**9.1 Weight loss**

The pin on disc was proposed to study and evaluate the tribological behaviour. The amount of wear loss due to dry sliding and wet sliding conditions of the novel hybrid composites fabricated by using stir casting technique and conducted the experiments. The results of the weight loss of wear test of all the hybrid composites and base alloy AZ91 magnesium are illustrated in the **Tables 8**–**11**. W1–initial weight of the sample before test(g) and W2-final weight of the sample after test(g). The test results obtained from the dry sliding wear test and wet sliding wear test were thoroughly


## **Table 8.**

*The weight loss of the hybrid composites and AZ91 alloy (dry and wet test) with load of 20 N.*


**Table 9.**

*The weight loss of the hybrid composites and AZ91 alloy (dry &wet Test) with load of 40 N.*


### **Table 10.**

*The weight loss of the hybrid composites and AZ91 alloy (dry &wet Test) with load of 60 N.*

*Experimental Investigation of Mechanical and Wear Behaviour of AZ91 Magnesium Hybrid… DOI: http://dx.doi.org/10.5772/intechopen.104703*


**Table 11.**

*The weight loss of the hybrid composites and AZ91 alloy (dry &wet Test) with load of 80 N.*

analysed to assess the effect of reinforcements and effect of load on the test samples. The sliding distance and velocity were maintained constant. The coefficient of friction is the ratio of frictional force to normal applied load. The coefficient of friction purely depends upon the materials properties and also on surface roughness, temperature of exposure, normal load and environment.

The wear test results (dry sliding & wet sliding) depicted in **Figure 12** (a)-(d) shows the weight loss of base alloy and all the hybrid composites fabricated by stir

### **Figure 12.**

*(a). Samples vs weight loss of hybrid composites and AZ91 (20 N). (b). Samples vs weight loss of hybrid composites and AZ91 (40 N). (c). Samples vs weight loss of hybrid composites and AZ91 (60 N). (d). Samples vs weight loss of hybrid composites and AZ91 (80 N).*

casting technique. Hybrid composites exhibited less weight loss under the load condition in both the dry and wet test condition than the reference alloy AZ91.

The lower weight loss may be attributed to the addition of reinforcements (Ti and Gr and TiO2 and Gr). Among the reinforcement's combinations, TiO2 and graphene reinforced hybrid composites showed less weight loss than Ti and graphene hybrid composite.

It was also inferred that as normal load increases, the weight loss also increased. Within the hybrid composites the weight loss slightly decreased with increase amount of titanium dioxide (2%). However, titanium and graphene hybrid system, it was found that weight loss increased with increase in the amount of titanium. It was also inferred from the results, weight loss in any test condition of the hybrid composites was not as high as AZ91 magnesium alloy. The similar trend of weight loss was found in dry and wet sliding condition of each hybrid composites.

## **9.2 Wear rate**

The wear rate was calculated for the hybrid composite samples and AZ91 magnesium alloy samples in both dry and wet sliding test conditions under the varying load condition of 20 N, 40 N, 60 N, and 80 N. The results are tabulated in the **Tables 12**–**15**. The AZ91 magnesium alloy and hybrid composites wear test results of the samples under load 20 N,40 N,60 N, and 80 N in both dry and wet condition revealed that the wear rate, wear mechanism, coefficient of friction and wear resistance of AZ91, A1, A2, B1 and B2 hybrid composite materials.

From the experimental test observation, it was inferred the wear rate was lower for all the hybrid composites than AZ91 magnesium alloy. Also the wear rate was lower for wet sliding than dry sliding condition due to the lubrication effect. The wear rate


**Table 12.**

*Wear rate of hybrid composites and AZ91 alloy (dry and wet 20 N).*


### **Table 13.**

*Wear rate of hybrid composites and AZ91 alloy (dry and wet 40 N).*

*Experimental Investigation of Mechanical and Wear Behaviour of AZ91 Magnesium Hybrid… DOI: http://dx.doi.org/10.5772/intechopen.104703*


### **Table 14.**

*Wear rate of hybrid composites and AZ91 alloy (dry and wet 60 N).*


### **Table 15.**

*Wear rate of hybrid composites and AZ91 alloy (dry and wet 80 N).*

**Figure 13.** *(a). Samples vs Wear rate for wet sliding Wear test. (b). Samples vs Wear rate for dry sliding wear test.*

of the test samples are depicted in **Figure 13** (a) and (b). AZ91 magnesium alloy showed high wear rate both in dry sliding and wet sliding conditions.

## **9.3 Effect of reinforcement and load**

The variation of wear rate as the function of load for all the test samples AZ91, A1, A2, B1, and B2 were plotted as shown in **Figures 14** and **15**. It was noticed that the addition of reinforcement decreased the wear loss of the contacting specimen (pin surface) against hardened steel disc, under all conditions of normal load applied in air.

