Analysis of wear characteristics of natural rubber nanocomposites

Wear 269 (2010) 152–166

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Analysis of wear characteristics of natural rubber nanocomposites Mithun Bhattacharya, Anil K. Bhowmick ∗ Rubber Technology Center, Indian Institute of Technology, Kharagpur 721302, India

a r t i c l e

i n f o

Article history: Received 6 August 2009 Received in revised form 21 March 2010 Accepted 22 March 2010 Available online 27 March 2010 Keywords: Nanocomposites Polymer-matrix composite Sepiolite Carbon nanofiber Abrasion resistance Elastomer

a b s t r a c t Tribological characteristics of natural rubber nanocomposites for wear resistant applications have been studied by sliding against a steel blade, in a specially designed abrader. Testing parameters have been optimized for minimum wear based on Taguchi orthogonal design with four important parameters, viz., nanofiller loading, applied normal load, speed and time of run. Amongst these, nanofiller loading has the most significant influence on wear characteristics. In spite of the high normal pressure acting at the line of contact, certain well established power law relations are found to obey in principle, as wear increases with normal load and frictional work, Fw . Analysis of the micrographs of the abraded surface and the corresponding debris reveals that the specific wear rate of both the nanocomposites (sepiolite and carbon nanofiber filled) is found to increase beyond a critical fractal dimension, i.e., with increasing structural complexity of the debris formed. The rate of wear decreases steeply with nanofiber loading as compared to sepiolite. The changes in temperature build up and dynamic coefficients of friction are found to be concomitant. The wear mechanism is found to be fatigue at low frictional work, followed by frictional wear at high Fw . © 2010 Elsevier B.V. All rights reserved.

1. Introduction Nanostructured elastomeric materials, one of the rapidly growing classes of materials, are being used increasingly for myriad applications where they experience friction and wear. Thus, the practical relevance of the wear of such rubbery nanocomposites cannot be over emphasized. A detailed scientific investigation of abrasion of such nanocomposites has not yet been undertaken. Even much of the knowledge on the tribological behavior of rubbery macrocomposite materials is empirical because of the complexity surrounding wear and its mechanism, which depends on many parameters like the physical and mechanical properties of interacting surfaces, temperature, pressure and the velocity at which the wear takes place. Sometimes it is further complicated by mechanochemical, thermo-mechanical and oxidative degradation. Different mechanisms have been proposed by various workers in this field to explain their observations [1–8]. Unidirectional abrasion of rubber leads to the formation of a characteristic surface pattern consisting of a series of periodic parallel ridges lying perpendicular to the sliding direction, looking like a wind-wrought pattern on sand, often referred to as abrasion pattern.

∗ Corresponding author. Present address: Indian Institute of Technology, Patna 800013, India. Tel.: +91 3222 283180/+91 612 2277380; fax: +91 3222 220312/ +91 612 2277384/+91 3222 277190. E-mail addresses: [email protected], [email protected] (A.K. Bhowmick). 0043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.03.022

Schallamach [1] and later, Grosch [3] reviewed abrasion of rubber and tire wear and they also correlated coarseness of the wear pattern to the rate of wear [4]. Champ, Southern, and Thomas [6] suggested that abrasion takes place due to cumulative growth of cracks by tearing under a repetitive process. Later, Gent and Pulford [9], using an apparatus similar (in principle) to the one used for this study, investigated carbon black filled elastomers, and found that the rate of wear increases with the measured frictional force raised to some power depending upon nature of the rubber and the filler. They observed polymer degradation under high frictional forces and concluded that abrasive wear by small tearing was not solely due to crack growth. Thavamani and Bhowmick [10] reported abrasion of carbon black filled natural rubber (NR), styrene butadiene rubber (SBR) and hydrogenated nitrile rubber (HNBR) and confirmed the above relationship between wear and frictional work, using an entirely different apparatus. Nayek et al. [11] also found the same to be applicable for silica filled NR and SBR compositions in tire application. Ridge formation which takes place on the rubber surface during abrasion is indicative of the mechanism of wear. Schallamach [2] suggested that the saw teeth were bent back and abraded from thin underside until torn off. Bhowmick [12] observed that ridges were generated by coalescence of particles, sequentially through the formation of ribs, rings and ruffles. Southern and Thomas [13] also described the mechanism of pattern formation. Pulford [7] and later Gent and Nah [14] studied the effect of blade type abrader on wear of rubber and vice versa. Fukahori and Yamazaki [15] emphasized

