Stiffness of Plasma Sprayed Thermal Barrier Coatings

Abstract

Thermal spray coatings (TSCs) have complex microstructures and they often operate in demanding environments. Plasma sprayed (PS) thermal barrier coating (TBC) is one such ceramic layer that is applied onto metallic components where a low macroscopic stiffness favors stability by limiting the stresses from differential thermal contraction. In this paper, the Young's modulus of TBC top coat, measured using different techniques, such as four-point bending, indentation and impulse excitation is reported, along with a brief description of how the techniques probe different length scales. Zirconia-based TBC top coats were found to have a much lower global stiffness than that of dense zirconia. A typical value for the as-sprayed Young's modulus was~23 GPa, determined by beam bending. Indentation, probing a local area, gave significantly higher values. The difference between the two stiffness values is thought to explain the wide range of TBC top coat Young's modulus values reported in the literature. On exposure to high temperature, due to the sintering process, detached top coats exhibit an increase in stiffness. This increase in stiffness caused by the sintering of fine-scale porosity has significant impact on the strain tolerance of the TBC. The paper discusses the different techniques for measuring the Young's modulus of the TBC top coats and implications of the measured values.

Measurements of elastic modulus and fracture toughness of an air plasma sprayed thermal barrier coating using micro-cantilever bending

“…The elastic modulus along the in-plane and out-of-plane direction is 144± 12and 109± 7GPa, respectively. Both values are significantly higher than the global modulus (10-70 GPa) of the APS TBCs reported in the literature [11,[20][21][22][23][24] . The difference is expected as the characteristic volume of the material that undergoes deformation during micro-cantilever bending is only in the range of ~ 60 -110 μm 3 , which is expected to contain far less defects than the macroscale TBC samples used for determination of the global modulus.…”

Relationship between mechanical properties and microstructure of yttria stabilized zirconia ceramics densified by spark plasma sintering

“…In the literature it is also proposed to shaped more porous TBC in order to reduce the thermal conductivity and reach low value as 0.4 W/mK for porosity higher than 30%vol [5]. Another important step for such ceramics is to evaluate their mechanical properties in order to bring their multifunctional properties on metallic substrates exposed to severe environment [6]. Many processes such as template method, slip casting, gel casting, direct foaming or partial sintering of powder [7][8][9][10] have already been used to produce porous ceramics.…”

Low‐temperature stiffening of air plasma‐sprayed 7 wt% Y2O3‐stabilized ZrO2

“…It has thus been of interest to examine the change in microstructure, stiffness, and thermal properties of the APS YSZ system subjected to continuous and cyclic thermal exposures. A large portfolio of scientific literature exists for both such attributes with exposure of free‐standing sprayed bodies to various temperatures and times . Specifically, a number of studies have reported the variation in the elastic modulus with sintering and attempted correlation with the variation in elastic modulus and microstructure evolution with sintering, but most of these studies have been carried out beyond 1000°C.…”

“…A large portfolio of scientific literature exists for both such attributes with exposure of free-standing sprayed bodies to various temperatures and times. [16][17][18][19][20][21] Specifically, a number of studies have reported the variation in the elastic modulus with sintering and attempted correlation with the variation in elastic modulus and microstructure evolution with sintering, but most of these studies have been carried out beyond 1000°C. Thomson et al 22,23 have shown the variation in bending modulus by cantilever bending of APS 8YSZ coating after sintering at temperatures ranging from 1000°C to 1400°C.…”