Ultra High Vacuum Diffusion Bonded Nb--Al sub 2 O sub 3 and Cu--Al sub 2 O sub 3 Joints--the Role of Welding Temperature and Sputter Cleaning

Polycrystalline Nb and Cu are welded in UHV (2 x 10 exp --10 mbar) to alumina (99.7 wt.%) at various temperatures. Prior to joining, the surfaces of the metal and ceramic to be welded are sputter-cleaned by Ar ions at 3-5 keV with a maximum dose of 5 x 10 exp 19 Ar exp + /cm exp 2 . The amount of bonded area at the interface depends on welding temperature and welding time. After 1 h welding time, the fraction of bonded area is 98% for Nb--Al sub 2 O sub 3 joints at 0.65T sub m (T sub m = melting point of the metal in K). The amount of unbonded regions at the interface of Cu--Al sub 2 O sub 3 joints decreases from 20% after 1 h welding time to 5% after 3 h welding time at 1010 deg C. Plastic flow and creep determine the pore closure mechanism at the metal/ceramic interface. Fracture energy G sub c and fracture resistance K sub c of the UHV bonded metal--ceramic joints depend strongly on welding temperature and the amount of bonded area. For Cu--Al sub 2 O sub 3 joints, sputter-cleaning is a prerequisite for reliable measurements of the fracture energy. Without sputter-cleaning, most of these joints did not withstand the cutting procedure during fabrication of four-point bend test beams. The fracture energy of Nb--Al sub 2 O sub 3 joints manufactured without sputter-cleaning is low and can only be determined for joints welded above 1500 deg C. Impurities at the metal/ceramic interface are assumed to be responsible for the decrease in bond strength of joints manufactured without sputter-cleaning.