0 2 4 6 8 10 12 0 100 200 300 400 500 600 Work of adhesion (mJ/m 2 ) 0 2 4 6 8 10 12 0 2 4 6 8 10 Interface fracture toughness (J/m2) Silver level (monolayer) Interface fracture toughness (J/m2) Gc ∝ Wad 3.0 (a) (b) Figure 2: Interface fracture toughness of niobium/sapphire interface as a function of (a) silver level at the interface, and (b) work of adhesion of the interface, and a least squares power law fit yielding n = 3.0. Microwedge scratch test Samples with and without silver at the interface experienced interface delamination under the microwedge scratch test. As shown in Fig. 3a, a portion of the film with a constant width was spalled. The tangential load at which the interface delamination occurred was recorded, and the strain energy release rate is calculated by Gerberich et al. [9] A decrease in interface fracture toughness due to the presence of silver at the interface is observed. The average interface fracture toughness for the sample without silver at the interface is 12.44 J/m2, while the average interface fracture toughness for the sample with silver at interface is 4.54 J/m2. The estimated values of the interface fracture toughness are consistent in magnitude with that of other metal/ceramic system where for W/SiO2 interface Gc was found to be 16 J/m2 and 4 J/m2 for 1.5 µm and 0.5 µm thick films, respectively [10]. This result is also consistent with the silver effect found by the microscratch test where the presence of silver weakened the interface of niobium/sapphire. (a) (b) 20 µm Figure 3: Micrographs of microwedge scratch tracks of (a) the PVD sample with 6.4 monolayer of silver at niobium/sapphire interface , and (b) a PVD sample with no silver at the interface. Analysis of buckles and curls in patterned lines Interface fracture toughness was also estimated from the delamination of patterned niobium lines on sapphire substrates. Both curls and buckles were found on some PVD samples. Buckling usually occurs when the film detaches from the substrate in the middle of the line, and curling occurs when the detachment is at one end of the line.
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