Oxidation of double glow plasma discharge coatings of iridium on molybdenum for liquid fuelled rocket motor casings

Dense and uniform coatings of iridium (Ir), 5-7 μm in thickness, were deposited onto molybdenum (Mo) substrates by double glow plasma discharge in the temperature range of 800-850uC at 35 Pa. During deposition, the Mo substrate was biased at a voltage of -300 V while the 99·9% Ir target was at a bias voltage of -800 V. After deposition, the Ir coating was ablated using an oxyacetylene torch with a flame temperature of ∼2000°C to determine the high temperature stability of the coated substrate. The morphology and microstructure of the Ir coating were observed using scanning electronmicroscopy while the composition and structure weremeasured using X-ray diffraction and energy dispersive spectroscopy. The thickness of the as deposited Ir coating was uniform and the interface between the coating and the substrate exhibited excellent adhesion with no evidence of delamination and cracks. After exposure to the flame, the surface of the as ablated coating presented imperfections including pores, bulges and cracks; however, the Ir coating retained sufficient adhesion to limit the weight loss of the Ir coated Mo substrate to 10 mg cm ^-2 s^-1.