. These advantages have allowed for the minimization of the throat diameter providing throat cross-sectional surface area approximately 2 times less in comparison with other liquid fuel HVOF torches presently used in the industry. The reduction in throat diameter results in significantly less consumption of fuel and oxygen, and also results in an overall decrease of the operating costs. Coating quality, deposition efficiency and deposition rate in this case are the same or better in comparison with other currently utilized liquid fuel HVOF processes.

Systematic measurements of heat fluxes into the substrate under the conditions of APS and HVOF have been performed with the use of a specially designed calorimeter. Results of measurements are presented in terms of the heat absorbed by a 150 mm diameter disk related to the electrical input power (EIP) and/or to the plasma/gas enthalpy. Depending on the process conditions heat flux into the substrate could reach as much as 20 per cent of the torch input power. Influences of gas composition, torch power and standoff distance on the heat transfer efficiency as well as the radial distributions of heat fluxes have been studied in detail and compared for three types of guns. Data obtained from the measurements of the radial distribution of heat fluxes in the plasma plume allowed estimating effective plasma jet diameter and the heat flux density, respectively. Rates of jet expansion differ significantly between the F4 and PlazJet plasma torches. Direct measurements of the substrate temperatures with the use of thermal paints have been performed and temperature patterns were compared with the results of 3D heat transfer modeling. Experimental values of heat fluxes were used in the calculations. Comparison showed that the experimental data on heat transfer efficiency could be used for accurate predictions of the substrate thermal conditions during thermal spraying.

The use of the wire arc spray process for the production of zinc and aluminum coatings as corrosion protection represents a rapidly growing market. In this paper, the effects of various spray parameters on the performance of the process for these materials are examined. The effects of arc voltage, current, and wire diameter have been studied for their effects on the main factors affecting process performance, spray performance, and application efficiency. The results of this study have led to the development of a spray system or a power supply to maximize the performance of the process for corrosion protection applications. Paper includes a German-language abstract.