TBCs for Gas Turbines under Thermomechanical Loadings: Failure Behaviour and Life Prediction

Abstract

Abstract. The present contribution gives an overview about recent research on a thermal barrier coating (TBC) system consisted of (i) an intermetallic MCrAlY-alloy Bondcoat (BC) applied by vacuum plasma spraying (VPS) and (ii) an Yttria Stabilised Zirconia (YSZ) top coat air plasma sprayed (APS) at Forschungszentrum Juelich, Institute of Energy and Climate Research (IEK-1). The influence of high temperature dwell time, maximum and minimum temperature on crack growth kinetics during thermal cycling of such plasma sprayed TBCs is investigated using infrared pulse thermography (IT), acoustic emission (AE) analysis and scanning electron microscopy. Thermocyclic life in terms of accumulated time at maximum temperature decreases with increasing high temperature dwell time and increases with increasing minimum temperature. AE analysis proves that crack growth mainly occurs during cooling at temperatures below the ductile-to-brittle transition temperature of the BC. Superimposed mechanical load cycles accelerate delamination crack growth and, in case of sufficiently high mechanical loadings, result in premature fatigue failure of the substrate. A life prediction model based on TGO growth kinetics and a fracture mechanics approach has been developed which accounts for the influence of maximum and minimum temperature as well as of high temperature dwell time with good accuracy in an extremely wide parameter range.

Mechanical and Thermo-physical Properties of Plasma-Sprayed Thermal Barrier Coatings: A Literature Survey

“…Consistently, the enhancement of crack length, due to crack coalescence as discussed above, must in turn result in a correlated decrease of the crack density. Comparable results were given by Beck et al [57], reporting a half-life threshold of 60 cracks per mm and a length of 40 lm, slightly lower.…”

“…Following this initial degradation, crack lengths stabilize indicating a huge decrease in crack growth rate until half-life (50%) from which a roughly exponential increase of crack length, corresponding to some coalescence, controls the overall damage of the TBC, leading to final failure. Other studies have reported the same trend of crack growth as function of the fraction of life [46,57].…”

“…Clearly, some parameters take on great effect on time to failure. Namely, they are the dwell time (cycling frequency) [43][44][45][46][47][48][49][50], the temperature of the bond coat [16,18,[43][44][45][46][47][48][49][50][51][52][53][54][55][56], the minimum temperature reached during cooling time [46,49,50,53,57], and the thickness of the thermal grown oxide [38, 46, 50, 52, 54-56, 58, 59].The quantity, morphology, and distribution of spallation cracks, lying parallel to the bond coat/YSZ interface, are parameters often analyzed in the literature. There are, for example, many data on the evolution of the maximum and average lengths and the number of cracks as a function of the aging of the TBC system [46,49,50,53,59,60].…”

Protective pre‐deposited slurry dip‐coating alumina layer on TBC system

“…The other alumina phases (γ, θ, and δ) are metastable and non‐protective . The non‐alumina oxides like, Cr 2 O 3 and NiO, tend to nucleate, grow and eventually react to form undesirable porous, brittle, and rapidly growing spinel, NiCr 2 O 4 /NiAl 2 O 4 , resulting in high local residual stresses generation, which accelerates TBC failure mechanisms …”

“…These techniques produce coatings with initial oxide layer, depending on the Al content within bond coat, which accelerates Al depletion . After the depletion of the Al in the bond coat, other non‐protective scales will start to form . Initial formed phases of the TGO are considered a critical factor influencing the life of the TBC system.…”