This invention, intended for use in the processing of turbine engine blades’ thermal barrier coatings, is a two-step procedure used to produce a thermally grown oxide that is completely devoid of lifetime-limiting spinel oxides. Both steps take place at the same temperature used in present day bond coat pre-oxidation, utilize everyday gases, and can be performed serially in the same furnace, in a matter of hours. In step one, pre-oxidation of a bond-coated blade yields a thermally grown oxide (TGO) layer that contains a limited amount of spinel. In step two, all spinel is removed in situ. In an industrial-scale setup, the entire process would take place in less than 24 hours, including ramp times to and from the exposure temperature. Once blade specimens are cooled and removed from the furnace, they are then ready to be coated with the thermally protective yttria-stabilized zirconia (YSZ) layer, using industry-standard techniques. Due to the nature of the process, no new spinel is expected to grow at the critical TGO–YSZ interface for as long as the part operates in service, which means that the blade will be completely spinel-less for its entire usable lifetime. By eliminating all spinel-related failure mechanisms, this may result in longer blade lifetimes and therefore significant cost reduction.
In any high temperature turbine engine, the proliferation of harmful spinel oxides in the TGO layer of engine blades’ thermal barrier coating systems can cause catastrophic failure of extremely expensive components. Spinel forms a weak interface with the thermally protective, ceramic YSZ outer layer, which may result in the YSZ detaching, thus hazardously exposing the metal blade to turbine combustion temperatures. Manufacturers therefore seek ways to mitigate the development of spinel both prior to and during engine operation.
Previous methods patented by manufacturers call for “pre-oxidation” routines that develop a low-spinel, high-a-aluminum-oxide TGO layer in a controlled laboratory environment. Reducing the amount of spinel in the TGO layer necessarily reduces the amount of spinel–YSZ interface, which delays the onset of the above-described failure mechanism. There are two main, existing categories of pre-oxidation. The most common takes place at high temperature in a vacuum furnace. The second involves depositing a thin, metastable-aluminum-oxide phase onto the metal bond coat (often in a vacuum chamber), coating it with YSZ, then heat-treating to form the desired a-aluminum-oxide TGO/YSZ system.
There are major disadvantages to these two methods. With the first method, using a vacuum furnace adds complexity and cost, and merely limits the amount of spinel produced; it does not eliminate spinel completely. With the second method, (1) an applied aluminum-oxide coating is not expected to adhere as strongly to the metal bond coat as a thermally grown oxide would, and (2) while the method does ensure a high volume ratio aluminum oxide at the YSZ–TGO interface, it does nothing to suppress spinel formation in the TGO that will naturally form beneath this layer upon first high temperature treatment; spinel would form and exist for the lifetime of the blade, now as part of a second TGO layer, providing another potentially problematic interface.
This technology eliminates the need for vacuum furnaces, ensures complete removal and subsequent suppression of spinel, and utilizes everyday gases, making it feasible for industrial-scale processing. The result is a low-cost, spinel-less TGO-YSZ interface.
By eliminating the need for vacuum furnaces and utilizing everyday gases at ambient pressure, processing is expected to be considerably less expensive than other methods
Completely removes – and subsequently suppresses – spinel, resulting in a spinel-less system for the lifetime of the part
There is flexibility in how the two steps are set up to complement each other, which allows the process to be optimized according to the manufacturer’s needs