5.1 Preamble
To meet such extensive demand, it is important to get copper from old products/
scraped items considering the depletion level of copper ores in the nature. But, old copper wires and machine parts coming out from industries and ship breakings usually contain a little amount of solder materials comprising of lead, tin, etc. It might have resulted in alteration of the functional ability including wear behavior of scraped copper materials.
In this context, scraped Cu requires the characterization of attainable properties to the surface behavior i.e., wear and friction properties; because major failure of a component occurs from surface wear, which induces the surface deformation and lowering of the strength. There are several studies dealing with wear resistance of Cu materials, few of them have addressed the effect of Pb in Cu on wear behavior [64-67] and few works have observed the wear properties of Cu-Sn alloys [68-69]. However, the effect of Sn and Pb together in Cu is yet to be characterized to obtain the most suitable rehabilitations of SnPb-solder affected scraped copper in producing the high value engineering products.
Moreover, Cu based bearings and associated fittings are being preferred for stainless steel shafts and for that the users are looking for reliable wear information. But the literature review has not provided any such work that has considered stainless steel as the counter surface for sliding wear tests against Cu based materials. Therefore, this chapter looks into the changes in friction and wear behavior of SnPb-solder affected old Cu against stainless steel counter surface at dry sliding conditions.
5.2 Experimental Details
To carry out the wear experiments using pin-on disk (POD) tribometer following ASTM Standard G99-05, the samples were machined with the size of 12 mm length and 5 mm diameter. These samples were also aged at 150°C for one hour to attend the homogeneity and stable hardness for all four materials. The end surface (5 mm diameter) of the pin samples were polished using emery papers in sequence with the course to fine
grits and finally of 1800 grits. Thereafter, the samples were cleaned in running water and dried up after the immersion in acetone. Since the stainless steel is normally used as shafts in marine applications, disks of SS 309s material containing 60% Fe, 23% Cr, 14% Ni, 2% Mn, 0.84% Si, 0.08% C, 0.05% P and 0.03% S were used as the counter- body, which had the hardness of 168 HV. The surfaces of the disk were ground by surface grinding machine and cleaned with acetone as well as dry cotton. Surface roughness of the disc at the time of experimental setup was on average, i.e., centre line average (Ra) 40µin (~1µm). During sliding wear tests, the end surface of the pin sample was pressed against horizontal rotating stainless steel disk with the applied load of 20 N which yielded nominal contact pressures of 1.02 MPa. The effect of loading was observed with variation of load from 5 N to 50 N. The track diameter of the rotating disk was 49 mm. The tests were conducted at the sliding speed of 0.513 ms-1 with sliding distances varied up to 2772 m without providing any lubricant at the contact surface. Experimental setup, few samples and one sample placed on the disk are shown in fig. 5.1.
Fig. 5.1 Experimental setup: (a) Pin-on-disk apparatus, (b) Few samples, and (c) One sample on disk.
The friction and wear experiments were carried out in three sliding environments such as dry sliding, fresh water (FW) wet and sea water (SW) wet conditions. The dry sliding condition was in ambient air with relative humidity 60% without lubrication. For FW condition, the stainless steel disk and the samples were immersed in distilled water.
Similarly for SW condition, the disk and the wear samples were immersed in sea water collected from the Bay of Bengal.
Every after designated period of test run, the wear track and the samples were cleaned with acetone and the pin was weighed for every test run using the weighing machine (model: Sartorius Entris 224-1S having pan diameter of 90 mm with the maximum weighing capacity of 220 g and of 0.1mg precision). Moreover, the length of the pin was measured using an ultrasonic thickness gauge (model: CG100 ABDL having measurement resolution of 0.01mm) to examine the volume loss for every wear test run.
