Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Patent
1994-12-14
1997-07-29
Robinson, Ellis
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
428212, 428215, 428217, 428220, 428469, 428472, 428699, 428701, 428615, 428621, 428622, 428623, 428632, 428633, 428635, 4272481, 427250, 4272557, 427535, B32B 1504, B32B 1800
Patent
active
056520441
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention is concerned with coatings for metallic substrates, and is particularly concerned with ceramic thermal barrier coatings for metallic components for use in gas turbine engines.
BACKGROUND OF THE INVENTION
It is known in the prior art to apply ceramic to a metallic substrate to produce a ceramic thermal barrier coating by the thermal, or plasma, spray process. In this technique the ceramic is applied onto a bond coat, for example a MCrAlY bond coat, which has been applied to the metallic substrate.
It is also known in the prior art to apply ceramic to a metallic substrate to produce a ceramic thermal barrier coating by the physical vapour deposition (PVD) process. In this technique the ceramic is applied onto a bond coat, for example a MCrAlY coating with an alumina interface layer, which has been applied to the metallic substrate. The ceramic thermal barrier coatings deposited by the PVD process have benefits over the ceramic thermal barrier coatings deposited by a thermal spray process. The main benefit is improved thermal shock resistance due to the columnar structure of the ceramic thermal barrier coating produced by the PVD process. Other benefits are improved erosion resistance and improved aerothermal performance.
However, despite these advantages, the ceramic thermal barrier coating deposited by the PVD process exhibits a thermal conductivity which is greater than that of a ceramic thermal barrier coating, of the same or similar composition, deposited by the thermal spray process. For example the thermal conductivity of a zirconia-8% yttria ceramic thermal barrier coating deposited by the PVD process is 2.0 W/m/K and the thermal conductivity for the same ceramic thermal barrier coating deposited by the thermal spray process is 0.8-1.0 W/m/K. If all other factors are the same for the two methods of deposition of the ceramic thermal barrier coating, the greater thermal conductivity of the ceramic thermal barrier coating deposited by the PVD process means that a greater thickness of ceramic is required to achieve the equivalent insulating effect when compared to the ceramic thermal barrier coating deposited by the thermal spray process. This is an undesirable property because this necessitates a greater weight of ceramic thermal barrier coating on the metallic components of the gas turbine engine, and this is particularly undesirable for rotating components e.g. turbine blades because the additional weight may limit the temperature of operation due to a corresponding reduction in the creep life of the metallic turbine blade.
A paper entitled "Microlaminate Composites as Thermal Barrier Coatings" by M. C.Radhakrishna, H. J. Doerr, C. V. Deshpandey and R. F. Bunshah was presented at the 15th International Conference on Metallurgical Coatings, at San Diego, USA, 11-15th Apr. 1988 and was subsequently presented in Surface and Coatings Technology, 36 (1988) 143-150. The paper discloses the use of microlaminate composites to reduce the thermal conductivity of physical vapour deposited thermal barrier coatings. The microlaminate composites comprise alternate layers of two different materials, e.g. two different metals, two different ceramics or a metal and a ceramic. The paper specifically describes the use of nickel layers interposed between layers of NiCoCrAlY and titanium layers interposed between layers of CoCrAlY. These microlaminates have thermal conductivities of 7.48 W/m/K for a 480 layer Ni/NiCoCrAlY coating system and 6.76 W/m/K for a 480 layer Ti/CoCrAlY coating system. The paper then suggests that the thermal conductivity of the microlaminate composite may be tailored to obtain thermal conductivity values similar to those of yttrium stabilised zirconia deposited by the thermal spray process by choosing appropriate metal and ceramic microlaminates.
A paper entitled "Sputter-Deposited Multilayered Ceramic/Metal Coatings" by J. W. Patten, M. A. Bayne, D. D. Hayes, R. W. Moss and E. D. McClanahan was presented at the International Conference on Metallurg
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Robinson Ellis
Rolls Royce plc
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