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Exploration of plasma treated stainless steel swarf to reduce the wear of copper-free brake-pads
Journal Pre-proof Exploration of plasma treated stainless steel swarf to reduce the wear of copper-free brake-pads Vishal Mahale, Jayashree Bijwe PII:
S0301-679X(19)30625-5
DOI:
https://doi.org/10.1016/j.triboint.2019.106111
Reference:
JTRI 106111
To appear in:
Tribology International
Received Date: 15 August 2019 Revised Date:
5 December 2019
Accepted Date: 7 December 2019
Please cite this article as: Mahale V, Bijwe J, Exploration of plasma treated stainless steel swarf to reduce the wear of copper-free brake-pads, Tribology International (2020), doi: https://doi.org/10.1016/ j.triboint.2019.106111. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
CRediT author statement
Vishal Mahale: Conceptualization, Methodology, Experimentation, Writing- Original draft preparation, Visualization, Investigation. Jayashree Bijwe: Conceptualization, Supervision, Investigation, Reviewing and Editing.
Exploration of Plasma Treated Stainless Steel Swarf to reduce the wear of Copper-free brake-pads Vishal Mahale*; Jayashree Bijwe1 Industrial Tribology Machine Dynamics and Maintenance Engineering Centre, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India. *Current address- Rane Brake-lining Limited, Chennai 600 058, India.
ABSTRACT In recent few years, the environmental issues especially related to the aquatic life are pushing the Friction Industry to develop NAO FMs (non-asbestos organic friction materials) without Cu. Stainless-steel swarf (SSS) having good corrosion resistance was tried as one of the replacing ingredients for Cu in FMs in Authors’ laboratory in earlier work. It proved to have a great potential for Cu replacement almost in all the performance properties but at the cost of slightly lower wear resistance and hence the theme of current research was to enhance the wear resistance capability of SSS. Hence, in the present study SSS were treated with air-plasma and used in brake-pads. Thus, two types of brake-pads with the identical composition but differing in type of SSS 10 wt. % (treated and untreated) were tribo-evaluated on a full-scale brake inertia dynamometer and treated SSS containing pads exhibited higher wear resistance (approx. 10 %) confirming success of the treatment, first time explored for the friction materials. Enhanced adhesion with resin was the main cause for the improved tribo-performance. KEYWORDS: - Non-asbestos organic (NAO); Copper-free; Plasma treatment; Stainless steel swarf. 1. INTRODUCTION Friction material (FM) is a heart of braking system which comprise of four major classes of ingredients viz. resin as binder, reinforcing fibers, functional fillers and space fillers. All these ingredients are added in FMs to achieve amalgam of desired performance properties. Some of the major desirable properties of FMs are; moderate and stable value of µ which will be least sensitive to the various operating conditions, resistance (as high as possible) to fade (FR), wear (WR), squeal and judder along with good counter-face friendliness and recovery performance. The WR due to economical point of view is a real concern to the Industries. Consistently a lot of
1
Corresponding author. Tel.: +91 11 26591280. E-mail address: [email protected] (J. Bijwe)
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research is being focused on the wear reduction of FMs either by adding appropriate ingredients or deleting those, which are contribute more to the wear of FMs [1-14]. Another major concern about the current non-asbestos organic (NAO) FMs is related to the copper as an important ingredient. The environmental issues especially related to the aquatic life in recent few years are pushing the Industry to develop NAO FMs without Cu. A lot of research is done on this aspect [7-13] and few patents are also available [16-19]. Authors have first time reported on the successful exploration of stainless steel swarf (SSS) as a substitution for Copper [13]. It proved to be quite competitive with Cu except wear resistance, which was lower by almost 10 %. Extracts of the finding are shown in Table 1. Table 1. % increase/ decrease in performance properties of brake-pads containing SSS/ Cu powder (10 wt. %) compared to the metal-free brake pad [13]. Brake pads → Performance Properties ↓ Density Hardness Thermal conductivity µ @ 80 kmph, 0.5 g % fade ratio Disc temperature Wear volume
S10
C10
%↑
%↓
%↑
%↓
14 2 42 3.7 17 8 -
0.5
27 6 68 5 17 8 -
9
The worn surface analysis of brake-pads indicated easy removal of SSS. It is well known that very high resistance of SS to the corrosion is due to its instantaneously generated Cr2O3 layer on the surface [13]. It is also known that metal oxides are formed consistent with minimizing surface energy [14] and hence do not have high adhesion to other surfaces or phenolic resin in the case of FMs. Hence, the particles get dug out very easily during wear- process. In case of Cu particles, such strong and homogeneous layer of CuO does not exist. It has hence better adhesion with resin and hence these particles offer higher wear resistance. If by any way, the adhesion of SSS to the resin is increased, the wear resistance may also increase. Such efforts, however, are not reported for FMs in the literature. Some researchers tried plasma treatment for increasing the surface adhesion of the stainless steel with the selected matrix such as Polyethylene terephthalate (PET) polymer [20]. Samad et al.
