**5. Effect of bonding variable on the mechanical properties of the joint**

## **5.1. Effect of bonding time the joint shear strength**

The shear strength of joints made as a function of bonding time is shown in Figure 12 (a). The graph show that the shear strength increased with increasing bonding time from 68 MPa at 1 minute to 137 MPa at 30 minutes. When a Ni-Al2O3 coating was used as the interlayer for a bonding time of 10 minutes, shear strength of 136 MPa was recorded. However when a pure Ni coating was used under that same bonding conditions, shear strength of 117 MPa was achieved [23]. The differences in joint shear strengths obtained were attributed to the presence of a nano-sized dispersion of Al2O3 particles within the joint and the precipitation of nickel aluminide phases within the joint region. Shen et al. [34] showed that the increase in yield strength of the nano-particle reinforced aluminum alloy is related to particulate–dislocation interaction by means of the Orowan bowing mechanism. Orowan theory suggests nano-sized particles act as barriers to dislocation motion. This mechanism leads to dislocation pile-up and an increase in the joint shear strength [35].

**Figure 12.** (a) Effect of bonding time on joint strength using 5μm thick Ni- Al2O3 coating at a bonding temperature of 600oC and (b) Effect of bonding temperature on joint shear strengths made using 5μm thick Ni-Al2O3 coatings for 10 minutes [23, 24].

The fractured surfaces of the shear tested joints were analyzed to identify the mechanism of joint failure. Figure 13(a) shows the fractured surface of a bond made for a bonding time of 1 minute. The fractograph shows an undulating surface containing shear plastic deformation with some cleavage facets indicating a mixed failure mode. The fracture appeared to have propagated through the bond-line. The result of the fractographic analyses suggests that the mechanism of failure transitioned from brittle to ductile as the bonding time increases. When the bonding time was increased to 10 minutes (see Figure 18b). The surface was characterized by an undulating appearance of plastic deformation indicative of ductile mode of failure. Additionally, fractured micro-Al2O3 particles were observed at the fractured surface, indicating a transgranular fracture through the bond-line.

XRD analyses of the fractured surfaces of bonds made at 1 and 10 minutes are shown in Figure 14. The results indicated the presence of peaks for phases such as AlFe6Si (2θ=38o), Al9FeNi and AlFeSi. The literature showed that these ternary compounds forms readily in Al-Mg-Si-Fe-Ni systems through various peritectic reactions [17]. In addition, binary crystal phases such as Al3Ni, Ni3Si (2θ=78o). Al3Si and Al2O3 compounds were also identified.

324 Advanced Aspects of Spectroscopy

strength [35].

MPa at 1 minute to 137 MPa at 30 minutes. When a Ni-Al2O3 coating was used as the interlayer for a bonding time of 10 minutes, shear strength of 136 MPa was recorded. However when a pure Ni coating was used under that same bonding conditions, shear strength of 117 MPa was achieved [23]. The differences in joint shear strengths obtained were attributed to the presence of a nano-sized dispersion of Al2O3 particles within the joint and the precipitation of nickel aluminide phases within the joint region. Shen et al. [34] showed that the increase in yield strength of the nano-particle reinforced aluminum alloy is related to particulate–dislocation interaction by means of the Orowan bowing mechanism. Orowan theory suggests nano-sized particles act as barriers to dislocation motion. This mechanism leads to dislocation pile-up and an increase in the joint shear

**Figure 12.** (a) Effect of bonding time on joint strength using 5μm thick Ni- Al2O3 coating at a bonding temperature of 600oC and (b) Effect of bonding temperature on joint shear strengths made using 5μm

The fractured surfaces of the shear tested joints were analyzed to identify the mechanism of joint failure. Figure 13(a) shows the fractured surface of a bond made for a bonding time of 1 minute. The fractograph shows an undulating surface containing shear plastic deformation with some cleavage facets indicating a mixed failure mode. The fracture appeared to have propagated through the bond-line. The result of the fractographic analyses suggests that the mechanism of failure transitioned from brittle to ductile as the bonding time increases. When the bonding time was increased to 10 minutes (see Figure 18b). The surface was characterized by an undulating appearance of plastic deformation indicative of ductile mode of failure. Additionally, fractured micro-Al2O3 particles were observed at the fractured

surface, indicating a transgranular fracture through the bond-line.

thick Ni-Al2O3 coatings for 10 minutes [23, 24].

