Powder Diffraction

X-Ray Diffraction

In situ stress analysis of multilayer environmental barrier coatings

B. J. Hardera1, J. Almera2, K. N. Leea3 and K. T. Fabera4

a1 Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois, USA

a2 Argonne National Laboratory, Advanced Photon Source, Argonne, Illinois, USA

a3 Rolls-Royce Corporation, Materials, Processes, and Repair Technology, Indianapolis, Indiana, USA

a4 Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois, USA

Abstract

The biaxial stress and thermal expansion of multilayer doped-aluminosilicate environmental barrier coatings were measured in situ during cooling using microfocused high-energy X-rays in transmission. Coating stresses during cooling from 1000 °C were measured for as-sprayed and thermally cycled samples. In the as-sprayed state, tensile stresses as high as 75 MPa were measured in the doped-aluminosilicate topcoat at 375 °C, after which a drop in the stress occurred accompanied by through-thickness cracking of the two outermost layers. After thermally cycling the samples, the stress in the topcoat was reduced to approximately 50 MPa, and there was no drop in stress upon cooling. This stress reduction was attributed to a crystallographic phase transformation of the topcoat and the accompanying change in thermal expansion coefficient. The addition of a doped aluminosilicate to the mullite layer did not lower the stress in the topcoat, but may offer increased durability due to an increased compressive stress.

(Received September 30 2008)

(Accepted January 30 2009)

Key Words:

  • Ceramic;
  • coatings;
  • in situ;
  • stresses;
  • transmission;
  • multilayer