Research On Oxidation Resistant Properties Of TiAlN Hard Coatings

In this study, TiAlN coatings were deposited on the top of YG6 cemented carbides with unbalanced magnetron sputtering technique and made these samples oxidized for half an hour in different temperature later, to research the oxidation mechanism and the oxidation resistance of TiAlN coatings. The microstructure, chemical composition and mechanics performance of the TiAlN coatings were examined by OM,SEM,XRD,XPS,scratch test and micro-hardness test.Experimental results shows that the coating is comprise of the phase of Ti3AlN. When the oxidation temperature rises up to 600℃, a new phase of Ti6O11 appears. The content of this phase decreases while the oxidation temperature rises up and the phase disappears in the temperature of 800℃. From the temperature of 800℃, the basal body begins to seriously oxidized, the basal body of cemented carbide can direct reacts with oxygen and produces CoWO4.From the temperature of 600℃, the samples have more weight after the oxidizing process compare with the samples that don't be dealt with, and the higher the oxidation temperature is, the weight enhanced more, the speed of oxidation is faster.SEM pictures shows that the thickness of these coatings between 1.8μm and 2.6μm, the coating have a tight microstructure in ambient temperature and from the pictures we don't find thick columnar crystals and cavitations. These samples possess well adherent coating morphology , the interface is distinct and continuous. Under the temperature of 800℃, oxidizing has no obvious influence on the morphology of fracture,but the morphology of fracture becomes rough when the oxidation temperature exceeds 800℃.In the face of coating, with the oxidation temperature increases, the content of the element of aluminum and oxygen had an upward tendency, but the element of nitrogen and the element of titanium had a downward tendency. In the highest oxidation temperature of 825℃, the main element are aluminum and oxygen, the element of nitrogen and titanium become tiny. In the ambient temperature, in the face of coating already have a mass of oxygen, when the oxidation temperature rise up, a mass of aluminum beneficiats in the face of coating and be preferential oxidized and produces Al2O3, the continuous and tight and stable layer of Al2O3 insulates the coating and the environment, the oxygen spreading inward and the elements of the coating spreading outward become more difficult, protects the coating and basal body from being persistent oxidized in high oxidation temperature. XPS also shows that the nitrides only exists in low oxidation temperature, when the oxidation temperature rise up, the nitrides increasingly break down, all the elements transform into oxyde in high oxidation temperature, so the process of oxidation is the process of nitrogen's break down and the producing of oxyde.Both the micro-hardness and adhesion strength decreases when the oxidation temperature rise up. This is due to the oxidation rate become higher, the coating became more rough and the higher interstress in high oxidation temperature.