CHAPTER 6 WEAR TESTING MEASUREMENT

Transcription

1 84 CHAPTER 6 WEAR TESTING MEASUREMENT Wear is a process of removal of material from one or both of two solid surfaces in solid state contact. As the wear is a surface removal phenomenon and occurs mostly at outer surfaces, it is more appropriate and economical to make surface modification of existing alloys than using the wear resistant alloys. 6.1 EXPERIMENTAL PROCEDURE OF WEAR TEST Dry sliding wear tests for different number of specimens was conducted by using a pin-on-disc machine (Model: Wear & Friction Monitor TR-20) supplied by DUCOM is shown in Figure 6.1. Figure 6.1 Wear testing machine

2 85 The pin was held against the counter face of a rotating disc (EN31 steel disc) with wear track diameter 60 mm. The pin was loaded against the disc through a dead weight loading system. The wear test for all specimens was conducted under the normal loads of 20N, 40N and a sliding velocity of 2 and 4 m/s. Wear tests were carried out for a total sliding distance of approximately 3000 m under similar conditions as discussed above. The pin samples were 30 mm in length and 12 mm in diameter. The surfaces of the pin samples were slides using emery paper (80 grit size) prior to test in order to ensure effective contact of fresh and flat surface with the steel disc. The samples and wear track were cleaned with acetone and weighed (up to an accuracy of gm using microbalance) prior to and after each test. The wear rate was calculated from the height loss technique and expressed in terms of wear volume loss per unit sliding distance. parameters: In this experiment, the test was conducted with the following 1. Load 2. Speed 3. Distance In the present experiment the parameters such as speed, time and load are kept constant throughout for all the experiments. These parameters are given in Table 6.1.

3 86 Table 6.1 Parameter taken constant during sliding wear test Pin material Al, Al/C, Al/C/3B 4 C, Al/C/6B 4 C, Al/C/9B 4 C Disc material EN 31 steel Pin dimension Cylinder with diameter 12 mm height 30 mm Sliding speed (m/s) 2, 4 Normal load 20, 40 Sliding distance (m) PIN-ON-DISC TEST In this study, Pin-on-Disc testing method was used for tribological characterization. The test procedure is as follows: Initially, pin surface was made flat such that it will support the load over its entire cross-section called first stage. This was achieved by the surfaces of the pin sample ground using emery paper (80 grit size) prior to testing Run-in-wear was performed in the next stage/ second stage. This stage avoids initial turbulent period associated with friction and wear curves Final stage/ third stage is the actual testing called constant/ steady state wear. This stage is the dynamic competition between material transfer processes (transfer of material from pin onto the disc and formation of wear debris and their subsequent removal). Before the test, both the pin and disc were cleaned with ethanol soaked cotton (Surappa et al 2007)

4 87 Before the start of each experiment, precautionary steps were taken to make sure that the load was applied in normal direction. Figure 6.2 represents a schematic view of Pin-on-Disc setup. Figure 6.2 Schematic views of the pin-on-disk apparatus Weight Loss The alloy and composite samples are cleaned thoroughly with acetone. Each sample is then weighed using a digital balance having an accuracy of ± 0.1 mg. After that, the sample is mounted on the pin holder of the tribometer ready for wear test. For all experiments, the sliding speeds are adjusted to 2 and 4 m/s. The specific wear rates of the materials were obtained by W = w where W denotes specific wear rates in mm 3 /N- w is the weight loss measured in grams, density of the worn material in g/mm 3 and F is the applied load in N. Weight loss of the alloy and composite samples in grams is shown in Table 6.2.

5 88 Table 6.2 Data of cumulative wear loss of alloy and composites Weight loss of alloy and composite Sliding Speed 2m/s Sliding Speed 4m/s S.No. Specimen Initial Final Weight Initial Final Weight Name weight weight loss weight weight loss (gm) (gm) (gm) (gm) (gm) (gm) 1 LM LM 25 + C LM 25+C + 3%B 4 C LM 25+C + 6%B 4 C LM 25+C + 9%B 4 C Figure 6.3 Weight loss of alloy and composite with 2 m/s

6 89 Figure 6.4 Weight loss of alloy and composite with 4 m/s Figures 6.3 and 6.4 show the cumulative weight loss of the alloy specimen after addition of graphite and boron carbide produced with the help of stir casting technique. After addition of reinforced material the sliding wear decreases significantly or says that weight loss is decreasing as the graphite and boron carbide addition is increasing as compared to matrix metal Wear Calculation 1. Area Cross sectional Area, 2. Volume loss Volume loss = Cross sectional Area x Height loss 3. Wear rate Wear rate = Volume loss / Sliding distance