Thereafter, the volume loss values and densities of the sample materials were used to obtain the wear loss values in µg unit. These wear loss values were rechecked with the weight loss values directly measured using the weighing machine to ensure the data reliability. The frictional forces and applied load for individual test run were measured by using the experimental setup arrangements. The frictional force was measured by a strain gauge loadcell mounted on the side of the lever arm of pin-on-disc tribometer and applied load was measured in the axial plane by a high resolution piezoelectric loadcell placed between pin assembly and lever arm. The test for each case was repeated at least three times to examine the weight and height (length) of sample pins, applied load, frictional force and coefficient of friction.
Specific wear rates were then calculated from the average of weight-loss values, the applied load and the sliding distance using equation (5.1) as follows, which is a modified expression based on the Archard equation [70,71].
WR = ∆W
SD × L (5.1) where,
WR = Specific wear Rate (µg N-1 m-1) ΔW = Wear Loss (µg)
SD = Sliding Distance (m) L = Load (N)
The coefficient of friction (COF) values were obtained in two ways. In one way, it was directly from the experimental setup. In another way, the obtained values of frictional force and normal load were then used in the very fundamental equation of frictional law to calculate the COF to ascertain the reliability of machine readings.
μ
=FF
L (5.2)
where,
FF = Frictional force (N) µ = Coefficient of friction
The metallographic images of the samples were examined before and after wear test using computer interfaced Optical Electronic Microscope (OEM) (model: Nikon BW- S500 having 4.19 Mpixel camera) at different magnifications to find out the microstructural changes in the surfaces during dry sliding experiments. Moreover, micrographs were also taken using field emission scanning electron microscope (FE- SEM) of model JEOL JSM-7600F to observe surface microstructures after wear tests.
5.3 Wear Loss in Dry and Wet Sliding Conditions 5.3.1 Wear in Dry Sliding Condition
Contacting asperities of metals deform plastically even at a very small load if it continues for a certain period and during relative motion, at first, material on the contacting surface gets displaced, but little or no material is actually lost. After certain time of sliding, material starts removing from a surface and wear occurs. Wear, the surface damage generally caused by ploughing, adhesion and asperity removal from one or both of two solid surfaces during sliding, is not fully a material property; rather it is a system response, where material property is only a part of that system. The system induced wear mechanisms include the progressive extrusion of material from the asperity junctions into thin chips as debris and/or the fracture of a thin surface layer leading to flake-like debris. Keeping this in mind the wear loss for Cu, Cu-Sn, Cu-Pb and Cu-Sn- Pb alloys have been observed against the sliding distance covered at the speed of 0.513 ms-1 with the normal load of 20 N in dry sliding condition and illustrated in fig. 5.2. Since wear by and large depends on applied load, sliding distance and hardness, the graphs obtained from the experiments have followed the rule of nature and thus the wear losses for all four sample materials have increased with the increase of sliding distance. It is because of temperature elevation at the contacts between rotating disk surface and sliding
surface of the specimen. Moreover, all four materials have not shown the same incremental rate of wear loss due to diverse surface configuration, different micro- hardness and time-variation for reaching to the plastic state of materials.
Fig. 5.2 illustrates that the wear loss values have been found to be increased gradually with the increase of sliding distance for all four sample materials in dry sliding condition.
Basically, during relative motion, material on the contacting surface has been displaced, but little or no material is lost at first. After covering certain sliding distance, material has started removing from the surface and wear occurs almost continuously. As such, the cumulative wear loss has been found to be increased with the increase of sliding period.