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[21] used air-plasma treatment on steel substrate to increase the adhesion with Ultra-High Molecular Weight Polyethylene (UHMWPE) and reported that wear life was enhanced by 10-12 times for plasma treated steel sample compared to the untreated sample. In further studies Samad et al. [22] compared the effect of piranha and air-plasma treatment on Si substrate to increase the adhesion with UHMWPE coating and reported that air-plasma treatment led to 25 times higher wear life than piranha treated samples. The claimed benefits of air-plasma treatment were about its simplicity, cost effectiveness and it does not use harmful chemicals such as phosphorous and sulphur unlike other pre-treatment procedures, and it can be easily adapted in industrial applications. Hence, in the present study SS was treated with air-plasma and used in FM for brake-pad. The SSS were exposed to plasma under vacuum for 500 seconds using a RF power of 18W as discussed in subsequent section. Thus, two multi-ingredient brake-pads with the identical composition, one with 10 wt. % plasma treated SSS and other with same amount of untreated SSS were developed to explore the influence plasma treatment on the performance of pads and tribo-evaluated on a full-scale brake inertia dynamometer and the results are presented in subsequent section. 2. EXPERIMENTAL DETAILS 2.1. Selection of Materials The theme ingredient SS swarf-SSS (K434) were procured from Kasturi Metals Pvt. Ltd. Amravati, India. Stainless steel (K434) coupons of dimensions 65 mm X 15 mm X 2 mm with 0.1 mm Ra value were used as substrates for measuring Lap shear strength (LSS). Two Cu-free FMs were developed in the form of brake-pads with identical composition and differing only in the plasma treated and untreated SS swarf. Composite designation details are given in Table 2. Table 2 Compositions and designations of FMs Ingredients by Wt.% Identical Composition* SS Swarf (SSS) Barite
Designations of FMs U- Untreated SSS
T- Plasma treated SSS
60 10 30
60 10 30
*Identical composition – binder: fibres: friction modifiers: functional fillers – 6:16:12:26
3
Brake-pad composition consisted of several ingredients viz. 6 wt.% straight phenolic resin as a binder, 10 wt. % SSS, 16 wt.% fibers (rockwool, aramid, Polyacrylonitrile), 12 wt.% friction modifiers (alumina and thermo-graphite), 26 wt.% functional fillers (cashew dust, promaxon, hydrated lime, crumb rubber, vermiculite) and 30 wt.% BaSO4. U and T denote the brake-pads with untreated and plasma treated SSS (10 wt. %) respectively. Brake-pads were prepared in the authors’ laboratory as per procedure discussed in the earlier research papers [12-13]. Standard mixing schedule [13] as shown in Table 3 was used for homogeneous mixing of ingredients. Mixture was cold pressed in the pre-former with the shape of brake-pads. Then hot pressing was done at 140 bars and 160o C for 9 minutes. Finally, pads were post cured for 120 °C for 2 hours and 160 °C for 5 hours. Table 3 Schedule of mixing of ingredients in plough shear mixer Steps Ingredients
Mixing duration (minutes)
A
Aramid
7
B
PAN + Rockwool
4
1. Aramid + BaSO4 2. Previous + (PAN + Rockwool)
5 (Batch-I)
3. Previous + SS swarf 4. Mixing of remaining ingredients 5. (Batch I) + (Batch-II) Total Duration
5 3
(Batch-II)
4 2 30 minutes
2.2. Plasma Treatment Procedure The plasma cleaning procedure included the following steps. SSS were cleaned with acetone and dried in the oven. The samples were then air-plasma treated using Harrick Plasma Cleaner (PDC32G-2). The SSS were exposed to plasma under vacuum for 500 seconds using a RF power of 18 W. Treatment time and power was chosen to keep total 9000 J energy constant as mentioned in the literature [21]. It was expected that the plasma treatment in vacuum would disintegrate Cr2O3 in elemental Cr on the surface. The metals have significantly higher surface energy compared to their oxides [23]. Higher surface energy of SSS with Cr on surface would lead to higher adhesion with the resin. However, exposure of treated SSS to the environment would instantaneously 4