7 90 4. Wear resistance Wear resistance = 1/ Wear rate 5. Specific wear rate Specific wear rate = Wear rate/load Graphs Table 6.3 Specimen vs wear rate (mm 3 /m) Specimen Wear rate (mm 3 /m) 2 m/s 4 m/s p g b b b Figure 6.5 Specimen vs wear rate (mm 3 /m) with 2 and 4 m/s

8 91 LM-25 and composites reinforced with boron carbide and graphite particles of size ranges (200 meshes) at a load of 20, 40 N and total time is 5 minutes. It can be attributed to the increase in hardness of the material due to the presence of hard ceramic particles. Material removal in a ductile material such as aluminium alloy matrix is due to the indentation and ploughing action of the sliding disc which is made from hard steel material (EN31 steel disc). Incorporation of hard graphite and B 4 C particles in the Al alloy LM25 restricts such ploughing action of hard steel counterpart and improves the wear resistance. Comparing the wear properties of composites reinforced with graphite and B 4 C particles, it is observed that despite their higher hardness, composites reinforced with graphite and B 4 C particles show improved wear resistance as compared to Al 6061 composites reinforced with SiC particles (Sanjeev Das et al 2006). Table 6.4 Specimen vs wear resistance (m/mm 3 ) Specimen Wear resistance (m/mm 3 ) 2 m/s 4 m/s p g b b b

9 92 Figure 6.6 Specimen vs wear resistance (m/mm 3 ) with 2 and 4 m/s Figure 6.6 shows the wear resistance as a function of time for the LM25 and composites reinforced with boron carbide and graphite particles of size ranges (200 meshes) at a load of 20, 40 N and total time is 5 minutes. It is observed that wear resistance of LM25 increased. Table 6.5 Specimen vs specific wear rate (mm 3 /Nm) Specimen Specific wear rate (mm 3 /Nm) 2 m/s 4 m/s p g b b b

10 93 Figure 6.7 Specimen vs specific wear rate (mm 3 /Nm) with 2 and 4 m/s Figure 6.7 shows the specific wear rate as a function of time for the LM25 and composites reinforced with boron carbide and graphite particles of size ranges (200 mesh) at a load of 20, 40 N and total time is 5 minutes. It is observed that specific wear rate of LM25 decreased. 6.3 SEM MICRO GRAPH OF AL/4 WT% C WITH 2 M/S The worn surface of the Al/4% graphite composite is shown in Figure 6.8. It clearly exhibits the presence of deep permanent grooves and fracture of the oxide layer, which may have caused the increase of wear loss. However, the worn surfaces of the two composites exhibit finer grooves and slight plastic deformation at the edges of the grooves. The surface also appears to be smooth because of the graphite reinforcement content.

11 94 (a) (b) Figure 6.8 (c) (d) Typical SEM micro graph of Al/4 wt% C with 2 m/s SEM Micro Graph of Al/4 wt% C/3, 6, 9 wt% B 4 C with 2 m/s The worn surfaces of the composite AlMMC s are shown in Figures 6.9 to Indistinct grooves and fine scratches were formed on the worn surface. The wear mechanism are characterised by the formation of the grooves, which are produced by the ploughing action of hard asperities on the counter disc and hardened worn debris. Increase in boron carbide would results in decrease in wear.

12 95 (a) (b) Figure 6.9 (c) (d) Typical SEM micro graph of Al/4 wt% C/ 3 wt % of B 4 C with 2 m/s (a) (b)

13 96 (c) (d) Figure 6.10 Typical SEM micro graph of Al/4 wt% C/ 6 wt % of B 4 C with 2 m/s (a) (b) (c) (d) Figure 6.11 Typical SEM micro graph of Al/4 wt% C/ 9 wt % of B 4 C with 2 m/s

14 SEM Micro Graph of Al/4 wt% C with 4 m/s (a) (b) (c) (d) Figure 6.12 Typical SEM micro graph of Al/4 wt% C/ 3 with 4 m/s The SEM image of aluminium composite was shown in Figure It shows that the worn surfaces of the two composites exhibit finer grooves and slight plastic deformation at the edges of the grooves. The surface also appears to be smooth because of the graphite reinforcement content.

