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Effects of glass-to-rubber transition of thermosetting resin matrix on the friction and wear properties of friction materials
Tribology International 54 (2012) 51–57
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Tribology International journal homepage: www.elsevier.com/locate/triboint
Effects of glass-to-rubber transition of thermosetting resin matrix on the friction and wear properties of friction materials Yaoqing Wu a, Ming Zeng a,b,n, Qingyu Xu c,d, Shuen Hou a,nn, Hongyun Jin a, Liren Fan a a
Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, PR China State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, PR China c Hubei Research Institute of Chemistry, Wuhan 430074, PR China d Haiso Technology Co. Ltd, Wuhan 430074, PR China b
a r t i c l e i n f o
abstract
Article history: Received 20 January 2012 Received in revised form 6 May 2012 Accepted 22 May 2012 Available online 31 May 2012
The present study is to examine the effects of glass-to-rubber transition of resin matrix on the friction and wear characteristics of friction materials, in relation to different types of thermosetting resins. Dynamic mechanical thermal analysis and friction test results revealed that glass-to-rubber transition of thermosetting resins influenced significantly the friction and wear behavior of the composite materials. There was a significant increasing tendency in friction coefficient and wear rate values for all composites when braking temperatures increased to 200 or 250 1C, accompanying the resin matrix converted from glassy state to rubbery state. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Benzoxazine Glass transition Friction Thermosetting resin
1. Introduction Automotive brake linings are one of the most important safety components in automobiles [1–5]. The commercial brake lining usually contains more than 10, up to 20 or 25 different constituents. The ingredients are categorized into four classes: binders, fillers, friction modifiers, and reinforcements [6,7]. Among the ingredients in the friction material, polymer binders (particularly thermosetting resin) play a crucial role in determining the friction characteristics during braking. Phenolic resin (PF) based automotive friction material has been extensively investigated [8–11]. However, the phenolic resin is brittle and is not resistant to high temperature, which often results in wear loss and fade under 350 1C for the friction materials [12,13]. Polybenzoxazine (BZ) is an addition polymerized phenolic system, exhibiting versatility in a wide range of applications, and having a wide range of interesting features and capability to overcome several shortcomings of conventional novolac and resole type phenolic resins. They exhibit near zero volumetric change upon curing, low water absorption, high char yield, and also possess thermal and flame retarding properties of phenolics n
Corresponding author at: Tel.: þ 86 15623351996. Corresponding author. Tel: þ 86 18971384510. E-mail addresses: [email protected], [email protected] (M. Zeng), [email protected] (S. Hou). nn
0301-679X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.triboint.2012.05.018
along with the mechanical performance [14–17]. Though benzoxazine based materials possess several advantages, they have not yet been widely used in the friction materials industries in case of the process problems, for instance, high curing temperature. The modified benzoxazine resins have been prepared by mixing benzoxazie with PF, epoxy, and nitrile butadiene rubber (NB) to manipulate the mechanical and thermal properties, and to lower the curing temperature [18,19]. Glass-to-rubber transition of polymer often leads to substantial change in stiffness, strength and other properties of polymeric materials. However, very little research has been focused on the effect of thermal transition behavior of resin matrix on the resultant wear performance of composite friction materials. The thermal effects as well as fade and wear characteristics of rubberbased friction materials with and without fiber reinforcements were examined recently [20]. The experimental data revealed that rubber-to-glass transition of rubber matrix influenced significantly the fade behavior and wear rate of the friction materials. In fact, the relationship between thermal transition behavior of thermosetting resins and tribological properties of friction materials should attract much attention, because thermosetting resins were mostly utilized as binder matrix in the field of friction composites. The present study is a generic study to examine the effects of the glass-to-rubber transition of resin matrices on the friction and wear characteristics of friction materials, in relation to different types of thermosetting resins. The main objective is to establish
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possible correlations between thermal transition behavior of resin matrix and tribological testing data of friction materials. The typical model system was elaborated from 30 v/v% reinforcing fibers, and 24 v/v% thermosetting resins before fabrication. In our laboratory, the combination of nano-meter sized potassium titanate and micro-meter sized ceramic fibers have been proved to be an effective way to improve overall performance of friction composites [21]. There are pure resins of PF and BZ, and blend resins of P-B (PF: BZ¼ 50: 50 (v/v%)) and P-B-N (PF: BZ: NB ¼40: 40: 20 (v/v%)). Therefore, the reinforcing fibers and binders were based on nano- and micro-meter sized ceramic fibers and pure or blend thermosetting resins. Furthermore, the curing and thermal transition behaviors of different types of thermosetting resins were characterized by DSC and DMA. The thermal and tribological properties of friction materials were measured by DMA and friction test, respectively.
2. Experimental 2.1. Material The phenolic resin (PF) with 10 v/v% crosslinker used in this work was purchased from Haiwos (Jinan Resin Co., Jinan, China). Benzoxazine resin (3,4-dihydro-3-cyclohexyl-6-t-butyl-1,3,2Hbenzoxazine, BZ) was prepared according to the Ref [22]. Nitrile butadiene rubber (NB) was from Sinopec Co., Nanzhou, China. The ceramic fiber (SaiDun Ceramic Fiber Co., Qingdao, China) was in the form of long fibers with an average length of 1–3 mm and an average diameter of 7–9 mm. Potassium titanate (Shanghai Potassium Titanate Fiber Co., Shanghai, China) was in the form of fine whiskers (needle shape), 10–20 mm in average length and 300–600 nm in average diameter. All chemical reagents were purchased from the commercial resources. The uncured blend resins of P-B and P-B-N were prepared in a ball grinder according to the formulation shown above. 2.2. Preparation of friction materials The friction materials were manufactured by mixing, hot press mounting, and post-curing. According to PF, BZ, P-B, and P-B-N resin binders, the friction materials were coded as f-PF, f-BZ, f-P-B and f-P-B-N, respectively. The relative volume percentages of ingredients in the friction material specimen are shown in Table 1. All ingredients were mixed in an electric blender for 5 min. The mixtures were molded at 200 1C under 20 MPa for 10 min, except PF-based friction materials were done at 160 1C with 20 MPa for 10 min according to its relatively lower curing temperature. To eliminate water vapor released from the friction materials, pressure should be released 3–5 times for the first 2 min of the thermo-molding process. Post-curing possesses were carried out by curing all samples again at 210 1C for 4 h in oven.
