Stress Histories of Yttria-Stabilized Zirconia and CoNiCrAlY Coatings during Thermal Spraying

Abstract

Thermal barrier coating (TBC) systems which are used for insulating the substrates of gas turbine blades from high temperature can be made by thermal spraying. The TBC system has residual stresses because of high temperature deposition and the thermal expansion mismatch in the system. In this study, how the residual stress occurs in TBC system was examined by both experimental measurement and FEM analysis. The Yttria-stabilized zirconia (YSZ) top coating was deposited by atmospheric plasma spraying (APS). CoNiCrAlY bond coatings were deposited by high velocity oxygen-fuel (HVOF) spraying and APS. The temperatures of YSZ, CoNiCrAlYs and substrates were measured during thermal spraying. The temperature of YSZ was the highest and that of CoNiCrAlY(HVOF) was the lowest among the three types of spray processes. The residual stresses were elastically calculated by FEM based on the measured temperature histories. The residual stress of YSZ and CoNiCrAlYs on two types of substrates were also measured by X-ray diffraction method. It was confirmed from FEM analysis that residual stress consisted of primary quench stress and secondary thermal mismatch stress. The quench stress was caused by the quenching of coating particles during deposition which occurs due to the huge thermal capacity of the substrate. The thermal mismatch stress was caused by the difference in linear expansion coefficients between coating and substrate. It was found that not only these two mechanisms but also microcrack formation caused by quench played an important role in the residual stress. The temperatures at which residual stresses might begin to occur in the coatings were shown based on the stress relaxation by microcrack formation. It was also found that peening effect played an important role in the residual stress of HVOF sprayed coating.

A Study of Delamination Mechanism of Thermal Barrier Coating under Thermal Cycling and Aging Condition by Residual Stress History

“…The temperature of coating and substrate was assumed to be identical. The physical values used in calculation were the same as previous paper [2]. Heat transfer coefficient depends on environmental flow.…”

“…Residual stress plays an important role on delamination of TBC [1]. Authors [2] had investigated the arising mechanism of residual stress during thermal spraying by both measurement of temperatures and FEM stress analysis. In this paper, it was examined how the residual stress changed after thermal cycling and thermal aging.…”

A Numerical Study on Generation Mechanism of Vertical Cracks in Top Coat of TBCs During APS Deposition

“…Of course, such large stress could not be generated in the real coating because of the strength of the sprayed top coat. Furthermore, it was reported that the residual stress in the top coat after spraying was several tens of MPa by x-ray diffraction method ( Ref 13,20,21). Then, a consideration of stress relaxation due to crack generation was necessary for analysis of the deposition process of ceramic coatings.…”