15 SEM Micro Graph of Al/4 wt% C/3, 6, 9 wt% B 4 C with 4 m/s (a) (b) (c) (d) Figure 6.13 Typical SEM micro graph of Al/4 wt% C/ 3 wt % of B 4 C with 4 m/s (a) (b)

16 99 (c) (d) Figure 6.14 Typical SEM micro graph of Al/4 wt% C/ 6 wt % of B 4 C with 4 m/s The SEM image of the aluminium composite is shown in Figures It provides that the presence of boron particle increases the hardness and reduces the metal removal rate reduced to 7% when compared to previous combination. (a) (b)

17 100 (c) (d) Figure 6.15 Typical SEM micro graph of Al/4 wt% C/ 9 wt % of B 4 C with 4 m/s At a sliding speed of 4 m/s, the wear rate shows a lowering trend which indicates the less removal of material from the surface. The micrograph shows the removal of material by delamination. Apart from this, cracks are generated along with particle pull out at the surface. Figure 6.15 shows the presence of a large number of grooves over the entire surface. 6.4 WEAR BEHAVIOUR The aim of the experimental plan is to find the important factors and the combination of factors influencing the wear process to achieve the minimum wear rate and COF. The experiments were developed based on an OA, with the aim of relating the influence of sliding speed, applied load and sliding distance. These design parameters are distinct and intrinsic feature of the process that influence and determine the composite performance. Taguchi recommends analyzing the S/N ratio using conceptual approach that involves graphing the effects and visually identifying the significant factors.

18 101 The above mentioned pin on disc test apparatus was used to determine the sliding wear characteristics of the composite. Specimens of size 12 mm diameter and 10 mm length were cut from the cast samples, and then machined. The contact surface of the cast sample (pin) was made flat so that it should be in contact with the rotating disk. During the test, the pin was held pressed against a rotating EN31 carbon steel disc by applying load that acts as a counterweight and balances the pin. The track diameter was varied for each batch of experiments in the range of 50 mm to 100 mm and the parameters such as the load, sliding speed and sliding distance was varied in the range given in Table 6.6. An LVDT (load cell) on the lever arm helps determine the wear at any point of time by monitoring the movement of the arm. Once the surface in contact wears out, the load pushes the arm to remain in contact with the disc. This movement of the arm generates a signal which is used to determine the maximum wear and the COF is monitored continuously as wear occurs and graphs between COF and time was monitored for both of the specimens, i.e., aluminium LM25, 4% of C, 3% of B 4 C, 6% of B 4 C, 9% of B 4 C. Further, weight loss of each specimen was obtained by weighing the specimen before and after the experiment by a single pan electronic weighing machine with an accuracy of g after thorough cleaning with acetone solution. The results for various combinations of parameters were obtained by conducting the experiment as per the OA and shown in Table 6.7. The measured results were analyzed using the commercial software MINITAB 15 specifically used in DOE applications.

19 102 Table 6.6 Process parameters and levels Level Load (N) Sliding Speed, S Sliding Distance, D (m/s) (m) PLAN OF EXPERIMENTS The dry sliding wear test was performed with three parameters: applied load, sliding speed and sliding distance and varying them for three levels. According to the rule that DOF for an OA should be greater than or equal to the sum of those wear parameters, a L9 OA which has 9 rows and 3 columns was selected as shown below: Table 6.7 Orthogonal array L 9 of Taguchi Experimental No. Column 1 Column 2 Column

20 103 The selection of OA depends on three items in order of priority, viz., the number of factors and their interactions, number of levels of the factors and the desired experimental resolution or cost limitations. A total of 9 experiments were performed based on the run order generated by the Taguchi model. The response of the model is wear rate and COF. In OA, the first column is assigned to applied loads, second column is assigned to sliding speed and third column is assigned to sliding distance and the remaining columns are assigned to their interactions. The objective of the model is to minimize the wear rate and COF. The Signal to Noise (S/N) ratio, which condenses the multiple data points within a trial, depends on the type of characteristic being evaluated. In this study, smaller the better characteristic was chosen to analyze the dry sliding wear resistance. The response table for signal to noise ratios show the average of selected characteristics of each level of the factor. This table includes the ranks based on the delta statistics, which compares the relative value of the effects. S/N ratio is a response which consolidates repetitions and the effect of noise levels into one data point. Analysis of variance of the S/N ratio is performed to identify the statistically significant parameters. 6.6 RESULTS AND DISCUSSIONS The aim of the experimental plan is to find the important factors and the combination of factors influencing the wear process to achieve the minimum wear rate and COF. The experiments were developed based on an OA, with the aim of relating the influence of sliding speed, applied load and sliding distance. These design parameters are distinct and intrinsic feature of the process that influence and determine the composite performance. Taguchi recommends analyzing the S/N ratio using conceptual approach that involves graphing the effects and visually identifying the significant factors.

