CHAPTER ROLE OF ORGANIC FIBERS IN FADE

Transcription

1 145 CHAPTER 7 INFLUENCE OF ARAMID, CELLULOSE AND ACRYLIC FIBERS IN A NAO DISC BRAKE PAD FORMULATION- EFFECT ON THERMAL STABILITY AND FRICTIONAL CHARACTERISTICS 7.1 ROLE OF ORGANIC FIBERS IN FADE Generally, commercial friction materials contain 5-25 wt by percentage of fibrous ingredients and the types and the relative amounts of the fibers affect many characteristics of brake accomplishment and wear life. A great effort was established to the effect of asbestos replacement on friction performance (Gopal 1996). The aramid pulp, particularly, has drawn much attention since the aramid pulp shows good filler retention (Briscoe 1988; Giltrow 1970; Gopal 1996; Anderson 1987). Also, it has a significant contribution in improving the operational life expectancy of the brakes, imparting low squeal, smooth friction performance, toughness and friction stability. In group of ceramic fibers, conversely, potassium titanate whiskers have been an ingredient of immense attention due to its thermal stability and good compatibility with binder resins. The potassium titanate whiskers offer thermal stability at higher temperatures due to their elevated melting point (Milevski 1987). The normal operating temperature of the rotor usually can reach up to o C for passenger cars and high temperatures up to C are registered in the front wheel disc pads. At such interfacial temperatures the friction coefficient (µ) suffers from a loss in effectiveness termed fade. Role of composition of friction material in the form of increased

2 146 thermal stability and oxidation resistance of the binder resin is of immense importance in controlling fade. Fibers play an important role in mitigating the drawbacks of resin or any other inbuilt compositional flaws resulting from inappropriate choice of ingredients in the formulation (Anderson 1992). Brass and copper powder in heavy-duty organic linings are reported to improve the fade resistance while aluminium and zinc powder are used for imparting better recovery characteristics. The other organic fibers namely acrylic and cellulose fibers are chosen because they present well as a processing aid supplying excellent green strength for a pre-form in spite of their poor resistance to heat. Hence, in the present chapter, three NAO composites are developed by varying organic fibers (Aramid, Cellulose, and Acrylic) which is compensated by barytes (inert filler) in a compensatory manner. Since, influence of thermal stability is best reflected in severe operating conditions, fade and recovery mode of performance testing is selected in the present work. 7.2 FIBRES CHARACTERIZATION Aramid fiber has a high mechanical strength, because of its hydrogen bonding molecular structure. It has anisotropic possessions due to the crystal structure with a rigid covalent bonding in the fiber direction and a weak hydrogen bonding in the transverse direction (Chawla 1998). Moreover they are relatively soft, light and exhibit excellent thermal stability till C with very good stiffness-to-weight ratio, superior wear resistance, friction stability and anti-fade properties. It forms molten viscous glassy layer/film on the area of contact during sliding under the severe conditions promoting a better adherence to the rotor disc. The interfacial friction film rheology tends to be more shear thinning under the severe pressure speed conditions that facilitates easier formation of sticky contact patches hence the adhesive

3 147 contribution to the overall coefficient to friction is increased resulting into an increase in tribological performance Acrylic fiber is light, has a high flexibility and high resistance against chemicals, though it is not strong enough under heat. Cellulose fiber is a natural organic-based polymer having many functional chemical groups which more likely enables it to interact with the binder efficiently. Cellulose fibre inclusion reduces squeal and imparts resiliency to the friction material apart from improving the pre-form strength. Moreover, it offers a high coefficient of friction and good recovery behavior in spite of its weak thermal stability. Table 7.1 Properties of acrylic, aramid and cellulose fibers Ingredient Typical Length (mm) Modulus (GPa) Test item Strength (MPa) Moisture Regain (%) Acrylic fiber Aramid fiber Cellulose fiber (a)

4 148 (b) (c) Figure 7.1 SEM image of (a) Aramid fiber, (b) Acrylic fiber, (c) Cellulose fiber 7.3 THERMO GRAVIMETRIC ANALYSIS OF THE THREE FIBERS The thermo gravimetric (TG) analysis is an important thermal analysis that shows the thermal stability of the materials. TG/DTG analysis of the fibers selected are carried out in a DTG 60 Schimadzu, Japan make using alumina pans (6.04 mg), under Zero air atmosphere ( 50 ml/min) at a heating rate of 10 0 C/min. Figure 7.2 a,b,c shows the TGA results of Aramid, Cellulose and Acrylic fibers in Zero air atmosphere.

