## 5 Nanoindentation of Diffusion-Bonded Interfaces
### Aluminium/Copper  
Another diffusion-bonded system that has been investigated using nanoindentation is Al/Cu, which is one of many metal intermetallic laminate (MIL) systems that have been investigated for potential application in armour materials [55]. The use of diffusion bonding to join Al and Cu has been the subject of studies by several researchers [54; 55].  
[PERSON] and [PERSON] [54] used nanoindentation to map the mechanical properties of Al and Cu bonded by vacuum hot-pressing. The specimens were diffusion bonded at temperatures of between 350 \({}^{\circ}\)C and 650 \({}^{\circ}\)C for a duration of 10 min. At temperatures below 500 \({}^{\circ}\)C, insufficient bonding between Al and Cu occurred. In contrast, the specimen bonded at 550 \({}^{\circ}\)C exhibited three distinguishable layers at the interface, which were identified using TEM as Al\({}_{2}\)Cu, AlCu/Al\({}_{3}\)Cu\({}_{4}\) and Al\({}_{4}\)Cu\({}_{9}\). The Al\({}_{2}\)Cu was seen closest to the Al parent metal, while Al\({}_{4}\)Cu\({}_{9}\) was in the reaction layer adjacent to the Cu parent. These two phases were separated by a mixed phase region composed of AlCu and Al\({}_{3}\)Cu\({}_{4}\). Nanoindentation arrays of 5 \(\times\) 15 indents were generated in the interfacial region. The maximum depth of the indents was 150 nm, which means that the maximum indent diagonal was \(\sim\)1.05 \(\upmu\)m. The indent spacing was not given. The nanoindentation measurements found that the hardness of all three intermetallic phases were significantly higher than both parent metals. The region having the highest hardness was the mixed AlCu + Al\({}_{3}\)Cu\({}_{4}\), the mean value of which was 6.33 GPa, which is more than four times the hardness of either parent metal. It was speculated that this region might confer a brittle character to the entire diffusion bond.  
[PERSON] et al. [55] used nanoindentation to map the mechanical properties across Al/Cu diffusion-bonded interfaces. Ten Al and Cu discs were alternately stacked and bonded under a dead weight (equating to a pressure of 12 MPa) in a vacuum furnace. The temperatures of the bonding operations ranged from 450 to 475 \({}^{\circ}\)C, while the durations were between 90 and 120 min. This resulted in the formation of three distinct intermetallic layers at the interface. Increased temperature and/or time resulted in an increasing total thickness of the intermetallic layer. Nanoindentation measurements (maximum load 10 mN) performed on sections of the interfaces found discernible variations in the properties of these phases. The variations in hardness and elastic modulus across the interface are shown in Figure 6. Al\({}_{4}\)Cu\({}_{9}\) exhibited the highest hardness of \(\sim\)1.5 GPa, which was significantly more than the corresponding values of either Cu (\(\sim\)0.8 GPa) or Al (\(\sim\)0.45 GPa).  
Figure 5: Nanoindentation of a composite heat treated at 550 \({}^{\circ}\)C for 180 min. (**a**) BSE SEM image showing an indented interface region, where all indents from Al, Layer 1 (\(\alpha_{\text{c}}\)-Al\({}_{15}\)(Fe,Cr,Mn)\({}_{3}\)Si\({}_{2}\)) and steel were placed within single-layer regions; (**b**) Measured hardness values from all indents located within single-layer regions. The black lines show average values, while the dashed grey lines show plus and minus one standard deviation; (**c,d**) Load–displacement curves including and excluding Al and steel, respectively [19].