2.3. Characterization 2.3.1. Differential scanning calorimetry (DSC) The curing behavior and glass transition temperatures of resins were measured by differential scanning calorimetry on a DSC Q20 (TA instrumental explorer, USA). The curing behaviors were determined from room temperature to 300 1C at heating rate of 10 1C/min under nitrogen flow of 50 mL/min. Then cured samples were rapidly cooled to 30 1C (60 1C/min). The second scan was measured at 10 1C/min from 30 to 300 1C to obtain glass transition temperature (Tg). Graphic determination of Tg was carried out by Wunderlich method. Tg corresponded to a half variation in calorific capacity during transition. The procedure was similar for all samples regardless of compositions. Three samples of each composition were measured and the average Tg values were calculated. 2.3.2. Dynamic mechanical thermal analysis (DMA) Dynamic mechanical thermal tests of the samples were conducted using a dynamic mechanical analysis machine (DMA/ SDTA861e, Mettler Toledo, Switzerland). The curing behaviors of resin cylinders (diameter: 10 mm, thickness: 0.3–0.4 mm) were measured from 30 to 230 1C at heating rate of 5 1C/min with the shear mode. Then cured resin samples were cooled to 30 1C. The second scan was performed at 1 Hz and at 5 1C/min from 30 to 350 1C to observe glass-rubber transition temperature (Tg). The friction material samples (50 mm 8 mm 4 mm) were measured in a three point bending mode with a vibration frequency of 1 Hz and heating rate of 5 1C/min from 30 to 350 1C. 2.3.3. Friction and wear test Friction tests were performed on a DSM constant speed friction testing machine (shown in Fig. 1) in accordance to China national standard GB5763-2008 [23]. The brake lining samples were cut to usual dimensions of 2.5 cm 2.5 cm 0.6 cm. A gray cast iron rotor disk with a friction radius of 0.15 m and Brinell hardenss of 185–210 was used. The disk rotation was kept fixed at 500 rpm. Two specimens with an area of 6.25 cm2 each were push-fitted on the surface of the rotor disk. The applied pressure on the samples was kept at 0.98 MPa. The friction disk was rotated for 5000 revolution at each testing temperatures of 100, 150, 200, 250, 300 and 350 1C. The average friction force during rotation was used for calculating friction coefficient (COF, m). The frictional coefficients at different temperatures were measured while heating and cooling the test samples. The thickness of the test samples was noted after each test period to calculate the wear rate of each period. The wear rate
Table 1 Formulation of investigated samples. Constituents
PF BZ NB Ceramic fiber Potassium titanate Friction modifiers graphite, coke, alumina, etc. Filler barium sulfate, calcium carbonate, etc.
Content vol.% f-PF
f-BZ
f-P-B
f-P-B-N
24 0 0 20 10 26 20
0 24 0 20 10 26 20
12 12 0 20 10 26 20
9.6 9.6 4.8 20 10 26 20 Fig. 1. The DSM constant speed friction testing machine.
Y. Wu et al. / Tribology International 54 (2012) 51–57
value was calculated as follows: V¼
1 A d1 d2 cm3 N1 m1 2pR N fm
ð1Þ
where R is the friction radius (R¼0.15 m), N is the disk rotation numbers of each test period (N ¼5000), A is the area of the specimen (A¼ 2.5 cm 2.5 cm), d1 and d2 are the average thickness of specimen before and after experiment (cm), respectively, and fm (N) is the average friction force measured during each test. Three parallel friction tests were carried out for every sample at each test condition.
3. Results and discussion 3.1. Curing behaviors of resins The typical non-isothermal DSC thermogram as a function of temperature for PF uncured resin was shown in Fig. 2. From the figure, the exothermic reaction began slowly around 121 1C (Tonset). Then the curing reaction reached a maximum curing reaction rate at the peak temperature of 143 1C (Tpeak). Finally, the curing reaction slowed down around 175 1C (Toffset) because less unreact materials were available to react. In addition, the endothermic reaction happened when the temperature was higher than 190 1C, resulting from the fragmentation of the polymer chains formed in the former stage. From Table 2, the Tonset, Tpeak, Toffset values of four types of thermosetting resins were measured. Compared with PF resin, the exothermic peak temperature of BZ resin was higher. The BZ resin occupied the curing reaction temperature of 218 1C. It is noted that P-B and P-B-N blend resins presented two exothermic peaks, one peak around 153 1C (Tpeak1) and another one at 207 1C (Tpeak2), which were due to the curing reactions of PF and BZ resins, respectively. The curing temperatures of blend resins lied in the range of both PF and BZ resins to show the miscibility of the components to a certain extent.
2 143°C
1
Heat flow Exo
0 -1
175°C
121°C
-2 -3 -4 -5 50
100
150
The typical DMA modulus curves as a function of temperature for PF uncured resin was shown in Fig. 3. The storage and loss modulus decreased quickly when the uncured resin melted and passed through the glass transition region, and then increased to about 10 MPa at the rubbery plateau due to the thermal polymerization. The curing temperature was determined at which the storage modulus equals the loss modulus. In the figure, the PF resin presented the curing temperature at 130 1C. The summary of the curing temperature results obtained from DMA measurements was also listed in Table 2. The BZ resin possessed the curing temperature at 180 1C. P-B and P-B-N blend resins presented curing temperatures around 170 and 135 1C, respectively. Compared with BZ resin, the distinctly decreased curing temperatures for P-B-N blend resins with PF resin and rubber added can be explained that there was chemical reactions occurred among the components to accelerate the curing process [24]. Both DMA and DSC results confirmed that adding PF resin and rubber could lower the curing temperature of BZ resin, which can improve the processibility of BZ resin. The above curing behaviors of thermosetting resins provided the important information to determine the thermomolding conditions of the resin based composites. According to the relatively lower curing temperature of PF, PF-based friction materials were thermomolded at 160 1C. However, the BZ and blend resins based composites were molded at 200 1C according to their relatively higher curing temperature.
3.2. Glass transition temperatures of resins The typical DSC heating thermogram of the second scan as a function of temperature for PF resin was shown in Fig. 4. The exothermic peak at 143 1C disappeared during the second scan, which was due to the complete curing reaction of the sample. The glass transition temperature of 165 1C (Tpeak) was measured from the midpoint of the heat capacity change in the figure. DSC measurement results of resins were shown in Table 3. The Tpeak increased in the order of PFoBZ oP-BoP-B-N, which suggested that adding PF resin and rubber could enhance chemical crosslinking of BZ resin to improve its glass transition temperature. Usually, mechanical loss factor (tan d) peak in the DMA thermogram, the a relaxation, reflects the glass transition, and may be analyzed to provide information about the inner structure and molecules motion. Fig. 5 presented tan d vs. temperature spectra of cured resins. PF and BZ cured resins presented Tg around 250 and 203 1C, respectively, which were obtained from the maximum tan d peak. The glass transition temperatures of cured resins were also summarized in Table 3. The experimental data were different from the DSC results, according to the different measurement method and applied mode. Normally,
200
Temperature [°C]
Table 2 The curing temperatures (oC) of uncured resins obtained from DSC and DMA results.
n
Tonset1
Tpeak1
Toffset1
Tonset2
Tpeak2
Toffset2
n
121 153 118 106
143 218 154 153
175 267 174 173
– – 174 173
– – 207 207
– – 238 248
130 180 170 135
Tcure: data obtained from DMA results.
Modulus [MPa]
10
Fig. 2. The typical thermogram of PF resin obtained from DSC measurement.
PF BZ P-B P-B-N
53
1
0.1
storage modulus loss modulus
Tcure 130°C
0.01 0
50
100
150
200
250
300
Temperature [°C] Fig. 3. The typical modulus curves of PF resin obtained from DMA measurement.
