National Conference on Recent Advances in Innovative Materials (RAIM-08)
Proceedings of the
National Conference on Recent Advances in Innovative Materials (RAIM-08) February 16-17, 2008
Editors Dr. Subhash Chand Dr. Kuldeep Kumar Sharma
Dr. A.S. Singha Dr. S.K.S. Parashar
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Influence of Brass as a Filler on Load-Speed Sensitivity of Polymer Based Friction Composites Mukesh Kumar and Jayashree Bijwe Industrial Tribology Machine Dynamics and Maintenance Engineering Centre, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, INDIA E-mail:
[email protected] Abstract Polymer based composite friction materials are widely used in brake systems of automobiles, locomotives etc. Metals are important fillers in friction materials since they control the thermal conductivity (TC) of composites apart from additional functions such as wear resistance, strength etc. Low TC of composites renders the tribosurface vulnerable for degradation of organic ingredients affecting the braking capability adversely while too high TC on the other hand, results in adverse effect on brake-fluid. The metallic fillers also affect the tribo performance of composites. Hence the contents are to be optimized to achieve best combination of performance properties and thermal conductivity. The optimum combination hence is to be tailored with right contents of metallic fillers. Hence in this paper the effect of brass particles in increasing amount on friction and wear performance of polymer based NAO friction composites was studied. Four friction composites with varying amount of brass (0, 4, 8 and 12 wt.% and barite in 35, 31, 27, and 23 wt.% in complementary manner) were developed as brake-pads. These were characterized for physical, chemical and mechanical properties. These brake pads were evaluated for their friction sensitivity towards load and speed in simulated braking conditions under variable operating parameters such as speed and pressure on a reduced scale prototype (RSP). Composite with 8% (by wt.) brass fibers proved to exhibit best combination of performance parameters related to friction and wear in this testing mode. Scanning electron microscopy (SEM) were employed to understand wear mechanisms. 1. Introduction Non-asbestos organic fiber reinforced-low metallic composites are increasingly being used in automotive brake disc pads, shoes, linings, blocks, clutch facings etc primarily because of awareness of health hazardness of asbestos. They are essentially multi ingredient systems in order to achieve the desired amalgam of performance properties [1, 2] and more than several hundred ingredients have been reported being used for friction composites in the literature. These are categorized as binder, fibers, friction modifiers and fillers based on the major function they perform. Binder is a resin mainly phenolic based, whose function is to hold all the ingredients very firmly so that they can contribute towards their major functions. While fibers such as mineral, ceramics, organic and metallic types provide mainly strength, wear resistance and raise the friction level, in general. Friction modifiers such as abrasives and solid lubricants are used to achieve the desired range of friction with minimal fluctuations. The class of fillers is again subdivided as functional fillers (to enhance the specific function such as resistance to fade, thermal conductivity etc) and space/inert fillers (mainly to cut the cost). A lot is reported on the influence of these ingredients in friction composites on tribo -performance in various testing situations such as fade and recovery, load, speed etc [3-7].
Metallic fillers contents are important in friction materials since they control the conductivity of composites apart from additional functions such as wear resistance; strength etc. Comparatively little is reported on metallic contents in non-asbestos friction materials in this context [8-10]. Thermo-physical properties such as specific heat, thermal conductivity (TC), diffusivity, thermal expansion etc. play vital roles in performance of brake pads, especially when braking is severe. Low TC of composite renders the tribo-surface vulnerable due to accumulation of frictional heat which leads to degradation of organic ingredients which in turn, affects the braking capability adversely. Too high TC on the other hand, results in adverse effect on brake-fluid leading to “spongy brakes” which is an unwanted phenomenon. The optimum TC of a composite hence is also very much essential feature. In spite of this fact, not much is reported in the literature on this aspect. Handa and Kato [8] studied the influence of variation of 3 ingredients (Cu powder, CNSL powder and BaSO4) in quaternary compositions. The studies were targeted to examine the influence of their variation on friction and wear properties on a reduced scale tribometer. The composites were of academic interest and testing conditions were also not realistic. No efforts were made to correlate the performance with
188 Recent Advances in Innovative Materials increase in thermal conductivity or changes in interface temperature as a result of inclusion of more Cu powder. Jang et. al [9] have observed the effect of metallic fibers such as low carbon steel, Al and Cu on performance of NAO friction composites using a small-scale friction tester. The studies were again focused on investigating the influence of addition of these fibers individually in composites under various operating conditions. Overall benefits or limitations of each fiber were discussed. The issues of optimal amount of fibers or thermal conductivity or effect on interface temperature, however, were not touched. In the present work four NAO composites were developed by varying brass fibers (0, 4, 8 and 12 %) and barite (inert filler) in a complementary manner keeping other ingredients unaltered. These composites were evaluated for their friction sensitivity towards load and speed in simulated braking conditions against a commercial disc under variable operating conditions such as speed and pressure on a reduced scale prototype (RSP). SEM studies were done to understand wear mechanisms. 2. Experimental and Results 2. 1. Fabrication of the composites The fabrication of composites containing 12 ingredients was based on keeping parent composition of 10 ingredients (65 % by wt) constant and varying two ingredients viz brass fiber and barite in a complementary manner as shown in Table 1 based on systematic increase in brass fibers ( 0, 4, 8 and 12 wt% ) and proportionate decrease in barite contents. The parent composition contained straight phenolic resin (10 %), functional fillers such as alumina, graphite, vermiculite, cashew dust (35 %) and fibers such as glass, PAN, Lapinus, Aramid and steel wool. These composites were designated as B0, B1, B2, and B3 accordingly (Table 1).