convert the Cr on surface to Cr2O3. Hence, air contact had to be avoided. To avoid re-formation of Cr2O3, plasma treated SSS from the chamber were instantaneously dipped into the phenolic resin solution (4 g resin in 100 ml acetone) so that a thin film of resin would cover the treated surfaces efficiently restricting to oxidation. Then acetone was allowed to evaporate in the oven and resin coated SSS were used for preparing the brake-pads. Care was taken to keep the total 6 wt. % of resin in both types of pads (U and T). 2.3. Sample Preparation for Lap Shear Strength (LSS) Analysis It was necessary to prove if this treatment increases adhesion of resin with the SSS. Hence, another experiment was designed which would justify the motive of treatment. Stainless-steel coupons of the grade identical to SSS (K434) with dimensions 65 mm X 15 mm X 2 mm were selected to investigate the adhesion between steel and resin. The small area 15 mm X 15 mm at the tip of each coupon (as per standard procedure ASTM D1002) was used for preparing LSS joints. The coupons were cleaned with acetone and dried followed by the treatment with airplasma. The treatment to coupons and SSS was identical in every respect. It was anticipated that the treated surface would have interacted with air leading to Cr2O3. To avoid this, the tip area (15 mm x 15 mm) of strips from one end was immediately immersed in phenolic resin solution to avoid oxidation of Cr layer. The LSS joints were then prepared in the specially designed mould [23]. As shown in Fig. 1 (a), uniform layer of phenolic resin powder was kept between the two SS coupons. After that these two coupons were placed in the mould (Fig. 1 b) and then in compression moulding machine. Gauge Length 15 mm
Gauge Width 15 mm
Phenolic resin
Grip distance 85 mm
Overall length 115 mm
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Figure 1 (a) Dimensions of LSS specimen as per ASTM D1002 Then it was heated up to temperature of 160 °C. After reaching the desired temperature 10 bar pressure was applied and allowed it to cure. It was expected that LSS (lap shear strength) joints when tested on an INSTRON 5582, 100 kN UTM machine as per standard procedure ASTM D1002, adhesion strength of treated SSS joints would be higher compared the joints containing untreated SSS.
Male part Female part
Fixture Figure 1 (b) Molding die for LSS sample preparation 2.4 Characterization of Brake-pads Developed brake-pads were characterized for density, porosity, hardness and acetone extraction. These characterizations tests were repeated for 3 times and average results are shown in Table 4. The details of tribological pair considered for brake inertia dynamometer testing are shown in Fig 2 (a). Tribological performance of brake-pads was evaluated on full scale brake inertia dynamometer, description was already given in our earlier work [12] as per JASO C 406 testing standards [13] shown in Table 5. The schematic diagram of brake inertia dynamometer is shown in Fig. 2 (b). Brake callipers and rotor disc of Maruti® Alto car were used for tribo-testing on dynamometer.
6
Table 4 Characterization of developed brake-pads Properties Density (g/cc) Oil porosity (%) (JIS D 4418) Water porosity (%) (JIS D 4418) Acetone extraction (%) (ASTM D 494) Hardness (HRR) (ASTM D 785)
U 2.135 12.84 14.16 0.212 83
T 2.183 12.67 14.84 0.391 82
Figure 2 (a) Schematic diagram of Brake pads and disc Table 5 JASO C-406 test standard
Bedding
Speed (kmph) 65
Pre-effect Effectiveness-I Re-Burnish-I Emer. Brake test
50 50,80,100 65 80
Baseline check Fade-I Recovery-I Re-Burnish-II
50 80 50 65
Baseline check Fade-II Recovery-II
50 80 50
Description
Deceleration Disc initial (g) Temp. (ºC) 0.35 120 ºC Effectiveness-I 0.3 80 ºC 0.1 to 0.8 80 ºC 0.35 120 ºC 0.1 to 0.25 80 ºC Fade & Recovery- I 0.3 80 ºC 0.45 60(first time) 0.3 80 ºC 0.35 120 ºC Fade & Recovery- II 0.3 80 ºC 0.45 60(first time) 0.3 80 ºC
Air Blower Off
No. of brake applications 200
Off Off Off Off
10 72* 35 8
Off Off On Off
3 10 12 35
Off Off On
3 15 12 7
*Three brake applications at each speed and deceleration value.