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its effect on the miscibility and crosslinking polymer network of the BZ blend resins. Both DMA and DSC results confirmed that adding PF resin and rubber could enhance chemical crosslinking of BZ resin to improve its glass transition temperature.
Heat flow [W/g]
0.0 -0.5 -1.0
3.3. Thermal behavior of friction materials Tg
-1.5 -2.0 0
50
100 150 200 Temperature [°C]
250
300
Fig. 4. The typical thermogram as a function of temperature of the second heating scan for PF resin obtained from DSC measurement.
Table 3 The glass transition temperatures (oC) of cured resins obtained from DSC and DMA results.
PF BZ P-B P-B-N n
Tonset
Tpeak
Tend
n
153 162 189 204
165 178 197 231
198 195 255 260
250 203 225 230
Tg
Tg: data obtained from the peak temperature of tan d from DMA results.
0.35 P-B-N P-B PF BZ
0.30 0.25
Tan δ
0.20 0.15 0.10 0.05 0.00 50
100
150
200
250
300
350
The storage modulus (E’) and mechanical loss factor (tan d) of the resin based friction materials as a function of temperature are shown in Figs. 6 and 7. In Fig. 6, the E’ values increased in the order of f-P-B-Nof-PFof-P-B of-BZ at 30 1C. The E’ value of the f-BZ material was higher than other materials in the glassy state, suggesting that BZ resin based composite material appeared more able to resist polymer segmental motion with filler reinforced. Then, E’ values of BZ resin based composite material showed a distinct decrease in storage modulus when temperature was higher than 150 1C, accompanying the thermal transition of resin matrix from glassy state to rubbery state. However, storage modulus were relatively stable for the friction materials f-P-B, f-PF and f-P-B-N even when the temperature was 200 1C, and then decreased significantly as the thermal glass-rubber transition occurred. PF and blend resins based composites kept stable under the increased temperatures, which were associated with relatively higher glass transition temperatures of corresponding resins as DMA results mentioned above. The temperature dependence of tan d from DMA was given in Fig. 7. Compared with the resins, all resin based composite materials showed that the thermal transition temperature ranges shifted to the higher temperature. The phenomena were interpreted by assuming that interactions between filler particles and the polymer matrix reduced molecular mobility and flexibility of the polymer chains in the vicinity of the interfaces. The glass transition temperatures were increased by hindering segmental motions of polymer chains with the addition of nano- and microsized fibers and other ingredients. The results were in good agreement with the references [26]. Moreover, all samples had a broad prominent damping peak, which reflected toughness and multi-molecular motion of the polymer based composite materials. The height and width of the a-relaxation peak may be analyzed in order to observe trends in the crosslinking density and network homogeneity [27]. The height of the tan d peaks for the friction materials f-PF, f-P-B and f-P-B-N decreased compared with that of the f-BZ material. Tan d is a ratio of viscous to elastic moduli, so it can be surmised that the decreasing height is related to a lower polymer molecular segmental mobility. In addition, the
Temperature [°C] Fig. 5. Tan d as a function of temperature for different cured resins.
f-P-B-N f-P-B f-PF f-BZ
18000 Storage Modulus [MPa]
DMA is more sensitive to observe the glass-rubber transition process than DSC method. If the two starting materials had phase separation and prevented interaction, there are two glass transition peaks for the blends [25]. Both blend resins showed one prominent broad damping peaks, attributing to the miscibility and chemical reaction of the components. Compared with BZ resin, the damping peaks of blend resins moved to higher temperature. Such increase in the Tg of the P-B copolymers was attributed to the crosslinking density improvement occurred from the additional cross-linking reaction between PF and the oxazine ring in the BZ resin. In addition, the NB rubber may be able to catalyze the reaction between BZ and PF to render a more complete network structure of P-B-N blend resin. The increasing temperature and broadening width of tan d peak for blend resins was indicative of relatively higher degree of crosslinking polymer network and thus was associated with relatively lower molecular motion. The above results certified that PF resin and rubber had
20000
16000 14000 12000 10000 8000 6000 4000 2000 0 50
100
150
200
250
300
350
400
Temperature [°C] Fig. 6. Storage modulus as a function of temperature for resin based friction materials.
Y. Wu et al. / Tribology International 54 (2012) 51–57
intensity of tan d peak of the f-P-B-N material obviously decreased and the transition temperature region shifted to higher temperature compared with that of the other friction materials. The relatively stronger intermolecular interactions between relatively complete polymer crosslinking network and fibers and other ingredients for the f-P-B-N material should be emphasized in here. Thus it is indicative of a higher degree of crosslinking polymer network structure formed when PF and rubber were added in the BZ resin.
3.4. Correlations between glass-rubber transition and friction properties The friction coefficient (COF) as a function of temperature for f-P-B-N, f-P-B, f-PF, and f-BZ composite friction materials were presented in Fig. 8. COF data (mean value (Mean) and standard deviation (S.D.)) related to the testing temperatures were summarized in Table 4. On the whole, the COF values increased to
0.26 f-P-B-N f-P-B f-PF f-BZ
0.24 0.22 0.20 0.18 Tanδ
0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 50
100
150 200 250 Temperature [°C]
300
350
400
Fig. 7. Tan d as a function of temperature for resin based friction materials.
55
reach the maximum value with the temperature increased from 100 to 200 or 250 1C, and then decreased with the continued increase of applied temperature. The adhesive resin decomposed or degraded to lose its ability to bind the fibers and other ingredients around 300 1C, resulting in the mechanical strength, the wear resistance, and the friction coefficient to dramatically decrease [28]. A certain temperature i.e. 100 1C below the glass transition temperature of all resins was examined. The COF increased in the order of materials f-BZof-P-B of-PF of-P-B-N, on the contrary, the storage modulus (Fig. 6) decreased in the order of materials f-BZ4f-P-B 4f-PF 4f-P-B-N at the same temperature. It can be concluded that the COF declined with an increase of storage modulus for the friction composites. At 100 1C where the resins owned glassy state, segmental motions of the polymeric chains were restricted significantly. It is difficult for the resin matrix to deform and conform to the counterface. It is assumed that when two rough surfaces contact with each other under the application of a normal load, actual contact occurs at the contact plateaus of the rough surfaces [29]. The friction generally increases with an increased area of real contact [30]. Under the same applied load, friction composites with relatively lower storage modulus tended to deform easily, and had relatively larger real contact area between friction pairs, which resulted in relatively higher friction force and COF. There was a significant increasing tendency in COF values of all composites when braking temperatures increased from 150 to 200 or 250 1C. The COF values of materials f-PF, f-BZ, and f-P-B increased rapidly and reached the maximum value at 200 1C. It is noted that the COF values of materials f-P-B-N attained the maximum value (0.46) at 250 1C. According to the results presented in Figs. 6 and 7, it was found that there also existed a critical temperature range i.e. 150–300 1C for all composites under which the glass-to-rubber transition of thermosetting resins happened. It is obvious that the glass-rubber transition temperature range was dependent on the types of resins used. The increased values of friction coefficient under the critical transition temperature range for all composites can be explained
0.55 f-P-B-N f-P-B f-PF f-BZ
0.4 Ware rate [10-7cm-1N-1m-1]
Friction coefficient μ
0.50 0.45 0.40 0.35 0.30
f-P-B-N f-P-B f-PF f-BZ
0.3
0.2
0.1
0.25 100
200
300
400
100
Temperature [°C] Fig. 8. Friction coefficient (m) as a function of temperature for resin based friction materials.
