Development of hot gas corrosion stable coating systems on non-oxide Si based ceramics for turbine applications

即使使用复杂的冷却概念，也不能期望基于高温合金的燃气涡轮机的效率的显著增加。因此，用非氧化物Si基陶瓷如Si 3 N4、SiC和SiC/SiC CMC替代它们是一种有前途的方法，从而允许更高的工作温度。除了较低的密度，这些陶瓷由于形成SiO2保护层而具有高的氧化稳定性。然而，主要燃烧产物之一的水蒸气会与该保护膜发生反应，导致其作为Si（OH）4挥发，从而影响陶瓷部件所需的长使用寿命。在Si 3 N4、SiC和SiC/SiC保护的合适候选材料中，二硅酸镱（Yb 2Si 2 O 7）涂层由于增加的热气体腐蚀稳定性和与这些陶瓷匹配的热膨胀系数而脱颖而出。最近，我们成功开发了一种致密的Yb 2Si 2 O 7基涂层，方法是将含有Silazane Durazane 1800（陶瓷粘结剂和SiO2源）以及Yb 2 O3和元素Si作为活性填料的悬浮液喷涂到Si 3 N4基材上，然后在1415 °C下在空气中进行反应性热解。然而，除了Yb 2Si 2 O 7之外，在其晶界处检测到SiO2相，这导致在实际热气体腐蚀测试（v = 100 m/s，1200 °C，200 h）期间涂层的劣化增强。因此，SiO2边界相的阻碍对于实现作为涡轮机部件的非氧化物Si基陶瓷在其预期使用寿命（t > 10，000 h）期间的适当保护是非常重要的。此后，将在本项目提案的范围内探讨两项有希望的战略。在策略1中，Yb、Sc或Hf金属或它们的氧化物纳米粉末与已经批准的涂层组分组合作为添加剂的使用应促进在热解期间在Yb 2Si 2 O 7的晶界处的SiO2相转化成合适的硅酸盐。在策略2中，用已经提到的元素对硅氮烷前体进行化学改性应当通过导致其在热解期间转化成相应的硅酸盐来避免在Yb 2Si 2 O 7边界相中形成游离SiO2。为了具体控制这两种策略的硅酸盐形成，并研究在热解过程中涂层组分之间的相互作用，首先对整料进行基本实验。随后，将选定的悬浮液喷涂和热解到Si 3 N4、SiC和SiC/SiC衬底上，以确保处理致密和厚（> 100 µm）的硅酸盐涂层。所有涂层系统的机械和物理性能的综合表征，特别是其对热气体腐蚀的稳定性，应决定其应用潜力。

A significant increase in the efficiency of gas turbines based on superalloys is not expected even by using sophisticated cooling concepts. Hence, their substitution by non-oxide Si-based ceramics as Si3N4, SiC and SiC/SiC CMCs is a promising approach, thus allowing higher service temperatures. Besides the lower density, these ceramics possess a high oxidation stability due to the formation of a SiO2 protective layer. However, water vapour, one of the main combustion products, reacts with this protective film and leads to its volatilization as Si(OH)4, thus compromising the demanded long service life of the ceramic components. Among the suitable candidates for the protection of Si3N4, SiC and SiC/SiC, Ytterbium Disilicate (Yb2Si2O7) coatings stand out due to the increased hot gas corrosion stability and the matching coefficient of thermal expansion to these ceramics. Recently, we have successfully developed a dense Yb2Si2O7-based coating by spraying suspensions containing the Silazane Durazane 1800 (ceramic binder and source of SiO2) as well as Yb2O3 and elemental Si as active fillers onto Si3N4 substrates, followed by reactive pyrolysis at 1415 °C in air. Besides Yb2Si2O7, a SiO2 phase was however detected at its grain boundaries, which led to the enhanced degradation of the coating during realistic hot gas corrosion tests (v = 100 m/s, 1200 °C for 200 h). Thus, the hindrance of a SiO2 boundary phase is of major importance to achieve a suitable protection of non-oxide Si-based ceramics as turbine components during their expected service life (t > 10,000 h). Hereafter, two promising strategies will be explored within the scope of this project proposal. In Strategy 1, the use of Yb, Sc or Hf metal or their oxide nanopowders in combination with the already approved coating components as additives should promote the conversion of the SiO2 phase at the grain boundaries of Yb2Si2O7 into suitable silicates during pyrolysis. In Strategy 2, the chemical modification of the Silazane precursor with the already mentioned elements should avoid the formation of free SiO2 within the Yb2Si2O7 boundary phase, by leading to its conversion into the respective silicates during pyrolysis. In order to specifically control the silicate formation for both strategies and to investigate the interactions of the coating components among each other during pyrolysis, basic experiments are first carried out on monoliths. Subsequently, the spraying and pyrolysis of selected suspensions onto Si3N4, SiC and SiC/SiC substrates will be performed, in order to assure the processing of a dense and thick (> 100 µm) silicate coating. A comprehensive characterization of the mechanical and physical properties of all coating systems but especially their stability against hot gas corrosion should determine their application potential.