Investigation On Oxidative Ablation Resistance Of C_f/ZrC-SiC Composites

Hypersonic aerospace vehicles has been widespread concerned by many military power in the world for its huge military value. With the development of hypersonic aircraft, we put forward higher requirements to the material.C_f/ZrC-SiC composites become the most potential ablative material at ultra-high temperature for its high-temperature ablation, light weight, toughness, thermal shock resistance, good chemical stability, etc.Ablation resistance of materials is a key indicator of its ability to assess the application in hypersonic aircraft, so it is necessary to study ablation resistance.In this paper, we assess C_f/SiC composites and C_f/ZrC-SiC composites by oxyacetylene flame and HVOF ablation, research the impact of the ratio of zirconium and silicon in the substrate, surface coating,flame temperature and speed, ablation angle, ablation time and other environmental factors on anti-ablative material properties and ablative layer structure.The results showed that when surface temperatures of C_f/SiC composites are about 1800 ℃ after two types of ablation, SiO2 ablation layer on the surface is not continuous, and ablation layer is thin. The mass loss rate and linear recession rate in HVOF is much greater than in oxyacetylene flame environment,they have a difference of an order of magnitude;the quantity of SiO2 accumulated on the surface oxide layer is less than oxyacetylene flame, grooves formed after ablation of carbon fiber is more apparent,ablative layer is more loose as there are holes in it.Due to the intense gas flux, the sample assessed by HVOF is more serious than oxyacetylene flame.The ratio of zirconium and silicon in matrix with 8: 1,6: 1,4: 1,2:1 of C_f/ZrC-SiC composites with SiC coating are assessed in oxyacetylene flame environment for 20 mm and 300 s, the surface temperature of the samples is at 2200 ℃ ~ 2300 ℃, the ablation rate is small, almost in the 10-3g?s-1 and 10-3mm?s-1 order of magnitude, which is slightly less than the C_f / SiC composite. coating in ablation center is failure, and the matrix begins oxidation and ablation, there is only ZrO2 on the surface which starts sintering and fusing, ablation layer is divided into three layers as ZrO2 layer, an intermediate layer of SiO2 filling in ZrO2 and another ZrO2 layer. With the increasing of the base silicon content, The mass loss rate is decreased, the dense extent of the surface oxide layer is reduced, the fusing of ZrO2 is increased, the thickness of ablative layer ablation layer is increased, so the samples’ ability of anti-ablation is ablation layer is decreased.The ratio of zirconium and silicon in matrix with16:1, 8: 1,6: 1,4: 1,2:1 of C_f/ZrC-SiC composites with SiC coating are assessed by HVOF for 20 mm, the surface temperature of all samples is about 1800 ℃, the ablation rates are smaller than the C_f / SiC composites. Through thickness the ablated composite was divided into a rich SiO2 layer on the surface, a rich ZrO2 layer which is denser in the middle, and lower layer is a individual ZrO2 layer. With the increasing of the base silicon content, density of the whole ablative layer is declined first and then ascended, thickness is decreased first and then increased. The samples of high level of zirconium or silicon showed poor performance of anti-erosion, but rather the samples with moderate ratio of zirconium and silicon(8: 1,6 :1) have stronger performance of anti-ablation.After comparing the two ablation assessment methods,we found that with the reducing of ablation distance, the ablation temperature and speed are all increasing, the degree of osteoporosis in the oxide layer is increasing of the samples ablatived by two assessment methods. The SiC coating on the surface can slow down the oxidation of the substrate to a certain extent.In the ablation of C_f / ZrC-SiC composites, both including oxidation of ZrC and SiC, volatilization of silicon oxide andgas flux,the difference between two methods is that gas stream flux which plays a major role in HVOF and the volatilization of silicon oxide play a major role in oxyacetylene flame environment.