200 300 Temperature [°C]
400
Fig. 9. Wear rate (V) as a function of temperature for resin based friction materials.
Table 4 Friction coefficient (m) values of friction materials samples. Samples
f-PF f-BZ f-P-B f-P-B-N
100 1C
150 1C
200 1C
250 1C
300 1C
350 1C
Mean
S.D.
Mean
S.D.
Mean
S.D.
Mean
S.D.
Mean
S.D.
Mean
S.D.
0.41 0.35 0.39 0.42
0.01 0.01 0.02 0.01
0.43 0.45 0.42 0.42
0.01 0.02 0.01 0.01
0.46 0.49 0.47 0.44
0.02 0.02 0.02 0.01
0.46 0.45 0.47 0.46
0.02 0.01 0.02 0.02
0.43 0.42 0.42 0.44
0.01 0.01 0.01 0.02
0.37 0.40 0.38 0.37
0.01 0.01 0.01 0.01
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Y. Wu et al. / Tribology International 54 (2012) 51–57
Table 5 Wear rate (V/10 7cm3N 1m 1) values of friction materials samples. Samples
f-PF f-BZ f-P-B f-P-B-N
100 1C
150 1C
200 1C
250 1C
300 1C
350 1C
Mean
S.D.
Mean
S.D.
Mean
S.D.
Mean
S.D.
Mean
S.D.
Mean
S.D.
0.14 0.12 0.11 0.08
0.01 0.01 0.01 0.01
0.13 0.14 0.13 0.09
0.01 0.01 0.01 0.01
0.17 0.14 0.13 0.11
0.02 0.02 0.01 0.01
0.21 0.13 0.14 0.15
0.02 0.01 0.01 0.02
0.29 0.16 0.21 0.22
0.02 0.02 0.02 0.02
0.39 0.26 0.34 0.36
0.03 0.02 0.03 0.02
by the storage and loss modulus behavior of the resin matrix. When resin matrix converted from glassy state to rubbery state, the loss modulus increased quickly, on the contrary, storage modulus decreased significantly. The number and area of contact plateaus increased with decreasing storage modulus. In addition, the matrix tended to absorb more energy during sliding. As a result, the interfacial interactions between contact pairs caused by the real contact area were enhanced to increase the COF values of friction composites [31]. Therefore, the glass-rubber transition of thermosetting resins can influence the tribological property of the friction composites. It is noted that the friction material f-P-BN occupied relatively higher glass-rubber transition temperature among four friction materials, resulting in better capability to stabilize the friction coefficient under relatively higher applied temperature. 3.5. Correlations between glass-rubber transition and wear properties The wear rate (V) as a function of temperature for friction materials f-P-B-N, f-P-B, f-PF, and f-BZ was presented in Fig. 9. The wear rate data (mean value (Mean) and standard deviation (S.D.)) related to the testing temperatures were summarized in Table 5. The figure illustrated the trend and variation of the specific wear rates of different composites against the applied temperature. On the whole, the wear rates for all friction composites increased slowly with an increase of temperature to 200 or 250 1C, and then increased dramatically when temperature was above 250 1C. Owing to the thermal decomposition effect of the resin binders above 300 1C, significant reduction in frictional coefficient and increase in wear rate were observed in Figs. 8 and 9. The wear rate of friction composites versus applied temperature showed a dramatic increase at 200 or 250 1C. The results were in accordance with COF results tendency dependent on the glass-to-rubber transition of resin matrix. As a matter of fact, this sudden change in the wear rate can be associated to the significant change of resin binders and the interactions between the matrix and fillers. The glass-to-rubber transition of thermosetting resins resulted in the polymer segmental motions increased significantly. Therefore, the resin matrix can deform easily to conform to counterface. This may lead to greater extent of interfacial interaction between contact pairs caused by higher real contact area and to higher extent of interlocking with the counterface asperities, which resulted in enhancement of the friction force and wear [32]. Therefore, the wear results obtained supported the friction results versus testing temperatures mentioned above.
4. Conclusions The effects of the glass-to-rubber transition of resin matrices on the friction and wear characteristics of friction materials were investigated. Pure and blend thermosetting resins were examined in the paper. The experimental results revealed that adding PF
resin and rubber could lower the curing temperature and improve glass transition temperature of BZ resin. Correspondingly, the increasing temperature and broadening width of tan d peaks for P-B-N resin based friction materials were owing to relatively stronger intermolecular interactions between relatively complete polymer crosslinking network and fibers and other ingredients. Both DMA and friction test results confirmed that glass-to-rubber thermal transition of thermosetting resins influenced significantly the friction behavior and wear rate of the composite friction materials. There was a significant increasing tendency in friction coefficient and wear rate values of all composites when braking temperatures increased to 200 or 250 1C, accompanying the resin matrix converted from glassy state to rubbery state. The glass-torubber transition of resin matrix led to that greater extent of interfacial interaction between contact pairs caused by higher real contact area and higher extent of interlocking with the counterface asperities, which resulted in enhancement of the friction force and wear. It is worth noting that P-B-N based friction material occupied relatively higher glass-rubber transition temperature, resulting in better ability to stabilize the friction coefficient and wear rate under relatively higher braking temperature.
Acknowledgment The authors acknowledge the SRF for ROCS, State Education Ministry, P R China, the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (contract grant number: CUGL090223 and CUGL100202), Hubei Provincial Department of Education (XD2010037), and the grant of the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (KF201106).
