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 fbr vacuum tight joints in linear accelerator c ells for the Next Linear Collider. High purity copper specimens were diffus ion bonded over a range of temperatures from 400 to 1000 degrees C, under h igh (3.45 MPa) and low (3.45 kPa) bonding pressures, and at two different d iamond machined surface finishes. Experiments showed that diffusion bonds w ith strengths equal to, or greater than, that of silver brazed joints could be made at temperatures greater than or equal to 700 degrees C at the 3.45 MPa bonding pressure, or greater than or equal to 800 degrees C at the 3.4 5 kPa bonding pressure. Partial strength diffusion bonds were made at tempe ratures as lon, as 400 degrees C at the high bonding pressure, whereas no b onding (zero strength) was observed at temperatures below 700 degrees C at the low bonding pressure. Observations of the fracture surfaces of the diff usion 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 hy diamo nd flycutting of unannealed surfaces, whereas at higher bonding pressures t he effect of surface finish was less important.