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the role of micro-vibrations and stick-slip motion in determining wear characteristics. In spite of the obvious possibility of utilization in wear resistant application, studies on tribological properties of novel rubber nanocomposites have not received their due attention. Nanofillers cannot be arbitrarily used in polymer matrices and expected to deliver wear resistance. Same nanofiller illustrates different behavior in different polymer matrices. For instance, nano zinc oxide increased the wear resistance of polytetrafluoroethylene [16], but it reduced the wear resistance when used in polyphenylsulfone [17]. Hence, we thought it would be appropriate to find whether the improvement in tear strength on inclusion of fibrous nanofillers (sepiolite and carbon nanofiber), as reported in our earlier paper [18], can be translated into improved wear resistance behavior in NR. Thus, the present work is not only scientifically relevant but also technologically important. Sepiolite is a fibrous needle like clay and eponymic carbon nanofibers too are fibrous. This similarity of morphological constitution and their strong work of adhesion with natural rubber by virtue of high surface energy (found from surface energetic analysis) dictated their selection as materials of choice for reinforcement. They had been earlier found to improve not only the mechanical and dynamic mechanical properties, but also to increase the hysteresis loss and hence the tearing energy, as well. That was addressed in great detail in our previous communications [18,19]. Furthermore, by virtue of its electrophilic nature, carbon nanofiber also presents the opportunity of quenching free radical generation and propagation processes, thereby reducing wear. Although Cho et al. [20] have used Taguchi method for tribological studies on polyphenylene sulfide, literature review reveals that no report is available on the effect of the process and testing parameters on wear characteristics of rubber nanocomposites. In this study, we have discussed the fundamentals of abrasion behavior of rubber nanocomposites. This has not been reported before, especially for rubber nanocomposites, which also satisfy most specifications for tire like applications in terms of their mechanical and dynamic mechanical properties. This paper also discusses the differences in behavior of micro- and nano-composites of elastomers in tire application. To the best of our knowledge, there is no such report in literature. Furthermore, through Taguchi design and analysis, we have been able to delineate and more importantly, for the first time quantify the factors affecting abrasion behavior of rubber nanocomposites meant for tire like applications. This study deals with the application of Taguchi robust design method to determine the influence of testing parameters on optimum wear behavior in natural rubber nanocomposites based on sepiolite and carbon nanofiber. Based on Taguchi orthogonal design, four important variables viz., nanofiller loading, applied normal load, speed and time of testing, have been studied as independent variables. An abrader specially designed in our laboratory, discussed elsewhere [11], is used to evaluate wear behavior of these nanocomposites against a steel blade abrader. The results on some black filled conveyor belt vulcanizates studied using this machine have already been published [21]. A blade type abrader was chosen because the ensuing wear processes seem to be quite consistent with the wear of actual tire treads in road testing [9,14]. Taguchi analysis is employed to identify the behavioral trend for each parameter and determine the optimum combination of testing parameters that yields optimum (minimum) wear. Analysis of variance is carried out to observe the level of significance of the factors. With the help of optical microscopy and computer aided image analysis, the surface morphology and metrology are studied to decipher the small-scale features on abraded surfaces and wear debris. The present study thus incorporates two new considerations: (a) an attempt to delineate the factors affecting wear of rubber nanocomposites by the specially designed blade

153

type abrader utilizing Taguchi robust design and (b) investigation of wear and its concomitant aggregated, hierarchal particulate debris products and abraded surfaces through morphological characterization. This has been achieved through the use of optical microscopy, computer aided image analysis and fractal analysis to comprehend the complex surface metrologies involved. 1.1. Taguchi method Taguchi technique [22,23] is a powerful tool for design of high quality systems based on orthogonal array experiments that provide much-reduced variance for the experiments with an optimum setting of process control parameters. This technique achieves the integration of design of experiments with the parametric optimization of the process, yielding the desired results. The orthogonal array requires a set of well-balanced (minimum experimental runs) experiments. The traditional method of calculating the desirable factor levels using the simple averages of the results does not capture the variability of the results within a trial condition. Taguchi’s method uses a logarithmic function of the desired output as the statistical measure of performance called signal-to-noise ratio (S/N), to serve as objective function for optimization. Defined as the ratio of the mean (signal) to the standard deviation (noise), it considers both the mean and the variability into account. There are three categories of S/N ratios: lower-the-better (LTB), higher-the-better (HTB) and nominal-the-best (NTB). The parameter-level combination that maximizes the appropriate S/N ratio is the optimal setting. For instance, in the case of minimization of wear, LTB characteristic needs to be used. The S/N ratio for wear is calculated using LTB criterion and the same is expressed as: S = −10 log N

   2 1 n

y

(1)

Furthermore, the statistically significant parameters are determined by analysis of variance (ANOVA) [24]. With the S/N ratio and ANOVA analysis, the optimal combination of the process parameters can be predicted. 1.2. Design factors and response variables This technique analyses the influence of process variables (which are also known as design factors) on the response variables and interprets the response as certain function of the process variables. There are numbers of factors that can control abrasion of rubbery materials, but the most prominent amongst them have been found to be (a) filler loading, (b) applied normal load, and (c) speed and (d) time of testing. These four factors are considered as main design factors in the present study. The primary response variable used to accomplish the present study is the volume loss. However, the effect of the testing conditions on several other variables has also been duly analyzed. In the present investigation, two predesigned L25 orthogonal arrays (based on Taguchi method) having the above four parameters at five levels for each parameter are chosen. Table 1 shows the design factors assigned to the respective columns along with their levels, and the corresponding sample designation. Each column represents a test parameter, while the rows indicate the test conditions. A full factorial experiment for the same set of parameters and levels would have required 45 = 625 runs. Since the experimental design is orthogonal, it is then possible to separate out the effect of each contributing parameter at different levels. The calculated main effects for the experimental design represent the effect of each factor, averaged over all levels and combinations of the other factors. These effects are then represented in the main effect plot, where the parameter has insignificant effect, if

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Table 1 Design of experiments. Test number

Sample designation a

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 a

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25

/F1 /F2 /F3 /F4 /F5 /F6 /F7 /F8 /F9 /F10 /F11 /F12 /F13 /F14 /F15 /F16 /F17 /F18 /F19 /F20 /F21 /F22 /F23 /F24 /F25

Filler loading (phr)

Weight (g)

Speed (rpm)

Time (min)

0 0 0 0 0 2 2 2 2 2 4 4 4 4 4 6 6 6 6 6 8 8 8 8 8

50 100 150 200 250 50 100 150 200 250 50 100 150 200 250 50 100 150 200 250 50 100 150 200 250

10 20 30 40 50 20 30 40 50 10 30 40 50 10 20 40 50 10 20 30 50 10 20 30 40

5 10 15 20 25 15 20 25 5 10 25 5 10 15 20 10 15 20 25 5 20 25 5 10 15

C: sepiolite; F: carbon nanofiber system.

the line for a particular parameter is near horizontal. On the other hand, a parameter for which the line has the highest inclination will have the most significant effect. Using Minitab [25], ANOVA is performed to determine which parameter significantly affects the performance characteristics. This type of analysis is not available in the literature and will be useful for analyzing the trend and selecting the testing parameters with an objective of understanding and optimizing wear behavior.