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[10] Sinha SK, Biswas SK. Friction and wear behavior of continuous fiber as cast kevlar phenolic resin composite. Journal of Material Science 1992;27(11): 3085–91. [11] Patnaik A, Kumar M, Satapathy BK, Tomar BS. Performance sensitivity of hybrid phenolic composites in friction braking: effect of ceramic and aramid fibre combination. Wear 2010;269(11–12):891–9. [12] Hong US, Jung SL, Cho KH, Cho MH, Kim SJ, Jang H. Wear mechanism of multiphase friction materials with different phenolic resin matrices. Wear 2009;266(7–8):739–44. [13] Kim SJ, Jang H. Friction and wear of friction materials containing two different phenolic resins reinforces with aramid pulp. Tribology International 2000;33(7):477–84. [14] Takeichi T, Guo Y, Rimdusit S. Performance improvement of polybenzoxazine by alloying with polyimide: effect of preparation method on the properties. Polymer 2005;46(13):4909–16. [15] Yagci Y, Kiskan B, Ghosh NN. Recent advancement on polybenzoxazine—a newly developed high performance thermoset. Journal of Polymer Science Part A 2009;47(21):5565–76. [16] Ishida H, Allen DJ. Gelation behavior of near-zero shrinkage polybenzoxazines. Journal of Applied Polymer Science 2001;79(3):406–17. [17] Kiskan B, Ghosh NN, Yagci Y. Polybenzoxazine-based composites as highperformance materials. Polymer International 2011;60(2):167–77. [18] Rimdusit S, Ishida H. Synergism and multiple mechanical relaxations observed in ternary systems based on benzoxazine, epoxy, and phenolic resins. Journal of Polymer Science Part B 2000;38(13):1687–98. [19] Jang J, Soe D. Performance improvement of rubber modified polybenzoxazine. Journal of Applied Polymer Science 1998;67:1–10. [20] Saffar A, Shojaei A, Arjmand M. Theoretical and experimental analysis of the thermal, fade and wear characteristics of rubber-based composite friction materials. Wear 2010;269(1–2):145–51.
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Contents lists available at SciVerse ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Effects of glass-to-rubber transition of thermosetting resin matrix on the friction and wear properties of friction materials Yaoqing Wu a, Ming Zeng a,b,n, Qingyu Xu c,d, Shuen Hou a,nn, Hongyun Jin a, Liren Fan a a
Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, PR China State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, PR China c Hubei Research Institute of Chemistry, Wuhan 430074, PR China d Haiso Technology Co. Ltd, Wuhan 430074, PR China b
a r t i c l e i n f o
abstract
Article history: Received 20 January 2012 Received in revised form 6 May 2012 Accepted 22 May 2012 Available online 31 May 2012
The present study is to examine the effects of glass-to-rubber transition of resin matrix on the friction and wear characteristics of friction materials, in relation to different types of thermosetting resins. Dynamic mechanical thermal analysis and friction test results revealed that glass-to-rubber transition of thermosetting resins influenced significantly the friction and wear behavior of the composite materials. There was a significant increasing tendency in friction coefficient and wear rate values for all composites when braking temperatures increased to 200 or 250 1C, accompanying the resin matrix converted from glassy state to rubbery state. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Benzoxazine Glass transition Friction Thermosetting resin
1. Introduction Automotive brake linings are one of the most important safety components in automobiles [1–5]. The commercial brake lining usually contains more than 10, up to 20 or 25 different constituents. The ingredients are categorized into four classes: binders, fillers, friction modifiers, and reinforcements [6,7]. Among the ingredients in the friction material, polymer binders (particularly thermosetting resin) play a crucial role in determining the friction characteristics during braking. Phenolic resin (PF) based automotive friction material has been extensively investigated [8–11]. However, the phenolic resin is brittle and is not resistant to high temperature, which often results in wear loss and fade under 350 1C for the friction materials [12,13]. Polybenzoxazine (BZ) is an addition polymerized phenolic system, exhibiting versatility in a wide range of applications, and having a wide range of interesting features and capability to overcome several shortcomings of conventional novolac and resole type phenolic resins. They exhibit near zero volumetric change upon curing, low water absorption, high char yield, and also possess thermal and flame retarding properties of phenolics n
Corresponding author at: Tel.: þ 86 15623351996. Corresponding author. Tel: þ 86 18971384510. E-mail addresses: [email protected], [email protected] (M. Zeng), [email protected] (S. Hou). nn
0301-679X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.triboint.2012.05.018
along with the mechanical performance [14–17]. Though benzoxazine based materials possess several advantages, they have not yet been widely used in the friction materials industries in case of the process problems, for instance, high curing temperature. The modified benzoxazine resins have been prepared by mixing benzoxazie with PF, epoxy, and nitrile butadiene rubber (NB) to manipulate the mechanical and thermal properties, and to lower the curing temperature [18,19]. Glass-to-rubber transition of polymer often leads to substantial change in stiffness, strength and other properties of polymeric materials. However, very little research has been focused on the effect of thermal transition behavior of resin matrix on the resultant wear performance of composite friction materials. The thermal effects as well as fade and wear characteristics of rubberbased friction materials with and without fiber reinforcements were examined recently [20]. The experimental data revealed that rubber-to-glass transition of rubber matrix influenced significantly the fade behavior and wear rate of the friction materials. In fact, the relationship between thermal transition behavior of thermosetting resins and tribological properties of friction materials should attract much attention, because thermosetting resins were mostly utilized as binder matrix in the field of friction composites. The present study is a generic study to examine the effects of the glass-to-rubber transition of resin matrices on the friction and wear characteristics of friction materials, in relation to different types of thermosetting resins. The main objective is to establish
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Y. Wu et al. / Tribology International 54 (2012) 51–57
possible correlations between thermal transition behavior of resin matrix and tribological testing data of friction materials. The typical model system was elaborated from 30 v/v% reinforcing fibers, and 24 v/v% thermosetting resins before fabrication. In our laboratory, the combination of nano-meter sized potassium titanate and micro-meter sized ceramic fibers have been proved to be an effective way to improve overall performance of friction composites [21]. There are pure resins of PF and BZ, and blend resins of P-B (PF: BZ¼ 50: 50 (v/v%)) and P-B-N (PF: BZ: NB ¼40: 40: 20 (v/v%)). Therefore, the reinforcing fibers and binders were based on nano- and micro-meter sized ceramic fibers and pure or blend thermosetting resins. Furthermore, the curing and thermal transition behaviors of different types of thermosetting resins were characterized by DSC and DMA. The thermal and tribological properties of friction materials were measured by DMA and friction test, respectively.
2. Experimental 2.1. Material The phenolic resin (PF) with 10 v/v% crosslinker used in this work was purchased from Haiwos (Jinan Resin Co., Jinan, China). Benzoxazine resin (3,4-dihydro-3-cyclohexyl-6-t-butyl-1,3,2Hbenzoxazine, BZ) was prepared according to the Ref [22]. Nitrile butadiene rubber (NB) was from Sinopec Co., Nanzhou, China. The ceramic fiber (SaiDun Ceramic Fiber Co., Qingdao, China) was in the form of long fibers with an average length of 1–3 mm and an average diameter of 7–9 mm. Potassium titanate (Shanghai Potassium Titanate Fiber Co., Shanghai, China) was in the form of fine whiskers (needle shape), 10–20 mm in average length and 300–600 nm in average diameter. All chemical reagents were purchased from the commercial resources. The uncured blend resins of P-B and P-B-N were prepared in a ball grinder according to the formulation shown above. 2.2. Preparation of friction materials The friction materials were manufactured by mixing, hot press mounting, and post-curing. According to PF, BZ, P-B, and P-B-N resin binders, the friction materials were coded as f-PF, f-BZ, f-P-B and f-P-B-N, respectively. The relative volume percentages of ingredients in the friction material specimen are shown in Table 1. All ingredients were mixed in an electric blender for 5 min. The mixtures were molded at 200 1C under 20 MPa for 10 min, except PF-based friction materials were done at 160 1C with 20 MPa for 10 min according to its relatively lower curing temperature. To eliminate water vapor released from the friction materials, pressure should be released 3–5 times for the first 2 min of the thermo-molding process. Post-curing possesses were carried out by curing all samples again at 210 1C for 4 h in oven.
