Expansion of the Applicable Range of HVOF Process Conditions

Abstract

Abstract A conventional GTV K2 kerosene fuel HVOF spraying system has been modified with the aim to expand the applicable range of process conditions in order to also cover the presently existing gap between conventional HVOF and cold gas spraying. Different measures have been applied in order to reduce heat transfer and increase momentum transfer to spray particles. Increase of momentum transfer results both in reduced particle temperature as a consequence of reduced dwell time in the hot flame area and high particle velocities. In detail expansion nozzles with conical shape that provide improved expansion of combustion gases, combustion chambers with reduced critical diameters that provide increased combustion chamber pressures and the injection of coolant media water and / or nitrogen into the combustion chamber are applied. The effect of modified spraying conditions on the process characteristics and coating properties have been studied for a variety of materials. Besides copper, titanium alloys and MCrAlYs also WC/Co(Cr) and Cr3C2/Ni20Cr have been sprayed and respective coatings have been analysed concerning their microstructure, gas content, microhardness and wear resistance. In particular a moderate increase of oxygen content in dense titanium alloy coatings compared to the powder feedstock from 0.41 to 0.59 wt.-% proves the high potential of the undertaken measures to expand the application field of HVOF spraying. Thereby existing systems only need relatively small modifications to achieve this expansion.