温度对B4C涂层氧化防护性能和防护机制的影响

目的 研究温度对B4C涂层氧化防护性能和防护机制的影响，得出B4C涂层最佳氧化防护温度范围，以及B4C涂层在不同温度的氧化防护机制演变。方法 以石墨为基体，采用放电等离子烧结法在石墨表面制备B4C涂层，通过不同恒温氧化试验(800、1 000、1 200、1 400 ℃)和室温至1400 ℃宽温域动态氧化试验来测试其氧化防护性能，并通过X射线衍射(XRD)、扫描电子显微镜(SEM)和能谱仪(EDS)对B4C涂层石墨试样氧化前后的物相组成、微观形貌、氧扩散等进行分析。结果 B4C涂层氧化后可生成B2O3玻璃膜，在800、1 000、1 200、1 400 ℃恒温氧化的防护效率分别为98.43%、98.61%、94.4%和92.8%，在室温至1 400 ℃宽温域动态氧化的防护效率为93.1%。B4C涂层在800 ℃以下主要依赖结构阻氧，800至900 ℃由结构阻氧向惰化阻氧转变，900 ℃以上主要依赖惰化阻氧。1 100 ℃以上，随温度升高B2O3玻璃膜的挥发加剧，B4C涂层惰化阻氧能力减弱。结论 B4C涂层的氧化防护效率随温度上升先增大后减小，结构阻氧机制逐渐降低，惰化阻氧机制先升高后降低。B4C涂层在800至1 100 ℃具有良好的氧化防护性能。

Effect of Temperature on Oxidation Protection Performance and Mechanism of B4C Coating

The oxidation of B4C coatings can generate a B2O3 glass film, which is a good oxidation protection material for carbon materials. The effect of temperature on the oxidation protection performance and mechanism of B4C coatings was studied, and the optimal oxidation protection temperature range of B4C coatings and the evolution of oxidation protection mechanism of B4C coatings at different temperature were obtained. In this paper, a B4C coating was prepared on a graphite surface by spark plasma sintering (SPS), and its oxidation behavior and oxidation protection mechanism were investigated by isothermal oxidation at different temperature (800, 1 000, 1 200, 1 400 ℃) and dynamic oxidation in a wide temperature range from room temperature to 1 400 ℃. The phase composition, micro-morphology and oxygen diffusion of B4C coated graphite samples before and after oxidation were analyzed with an X-ray diffraction (XRD), a scanning electron microscopy (SEM) and an energy dispersive spectroscopy (EDS). The results showed that the B4C coating was oxidized at 800, 1 000, 1 200 and 1 400 ℃ for 100 min, the oxidation protection efficiency was 98.43%, 98.61%, 94.4% and 92.8%, respectively. When the B4C coating was oxidized at 800 ℃, the rate of coating oxidation and glass film formation were slow, so oxygen diffused to the interior of the coating and the substrate, resulting in rapid reduction of oxidation protection efficiency at the initial stage of oxidation; when continuous B2O3 glass was formed, the pores in the coating were reduced, and the protective efficiency of the coating rebounded; after oxidation for 60 min, the oxidation protection efficiency was basically stable. The B4C coating had better protection effect at the initial stage of oxidation at 1 000 ℃ than other temperature; with the duration of oxidation, its protection efficiency slowly decreased, and the oxidation protection efficiency was basically stable after oxidation for 60 min. The protection efficiency only decreased by 1.39% after oxidation for 100 min at 1 000 ℃. When B4C coated graphite samples were oxidized at 1 200 and 1 400 ℃, the oxidation protection efficiency of the B4C coating decreased rapidly with oxidation time; after oxidation for 100 min, the protection efficiency was 94.4% and 92.8%, respectively. The B4C coating had the best film-formation effect and the highest protection efficiency at 1 000 ℃. The B4C coated graphite sample was oxidized from room temperature to 1 400 ℃, and the mass first remained stable (<700 ℃), then increased rapidly (700-900 ℃), and finally increased slowly (>900 ℃). The protection efficiency of the B4C coating remained stable below 600 ℃, decreased slowly at 600-700 ℃, decreased rapidly at 700-800 ℃, rebounded rapidly at 800-900 ℃, remained basically stable at 900-1 100 ℃, and weakened slowly above 1 100 ℃. Combined with isothermal oxidation and wide temperature range oxidation, it can be concluded that the B4C coating has good oxidation protection effect in the temperature range of 800-1 100 ℃. The oxidation protection mechanism of the B4C coating below 800 ℃ is mainly structural oxygen-blocking; the protection mechanism changes from structural oxygen-blocking to inert oxygen-blocking at 800-900 ℃; and the protection mechanism is mainly inert oxygen-blocking above 900 ℃. When the temperature exceeds 1 100 ℃, the volatilization of glass film intensifies and the inert oxygen-blocking weakens.