Fabrication And Properties Of The Silicon Carbide Based Coatings Via Gel-Casting And Gas Silicon Infiltration

In the background of the application of the C/SiC mirror,the fabrication of the Si/SiC optical coating is studied in this paper.A combination of gel-casting(GC)and gas silicon infiltration(GSI)is firstly used in the preparation of the optical coating in order to solve the problem of inhomogeneity,low-roughness and low-thickness.The problem of the gelation reaction with carbon black is solved and the gel-casting process is successfully used in the preparation of the coating,the ceramic and the C/SiC joint technology.Effects of the different variables on the GC-GSI process are also studied.Effect of the carbon black on the gelation reaction is studied and a process optimization is carried out.Results show that carbon black has both capturing and accelerating effect on radicals revised by the initiation reaction.In this study,a new initiator AIBA is selected to solve the problem above.The initiation temperature of gelation reaction was 70?,the amount of initiator was 20?l per gram of slurry,the conversion rate of AM was 98.77%,and the dwell time was 50 min.The mechanism of GSI is explored,and the silicon content used in the process is also studied.There are two steps for GSI,firstly,gaseous Si is adsorbed by C and ?-SiC is formed on the surface of the ?-SiC;secondly,C transfers through the ?-SiC layer and the reaction occurs,?-SiC grains with less than 0.5?m are formed.The reaction is not complete when the silicon content is insufficient,on the contrary,the excessive residual silicon content in the samples can cause volume expansion.During the preparation of the coating,the silicon content is 0.8 times the mass of the green body.The one-step method and two-step method are studied by measuring the bonding strength between the coating and the composite.The coatings by both the methods show good bonding properties,the shear strength are over 20 MPa.However,the coated composite by one-step method bonds to each other tightly,cracks occurs during the preparation of the middle-size mirror;the other one is more suitable to apply in the process of large-size mirror.The GC-GSI process can be also applied in the joint technology.Effects of the AM content on the process of GC-GSI are studied.When the AM content is insufficient,a complete polymer net cannot be formed,defects occur in the green body and cannot be filled in the GSI process,and silicon deposits in the micro cracks,which lead to large shrinkage of green body and low mechanical properties of the ceramic.Excessive AM content is also detrimental to the properties of the samples,the viscosity of the slurry increases dramatically.The AM content of 25 wt.% is choosed for the preparation of the coating.Effects of the particle size distribution on the process of GC-GSI are studied.When pH=9~11,the Zeta potential of the slurry is around-65 mV,corresponding to the TMAH content of 0.6~0.7wt.% for the ceramic powder.When F240/F1200=1.5,SiC powders come to the most compact stack state,and it can be considered that coarse powders are closely packed,fine particles are filled in the space homogeneously.In this case,there is no particle aggregation of only F240 or F1200,the viscosity of the slurry come to a minimum,and the sample shows the best comprehensive performance after GSI process.Effects of the solid content on the process of GC-GSI are studied.When the solid content is low,the green body shows high shrinkage and cannot stack compactly,leading to the low-density and high-porosity;when the solid content is over high,particles show uneven distribution in slurries,bubbles cannot be degassed,the high-viscosity slurries cannot be spreading smoothly.During the preparation of the coating,a solid content of 60 vol.% is selected.Effects of the carbon black content on the process of GC-GSI are studied.The carbon shows good dispersion property when the dispersant PVP K30 content is 0.5wt.%.With the increase of the carbon black content,the viscosity of the slurry increases,the linear shrinkage of the green body decreases and the density increases.The composition of the green body can be designed to influence the composition after GSI process.The higher the carbon black content is,the density and porosity of the green body is more close to theoretical value,and the SiC content is higher after GSI process.The continuous phase changes from Si to ?-SiC,the capacity to resist bending strength and thermal shock becomes higher.Considering the fluidity of slurry and the thermal match with the composites,during the preparation of the coating,a carbon black content of 15 wt.% is selected.The optimized GC-GSI process is used to prepare ?160mm Si/SiC coated C/SiC composite mirror with excellent optical properties,RMS=0.024?,Ra=0.57 nm.