cured in a compression-molding machine under a pressure of 8 MPa for 7-8 minutes at a curing temperature of 150oC. Three intermittent ‘breathings’ were also allowed during the initiation of curing to expel volatiles. The pads were then removed and were post-cured in an oven at 100° C for 8 hours. The surfaces of the pads were then polished with a grinding wheel to attain the desired thickness and surface finish. 2. 2. Characterization of the composites For mechanical strength testing, test bars were fabricated as per ASTM standards. Composites were characterized for physical (density, water swelling, heat swelling and void contents) and chemical (acetone execration) properties. Details of measurement procedure of these properties are discussed elsewhere [11]. Void contents were calculated by measuring actual density and theoretical density. Hardness was measured by a Rockwell hardness tester on R scale. Thermophysical properties were measured as per ASTME1461-01 standard on FL-3000 Flash line instrument supplied by Anter Corporation, USA. Samples of square size (10 x 10 mm) and thickness 2-2.5 mm were used for these measurements at room temperature. 2. 3. Test set-up and procedure for studying sensitivity of friction to load and speed The friction and wear tests were done on a horizontal loading assisted pad-on-disc type reduce scale prototype (RSP) shown in Fig.1. The RSP essentially consists of a pearlitic grey cast iron rotor disc of the passenger car. The disc was connected disc was connected through an interchangeable flange to the flywheel of mass of 100 kg (moment of inertia 6 kg m2) on the shaft.
Table 1. Details of the formulated composites based on the variation in amount of brass fibers and barite Ingredients Composites designation Space filler- BaSO4 Metallic filler- Brass Total amount of two
composition by wt. B0 B1 B2 B3 35 31 27 23 0 4 8 12 35 35 35 35
The total metallic contents (steel and brass) in B0, B1, B2 and B3 thus were; 8, 12, 16 and 20 %. The ingredients were mixed in a plough type shear mixer to ensure the macroscopic homogeneity. The mixing schedule was of ten minutes duration. The mixture was then placed into a four-cavity mould supported by the adhesive-coated back plates. Each cavity was filled with approximately 80g of the mixture and heat
Fig. 1. Schematic of reduce scale proto-type(RSP) The flywheel was connected to a 7.5 HP AC motor via a cross coupling. The disc rpm was controlled by controlling the AC motor input into the drive, which was pre-set through the control panel capable of imparting a speed up to 1400 rpm to the disc. A pair of samples of square size (24 x 24 mm2) was cut from the brake pads along with the back
Influence of Brass as a Filler on Load-Speed Sensitivity of Polymer Based Friction Composites 189 plates and push fitted in the sample holders connected to a pressure actuator. The specimens were placed in diametrically opposite locations on the same side of the disc. The load on the pads could be manipulated by controlling the applied pressure on the pads ranging between 1 and 6 MPa in the steps of 0.1 MPa. The load cell attached to the frame carrying the specimen holder measured the frictional force. The operating variables conforming to a particular experimental design such as applied pressure, rubbing speed, number of braking cycles and braking duration could be pre-set in the programmer on the controller. The specimens were ground to a thickness of approximately 1.5 cm (including back plate). The uniform contact of the friction surface was assured through a few cycles of initial bedding-in operation under a nominal pressure of 1 MPa and at a linear speed of 5.03 m/s on the disc. This was done to ensure more than 80% of conformal contact. The samples were cleaned to remove the loose wear debris followed by the initial weighing of the sample after cooling. The specimens were then subjected to braking cycles on the RSP under a series of braking pressure and speed as described in Table 4. Braking duration (touch time) and number of braking in each cycle were constant as 1 sec and 25 respectively. Table 2. Experimental design for tribo-evaluation Operating variables Pressure (MPa) Linear Speed
Experimental design 2, 3 and 4 10.0/36, 12.5/45 and 15.0/5
Initial temperature of the disc was 250 C. The temperature of specimen and disc was measured with laser gun (Quick temp 860-T3, Testo Co. Germany make) after each experiment to have a rough idea about trends in increase in temperature after completion of braking cycles and are shown in Table 5. The frictional force was recorded continuously on the PC and the built in software calculated the average value. In the final display 25 values of µ were recorded as a function of 25 cycles. The µ stabilized after first 10 cycles in general. Hence the mean µ of last 15 cycles was calculated and considered as representative value (µm) for that experiment. The variations in µm at various speeds and pressures is shown in Table 6. The sensitivity of µm to speed was analyzed by calculating the extent of decrease in µm with respect to transitions in speed from one level to the successive level. These were designated as ∆µmv1–2 and ∆µmv2–3, which were the decrements in µm with the increase in speed from 10 m/s (v1) to 12.5m/s (v2) and 12.5m/s (v2) to 15.0m/s represented in Fig. 2a.