The graphs illustrated by Molian et al (1991) depicts the wear loss both in linear and non- linear pattern [64], but the present wear loss results do not have such linearity against sliding distance. Furthermore, the occurrence of wear from one or both of two solid surfaces during sliding is caused by ploughing or/and asperity removal, which is not fully a material property; rather it is a system response where material property is only a part of that system [72]. Besides, the system induced wear mechanisms include the progressive extrusion of material from the asperity junctions into debris and/or the fracture of a thin surface layer leading to flake-like debris [73]. The present results indicate that the addition of Sn or/and Pb in Cu has reduced the wear loss in dry sliding condition, which agrees with Pathak & Tiwari [65] and Kumar et al [69]. As per Fig. 5.2, the alloying effect on wear loss is clearly visible with the increase of sliding distance and after the sliding distance of 2772 m the wear losses of four sample materials have been found to be 19.9 mg, 13.7 mg, 18 mg and 15.8 mg with the corresponding standard deviation of 0.94, 0.67, 0.85 and 0.73, respectively. It shows that the lowest wear loss trend is observed for Cu-Sn alloy which is followed by Cu-Sn-Pb alloy and then Cu-Pb alloy; the highest wear loss is observed for pure Cu. Therefore, tiny alloying elements such as 1.15% Sn, 1.17% Pb and 1.26% Sn + 1.19% Pb with Cu have reduced the wear losses of Cu in the dry sliding friction by 31.2%, 9.6% and 20.6%, respectively. Here, Sn is found as dominant to alter the wear behavior of Cu.
For the thermal ageing condition from room temperature to 150°C, the sequence of sample materials from high to low hardness is as Cu-Sn alloy > Cu-Sn-Pb alloy > pure Cu > Cu-Pb alloy. But the wear loss sequence after sliding distance of 200m or more is as Cu-Sn alloy < Cu-Sn-Pb alloy < Cu-Pb alloy < pure Cu. Considering the behaviour of
pure Cu and Cu-Pb alloy, the correlation between wear loss and micro-hardness have not been significantly noticed at this thermal conditions. However, frictional wear losses are found to be inversely correlated with the coefficient of -0.97 to the micro-hardness values of four sample materials at elevated temperature like 300°C as seen in fig. 3.5, i.e., the hardest material (Cu-Sn alloy) has shown the lowest wear loss (highest wear resistance), which is followed by Cu-Sn-Pb alloy as the second hardest, then Cu-Pb alloy, and pure Cu is observed to be the least wear resistant with the highest wear loss in dry sliding friction.
Fig. 5.2 Wear loss against the sliding distance at the sliding speed of 0.513 ms-1 with the normal load of 20 N (with the pressure of 1.02 MPa) in dry sliding condition.
5.3.2 Wear in Fresh Water Sliding Condition
Fig. 5.3 illustrates that the wear loss at fresh water sliding condition is increased gradually with the increase of sliding distance for all four sample materials. The increasing trend of wear loss in wet sliding condition is very similar to that of dry sliding
0 500 1000 1500 2000 2500 3000
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Wear Loss, mg/g
Sliding Distance, m Cu
Cu-Sn Cu-Pb Cu-Sn-Pb
but the magnitude is less. It is because of fresh water lubrication effect on the mating surfaces. Here also, little time is elapsed before ploughing up the surface material. After covering certain sliding distance, material has started removing from the surface and wear occurs almost continuously. As such, the cumulative wear loss has been found to be increased with the increase of sliding period but in a non-linear form which does not fully agree with Molian et al (1991) [64]. As per fig. 5.3, the wear losses of four sample materials for the sliding distance of 2772 m have been found to be 12.5 mg, 10.8 mg, 14.9 mg and 11.9 mg respectively. It shows that the lowest wear loss trend is observed for Cu-Sn alloy which is followed by pure Cu, and then Cu-Sn-Pb alloy and Cu-Pb alloy has shown the highest wear loss.
Fig. 5.3 Wear loss against the sliding distance at the sliding speed of 0.513 ms-1 with the normal load of 20 N (with the pressure of 1.02 MPa) in fresh water wet sliding.
