Preparation,Microstructure And Properties Of Zr Modified Silicide Composite Coatings

Due to possessing high melting points,low densities and favorable mechanical performance both at high temperature and room temperature,Nb-Si based ultrahigh temperature alloys are promising canditates for high-temperature structural materials applied in future aircraft engines.However,their poor oxidation performance in high-temperature environment can not satisfiy the partical demands,although adding Ti,Cr,Al and Hf etc.elements can ameliorate oxidation resistance of Nb-Si based ultrahigh temperature alloys to some extent.Moreover,excessive additions of alloying elements could be detrimental to mechanical performance of these alloys.Consequently,applying a protective coating on Nb-Si based ultrahigh temperature alloys is an essential and effective method.The motivation for this work is to develop a Zr modified silicide composite coating on Nb-Si based ultrahigh temperature alloys.A TiSi₂-NbSi₂composite coating and a（Nb,Zr）Si₂-NbSi₂ composite coating（The coating is named as（Nb,Zr）Si₂-NbSi₂ composite coating,because Zr is mainly located at the outer layer）were respectively prepared by a two-step process including firstly magnetron sputtering a Zr-containing alloy film and then Si-Y co-deposition.The effect of technological parameters（holding temperature and holding time）on the coating structure was systematically studied.Moreover,the high-temperature oxidation resistance,hot corrosion behavior and adhesion performance of coatings prepared by optimized technological parameters were respectively investigated.Eventually,the role of Zr on the oxidation/hot corrosion process of coatings was revealed.For comparsion,the oxidation and hot corrosion behaviors of the substrate alloy were investigated.The main conclusions are as follows:The oxidation resistance（at 900 and 1250 ^oC）and hot corrosion behavior（at 900^oC）of the alloy were investigated.The oxidation of the alloy follows parabolic kinetics at 1250 ^oC and all the scale formed on the alloy spall out after cooling.The mass gain of the alloy after oxidation at 1250 ^oC for 100 h reaches up to 180 mg/cm².The oxidation process at 900 ^oC of the alloy involves an initial parabolic stage（0～50 h）and a rapid linear stage（after 50 h）.Meanwhile,“pest”degradation phenomenon is observed on the alloy surface after oxidation at 900 ^oC for 100 h.The hot corrosion process of the alloy contains two stages:an initial parabolic stage（0～20 h）and a rapid linear stage（after 20 h）.Moreover,catastrophic scale disintegration occurs after hot corrosion at 900 ^oC for 100 h.The Zr based alloy（Zr-20Ti-5Al,at.%）and Ti based alloys Ti-5Al-10Nb-x Zr（x=0,2,5,10,15 and 20 at.%）were prepared by vacuum high frequency induction melting.Silicide coatings were respectively prepared on these alloys via Si-Y co-deposition.The silicide coating on the Zr based alloy is mainly composed of a thick（Zr,Ti）Si₂ layer and a fairly thin（Zr,Ti）Si inner layer.Oxidation results reveal that severe spallation phenomenon occurs on this coating specimen after oxidation at 800 ^oC for 1 h,indicating that ZrSi₂ is not appropriate as oxidation-resistant coatings to protect Nb-Si based alloys from oxidation.The structure and oxidation resistance of coatings prepared on the Ti based alloys shows a significant change with increasing Zr contents in the alloys.0Zr coating has a three-layer structure,i.e.,a TiSi₂ outer layer,a relatively thin TiSi middle layer and a Ti₅Si₄ inner layer;With increase in Zr content from 2 to 10 at.%,the coatings include a（Zr,Ti）Si₂+（Ti,Zr）Si₂ outer layer,a（Ti,Zr）Si+（Ti,Zr）Si₂ or（Zr,Ti）Si+（Ti,Zr）Si₂ middle layer and a（Ti,Zr）₅Si₄ inner layer;The coating specimens15 Zr and 20 Zr consist of a（Zr,Ti）Si₂+（Ti,Zr）Si₂ outer layer and a（Ti,Zr）₅Si₄ inner layer.Isothermal oxidation results reveal that 2Zr coating specimen shows a better oxidation resistance than any other coating specimen,adding 2 at.