Crack propagation studies and bond coat properties in thermal barrier coatings under bending

Abstract

Ceramic based thermal barrier coatings (TBC) are currently considered as a candidate material for advanced stationary gas turbine components. Crack propagation studies under bending are described that were performed on plasma sprayed ZrO 2 , bonded by MCrAlY layer to Ni base superalloy. The crack propagation behaviour of the coatings at room temperature in as received and oxidized conditions revealed a linear growth of the cracks on the coating till the yield point of the super alloy was reached. High threshold load at the interface between the ceramic layer and the bond coat was required to propagate the crack further into the bond coat. Once the threshold load was surpassed the crack propagated into the brittle bond coat without an appreciable increase in the load. At temperatures of 800°C the crack propagated only in the TBC (ceramic layer), as the ductile bond coat offered an attractive sink for the stress relaxation. Effects of bond coat oxidation on crack propagation in the interface region have been examined and are discussed.

Isothermal Oxidation Behavior of Plasma Sprayed MCrAlY Coatings

“…An Al-depleted zone at the overlay coat/substrate interface as well as just below the TGO along the TGO/MCrAlY interface was observed in the heat exposed specimens. Similar observations were also made by previous investigators in studying the oxidation behavior of sprayed MCrAlY coatings in air (Brandl et al, 1996;Ray & Steinbrech, 1999;Ray et al, 2001). They have observed Al depleted zone below the MCrAlY coating and also below the TGO scale after a long time oxidation of 500 h. The MCrAlY coat/substrate interface revealed the presence of porosity and carbide precipitations (Ray & Steinbrech, 1999;Ray et al, 2001) in heat exposed specimens.…”

“…Previous authors have also reported similar observations in such MCrAlY overlaid substrates (Brandl et al, 1996;Ray & Steinbrech, 1999;Ray et al, 2001). 11 shows the surface roughness of as-sprayed and exposed coatings with increasing exposure time.…”

“…The metal matrix shows grain growth with carbide precipitation within the grains. Possibly secondary carbides precipitate out along the grain boundaries and primary carbides in the center of the grains as described in some references (Ray & Steinbrech, 1999;Ray et al, 2001). Due to the high oxygen conductivity of scales at high temperature and the presence of porosity in the TGO and in the MCrAlY coating, the Al in the MCrAlY coat oxidizes to Al 2 O 3 at the TGO/MCrAlY coat interface.…”

Power-law characteristics of damage and failure of ceramic coating systems under three-point bending

“…Note that the elastic modulus of the coatings is assumed to be a constant as a material property parameter based on the composite beam theory as did in Ref. [11,15,18], the effect of strain on the elastic modulus and the residual deformation are not considered here.…”

Fracture Characteristics and Damage Evolution of Coating Systems Under Four‐Point Bending

“…Therefore, investigating the fracture mechanism and damage evolution behavior of TBC systems (or environmental barrier coatings and other coating systems) is important. Recent studies have shown that the typical failure modes of TBC or other coating systems include transverse cracking (perpendicular to the interface) in the ceramic top coat and delamination of the interface between the top coat and bond coat or between the bond coat and substrate after mechanical (i.e., tension and bending) [7][8][9][10][11][12][13][14][15] and thermal loadings. [16][17][18][19][20] The fracture modes are usually not independent of each other.…”