Correlation Between Surface Characters Of C/SiC Composites Substrate And Properties Of SiC Coating

The carbon fiber reinforced silicon carbide(C/SiC)composites have wide and promising applications fields,such as the aeronautic and astronautic engines,the high temperature thermal protection systems,for its excellent performance.However,there exists a deficiency on high-temperature oxidation resistance of C/SiC composites.It is an effective way to make further promotion on the oxidation resistance of C/SiC composites by preparing SiC coating on the surface of C/SiC substrate to seal the holes and cracks.The surface of C/SiC composites is diversiform and complicated,therefore,surface characters have a direct influence on the interface bonding strength and the thermal shock resistance of SiC coating.The correlations between the surface characters of C/SiC composites and properties of SiC coating were in investigated in this paper.The impacts of fiber texture and surface grinding on the surface characters as well as the interface bonding strength,the thermal shock resistance were highly fouced.In addition,the service character of SiC coating on the C/SiC composites nozzle was analyzed according to the application practice,and the failure mechanism was discussed.The open porosity of the needle-punching felt,carbon cloth laminated and three-dimensional braided composites fabricated by PIP process were 14.28%,15.81% and 17.43%,respectively,and the flexural strength severally were 193 MPa,236MPa and 316 MPa.The influences of the fiber textures on the surface characters of C/SiC substrates were investigated.It was found that the entire surfaces of substrates were covered by SiC ceramic lamellas;all of the three substrates had a “fur-core” structure apparently.Among them,carbon cloth laminated had a surface state with smaller and uniform SiC lamellas,and the surface of the needle-punching felt existed with amounts of pinpricks.Phases of the substrate surface were formed with SiC crystallite,free carbon and adsorbed oxygen,which were independent on fiber texture.The deposition rate of SiC coating was declined in needle-punching felt,carbon cloth laminated and three-dimensional braided order.There were amounts of pinpricks on the surface of needle-punching felt and small size SiC lamellas on the surface of carbon cloth laminated,which supplied SiC deposition with much more nucleations and growing points.Tensile method was adopted to measure the bonding strength between coating and substrate,results of the needle-punching felt,carbon cloth laminated and three-dimensional braided were 11.87 MPa,10.45 MPa and 10.13 MPa,respectively,measurements were the strength range from the “fur” layer to the “core” layer,actually.The optimalizing binding strength of the needle-punching felt was rooted in large contact area and pinning effect which formed by a mass of needle pores.The thermal shock resistance of C/SiC with SiC coating was assesed.Results showed that both the mass and bending stregth went down after 10 times thermal shock from 1600 ?(soaking 10min)to room temperature(RT),espically for the needle-punching felt.There were many thermal mismatch cracks on the SiC coating during the process,which caused effective protection on the substrate losed,C fibers oxidized,mass loss and strength degration appeared.Furthermoe,the specific pinpricks and Z-pin fibers provided much more channels for oxygen,and the damage exacerbated.SiC “fur ” layers were removed by wheel grinding,and the influences of grinding on the substrate surface characters were analyzed.The carbon cloth laminated had a higher surface flatness with lots of bare fibers.There were fewer fibers on the grinding surface of the needle-punching felt,and fibers vertical to the grinding surface existed.Combination between the grinding surface and the SiC coating was systematically investigated.SiC coating deposited on the surface of the needle-punching felt was impact and well integrated with the substrate,and there were many obvious micro-cracks on the surface of SiC coating based on the carbon cloth laminated substrate.The bonding strength of the needle-punching felt,carbon cloth laminated and three-dimensional braided substrates were 9.26 MPa,6.54 MPa and 8.35 MPa,respectively,which were inferior to the bonding strength between the “fur” layer and the “core” layer.The carbon cloth laminated had the minimum strength,which was caused by the flat grinding surface,amounts of bare fibers and the thermal mismatch between SiC coating and C fibers.The thermal shock resistance of the grinding C/SiC with SiC coating was assesed.After 10 cycles thermal shock,there was a relatively serious mass loss of the needle-punching felt and carbon cloth laminated because of C fiber oxidation and coating dropped,and the three-dimensional braided had a lighter oxidation weightless for the well-prepared SiC coating.The flexural strength of these three substrates was declined because the fiber skeletons were impaired.It was found that both the mass retention ratio and the flexural strength retention ratio were less than the substrates without wearing,and it should be avoided to wearing the surface of C/SiC composites.The service characters of SiC coating coated on the C/SiC composites nozzle was tested under the hot firing test.After tested at 1700? for total 76 s pulse program,SiC coating was oxidized and flaked partly.The primary causes of SiC coating failure at different locations of the nozzle were disparate.There were some thermal mismatch cracks at the combustor straight section.As for the contraction section,coating was torn down,substrate and carbon fiber appeared.Gone the coating at the throat,high temperature oxidation and gas current rush were the main reasons which caused the coating failure.