Research On Thermally Stability And High-Temperature Oxidation Behaviour Of Nanocrystalline Coating

Nanocrystalline coatings possess high resistance against high-temperature oxidation,and are capable of avoiding coating/alloy elements interdiffusion which is inevitable for traditional metallic coatings,and therefore are a promising solution for the protective technologies at very high temperatures.A potential risk of the nanocrystalline coatings is grain coarsening during the long-term service at high temperatures.The purpose of this work is to investigate the effects of small amounts of oxygen doping in the nanocrystalline coatings on improving the thermal stability and oxidation behavior.The nanocrystalline coatings doped with small amounts of oxygen were prepared by magnetron sputtering in gas mixture of Ar and O2,while the coating free of oxygen was prepared in Ar only.The grain coarsening behavior of the coatings with or without oxygen doping was comparatively studied at high temperatures,and were discussed in associated with elements segregation and second phase-particle pinning.Finally,the corrosion experiment of the coatings in containing NaCl and water vapor environments at 700? was carried out,and its corrosion resistance mechanism was proposed.A theoretical system was established in this work for designing nanocrystalline coatings with high oxidation resistance,thermal stability while avoiding elements interdiffusion with the alloy substrate.This work may provide theoretical base for the application of nanocrystalline coatings at high temperatures,e.g.protective coatings for components of gas turbines.To understand the relationship between the thermal stability and high-temperature oxidation behavior of ordinary nanocrystalline coatings,a nanocrystalline coating with an Al content of 3.6 wt%was prepared by magnetron sputtering,and oxidation experiments at 1000? was carried out.The results showed that protective alumina scale was formed on the nanocrystalline coating at the initial oxidation stage.After 100 h exposure,however,generation and propagation of cracks were triggered by formation of TiO2 in the alumina scale,and led to local scale spallation.In the spallation zones,growth of less protective oxides,e.g.TiO2?NiCr2O4?NiTiO3,instead of alumina healing,occurred,because the coating grains have grown to the micron level and the Al contents beneath the spallation zones was too low to form alumina.The above studies suggest that higher thermally stability of nanocrystalline coating may be beneficial to avoiding spallation of the alumina scales.The oxygen has low solubility in the nickel-based superalloy and is prone to segregating and precipitating oxide particles in grain boundaries,which may help to inhibit grain growth.Therefore,a small amount of oxygen was introduced during process to prepare oxygen-doped nanocrystalline coating.The microstructure evolution of the oxygen-doped coatings in the vacuum annealing experiment at 800-1100? was studied.The results showed that the thermal stability of the coating was improved by increasing oxygen content in the coatings,and low rates of grain growth were found even at temperature up to 1100?.The excellent thermal stability of the oxygen-containing coatings was attributed to the interplay of thermodynamic and kinetic factors.During annealing,the dissolved oxygen in the deposited coating was redistributed,some segregated at the grain boundary to reduce the grain boundary energy,and some others reacted with alloy elements to form oxide phase to inhibit grain boundary migration.Isothermal oxidation and cyclic oxidation experiments of the coatings were carried out at 1000?.The results showed that the amounts of oxygen in the coatings had great effects on the oxidation behavior.Breakaway oxidation of the 0-14 coating occurred quickly,where O-14 is referred to the coating contained 14 at%oxygen.The mechanism is that most of the Al atoms in this coating existed as alumina,resulting in insufficient Al to support formation of the external alumina scale.The O-2 and O-8 coatings showed better oxidation resistance than oxygen free coatings,especially O-8 coating performed the best.The mechanism is in associated with low rates of gain growth in the O-2 and O-8 coatings,which reduced the critical aluminum concentration required for maintaining the growth of the alumina scale,less than half of than in the O-free coating.In addition,the O-2 coating had oxidation rates higher than the O-8 coating because a little Al2TiOs formed within the oxide scale on the O-2 coating.The O-8 and duplex(O-8/O-0)coating are resistant to scale spallation.The alumina nano-particles within the O-8 coating reduced the thermal expansion coefficient mismatch between the coating and the oxide scale,thereby decreased the thermal stresses.In the duplex coating,the outer O-8 layer reduced the thermal stress in the coating,and the inner O-free layer provided better resistance to deformation,so the duplex coating has excellent spallation resistance.The corrosion behavior of the oxygen-doped coating covered by NaCl film in humid environments at 700? was studied.The results showed that K38G alloy exhibited relative poorer corrosion resistance than coatings,and its corrosion products were composed of a loose NiO outer layer and a dense NiCr2O4 inter layer.The loose oxide scale cannot effectively prevent the invasion of corrosive medium,and was prone to peel off.The oxide scales on the O-free and O-8 coatings were mainly composed of Cr2O3.In addition,the two coatings maintained nanocrystalline structure at 700?,a large grain boundary density accelerated the inward diffusion rate of the corrosive medium.Therefore,the O-free coating suffered severe internal corrosion.However,the Al2O3 nano-particles precipitated along the columnar grain boundary of the O-8 coating,and effectively reduced the inward diffusion of the corrosive medium along the grain boundary.Thus,almost no internal corrosion occurred in the O-8 coating.