2. Experimental materials and methods 2.1. Materials Natural rubber, (Mooney Viscosity, ML1+4 @100 ◦ C = 60) was supplied by the Rubber Board, Kottayam, Kerala, India. The fillers used were: organomodified nanoclay–Pangel B20 (Sepiolite clay from Tolsa S.A., Madrid, Spain), and carbon nanofiber-Pyrograf III, PR-24 (Vapour Grown Carbon Fiber from Pyrograf® Products Inc., Ohio, USA). Standard rubber grade zinc oxide, sulphur, benzene and toluene were procured from Merck Ltd., Mumbai, India. Stearic acid was supplied by Shreeji Fine Chemicals, Mumbai, India and N-cyclohexyl-2-benzothiazyl sulfenamide (CBS) by ICI India Ltd., Chemicals Division, Mumbai, India. N-isopropyl-N -phenylp-phenylenediamine (IPPD) was provided by Bayer Chemicals AG (presently, Lanxess), Leverkusen, Germany. Chemlok 205 (primer) and 220 (adhesive) used for rubber to metal adhesion were bought from Lord India Chemical Products Pvt. Ltd., Nasik, India. Microtome blades (High Profile-Personna Plus, American Safety Razor Company, Staunton, USA) used as abrader were supplied by Personna U.K. Ltd. The recipe used in this investigation is tabulated in Table 2.

The basic recipe was kept common for all the systems, in accord with our previous publication [18]. There was no variation in the loading of anti-oxidant, curatives and cure accelerators. The samples and their designations are illustrated in Table 1. 2.2. Preparation of nanocomposite The nanofiller was initially mixed with the rubber in a Brabender Plasticorder (PLE 330) at 80 ◦ C at 60 rpm for 2 min. The remaining compounding ingredients, except the curative package, were then added and mixed in the Brabender under the above mentioned conditions for 3 min. The curatives were subsequently added to the resulting masterbatch in a two-roll mill (Schwabenthan, Berlin), following standard mixing sequence. 2.3. Preparation of rubber specimen The rubber specimens as shown in Fig. 1 were prepared by molding in a David-Bridge hydraulic press (supplied by Castleton, Rocchdle, England) at a pressure of 5 MPa at 150 ◦ C. To accommodate for the thickness and the metal insert, curing was performed till twice the t90 value (optimum cure time) obtained from a Mon-

Table 2 Rubber formulation. Ingredient

Rubber

Nanofiller

Zinc oxide

Stearic acid

IPPD

CBS

Loading in phr

100

2,4,6,8

5

2

1

0.8

Fig. 1. Rubber sample.

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Fig. 2. Schematic diagram of the machine.

santo oscillating disc rheometer (ODR-100 s). The specimens were conditioned at room temperature for 16 h before carrying out the testing. 2.4. Description of the experimental set-up for wear investigation The experimental set-up was designed to rotate a circular rubber disc against a rigidly held razor blade under different magnitudes of applied normal load, sliding speed and time (Fig. 2). The rotation of the rubber disc at different controlled speeds was provided through a single phase motor of 0.2 kW power connected through a pulley. The metal blade sample holder was connected to a long cantilever arm through an octagonal ring type dynamometer, from which the normal and the tangential components of the force were derived using the Wheatstone bridge principle. Normal load was applied at the rubber-blade interface by hanging weights at the free end of the cantilever beam. In the present design of the experimental set up, the actual normal load acting at the interface was not equal to the weight hung at the free end; instead it was also dependent on the distance from the axis of the blade holder. Therefore, in order to measure the magnitude of the load at the interface, calibration was done by recording the response of the load cells with accurately known weights. The other details of the abrader have been discussed in an earlier publication, using different rocks as abrading material [11]. The amount of wear was measured as weight loss before and after abrasion and subsequently converted to volume loss. 2.5. Experimental procedure for investigations of wear of rubber The full arrangement of the experimental set up during wear testing of the rubber sample is shown in Fig. 2. At first, the rubber disc was fixed tightly to the shaft of the pulley by the set-screw. The fresh razor blade sample was then clamped into the blade holder. The length of projected portion of the blade outside the sample holder was adjusted so as to match the position maintained during normal load calibration. Different weights were mounted on the hanger of the cantilever beam and the corresponding normal force exerted and the frictional force generated were grabbed from the dynamometer using an external circuit, amplified and projected on to a computer attached

with the machine. The frictional force was monitored continuously by means of the slight tangential deflection of the strain gauges cemented on the dynamometer. During testing, the razor blade sticks to the rubber surface until there is a sudden break as a result of the gradually increasing pull which causes a very rapid slip. The blade sticks again and the process is repeated indefinitely and thus the resulting frictional force is not constant. This characteristic of abrasion of rubber by blade type abrader is called stick-slip motion (shown later). To obviate the effect of changing abrading capabilities of the razor blade, due to the wearing out of the blade and its inherent protective layer with run time, the blade was changed after every two runs. Such blades have earlier been used for similar experiments by Gent and Pulford [9] and Zhang [26]. The exact test conditions and materials (rubber wheel, blade and brush for debris removal) are in accord with previous publications. The brushing was done in accord with available literature [11,27] by holding the soft polymeric bristles tangentially to the rotating wheel surface. This ensured that no drag force, which could contribute towards abrasion, is imparted. Frequent brushing was done to remove the debris particle from clinging on to the surface and the wear debris particles were collected from the wake zone of the experiment. Wear studies were carried out in accord with the orthogonal Taguchi design at room temperature and the weight loss and the temperature of the abraded surface of the rubber samples were measured. The temperature developed at the rubbing interface was measured with a non-contact infra red thermometer (Model MT10, Metravi, Kolkata, India). The dynamic coefficient of friction (), frictional work (Fw ) and abrasion loss (V) were computed from the primary observations. 2.6. Studies on abraded surface and wear debris The tested specimens were cut for microscopic examination. The nature of abraded surfaces, and the particulate and structured wear debris of each and every nanocomposite were collected on a clean paper for studying with an optical microscope (WILD M8, Wild Heerbrugg, Switzerland). The images were captured using Moticam1000, Motic China Group Co. Ltd., Xiamen, China. Due to the large force exerted at the line of contact of the blade with the rubber sample (around three orders of magnitude higher than those for rock type abrader [11]), the characteristic wear

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patterns were discernible and could be resolved through optical microscope itself.

