The effect of microstructure and chemistry on the tensile strength of Nb/sapphire interfaces with and without interlayers of Sb and Cr

Tensile strengths of interfaces between sputter-deposited Nb coatings and (0001) surfaces of sapphire, with and without 1--40 angstrom thick interlayers of Sb and Cr are measured by a modified laser spallation experiment. The average tensile strength of 1.22 GPa for the Nb/sapphire interface can be continuously increased to 2.3 GPa by controlling the thickness of the Cr interlayer, whereas by using Sb interlayers, the strength can be controllably decreased to 0.1 GPa. Although annealing the Nb/Cr/sapphire samples at 600 C for 24 h resulted in no change in the tensile strength, heating at 1,200 C for 10 min increased the strength to 2.65 GPa, due primarily to the formation of the interfacial reaction compound Cr2Nb. Similarly, annealing the Nb/Sb/sapphire samples at 400 C for 24 h marginally increased the strength to 0.3 GPa. The locus of failure upon spallation, as determined via AES and XPS analysis, was between the sapphire and the adjoining metallic layer in all the samples. Together with companion studies where the interface atomic structure and chemistry were obtained for the above interfaces using high resolution transmission electron microscopy, a strength-structure-chemistry relation is obtained for the Nb/sapphire interface, with and without interlayers of Sb and Cr