21 Results of Statistical Analysis of Experiments The results for various combinations of parameters were obtained by conducting the experiment as per the OA. The measured results were analyzed using the commercial software MINITAB 15 specifically used in DOE applications. Tables 6.8 to 6.22 shows the experimental results average of two repetitions for wear rate and COF. To measure the quality characteristics, the experimental values are transformed into a signal to noise ratio. The influence of control parameters such as load, sliding speed and sliding distance on wear rate and COF has been analyzed using signal to noise response table. The ranking of process parameters using signal to noise ratios obtained for different parameter levels for wear rate and COF are given for aluminium LM25, 4% of C, 3% of B 4 C, 6% of B 4 C, 9% of B 4 C. The control factors are statistically significant in the Signal to Noise ratio and it could be observed that the sliding distance is a dominant parameter on the wear rate and COF followed by applying load and sliding speed. The analysis of these experimental results using S/N ratios gives the optimum conditions resulting in minimum wear rate and COF Analysis of Variance Results for Wear Test The experimental results were analyzed with ANOVA, which is used to investigate the influence of the considered wear parameters, namely, applied load, sliding speed and sliding distance that significantly affects the performance measures. By performing analysis of variance, it can be decided which independent factor dominates over the other and the percentage contribution of that particular independent variable. Aluminium LM25, 4% of C, 3% of B 4 C, 6% of B 4 C and 9% of B 4 C of the ANOVA

22 105 results for wear rate and COF for three factors varied at three levels and interactions of those factors. This analysis is carried out for a significance = 0.05, i.e. for a confidence level of 95%. Sources with a P-value less than 0.05 were considered to have a statistically significant contribution to the performance measures. Table 6.8 Responses table for S/N ratio for wear (Al LM 25) S.No. S/N S/N Load Speed Distance Wear ratio C.O.F ratio (N) (m/s) (m) (mm 3 /m) wear C.O.F rate

23 106 Table 6.9 Responses table for S/N ratio of coefficient of friction (Al LM 25) Level Load (N) Speed (m/s) Distance (m) Delta Rank Table 6.10 Main effects for plot for S/N ratios - coefficient of friction Level Load (N) Speed (m/s) Distance (m) Delta Rank Figure 6.16 Main effects for plot for S/N ratios - wear rate

24 107 Figure 6.17 Main effects for plot for S/N ratios - wear rate Figure 6.18 Main effects for plot for S/N ratios - coefficient of friction

25 108 Figure 6.19 Main effects for plot for S/N ratios - coefficient of friction Table 6.11 Responses table for S/N ratio for wear (Al-LM 25/4% C) S/N S/N Load Speed Distance Wear Ratio S.No. (N) (m/s) (m) (mm 3 C.O.F Ratio /m) Wear c.o.f Rate

26 109 Table 6.12 Responses table for S/N ratio of coefficient of friction (Al - LM 25/4% C) Level Load(N) Speed(m/s) Distance(m) Delta Rank Table 6.13 Main effects for plot for S/N ratios - coefficient of friction Level Load(N) Speed(m/s) Distance(m) Delta Rank Figure 6.20 Main effects for plot for S/N ratios - wear rate

27 110 Figure 6.21 Main effects for plot for S/N ratios - wear rate Figure 6.22 Main effects for plot for S/N ratios - coefficient of friction

28 111 Figure 6.23 Main effects for plot for S/N ratios - coefficient of friction Table 6.14 Responses table for S/N ratio for wear (Al LM 25/4% C/3% B 4 C S/N S/N S.No. Load Speed Distance Wear C.O.F Ratio (N) (m/s) (m) (mm 3 ratio /m) Wear C.O.F Rate

29 112 Table 6.15 Responses table for S/N ratio of coefficient of friction (Al LM 25/4% C/3% B 4 C)) Level Load (N) Speed (m/s) Distance (m) Delta Rank Table 6.16 Main effects for plot for S/N ratios - coefficient of friction Level Load (N) Speed (m/s) Distance (m) Delta Rank Figure 6.24 Main effects for plot for S/N ratios - wear rate

30 113 Figure 6.25 Main effects for plot for S/N ratios - wear rate Figure 6.26 Main effects for plot for S/N ratios coefficient of friction

