Oxidation Resistance Of Rare-earth-oxide Modified MoSi₂-Based Coating For C/C Composites

Carbon/carbon（C/C）composites are currently one of the few candidate materials for applications above 1600℃benefiting from its excellent properties such as light weight,high strength,good resistance to friction and thermal shock,low thermal expansion coefficient,high thermal conductivity and dimensional stability,etc.However,the oxidation sensitivity of C/C composites is becoming the biggest bottleneck restricting its application in an aerobic environment.At present,employing the surface coating is the most effective means for solving this problem.MoSi₂ is a preferred protective coating material for C/C composites due to its high melting point,high thermal stability,and the formation of a Si O₂ glass protective layer during oxidation at elevated temperatures.It is known that the high-temperature protection capability of MoSi₂ coating mainly derives from the Si O₂ glass film formed by oxidation to block the penetration of oxygen,but Si O₂ glass usually has good fluidity and stability only in the temperature range of 1200-1800℃,exhibiting a relatively narrow protection temperature domain.Meanwhile,during the thermal cycles between high and low temperatures,MoSi₂ is prone to phase transformation,and is accompanied by brittle cracking problems at meso-low temperature as well as“pesting”powdering oxidation behavior,which greatly limits its applications in the field of long-term oxidation resistance with a wide temperature range.To broaden the serviceable scope of MoSi₂coatings,this project selected rare earth oxides as modifiers and applied supersonic atmospheric plasma spraying to prepare a variety of rare-earth-oxide modified MoSi₂-based composite coating systems,and their antioxidant performance was tested in multiple oxidizing environments.XRD,SEM,EDS,XPS,TEM,scratch experiment and roughness test were employed to characterize and analyze the phase composition,morphology characteristics,element distribution and mechanical properties of MoSi₂-based coating with rare-earth modification before and after oxidation.The effects of rare earth oxides on the microstructure and oxidative protection performance of MoSi₂coatings were investigated.The mechanism of high-temperature oxidation prevention of MoSi₂-based coatings with rare-earth-oxide modification was also further analyzed.The main research contents and conclusions of this paper are summarized as follows.A single-phase MoSi₂ coating was fabricated using the high-efficiency supersonic atmospheric plasma spraying（SAPS）method,and the main process parameters were optimized and adjusted.The effects of spraying power,primary gas（Ar）flow rate and powder feeding rate on the microstructure of the coating was investigated.The results shown that the coating prepared with 50-55k W（spraying power）,75-90L/min（primary gas flow rate）and20-30g/min（powder feeding rate）possessed a good overall quality with low porosity,dense structure and good interface combination.The thermogravimetric curve of the MoSi₂ coating appeared an inflection point around 750℃,and then the weight loss rate increased rapidly,suggesting that the“pesting”powdering oxidation behavior occurred,and the final weight loss reached 7.21%.At 900℃,1200℃,1500℃static environments,with the increasing of oxidation temperature,the fluidity of the Si O₂ glass phase formed on the coating surface elevated,together with an enhanced ability to heal defects,which was easier to develop a continuous Si O₂ glass film.Furthermore,MoSi₂ was accompanied by the transition from low-temperature brittleness to high-temperature plasticity,which was beneficial to suppress the formation of penetrating cracks in the coating,thereby improving the oxidation resistance of the coating.Therefore,the service life of the coating was extended from 15h at 900℃to50h at 1500℃.Considering that the drawbacks of low-temperature“pesting”pulverization oxidation for MoSi₂ coating and short protection time at elevated temperatures,an innovate concept that modifying MoSi₂coating with rare-earth