2.3. Characterization 2.3.1. Differential scanning calorimetry (DSC) The curing behavior and glass transition temperatures of resins were measured by differential scanning calorimetry on a DSC Q20 (TA instrumental explorer, USA). The curing behaviors were determined from room temperature to 300 1C at heating rate of 10 1C/min under nitrogen flow of 50 mL/min. Then cured samples were rapidly cooled to 30 1C (60 1C/min). The second scan was measured at 10 1C/min from 30 to 300 1C to obtain glass transition temperature (Tg). Graphic determination of Tg was carried out by Wunderlich method. Tg corresponded to a half variation in calorific capacity during transition. The procedure was similar for all samples regardless of compositions. Three samples of each composition were measured and the average Tg values were calculated. 2.3.2. Dynamic mechanical thermal analysis (DMA) Dynamic mechanical thermal tests of the samples were conducted using a dynamic mechanical analysis machine (DMA/ SDTA861e, Mettler Toledo, Switzerland). The curing behaviors of resin cylinders (diameter: 10 mm, thickness: 0.3–0.4 mm) were measured from 30 to 230 1C at heating rate of 5 1C/min with the shear mode. Then cured resin samples were cooled to 30 1C. The second scan was performed at 1 Hz and at 5 1C/min from 30 to 350 1C to observe glass-rubber transition temperature (Tg). The friction material samples (50 mm 8 mm 4 mm) were measured in a three point bending mode with a vibration frequency of 1 Hz and heating rate of 5 1C/min from 30 to 350 1C. 2.3.3. Friction and wear test Friction tests were performed on a DSM constant speed friction testing machine (shown in Fig. 1) in accordance to China national standard GB5763-2008 [23]. The brake lining samples were cut to usual dimensions of 2.5 cm 2.5 cm 0.6 cm. A gray cast iron rotor disk with a friction radius of 0.15 m and Brinell hardenss of 185–210 was used. The disk rotation was kept fixed at 500 rpm. Two specimens with an area of 6.25 cm2 each were push-fitted on the surface of the rotor disk. The applied pressure on the samples was kept at 0.98 MPa. The friction disk was rotated for 5000 revolution at each testing temperatures of 100, 150, 200, 250, 300 and 350 1C. The average friction force during rotation was used for calculating friction coefficient (COF, m). The frictional coefficients at different temperatures were measured while heating and cooling the test samples. The thickness of the test samples was noted after each test period to calculate the wear rate of each period. The wear rate
Table 1 Formulation of investigated samples. Constituents
PF BZ NB Ceramic fiber Potassium titanate Friction modifiers graphite, coke, alumina, etc. Filler barium sulfate, calcium carbonate, etc.
Content vol.% f-PF
f-BZ
f-P-B
f-P-B-N
24 0 0 20 10 26 20
0 24 0 20 10 26 20
12 12 0 20 10 26 20
9.6 9.6 4.8 20 10 26 20 Fig. 1. The DSM constant speed friction testing machine.
Y. Wu et al. / Tribology International 54 (2012) 51–57
value was calculated as follows: V¼
1 A d1 d2 cm3 N1 m1 2pR N fm
ð1Þ
where R is the friction radius (R¼0.15 m), N is the disk rotation numbers of each test period (N ¼5000), A is the area of the specimen (A¼ 2.5 cm 2.5 cm), d1 and d2 are the average thickness of specimen before and after experiment (cm), respectively, and fm (N) is the average friction force measured during each test. Three parallel friction tests were carried out for every sample at each test condition.
3. Results and discussion 3.1. Curing behaviors of resins The typical non-isothermal DSC thermogram as a function of temperature for PF uncured resin was shown in Fig. 2. From the figure, the exothermic reaction began slowly around 121 1C (Tonset). Then the curing reaction reached a maximum curing reaction rate at the peak temperature of 143 1C (Tpeak). Finally, the curing reaction slowed down around 175 1C (Toffset) because less unreact materials were available to react. In addition, the endothermic reaction happened when the temperature was higher than 190 1C, resulting from the fragmentation of the polymer chains formed in the former stage. From Table 2, the Tonset, Tpeak, Toffset values of four types of thermosetting resins were measured. Compared with PF resin, the exothermic peak temperature of BZ resin was higher. The BZ resin occupied the curing reaction temperature of 218 1C. It is noted that P-B and P-B-N blend resins presented two exothermic peaks, one peak around 153 1C (Tpeak1) and another one at 207 1C (Tpeak2), which were due to the curing reactions of PF and BZ resins, respectively. The curing temperatures of blend resins lied in the range of both PF and BZ resins to show the miscibility of the components to a certain extent.
2 143°C
1
Heat flow Exo
0 -1
175°C
121°C
-2 -3 -4 -5 50
100
150
The typical DMA modulus curves as a function of temperature for PF uncured resin was shown in Fig. 3. The storage and loss modulus decreased quickly when the uncured resin melted and passed through the glass transition region, and then increased to about 10 MPa at the rubbery plateau due to the thermal polymerization. The curing temperature was determined at which the storage modulus equals the loss modulus. In the figure, the PF resin presented the curing temperature at 130 1C. The summary of the curing temperature results obtained from DMA measurements was also listed in Table 2. The BZ resin possessed the curing temperature at 180 1C. P-B and P-B-N blend resins presented curing temperatures around 170 and 135 1C, respectively. Compared with BZ resin, the distinctly decreased curing temperatures for P-B-N blend resins with PF resin and rubber added can be explained that there was chemical reactions occurred among the components to accelerate the curing process [24]. Both DMA and DSC results confirmed that adding PF resin and rubber could lower the curing temperature of BZ resin, which can improve the processibility of BZ resin. The above curing behaviors of thermosetting resins provided the important information to determine the thermomolding conditions of the resin based composites. According to the relatively lower curing temperature of PF, PF-based friction materials were thermomolded at 160 1C. However, the BZ and blend resins based composites were molded at 200 1C according to their relatively higher curing temperature.
3.2. Glass transition temperatures of resins The typical DSC heating thermogram of the second scan as a function of temperature for PF resin was shown in Fig. 4. The exothermic peak at 143 1C disappeared during the second scan, which was due to the complete curing reaction of the sample. The glass transition temperature of 165 1C (Tpeak) was measured from the midpoint of the heat capacity change in the figure. DSC measurement results of resins were shown in Table 3. The Tpeak increased in the order of PFoBZ oP-BoP-B-N, which suggested that adding PF resin and rubber could enhance chemical crosslinking of BZ resin to improve its glass transition temperature. Usually, mechanical loss factor (tan d) peak in the DMA thermogram, the a relaxation, reflects the glass transition, and may be analyzed to provide information about the inner structure and molecules motion. Fig. 5 presented tan d vs. temperature spectra of cured resins. PF and BZ cured resins presented Tg around 250 and 203 1C, respectively, which were obtained from the maximum tan d peak. The glass transition temperatures of cured resins were also summarized in Table 3. The experimental data were different from the DSC results, according to the different measurement method and applied mode. Normally,
200
Temperature [°C]
Table 2 The curing temperatures (oC) of uncured resins obtained from DSC and DMA results.
n
Tonset1
Tpeak1
Toffset1
Tonset2
Tpeak2
Toffset2
n
121 153 118 106
143 218 154 153
175 267 174 173
– – 174 173
– – 207 207
– – 238 248
130 180 170 135
Tcure: data obtained from DMA results.