31 114 Figure 6.27 Main effects for plot for S/N ratios - coefficient of friction Table 6.17 Responses table for S/N ratio for wear (Al - LM 25/ 4% C/ 6%B 4 MMC) S/N S/N ratio Load Speed Distance Wear S.No. (N) (m/s) (m) (mm 3 C.O.F ratio wear /m) C.O.F rate

32 115 Table 6.18 Responses table for S/N ratio of coefficient of friction (Al - LM 25/ 4% C/ 6%B 4 MMC) Level Load(N) Speed(m/s) Distance(m) Delta Rank Table 6.19 Main effects for plot for S/N ratios - coefficient of friction Level Load(N) Speed(m/s) Distance(m) Delta Rank Figure 6.28 Main effects for plot for S/N ratios - wear rate

33 116 Figure 6.29 Main effects for plot for S/N ratios - wear rate Figure 6.30 Main effects for plot for S/N ratios - coefficient of friction

34 117 Figure 6.31 Main effects for plot for S/N ratios - coefficient of friction Table 6.20 Responses table for S/N ratio for wear (Al - LM 25/ 4% C and 9% B 4 MMC) S/N S/N Load Speed Distance Wear Ratio S.No. (N) (m/s) (m) (mm 3 C.O.F Ratio /m) Wear C.O.F Rate

35 118 Table 6.21 Responses table for S/N ratio of coefficient of friction (Al LM 25/ 4% C and 9% B 4 MMC) Level Load(N) Speed(m/s) Distance(m) Delta Rank Figure 6.32 Main effects for plot for S/N ratios - wear rate

36 119 Table 6.22 Main effects for plot for S/N ratios - coefficient of friction Level Load(N) Speed(m/s) Distance(m) Delta Rank Figure 6.33 Main effects for plot for S/N ratios - wear rate

37 120 Figure 6.34 Main effects for plot for S/N ratios coefficient of friction Figure 6.35 Main effects for plot for S/N ratios coefficient of friction

38 121 The interaction terms have little or no effect on the coefficient of friction & the pooled errors accounts only 0.5% & 1.4%. From the analysis of variance & S/N ratio, it is inferred that the sliding distance has the highest contribution on wear rate & COF followed by load & sliding speed. 6.7 ANOVA Table 6.23 Analysis of variance for wear (coded units) with LM 25 Source DF Seq SS Adj SS Adj MS F P Main Effects Way Interactions Residual Error Total It can be observed that for AMMCs that the sliding distance has the highest influence on wear rate. Hence sliding distance is an important control factor to be taken into consideration during the wear process followed by applied loads and sliding speed respectively, it can observe that the load has the highest contribution, followed by sliding distance and sliding speed for Al LM25 with reinforcement combination of MMCs. The interaction terms have little or no effect on COF & the pooled errors accounts. From the analysis of variance and S/N ratio, it is inferred that the sliding distance has the highest contribution on wear rate & COF followed by load & sliding speed. These values are shown in the Tables 6.23 to 6.32.

39 122 Table 6.24 Analysis of variance for C.O.F (coded units) with LM 25 Source DF Seq SS Adj SS Adj MS F P Main Effects Way Interactions Residual Error Total Table 6.25 Analysis of variance for wear (coded units) with LM25/C Source DF Seq SS Adj SS Adj MS F P Main Effects Way Interactions Residual Error Total Table 6.26 Analysis of variance for C.O.F (coded units) LM25/C Source DF Seq SS Adj SS Adj MS F P Main Effects Way Interactions Residual Error Total

40 123 Table 6.27 Analysis of variance for wear (coded units) with LM25/C/3B 4 C Source DF Seq SS Adj SS Adj MS F P Main Effects Way Interactions Residual Error Total Table 6.28 Analysis of variance for C.O.F (coded units) with LM25/C/3B 4 C Source DF Seq SS Adj SS Adj MS F P Main Effects Way Interactions Residual Error Total 8 Table 6.29 Analysis of variance for wear (coded units) with LM25/C/6B 4 C Source DF Seq SS Adj SS Adj MS F P Main Effects Way Interactions Residual Error Total

41 124 Table 6.30 Analysis of variance for C.O.F (coded units) with LM25/C/6B 4 C Source DF Seq SS Adj SS Adj MS F P Main Effects Way Interactions Residual Error Total Table 6.31 Analysis of variance for wear (coded units) with LM25/C/9B 4 C Source DF Seq SS Adj SS Adj MS F P Main Effects Way Interactions Residual Error Total Table 6.32 Analysis of variance for C.O.F (coded units) with LM25/C/9B 4 C Source DF Seq SS Adj SS Adj MS F P Main Effects Way Interactions Residual Error Total