Effects Of Silicon And Nickel Addition On High Temperature Oxidation Resistance And Mechanical Properties Of Ti(Cr)AlN Coatings

Nitride coatings have been widely used in machining,aerospace,automotive,chipmaking and other industries due to their high hardness,excellent wear resistance and good chemical stability.Efforts on achieving higher hardness and toughness,enhanced thermal stability and high temperature resistance of the nitride coatings by modification of the composition and the microstructure of the coatings have never been stopped and are always the focus of the researchers.In this dissertation,silicon modified TiAlN and CrAlN coatings were fabricated to protect γ-TiAl based alloys from high temperature oxidation.The addition of silicon to the coatings not only promoted the formation of protective oxide scale on the coatings at the initial stage of oxidation but also helped the scale maintain protectiveness during the long-term oxidation,so decreased the oxidation rate of the coatings.Furthermore,the Si addition to the TiAlN coating also promoted the formation of a Ti5Si3 diffusion barrier layer at the coating/γ-TiAl based alloys interface,which inhibited the inner diffusion of nitrogen from the coating to the substrate alloy and helped the coating maintain long-term stability at high temperature.Nickel was added to TiAlN coating to improve the mechanical properties of coating.The influences of nickel on the mechanical properties of TiAlNiN coating were studied.Firstly,the effects of silicon on the oxidation resistance of the TiAlSiN coatings deposited on γ-TiAl based alloys were studied.The multi-arc ion plating system was used to deposit Ti0.50Al0.40Si0.10N,Ti0.50Al0.45Si0.05N and Ti0.50Al0.50N coatings on the surface of γ-TiAl based alloys.Ti0.50Al0.50N coating showed poor oxidation resistance.The oxidation resistance of the Ti0.50Al0.45Si0.05N coating was better and that of the Ti0.50A10.40Si0.10N coating was significantly better than that of Ti0.50Al0.50N coating.Especially,after oxidation for 1000 cycles at 900℃,the oxide scale formed on the Ti0.50Al0.40Si0.10N coating was still thin,dense and adherent.The excellent oxidation resistance of the Ti0.50Al0.40Si0.10N coating was mainly attributed to the addition of silicon in the coating promoted the selective oxidation of Al at the initial stage of oxidation and the formation of the outer continuous α-Al2O3 layer on the surface of the coating which hindered the internal diffusion of oxygen.During the long-term oxidation process,the aggregation of SiO2 at the columnar grain boundaries of the a-TiO2 layer in the oxide film inhibited the growth of a-TiO2 grains and their transformation to rTiO2 and ensured long-term stability of the above α-Al2O3 layer.The formation of SiO2 layer at the oxide film/coating interface and the segregation of SiO2 at the degraded coating and the a-TiO2 layer inhibited the internal diffusion of oxygen and the external diffusion of titanium and reduced the oxidation rate of the coating.Another reason for the excellent high-temperature oxidation resistance of Ti0.50Al0.40Si0.10N coating at 900℃ is the in-situ formation of Ti5Si3 diffusion barrier at the coating/matrix interface at the initial stage of oxidation due to inner diffusion of silicon from the coating to TiAl alloy.The Ti5Si3 diffusion barrier layer was stable and firstly found that could effectively hindered the internal diffusion of N element in the coating and the external diffusion of Cr,Nb,Mn and other elements in the alloys during the long-term cyclic oxidation process at 900 ℃.The main reason that the Ti5Si3 layer prevented nitrogen diffusion into the alloy is that the Gibbs free energy for the nitriding of Ti5Si3 is higher than that for TiAl.Also,the substitutional dissolution of Mn,Nb and Cr in the Ti5Si3 layer inhibited their outer diffusion to the coating.Secondly,the effects of silicon on oxidation resistance of CrAlSiN coatings deposited on TiAl based alloy were studied.The multi-arc ion plating system was used to deposit Cr0.27Al0.73N,Cr0.50Al0.50N,Cr0.28Al0.62Si0.10N and Cr0.40Al0.50Si0.10N coatings on the Ti-48Al-2Cr-2Nb alloy.During cyclic oxidation at 900℃,there is a large number of cracks formed in Cr0.27Al0.73N and Cr0.28Al0.62Si0.10N coatings due to the large difference in thermal expansion coefficient of the coatings with the TiAl alloy,a large number of cracks formed in coatings during cyclic oxidation at 900℃.During cyclic oxidation at 900℃ and 950℃ a continuous outer Cr2O3 scale and an internal oxidation zone dominated by Al2O3 formed on the Cr0.50Al0.50N coatings.The internal oxidation zone thickened with the increasing of the oxidation temperature and time.After oxidation for 1000 cycles at 950℃,Cr0.50Al0.50N coating degraded seriously due to the oxidation and the inter-diffusion.The Cr0.40Al0.50Si0.10N coating showed excellent oxidation resistance during thermal cyclic process at both 900℃ and 950℃.After oxidation for 1000 cycles at 950℃,the oxide scale formed on the surface of the Cr0.40Al0.50Si0.10N coating was still thin.A continuous Al2O3 layer instead of the internal oxidation zone formed under the outer Cr2O3 layer in the oxide scale on the Cr0.40Al0.50Si0.10N coating.Although the Cr0.40Al0.50Si0.10N coating with c-Cr(Al,Si)N structure decomposed into c-CrN,c-Cr2N,Cr and h-AlN,and h-AlN phases precipitated at the grain boundaries of Cr-rich phases,the crystal grains in the coating were very fine even after 1000 cycles oxidation.Therefore,it was considered that the presence of silicon in Cr0.40Al0.50Si0.10N coating refined the crystal grains and inhibited the growth of the grains during cyclic oxidation.The outer diffusion of aluminum and the formation of the continuous Al2O3 layer was promoted,which inhibited the internal diffusion of oxygen.During the cyclic oxidation processes both at 900℃ and 950℃,serious inter-diffusion occurred at the interface of CrAl(Si)N coating/Ti-48Al-2Cr-2Nb alloy and a thick inter-diffusion zone formed.Finally,the influences of nickel on the mechanical properties of TiAlNiN coating which deposited using arc ion plating system were studied.The TiAlNiN coating presented columnar crystal structure and nickel was dissolved in c-TiN lattice.Deposition parameters such as negative bias voltage and nitrogen partial pressure have significant effects on the mechanical properties of the coating.The hardness,elastic modulus and residual compressive stress decreased and the toughness index H3/E2 of the TiAlNiN coatings increased with the increasing of nickel content.It is considered that nickel was dissolved in TiN lattice in metallic state instead of forming Ni-N bond in the TiAlNiN coating which affected the valence bond state in the coating thus affected the mechanical properties of the coating.