Study On Structure And Ablation Properties Of Iridium Coating On Carbon And Tungsten Carbide

In this work, double glow plasma technique was used to produce iridium (Ir) coating on carbonaceous and tungsten carbide (WC) substrates, respectively. The microstructure and the morphology of the coatings were investigated by scanning electron microscope and X-ray diffraction, respectively. The mechanical properties of the coatings on the WC substrates were measured by the scratch testing and nanoindendation, respectively. The residual stresses in the monolayer and tri-layer Ir coatings on WC substrates were measured using X-ray diffraction technique. The ablation performance of Ir coating was evaluated by oxy-acetylene torch at 2000℃. The coating was composed of a polycrystalline columnar structure with (220) preferential growth orientation. Ir coating failed predominantly by grain boundary brittle fracture at room temperatures. Brittle intergranular fracture did not depend on grain size. Intergranular fracture in the coating could arise from low cohesive strength of the grain boundaries. The interface between the coating and substrate exhibited excellent adhesion. The hardness and elastic modulus of Ir coating were about 800HV and 644GPa, respectively. Because of the good adhesion, Ir coating exhibited excellent scratch resistance. The failure mode of Ir coating was buckling spallation failure by scratch testing at room temperature. The compressive stress in monolayer coating was -1.6GPa. The compressive stress in tri-layer coating was -1.1GPa. Although some pores and the rumpling appeared on the coating after ablation, the integrality and adhesion of the coating on the carbon kept up. The rumpling could be attributed to plastic deformation of Ir coating which resulted from expansion or contraction mismatch of the coating and the substrate. The coating/substrate interface exhibited poor adherence after ablation. The ablation results indicated that Ir coating could afford the high temperature up to 2000℃and protect the carbonaceous substrate from quick oxidizing. Graphite (carbon composites) has been coated directly with Ir, but employing an initial coating of tungsten (W) promoted bonding. Ir/W multilayer coatings could be more farorable for a protection against oxidation at high temperature. Monolayer coating on the WC could not afford the high temperature up to 2000℃by oxidation flame, but tri-layer coating could protect the substrate for 90s at the high temperature oxidation. Monolayer coating could remain intact by carbonizing flame at 2000℃.