Preparation And Properties Of The Ablation-resistant And Corrosion-resistant Coatings On Molybdenum Metal Surface

In this paper,MoSi2/SiO2 and Al-modified MoSi2/Al2O3 coatings obtained by halide-activated pack cementation combined with preoxidation method were prepared on molybdenum substrate by using SK-G08123K tube atmosphere furnace and SX2-4-13GJ muffle furnace.The coating structure and formation mechanism were investigated by means of an optical microscope,scanning electron microscope,X-ray diffractometer,and electron probe.Thereafter,the ablation resistance and corrosion resistance were evaluated using a GTD111 superalloy.Based on microstructure characters and analysis,the failure mechanism was proposed during the casting process.The MoSi2 coatings with different thicknesses were oxidized at 1300?/20 h.After oxidation,the coating is composed of SiO2,MoSi2,Mo5Si3,and Mo3Si.The phase composition of the oxidized coating varies with the MoSi2 thickness,and its structure determines the service life.The coating structure of 40 ?m MoSi2 oxidized at 1300?/10 h consists of SiO2,MoSi2,and Mo5Si3,and SiO2 is a cristobalite phase.In the MoSi2/SiO2 coating,the cristobalite can resist the corrosion of GTD111 alloy melt,as well as accompanied by the cristobalite layer partially destroyed at the core/superalloy interfaces.Due to the chemical reactions between GTD111 alloy and quartzite,tensile stress is formed during the alloy solidification stage,inducing the failure of the cristobalite layer.The Al-modified MoSi2 coating is composed of Mo(Si,Al)2,Al8Mo3,and AlMo3.The Al8Mo3 is first formed on the molybdenum surface.Then the diffused Si replaces the Al site of the Al8Mo3 to form Mo(Si,Al)2,and Al8Mo3 degenerates to AlMo3 in the front part of Mo/Al8Mo3 interface.The microstructure evolution of as-selected 44 um Al-modified MoSi2 coating oxidized at 1300? is systematically investigated.In the Mo(Si,Al)2/Al8Mo3 interface,the mutual diffusion of Si and Al makes Al8Mo3 form Mo5(Si,Al)3.In the Al8Mo3/Mo interface,Al8Mo3 decomposes AlMo3 due to the high Al diffusion potential energy.The Al-modified MoSi2/Al2O3 coating without Al8Mo3 phase can resist the corrosion of GTD111 alloy melt.However,the Al-modified MoSi2/Al2O3 coating with Al8Mo3 phase coating is seriously destroyed during the cast process.The failure of the coating is a result of the low melting point Al8Mo3 phase.This work plays a key role in promoting the applications of refractory metal core and lays a foundation for turbine blades with the higher efficient air-cooled structures.