Method for thermally spraying crack-free mullite coatings on ceramic-based substrates

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.