Preparation of Multilayer Coatings to Protect Carbon-Carbon Composites from Oxidation.pdf

1、Based on their good mechanical performances, potential uses of structural carbon materialshave been identified in high temperature applications.Many applications require extendedperiods ranging from a few hours (at high

2、temperature) to a few thousands of hours (at lowtemperature) in oxidizing environments where unprotected carbon/carbon composites deterioraterapidly.Therefore numerous investigations have been conducted in an attempt to

3、protect carbon-carbon composites against oxidation.The main protection mechanism relies on the use of abarrier against oxygen diffusion.
　　The problem, when applying this concept, is the thermal mismatch between coati

4、ng andcarbon substrate which results in cracking of the coatings.Thereby, oxygen diffuses through thecracks, causes oxidation of the underlying carbon which leads to the spallation of the coatings.
　　Therefore, multil

5、ayer coatings including inner and outer layer have been developed to protectCarbon/Carbon eomposites from oxidation.In the multilayered coatings, MoSi2 makes up theouter layer while SiC forms the inner buffer layer.
　

6、　The peripheral multilayer coating containing MoSi2 exhibits an excellent oxidationprotective ability for Carbon/Carbon composites at 1500-1600℃.However, generation of micro-cracks in the MoSi2 coating cannot be competen

7、t for a long time at 800-1000℃ due to themismatch of thermal expansion coefficient between the SiC bonding layer and the MoSi2 outercoating.Therefore, the protective temperature range of multilayer coating containing MoS

8、i2 wastoo narrow that limits its protection ability at high temperature.The coefficient of thermalexpansion of MoSi2 (～8.1×10-6/K) is much larger than that of Carbon/Carbon composites(～1.0×10-6/K).Therefore, the mismatch

9、 between CTEs of the outer MoSi2 layer and the innerSiC coated C/C matrix causes degradation of the coating under thermal cycling and ultimatelylowers the coating durability.Another major reason of coating failure is the

10、 '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 oncarbon/carbon composites to improve their oxi

11、dation resistance at different temperatures from1273-1873K respectively.The Carbon/Carbon composites were coated with a SiC inner layerand a MoSi2-Si3N4 outer layer by the slurry and pack cementation method.Carbon/Carbon

12、composites were coated with SiC inner layer by pack cementation method while, MoSi2 andSi3N4 were used as outer layer prepared by slurry sintering method.At high temperature,Si3N4was found to play an important role to in

13、crease the oxidation and thermal shock resistance as aresult of the formation of a dense SiO2 layer.Due to small size and good fluidity of SiO2, it canseal all the micro-cracks formed during the volatilization of MoO3, C

14、O2 and N2.The phasecomposition, microstructure and spot elements of the coating were analyzed by SEM, XRD, EDSand Raman spectra.Results showed that as prepared coating was dense and homogeneous withno more cracks or cavi

15、ties on the surface of coating.
　　In addition, XRD results confirmed the presence of different peaks of coated materialsbefore and after oxidation test.The Raman spectra result also confirms the formation of SiO2 inth

16、e MoSi2-Si3N4 coating after oxidation at 1773K.It was found that peak with wave number of228 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 re

17、sult.To evaluate the oxidation protective ability of coating,oxidation resistance tests were conducted at different time and temperatures.Oxidation testsshow that the multilayer coating could protect the carbon/carbon ma

18、trix at 1273-1773K for 50-150h with a minute loss and that coating could withstand 40 thermal cycles between 1773K androom temperature.The weight loss of the coated carbon/carbon composites has been ascribed tothe format

19、ion of micro-cracks and volatilization of SiO2, MoO3, and N2 in the coating at hightemperatures.
　　The purpose of this work is to examine the oxidation of carbon-carbon composites and toevaluate the inherent limitatio

20、ns of the various current coatings.Recently due to these limitations,composite coatings have been studied.It is evident from the results the multilayer coatingsproved an excellent oxidation and thermal shock resistance.<