Concurrent with the development of new generation of gas turbines, many attempts have been made to introduce advanced thermal barrier coatings with lower thermal conductivity and higher temperature stability. Most of the research to improve TBCs performance are based on two general approaches: structural modifications and chemical modifications. In most cases, the improvements in some properties are at the expense of loss of some other properties. Changing in the TBCs architecture and the application of multilayer coatings, consisting of layers with engineered properties based on the requirements, is a solution to achieve a combination of desired properties. In all of these development methods it is to be understood that the principle is reducing the possibility of formation of cracks, but, once formed, all such cracks can grow under and thermal cycles and eventually lead to coating delamination and spallation. Self-healing is the most precious phenomenon to overcome this problem.
TopIntroduction
A large number of industrial processes operate under the most arduous conditions of temperature and stress. Some of the features of such environments include high temperature, increased temperature gradients, high pressure, large stresses on individual components, the presence of an oxidizing and corroding atmosphere, and also the possibility of foreign object damage (FOD). A few examples of such processes are steam turbines, aero, marine, and industrial gas turbines (Bose, 2011; X. Chen et al., 2003).
Turbine inlet temperature (TIT) is the critical parameter in improving gas turbine engine power and fuel efficiency. Even a small increase in TIT leads to less fuel consumption to perform the same function and thus will improve turbine efficiency (Rahman, Ibrahim, & Abdalla, 2011; Stöver & Funke, 1999; Vassen, Stuke, & Stöver, 2009). Investigation on Rolls-Royce aero gas turbine engines shown that via increasing turbine inlet temperature as much as approximately 700 ̊C the efficiency increased from 12% to 50% during the period 1970–2000 (Nicholls, 2003). Today, for J-class turbine engines, the TIT and the efficiency of 1650 ̊C and 61% were validated (Ai, Masada, & Ito, 2014). But despite all the advances in the gas turbine industry, this trend of increasing temperature is continuing such that it is estimated that over the next 20 years a 200 ̊C increase in turbine inlet temperature will be required to meet the demand for improved performance (Singh, 2014). These improvements will be achieved by three principal routes: Ι) materials developments, П) advances in cooling systems and III) development of advanced protective coatings and deposition technologies (D. Clarke & Levi, 2003).
Material Development and Advances in Cooling System
In the past decades, there have been extensive efforts to development of advanced high-temperature materials for turbine engines. Nowadays, combustors and high-pressure turbine blades and vanes which work in the harshest parts of gas turbine engines are manufactured from nickel-based superalloys. The wrought superalloys were the primary forms that developed and applied to turbine blades. In the process of development, the cast forms of superalloys were preferred, because of the improvement in the temperature capability and mechanical properties. In the following, with the introduction of advanced casting technologies, directionally solidified (DS) and single-crystal (SC) superalloys are introduced and used in modern turbine engines. The investigations have shown that through improved alloy compositions and innovation in processing, the temperature capability of superalloys has been enhanced at the average rate of 6.7 ̊C per year (Harada, 2003; Reed, 2006).
A further increase in the temperature tolerance of superalloys, such as nickel-based or cobalt-based superalloys, has become very difficult in recent years. The reason is that in the current generation of superalloys the temperature capability has been increased close to their maximum permissible (initial melting temperature: below 1500 K). On the other hand, according to published reports, each super-alloy generation can cost up to $ 10 million. This approximate amount only relates to the condition that the new generation is based on an adding material or a slight processing change from the previous generation. While if the change is fundamental, for example, a change in the casting process, will cost much more than this amount (Council, 1996; Stern, 1996).
In recent years, a significant part of the increase in turbine inlet temperature has been achieved due to the cooling of hot turbine components via internal flows of compressed air. Although compressing and pumping the air through the cooling system requires energy, and also the manufacture of turbine blades with internal cooling passages requires the use of sophisticated and costly casting processes, the science of air cooling is well developed, and now the role of cooling systems in increasing the turbine operating temperature is approximately 400 ̊C (Council, 1996).