Use of Cobalt based brazing mixture to avoid interaction between MCrAlY coating and brazed area. Name: SiCoBraze
Original Publication Date: 2005-Jan-25
Included in the Prior Art Database: 2005-Jan-25
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Nickel based mixtures are used for brazing of Nickel based materials. The mixture consists of a high melting powder, typically the same materials as the substrate, and a low melting powder, typically a composition that matches the substrate with addition of Boron as a melting point suppressant. Brazing is widely used in gas turbine components to repair cracks and to build up surfaces. After repair, the components are coated with a MCrAlY (Metal Bond Coat (MCrAIY, M = Ni, Co)) coating for oxidation resistance. A thermal barrier coating (TBC) layer may also be applied on top of the MCrAlY coating for reduction of the metal temperature, which allows the components to operate under higher temperatures. Since the solubility of Boron on the Nickel matrix is very limited, a large amount of Boron is available to diffuse freely from the brazing into the MCrAlY coating. There, it reacts with Chromium, forming a Chromium-Boride (CrB) phase that has low ductility at the operating temperature. CrB are observed both as a continuous layer at the interface between the substrate and the MCrAlY. Premature coating spallation may occur due to the low ductility of the continuous layer at the interface, resulting in a reduced part life. Also, Cr content available for oxidation resistance of the coating is reduced, since CrB phase is very stable. This leads to a reduced oxidation resistance, which can result in earlier spallation of the TBC due to the enhanced oxide formation. As a result, the metal temperature equals the operating temperature.