## 5 Nanoindentation of Diffusion-Bonded Interfaces
### Steel/Tungsten
Increased bonding time resulted in higher tensile strength of the bond, which was attributed to increased interdiffusion of W/Cu and Cu/(Fe,Cr,Ni) atoms across the respective interfaces. Fracture was found to take place in the centre of the interlayer, occurring in a ductile manner. Figure 7: Nanoindentation measurements showing the variation in hardness across the (**a**) W/Ni and (**b**) Ni/ferritic steel interfaces [17]. A Cu interlayer was also used by [PERSON] et al. [PERSON] et al. (2018) to bond a W-7 Ni-3 Cu alloy to Type 304 stainless steel. The interlayer thickness was 0.1 mm and the bonding was carried out at 900 \({}^{\circ}\)C and 20 MPa for 2 h. Examination of the interface revealed the interdiffusion and formation of W(Cu) and \(\gamma\)-Fe(Cu) solid solutions with no intermetallic phases. Nanoindentation also indicated solid solution strengthening with elevated hardness values recorded in the regions of interdiffusion either side of the Cu interlayer. The presence of the interlayer was found to increase the tensile strength of the bond by 58% compared to W bonded to stainless steel in the absence of an interlayer. The bond was found to fracture close to the Cu/steel interface by a mixed mode of plastic fracture and intergranular fracture. In another study, [PERSON] and [PERSON] [PERSON] and [PERSON] (2018) studied the bonding of W to 9% Cr-1% W steel. A Nb interlayer was used to reduce the residual stresses arising from the mismatch in thermal expansion coefficient between W and steel. The bonding was carried out at 1050 \({}^{\circ}\)C for 1 h followed by a post-bond heat treatment of 760 \({}^{\circ}\)C for 90 min. Nanoindentation measurements across the steel/Nb interface showed the reaction layer, the hardness of which (\(\sim\)5.5 GPa) was significantly higher than either steel or Nb, both being \(\sim\)3.0 GPa. This reaction layer was suggested to be niobium carbides (Nb\({}_{2}\)C and Nb\({}_{6}\)C\({}_{5}\)) and its thickness was 7-8 \(\upmu\)m. It was brittle in nature, as demonstrated by Charpy impact tests and tensile tests. Fracture of the specimen occurred at the steel/Nb interface with no apparent plastic deformation. The same authors [PERSON] and [PERSON] (2018) also investigated the use of a vanadium interlayer to bond 9% Cr steel to tungsten. A 1 mm thick V interlayer was used between steel and W. Bonding was carried out under the same conditions as in the previous study [PERSON] and [PERSON] (2018). Nanoindentation was used to study the hardness variation across the steel/V and V/W interfaces; no information was given on the nanoindentation test conditions. Examination of the interfaces in transverse sections did not reveal any unbonded regions of microcracks; no reaction layers were seen at the V/W interface. In contrast, two regions of 8 \(\upmu\)m and 6 \(\upmu\)m in thickness were seen at the steel/V interface; they were inferred to be reaction layers formed by interdiffusion across the interface. Auger electron spectroscopy (AES) of the interface revealed that V had diffused into the steel, resulting in the formation of a vanadium carbide (V\({}_{2}\)C) layer, the thickness of which was approximately 6 \(\upmu\)m. A second layer adjoining the carbide, which was ascribed to sigma phase containing Cr from the steel, was also seen. On the V/W interface there was evidence of a solid solution being formed. Nanoindentation measurements across the bond interfaces (see Figure 9) revealed that the hardness of the carbide layer at the V/steel interface (\(\sim\)14 GPa) was approximately five times that of the  
Figure 8: Nanoindentation measurements showing the variation in hardness across the stainless steel/copper and copper/tungsten interfaces Basuki and Aktaa (2018). steel (\(\sim\)3 GPa). At the V/W interface, a continuous increase in hardness (see Figure 9) was observed from the V (2.2 GPa) to the W (7 GPa); the intermediate values were cited as evidence for the absence of any intermetallic phases at this interface.