An HVOF extension gun for spraying internal diameters down to 5” (127 mm) has been developed and evaluated. The spray gun utilizes vortex stabilization of the flame which permits operation with a minimal length combustion chamber. Small spray distances and subsequent reduced particle dwell times present a challenge for heating and accelerating powders to achieve typical HVOF coating benchmark properties such as hardness, density and residual stresses. Initial coating characteristics and process/system capabilities are discussed.

In this study, titanium carbonitride (TiCN) coatings are obtained by atmospheric plasma spray synthesis, or reactive plasma spraying. In order to promote reactions between Ti particles and reactive gases, an extended gas tunnel was mounted at the end of a conventional plasma gun. The oxidation behavior of the TiCN coatings was investigated over a wide temperature range, showing that the coatings suffered severe oxidation at temperatures above 700 °C and were entirely oxidized to the TiO 2 phase at 1100 °C. The principal oxidation mechanism was revealed, indicating that oxygen can penetrate into the TiCN coatings at high temperatures. Changes in microhardness were also investigated as a function of temperature, showing a precipitous drop over the range of 700-1100 °C.