3. Results and discussion 3.1. Taguchi analysis of wear characteristics

2.7. Image analysis ImageJ software developed by the National Institutes of Health (U.S.A.) and its various plugins available in the public domain were extensively used for the purpose of image analysis and surface metrology of the optical photomicrographs of the rubber debris and abraded rubber surface. After binarization of the image fields, perimeter and feret diameter of all the wear debris were calculated by using the Analyze Particles plugin [28]. The roughness analysis was done using the Roughness Calculations plugin [29], whereas the fractal dimensions of the grayscale images were calculated by the help of the FracLac plugin [30]. The results given here are the average of six samples. The standard deviations for perimeter, feret diameter, roughness and fractal dimensions for these sets of observations are ±0.6, ±0.14, ±6.1 and ±0.08 units, respectively.

In order to perform Taguchi analysis, volume loss, coefficient of friction (COF) and temperature build up (TBU) are plotted in Figs. 3a–c and 4 as a function of the parameters used in this study. These plots, as described earlier, are known as the main effects plot. Each point here represents the mean of the five observed values obtained for a particular parameter while using different values of the other parameters. Fig. 3a–c corresponds to sepiolite and Fig. 4 to nanofiber filled systems. From Fig. 3a, it is seen that the abrasion loss decreases almost linearly with the incorporation of sepiolite. It, however, exhibits just the opposite behavior with increase in applied normal load, speed and time. From Fig. 3b and c, it is seen that the sepiolite loading also favorably influences the temperature build up and the dynamic coefficient of friction in these nanocomposites. As expected, there is an increment in

Fig. 3. Main effects plot—sepiolite.

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157

Fig. 4. Main effects plot—nanofiber.

Table 3 ANOVA for sepiolite systems. Source

DF

Seq SS

Adj MS

Loading Norm load Speed Time Residual error Total

4 4 4 4 8 24

3541 1933 1687 2235 1316 11412

935.3 483.4 421.9 683.6 164.5 –

both volume loss and temperature with increase in normal load, speed and time. These parameters, however, fail to influence the COF of these nanocomposites. This observation is in agreement with those reported earlier [14,15,20]. The coefficient of friction against very smooth surfaces is high because of high adhesion between the mating surfaces. Thus, with increase in nanofiller loading, the increase in surface roughness and the concomitant loss in rubber-blade adhesion lower the COF. Thus, these plots indicate the effectiveness of this mineral filler in lowering the volume loss, coefficient of friction and to certain extent also the TBU in these nanocomposites. It is interesting to note that for carbon nanofiber filled nanocomposites, Fig. 4, the abrasion loss decreases remarkably even with the incorporation of 2 phr of carbon nanofiber, but is not influenced appreciably on further increasing the loading up to 8 phr. These nanocomposites exhibit the same trends in wear, TBU and COF with normal load, speed and time. The graphs for COF and TBU have been

F

P

5.86 2.94 2.56 3.98 – –

0.012 0.091 0.120 0.056 – –

%Contribution 31 17 15 20 12 –

provided as supporting information (S1). Carbon nanotubes based materials have also been earlier reported to have lower COF and wear rates, as well [31]. 3.2. Analysis of variance (ANOVA) of means Tables 3 and 4 show the ANOVA result for wear of NR nanocomposites. ANOVA calculates the F-ratio, which is the ratio between the regression mean square and the mean square error, as the measure of significance of the parameters under investigation with respect to the variance. In general, when F value increases, the significance of the parameter also increases [23]. ANOVA tables show the percentage contribution of each parameter. It is seen that parameter A (nanofiller loading) has got the most significant influence on wear characteristics at the confidence level of 90%. The corresponding high F-ratio and p value corroborate this result. This type of analy-

Table 4 ANOVA for carbon nanofiber systems. Source

DF

Seq SS

Adj MS

F

P

Loading Norm load Speed Time Residual error Total

4 4 4 4 8 24

5258 1325 1622 1765 2324 12293

1314.5 331.2 405.5 441.2 290.5 –

4.53 1.14 1.40 1.52 – –

0.033 0.404 0.318 0.285 – –

%Contribution 43 11 13 14 19 –

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Fig. 5. NR sepiolite: volume loss against (a) frictional work and (b) normal pressure.

sis is not available in the literature and will be useful for assigning factors while studying wear debris and surface formation within the particular range of testing parameters with the objective of optimizing wear.