5.3.3 Wear in Sea Water Sliding Condition
Fig. 5.4 demonstrates the non-linear increasing wear loss trends of all four sample materials for the sliding in presence of sea water on the mating surface against the
0 500 1000 1500 2000 2500 3000
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Cu Cu-Sn Cu-Pb Cu-Sn-Pb
Sliding Distance, m
Wear Loss, mg/g
increase of sliding distance. The pattern of wear loss in wet sliding condition is very similar to that of dry sliding and fresh water sliding but the magnitude is the least amongst the three environments. It is because of sea water lubrication effect on the mating surfaces. In this environment also, removal of material from the surface has started after covering a certain sliding distance and continues throughout. As such, the cumulative wear loss values are increased for sliding distance. As per Fig. 5.4, the wear losses of four sample materials for the sliding distance of 2772 m in sea water environment have been found to be 10.9 mg, 9.7 mg, 14.5 mg and 11.6 mg respectively. It shows that the lowest wear loss trend occurs for Cu-Sn alloy, which is followed by pure Cu, and then Cu-Sn-Pb alloy, where Cu-Pb alloy has shown the highest wear loss.
Fig. 5.4 Wear loss against the sliding distance at the sliding speed of 0.513 ms-1 with the normal load of 20 N (with the pressure of 1.02 MPa) in sea water wet sliding.
0 500 1000 1500 2000 2500 3000
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Cu Cu-Sn Cu-Pb Cu-Sn-Pb
Sliding Distance, m
Wear Loss, mg/g
Fig. 5.5 Comparison of weight loss for the sliding distance of 2772 m at the speed of 0.513 ms-1 with normal load of 20 N: (a) by materials in particular environment
and (b) by sliding environments for particular material.
5.3.4 Comparison of Wear Loss amongst the Sliding Environments
Since the wear (weight loss) values are different from environment to environment, a comparison is made amongst the results in the present study. Fig. 5.5 show two aspects of comparison, such as, (i) sample-wise comparison amongst environments and (ii) environment-wise comparison amongst the samples. Fig. 5.5 (a) illustrate that pure Cu losses the highest material from its surface whereas in other two sliding conditions in dry sliding condition after covering the sliding distance of 2772 m. Whereas, Cu-Pb alloy is showing the highest loss of surface material after covering the same sliding distance. On the other hand, Cu-Sn alloy is found to be the most wear resistant material amongst the four samples by losing the least amount of material from its surface for all three sliding environments. Fig. 5.5 (b) demonstrate that all sample materials are showing the highest wear in dry sliding, then fresh water wet sliding and the least in sea water wet sliding.
The reduction of weight losses in fresh water and sea water environments for pure Cu are 37.18% and 45.23% respectively, for Cu-Sn alloy 21.17% and 29.20% respectively, for Cu-Pb alloy 17.22% and 19.44% respectively, and for Cu-Sn-Pb alloy 24.68% and 26.58% respectively. It indicates that fresh acts as a lubricant very significantly and sea water is even further. Therefore, solder affected Cu can easily be utilised in such water environments for bush or bearing with stainless steel shaft.
5.4 Specific Wear Rates
5.4.1 Wear Rate in Dry Sliding Condition
Specific wear rate magnitudes of all four sample materials have calculated using wear loss data in Eq. (5.1) for the corresponding sliding distance, and the results are found to be depended on the surface roughness and relative hardness of the two contacting surfaces, and on the size, shape and hardness of wear debris and reaction products trapped between them [65]. The results presented in fig. 5.6 elucidate that wear rate at the onset of sliding is largely controlled by the coefficient of static friction and thus the initial wear rate values are sharply higher for all sample materials. Fig. 5.6 also depicts that pure Cu has the poorest wear performance with the highest wear rate amongst four sample materials under dry sliding condition, which is followed by high Cu-Pb aloy and then high Cu-Sn-Pb alloy, where high Cu-Sn alloy has shown the best wear performance. The specific wear rates of Cu, high Cu-Sn alloy, high Cu-Pb alloy and SnPb-solder affected
Cu for the operation period of 90 minutes covering the sliding distance of 2772 m have been found to be 358.9510-3 µgN-1m-1, 247.1110-3 µgN-1m-1, 324.6710-3 µgN-1m-1 and 284.5010-3 µgN-1m-1, respectively. As such, Cu-Sn alloy, Cu-Pb alloy and SnPb- solder affected Cu have shown 31.16%, 9.55% and 20.60% less specific wear rates than that of pure Cu. It means addition of small amount of alloying element like Sn or/and Pb in Cu plays an important role to reduce the wear rate.
