Solid-state reaction bonding of silicon carbide (HIP-SiC) below 1000 degrees C

A solid-state method of joining Si-free hot isostatically pressed SiC (HIP-SiC) at relatively low temperatures is described. The bonding method involved prereacting of HIP-SiC specimens with Cr at 1000 degrees C to produce the required thermodynamically stable ternary phase (tau, Cr5Si3C) and then diffusion bonding of the coated specimens employing Ni foil as an intermediate layer. The diffusion bonding of the prereacted specimens was carried out in a locally heated diffusion bonding equipment in a temperature range 700-1000 degrees C. The quality of the joint was assessed by microstructural characterisation. The coatings obtained on the HIP-SiC specimens acted as a diffusion barrier and avoided excessive reaction between Ni and SiC. Optimum bonding conditions were 940 degrees C and 3 h. Scanning electron microscopy (SEM)/energy-dispersive X-ray microanalysis (EDX) and metallographic analysis showed good metallurgical compatibility and the absence of discontinuities and micropores at the interfaces in the bonded assembly. The proposed bonding scheme is superior to bonding SiC using Ni or Ni-Cr foils alone. The excellent control of the reaction zone width and morphology also opens the prospect of joining SiC to Ni base alloys.