**Figure 14.** *Load/samples vs Wear rate(x10<sup>3</sup> ) for dry wear test.*

**Figure 15.** *Load/samples vs Wear rate (x10<sup>3</sup> ) -wet sliding wear test.*

It was found the wear resistance of the hybrid composites showed improved performance than the unreinforced base alloy. This behaviour in the hybrid composites was attributed due to two main reasons are (i) the addition of reinforcements and (ii) refinement in the grain size of the hybrid composites.

The addition of titanium and graphene had added enough hardness to the novel material to resist the wear due to sliding. At higher loads, AZ91/Ti/0.5% Gr showed more evident result in difference in wear rate than AZ91 reference alloy. Within the hybrid composite, varying weight percentage in titanium exhibited better wear resistance. Similar results were reported in the previous work [27]. But the addition graphene along with titanium in the AZ91 base alloy had better results.

*Experimental Investigation of Mechanical and Wear Behaviour of AZ91 Magnesium Hybrid… DOI: http://dx.doi.org/10.5772/intechopen.104703*

The other system AZ91 reinforced with titanium dioxide and graphene revealed less wear loss even at higher load conditions. The presence of titanium dioxide in the matrix along with graphene, acts as solid lubricants protecting the pin sample from the direct contact with the disc surface and also the material loss [28]. Within the hybrid composites, varying weight percentage of titanium dioxide exhibited better wear resistanceAZ91/1% TiO2/0.5%Gr. At higher loads, it was inferred that the both the combination of reinforcements with base alloy showed similar behaviour of wear loss.

The wear resistance and specific wear resistance each of hybrid composites and AZ91 magnesium alloy are illustrated in the **Tables 16**–**19** under different load


### **Table 16.**

*Wear resistance and specific wear rate of hybrid composites and AZ91 alloy (dry and wet 20 N).*


**Table 17.** *Wear resistance and specific wear rate of hybrid composites and AZ91 alloy (dry and wet 40 N).*


### **Table 18.**

*Wear resistance and specific wear rate of hybrid composites and AZ91 alloy (dry and wet 60 N).*


**Table 19.**

*Wear resistance and specific wear rate of hybrid composites and AZ91 alloy (dry and wet - 80 N).*

### **Figure 16.**

*(a). Samples vs Wear resistance(x10<sup>3</sup> ) for wet sliding Wear test. (b). Samples vs Wear resistance(x10<sup>3</sup> ) for dry wear test.*

condition. It was also inferred that the wear loss was remarkably lower in wet sliding condition than dry sliding as depicted in **Figure 16** (a) and (b). Hence the wear resistance was greatly improved in wet sliding condition.

The reports of earlier researches represent that increase in load, increases the wear loss. In sight about the results clearly depicts that presence of titanium or titanium dioxide with 0.5% graphene as reinforcements for the hybrid composites as hard phase in the magnesium matrix results in strengthening. As a result, it provides improved resistance against plastic deformation.

## **9.4 Surface analysis of Wear samples**

In order to explore the wear behaviour of the novel hybrid composites under different test condition and also the presence of reinforcements nanoparticles effect, wear track of test samples are recorded using optical microscope for dry sliding and wet sliding wear test under the load conditions of 20 N, 40 N, 60 N, and 80 N. As well as SEM analysis was done for clear understanding of wear mechanism. The **Figures 17**–**24** depicts the wear track of AZ91, hybrid composites A1, A2, B1, and B2in the dry and wet sliding wear condition respectively.

*Experimental Investigation of Mechanical and Wear Behaviour of AZ91 Magnesium Hybrid… DOI: http://dx.doi.org/10.5772/intechopen.104703*

**Figure 17.** *Optical microscope image of worn surface of AZ91 for load 20 N, 40 N, 60 N, and 80 N (dry condition).*

By Archard's law, as the hardness of the material increases, the wear resistance property is greatly improved and wear rate is less. Considering all the test condition, it was found from the worn surface morphology of the test samples that there were three wear mechanism abrasion, delamination and oxidation. Adhesion was not very significantly found in the hybrid composites. This was mainly due to the presence of graphene which acts as solid lubricant. In addition to this, titanium dioxide not only imparts hardness to the materials but also act as lubricant. This may be one of the reasons for wear loss to be least in hybrid composites fabricated in this combination AZ91/TiO2/0.5%Gr.

The SEM images of the worn surface were shown in **Figures 25**-**29** in dry and wet condition to evalute the wear behaviour. It was inferred clearly that dry sliding test produced grooves on the pin surface incurring heavy material loss due to ploughing action of harder surface. It was also found that AZ91 magnesium alloy exhibited severe deformation with deep groove and cracks. The titanium and graphene reinforced hybrid composites showed grooves but at high weight percentage the grooves were deeper which depicts either detachment of reinforcement and breaking

**Figure 18.**

brittle layer formed. But in titanium dioxide and graphene reinforced hybrid composite showed only fine grooves in dry condition and very fine scratches in wet condition. It evident that hybrid composite reinforced with titanium dioxide and graphene possess better wear resistance.