**Figure 13.** SEM micrograph the fractured surface of a bond made at 600oC with 5 μm thick Ni–Al2O3 for: (a) 1 minute and (b) 10 minutes

**Figure 14.** XRD analysis of the fractured surface of a bond made with 5 μm thick Ni–Al2O3 for (a) 1 minute and (b) 10 minutes and

#### **5.2. Effect of temperature on joint shear strength**

Joint shear strengths measured as a function of bonding temperature were obtained using a single lap shear test. A comparison of the joint shear strengths of bonds made at 570, 590, 600 and 620°C is shown in Figure 12(b). The test result show that the shear strength increased with increasing bonding temperature from 45 MPa at 570oC to 138 MPa 600oC. This increase in joint strength was attributed to the presence of nano-sized ceramic particles and the precipitation of intermetallic phases within the joint region. The formation of these nickel aluminide phases increased with increasing bonding temperature. Specimens bonded at 620oC gave the highest bond strength of 136 MPa. The effect of the nano-particles and the precipitated intermetallics on the composite was discussed by Zhang and Chan [45] and results in Orowan strengthening as discussed in the previous sections.

In order to compare the effect of bonding temperature on joint failure mechanisms the fractured surfaces were examined using SEM. The results collected suggested that the ductility of the joint increased with increasing bonding temperature. For a bonding temperature of 570°C a mix failure mode was observed with both shear rupture dimples and cleavage planes (Figure 15a). An XRD analysis of the fractured surface indicated a high concentration of Al2O3 particles (see Figure 16a). This indicated that at this temperature the matrix-particle (M-P) interface was the weakest point for crack propagation giving the lowest joint strength of (53 MPa). Fracture propagation was observed through the bond-line. When the bonding temperature was increased to between 590°C and 620oC XRD analyses of the fracture indicated that the amount of intermetallic formed within the joint increased to include the binary compounds Al3Si, Al3Ni and Ni3Si which suggested that failure propagated through the bond-line. Additionally, peaks of the ternary phases AlFeSi (2θ=37 and 68o) and Al9FeSi (2θ=78 and 84o). The presence of these compounds confirms the formation of peritectic reactions during bonding. At 620oC the fractured surface was characterized by shear ruptured dimples indicating a ductile failure mode which occurred in the parent metal adjacent to the bond-line.

**Figure 15.** (a) SEM micrograph of the fractured surface of a joint made using a 5 μm thick Ni-Al2O3 coatings at 570oC and (b) 620oC.

**Figure 16.** XRD spectrum of the fractured surface of a joint made using a 5 μm thick Ni-Al2O3 coatings at (a) 570oC and (b) 620oC.

#### **5.3. Effect of coating thickness on shear strength measurements**

326 Advanced Aspects of Spectroscopy

600 and 620°C is shown in Figure 12(b). The test result show that the shear strength increased with increasing bonding temperature from 45 MPa at 570oC to 138 MPa 600oC. This increase in joint strength was attributed to the presence of nano-sized ceramic particles and the precipitation of intermetallic phases within the joint region. The formation of these nickel aluminide phases increased with increasing bonding temperature. Specimens bonded at 620oC gave the highest bond strength of 136 MPa. The effect of the nano-particles and the precipitated intermetallics on the composite was discussed by Zhang and Chan [45] and

In order to compare the effect of bonding temperature on joint failure mechanisms the fractured surfaces were examined using SEM. The results collected suggested that the ductility of the joint increased with increasing bonding temperature. For a bonding temperature of 570°C a mix failure mode was observed with both shear rupture dimples and cleavage planes (Figure 15a). An XRD analysis of the fractured surface indicated a high concentration of Al2O3 particles (see Figure 16a). This indicated that at this temperature the matrix-particle (M-P) interface was the weakest point for crack propagation giving the lowest joint strength of (53 MPa). Fracture propagation was observed through the bond-line. When the bonding temperature was increased to between 590°C and 620oC XRD analyses of the fracture indicated that the amount of intermetallic formed within the joint increased to include the binary compounds Al3Si, Al3Ni and Ni3Si which suggested that failure propagated through the bond-line. Additionally, peaks of the ternary phases AlFeSi (2θ=37 and 68o) and Al9FeSi (2θ=78 and 84o). The presence of these compounds confirms the formation of peritectic reactions during bonding. At 620oC the fractured surface was characterized by shear ruptured dimples indicating a ductile failure mode which occurred

**Figure 15.** (a) SEM micrograph of the fractured surface of a joint made using a 5 μm thick Ni-Al2O3

results in Orowan strengthening as discussed in the previous sections.

in the parent metal adjacent to the bond-line.

coatings at 570oC and (b) 620oC.