3.3. Effect of friction, load and viscoelasticity Studies on wear of NR nanocomposites against a razor blade have been carried out at different normal loads. The extent of wear, temperature development, frictional force and dynamic coefficient of friction have been determined. In the present work, both the zones of transient and steady abrasion have been considered, in line with Zhao and Bahadur [32]. It was observed that the volume loss became independent of the number of revolutions, and constancy in the distance between ridges too was established at high severity. Prior to that, under low wear conditions, though the wear loss was low, but volume loss was not strictly constant and microridges were still being formed. Unlike the previous authors [26,33], we have spanned both the states of abrasion because one of our primary objectives was to design (L25 Taguchi design) the experiments scientifically to delineate quantitatively the factors governing wear in rubber nanocomposites. We were not only able to quantize the statistically

significant factors and their effect in this way, but also highlight the efficacy of the two nanofillers (at low loadings) in arresting the wear. On application of normal load, as designed in the Taguchi array (Table 1), large forces act on the line of contact which acts as the rubber-blade interface, resulting in normal pressure ranging from 10 to 31 MPa. The relations between the volume loss and the frictional work and normal pressure for sepiolite filled NR nanocomposite vulcanizate are shown in Fig. 5, while Fig. 6 deals with the same for carbon nanofiber filled system. Since from the preceding discussion it was concluded that amongst all factors nanofiller loading exerts the most significant contribution towards wear, these studies are performed at the different nanofiller loadings. Wear rate plotted against the frictional work input per unit revolution (and also normal pressure) in logarithmic scales yields linear relationships. The following power law equation describes the relationship between frictional work input per unit revolution and volume loss (V).

n V = kFw

Fig. 6. NR C-nanofiber: volume loss against (a) frictional work and (b) normal pressure.

(2)

M. Bhattacharya, A.K. Bhowmick / Wear 269 (2010) 152–166 Table 5 Coefficients and exponents for NR sepiolite nanocomposites. Filler loading (phr)

k

n

ˇ

˛

0 2 4 6 8

6.5E−16 3.2E−12 3.8E−11 1.1E−10 1.7E−09

7.25 2.36 2.25 0.75 0.22

1.1E−17 1.1E−13 6.1E−12 2.6E−11 3.9E−10

8.51 2.66 2.88 2.22 0.75

while Eq. (3) depicts the relationship between normal load and volume loss. V = ˇN ˛

(3)

Higher wear at higher normal load is due to more energy input to the system as a result of which the material reaches its critical fracture energy of tearing quickly. The corresponding coefficients and exponents (k, n, ˇ, ˛) values are tabulated in Tables 5 and 6. Characterization and quantization of rubber wear through power law coefficients and exponents has been in vogue in the literature [6,9,10]. These parameters enumerate the effect of frictional and normal load on the wear loss and are known to be characteristics of the rubber-filler-abrader systems, i.e., they depend on the nature of abrader and also the nature of the rubber compounds. The exponents (n and ˛) are found to be decidedly smaller for the nanofiller containing compounds, testimonial of the lesser dependence of the wear rate on the normal load and the subsequent frictional work generated. Gent and Pulford [9] found ‘n’ of filled compound (1.5) was lowered from that of the unfilled SBR (2.9). The corresponding ‘k’ values were of 2.0 × 10−12 and of 7.0 × 10−16 , respectively. Champ et al. [6] observed for NR the coefficient and exponent were 1.1 × 10−13 and 2.6, respectively. Thavamani and Bhowmick [10] reported the value of ˛ in the range of 1.57–2.2 and ˇ in the range of 1.11 × 10−16 to 5.78 × 10−14 and those of ‘n’ in the range of 1.82–2.20 and value of ‘k’ in the range of 3.10 × 10−14 to 6.82 × 10−16 for black filled NR compounds, although they have used an entirely different abrader. Although the presence of carbon nanofiber arrests the wear rate more (Figs. 3 and 4), it can be inferred from Figs. 5 and 6 that the change in volume loss with frictional work (and also normal pressure) is more significantly influenced by the presence of sepiolite than carbon nanofiber. Also, the 8phr sepiolite loaded sample has the flattest of curves with the lowest of exponents (Table 5), indicating clearly a near invariance with increasing frictional work (and also normal pressure). It is also interesting to note that beyond a certain critical value of frictional work (and also normal pressure), there exists a reversal in the relative wear rates. Thus, extrapolation of measured data suggests that wear rate of sepiolite and carbon nanofiber filled composites would be greater than that of their unfilled counterpart for low frictional and normal loads. Similar observation was made by other authors while working with different types of abraders [9,11]. Wear process, it must be remembered, involves a small scale tearing process analogous to crack growth under repeated stressing and a direct failure on a single stress application, which can be correlated to the tensile strength. This reversal of the relative wear rates is probably because of change in the wear mechanism which reflects that the differTable 6 Coefficients and exponents for NR carbon nanofiber nanocomposites. Filler loading (phr)

