Thermal Stability, Oxidation Behavior And Corrosion Properties Of AlN-based Hard Coatings

Abstract:Some metal nitride coatings, owing to their superior characteristics such as high hardness and wear resistance, superior oxidation resistance and thermal stability, relatively low friction coefficient and superior chemical stability against metals, have been widely used as protective hard coatings in various harsh working environment, e.g. high temperature, humid, and corrosive atmosphere。Al0.66Ti0.34N, Al0.66Cr0.34N and Al0.65Cr0.30Si0.04W0.01N hard coatings were fabricated by cathodic arc evaporation technology. GK05, GU20and GU25UF cemented carbides with different grain size were used as the substrate. The microstructures and mechanical properties were observed and tested. For the comparison research of the thermal stability and oxidation resistance, the coated cemented carbide specimens were treated in Ar and ambient atmosphere at700,800,900and1100℃for2h, respectively. The characteristics associated with the phase evolution were determined by scanning electron microscopy, energy dispersive spectroscopy, X-ray photoelectron spectroscopy, X-ray diffractometer and nanoindentation tester. The change in the composition on the cross section and the thickness of the oxidized layer were analyzed by electron probe micro-analysis. The corrosion behaviors of the hard coatings were investigated by electrochemical impedance spectroscopy (EIS) measurement and potentiodynamic polarization in3.5wt.%NaCl solution at25℃. The main results are summarized as following:1. The results of microstructures and mechanical properties show that, the surface of AlTiN coating shows some droplets and holes. The roughness of the AlTiN, AlCrN and AlCrSiWN coatings is approximately42.5nm,27.7nm and6.3nm, respectively. Among the three coatings, AlTiN is the roughest. The surface quality of AlCrN and AlCrSiWN coatings is improved, however the AlCrSiWN coating has some big but not deep craters. The AlTiN coating has a columnar porous structure, however the coatings of AlCrN and AlCrSiWN have uniform and dense structure. The as-deposited AlCrSiWN coating has the highest hardness value of33.96GPa, followed by AlCrN (32.03GPa) and AlTiN (25.9GPa). The fact that the addition of Si results in an increase in hardness can be ascribed to the formation of the nanocomposite structure of AlCr(SiW)N embedded in amorphous Si3N4along the grain boundaries.2. The three coatings as-deposited are crystallized into an A1N based cubic structure. For the AlTiN coating, due to the strong effect of the WC diffraction peaks and serious broadening phenomenon of the peaks corresponding to AlN, AlN based phase is hard to be identified. No diffraction peaks of Si3N4were observed in the XRD pattern of the AlCrSiWN coating. Nevertheless, from the existence of Si-N chemical bonding indicated by the XPS result, Si might exist in the form of amorphous Si3N4.3. Annealing in Ar, AlTiN experiences an isostructural decomposition which results in the formation of fcc-TiN and fcc-AIN phases from700℃to900℃. At900℃, AlTiN acquires a maximum hardness of29.48GPa which is the highest among the three coatings. At1100℃, softer hcp-AIN is detected due to the phase transformation. The hardness of AlCrN and AlCrSi WN drops substantially at the temperature higher than700℃. Nevertheless, AlCrN outperforms AlCrSiWN in terms of the thermal stability. At1100℃, the hardness of AlCrSiWN reaches to18.99GPa, which is the lowest among the three coatings.4. The outward diffuse of Al, Ti, Cr and Si atoms and inward diffusion of oxygen and N atoms are observed. For the AlTiN coating, a two-layered oxidation structure exists, namely Al2O3dominated top layer and Al2O3and TiO2mixted sublayer. A mixed Al2O3and Cr2O3oxide layer is formed on the AlCrN and AlCrSiWN coatings. Additionally, the mixed oxide layer of Al2O3, Cr2O3and SiO2can more effectively hinder the elemental interdiffusion, which can improve the oxidation resistance. The following ordering of the three coatings according to oxidation resistance is found:AlCrSiWN>AlCrN>AlTiN.5. Compared with the bare cemented carbide substrate, the corrosion resistance of the three coatings has been improved. Among the three coatings, AlTiN coating exhibites the worst corrosion resistance due to its columnar structure accompanied by a large number of pinholes. All these defects form a direct diffusion path for the corrosive electrolytes to pass through. AlCrSiWN coating with a dense microstructure exhibits the best corrosion resistance due to a segregation of amorphous SiNx phases from the metastable Al-Cr-Si-W-N substrate.6. As the electrode potential becomes more positive, corrosion reaches the substrate though the pinholes and an oxide film is generated as a passivation coating. Passivation coating breakdowns as the immerse time or the applied potential increases, then the pitting corrosion becomes the main corrosion mechanism.