Figure 17 (a) shows the variation in joint shear strength values as a function of coating thickness. The graph shows that the shear strength increased with increasing coating thickness from 53 MPa at 1 μm to 144 MPa at 11 μm. This increase in joint strength was attributed to three phenomena: the presence of nano-size Al2O3 particles in the joint center, the segregation of micro Al2O3 particle to the joint zone and the precipitation of intermetallic phases such as Al3Si, Ni3Si, Al3Ni, and Al9FeNi within the joint region. As discussed in the previous sections, for composites containing nano-sized particles, strengthening is often explained by the Orowan mechanism [36, 37]. Orowan bypassing theory shows that when smaller particle reinforcements are used the result is a more effective pinning of dislocation motion compared to when micro-particles are used. This mechanism leads to an increase in joint strength and hardness.

**Figure 17.** (a) Joint shear strengths as a function of particle size using 5μm thick coatings (b) Shear strength profile plotted as a function of Ni-Al2O3 coating thickness.

When the coating thickness was increased beyond 11 μm, a decline in the strength of the joints was observed. At a coating thickness of 13 μm, a joint strength of 80 MPa was recorded. This reduction in joint strength was attributed to the formation of densely packed micro-Al2O3 particle-rich regions along the bond interface and also due to an increase in the volume of intermetallics compounds such as AlFe3Si within the joint. The literature shows that the volume fraction of the micro-Al2O3 particles within the joint is inversely proportional to the joint ductility. Therefore, as the width of the particle segregated zone increased the ductility of the joint decreases. This leads to embrittlement of the joint region and causes a reduction in joint strength [38,39, 40, 41]. The findings published in the scientific literature supports the results collected in this study.

Fractured surfaces of the shear tested joints were analyzed to identify the mechanism of joint failure and the composition of the fractured surfaces. Figures 18 shows the micrographs of the typical fractured surfaces obtained for joints that were bonded using coating thickness ranging from 1 to 3μm, respectively. Fractographic analyses revealed that the fractured surface contained cleavage planes, which propagated through the bond-line. In addition, Al2O3 particles were visible at the fractured surface. Examination of the fractured surfaces revealed characteristics of a brittle fracture which suggest that insufficient eutectic liquid is formed when using coating thickness between 1 and 3μm. Composition analysis of the fractured surfaces using XRD indicated the presence of peaks for Al2O3, Ni3Si and Ni17Al3.9Si5.1O48 compounds.

When an interlayer thickness of 11μm (see Figure 19a) was used, the fractured surface showed evidence of both shear plastic deformation and fractured micro- Al2O3 particles indicating a ductile transgranular fracture. Samples bonded at this condition had the highest shear strength. This indicated a critical combination of segregated micro- Al2O3 particles and nano-Al2O3. Crack propagation occurred in the base metal adjacent to the bond-line. The results suggest that within coating thickness range of 5 to 11μm, sufficient eutectic liquid is produced, which facilitate good particle to matrix bonding resulting increased joint strength. The XRD spectrum shown in Figure 19 (b) indicated the presence of Ni3Si, MgAl2O4 and Al2O3 compound at the fractured surface. Increasing the coating thickness beyond 11μm resulted in the gradual transition of the fracture mode from ductile to brittle. At a coating thickness of 13μm thick Ni-Al2O3 coating (see Figure 20), the surface is characterized by dimples along the interparticle regions indicating a ductile failure through the particle-rich regions along the bond-line. XRD analyses of the fractured surfaces indicate the presence of peaks for Al2O3, NiAl2O4 and AlFe3Si compound at the fractured surface (see Figure 20b).

The results suggest that the ductility of the joint region increased with increasing coating thickness up to 9 μm. When the coating thickness was increased beyond 9 μm the joint region transitioned from ductile to brittle. These transitions were attributed to an increase in the volume of eutectic liquid that forms with increasing coating thickness leading to interparticle contact.

and Ni17Al3.9Si5.1O48 compounds.

Figure 20b).

interparticle contact.