5 149 (a) (b) (c) Figure 7.2 TGA curves for (a) Acrylic fibers, (b) Aramid fibers (c) Cellulose fibers

6 150 Acrylic fiber shows two stages of weight loss. In the first stage, the material shows a maximum weight loss of 40% from C up to C. In the second stage, C, it completely decomposes leaving no residual mass and this can be attributed to thermo-oxidative reactions. Hence, Acrylic fiber starts to depolymerize at around C. In the case of Aramid fiber, the chemical reaction occurs abruptly compared to the gradual reaction in the case of acrylic fiber. Aramid fiber begins to be depolymerized at around C. The cellulose fiber commences degrading at around C itself showing its poor thermal stability. These results present the characteristic performance of the fibers according to heat. 7.4 FABRICATION OF THE BRAKEPADS The fabrication of the composites is carried out in keeping fixed parent dry mix formulation containing binder resin (10%), metal content (9%), space fillers (30.7% ), friction modifiers (18.5% ) and abrasives ( 2%) amounting to 70.2% by weight. The rest 29.8 % is adjusted by varying the organic fibers namely Aramid (Kevlar Du point 979 IF 538), Cellulose (Arbocel-ZZZ8-1R), Acrylic fibers (CPF207 Sterling Fibers Co.) which is compensated by adding the inert filler namely the Barytes. The combination of organic fibers should be kept around 10 to 12% by weight. The Kevlar has better thermal stability with higher cost. The remaining two fibers are very much essential for getting the good preform strength. Hence, in spite of their poor resistance to heat, they are added in smaller amounts considering the manufacturing aspect. The detail of the fiber combinations is given in the Table 7.2.

7 151 Table 7.2 Varying ingredients of NAO brake pad S.no Raw Material Name NA01 by Wt% NA02 by Wt% NA03 by Wt% 1 Kevlar Acrylic Fiber Cellulose Barytes Fine MOLDING IN HYDRAULIC PRESS The ingredients are mixed in a plough type shear mixer. The brake friction composites in the form of Pads are molded in hydraulic Press (Glasnost Hydraulics) of 150 ton capacity. Mix weight of 1.5 kg is taken and put in a compaction die. The top and bottom temperature of the die is maintained between 150 o C and 160 o C. A pressure of 160 to 170 Kg/cm 2 is applied. The press curing cycle is maintained for 8 minutes. Five breathing cycles are followed by final curing. The breathing cycles help to remove entrapped gases evolved during cross linking reaction of the resin. The molded composites are post baked at different temperatures for different periods as shown in the table 7.3. Table 7.3 Baking schedule Ambient to 140 o C Between 140 o C and 145 o C Raise 150 o C Between 150 o C and 155 o C Total 120 minutes 90 minutes 90 minutes 90 minutes 05 hours

8 152 The surfaces of the pads are then polished with the grinding wheel to attain the desired thickness and remove the resinous skin. Visual appearance characteristics like gapping (between material and plate), splits, material flash, un-ground material surface and surface blisters are checked. 7.5 CHARACTERIZATION OF THE BRAKEPADS Basic properties of the specimens are measured following Indian standard IS 2742: Composites are characterized for physical (specific gravity, heat swelling, water swelling, Loss on ignition) and chemical properties (acetone extraction). All the results are shown in the table 4. The hardness is measured using the Rockwell hardness machine with S scale. The cold shear strength between the pad and the back plate is measured by using the Universal Testing machine by applying the shear force with speed of 10mm/min. Thermo gravimetric analysis (TGA) is performed to examine the thermal decomposition of the friction material using a thermal analyzer (Shimadzu DTG-60) in an atmospheric condition. The temperature raise during normal passenger car application will be between o C and this temperature zone is very critical to determine the thermal stability of the product considering the brake application. Hence, the temperature range is selected between 150 and 400 o C at a heating rate of 10 0 C/min. Friction and wear tests are conducted on a chase type friction testing machine from Link Engineering, USA, which is in compliance with Society of Automotive Engineer SAE J661 test procedures. All the tests mentioned above are conducted at Technology Center, Hindustan Composites Limited, Aurangabad, India.

