Sliding wear – material loss, surface roughness and surface topography
The additive manufactured samples are investigated for the high temperature sliding wear and the moss loss measured. The mass loss indicates that the frictional energy developed during the sliding has highly influenced towards the material loss. Figure 3 represents the measured mass loss with respect to the sliding time at different applied load conditions. It is clear that the minimum material loss during the friction is 0.103 g at applied load 5 N. For the same case, the mass loss increases with increase in temperature. It infers that, when there is increase in temperature the material sensitivity has influenced towards deformation and material loss occurs. Subsequently, with increase in applied load the sensitivity has highly influenced on surface wear and the material loss trend increases. At an applied load of 15 N the maximum material loss was recorded 0.2 to 0.31 g.
The high temperature sliding wear test samples after materials deformation is subjected to measure the surface roughness for discussion. At this state the severity of the working condition has made the material to undergo severe surface degradation and the responses are revealed in the form of wear tracks. Figure 4 depicts the surface roughness value measured at each test conditions. Maximum surface roughness of 1.76 µm is recorded from the investigation for the maximum applied load of 15 N. While investigation, the material in contact with the counter surface are prone to severe surface roughness with increase in applied load and temperature. In addition, the wear debris at the contact surface may also influence over roughening the area. In the same case for minimum applied load surface roughness measured are in the range of 0.5–0.57 µm. During investigation the samples do slide with a centrifugal force and the mechanical action between the pin and the disc will lead to surface damage.
The test samples are analysed with the support of scanning electron microscope (SEM) to reveal the surface topography. Figure 5 illustrates the profile of worn surface at different process conditions. These samples are chosen based on the results of surface roughness and the conditions are given in the inset of the Fig. 5 caption. The minimal roughness was notices with 200 ºC – 5 N – 10 min process condition. As the wear tracks are less but the wear scars are deep in penetration. The depth of penetration might be due to the temperature sensitised by the material during the experimentation. For minimum applied load with low working temperature the surface roughness was found to be average after 30 min of experimentation at the process condition of 100 ºC – 5 N – 30 min. The wear scars are less and uniform throughout the contact surface. Similarly, for the 150 ºC – 15 N – 10 min process condition the roughness revealed close to 100 ºC – 5 N – 30 min. On continuous process the wear has extended in severe for 150 ºC – 15 N at the end of 30 min. The contact surface has prone to severe wear and the surface topography revealed with maximum wear tracks. The exposed contact area has a severe worn surface with material loss due to abrasion and ablate of material in bulk. This is the hybrid action of mechanical and metallurgical transformation held over the contact surface. In order to control material behaviour, the post processing of 3D printed materials can be one the scope in future to lead the proposed work study.
Figure 5: Electron image of test samples after high temperature sliding wear analysis
Statistical Analysis and Process optimization
The experimental data derived from the investigation are used to evaluate using statistical analysis tool and response surface methodology (RSM) to optimize the process parameter for surface roughness and material loss. The statistical analysis is performed using MINITAB software for the twenty seven set of experiments as given in the Table 2.
Table 2
Experimental plan and response for the sliding wear analysis
Exp No.
