## 5 Nanoindentation of Diffusion-Bonded Interfaces
### Steel/Tungsten  
Diffusion bonding has also been used to join tungsten to steel for possible use as blankets and divertors in nuclear fusion reactors [58] which are subjected to highly energetic particles resulting from the fusion reactions [57]. The properties of tungsten are exploited in applications requiring resistance to high heat fluxes and physical and chemical sputtering processes [58]. For use in these applications the tungsten must be bonded to a reduced-activation ferritic/martensitic (RAFM) alloy such as EUROFER97, a 9% Cr-1% W steel. However, when [PERSON] and [PERSON] [56] bonded tungsten to EUROFER97, they found that although the bond exhibited a high tensile strength it was brittle owing to the presence of a thin layer of FeW and metal carbides at the interface. To prevent the formation of these reaction layers, numerous interlayers have been used by various researchers with nanoindentation used to assess the properties of the interfaces, details of which will be discussed below. [PERSON] et al. [17] used a Ni interlayer in the diffusion bonding of ferritic steel to tungsten for plasma-facing components in nuclear fusion reactors. The interlayer was in the form of a foil 0.3 mm in thickness, its function being to reduce the residual stresses caused by the mismatch in thermal expansion coefficient and to prevent the formation of brittle intermetallics FeW and Fe\({}_{7}\)W\({}_{6}\). Diffusion bonding was carried out at temperatures of between 850 and 950 \({}^{\circ}\)C for 1.5 h in a vacuum hot press furnace. Microstructural examination of the bonded interfaces revealed a Ni-rich solid solution (Ni(W)), 3.5-6.0 \(\upmu\)m in thickness, formed at the Ni/W interface, together with a thin layer of Ni\({}_{4}\)W which had formed at the highest bonding temperature (950 \({}^{\circ}\)C). In contrast, at the Ni/steel interface, no intermetallic phases were seen; instead, solid solution-type elemental concentration profiles were observed across the interface. This was attributed to the good compatibility between steel and nickel, as predicted by the Fe-Ni and Cr-Ni phase diagrams. Nanoindentation of the interfaces was carried out using a Berkovich indenter at a maximum load of 30 mN. The measurements (see Figure 7) showed a step-change across the W/Ni interface. The Ni(W) was slightly lower than that of W but significantly higher than that of Ni. The thickness of Ni\({}_{4}\)W was insufficient to enable its mechanical properties to be measured. The diffusion zone between the Ni and ferritic steel exhibited a higher hardness than either parent material, reaching a maximum about 5 \(\upmu\)m from the interface between the diffusion  
Figure 6: Hardness and elastic modulus variation across the interface of a diffusion-bonded Al/Cu joint [55]. zone and ferritic steel. The maximum indent depth varied from 400 nm (W) to 700 nm (Ni), and the indent sizes were between 2.8 \(\upmu\)m (W) and 4.9 \(\upmu\)m (Ni). [PERSON] et al. [59] investigated the diffusion bonding of W to stainless steel type 316L. A Cu foil, 0.25 mm in thickness, was used as an interlayer. Bonding was carried out at 1050 \({}^{\circ}\)C and a pressure of 150 MPa for between 0.5 and 2 h. Unlike other studies, no intermetallic compounds were formed at the interface. Instead, diffusion of Cu into both parent metals was observed using SEM/EDS, which resulted in solid solution strengthening of regions of the parent metals adjacent to the interface. This was also reflected in nanoindentation measurements (see Figure 8) which showed elevated hardness of these regions, particularly in the W, which increased to \(\sim\)10.2 GPa (c.f. \(\sim\)7.5 GPa for base W). This was also seen in the SS316L/Cu interface, albeit to a lesser extent. The region of lowest hardness in the entire joint was the Cu interlayer, which was \(\sim\)1.5 GPa. No information was provided on the conditions used in the nanoindentation measurements.