Development of improved cold spray and HVOF deposited coatings

The overall aim of this research project was to expand the understanding of the deposition of titanium and the nickel-based superalloy Inconel 718 by spray deposition methods. The spray processes employed were cold spraying and high velocity oxy-fuel (HVOF) thermal spraying. The first part of the work was undertaken to expand the understanding of the deposition of titanium by cold spraying; the HVOF process is unsuitable for Ti because of the metal's high reactivity. The deposits were produced from commercially pure titanium using cold spray equipment designed in the University. Using helium gas, the effects of different powder particle size ranges, types of substrate, substrate preparation methods, and spray parameter conditions on deposit formation were investigated. Using a simple one-dimensional model of compressible gas flow and particle acceleration, particle velocity distributions were calculated to aid interpretation of experimental data. Results show that titanium can be successfully cold sprayed onto substrates of Ti6AI4V and mild steel, with the critical velocity for deposition of this powder type of approximately 690 m s-1. The level of porosity was generally in the range of 13-23% and the adhesive bond strength was dependent on surface preparation but independent of gas pressure with values ranging from 22 MPa to 10 MPa for ground and grit blasted substrates respectively. This compares with a value of around 80 MPa which is typical for well adhered HVOF sprayed coatings. The second part of the study was concerned with comparing the deposition of Inconel 718 by cold spraying and HVOF thermal spraying; the latter employed a JP5000 liquid fuel gun. A Tecnar DPV-2000 instrument was used to systematically investigate the effect of changes in spray parameters (spraying stand-off distance, oxygen/fuel ratio, total mass flow rate, combustion pressure), on particle velocity and temperature during HVOF spraying. It was found that generally the particle velocity was more strongly affected by the stand-off distance and combustion pressure of the spraying gun whereas the particle temperature was mostly influenced by the particle size and combustion pressure. The microstructures of coatings sprayed under 4 different well controlled conditions were investigated and changes in the morphology of splats and partially melted particles in the coating were related to the particle temperature and velocity at impact. The HVOF had high bond strength and low oxygen level of typically 0.45 wt% (corresponding to an oxide content of less than 1.6 wt.%). By contrast, in the cold sprayed coatings, the bonding was considerably low (-14 MPa), independently from the process conditions. It was found that the process parameter that mainly affected the properties of the cold sprayed deposits was the gas pressure. More specifically, the microhardness of the coatings increased with the pressure whereas the relative porosity decreased.