(1)
∆µmv1–2
(2) ∆µmv2–3
Fig. 2a. Speed sensitivity -Variation in ∆µmv of the composites corresponding to the sliding speed transitions (1) from v1 to v2 (10.0 to 12.5 m/s) and (2) from v2 to v3 (12.5 to 15.0 m/s). Similarly sensitivity of µm to pressure was analyzed by calculating the extent of decrease in µm with respect to transitions in pressure from one level to the successive level. These were designated as ∆µmp1–2 and ∆µmp2–3, which were the decrements in µm with the increase in pressure from 2MPa (p1) to 3MPa (p2) and 3MPa (p2) to 4MPa (p3) m/s respectively at a constant speed and the data are represented in Fig. 2b.
(1) ∆µmp1–2
(2) ∆µmp2–3
Fig. 2b. Pressure sensitivity- Variation in ∆µmp of the composites corresponding to the pressure transitions (1) from P1 to P2 (2 to 3 MPa) and (2) from P2 to P3 (3 to 4 MPa).
190 Recent Advances in Innovative Materials The data on wear volume as a function of PV value (pressure and velocity) are are shown in Fig.3. Wear was calculated after each experiment of 25 cycles and plotted individually. Studies on worn surfaces by SEM are shown in Fig.4. 3. Discussion on Results Physical and mechanical properties As seen from the Table 2, it was observed that the density of composites showed increasing trend because of addition of brass fibers which is heavier than the filler Barite. Acetone exaction indicates amount of uncured resin in the composites which was negligible in all the composites except B0 where it was highest (1.1%). Table 2. Physical and mechanical properties of the selected composites Properties Density (g/cc) Acetone extraction (%) Water absorption (%) Void content (%) Heat swelling (%) Tensile strength (MPa) Tensile Modulus (GPa) Flexural strength (MPa) Flexural Modulus (GPa) Rockwell Hardness (R-
B0 2.27 1.18 0.73 1.98 1.47 11.6 0.42 37.3 7.35 118
B1 2.29 0.40 0.32 2.08 1.78 10.9 0.35 29.6 6.31 116
B2 2.33 0.35 0.56 3.03 2.23 10.0 0.39 28.9 5.97 116
B3 2.38 0.60 0.70 3.52 2.52 8.2 0.33 27.2 5.77 114
92 730 1.53
100 737 1.69
105 833 2.04
108 855 2.20
scale)
Diffusivity (cm2/sec) x 4 Sp. Heat (J/kg K) Conductivity (W/m K)
Void contents increased slowly with increase in brass fibers because of larger size of brass fiber as compared to barite powdery particles. The heat swelling also showed the same trend as expected because of higher expansion of brass fibers. Mechanical properties, (tensile and flexural) decreased with increase in contents of brass basically because of increase in filler content of bigger size with simultaneous decrease in a filler content of smaller size. Similarly thermal conductivity, diffusivity and specific heat increased with increasing amount of brass fibers. Similar trends were observed in our earlier investigations on similar series of composites with increasing steel wool [11]. Inclusion of metal fibers with reduction of equivalent amount of powdery filler (barite in both the cases) reduced the strength, modulus and hardness while increased the density, void contents, heat swelling and thermo-physical properties. The water absorption and amount of uncured resin, however, did not show
Fig. 3- Histogram showing the variation in wear of the composites at various PV (pressure & velocity) values. regular trends in both the series of steel wool and brass fibers. 3. 1. Load- speed sensitivity studies on Reduced Scale Prototype (RSP) Tribo-performance As seen from Table 5, it was observed that pad temperature was higher than the disc temperature. Moreover, with increase in speed and pressure temperature of the surfaces of discs and pads increased as expected. It was, however, interesting to note that as brass contents in composites increased, the temperature on surfaces of discs and pads decreased slowly, in general with few exceptions which are underlined in Table 5. In case of pad surface exceptions are very few. Thus the basic aim of the research