One of the areas of research that continue to attract researchers worldwide is the development of thermal barrier coatings (TBCs) especially associated with the design of new ceramic topcoats with low thermal conductivity and a high coefficient of thermal expansion. The purpose of this chapter is to present the advances that have been achieved regarding ceramic topcoats in the last decades, making a historical journey that culminates with the contributions of this decade. The introduction of new crystalline structures and chemical compositions have opened the door to the real possibilities of replacing yttria-stabilized zirconia (YSZ) to ensure the optimal thermomechanical-chemical properties required by TBCs. Future research directions associated with this topic are also provided.
TopIntroduction
Thermal barrier coatings (TBCs) can be defined as advanced deposited material systems, operating at high temperatures to offer improved thermal stability and lower thermal conductivity These are placed on metal surfaces to protect the components of the hot section of gas turbine engines, and thereby, achieving higher fuel efficiency and lower emission objectives (Backman, 1992; Padture, 2002; Herzog, 2006). These coatings additionally improve erosion and impact resistance, which are crucial to increase engine durability and performance. Unfortunately, these coatings are susceptible to accelerated degradation due to deposition of silicates (known as CMAS) by environmental debris such as dust, sand, and ash that adheres to them (Vaßen, 2012; Poerschke, 2017). The advanced materials used for this purpose are based on combinations of doped transition metal oxides with rare earth oxides, which reduce oxidation and thermal fatigue in the metal part. The most commonly used oxides are yttria stabilized zirconia (ZrO2-Y2O3), mullite (3Al2O3-2SiO2), alumina (Al2O3), ceria (CeO2), lanthanum zirconate (La2Zr2O7), lanthanum oxide (La2O3), oxide niobium (Nb2O5), and praseodymium oxide (Pr2O3) (Vasen, 2000; Cao, 2004; Tarasi, 2011). New and innovative materials for TBCs are being introduced such as LaTi2Al9O19 (LTA) (Xie, 2011), lanthanide tantalate (RETa3O9) (where RE = Ce (Cerium), Nd (Neodymium), Sm (Samarium), Eu (Europium), Gd (Gadolinium), Dy (Dysprosium), Er (Erbium)) (Chen L, 2018), dysprosium-tantalum oxide (DTO) (Wu, 2018), magnesium-silicon oxide (MSO) (Chen S, 2019), lanthanide niobate (Ln3NbO7) (LNO) (where Ln or L = Dy (Dysprosium), Er (Erbium), Y (Yttrium), Yb (Ytterbium)) (Yang, 2019), zirconium lanthanate (Zr3Ln4O12) (where Ln = La (Lanthanum), Gd (Gadolinium), Y (Yttrium), Er (Erbium), and Yb (Ytterbium)) (Zhao M, 2019), magnetoplumbite (LnMgAl11O19) (where Ln = La (Lanthanum), Pr (Praseodymium), Nd (Neodymium), Sm (Samarium), Eu (Europium), Gd (Gadolinium)) (Zhao Y, 2019), and gadolinium-zirconium oxide (GZO) (Vaßen, 2020).