Preparation And Properties Of Ablation And Oxidation Resistant Multilayer Coatings On Graphite

With the development of aerospace technology, increasingly severe aircraft service environment of aircraft is more serious, the demands for thermal barrier coating with higher performance are proposed. Carbon based materials, such as C/C composite material, graphite, etc., with light weight, high strength, high modulus, good high temperature stability and thermal shock resistance, are widely used in aerospace domain. However, carbon based materials tend to be oxidized when the temperature is higher than 500 oC. Coating technology is an effective way to improve the oxidation resistance of carbon based materials. Ultra high temperature materials is a class of special materials that could maintain the physical and chemical stability in ultra high temperature（>2000 oC） and reactive atmosphere（atomic oxygen environment, plasma, etc.）. ZrB₂–SiC based ultra high temperature ceramic has good high temperature strength, oxidation resistance and ablation resistance, etc., which is currently studied most extensively. Both carbon–based materials and ZrB₂–SiC based ceramics have their unique advantages and disadvantages. Combining the light weight, good thermal shock resistance of carbon matrix with the good oxidation and ablation resistance of ZrB₂–SiC ceramic is a good and reasonable way to solve the problem of poor oxidation resistance of carbon materials and poor thermal shock resistance and brittleness of ZrB₂–SiC ceramic. In this way, a new material with good thermal shock resistance, oxidation and ablation resistance, light weight and other good properties could be obtained.Among the conventional methods for preparing coatings on carbon based materials, ZrB₂, in the form of particles, are usually added directly leading to incompact coating structure which influenced the the coating performance. So the problem needs to be solved first is how to in situ synthesize ZrB₂ on the surface of matrix. But the coatings prepared by CVD always have high density, so high thermal residual stress will be caused by the big coefficient of thermal expansion mismatch between matrix and outer coating deposited on graphite directly, which will influence the performace of the coating. The coating prepared with loose structure and small residual stress by pack cementation is suitable for buffer and bonding layer for eliminating thermal stress in multilayer coating. So, in this research, the coating prepared by pack cementation was used for buffer layer, while the coating prepared by CVD was used for outer layer.Firstly, ZrB₂ coating was prepared on graphite on graphite by chemical vapor deposition（CVD） using Zr Cl₄, BCl₃ and H₂ as reaction gases, Ar as carrier gas and diluting gas. ZrB₂ coating covering the graphite evenly was obtained by CVD under the conditions of 240 L/h Ar/H₂（5%） gas mixture, 240 L/h Ar gas, 5 L/h BCl₃ and 50.73 g Zr Cl₄ at 1200 oC for 2h. The coating thickness was about 12 μm, and no impurity phase was detected.ZrB₂/SiC coating on the surface of graphite was prepared using two–step chemical vapor deposition. The SiC layer with homogenous and dense structure completely covered on ZrB₂ layer. During the deposition process, SiC filled the pores in ZrB₂ layer; the two layers tightly bounded together. However, due to the loose structure of ZrB₂ layer, there was big thermal stress in SiC layer leading to cracks.To relax the thermal stress between coating deposited by CVD and matrix, the research on preparation of ZrB₂–SiC coating by pack cementation was carried out. The inner SiC layer was fabricated by pack cementation using Si, graphite and Al2O3 powders as raw materials at 1800 oC for 2h. The outer ZrB₂–SiC layer was prepared on SiC coated graphite using ZrB₂, Si and graphite powders at 2000 oC for 2h. Finally, SiC/ZrB₂–SiC（ZS50–2） coating with good oxidation and thermal cycling resistance was obtained. The weight retention of ZS50–2 coating was 93.5% and 97.1% respectively after thermal shock test between 1500 oC and room temperature for 15 cycles and oxidation in air for 19 h. Ferrocene could improve the porosity of ZrB₂–SiC coating, and further improved its performance. The weight retention of the coated sample with 3 wt. % ferrocene（ZS50–3f） was 96.2% after 15 thermal cycles between 1500 oC and room temperature, and the weight retention was 99.8% after 19 h oxidation in air at 1500 oC. SiC constituted the main structure of SiC/ZrB₂–SiC coating and the loose structure was beneficial for relaxing thermal stress. Finally, SiC/ZrB₂–SiC/ZrB₂/SiC coating was prepared by depositing ZrB₂/SiC coating on SiC/ZrB₂–SiC（ZS50–3f） coated specimen.Under the ablation condition of 1800 L/h O2 and 1900 L/h C2H₂, CVD ZrB₂/SiC coating failed totally after ³3 s ablation; under the ablation condition of 1300 L/h O2 and 1900 L/h C2H₂, CVD ZrB₂/SiC coating also failed totally after ¹70s ablation. SiC/ZrB₂–SiC（ZS50–3f） coating also had better ablation resistance under temperature higher than 1850 oC for ²00 s. The linear and mass ablation rates at center area were 4.647 μm/s and 1.216 mg/s, respectively. Under the ablation condition of 1800 L/h O2 and 1900 L/h C2H₂, SiC/ZrB₂–SiC/ZrB₂/SiC coating showed excellent ablation resistance. The mass ablation rate and the linar ablation rate after 298 s ablation were 0.27 mg/s and 0.57 μm/s, respectively.