elements was creatively proposed,and rare-earth Y₂O₃ was firstly introduced into MoSi₂ coating by SAPS process to obtain a Y₂O₃-MoSi₂composite coating.This study found that the introduction of rare-earth Y₂O₃effectively relieved and even inhibited the phase transformation,high-temperature oxidation and decomposition behavior of MoSi₂ in the coating during spraying,which increased the density and interface bonding strength of the coating.The temperature corresponding to the inflection point in the thermogravimetric curve of the MoSi₂-Y₂O₃ coating increased to around 1120℃,that is,the“pesting”pulverization oxidation behavior of MoSi₂ was alleviated to a certain extent,and the final weight loss of the coating was reduced to 2.36%.In 1500℃and 1700℃static air,owing to the formation of Si-O-Y multiphase glass layer,the operating duration of MoSi₂-20wt.%Y₂O₃ coating was respectively extended to 100h（1500℃）and 12h（1700℃）,and the corresponding mass loss was 1.92%and 13.74mg/cm²,exhibiting better oxidation resistance than single-phase MoSi₂ coating.To further investigate the modification action of lanthanides on MoSi₂ coating,rare-earth Yb₂O₃ and La₂O₃ were selected as modifiers,and Yb₂O₃-MoSi₂ and La₂O₃-MoSi₂ composite coatings were prepared by SAPS method.The effects of Yb₂O₃doping content on the microstructure and oxidation resistance of Yb₂O₃-MoSi₂ coating were studied as well as the evolution behavior of La₂O₃-MoSi₂ coating in multiple oxidation environments.Studies have shown that the incorporation of rare-earth Yb₂O₃ and La₂O₃ decresed the number of pores in the coating,together with an increased density and bonding strength of the coating,showing an enhanced oxidation resistance relative to the single-phase MoSi₂ coating.During the high-temperature oxidation process,due to the presence of rare-earth Yb₂O₃,a Si-O-Yb multiphase glass layer with Si O₂ glass as the main phase and embedded with high melting point oxides such as rare-earth silicate Yb₂Si₂O₇ was produced on the coating surface.Also,the rare-earth La₂O₃ promoted the formation of Si-O-La multiphase glass layer inlaid with high melting point phases involving rare-earth lanthanum silicates（La₂Si O₅,La₂Si₂O₇）.They can not only stabilize the Si O₂ glass phase to reduce its high-temperature volatilization consumption,but also effectively suppress the development of longitudinal penetrating cracks within the coating,thereby blocking the oxygen diffusion and enhancing the high-temperature service capability of the coating.Aiming at the issues that monobasic rare earth oxides are enormously easy to agglomeration together with a limited effect when modifying MoSi₂ coating,the application of binary rare earth oxides to play a synergistic role among different rare-earth elements is expected to make up for the modification deficiency of single rare earth oxide.Therefore,the binary rare-earth modified MoSi₂-based coatings such as MoSi₂-Y₂O₃-La₂O₃（MYL）,MoSi₂-Yb₂O₃-La₂O₃（MYb L）,MoSi₂-Y₂O₃-Yb₂O₃（MYYb）were prepared using SAPS method.The results shown that the service life of MYL,MYb L,MYYb coatings at 1500℃and 1700℃were respectively increased to 150h and 18h,superior to the unitary rare-earth modified coatings.This phenomenon can be mainly attributed to the following two aspects.For one thing,a variety of high melting point rare-earth silicates such as Y₂Si O₅,La₂Si₂O₇,Yb₂Si₂O₇,Y₂Si₂O₇,La₂Si O₅ produced during the oxidation process with Si O₂ glass phase forming Si-O-Y-La,Si-O-Yb-La,Si-O-Y-Yb multiphase glass layer,which combines the excellent characteristics of Si-O-Y,Si-O-Yb and Si-O-La glass,effectively reducing the volatilization consumption of Si O₂ glass phase,thus improving the high-temperature stability of the composite glass layer.For another,the solid solutions(（La₂Mo₂O₉）_0.889,Yb₂（Mo O₄）₃,La₁₂Mo₆O₃₅,Y₆Mo O₁₂,La₂Mo₃O₁₂)derived from the reaction between rare earth oxide and Mo O₃with low melting point possess a good stabilizing effect on Mo O₃,and then decreasing the severe volatilization of Mo O₃ at elevated temperatures,exhibiting a more durable oxidation protection capability.