Fracture Toughness of Plasma‐Sprayed Thermal Barrier Ceramics: Influence of Processing, Microstructure, and Thermal Aging

Abstract

Fracture toughness of thermal barrier coatings (TBCs) has gained significant interest in recent years as one of the dominant design parameters dictating selection of materials and assessing durability. Much progress has been made in characterizing and understanding fracture toughness of relevant TBC compositions in their bulk form, but it is also apparent that the toughness is significantly affected by process‐induced microstructural defects. In this investigation, a systematic study of the influence of coating microstructure on the fracture toughness of atmospheric plasma‐sprayed TBCs has been carried out. Yttria partially stabilized zirconia (YSZ) coatings were fabricated under different process conditions inducing different levels of porosity and defect densities. Fracture toughness was measured on free‐standing coatings in as‐processed and thermally aged conditions using the double torsion technique. Results indicate significant variance in fracture toughness among coatings with different microstructures including changes induced by thermal aging. Comparative studies were also conducted on an alternative composition, Gd2Zr2O7 which, as anticipated, shows significantly lower fracture toughness compared to YSZ. The results not only point toward a need for process and microstructure optimization for enhancing TBC performance, but also a framework for establishing performance metrics for promising new TBC compositions.

A comprehensive sintering mechanism for TBCs‐Part I: An overall evolution with two‐stage kinetics

“…Firstly, a intervening bond‐coat is deposited on superalloy to facilitate strong adhesion between substrate and a top‐coat. Subsequently, the ceramic top‐coat is deposited to provide the thermal insulating function . A common material for the top‐coat is the yttria‐stabilized zirconia (YSZ), since it exhibits a low thermal conductivity [~2.5 W·(m·K) −1 ] and a high thermal expansion coefficient (~11 × 10 −6 K −1 ) .…”

SiC fiber and yttria-stabilized zirconia composite thick thermal barrier coatings fabricated by plasma spray

“…The fracture toughness of five YSZ coatings showed significant differences ranging from 1.11 to 1.17 MPa √ m. It was reported that the fracture toughness of APS YSZ coatings processed by indentation method is 1.35-3.30 MPa √ m [12,23]. The fracture toughness for YSZ coatings was reported to be much larger, probably due to the thinner coating thickness [23][24][25]. YSZ coatings average fracture toughness was 1.13 MPa √ m. And as shown in Table 2, there is an approximately 48% lower value compared to thick SiC fiber/YSZ composite coatings that had a average fracture toughness of 1.67 MPa √ m, because fiber-reinforced composites have a better fracture toughness [26].…”

“…The fracture toughness of five YSZ coatings showed significant differences ranging from 1.11 to 1.17 MPa √ m. It was reported that the fracture toughness of APS YSZ coatings processed by indentation method is 1.35-3.30 MPa √ m [12,23]. The fracture toughness for YSZ coatings was reported to be much larger, probably due to the thinner coating thickness [23][24][25].…”

Failure mechanisms in APS and SPS thermal barrier coatings during cyclic oxidation and hot corrosion

“…It is thermodynamically compatible with the protective thermally grown oxide (TGO). It also exhibits high fracture toughness over 2 MPa.m 1/2 [11,12]. All these have made YSZ the preferred choice for top coat material in TBC applications for many decades.…”