work to minimize the temperatures at the surface of pad and disc with the help of metal contents was fulfilled. However, it was also observed that the interface temperature is not the only criteria to decide the overall performance of the composites. Had it been true, B3 should have shown the best performance in wear mode also. This confirms that the metal contents which are added to control the interface temperature interfered with other performance properties also making the formulation-design task more difficult. Hence selection of ingredients in formulation in optimized contents becomes more imperative. As seen in Table 6, it was observed that with increase in pressure μ of composites decreased In case of speed increase, no fixed trends were observed because speed is known to have more complex influence on μ rather than the pressure. Some general trends emerged from this table as follows. • Friction behavior at lowest speed (10m/s) –B2 and B1 showed higher μ and B3 showed lowest..
Influence of Brass as a Filler on Load-Speed Sensitivity of Polymer Based Friction Composites 191 • Friction behavior at moderate speed (12.5m/s)–B3 started showing higher μ especially at higher pressures, followed by B2. B0 showed the lowest μ. • Friction behavior at highest speed (12.5m/s)- B3 and B2 showed highest μ in general . B0 showed the lowest μ. These trends indicate that inclusion of metal contents and hence TC of pads increased the μ of composites in general. Increase in TC reduced the interface temperature of the tribo-couple and hence the fading tendency in μ. Composite B0 which had no brass fibers hence showed very low μ as operating conditions became more severe (high P-V conditions). Composites B2 and B3 thus proved more efficient in protecting μ from lowering down under severe operating conditions.
Fig 2 indicates the sensitivity of μ towards operating conditions. For ideal materials such curves should be as straight, parallel and very near to x-axis confirming least sensitivity towards operating parameters. As seen from Fig 2a1, for lower to moderate speed transition, performance in general followed the order; B1 =B0 > B3 > B2 For moderate to high speed transition (Fig 2a2), B3 >B2 > B0> B1 For sensitivity to pressure as seen from Fig 2b1, performance order in general was; B3 =B0 > B2 > B1 For moderate to high speed transition (Fig 2b2), performance order was; B3 >B1 > B2 > B0
Table 5. Increase in the temperature of surfaces of discs and composites tested on RSP under various conditions of loads and speeds Parameters Disc Temperature (0C) /samples Speed-10 m/s (rpm800) Speed-12.5 m/s (rpm1000) Speed-15 m/s (rpm 1200) B0 B1 B2 B3
2MPa 56 55 53 48
3MPa 64 65 62 57
4MPa 67 76 63 67
B0 B1 B2 B3
108 100 65 80
122 114 112 103
114 144 123 118
2MPa 3MPa 4MPa 63 83 83 71 84 104 65 89 95 66 83 91 0 Pad Temperature ( C) 113 152 191 138 168 203 114 180 190 116 174 183
2MPa 75 83 82 74
3MPa 107 106 110 91
4MPa 108 115 110 103
138 165 158 139
179 210 206 201
239 251 243 234
Table 6. Stabilized coefficient of friction (μ) for the selected composites. Parameters/ Samples B0 B1 B2 B3
Speed-10 m/s (rpm800) 2MPa 3MPa 4MPa 0.401 0.388 0.372 0.431 0.394 0.381 0.410 0.397 0.382 0.396 0.385 0.376
Speed-12.5 m/s (rpm1000) 2MPa 3MPa 0.410 0.377 0.449 0.382 0.440 0.396 0.417 0.396
Thus, overall composite B3 appears to be the best from least sensitivity of friction coefficient to load – speed variation. B2 was moderate performer in this respect. Figure 3 shows wear of composites at different PV values. In general it was observed that B0 and B3 showed higher wear than other two composites. B2 appeared to be most wear resistant in most of the cases. SEM studies Surfaces of specimens worn under highest PV condition (60 MPa-m/s) were selected for SEM studies. The micrographs are arranged in the order of their increase wear performance (B2> B1 > B3 > B0).
