How to Improve Thermal Barrier Properties of YSZ Coating

How to Improve Thermal Barrier Properties of YSZ Coating
M.Saremi*, Z. Valefi and A. Keyvani
Faculty of Metallurgy & Materials Engineering, University of Tehran, Tehran, Iran
Accepted for publication on 12th March 2014
Thermal barrier coatings are used in gas turbine engine to improve fuel efficiency and also to protect
the base material from corrosive environment of combustion chamber. Yttria Stabilized Zirconia
(YSZ) is mainly used as TBC because of its low thermal conductivity and near to metals coefficient
of thermal expansion. However the YZS is subject to early failure due to over growth of TGO, hot
corrosion due to the impurities in fuels and thermo-mechanical stresses. Research on improving
TBCs are conducted in two directions; first on improving stability and durability of present YSZ and
second on introducing new materials to be used at higher temperature to increase inlet temperature in
turbine engine.
Among different techniques to improve durability of YSZ, structural modification toward
nanostructure and alteration through composition with Alumina and Ceria has shown promising
results (xx). We have shown that application of 100 microns of Alumina on YSZ improves its
resistance to TGO growth and reduces hot corrosion effects. Destabilization of YSZ occurs, as a
result of Yttrium depletion due to the formation of YVO4 during hot corrosion. In nano structured
YSZ the migration of oxygen gas or oxygen ions toward BC-YSZ interface and penetration of fused
salts causing hot corrosion is reduced because of a packed structure of YSZ.
In a novel effort to study thermo-mechanical stresses on YSZ we measured the coefficient of thermal
expansion of micro and nano structured YSZ and YSZ –Alumina composite coatings during heating
and cooling regimes to 1050 oC and found that: first the expansion during heating is longer than
retraction during cooling in YSZ and second this difference is very short for nano structured YSZ but
it is considerable for micro and composite structures. Consequently, it was concluded that this
parameter can be considered as a stress raiser in the interface enhancing spallation of the TBC.
In the next effort our research group applied SPPS technique using different precursors to modify the
TBC structure and compare it with coatings obtained by usual plasma spray method. The SPPS has
the capacity to engineer the TBC composition and structure in order to get layered and functionally
graded YSZ-Alumina TBC.
Also high temperature oxidation, thermal shock resistance and thermal conductivity of thermal
barrier coatings deposited by atmospheric plasma spraying of nano agglomerated powder (NP),
pyrolyzed precursor powder (PP) and solution precursor (SP) feedstock were characterized in this
study. Findings showed that SPPS coating, because of vertical crack formation and small sizes of
pores and splats, has lower thermal conductivity and higher oxidation and thermal shock resistance
compared to the other coatings studied.
Layered and functionally graded YSZ-Alumina TBC coatings were also applied on In-738 Ni super
alloy using SPPS. In layered YSZ-Alumina coatings because of difference in thermal expansion
coefficient of YSZ and Alumina and stresses developed at the interface, the tensile strength and
consequently thermal shock resistance of the coating was not promising and failure of coating
occurred at YSZ-Alumina interface. However in Functionally Graded TBC the distribution of
stresses in the graded layers resulted a better thermal shock resistance with longer time to spallation.
It can be concluded that by modification of YSZ structure and composition it is possible to improve
both stability and thermal conductivity of such TBC.