Microscratch Experiments Four sets of samples were made where a set constituted a series of depositions with varying interface silver level. All the films have a thickness of about 100 nm, as measured by the Dektak3 profilometer. The amount of silver at the niobium/sapphire interface varies from less than 0.8 monolayers to 6.4 monolayers as measured by RBS. Details of the experiment are given in ref. [5]. All samples with measurable silver level (>0.8 monolayer) at the interface failed during the scratch test with the niobium films delaminating from the sapphire substrates in a “brittle manner”. The failure was characterized by multiple spallations as the film detached from the substrate (Fig. 1a). Buckling was also observed along the scratch track in areas prior to the first spall. The tangential load shows abrupt changes corresponding to the spalls and the breakthroughs of pile-up material in front of the indenter. Interfacial toughness can be obtained from the geometry of the first spallation and the tangential load at which it happened. On the other hand, there was no indication of interfacial failure during the scratch test for the sample with <0.8 monolayer of silver at the interface, as well as the sample without silver . The film underwent ploughing with no evidence of delamination (Fig. 1b). No value of interface fracture toughness was estimated for these two samples because of the absence of interfacial failure. (a) (b) Figure 1: SEM micrographs of scratch tracks for (a) sample with interface delamination ( PVD, 4.2 monolayers of silver), and (b) sample without interface delamination ( PVD, < 0.8 monolayers of silver). The results from microscratch test showed a strong correlation between the interface fracture toughness and the amount of silver at the interface, as plotted in Fig. 2a. This result is in good agreement with the general trend of the interface fracture toughness as a function of the work of adhesion. As been pointed out by Elssner et al. [2], the interface fracture toughness Gc, increases exponentially with the work of adhesion Wad: , where the exponent n is a function of the orientation of the metal constituent. They found that the value of n for the (100) interface plane of niobium is 1.9, and that for the (110) interface plane of niobium is 3. Gc ∝WAd n The leveling off in interface fracture toughness at a silver level of 4.2 monolayers implies that the silver coverage reached 100% at a level of 4.2 monolayers. For the interface without silver, the work of adhesion (Wad) is that of pure niobium/sapphire: 800 mJ/m2 [6]. For interfaces that have 100% coverage of silver, it was found that the interface fracture occurred at the silver/sapphire interface [7], which has a work of adhesion of 150 mJ/m2 [8]. Assuming a linear interpolation of the work of adhesion for intermediate value of silver coverage, Fig. 2a can be replotted as a function of the work of adhesion (Fig. 2b). A least square fit of this curve using a power law function gave a value of 3.0 for the exponent n in Eq. 3. This result is in very good agreement with the value of 3.0 obtained by Elssner et al. [2].
RkJQdWJsaXNoZXIy MjM0NDE=