**4.4. Effect of nano-sized particle on joint properties**

The effect of nano-sized particles on the microstructural development across the joint region was studied using energy dispersive spectroscopy (EDS). Figure 9(a) shows the microstructure of a joint bonded using a 5μm thick Ni coating dispersed with 50 nm Al2O3 particles. From the micrograph a 50 μm wide particle segregated zone was seen within the joint center. Also present at the center of the joint are dark clumps, which EDS analysis suggested are oxide particles (Figure 10 a and b). The presence of the oxide clusters observed, are likely Al2O3 particles which agglomerated during the deposition process. When the coating particle size was increased to 500 nm Al2O3, a similar result was obtained (see Figure.9b). WDS analyses of the joints as a function of particle size indicated that the Ni concentration of 0.95 wt% and 0.79 wt% for samples bonded using 500 nm and 50 nm respectively. The lower concentration obtained when 50 nm particles are used suggest a faster diffusivity of Ni during the bonding process. This was attributed to greater surface contact between the uncoated Al-6061 sample and the Ni-Al2O3 coating surface. Analysis of the roughness using SEM indicated that the surface roughness increased from 0.1 μm for coatings containing 50 nm particles to 0.25 μm for coating containing 500 nm particles.

**Figure 9.** Microstructure of joints bonded at 600oC for 10 min using (a) 5μm thick Ni-(50nm) Al2O3 (b) 5μm thick Ni-(500nm) Al2O3 [32].

5μm thick Ni-(500nm) Al2O3 [32].

solid/liquid interface during isothermal solidification [5].

**4.4. Effect of nano-sized particle on joint properties** 

The increase in the width of the segregated zone was attributed to increased liquid formation with increasing coating thickness. As the width of the eutectic liquid increases more Al2O3 particles are immersed in the liquid phase. These particles are pushed by the

The width of the particle segregated zone was significantly lower than that achieved when pure Ni-coatings are used as the interlayer. The difference in the width of the segregated zone between joint bonded using pure Ni coating and Ni-Al2O3 coating was attributed to the presence of nano-size Al2O3 particle in the joint center and a reduction in the concentration

The effect of nano-sized particles on the microstructural development across the joint region was studied using energy dispersive spectroscopy (EDS). Figure 9(a) shows the microstructure of a joint bonded using a 5μm thick Ni coating dispersed with 50 nm Al2O3 particles. From the micrograph a 50 μm wide particle segregated zone was seen within the joint center. Also present at the center of the joint are dark clumps, which EDS analysis suggested are oxide particles (Figure 10 a and b). The presence of the oxide clusters observed, are likely Al2O3 particles which agglomerated during the deposition process. When the coating particle size was increased to 500 nm Al2O3, a similar result was obtained (see Figure.9b). WDS analyses of the joints as a function of particle size indicated that the Ni concentration of 0.95 wt% and 0.79 wt% for samples bonded using 500 nm and 50 nm respectively. The lower concentration obtained when 50 nm particles are used suggest a faster diffusivity of Ni during the bonding process. This was attributed to greater surface contact between the uncoated Al-6061 sample and the Ni-Al2O3 coating surface. Analysis of the roughness using SEM indicated that the surface roughness increased from 0.1 μm for coatings containing 50 nm particles to 0.25 μm for coating containing 500 nm particles.

**Figure 9.** Microstructure of joints bonded at 600oC for 10 min using (a) 5μm thick Ni-(50nm) Al2O3 (b)

of Ni (81.6 wt%) present in the coating, when Ni-Al2O3 is used (see Figure 13b).

**Figure 10.** EDS analyses of joints bonded at 600oC for 10 min using (a) 5μm thick Ni-(500 nm) Al2O3 (b) 5μm thick Ni-(50 nm) Al2O3 [33]

The micrograph shown in Figure 9 a thin segregated zone was formed at the joint center in both cases. The hypothesis is that the difference in the width of the segregated zone obtained is dependent on the differences in particle size, surface roughness of the coating and the distribution of the nano-sized particles in the joint zone during bonding. The presence of nano-Al2O3 along the interface was confirmed by TEM analyses which indicated that the nano-particles are arranged along the grain boundary as shown in Figure 11 which would impart a pinning effect as described by Orowan [24].

**Figure 11.** (a) TEM image of the bonded joint when nano-sized Al2O3 particles are used in the interlayer and (b) TEM image of a nano-Al2O3 particle located at a grain boundary [33].