%Zr in the substrate alloy can facilitate the formation of a dense SiO₂ layer,while excessive Zr in the coating will lead to the deterioration of the scale adhesion performance.A TiSi₂-NbSi₂ composite coating was prepared on Nb-Si based ultrahigh temperature alloys via firstly magnetron sputtering Ti-5Al-10Nb-2Zr film and then Si-Y co-deposition.The coating is mainly composed of a TiSi₂ outer layer,a（Nb,X）Si₂inner layer（X represents Ti,Zr,Cr etc.elements）and a fairly thin（Ti,Nb）₅Si₄transitional layer as well as（Cr,Al）-rich zone.The formation and growth of the coating may mainly rely on the inward diffusion of Si during pack cementation process.At the initial stage,the Ti₅Si₄ layer forms preferentially by Si atoms permeating into the alloy film and reacting with it.After a short period of holding at the pack cementation temperature,（Ti,Nb）₅Si₄ transitional layer forms by Si atoms reacting with the substrate.With prolonging of the holding process,since the Ti₅Si₄ layer has completely transformed into the Ti Si₂ outer layer,the coating thickening is dominated by the growth of（Nb,X）Si₂ layer.The coating can effectively protect the Nb-Si based alloy from oxidation at 1250 ^oC for at least 100 h and the scale is mainly composed of TiO₂,SiO₂ and Cr₂O₃.However,the TiSi₂ outer layer has completely degradated into a porous Ti₅Si₄ layer after oxidation for 20 h and the consumption of the coating depends on the deagradation of（Nb,X）Si₂ inner layer during the later oxidation process.The coating has a superior hot corrosion resistance than the simple silicide coating（Si-Y coating）and the mass gain is 4.1 mg/cm² after exposure of 100 h.The scale consists mainly of TiO₂ and amorphous silicate.Meanwhile,NaNbO₃ phase is found in the scale after exposure of 100 h.The presence of Zr in the scale can restrain sulphur and chlorine from reacting with other elements,and suppress the formation of Nb₂O₅ at the scale/coating interface.The average critical loads for adhesion performance tests of as-prepared and annealed Ti-Al-Nb-Zr film specimens and TiSi₂-NbSi₂ composite coating are respectively about 9.5,15.6 and 77 N,indicating that the composite coating possesses excellent adhesion.A（Nb,Zr）Si₂-NbSi₂ composite coating was prepared on Nb-Si based ultrahigh temperature alloys via firstly magnetron sputtering Nb-22Ti-5Al-2Zr-15Si film and then Si-Y co-deposition.The coating consists mainly of a（Nb,Zr）Si₂ outer layer（by inward diffusion of Si into the sputtered film）,a fairly thicker（Nb,X）Si₂ inner layer（by inward diffusion of Si into the substrate）and a thin transitional layer.The concentration of Zr in the outer and inner layer is respectively 0.58～0.67 at.%and 0.31～0.35 at.%.The composite coating exhibits an excellent oxidation resistance at 1250 ^oC and the mass gain of the coating specimen after oxidation for 100 h was only 1.6 mg/cm².The scale is compact and dense,and is mainly composed of TiO₂ outer layer,SiO₂ matrix layer and Cr₂O₃ inner layer.Zr exsits in the form of（Y,Zr,Ti）O_x particles and solution in the scale.Adding a minute amount of Zr could promote the formation of a dense and continuous SiO₂ scale and retard outward diffusion of metal ions.Meanwhile,the tiny Zr-rich oxides at the scale/coating interface could improve the scale adhesion and ameliorate the interface defects to some extent.The hot corroded scale is mainly composed of NaNbO₃,TiO₂ and amorphous silicate.The average values of the critical load of the adhesion performance tests for Nb-Ti-Al-Zr-Si alloy film,the composite coating and oxidized coating specimens are found to be 13.2,94.6 and 21.6 N,respectively.