Download Thermal Barrier Coatings

Thermal Barrier Coatings (TBCs)
The basic concept of TBCs was conceived during the 1950s when the aim was to protect
the critical air-cooled metal components of engines, from high temperature degradation
[29-31]
.
The lives of metal components decrease because of the thermal stresses generated in them in a
high temperature dynamic environment. Engines of all types and in particular those in Aircrafts, Marines and Gas turbine power generators were, the most demanding in terms of
durability and reliability. The hot section airfoils of such engines are expected to have life times
of thousands of hours under high thermo-mechanical stresses, in aggressive gas environments at
high temperature.
Turbine airfoil degradation is generally caused by oxidation, hot corrosion, inter
diffusion and thermal fatigue of varying severity, depending upon the applications. Aircraft
engines are generally exposed to all the above hazards and among them the high temperature
oxidation and thermal fatigue are the most severe. In order to overcome these problems in the
early fifties, almost all engines relied on the use of cermet and metal coatings to combat
environmental degradation. The research efforts in various laboratories were then directed
towards the development of ceramic components as replacements to metals in engines. However
despite attaining improved performance capabilities, (because of high thermal insulation and
corrosion resistance) poor machinability, low toughness and a host of other problems prevented
the successful development of all ceramic engines. Efforts are still on towards achieving the
ultimate goal. Research was directed towards the development of super alloy turbine blades,
capable of operating at elevated temperatures with improved performance and efficiency in high
pressure section of aircraft turbine engines. The maximum working temperature capability for
these components could be attained through simple alloying or directional solidification
[32-34]
.
Ni-Al-Mo based directionally solidified alloys possessed much superior high temperature
strength and machinability than conventionally cast super alloys. However the gain in higher
working temperature was generally offset by their inferior resistance to oxidation and corrosion.
Providing aluminised coatings was the next phase development. For eg the Ni based alloys,
because of the increased high temperature strength these materials were generally offset by their
inferior resistance to oxidation and corrosion. The next phase, with aluminium, a surface
enrichment resulted that could provide the necessary protection to super alloy gas turbine airfoil
[35]
.
Although such coating provide considerable relief to the then existing immediate
problems, their scope for general applications were limited particularly at temperatures of the
order of 11000C and in severe hot corrosion environments. Other alternatives such as surface
enrichment with chromium or outward diffusion of silicon in Ni based alloys have also been
considered but without much success. Efforts in developing suitable alloy composition resulted
in several new systems based on MCr, MCrAl and MCrAlY
[35-37]
. The alloy composition could
be tailored to suit a combination of requirements such as corrosion resistance and durability
coupled with moderate oxidation stability at high temperature. These compositions in the form of
overlay coatings give rise to increased ductility characteristics. This advantage was put to
maximum use over the subsequent decades when ceramic coatings were introduced for surface
protection. In particular, MCrAlY came to be used as an intermediate layer or coating,
designated as ‘bond coat’ in order to tackle the problems of thermal expansion mismatch
between the metal substrate and ceramic coating[35,38]. In spite of the several advances made in
developing suitable metal alloy coating composition and ceramic components for several
decades, the high temperature oxidation and thermo-mechanical fatigue of the metal components
remain problematic. For overcoming these problems the solution was the use of TBCs.
TBCs were first developed at NASA Lewis (Lewis Research centre, National
Aeronautics and Space Administration, Ohio, USA) and a few other research laboratories in the
world. Conceptually, TBCs were meant to drive the advantage offered by both metals and
ceramics simultaneously, in the best possible way without making any additional compromises
[39-43]
. In a broad sense, TBCs are ceramic over layers, which provide thermal insulation to the
metal on which they are applied. Thermal barrier coating consists of a metal substrate coated
with ceramic layer and an intermediate metallic bond coat layer (Fig 2.1).
Fig 2.1 Schematic diagram of thermal barrier coating.
The properties of given TBC coating depend upon the specific application for which it is
designed. They are achieved through a proper selection of the materials and the process
parameters. In general a good TBC should exhibit some essential characteristics. Most
prominently, it should withstand high temperature and should remain strongly adhere to the
metal substrate on which it is coated, without peeling off during thermal cycling. Accordingly
the TBC material should have low thermal conductivity, a relatively high coefficient of thermal
expansion (to match the expansion coefficient of metal substrate), thermodynamic stability in the
environment of its use and thermo-mechanical stability. It has been observed that only ceramic
coatings posses such a combination of favourable characteristics at high temperatures, which is
evident in their resistance to oxidation, corrosion, erosion and wear. However the critical
requirement of a TBC is its heat insulating capability of the material ie low value of thermal
conductivity of ceramics is of great importance, but a majority of them are poor in adhesion to
the metal substrates on which they are to be coated. The low thermal expansion and poor thermomechanical stability of ceramics at high temperature also pose problems and these limitations are
overcome to a significant extent by the use of bond coats.
Factors affecting TBC performance
Miller etal [65] have critically reviewed the present state of work on TBCs for Gas turbine
engines employed in electric power generation. Although TBCs based on Y2O3-ZrO2 ceramics
coated over a layer of MCrAlY bond coat were found to perform satisfactorily in clean fuel
based electric cutting applications, they were not found fit for use in propane fuel that consists of
salts which would melt at the temperatures of the burning fuel. Bennet [66] has discussed a variety
of mechanisms that degrade TBCs among which some are specific to the bond coat and others to
the TBC ceramic. The primary degradation mechanisms are the following;
(i) Oxidation and the consequent degradation of the bond coat
(ii) De-bonding and separation of the ceramic coating from the bond coat due to the
compressive stress developed at the bond coat/ ceramic interface and a tensile stress normal to
the plane of the bond coat.
(iii) Chemical instability of the ceramic coating which has been reported to occur in
composition like 24 % Mgo-ZrO2.
A rapid precipitation of magnesia appears to occur when this composition is exposed to
temperatures higher than 9500C. This is because the cubic phase of MgO-ZrO2 solid solution
provides thermal insulation but the free MgO precipitate promotes thermal conduction. Increase
of thermal conductivity is not desirable for TBC. In the presence of molten salt ( for eg in the
flames of impure fuel) it has been reported that Y2O3 reacts with the salt leading to
destabilization of the Y2O3-ZrO2 cubic phase even when there is no reaction, as observed in case
of MgO-ZrO2, the molten salt can penetrate the coating, oxidize the underlying bond coat and
spall the TBC. In the YSZ system, 20% Y2O3-ZrO2 composition coated by conventional plasma
spraying techniques exhibits structures with high densities of micro cracks near bond coat
interface. Therefore these coatings perform poorly in thermal cyclic tests [67]. The combination of
8 % Y2O3-ZrO2 with a highly oxidant bond coat produced by plasma spraying appears to provide
the best combination as a TBC system and provide adequate durability in turbine blade airfoil
applications. Apart from mechanical degradation during thermal cycling, these coatings are also
damaged by a chemical reaction in a reactive environment containing vanadium and sulphur.
Another important problem associated with yttria partially stabilized zirconia (YPSZ) TBCs
arises from the high temperature ionic conductivity of zirconia because of its diffusivity for
oxygen which promotes oxidation of the bond coat. Of the several factors which limit the life of
TBCs, oxidation of the bond coat is perhaps the most important.