Effect of surface condition and bonding pressure on quality of diffusion bonded high purity copper for linear collider accelerator structures

Tests have been carried out to assess the feasibility of diffusion bonding as a fabrication technology for vacuum tight joints in linear accelerator cells for the Next Linear Collider. High purity copper specimens were diffusion bonded over a range of temperatures from 400 to 1000°C, under high (3.45 MPa) and low (3.45 kPa) bonding pressures, and at two different diamond machined surface finishes. Experiments showed that diffusion bonds with strengths equal to, or greater than, that of silver brazed joints could be made at temperatures ≥700°C at the 3.45 MPa bonding pressure, or ≥800°C at the 3.45 kPa bonding pressure. Partial strength diffusion bonds were made at temperatures as low as 400°C at the high bonding pressure, whereas no bonding (zero strength) was observed at temperatures below 700°C at the low bonding pressure. Observations of the fracture surfaces of the diffusion bonded specimens showed that bonding begins by point asperity contact. At low bonding pressures, surfaces created by diamond turning of annealed copper specimens produce higher strength bonds than those created by diamond flycutting of unannealed surfaces, whereas at higher bonding pressures the effect of surface finish was less important.