Braze microstructure evolution and mechanical properties of electron beam joined ceramics

A study was conducted to demonstrate the feasibility of ceramic joining using a high-energy (10 MeV) electron beam. Special emphasis was paid to define the beam operating parameters necessary to produce strong, hermetic seals. Electron beams with energies in this range penetrate through a significant thickness of ceramic (>1 cm), depositing their energy deep within a part. Thus, this process provides a special capability for producing joints at buried interfaces. Scanning electron microscopy (SEM) and an energy dispersive X-ray spectrometer (EDS) were used to evaluate the degree of reaction between buried braze metals and ceramic substrates. The mechanical integrity of cylinder joints was evaluated using a standard tensile strength test. The hermeticity of the joint was also tested. Specimens joined by electron beams were compared to specimens fabricated by conventional oven brazing. Prior to strength testing or sectioning for microstructural examination, some samples were examined non-destructively using ultrasound to evaluate the degree of bonding in the joint area. The microstructures of these ultrasonically identified ‘strongly’ and ‘weakly’ bonded regions were studied and compared. The sequence of phase formation at the ceramic-braze interface is explained in terms of changes in the chemical activity of titanium and microstructure in the solidified brazes. Mechanical strength and microstructural evolution appear to correlate well with the reaction kinetics involved at the ceramic-braze interface.