Low-diffusion And Oxidation-Resistant Coating For Single Crystalline Superalloy

The insatiable quest for higher efficiency and better reliability of gas-turbine engines is pushing the increment of gas inlet temperature,which,in turn,motivates the development of advanced coating systems for protecting single-crystal superalloy components from progressively rigorous working environment.Thermal barrier coating system(TBCs),which consists of an Al-rich metallic bond coat(BC)providing oxidation resistance and an overlying ceramic coat(TC)for thermal insulation,is considered as the most advanced and structurally-complicated coating system hitherto.As the crucial layer of TBCs,the metallic bond coat is always expected to possess excellent oxidation and rumpling resistance and meanwhile limited interdiffusion with alloy substrate.Under traditional wisdom,oxidation resistance of a coating is always promoted by increasing Al content,which,unfortunately,leads to severe interdiffusion with alloy substrate.Moreover,surface distortion of the coatings always brings about great troubles.In this work,modified nanocrystalline coatings are developed to cope with aforementioned challenges.Firstly,three nanocrystalline coatings with phase constitutions of γ/γ’,single γ’,andγ’/β,respectively,tailored by adjusting the Al content,are developed on the widely used single-crystal superalloy René N5.Owing to the high content of refractory elements in these coatings,the σ-(Cr,W,Mo,Re)phases containing moderate amount of Y precipitate out during annealing.Isothermal oxidation behaviors at 1050℃indicate that precipitation and redissolution of the Y-containing σ-phases during oxidation are strongly dependent on the original Al content of a coating,which leads to different segregation behavior of Y-enriched oxides in alumina scales and oxidation kinetics.Higher Al content inhibits the intensive oxidation of Y,but causes elements interdiffusion with the alloy substrate.The nanocrystalline coating consisted of singleγ’ phase is found to possess low oxidation rate and meanwhile little elements interdiffusion.In order to understand failure mechanisms of the oxide scales on nanocrystalline coatings with different phase constitutions under thermal stress,cyclic oxidation is carried out at 1100℃.The oxide scale on γ/γ’ coating exhibits serious surface rumpling accompanied by frequently cracking at the crests of undulations after 500 cycles.The premature degradation of the coating in microstructure(i.e.γ’ phase dissolution and concurrent grain coarsening),arising from high oxidation rate and low original Al content,not only increases its CTE to introduce larger thermal stress,but also decreases its hardness and elastic modulus,making it sensitive to rumpling and further inducing cracking of oxide scale.Comparatively,benefited from lower oxidation rate and higher Al content,the degradation of γ’ and γ’/β coatings is less significant.They are subjected to lower thermal stress but possess higher deformation resistance during cycling.However,local spallation of the oxide scale occurs on γ’/βcoating since it is insusceptible to rumpling and thus the thermal stress is difficult to release in time by plastic deformation.The γ’ coating gains a good balance between thermal stress generation and stress relaxation by plastic deformation,presenting high resistance to scale cracking and spallation.Since the γ’ coating outperforms its counterparts of γ/γ’ and γ’/β coatings and exhibits the best comprehensive properties,ceramic top coat is further deposited on it to evaluate its performance in TBCs.Compared with conventional NiCrAlY coating,the nanostructure of γ’ coating facilitates the rapid formation of Al2O3 on its surface,which is beneficial to interface adhesion between TGO and YSZ.Additionally,the degradation rate of the γ’ coating is effectively postponed due to little interdiffusion with alloy substrate.Therefore,the γ’ coating is recommended to be a promising candidate as a BC in TBCs.Finally,Pt-modified γ’ nanocrystalline coatings are prepared and cyclic oxidation behaviors are investigated at 1100℃ and 1150℃.Addition of Pt makes the γ’nanocrystalline coating achieve a highly purified oxide scale free of oxide impurities,which effectively decreases the formation of microcracks in the interior of the oxide scale originating from stress concentration and thus avoids the premature failure of the TGO.In comparison with Pt-free γ’ coating,the Pt-modified coating exhibits decreased CTE and increased Young’s modulus and hardness,whereupon it is subjected to smaller stress during thermal cycling and more resistant to surface distortion,displaying mitigated surface rumpling.Furthermore,the degradation rate of the coating is successfully alleviated by Pt addition,which predominantly benefits from the uphill diffusion of Al.