Modulus [MPa]
10
Fig. 2. The typical thermogram of PF resin obtained from DSC measurement.
PF BZ P-B P-B-N
53
1
0.1
storage modulus loss modulus
Tcure 130°C
0.01 0
50
100
150
200
250
300
Temperature [°C] Fig. 3. The typical modulus curves of PF resin obtained from DMA measurement.
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Y. Wu et al. / Tribology International 54 (2012) 51–57
its effect on the miscibility and crosslinking polymer network of the BZ blend resins. Both DMA and DSC results confirmed that adding PF resin and rubber could enhance chemical crosslinking of BZ resin to improve its glass transition temperature.
Heat flow [W/g]
0.0 -0.5 -1.0
3.3. Thermal behavior of friction materials Tg
-1.5 -2.0 0
50
100 150 200 Temperature [°C]
250
300
Fig. 4. The typical thermogram as a function of temperature of the second heating scan for PF resin obtained from DSC measurement.
Table 3 The glass transition temperatures (oC) of cured resins obtained from DSC and DMA results.
PF BZ P-B P-B-N n
Tonset
Tpeak
Tend
n
153 162 189 204
165 178 197 231
198 195 255 260
250 203 225 230
Tg
Tg: data obtained from the peak temperature of tan d from DMA results.
0.35 P-B-N P-B PF BZ
0.30 0.25
Tan δ
0.20 0.15 0.10 0.05 0.00 50
100
150
200
250
300
350
The storage modulus (E’) and mechanical loss factor (tan d) of the resin based friction materials as a function of temperature are shown in Figs. 6 and 7. In Fig. 6, the E’ values increased in the order of f-P-B-Nof-PFof-P-B of-BZ at 30 1C. The E’ value of the f-BZ material was higher than other materials in the glassy state, suggesting that BZ resin based composite material appeared more able to resist polymer segmental motion with filler reinforced. Then, E’ values of BZ resin based composite material showed a distinct decrease in storage modulus when temperature was higher than 150 1C, accompanying the thermal transition of resin matrix from glassy state to rubbery state. However, storage modulus were relatively stable for the friction materials f-P-B, f-PF and f-P-B-N even when the temperature was 200 1C, and then decreased significantly as the thermal glass-rubber transition occurred. PF and blend resins based composites kept stable under the increased temperatures, which were associated with relatively higher glass transition temperatures of corresponding resins as DMA results mentioned above. The temperature dependence of tan d from DMA was given in Fig. 7. Compared with the resins, all resin based composite materials showed that the thermal transition temperature ranges shifted to the higher temperature. The phenomena were interpreted by assuming that interactions between filler particles and the polymer matrix reduced molecular mobility and flexibility of the polymer chains in the vicinity of the interfaces. The glass transition temperatures were increased by hindering segmental motions of polymer chains with the addition of nano- and microsized fibers and other ingredients. The results were in good agreement with the references [26]. Moreover, all samples had a broad prominent damping peak, which reflected toughness and multi-molecular motion of the polymer based composite materials. The height and width of the a-relaxation peak may be analyzed in order to observe trends in the crosslinking density and network homogeneity [27]. The height of the tan d peaks for the friction materials f-PF, f-P-B and f-P-B-N decreased compared with that of the f-BZ material. Tan d is a ratio of viscous to elastic moduli, so it can be surmised that the decreasing height is related to a lower polymer molecular segmental mobility. In addition, the
Temperature [°C] Fig. 5. Tan d as a function of temperature for different cured resins.
f-P-B-N f-P-B f-PF f-BZ
18000 Storage Modulus [MPa]
DMA is more sensitive to observe the glass-rubber transition process than DSC method. If the two starting materials had phase separation and prevented interaction, there are two glass transition peaks for the blends [25]. Both blend resins showed one prominent broad damping peaks, attributing to the miscibility and chemical reaction of the components. Compared with BZ resin, the damping peaks of blend resins moved to higher temperature. Such increase in the Tg of the P-B copolymers was attributed to the crosslinking density improvement occurred from the additional cross-linking reaction between PF and the oxazine ring in the BZ resin. In addition, the NB rubber may be able to catalyze the reaction between BZ and PF to render a more complete network structure of P-B-N blend resin. The increasing temperature and broadening width of tan d peak for blend resins was indicative of relatively higher degree of crosslinking polymer network and thus was associated with relatively lower molecular motion. The above results certified that PF resin and rubber had
20000
16000 14000 12000 10000 8000 6000 4000 2000 0 50
100
150
200
250
300
350
400
Temperature [°C] Fig. 6. Storage modulus as a function of temperature for resin based friction materials.
Y. Wu et al. / Tribology International 54 (2012) 51–57
intensity of tan d peak of the f-P-B-N material obviously decreased and the transition temperature region shifted to higher temperature compared with that of the other friction materials. The relatively stronger intermolecular interactions between relatively complete polymer crosslinking network and fibers and other ingredients for the f-P-B-N material should be emphasized in here. Thus it is indicative of a higher degree of crosslinking polymer network structure formed when PF and rubber were added in the BZ resin.
3.4. Correlations between glass-rubber transition and friction properties The friction coefficient (COF) as a function of temperature for f-P-B-N, f-P-B, f-PF, and f-BZ composite friction materials were presented in Fig. 8. COF data (mean value (Mean) and standard deviation (S.D.)) related to the testing temperatures were summarized in Table 4. On the whole, the COF values increased to
0.26 f-P-B-N f-P-B f-PF f-BZ
0.24 0.22 0.20 0.18 Tanδ
0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 50
100
150 200 250 Temperature [°C]
300
350
400
Fig. 7. Tan d as a function of temperature for resin based friction materials.