It consists of bedding of brake pads by 200 stops at 65 kmph and 0.35 g followed by effectiveness and Fade & Recovery section. In effectiveness section temperature was kept constant and pressure-speed were varied to emerge out influence of pressure and speed on the performance of brake pads. On the other hand, in Fade & Recovery section pressure-speed were kept constant and temperature allowed to increase to emerge out the influence of temperature on performance of brake pads. In this test schedule, three brakes were applied at each pressure and speed and average of three µ values was considered for presenting the final results.
1. 2. 3. 4. 5. 6. 7. 8.
A.C Power Supply Frequency Drive D.C. Motor Main Shaft Encoder Inertia Wheels Emergency Brake Rotor Disc
9. 10. 11. 12. 13. 14. 15. 16.
Brake Calliper holding brake pads Brake fluid hose pipe Blower Compressor Filter Pneumatic pressure Sensor Hydraulic piston Cylinder Brake Fluid Receptacle
17. 18. 19. 20. 21. 22. 23. 24.
Hydraulic pressure Sensor Disc Temperature sensor Brake Pad Temp. thermocouple Torque Sensor NI 6008 Card Control Unit Electronic EPB control LabVIEW based Computer Display
Figure 2 (b) Schematic diagram of brake inertia dynamometer
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2.5 Contact angle measurement The wettability property of the SSS with the resin is an important parameter for attaining good adhesion with the binder. This was indirectly done with coupons of the same grade of SSS. The DI water contact angle measurements were carried out for untreated and plasma treated SS coupons and the contact angle images are shown in Fig 3.
T
U
Figure 3 Contact angle for untreated and plasma treated SS coupons 3. RESULTS AND DISCUSSION The results of LSS test and contact angle measurement along with physical, chemical, mechanical and tribological characterization of the pads are discussed in Figs. 3-11 while worn surface analysis is shown in Figs. 12-13.
3.1 Wettability Studies As seen in Fig. 3, contact angles for untreated and treated coupons were 75° and 45°. The remarkable decrease by 30° indicated increased hydrophilicity, which in turn indicates increase in the surface energy and hence possibility of increased adhesion with the resin. This was further confirmed from LSS studies.
3.2 LSS Results Table 6 shows the significant increase in LSS (62 %) due to plasma treatment of SS coupons as compared to untreated coupons which confirmed increase in the adhesion between SS coupons and phenolic resin due to plasma treatment. 9
Table 6 Results of LSS test on coupons SS Coupons
Lap shear strength (MPa)
Untreated
2.32
Treated
3.76
3.3 Characterization of Brake-pads Table 3 shows the results of characterization of brake-pads. There was no remarkable difference in most of the properties of U and T barring density. Density of treated SS –pads was higher (2.2 %) supporting increased bonding between SSS and resin.
3.4 Tribo-Characterization of Brake-pads Results of effectiveness performance i.e. sensitivity of friction towards pressure and speed are shown in Figs. 4 and 5 respectively. Figures. 6-10 show the results of fade and recovery (F&R) test. Wear behaviour is shown in Fig. 11. Effectiveness performance Sensitivity of µ towards pressure Figure 4 is the effectiveness graph showing changes in coefficient of friction (µ) with deceleration (0.1g–0.8g) at 3 speeds (50, 80 and 100 kmph). Salient features are as follows. •
Overall µ varied from 0.42 to 0.34 for both the brake-pads. Treated SS pads showed higher µ in general, especially when deceleration was high (beyond 0.3) than the untreated one. It could be because of stronger hold of resin on SSS due to treatment.
10
0.42
µ
0.39 (a) 50 kmph 0.36
T
U
0.42
µ
0.39 (b) 80 kmph 0.36
0.42 (c) 100 kmph
µ
0.39 0.36 0.33 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Deceleration (g) Figure 4 Variation in µ as a function of deceleration for (a) 50 kmph (b) 80 kmph and (c) 100 kmph •
Pressure fade i.e. decline in µ with pressure (deceleration in this case) mainly depended on the type of pad and speed. Both the brake-pads showed almost similar sensitivity for pressure fade at lowest speed. For higher speeds T showed superior performance.
•
When speed sensitivity at different pressure levels was investigated (moving from lowest speed to highest speed for a selected g level- as marked for 0.5g), it was observed that plasma treated SS-pad proved better than untreated SS-pad.
