# Advanced Metal Matrix Nanocomposites
## 7 Tribological Properties of MMNCs  
MMNCs display superior tribological properties, such as surface hardness and wear resistance [184; 185; 186; 95; 187; 188]. Ultrasonic treatment has been advantageous to enhance the surface properties by uniform dispersion of nanoparticles and avoid the formation of porosity. Improvement in the surface hardness by 75% has been achieved with a reinforcement of 5 wt% of SiC in Mg alloy (AZ91D) by ultrasonic-assisted casting [131]. Using a similar casting process, [PERSON] et al. [122] observed a 20% hardness improvement in Al 356 alloy with the addition of 2 vol% SiC nanoparticles. However, very few studies have been conducted on the tribological properties of MMNCs produced by ultrasonic-assisted casting. In one of the studies on Al alloy, [PERSON] et al. [187] produced the nano B\({}_{4}\)C-Al composite by ultrasonic-assisted stir casting and conducted the dry sliding friction and wear test against EN31 hardened steel; the addition of 8 wt% B\({}_{4}\)C decreased the wear rate by \(\sim\)100% compared with pure Al (Figure 23a). The coefficient of friction has also decreased with the addition of B\({}_{4}\)C in Al, as exemplified for Al-8 wt% B4C composites (Figure 23b). This lowering of the wear rate and the coefficient of friction was observed due to formation of a mechanically mixed layer during sliding, and such a layer acted as a solid lubricant film and protected the surfaces. B\({}_{4}\)C-AA6061 composites were fabricated by ultrasonic-assisted casting [188] and, where the friction and wear were measured; AA6061 reinforced with 1.5 vol% of nano B\({}_{4}\)C particle had lower weight loss (136% lesser than the unreinforced AA6061 alloy) possibly due to the improved hardness by the uniform distribution of nanoparticles that served as load-bearing elements in the matrix. Zinc-aluminum (ZA) cast alloys, known for good castability and high wear resistance, have been reinforced with nano-sized Al\({}_{2}\)O\({}_{3}\) particles using ultrasonic-assisted stir casting [189]. Three different composites with varying amount of reinforcement (1, 2, and 3 wt%) were prepared and evaluated via the dry sliding test for a range of speed (1-2.5 m/s), load (15-60 N), and sliding distance (750-3000 m), and the wear loss was analyzed using the analysis of variance (ANOVA) to understand the significance of aforementioned parameters on wear volume. It was observed that the most important influencing parameter was sliding distance with percentage contribution (74%) followed by reinforcement content (14.4%), speed (6.5%), and load (3.5%). The higher contribution from reinforcement suggests that nano-Al\({}_{2}\)O\({}_{3}\) reduced the wear volume. Other than tribological test parameters and reinforcement content, the size of the nano-reinforcement is also important to study wear resistance. [PERSON] et al. [168] studied the effect of SiC particle size (0.5, 1, 1.5, and 2 wt%) on stir-casted AA2219 nanocomposites fabricated by the ultrasonic-assisted process; 50 nm and 250 nm sizes of SiC were used in the matrix with the observation that the 50 nm SiC-AA2219 had 25% lower wear volume compared with the 150 nm SiC-AA2219 composites. These results also reaffirmed that  
Figure 23: Tribological properties of Al-B\({}_{4}\)C composite fabricated by ultrasonic-assisted casting: (a) wear rate, and (b) coefficient of friction variation [187]. ultrasonic fabrication can certainly provide uniform reinforcement of a wide range of nanoparticles sizes and improves the tribological properties. In general, wear properties of materials are related to surface hardness; higher surface hardness provides increased wear resistance. [PERSON] et al. (2018) also corroborated the same trend by witnessing that the addition 20 vol% of Al\({}_{2}\)O\({}_{3}\) (50 nm) in 6082 Al alloy increased the hardness by \(\sim\)78%, which subsequently resulted in lower weight loss when sliding against tool steel (Figure 24).