k

n

ˇ

˛

0 2 4 6 8

2.1E−19 1.3E−17 4.9E−16 3.5E−16 1.3E−16

10.40 7.76 7.17 7.01 6.87

5.5E−22 5.6E−21 2.7E−20 2.6E−19 6.4E−19

12.25 9.37 8.97 8.69 8.25

159

ences in the strength and extensibilities at high severities plays the dominant role, while the differences in crack growth rates and fatigue seem to be the determining factor at low severities. The low severity conditions correspond to the transient state of abrasion, while the high severity ones correspond to steady state abrasion, as discussed earlier. This is also the reason why the wear dynamics and mechanism are different under low and severe wear conditions. Thavamani et al. [34] had earlier found that at low loads fatigue wear dominated, whereas at higher ones frictional wear was more prominent. Fatigue wear is caused by repeated stress driven crack initiation and propagation mechanism at the surface and sub-surface regions where the stress is maximum. In well dispersed nanocomposites such stress concentration may take place at the edges/terminal end of the nanofillers initiating cracks. In our earlier studies [18], we found that the tensile strength and modulus improved tremendously on incorporation of nanofillers. The strain energy density (W) also registered large increments of the order of 80%, in their presence. Since fatigue life (N) of strain crystallizing rubbers is inversely related to the strain energy density (to be precise, N ∼ W−2 ) [35], the NR nanocomposites exhibit lowering of fatigue life and greater crack growth/initiation tendencies. Thus, in low severity conditions which are dominated by fatigue wear, abrasion resistance of the NR compounds showed a reversal in trend, i.e., the gum exhibited low abrasion, while the nanocomposites wore away faster. On the other hand, under high severity conditions, the actual stress is closer to ultimate tensile stress, deformation levels are much higher and stress cycles are very frequent. Thus abrasive wear comes into play under such conditions. From earlier studies it is known that the viscoelastic behavior of a rubbery material can be used for prognosis of its wear characteristic [36,37]. Furthermore, the tan delta at low temperatures (∼−60 ◦ C) can be correlated with the abrasion resistance, while those at 0 ◦ C and 60 ◦ C correspond to the wet skid resistance and the rolling resistance in tire applications [38]. Since Tg of these nanocomposites have earlier [18] been found to coincide with the zone assigned to the measure of abrasion resistance, the volume loss was also studied against the tan delta at Tg in Fig. 7. A double logarithmic plot of the volume loss against tan delta at Tg (Fig. 7) clearly shows linear dependence. The wear rate decreases with increase in rubber-filler bonding, which causes the nanocomposite to have lower tan delta at Tg . The viscoelastic behavior of carbon nanofiber filled nanocomposites appears to influence the volume loss more prominently (Fig. 7b) than their sepiolite counterparts (Fig. 7a). 3.4. Studies of debris and abraded surface Fig. 8 displays the representative photomicrographs of the debris and the abraded surface produced during abrasion of sepiolite filled NR nanocomposites. The wear process generating such representative features are indicated in the sample designation, in accord with Table 1. Fig. 9a is a typical spectrum of tangential frictional force against sliding time, representing stick-slip motion, in which zones A and B (Fig. 9b) correspond to the stick and slip phase of stick-slip motions, respectively. Fig. 10 assimilates the detail analysis of all such debris and abraded surfaces. Figs. 11 and 12 are the carbon nanofiber analogues of those in Figs. 8 and 10. Tables 7 and 8 list the wear characteristics in terms of their volume loss, COF, TBU and debris features of sepiolite and carbon nanofiber containing NR nanocomposites, respectively. Two major types and sizes of debris are observed: (A) intrinsic particles and (B) aggregates. Intrinsic particles (Figs. 8b and 11a and b) originate from two sources—either

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Fig. 7. Dependence of volume loss on viscous loss properties at Tg in (a) sepiolite and (b) C-nanofiber filled nanocomposites.

the growth of microflaws or from asperity induced micro-tearing, while aggregates (Figs. 8a, c, h and 11c, h) result from the tacky interactions which occur during the slip induced tumbling process, and the periodic tearing away of tongues forming the abrasion

pattern [1,39], resulting in larger particles. The detachment of the basic intrinsic particles results in the eventual formation of the ridges on the abraded surface by cumulative abrasion. In the case of abrasion by blade, there occurs intermittent contact

Fig. 8. Topography—sepiolite nanocomposites.

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161

Fig. 9. Stick-slip motion: zone A, stick; zone B, slip.

between the blade and the surface resulting in a stick-slip pattern [15]. The periodic bumping during alternating contact in slip phase of the stick-slip motion (Fig. 9) causes the initial microcracks or microridges and later these propagate through the stick phase. As elucidated by Schallamach, the ridges fold over on each passing of the abrader, thereby protecting the trailing edge (wake zone) from further wear. The tongue thus formed is shown in Fig. 8f and g. Champ et al. [6] have proposed that due to mechanical fatigue caused by repeated straining, the tongue tip tears away eventually

leading to formation of the larger debris, which in fact accounts for the major volume of the wear. The wave patterns on the abraded surface are the imprints of the causative wear mechanism, and a careful look at those reveal that the angle formed by the abrasion patterns varies with compound composition. In fact, for gum compounds it is around 33 ± 3◦ , whereas this angle increases with filler loading up to 46 ± 2◦ for the 8 phr sepiolite filled systems. This is also seen in the representative figures of the abraded nanofilled samples (Figs. 8f, g and 11f, g). Fig. 8f and g represents the gum compound (C2) and 8 phr sepiolite

Fig. 10. Debris and abraded surface metrology in NR sepiolite nanocomposites.

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Fig. 11. Topography—carbon nanofiber nanocomposites.

filled NR (C24), while Fig. 11f and g correspond to the gum compound (F3) and 6 phr carbon nanofiber filled NR (F22), respectively. It is thus observed that the larger the angle, the more is the protection available to the wake zone from exposure to the blade and lesser is the total wear. Due to the motion of the wheel, the applied normal force acts obliquely on the rubber surface and the cosine component of the resultant vector is responsible for the tangential frictional force generated. Thus, greater is the angle created by the tongue formed by the abrasion process, lesser is the frictional force and hence the associated wear. The influence of the frictional work on the size distribution, in terms of the average perimeter projection, of the debris is demonstrated at a normal pressure of 20 MPa (Fig. 10a). Because of their irregularities, the debris particles can seldom be accurately represented by diameter or length. The projection of a debris particle generates its perimeter. Hence, the perimeter has been used to describe the surface metrology as it yields a better and more objective representation of the debris. The particle size of the aggregates increases with frictional work, but those of the intrinsic particles remain invariant. Such bimodal distribution has been reported earlier to be characteristics of wear debris [9,10]. The feret diameters

and the average roughness of these particles are listed in Table 7 and they were found to marginally increase with the frictional work per revolution. Fig. 10b describes the relation between ridge spacing (as seen in Figs. 8d, e and 11d, e) and the temperature build up and volume loss on a logarithmic plot. Fig. 10c and d illustrates the effect of viscoelastic properties of the nanocomposites on the ridge spacing and hence, also on the wear behavior. It is seen that with ridge spacing, both the wear rate and the temperature build-up increase. Thus, as a corollary, both these parameters can be expected to show rising trends with frictional work, as well. Schallamach [40] explained the formation of ridges on the worn surface of NR compounds and related the ridge spacing (Rs ) to the normal load (P) and modulus (E) of rubber by the relation Rs = Const

P E

rd2

1/3 (4)

Or, Rs ∼E −1/3

(5)

Or, Rs ∼P 1/3

(6)

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163

Fig. 12. Debris and abraded surface metrology in NR C-nanofiber nanocomposites.

