Failure Mechanism for Thermal Fatigue of Thermal Barrier Coating Systems

Abstract

Thick thermal barrier coatings (TBCs), consisting of a CoNiCrAlY bond coat and yttria-partially stabilized zirconia top coat with different porosity values, were produced by air plasma spray (APS). The thermal fatigue resistance limit of the TBCs was tested by furnace cycling tests (FCT) according to the specifications of an original equipment manufacturer (OEM). The morphology, residual stresses, and micromechanical properties (microhardness, indentation fracture toughness) of the TBC systems before and after FCT were analyzed. The thermal fatigue resistance increases with the amount of porosity in the top coat. The compressive in-plane stresses increase in the TBC systems after thermal cycling; nevertheless the increasing rate has a trend contrary to the porosity level of top coat. The data suggest that the spallation happens at the TGO/top coat interface. The failure mechanism of thick TBCs was found to be similar to that of conventional thin TBC systems made by APS.

Comparison of Thermal Shock Behavior of 7ysz, 15ysz and Sysz Thermal Barrier Coatings Produced by Aps Method

“…Beginning of the failure from the edges was due to the extreme heating and cooling conditions encountered and the singularity of thermal stresses at the edges. Other studies [4,26,27] have also mentioned the edge effect on failure during thermal cycling. Figure 6 presents the weight changes as a function of cycle number for three kinds of nanostructured samples during thermal shock testing.…”

“…Previous studies [27][28][29] showed that oxidation of the bond coat and thermal mismatch stress were the main factors causing degradation of TBCs. EDS analyses of top coat/bond coat interfaces of the three nanostructured TBCs after thermal shock testing are shown in Figure 8.…”

“…In recent years, nanostructured zirconia based TBCs deposited by atmospheric plasma spraying have been the focus of attention. It was reported that nanostructured thermal barrier coatings had high bonding strength [20], low thermal conductivity [20][21][22][23][24], and prolonged thermal cycling lifetime [25][26][27]. Accordingly, nanostructured TBCs are expected to provide better performance than the conventional TBCs.…”

The influence of the spraying angle on properties of thermally sprayed HVOF cermet coatings

“…The technique of indentation fracture toughness measurement, described in detail by Ponton and Rawlings [13], can be successfully used for evaluation of thick brittle coatings [14][15][16]. In this case, the model of Lawn and Swain [17] was used to determine the number of K Ic .…”

Effect of Particle Size on the Thermal Shock Resistance of Plasma-Sprayed YSZ Coatings

“…The pores in a plasma-sprayed coating can be divided into three types with various orientations and shapes: intra-lamellar cracks, inter-lamellar pores and globular pores [7][8][9]. In addition, the pore structure of the coating may evolve during the thermal cycling due to the effect of sintering and thermal stress [10,11]. The increase in stiffness caused by the sintering of fine-scale porosity has significant impact on the strain tolerance of the TBC [12].…”