9 DISCUSSION ON PHYSICAL AND MECHANICAL PROPERTIES The physical and mechanical properties of all friction materials are listed in Table 7.4. The specific gravity and hardness depend on the ingredients and weight percentage used as well as the manufacturing process parameters. As seen from the Table 7.4, higher the organic fiber content, the lower is the specific gravity and hardness. The lesser value of the heat swell and the Loss on Ignition indicates the resistance to thermal degradation by the respective fibers and the supporting ingredients in the formulation. These results present the characteristics of the acrylic fiber with carbon-nitrogen triple bonding and Para-aramid with hydrogen bonding with respect to heat. Here, NA03 which has the high organic fiber content has good thermal stability. Table 7.4 Physical and mechanical properties of the various compositions Properties Unit NA-01 NA-02 NA-03 Specific Gravity Hardness HR S Acetone Extract % Heat Swell In mm Water swell In mm Nil Loss on Ignition at C % Cold Shear Test MPa Hot Shear C / 45 Minutes Weight loss in range of C MPa %

10 THERMAL ANALYSIS OF THE DEVELOPED BRAKE PADS Figure 7.3 TGA for composite NA01 Figure 7.4 TGA for composite NA02

11 155 Figure 7.5 TGA for composite NA03 TGA test is normally used for testing of Thermal Stability of raw and finished materials. Figures show thermo gravimetric analysis ( TGA) results of the three friction composites which reveal the weight loss % when exposed to C.The weight loss% of NA01 during C is For NA02, it is and for NA03, the loss is Hence, NA03 has better thermal stability than the other two composites. It is observed that higher the organic fiber content, higher is the thermal stability. Now, it becomes interesting to check whether the thermal stability of the friction material paves way for higher frictional stability by carrying out the frictional test. 7.8 FADE AND RECOVERY CHARACTERISTICS OF THE DEVELOPED BRAKE PADS Results of fade and recovery behavior of composites NA-01, NA- 02 and NA-03 are presented in the form of the plots in the graph.in this investigation, the fade behavior is evaluated by the variation of COF with temperatures during the fade and the recovery stage of friction test.

12 156 Figure 7.6 Fade and recovery plot of NA01 Figure 7.7 Fade and recovery plot of NA02

13 157 Figure 7.8 Fade and recovery plot of NA03 Figure ( ) illustrates the variation of COF with temperature in the entire cycle that is the baseline, Fade and Recovery. The initial instability of the friction coefficient in all the plots in the initial baseline is not unusual and normally disappears after a certain number of brake applications in the real braking situation. The increase in coefficient of friction at the beginning is due to the changes of the real contact area at the sliding interface. Due to the topography of all composites, the area of real contact is confined within the contact plateaus. The increase in friction with increasing temperature is primarily correlated to the formation of primary plateaus. Broken fibers and other hard inclusions which are embedded on the surface of the composites represent the primary plateaus while smooth protruding patches on the surface represent the secondary plateaus. The primary plateaus are the lower removal rate of the mechanically stable and wear resistant ingredients of the pad. When the rough surface of the composite is worn, the primary plateaus form, thereby increasing the possible area of real contact between the composite and the counter face.