Speed-15 m/s (rpm 1200) 4MPa 2MPa 3MPa 4M 0.357 0.409 0.334 0.31 0.379 0.432 0.359 0.34 0.377 0.450 0.365 0.35 0.381 0.423 0.362 0.35
As seen from micrograph (4B0a-X 500), the surface of B0 (which showed lowest wear performer and highest wear) was fully covered with secondary plateaus. The micrographs 4B3a and 4B3b (BS) (both X 500) indicated the dominance of secondary plateaus which support higher wear of B3. Micrograph 4B3c (BS) (X 500) at different location confirms the existence of various ingredients and few primary plateaus. Micrographs 4B1a and 4B1b (BS) (both X 500) confirms more amount of primary plateaus supporting its good wear resistance. Surface of B2, which showed lowest wear, however, was distinctly different from other surfaces as seen in micrographs 4B2a and 4B2c. SEM and BS images
192 Recent Advances in Innovative Materials (4B2a and 4B2b –X 100) indicate wavy nature of the surface and micro-voids. The primary plateaus (left corner) are also seen. The secondary plateaus are small in size and uniformly distributed. 4. Conclusions Based on studies conducted on composites with increasing brass contents under various operating conditions in P-V sensitivity mode, following conclusions were drawn • Increase in brass contents led to deterioration in strength and modulus, hardness etc and increase in density, void contents, heat swelling, water
• absorption, specific heat, thermal diffusivity and conductivity (TC). • Higher TC lead to lower interface temperature. μ decreased with load. However, no fixed trends were observed for speed. • Composite with 8 % (B2) proved to have best combination of performance properties. • It was thus finally concluded that enhancement in TC of a composite does not necessarily improve its fade behavior and overall tribo-performance. Tribo-performance, in general, improves up to a certain level of brass contents (in this case up to 8%) and then drops down. Various mechanisms were found to be responsible for these observations.
4B0a
4B3a
4B3b
4B3c
4B1a
4B1b
Influence of Brass as a Filler on Load-Speed Sensitivity of Polymer Based Friction Composites 193
4B2a
4B2b
Fig. 4. SEM studies on the surfaces of composites worn under highest PV condition (60 MPa.m/s) in RSP testing References [1] J. Bijwe, “Composites as friction materials: Recent developments in non-asbestos fiber reinforced friction materials-A review”, Polym. Compos., 18, 3 (1997), 378-396. [2] D. Chan and G. W. Stachowiak, “Review of automotive brake friction materials”, Proc. Instn. Mech. Engrs. Part D: Jr. of Automobile Engineering, 218 (2004), 953-966. [3] F. Dong, F. D. Blum and L. R. Dharani, "Lapinus fiber reinforced phenolic composites: flexural and frictional properties," Polym. & Polym. Compos., 4 (1996) 155-161. [4] B. K. Sathapathy, “Performance Evaluation of Non-Asbestos Fiber Reinforced Organic Friction Materials “Ph .D. Thesis, Indian Institute of Technology Delhi, (2003). [5] L. Gudmand-Hoyer, A. Bach, G.T. Nielsen and P. Morgen, “Tribological properties of automotive disc brakes with solid lubricants”, Wear, 232 (1999) 168-175. [6] P. Gopal, L. R. Dharani and F. D. Blum, "Load, speed and temperature sensitivities of a carbon fiber reinforced phenolic friction material," Wear 181-183 (1995) 913-921.
[7] S.J. Kim, M.H. Cho, D.S. Lim, and H. Jang, “Synergistic effect of aramid pulp and potassium titanate whiskers in the automotive friction materials”, Wear, 251 (2001) 1484-1491. [8] Y. Handa and T. Kato, “Effects of Cu powder BaSO4 and cashew dust on the wear and friction characteristics of automotive brake pads”, Tribol. Trans., 39, 2 (1996) 346-353. [9] H. Jang, K. Koa, S. J. Kim, R.H. Basch, and J.W. Fash, “The effect of metal fibers on the friction performance of automotive brake friction materials”, Wear 256 (2004) 406-414. [10] X. Qu, L. Zhang, H. Hing, and G. Liu, “The effect of steel fiber orientation on Frictional properties of asbestos-free friction materials”, Polymer Comp., 25, 1 (2004), 94-101. [11] J. Bijwe, M. Kumar, “Optimization of Steel Wool Contents in Non-Asbestos Organic (NAO) Friction Composites for Best Combination of Thermal Conductivity and Tribo-performance”, Wear 263 (2007) 1243-1248
