| Based on their good mechanical performances,potential uses of structural carbon materials have been identified in high temperature applications.Many applications require extended periods ranging from a few hours(at high temperature)to a few thousands of hours(at low temperature)in oxidizing environments where unprotected carbon/carbon composites deteriorate rapidly.Therefore numerous investigations have been conducted in an attempt to protect carbon-carbon composites against oxidation.The main protection mechanism relies on the use of a barrier against oxygen diffusion.The problem,when applying this concept,is the thennal mismatch between coating and carbon substrate which results in cracking of the coatings.Thereby,oxygen diffuses through the cracks,causes oxidation of the underlying carbon which leads to the spallation of the coatings.Therefore,multilayer coatings including inner and outer layer have been developed to protect Carbon/Carbon composites from oxidation.In the multilayered coatings,MoSi2 makes up the outer layer while SiC forms the inner buffer layer.The peripheral multilayer coating containing MoSi2 exhibits an excellent oxidation protective ability for Carbon/Carbon composites at 1500-1600?.However,generatiol of micro?cracks in the MoSi2 coating cannot be competent for a long time at 800-1000? due to the mismatch of thermal expansion coefficient between the SiC bonding layer and the MoSi2 outer coating.Therefore,the protective temperature range of multilayer coating containing MoSi2 was too narrow that limits its protection ability at high temperature.The coefficient of thermal expansion of MoSi2(?8.1×10-6/K)is much larger than that of Carbon/Carbon composites(?1.0×10-6/K).Therefore,the mismatch between CTEs of the outer MoSi2 layer and the inner SiC coated C/C matrix causes degradation of the coating under thermal cycling and ultimately lowers the coating durability.Another major reason of coating failure is the 'pest phenomenon',occurring due to its incompatibility between the matrix and the coating.To overcome this problem,A MoSi2-Si3N4/SiC multilayer coating was prepared on carbon/carbon composites to improve their oxidation resistance at different temperatures from]273-]873K respectively.The Carbon/Carbon composites were coated with a SiC inner layer and a MoSi-Si3N4 outer layer by the slurry and pack cementation method.Carbon/Carbon composites were coated with SiC inner layer by pack cementation method while,MoSi2 and Si3N4 were used as outer layer prepared by slurry sintering method.At high temperature,Si3N4 was found to play an important role to increase the oxidation and thermal shock resistance as a result of the formation of a dense SiO2 layer.Due to small size and good fluidity of SiO2,it can seal all the micro-cracks formed during the volatilization of MoO3,CO2 and N2.The phase composition,microstructure and spot elements of the coating were analyzed by SEM,XRD,EDS and Raman spectra.Results showed that as prepared coating was dense and homogeneous with no more cracks or cavities on the surface of coating.In addition,XRD results confirmed the presence of different peaks of coated materials before and after oxidation test.The Raman spectra result also confirms the formation of SiO2 in the MoSi2-Si3N4 coating after oxidation at 1773K.It was found that peak with wave number of 228 and 414 cm-1 is the cristobalite while the wave number of 1034 cm-1 is the amorphous silica,which is accordance with the XRD result.To evaluate the oxidation protective ability of coating,oxidation resistance tests were conducted at different time and temperatures.Oxidation tests show that the multilayer coating could protect the carbon/carbon matrix at 1273-1773K for 50-150h with a minute loss and that coating could withstand 40 thermal cycles between 1773K and room temperature.The weight loss of the coated carbon/carbon composites has been ascribed to the formation of micro-cracks and volatilization of SiO2,MoO3,and N2 in the coating at high temperatures.The purpose of this work is to examine the oxidation of carbon-carbon composites and to evaluate the inherent limitations of the various current coatings.Recently due to these limitations,composite coatings have been studied.It is evident from the results the multilayer coatings proved an excellent oxidation and thermal shock resistance. |
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