Method for thermally spraying crack-free mullite coatings on ceramic-based substrates
First Claim
1. A method comprising the steps of:
- thermal spraying a mullite powder to form a mullite layer on a substrate, the thermal spraying step being performed so that the mullite powder is heated to a deposition temperature between the peritectic and liquidus temperatures of mullite, by which the mullite powder melts to form a silica-rich liquid phase and solid alumina phases, and the mullite layer is deposited to contain alumina phases in a silica-rich mullite matrix;
following deposition of the mullite layer but before the mullite layer cools from the deposition temperature to a temperature at which cracks form in the mullite layer, heating the mullite layer in-situ for a duration and at a temperature sufficient to convert the alumina phases to mullite and thereby substantially achieve phase equilibrium within the mullite layer; and
then cooling the mullite layer to room temperature, the mullite layer being substantially free of alumina phases and silica-rich phases and free of through-thickness cracks.
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Abstract
A process for depositing a mullite coating on a silicon-based material, such as those used to form articles exposed to high temperatures and including the hostile thermal environment of a gas turbine engine. The process is generally to thermally spray a mullite powder to form a mullite layer on a substrate, in which the thermal spraying process is performed so that the mullite powder absorbs a sufficient low level of energy from the thermal source to prevent evaporation of silica from the mullite powder. Processing includes deposition parameter adjustments or annealing to maintain or reestablish phase equilibrium in the mullite layer, so that through-thickness cracks in the mullite layer are avoided.
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Citations
9 Claims
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1. A method comprising the steps of:
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thermal spraying a mullite powder to form a mullite layer on a substrate, the thermal spraying step being performed so that the mullite powder is heated to a deposition temperature between the peritectic and liquidus temperatures of mullite, by which the mullite powder melts to form a silica-rich liquid phase and solid alumina phases, and the mullite layer is deposited to contain alumina phases in a silica-rich mullite matrix;
following deposition of the mullite layer but before the mullite layer cools from the deposition temperature to a temperature at which cracks form in the mullite layer, heating the mullite layer in-situ for a duration and at a temperature sufficient to convert the alumina phases to mullite and thereby substantially achieve phase equilibrium within the mullite layer; and
thencooling the mullite layer to room temperature, the mullite layer being substantially free of alumina phases and silica-rich phases and free of through-thickness cracks. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
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Specification