When the coating thickness was increased beyond 11 μm, a decline in the strength of the joints was observed. At a coating thickness of 13 μm, a joint strength of 80 MPa was recorded. This reduction in joint strength was attributed to the formation of densely packed micro-Al2O3 particle-rich regions along the bond interface and also due to an increase in the volume of intermetallics compounds such as AlFe3Si within the joint. The literature shows that the volume fraction of the micro-Al2O3 particles within the joint is inversely proportional to the joint ductility. Therefore, as the width of the particle segregated zone increased the ductility of the joint decreases. This leads to embrittlement of the joint region and causes a reduction in joint strength [38,39, 40, 41]. The findings published in the

Fractured surfaces of the shear tested joints were analyzed to identify the mechanism of joint failure and the composition of the fractured surfaces. Figures 18 shows the micrographs of the typical fractured surfaces obtained for joints that were bonded using coating thickness ranging from 1 to 3μm, respectively. Fractographic analyses revealed that the fractured surface contained cleavage planes, which propagated through the bond-line. In addition, Al2O3 particles were visible at the fractured surface. Examination of the fractured surfaces revealed characteristics of a brittle fracture which suggest that insufficient eutectic liquid is formed when using coating thickness between 1 and 3μm. Composition analysis of the fractured surfaces using XRD indicated the presence of peaks for Al2O3, Ni3Si

When an interlayer thickness of 11μm (see Figure 19a) was used, the fractured surface showed evidence of both shear plastic deformation and fractured micro- Al2O3 particles indicating a ductile transgranular fracture. Samples bonded at this condition had the highest shear strength. This indicated a critical combination of segregated micro- Al2O3 particles and nano-Al2O3. Crack propagation occurred in the base metal adjacent to the bond-line. The results suggest that within coating thickness range of 5 to 11μm, sufficient eutectic liquid is produced, which facilitate good particle to matrix bonding resulting increased joint strength. The XRD spectrum shown in Figure 19 (b) indicated the presence of Ni3Si, MgAl2O4 and Al2O3 compound at the fractured surface. Increasing the coating thickness beyond 11μm resulted in the gradual transition of the fracture mode from ductile to brittle. At a coating thickness of 13μm thick Ni-Al2O3 coating (see Figure 20), the surface is characterized by dimples along the interparticle regions indicating a ductile failure through the particle-rich regions along the bond-line. XRD analyses of the fractured surfaces indicate the presence of peaks for Al2O3, NiAl2O4 and AlFe3Si compound at the fractured surface (see

The results suggest that the ductility of the joint region increased with increasing coating thickness up to 9 μm. When the coating thickness was increased beyond 9 μm the joint region transitioned from ductile to brittle. These transitions were attributed to an increase in the volume of eutectic liquid that forms with increasing coating thickness leading to

scientific literature supports the results collected in this study.

**Figure 18.** (a) SEM micrograph (b) cross-section (Mag. X10) and (c) XRD analysis of the fractured surface for a bond made with a 3 μm thick Ni–Al2O3 coating for 10 minutes at 600oC.

**Figure 19.** (a) SEM micrograph (b) cross-section (Mag. X10) and (c) XRD analysis of the fractured surface for a bond made with an 11 μm thick Ni–Al2O3 coating for 10 minutes at 600oC.

**Figure 20.** (a) SEM micrograph and (b) XRD analysis of the fractured surface for a bond made with a 13 μm thick Ni–Al2O3 coating for 10 minutes at 600oC.

#### **5.4. Effect of interlayer particle size shear strength measurements**

Figure 17(b) show the joint shear strength graph as a function of interlayer particle size. The result indicated that joint shear strength increased with decreasing particle size from 138 MPa with 500 nm to142 MPa with 50 nm. This increase in joint shear strength was attributed to better distribution of nano-sized particles within the interlayer when smaller particle sizes are used. In both cases higher shear strengths were obtained than when pure Ni coating is used (117 MPa) [23]. The results indicate that joint strength of up to 90% that of the base metal (BM) is achievable when using a 50 nm diameter nano-sized particle-reinforced interlayer. Tjong [42] showed that the nano-particle size has a strong effect on the yield strength. The author suggested that a particle size of 100 nm is a critical value for improving the yield strength of nano-composites. Below this critical value the yield strength increases significantly with decreasing particle size. Similar results were obtained by Gupta and coworkers [43, 44]. Zhang and Chen [45] showed that the Orowan stress plays a major role in strengthening the nano-composites.

Figure 21 shows the fractured surface for a bond made using a 5μm Ni- 50 nm Al2O3 particle. The fractograph showed shear plastic deformation, indicative of ductile fracture with a crack propagating primarily through the bond-line and a section of the base metal adjacent to the bond-line. A similar result was obtained when a dispersed particle size of 50 nm were used in the coating. XRD analyses of the fractured surfaces indicated the presence of peaks for Al2O3, NiAl2O4 and Al11Ni9 compound at the fractured surface.

**Figure 21.** (a) SEM micrograph and (c) XRD analysis of the fractured surface for a bond made with 5 μm thick Ni–(50nm) Al2O3 for 10 minutes at 600oC.
