C / C Composite Study Of The Sic Coating

Although C/C composites and graphites have a series of excellent properties such as low density,high temperature resistance,etc.,they are limited to be used in high temperature situation for their poor oxidation resistance and weak erosion resistance.Coating protection is regard as one of the most important resolution schemes.Because SiC coating has high rigidity preferable high temperature intensity, favourable heat conductance , thermal stability and good chemical compatibility with C/C,it is very important in practice to launch research on SiC coating for C/C composites and graphites.In this paper,SiC coating was prepared by different methods(paste coating,dip coating and vapor reaction coating) on the surfaces of C/C composites and graphites.Paste coating and dip coating are both ordinary methods in wet technics,whose key technology is to make suitable slurry with appropriate floating stability and rheology,which can be obtained by adjust its PH value and density. The best coating prepared by paste method can coat C/C through high temperature heat treatment with simple slurry which was made of silica sol and Si powder. There are more control parameters in dip coating technics,and the SiC coating thickness can be foresaw via controlled dip weight because dip weight was relative with slurry density and varieties of C/C in a given period of time.The best parameters of slurry for dip coating were PH=9 and density of 1.62 gcm-3.Vapor reaction coating (VR coating) is a more complex technology. The compact coatings by vapor reaction (VR) were gained on the surface of C/C at different temperature by introducing SiO vapor ,which generated by Si and auxiliary SiO₂ at high temperature in inertial atmosphere, into reaction chamber, where it occurred VR with C/C through diffusion. The best VR coating was obtained by the following technics: room temperature- 1700/℃2hrâ†'1700℃/lhrâ†'1700℃-1900 ℃/lhrâ†'1900℃/lhr , which is thought of a kind of perfect coating not only because of its continuous distribution and uniformity of SiC tissues and eliminating cracks produced during period of preparing process by better graded distrubution,but also because of the SiC whiskers preparedaccompanyly in the coating.In a further investigation,SiC whiskers with much larger slenderness ratio were also prepared in the VR coating by soaking C/C composites and graphites,the parent materials,in Sol of Fe3+ or Co2+ or Ni2+ for a preset time .The performance of these coatings was comprehensively checked by heat shock resistance (HSR) and oxidation resistance(OR).It showed that the VR coating has a best HSR.The HSR is affected by SiC whiskers and grains of coating together because of their affecting the flexural strength of coating materials?the collapse of SiC whiskers weakened the flexural strength while recrystallization of grains swelled it. The HSR of coating with rich SiC whiskers was more excellent than any other ones either in rapid or slow cooling HSR condition,while those with poor SiC whiskers showed its high HSR only in rapid cooling condition. The OR of VR coatings is defferent due to technics,the VR coatings without SiC whiskers have the best OR in the initial stage while those with SiC whiskers which were not obtained by catalytic reaction have the best OR in the final stage.The mass loss ratios of dip and VR coating were different from each other during process of oxidation resistance,the former underwent a period of mass increase which indicating a best OR in the initial stage while the latter was always in mass loss stages.