Microstructure evolution and improved mechanical strength of the C/C-Mo joint by adjusting the joining temperature in a wide temperature range

Abstract In order to reduce the residual thermal stress to obtain the C/C-Mo joints with high mechanical properties, the composition and distribution of the phases were optimized by adjusting the joining temperature in a wide range. As the joining temperature increases from 1180 °C to 1380 °C, the joint strength first increases and then decreases, which reaches a maximum (47.84 MPa) at 1330 °C, 42.30% higher than the joint brazed at the common utilizing temperature of the BNi-5 filler (1180 °C). The interlayer of the joint is divided into two regions: Zone 1 with Ni-rich MoNiSi and a little Ni solid solution (Niss), and Zone 2 with Mo-rich MoNiSi and high content of Niss. As the temperature increases, the thickness of Zone 2 increases while that of Zone 1 reduces due to the phase transition from Ni-rich MoNiSi to Mo-rich MoNiSi. This microstructure evolution introduces more Niss in the joint, improving the joint strength by reducing residual thermal stress via plastic deformation. Furthermore, a Ni-based interfacial layer including complex compounds with smaller sizes in the joints brazed at higher temperature leads to the stronger bonding between C/C composites and braze. The straight fracture path along the interface changed to the tortuous path in the braze seam, leading to more energy consumption, which results in the improved joint strength. The excessive temperature (1380 °C) would lead to the big sizes of the brittle MoNiSi phase, resulting in the formation of pores and cracks in the joints and poor joint strength.