Preparation And Properties Of MoSi₂-based High Temperature Oxidation Resistant Coating

Niobium-based superalloys have received a great deal of attention due to their high melting points,low densities and good mechanical properties at elevated temperatures.However,their poor oxidation resistance has been a major barrier to their application.Molybdenum disilicide(MoSi₂)is a promising material that is resistant against high-temperature oxidation due to the formation of a continuous,adherent and self-healing glassy silica(SiO₂)layer on the surface.Thus,molybdenum disilicide coatings have been added on the surface of superalloys to prevent the permeation of oxygen at high temperatures.While there are several problems during the application of MoSi₂ coatings:(1)MoSi₂ coatings produced by different methods possess different oxidation capacities,so the proper coating deposition method need to be established firstly,and the preparation parameters should be adjusted;(2)The "PEST" oxidation problem at low temperatures.The "PEST" oxidation phenomenon was reported previously,but the oxidation mechanism is unsatisfactory;(3)Because of the low toughness as well as the CTE mismatch between the coating and the substrate,it will introduce cracks into the MoSi₂ coating;(4)Si diffuses rapidly in the MoSi₂ coating at high temperature,and the MoSi₂ phase transforms into lower silicide,consequently,the oxidation resistance is weakened.On account of the above problems,we developed a two-step method to prepare the MoSi₂ coating and the(Mo,W)Si₂ coating:In the first step,Mo layer and MoW layer were deposited by the arc ion plating(AIP)method,then MoSi₂ and(Mo,W)Si₂ coatings were prepared by pack cementation of Si.Isothermal oxidation temperatures were 500?,800?,1000?,1200?,1350? and 1500?,and the thermal shock temperature was 1500?.The main conclusions were as below:After analysis of the effect of the key parameters of the AIP process on the structures of the Mo layer,we can obtain a Mo layer with good surface morphology and high deposition rate under the experimental parameters of arc current 120 A,Ar pressure 1.5Pa direct bias voltage-50V,pulsed bias voltage-100V.The SiO₂ film formed on the surface of the MoSi₂ coating is not compact at low temperatures of 500? and 800?,and the selective oxidation is inhibited,which results in the weight loss of the coating.The SiO₂ film is continuous and compact if the oxidation temperature is above 1000?,and the selective oxidation will carry out at the interface of the coating and the SiO₂ film,and the oxidation resistance of the oxidation film is good.When the SiO₂ film forms on the MoSi₂ coating,the oxidation resistance of the coating depends on the oxidation resistance of the cracks and the degradation rate of the coating.By analyzing the oxidation resistance of the coatings with two different structures,we find that the selective oxidation types are different at the interface of the oxidation film and the coating for the two types coatings.Diffusion rate of Si and degradation of the coating is slow if the diffusion barrier exists in the coating.Attributed to the CTE mismatch between the coating and the substrate,thermal stress exceeds the strength of the coating,and it will bring cracks into the coating.During the isothermal oxidation process at 1500?,Si diffuses rapidly not only towards the coating surface to form SiO₂,but also into the substrate to form lower silicide phases,which will accelerate the degradation of the MoSi₂ coating.Mo3Si phase forms on the surface of the coating after oxidation for 10 h,which reduces the oxidation resistance.After 1500?thermal shock for 100 times,amounts of cracks attribute within the coating.After comparing the oxidation properties of the(Mo,W)Si₂ coatings with different W contents with the oxidation resistance of MoSi₂ coating,it is concluded that:a(Mo,W)O3·SiO₂ oxide film with stable structrue formed on the coating during isothermal oxidation at 500?,the oxide prefer to form at the grain boundaries and in the cracks,which will cause large stress,besides,oxygen diffuses through the oxide film to the crack tip causing oxygen brittleness,which will accelerate the crack propagation,and the "PEST" oxidation occurs at last.(Mo,W)Si₂ coating degraded during isothermal oxidation at 1000?,and the degradation direction is:(Mo,W)Si₂?(Mo,W)5Si3?(Mo,W)3Si.During isothermal oxidation at 1200?,Si of the(Mo,W)Si₂ coating prefers to diffuse along with the grain boundaries,and lower silicide phases prefer to nucleate and grow at the grain boundaries.After isothermal oxidation at 1500?for 16h,the coating surface degraded into(Mo,W)5Si3 phase,and the degradation rate is lower than that in MoSi₂ coating.There exists large size crack in the coating after oxidation,and the thickness of the coating near the large crack decreased,which leads to lower oxidation resistance.After 1500? thermal shock for 100 times,a lot of voids distributed in the coating,which is beneficial for the coating's toughness.The average crack number of the(Mo,W)Si₂ coating is far lower than that of the MoSi₂ coating after the thermal shock.