|
Input Parameter
|
Responses
|
Load (N)
|
Temp (ºC)
|
Time (min)
|
SR (µm)
|
mass loss (g)
|
1
|
5
|
100
|
10
|
0.316
|
0.046
|
2
|
5
|
100
|
20
|
0.487
|
0.068
|
3
|
5
|
100
|
30
|
0.568
|
0.104
|
4
|
5
|
150
|
10
|
0.412
|
0.057
|
5
|
5
|
150
|
20
|
0.489
|
0.068
|
6
|
5
|
150
|
30
|
0.577
|
0.108
|
7
|
5
|
200
|
10
|
0.298
|
0.071
|
8
|
5
|
200
|
20
|
0.347
|
0.083
|
9
|
5
|
200
|
30
|
0.509
|
0.132
|
10
|
10
|
100
|
10
|
0.587
|
0.059
|
11
|
10
|
100
|
20
|
0.875
|
0.088
|
12
|
10
|
100
|
30
|
1.384
|
0.142
|
13
|
10
|
150
|
10
|
0.508
|
0.065
|
14
|
10
|
150
|
20
|
0.789
|
0.079
|
15
|
10
|
150
|
30
|
1.26
|
0.155
|
16
|
10
|
200
|
10
|
0.487
|
0.049
|
17
|
10
|
200
|
20
|
0.724
|
0.087
|
18
|
10
|
200
|
30
|
1.226
|
0.192
|
19
|
15
|
100
|
10
|
0.759
|
0.093
|
20
|
15
|
100
|
20
|
0.987
|
0.170
|
21
|
15
|
100
|
30
|
1.693
|
0.233
|
22
|
15
|
150
|
10
|
0.865
|
0.130
|
23
|
15
|
150
|
20
|
1.216
|
0.193
|
24
|
15
|
150
|
30
|
1.766
|
0.307
|
25
|
15
|
200
|
10
|
0.814
|
0.087
|
26
|
15
|
200
|
20
|
1.112
|
0.120
|
27
|
15
|
200
|
30
|
1.644
|
0.240
|
From the experimental investigation, the responses on surface roughness – SR and material loss are evaluated for the input process parameters – load (N), temperature (ºC) and sliding time (min). The significant reason for the material loss and the surface roughness of the worn samples are discussed in the previous section. In order to study the influence of input process parameter in detail, the statistical tool is used to study the contribution through analysis of variance (ANOVA). From the analysis of variance, the influence of input parameters evaluated are given in Fig. 6. It is clear to infer that the applied load (N) and the sliding time (min) are the two predominant factors to influence the responses on material loss (g) and surface roughness (µm). For material loss has 53.6% of contribution for sliding time and 43% for applied load; it shows that the time has highly influenced towards the wear and material loss. Subsequently the applied load has 53% and 36% for sliding time. Roughness has impact on applied load and the contact area. At the same time, the contribution for the temperature has not influenced towards material loss. Temperature for the nickel alloy has less impact which is negotiable compare to the other two process parameters (load and time). The data generated from the experiments are evaluated to check the fitness and it is confirmed that the data falls in tract. The R2 (residual value) value for the proposed design of experiment is 90.26% for material loss and 86.05% for surface roughness. The regression equation derived from the design of experimental analysis is as follows.
SR(Ra) = -0.458 + 0.07614 Load − 0.000550 Temp + 0.03101 Time
Mass Loss = -0.0896 + 0.00928 Load + 0.000066 Temp + 0.005313 Time
The regression equation has used to find the nearest value for the proposed design of experiments for unknown set of experiments. It has very less difference and the values are acceptable. This technique can be used to study unknown value of test data within the boundary limit of proposed design.
The interrelation between the input process parameters and the response outcomes are discussed with contour plots. The surface roughness value for the proposed experimental design are presented in Fig. 7. As discussed from the analysis of variance, the time and load factors are the major presider to produce wide variation in surface roughness. The interrelation between the load and the temperature has very meagre influence over surface roughness. At the same the relation between the temperature and the time are in negotiable relations. The same pattern was recorded for the material loss studies having strong influence between the load and sliding durations. All other two combinations negotiable and or steady to ignore for response methodology. Figure 8 infers the results for mass loss in similar to surface roughness.
In continuation to the surface plot for process parameters interrelation, the response surface methodology was adopted to predict the optimal process parameter. Based on the experimental design, for the twenty-seven set (three factors) of trial run response surface methodology was design and executed. From the execution the experimental data generated from the investigation falls in line with the normal probability line. Residual plot for the response surface methodology is depicted in the Fig. 9. Similar graphical data was extracted for material loss and it has been given in Fig. 10. Data generated for the also meets the desired expectations and the quality data are validated. From the analysis it has been noticed that the best process condition was 10 N applied load at a working temperature of 100 ºC for a sliding time of 30 min. It is also merit to say that the responses aligned near to the experimental data for the same process conditions. Therefore, the demand for confirmation test has been neglected to redo the work.