During abrasion, the rubber surface is subjected to shearing and hence E may be replaced by the shear modulus, G . The logarithmic plot of ridge spacing versus G for different sepiolite containing NR nanocomposites at constant normal load yields a straight line with a slope of −0.32, as shown in Fig. 10c. As in the case of volume loss (Fig. 10b), the ridge spacing is also found to vary linearly with the loss factor (Fig. 10d), bearing the following relation: Rs = 1.82 tan ı0.67

(7)

Similar studies on the carbon nanofiber filled NR nanocomposites reveal that all the above observations hold true (Figs. 11 and 12 and Table 8). Fig. 10a and Fig. 12a reveal that the intrinsic particles formed from both the composites containing nanofillers are not only invariant with frictional work, but they are also of similar perimetric measure (3 ± 0.3 mm).

During debris formation, as the length of contact grows in the slip direction (indicated by arrows in Figs. 8d and e and 11d and e), the aggregates undergo progressive axial lengthening depending on the tackiness of the initial debris. This is particularly true for the carbon nanofiber filled samples which possess higher degree of tackiness, resulting in rolled up surfaces and elongated fibrillar debris, seen in Fig. 11 (c, h). Carbonaceous materials like fullerenes are electrophilic in nature and can participate in arresting wear through free radical generation mechanism. Such mechano-degradative processes in the presence of carbon nanofibers may result in increased tackiness and concomitant axial lengthening. The correlation between the COF and temperature build up has already been discussed in the previous section. Fig. 12b brings to fore another unique observation relating the temperature build up to the roughness of the debris particle. It is seen here that the near exponential drop in temperature co-occurs with a similar

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Table 7 Wear characteristics of NR sepiolite nanocomposites. Sample

Vol. loss (cm3 )

COF

Temp. diff. (◦ C)

Designation C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 a

0.4 1.0 3.5 6.0 10.3 1.9 3.7 6.2 1.2 1.2 2.5 0.7 2.4 1.7 4.1 1.1 4.3 1.6 4.0 1.2 1.6 0.5 0.3 0.8 2.6

1.26 1.15 1.15 1.15 1.16 1.13 1.14 1.13 1.08 1.10 1.09 1.13 1.07 1.13 1.13 1.08 1.07 1.10 1.11 1.07 0.91 0.96 0.99 0.94 0.93

1 4 5 7 10 3 6 7 8 9 3 4 6 4 6 2 4 6 4 4 3 2 2 4 7

Feret Diameter (mm)

Roughness

Agg.

Int.

RMSa , Rq

Average, Ra

2.75 2.41 4.78 2.95 2.95 2.04 2.95 5.62 3.38 2.90 3.97 3.86 2.09 3.43 3.34 3.18 2.77 2.46 4.45 3.04 3.18 2.45 3.13 3.16 1.96

1.33 0.93 1.96 0.83 0.97 0.79 0.80 1.09 0.91 1.07 0.95 0.79 0.82 1.05 0.96 1.10 0.94 0.73 0.90 0.77 1.29 0.93 1.06 1.02 0.79

96 87 81 95 76 92 98 94 108 107 104 86 76 83 80 75 74 79 92 100 89 86 78 98 86

90 80 70 79 63 85 88 85 97 99 100 81 71 75 72 66 67 72 85 91 81 80 71 88 77

RMS: Root Mean Square, Agg.: Aggregate, Int.: Intrinsic particle.

increase in debris roughness. Thus, it can be inferred that significant portion of the energy is dissipated in generating the irregular surface contours, thereby alleviating the increment in temperature. Table 9 compares the wear characteristics of the sepiolite and nanofiber filled systems numerically. These equations clearly elicit the determining factors which influence the wear behavior. The wear rate (volume loss= V) and ridge spacing (Rs ) are found to be related to the tan delta at Tg by the empirical equations V = a1 tan ıb1 and Rs = a2 tan ıb2 . However, it needs to be mentioned here that the both volumetric loss and ridge

spacing are influenced much strongly by the loss factor in nanofiber filled systems than in the case of sepiolite (Figs. 7b and 12d). During abrasion some part of the frictional work is converted to heat energy and the remaining part is used for tearing the sample and removal of the material from the surface. The part converted to heat energy manifests in form of the increased temperature on the abraded surface. The temperature development is measured under different loads and found to be an exponential function of the ridge spacing for both sepiolite and nanofiber filled NR nanocomposites (Figs. 10b and 12b).

Table 8 Wear characteristics of NR carbon nanofiber nanocomposites. Sample

Vol. loss (cm3 )

COF

Temp. diff. (◦ C)

Feret diameter (mm)

Roughness

Agg.

Int.