14 158 In the case of NA 01, beyond C ( F), there is a steep fall in the µ value from 0.42 to The trend of friction coefficient goes from decreasing slightly at above C (250 0 F ) and continues the same trend until C(534 0 F) during the first fade. During that time, as the temperature of the friction surface goes up to C(534 0 F), the resin starts to depolymerize judging from that the beginning of the pyrolysis temperature in resin is around C(482 0 F). The recovery is quicker in almost all the composites. The second fade behavior at elevated temperatures is attributed to the thermal decomposition of ingredients, mostly the organic fibers and rubber components and is followed by the subsequent destruction of contact areas at the sliding interface (Kim 2000 ) Less content of the fibers causes few fibers to appear on the surface there by reducing the true contact area. From fig 7.6, the µ value of NA-01 during second fade test goes as low as 0.21 not even near the mediocre performance, which is attributed due to the poor thermal stability caused due to higher amount of weight loss as found from TGA studies due to the lesser amount of the organic fibers in the formulation. In the case of NA 02, a slightly better behavior is observed in both the fade and recovery mode. Since no abrupt changes in the Coefficient of friction of NA03 composites occurred up to C(650 0 F), (Figure 7.8), it can be concluded that high temperature fade resistances are good. More content of these fibers in the brake composite NA03 causes more fibers to appear on the friction surface. This made the true contact area between the fibers and the brake disc increase, so a higher friction coefficient is achieved. Also, TGA studies revealed that weight loss is less (9%) only in NA03 composite which is having the highest percentage of organic fibers, which indicates that less weight loss during the elevated temperatures causes more thermal stability. This thermal stability paved way for the frictional stability in NA 03.

15 FRICTIONAL RESPONSE OF THE BRAKE PADS Table 7.5 SAE J661 (or) equivalent Indian IS 2742 frictional response of the friction composites Performance Attributes NA-01 NA-02 NA-03 Hot friction or Fade µ Normal or Recovery µ First Fade by Calculation (%) First recovery by Calculation (%) Second Fade by Calculation ( % ) Second Recovery by Calculation (%) Wear loss by wt % WEAR OF THE BRAKE PADS EVALUATED UNDER FADE- RECOVERY CONDITIONS Wear (loss by weight %): NA-01>NA-02>NA-03. Figure 7.9 Wear loss by wt% of the various compositions

16 160 Generally, the harder samples are supposed to have a lower average thickness loss (lower wear). But in this investigation, it is found that this postulation does not apply to the friction materials. Thus, it could be concluded that there is no direct correlation between hardness with average thickness wear loss. It also has been reported that hardness of brake materials cannot be simply related to the content of structural constituents, and there is no correlation between hardness and wear resistance (Filip 2003). Wear in general, relies on several aspects like temperature, applied load, properties of the mating materials and durability of the transfer layer. In the friction materials, the resin, organic fibers, rubber components and other organic friction modifiers play decisive roles in the wear of the friction materials (Shin 2010). The reason is due to the high temperature that is generated above the transformation temperatures of these organic ingredients. Figure 7.10 Schematic representation of plateau formation

17 161 (a) (b) (c) Figure 7.11 SEM micrographs of the worn surfaces of the composites (a) NA 01 (b) NA 02 and (c) NA 03

18 162 Microscopic observation of worn surface morphology suggested that presence of higher amount of fiber contents in the friction material NA03 play a crucial role in producing major friction films on the rubbing surface. On the other hands, the friction material with lesser amount of fibers in NA01 and NA 02 show smooth worn surface topography without exhibiting locally primary contact plateau. The selected organic fibres and its amount played an important role in justifying the drawbacks of resin and other organic components which is reflected in lower wear percentage of NA 03. The Figures (7.11a 7.11c) show a clear result indicating less wear loss for the friction material with more amounts of fibers SUMMARY Based on the studies conducted on composites with an increase in fiber contents under various operating conditions and two testing modes, namely Fade and Recovery & TGA-DTG, the following conclusions are drawn: Increase in organic fiber content reduced the hardness and increased the specific gravity Increase in fiber content lead to increase in µ value as found in the composite NA03 when evaluated under SAE J661 schedule conducive to precipitate the fade and recovery behavior of the materials In case of TGA and DTG studies, the % weight loss in NA03 is less compared to other two compositions which indicate higher thermal stability

19 163 Hence, it is concluded that composite NA03 proves to be more efficient than the other two versions for operating in severe conditions leading to fading. The wear rate is also lower than the other two composites.