Effects of thin film Nb interlayer in Cu/sapphire bonds

The effects of a thin film Nb interlayer on the microstructure and fracture energies of Cu/Nb/Al2O3 diffusion bonds have been studied by four-point bend testing, TEM and SEM analyses. Single crystalline Cu and α-Al2O3 with different crystallographic orientations at the interface were used to determine the influence of the orientation relationship (OR) on fracture resistance. The Nb film interlayer deposited by electron beam evaporation on the ceramic side prior to diffusion bonding was found to be polycrystalline and fiber-textured after diffusion bonding, with the close-packed plane (110) being parallel to the (0001) basal plane of sapphire. The introduction of the thin film Nb interlayer greatly improves the Cu/Al2O3 bond strength, whereas diffusion bonding can be performed at a relatively low temperature (900°C). The orientation relationship between Cu and α-Al2O3 crystals at the interface strongly influences the fracture energy of the Cu/Nb/Al2O3 bonds. The bonds with the OR Cu(100)[011]∥Al2O3(0001)[112̄0] possess the highest fracture energies, exhibiting a ductile interface fracture mechanism. The much higher fracture energies obtained for the Cu/Nb/Al2O3 bonds relative to either Cu/Al2O3 or Nb/Al2O3 bonds are attributed to the strong adhesion of Nb to α-Al2O3 combined with a larger plastic deformation in the metal side during fracture.