Zirconia based thermal barrier coatings for extended temperature ranges

该研究项目将集中在提高燃气轮机运行温度的两个主要方面。一方面，将研究将标准材料8YSZ的工作温度提高到1250℃以上的可能性。另一方面，将生产由替代陶瓷制成的等离子喷涂单层，与8YSZ相比，它在高温下具有更好的相稳定性，适用于1600°C的应用。第一个重点是研究在变化的工艺和操作条件下的转化过程，这对于评估未来热障涂层在延长运行状态下的寿命是必要的。为了延长8YSZ在1250°C以上的运行状态，将研究在高温下标准材料8YSZ的显微组织对相变和烧结行为的影响。由于热障涂层在高温下的高温稳定性和寿命在很大程度上取决于相变和烧结行为，因此了解这些性能的潜在机制和随时间的发展对潜在的应用领域具有重要意义。这两个过程都依赖于温度并在高温下加速，然而，有迹象表明，具有特别细粒度的微观结构，相变和烧结都可以被抑制。此外，非常快的冷却速率可以抑制相变。因此，一个目标是确定潜在的机制，以确定合适的工艺参数选择或特殊的涂层工艺，以生产涂层，这表明由于其微观结构特征改善了相稳定性和烧结性能。冷却速率对相变的影响将通过具体的实验来阐明，并可能导致标准材料8YSZ的工作范围的扩展。该项目的第二部分是处理1600°C下操作的新材料，是开发一种替代锆基陶瓷的热障涂层。为此，从共掺杂氧化锆中选择了四种具有代表性的候选材料，与8YSZ相比，它们具有较长的温度稳定性。选择一方面包括简单的体系，其中掺杂策略作为四方相的稳定，增加断裂韧性，或改善四方性（Ta-Y, Ce/Sc-Y）。此外，还将研究与氧化铝的共掺杂。最后，将选择一个已经商业化的复杂掺杂系统作为基准。本文也没有深入分析工艺、微观结构和操作条件对系统稳定性的影响。

The research project will focus on two major aspects to increase the operation temperature of gas turbines. On the one hand, possibilities to increase the operation regime of the standard material 8YSZ to temperatures above 1250°C will be investigated. On the other hand, plasma-sprayed monolayers made out of alternative ceramics will be produced, which have compared to 8YSZ an improved phase stability at high temperatures and are suitable for an application at 1600°C. A first focus is a study of the transformation processes under changed process and operation conditions, which are necessary for an evaluation of lifetime in the extended operation regime of future thermal barrier coatings.For an extension of the operation regime of 8YSZ above 1250°C the influence of microstructure on the phase transformation and the sintering behavior of the standard material 8YSZ will be investigated at very high temperatures. As the high temperature stability and hence the lifetime of the thermal barrier coatings at very high temperatures are depending strongly on phase transformation and the sintering behavior, an understanding of the underlying mechanisms and of the development with time of these properties is of fundamental importance for the possible application fields. Both processes are temperature dependent and accelerated at high temperatures, however, there are hints, that with an especially fine-grained microstructure both phase transformation and sintering can be suppressed. In addition, very fast cooling rates can suppress the phase transformation. One objective is therefore, to identify underlying mechanisms to determine a suitable selection of process parameters or special coating processes for a production of coatings, which show due to their microstructural features improved phase stability and sintering behavior. The influence of the cooling rate on the phase transformation will be clarified by specific experiments and could additionally lead to an extension of the operation regime of the standard material 8YSZ.The second part of the project, which is dealing with new materials for an operation at 1600°C, is the development of a thermal barrier coating out of an alternative zirconium based ceramic. For that four representative candidates from the co-doped zirconium oxides were selected, which have compared to 8YSZ an extended temperature stability. The selection includes on the one hand simple systems, in which strategies for doping as stabilization of the tetragonal phase, increasing the fracture toughness, or improving the tetragonality are applied (Ta-Y, Ce/Sc-Y). In addition, the co-doping with alumina will be investigated. Finally, an already commercial available complex doped systems will be selected as a benchmark. Also here it has not been analyzed in depth how processing, microstructure and operation conditions influence the stability of the system.