Fig. 5.6 Wear rate against the sliding distance at the sliding speed of 0.513 ms-1 with the normal load of 20 N (with the pressure of 1.02 MPa) in dry sliding condition.
The hardness results (shown in figures 3.3 to 3.5) and the wear results (Fig. 5.6) does not fully agree with common understanding on ‘the harder has the lesser wear’ for sample materials. Because, the specific wear rates of both harder alloys (Cu-Sn alloy and SnPb- solder affected Cu) as well as softer alloy (Cu-Pb alloy) have been found to be lesser than that of pure Cu and it disproves the universal acceptance of reverse proportionality of wear rate against hardness. However, solder affected copper has shown moderate position amongst the four materials and it may be considered suitable to use for machine parts having sliding motions. Furthermore, fig. 5.6 demonstrates certain level of wear rate sensitivity over the sliding distance covered during the experiment and the wear rates are found to be increased against the sliding distance. It might be the reason of adhesion
0 500 1000 1500 2000 2500 3000
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
Specific Wear Rate, g/Nm
Sliding Distance, m Cu
Cu-Sn Cu-Pb Cu-Sn-Pb
occurring at the asperity contacts in the interface resulting in the detachment of a fragment from one surface and might get away or attached to the other surface. As the sliding process continues further, the fragments that have transferred to the other surface might also be finally removed as loose wear particles.
5.4.2 Wear Rate in Fresh Water Sliding Condition
Fig. 5.7 indicates that wear rate of pure Cu has not been affected remarkably with the change of sliding environment from dry to FW wet. However, other three sample materials have shown increased wear rates in FW wet sliding condition than those of dry sliding condition, and it has happened more for alloys where lead is present (both Cu-Pb and Cu-Sn-Pb). Basically, wear rate against the sliding distance is the highest for Cu-Pb alloy throughout. For initial sliding, it is followed by Cu-Sn-Pb alloy at FW wet sliding condition. After a certain sliding distance (about 700 m), the wear rates of Cu-Sn-Pb alloy comes down below that of Cu-Pb alloy in FW sliding and continues to maintain this level. Wear rate values of all four materials are then observed to be increasing up to total sliding length of 2772 m. At the end, the wear rate values of pure Cu, high Cu-Sn alloy, high Cu-Pb alloy and SnPb-solder affected Cu for the operation period of 90 minutes covering the sliding distance of 2772 m are 225.4710-3 µgN-1m-1, 194.8110-3 µgN-1m-1, 268.7610-3 µgN-1m-1 and 214.6510-3 µgN-1m-1, respectively. As such, Cu- Sn alloy and SnPb-solder affected Cu have shown 13.6% and 4.8% less specific wear rates than that of pure Cu. On the other hand, Cu-Pb alloy has shown 19.2% more specific wear rate than that of pure Cu. It is a contrast result of pure Cu and Cu-Pb alloy from the results of dry sliding condition, where Cu-Pb alloy have shown lower wear rate than that of pure Cu samples as seen in fig. 5.6. So, the results indicate that the wear rate is affected by the inclusion lead element in Cu for wet sliding condition. At the same time, the wet sliding in fresh water shows that the wear rate values of all four sample materials are reduced by 37.18%, 21.17%, 17.22% and 24.68% respectively. As a whole, the presence of fresh water has reduced the wear rate, and the highest wear rate is observed for Cu-Pb alloy, which is followed by pure Cu, then Cu-Sn-Pb alloy and the lowest for pure Cu at FW wet sliding condition.