Citation:
R. Wellman, G. Whitman, J.R. Nicholls, CMAS corrosion of EB PVD TBCs: Identifying the minimum level to initiate damage, International Journal of Refractory Metals and Hard Materials, Volume 28, Issue 1, Tribology of Hard Coatings, January 2010, Pages 124-132
Abstract:
Over the last decade a significant amount of research has been conducted into
the durability of thermal barrier coatings (TBCs) focusing mainly on issues of
oxidation, erosion and foreign object damage (FOD). However, as the performance
and durability of TBCs has improved the temperatures at which they operate has
increased. This increase in temperature has resulted in another lifing issue for
EB PVD TBCs, namely that of CMAS attack. Calciumâ  magnesiumâ  alumino-silicate
(CMAS) attack occurs when atmospheric dust that has deposited on the surface of
turbine blades melts and wicks into the columns of the TBC. This occurs at
temperatures above 1240â  1260 à °C and results in the degradation of the
columnar microstructure of the TBCs. Due to the fact that TBCs operate in a
temperature gradient CMAS only infiltrates part of the coating before
solidifying. There are a number of issues associated with CMAS attack, both
chemical and mechanical. From a chemical point of view CMAS attack of electron
beam (EB) physical vapour deposited (PVD) TBCs can be considered as a form of
corrosion; when there is a lot of excess CMAS on the surface of a coated
component Yttria diffuses out of the TBC into the molten CMAS resulting in a
tâ ² to monoclinic phase transformation in the yttria stabilised zirconia (YSZ),
CMAS attack also results in localised melting and subsequent re-precipitation of
the coating resulting in a loss of the defined columnar microstructure. While
from a mechanical point of view the CMAS, once re-solidified, reduces the strain
compliance of the EB PVD and can result in spallation of the TBC on cooling.
Furthermore, current studies have indicated that small amount of CMAS
infiltration significantly increases the erosion rate of EB PVD TBCs. This paper
covers various aspects of CMAS attack of EB PVD TBCs, specifically looking at
minimum levels of CMAS required to initiate damage, as well as investigating it
from an erosionâ  corrosion p