RMS, Rq

Average, Ra

0.4 1.0 3.5 6.0 10.3 0.8 1.2 2.0 0.6 0.4 0.8 0.4 0.1 0.3 1.0 1.9 1.1 0.9 1.5 0.3 0.1 0.3 0.1 0.3 1.4

1.26 1.15 1.15 1.15 1.16 0.90 0.95 0.89 0.94 0.95 0.84 0.92 0.94 0.93 0.92 0.83 0.87 0.91 0.86 0.90 0.88 0.93 0.93 0.99 0.91

1 4 5 7 10 2 6 6 5 4 7 3 8 4 4 4 8 4 5 3 7 7 7 6 7

2.75 2.41 4.78 2.95 2.95 2.63 2.79 2.24 2.50 2.70 2.13 2.86 2.50 2.70 2.81 2.55 1.90 2.70 2.56 2.88 2.03 3.22 2.59 2.79 2.60

1.33 0.93 1.96 0.83 0.97 0.74 0.86 0.79 0.67 0.82 0.65 0.96 0.81 0.78 0.85 0.93 0.67 0.88 0.57 0.85 0.54 0.82 0.69 0.66 0.87

96 87 81 95 76 156 150 149 139 123 139 128 120 125 107 145 96 137 129 144 133 132 117 112 134

90 80 70 79 63 152 146 146 134 120 134 123 116 121 104 140 94 133 126 138 130 128 114 108 129

Designation F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12 F13 F14 F15 F16 F17 F18 F19 F20 F21 F22 F23 F24 F25

M. Bhattacharya, A.K. Bhowmick / Wear 269 (2010) 152–166 Table 9 Comparison between sepiolite and nanofiber. Nanofiber

4. Conclusions Sepiolite

9.0

V = 0.008 tan ı V = 1.44Rs0.89 Rs = 1.57G −0.32 Rs = 0.32 tan ı2.48 T = 4.46Rs0.23

165

2.5

V =1.08 tan ı V = 0.25Rs1.93 Rs = 2.49G −0.32 Rs = 1.82 tan ı0.67 T = 1.75Rs0.91

Fig. 13. Variation of specific wear rate with fractal dimension of the debris formed.

Numerical descriptors of wear particles could obviate the need for expert interpretation of wear particle morphology and greatly simplify the diagnostic procedure. Fractal parameters (which are a measure of the irregularity and complexity) provide this opportunity and have been earlier applied to fine particles [41]. Although, the morphology of a wear particle can be moderately characterized by the description of its boundary profile, surface characteristics and size, yet they may differ greatly in surface texture. Hence, the differences in the particle morphology which is concomitant of the causative wear processes involved in their formation are not reflected in the linear fractal dimensions derived from their boundary profiles. Thus, the “‘texture fractal dimension” of the particles in the photomicrograph is determined using the variation in gray level (light intensity) data of the points on the particle surface using the FracLac plugin of ImageJ. Fractal analysis can be successfully applied to describe the texture and shape complexity of wear particle boundaries and surfaces. Fig. 13 is the plot of the specific wear rate (volumetric loss per unit revolution normalized against constant normal load) versus fractal dimension (Df ) of the corresponding wear debris and shows a monotonous increase. Similar observations and subsequent prognosis were made by Zhang et al. [42]. In fact, a closer look reveals, a stair-like curve in the operating zone, where two plateau regions joined by steep inclined segments. The plateau region possibly signifies the formation of some steady state in terms of generation of surface irregularity and wear rate, as well. The graph clearly indicates that although wear rate increases unilaterally with fractal dimension, there exists certain critical values of Df at which sharp upturns are seen. These regions correspond to generation of debris by the slip induced tumbling process and/or the periodic tearing away of tongues forming the abrasion pattern, which enhances the wear rates significantly. The tongue tearing phenomenon not only contributes to the wear volumes, but also exposes more and more surface (in the wake zone) to intensive abrasion.

1. Design of experiments approach by Taguchi method enables us to analyze successfully the trends in friction and wear behavior of nanocomposites comprised of two different nanofillers, with filler loading, applied normal load, speed and time as the variables with much fewer experiments than that would otherwise be needed. 2. ANOVA provides us the percentage contribution of each parameter towards wear. Wear decreases with nanofiller loading, while it increases with the rest of the parameters. Amongst all the parameters only nanofiller loading has a statistically significant contribution. 3. Both nanofillers lower the wear characteristics of NR nanocomposites. The reduction in wear is, however, much greater even at very low nanofiber loading, as compared to that in the case of sepiolite. 4. The coefficient of friction is lowered with the addition of these nanofillers, due to the reduction in rubber-blade counterface adhesion and the trends in coefficient of friction and temperature build up shows significant correspondence with changes in the design factors. 5. The volume loss (V) is related to the normal load (N) by the equation V = ˇN˛ and with frictional work (Fw ), by the relation Fw n . The values of the coefficients and exponents are in by V = kFw agreement with those reported earlier, and the exponents are decidedly smaller for the nanofiller containing compounds, testimonial of the reduced dependence of the wear rate on the normal and frictional loads. 6. Abrasion follows stick-slip process. Fatigue wear dominates in low frictional work region, whereas friction based mechanism assumes prominence up field. 7. Bimodal distribution of wear debris size is observed, resulting in the formation of intrinsic and aggregated particles. Only the resulting aggregated particle size increases linearly with frictional work, while the intrinsic particles remain unaffected. Carbon nanofiber containing nanocomposites exhibit wear by roll formation. 8. Characteristic abrasion patterns are formed on the surface of the abraded surface. It is seen that with ridge spacing, wear rate and temperature build up increase, same as with frictional work. All these parameters are decisively influenced by the viscoelastic properties of the nanocomposites. The wear rate and ridge spacing (Rs ) are related to the tan delta at Tg as power law functions. 9. The specific wear rate shows a unilateral increase with fractal dimension of the corresponding wear debris generated, forming a step-like curve exhibiting critical values of fractal dimensions at which significant jump in wear rate occurs. Supporting information The graphs for the variation of COF and TBU of the carbon nanofiber filled compounds have been provided as supporting information (S1). Acknowledgement We gratefully acknowledge Mr. Satyajit Chatterjee of Indian Institute of Technology Kharagpur, India, for his assistance in Taguchi analysis. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.wear.2010.03.022.

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