This invention relates to a thermal barrier coating material to be used for high temperature members like moving and stationary blades and combustors of a gas turbine, and a method of producing the same.
Complying with the requirement of effective utilization of energy resources, a research and development directing to make gas turbines highly efficient is aggressively going on in the field thereof. It is known that the higher the temperature of exhaust gas of the combustor is, the efficiency of gas turbine power generators will be higher.
It is therefore recognized to be important attempting to make the inlet temperature of gas turbines high for making gas turbines highly efficient, thus facilitating wide range of research and development to reduce the temperature of materials and improve heat resistance of high temperature members such as moving and stationary blades and combustors of gas turbines.
Countermeasures for reducing the temperature of materials include such constructive improvements as insulating the heat from the combustion gas using a film cooling mechanism, improving cooling characteristics using an impinge cooling mechanism or increasing cooling cross section of moving and stationary blades using a return-flow mechanism.
Countermeasures for improving heat resistance includes various improvements of materials, for example development of high temperature materials using a super alloy of Ni, Co and Fe for the purpose of increasing the strength at high temperature. However, there were some limitations in these high temperature materials that they can be hardly used at a temperature range of 900 to 950 C. where their strength begins to decrease due to softening or recrystallization ascribed to the characteristics of the materials.
For solving this problem, an art using a thermal barrier coating (TBC) has been noted. In TBC method, a thermal barrier ceramic layer of, for example, zirconia having a low thermal conductivity and being chemically stable is coated on a metallic substrate comprising a super alloy.
One example of this thermal barrier coating member has a construction provided with a metallic member (substrate) made of a super alloy mainly composed of Ni, Co or Fe, an intermediate layer covering this substrate and comprising MCrAlY (M denotes at least one of Ni, Co and Fe) being excellent in chemical resistance and anti-oxidation and a ceramic layer mainly composed of a stabilized zirconia on the intermediate layer, wherein temperature increase of the substrate can be suppressed by this ceramic layer. The intermediate layer serves for protecting the metallic substrate as well as enhancing adhesion of the substrate with the ceramic layer.
It is reported in such thermal barrier coating members that the surface temperature of the metallic substrate can be reduced by 50 to 100 C. by a thermal barrier ceramic layer with a thickness of, for example, hundreds microns (for example, Japanese Unexamined Patent Publication No. 62-211387). Accordingly, when this thermal barrier coating member is applied for the high temperature member of a gas turbine, it is possible to increase the driving gas temperature of the gas turbine. Moreover, since heat flux from the combustion gas side to the cooling gas side of the gas turbine can be more reduced, the quantity of cooling gas flow for the metallic member can be also advantageously reduced besides suppressing the temperature increase of the metallic member.
Conventional thermal barrier coating members, however, were so liable to be damaged by cracks or peeling of ceramic layers that their function as a thermal barrier was deteriorated, hence the temperature of the metallic member was increased, thereby sometimes causing troubles that the metallic member was melted or broken at worst. This is by no means preferable in operating the turbine system.
For preventing such troubles from occurring, a number of countermeasures are proposed directing to the causes of cracks and peeling of the ceramic layer, for example difference of thermal expansion between the metallic substrate and ceramic layer, or oxidation and corrosion of the intermediate layer.
Regarding to the countermeasure for the difference of thermal expansion, a method of reducing thermal strain is proposed, wherein, for example, residual stress caused by compression is impressed to the ceramic layer having low thermal expansion coefficient by applying a heat treatment after coating a ceramic layer on the metallic substrate. Since the ceramic layer is used in the residual stress field caused by compression, appearance of cracks and peeling due to the difference in thermal expansion described above may be reduced.
As a countermeasure for oxidation and corrosion of the intermediate layer, a method of improving corrosion resistance of the intermediate layer is proposed, wherein, for example, an oxide layer mainly composed of Al is previously formed on the surface of the intermediate layer to suppress oxygen from invading into the intermediate layer. Progress of oxidation and corrosion in the intermediate layer is prevented via the oxide layer in this case, thereby making it possible to alleviate peeling of the ceramic layer caused by oxidation and corrosion.
Besides the trouble causes above, cracks may often appear by a reaction sintering of the surface film of the ceramic layer, the countermeasure for which has been scarcely noted or investigated. The ceramic layer, mainly composed of zirconia (ZrO.sub.2) stabilized with Y.sub.2 O.sub.3, MgO and CaO that have low thermal conductivity and are chemically stable, have a porous structure containing many air cavities by taking reduction of heat conductivity into consideration. Therefore, thermal conductivity may be increased and heat resistance may be deteriorated when this ceramic layer has turned into a dense structure containing less air cavities.
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