致密SiC涂层在水蒸气及空气混合气氛中的氧化研究

Study on the oxidation of dense SiC coating in water vapor and air mixture atmosphere

摘要:

石墨是高温气冷堆中重要的结构材料和慢化材料,提高石墨的抗氧化性能是提高高温气冷堆安全性的重要保证。SiC涂层是目前应用最广泛的涂层材料体系之一,本文主要针对球形燃料元件基体石墨样品上流化床化学气相沉积工艺所制备的致密SiC抗氧化涂层,在1400°C的高温环境中,进行水蒸气和空气环境下的氧化行为研究与机理分析。结果表明,气氛对氧化层厚度有显著影响。单一水蒸气情况下,所得氧化层的厚度均比混合气氛的氧化层厚度薄。单一水蒸气氧化后的表面比混合气氛氧化的表面具有更平整表面、更轻微的点状开裂和更小的线性裂纹。致密SiC涂层氧化均发生在表面,没有破坏致密层的结构。氧化机理分析表明,致密SiC涂层的表面形貌变化、裂纹的产生以及厚度变化等均与氧化竞争反应和氧化中间产物有关。致密SiC涂层在空气和水蒸气等不同环境的氧化过程中,均能在表面形成一定厚度的SiO₂自愈合层,且10h的氧化后,涂层的氧化深度仅在表层的3.5 μm以内,没有破坏致密层结构。

Abstract:

Graphite is an important structural material and moderating material in high temperature gas-cooled reactor (HTGR). Improving the oxidation resistance of graphite is an important guarantee to improve the safety of HTGR. SiC coating is one of the most widely used coating materials at present. In this paper, a dense SiC antioxidant coating was prepared by fluidized bed chemical vapor deposition process on matrix graphite sphere. And its oxidation behavior and mechanism were analyzed at 1400℃. The results show that the atmosphere has a significant effect on the oxide layer thickness. In the case of single steam, the thickness of the oxide layer obtained is thinner than that in the mixed atmosphere. The surface oxidized by single steam has a smoother surface, slighter point cracks and smaller linear cracks than that oxidized by mixed atmosphere. The oxidation of dense SiC coatings occurs on the surface without destroying the structure of the dense layer. The oxidation mechanism analysis shows that the surface morphology, crack formation and thickness change of the dense SiC coating are all related to the oxidation competition reaction and oxidation intermediates. In the oxidation process of dense SiC coatings in different environments such as air and water vapor, a certain thickness of SiO2 self-healing layer can be formed on the surface. After 10 hours of oxidation, the oxidation depth of the coating is only within 3.5 μm of the surface, and the structure of the dense layer is not destroyed.