Liquid phase bonding of siliconized silicon carbide

Abstract

Aluminium was used as a braze to join siliconized silicon carbide to itself. Brazes were carried out in the 700–1100 °C temperature range, in vacuum. A thick reaction layer forms in the ceramic adjacent to the braze film, due to reaction between the metal braze and the free silicon present in the ceramic matrix. The silicon concentration of the braze film reaches values well above the maximum liquid solubility at the brazing temperature. A pseudotransient aluminium-silicon liquid phase promotes the formation of a 100% silicon braze film when either high temperatures, long holding times or very slow cooling rates are used. The dominant mechanism responsible for the formation of the braze microstructure is the preferential unrestrained solidification growth of Si plates on the braze plane, supported by fast liquid Si diffusion. Strong joints were produced and, when pure silicon brazes formed, four-point bend strengths over 200 MPa were obtained at testing temperatures as high as 700 °C. Fracture occurs either in the reaction layer-ceramic boundary or in the braze, the crack propagation plane changing from one side of the braze-ceramic interface to the other and through the braze itself.