55
reach the maximum value with the temperature increased from 100 to 200 or 250 1C, and then decreased with the continued increase of applied temperature. The adhesive resin decomposed or degraded to lose its ability to bind the fibers and other ingredients around 300 1C, resulting in the mechanical strength, the wear resistance, and the friction coefficient to dramatically decrease [28]. A certain temperature i.e. 100 1C below the glass transition temperature of all resins was examined. The COF increased in the order of materials f-BZof-P-B of-PF of-P-B-N, on the contrary, the storage modulus (Fig. 6) decreased in the order of materials f-BZ4f-P-B 4f-PF 4f-P-B-N at the same temperature. It can be concluded that the COF declined with an increase of storage modulus for the friction composites. At 100 1C where the resins owned glassy state, segmental motions of the polymeric chains were restricted significantly. It is difficult for the resin matrix to deform and conform to the counterface. It is assumed that when two rough surfaces contact with each other under the application of a normal load, actual contact occurs at the contact plateaus of the rough surfaces [29]. The friction generally increases with an increased area of real contact [30]. Under the same applied load, friction composites with relatively lower storage modulus tended to deform easily, and had relatively larger real contact area between friction pairs, which resulted in relatively higher friction force and COF. There was a significant increasing tendency in COF values of all composites when braking temperatures increased from 150 to 200 or 250 1C. The COF values of materials f-PF, f-BZ, and f-P-B increased rapidly and reached the maximum value at 200 1C. It is noted that the COF values of materials f-P-B-N attained the maximum value (0.46) at 250 1C. According to the results presented in Figs. 6 and 7, it was found that there also existed a critical temperature range i.e. 150–300 1C for all composites under which the glass-to-rubber transition of thermosetting resins happened. It is obvious that the glass-rubber transition temperature range was dependent on the types of resins used. The increased values of friction coefficient under the critical transition temperature range for all composites can be explained
0.55 f-P-B-N f-P-B f-PF f-BZ
0.4 Ware rate [10-7cm-1N-1m-1]
Friction coefficient μ
0.50 0.45 0.40 0.35 0.30
f-P-B-N f-P-B f-PF f-BZ
0.3
0.2
0.1
0.25 100
200
300
400
100
Temperature [°C] Fig. 8. Friction coefficient (m) as a function of temperature for resin based friction materials.
200 300 Temperature [°C]
400
Fig. 9. Wear rate (V) as a function of temperature for resin based friction materials.
Table 4 Friction coefficient (m) values of friction materials samples. Samples
f-PF f-BZ f-P-B f-P-B-N
100 1C
150 1C
200 1C
250 1C
300 1C
350 1C
Mean
S.D.
Mean
S.D.
Mean
S.D.
Mean
S.D.
Mean
S.D.
Mean
S.D.
0.41 0.35 0.39 0.42
0.01 0.01 0.02 0.01
0.43 0.45 0.42 0.42
0.01 0.02 0.01 0.01
0.46 0.49 0.47 0.44
0.02 0.02 0.02 0.01
0.46 0.45 0.47 0.46
0.02 0.01 0.02 0.02
0.43 0.42 0.42 0.44
0.01 0.01 0.01 0.02
0.37 0.40 0.38 0.37
0.01 0.01 0.01 0.01
56
Y. Wu et al. / Tribology International 54 (2012) 51–57
Table 5 Wear rate (V/10 7cm3N 1m 1) values of friction materials samples. Samples
f-PF f-BZ f-P-B f-P-B-N
100 1C
150 1C
200 1C
250 1C
300 1C
350 1C
Mean
S.D.
Mean
S.D.
Mean
S.D.
Mean
S.D.
Mean
S.D.
Mean
S.D.
0.14 0.12 0.11 0.08
0.01 0.01 0.01 0.01
0.13 0.14 0.13 0.09
0.01 0.01 0.01 0.01
0.17 0.14 0.13 0.11
0.02 0.02 0.01 0.01
0.21 0.13 0.14 0.15
0.02 0.01 0.01 0.02
0.29 0.16 0.21 0.22
0.02 0.02 0.02 0.02
0.39 0.26 0.34 0.36
0.03 0.02 0.03 0.02
by the storage and loss modulus behavior of the resin matrix. When resin matrix converted from glassy state to rubbery state, the loss modulus increased quickly, on the contrary, storage modulus decreased significantly. The number and area of contact plateaus increased with decreasing storage modulus. In addition, the matrix tended to absorb more energy during sliding. As a result, the interfacial interactions between contact pairs caused by the real contact area were enhanced to increase the COF values of friction composites [31]. Therefore, the glass-rubber transition of thermosetting resins can influence the tribological property of the friction composites. It is noted that the friction material f-P-BN occupied relatively higher glass-rubber transition temperature among four friction materials, resulting in better capability to stabilize the friction coefficient under relatively higher applied temperature. 3.5. Correlations between glass-rubber transition and wear properties The wear rate (V) as a function of temperature for friction materials f-P-B-N, f-P-B, f-PF, and f-BZ was presented in Fig. 9. The wear rate data (mean value (Mean) and standard deviation (S.D.)) related to the testing temperatures were summarized in Table 5. The figure illustrated the trend and variation of the specific wear rates of different composites against the applied temperature. On the whole, the wear rates for all friction composites increased slowly with an increase of temperature to 200 or 250 1C, and then increased dramatically when temperature was above 250 1C. Owing to the thermal decomposition effect of the resin binders above 300 1C, significant reduction in frictional coefficient and increase in wear rate were observed in Figs. 8 and 9. The wear rate of friction composites versus applied temperature showed a dramatic increase at 200 or 250 1C. The results were in accordance with COF results tendency dependent on the glass-to-rubber transition of resin matrix. As a matter of fact, this sudden change in the wear rate can be associated to the significant change of resin binders and the interactions between the matrix and fillers. The glass-to-rubber transition of thermosetting resins resulted in the polymer segmental motions increased significantly. Therefore, the resin matrix can deform easily to conform to counterface. This may lead to greater extent of interfacial interaction between contact pairs caused by higher real contact area and to higher extent of interlocking with the counterface asperities, which resulted in enhancement of the friction force and wear [32]. Therefore, the wear results obtained supported the friction results versus testing temperatures mentioned above.
4. Conclusions The effects of the glass-to-rubber transition of resin matrices on the friction and wear characteristics of friction materials were investigated. Pure and blend thermosetting resins were examined in the paper. The experimental results revealed that adding PF
resin and rubber could lower the curing temperature and improve glass transition temperature of BZ resin. Correspondingly, the increasing temperature and broadening width of tan d peaks for P-B-N resin based friction materials were owing to relatively stronger intermolecular interactions between relatively complete polymer crosslinking network and fibers and other ingredients. Both DMA and friction test results confirmed that glass-to-rubber thermal transition of thermosetting resins influenced significantly the friction behavior and wear rate of the composite friction materials. There was a significant increasing tendency in friction coefficient and wear rate values of all composites when braking temperatures increased to 200 or 250 1C, accompanying the resin matrix converted from glassy state to rubbery state. The glass-torubber transition of resin matrix led to that greater extent of interfacial interaction between contact pairs caused by higher real contact area and higher extent of interlocking with the counterface asperities, which resulted in enhancement of the friction force and wear. It is worth noting that P-B-N based friction material occupied relatively higher glass-rubber transition temperature, resulting in better ability to stabilize the friction coefficient and wear rate under relatively higher braking temperature.
Acknowledgment The authors acknowledge the SRF for ROCS, State Education Ministry, P R China, the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (contract grant number: CUGL090223 and CUGL100202), Hubei Provincial Department of Education (XD2010037), and the grant of the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (KF201106).
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