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，基于高温合金的燃气轮机的效率也不会有显着的提高。因此，用非氧化物硅基陶瓷替代氮化硅、碳化硅和碳化硅/碳化硅CMCs是一种很有前途的方法，从而允许更高的使用温度。除了密度较低外，由于形成了一层SiO_2保护层，这些陶瓷还具有高的氧化稳定性。然而，作为主要燃烧产物之一的水蒸气与这种保护膜发生反应，导致它以Si(OH)4的形式挥发，从而影响了陶瓷部件所需的较长使用寿命。在氮化硅、碳化硅和碳化硅/碳化硅涂层中，Yb2Si2O7涂层由于具有更高的热气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，我们成功地开发了致密的Yb2Si2O7基涂层，方法是在Si3N4基片上喷涂含有硅氮烷Durazane 1800(陶瓷粘结剂和二氧化硅的来源)以及Yb2O3和元素硅作为活性填料的悬浮液，然后在1415℃的空气中进行反应热解。然而，除了Yb2Si2O7外，在其晶界还发现了一种SiO_2相，这导致了涂层在真实的热气体腐蚀试验(v=100m/S，1200℃，200h)中加速退化。因此，对于作为涡轮部件的非氧化物硅基陶瓷在其预期使用寿命(t&gt；10,000 h)内实现适当的保护，SiO_2边界相的阻碍是非常重要的。此后，将在本项目提案的范围内探索两个有希望的战略。在战略1中，将Yb、Sc或Hf金属或其氧化物纳米粉末与已经批准的涂层成分作为添加剂结合使用，应能促进Yb2Si2O7晶界的SiO_2相在热解过程中转化为合适的硅酸盐。在战略2中，用上述元素对硅氮烷前驱体进行化学修饰，应避免在Yb2Si2O7边界相内形成游离的二氧化硅，使其在热解过程中转化为相应的硅酸盐。为了具体控制这两种策略的硅酸盐形成，并研究热解过程中涂层组分之间的相互作用，首先在整体上进行了基础实验。随后，将在Si3N4、碳化硅和碳化硅/碳化硅衬底上进行选定悬浮液的喷涂和热解，以确保加工出致密而厚(&gt；100微米)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，特别是它们对热气体腐蚀的稳定性，应该决定它们的应用潜力。

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.