•
Speed fade was always higher than pressure fade.
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Sensitivity of µ towards speed Sensitivity of speed was represented in the form of speed spread (% SS). Figure 5 shows the changes in speed spread with deceleration and salient features are as follows. Speed spread is the percentage ratio of friction coefficient at higher speed to lower speed. Ideally, the slope of % SS curve should be lowest with minimum undulations. Higher value of % SS represents the better performance of FMs.
105
Speed Spread (%)
(a) Mild 100
95
90
T
Speed Spread (%)
85 105
U
(b) Severe
100
95
90
85 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Deceleration (g) Figure 5 Variation in speed spread as a function of deceleration for (a) mild conditiontransition from 50 to 80 kmph and (b) severe condition- transition from 50 to 100 kmph
12
•
Pad T showed significantly higher speed-spread (lower tendency to speed) than pad U in both mild and severe conditions.
•
The speed spread (sensitivity of µ towards speed) depended on speed transitions also. It is shown in Table 7. Table 7 Trends of speed spread % at selected g levels
Selected value of g 0.1 0.5 0.8
Performance % SS (Mild condition) trend Transition from 50-80 kmph U T 95 101 96 102 97 97
% SS (Severe condition) Transition from 50-100 kmph U T 91 95 94 94 91 92
Fade and recovery performance Figures 6 and 7 show the results of F&R test at first and second F&R cycle respectively. Deterioration in µ during fade is also visible. The smaller circles show the changes in µ in most severe conditions. With help of thermocouple located in the centre of the pad, increase in the bulk temperature was monitored and presented in these Figs. Salient observations of F&R results (Figs. 6 and 7) are shown below. 0.40
400 Fade I
T(µ) U(µ) T(Pad temp.) U(Pad temp)
Recovery I
350
µ
250 200 0.30
Pad Temp.(°C)
300 0.35
150 100 0.25
50 2
4
6
8
10
12
14
16
18
20
22
Brakings Figure 6 Sensitivity of µ for temperature for F&R I 13
400
0.40 Fade II
Recovery II
T(µ) U(µ) T(Pad temp.) U(Pad temp)
350
µ
250 200 0.30
Pad Temp.(°C)
300 0.35
150 100 0.25
50 2
4
6
8
10 12 14 16 18 20 22 24 26
Brakings Figure 7 Sensitivity of µ for temperature for F&R II •
Tendency of fading depended on fade cycle. In second cycle, it was lower than the first one, which is as per general trend (13, 15). During first cycle the µ decreased because of degradation of organic contents in the top layer. Although this layer was removed before initiation of second cycle (in built in testing schedule), thermal stresses had percolated in sub-surfaces and metallic ingredients could have been work hardened. These started contributing abrasive action during brakings in II F&R cycle, thereby compensating for the loss in µ due to the degradation of organic contents to some extent. Hence fading tendency was lower in second F&R cycle as compared to first.
•
The small circles focus on performance when temperature was highest. In first F&R cycle, pad U proved better than pad T, while in second F&R cycle the performance trend was opposite.
Figures 8 - 10 show the extracts of the data from the F&R test. Figure 8 shows fade µ and recovery µ for both cycles while Fig. 9 shows % fade ratio and % recovery ratios. Figure 10 shows rise in disc temperature during fade cycle. Lower rise in the disc temperature is desirable 14
for better performance of brake-pads. As seen from Fig. 8 the fade µ and recovery µ for pads showed following order. •
Fade µ- For F&R I – U (0.282) > T (0.276) ; For F&R II – T (0.299) > U (0.286)
•
Recovery µ- For F&R I – T (0.366) > U (0.339); For F&R II – T (0.347) > U (0.321)
Fade µ should be as equal as performance µ. Generally higher the µ (not exceeding the desired value), better is the rating of the friction-material.
0.40
0.40 T
T
(a)
Fade µ
Recovery µ
0.35
U
0.30
0.25
U
(b)
0.35
0.30
0.25 F&R-I
F&R-II
Fade & Recovery cycles
F&R-I
F&R-II
Fade & Recovery cycles
Figure 8 Fade µ and recovery µ for F&R cycle I and II Figure 9 summarizes % fade and recovery ratios (percent ratio of minimum friction coefficient to maximum friction coefficient in the corresponding cycle) of the pads and it should be higher for better performance. In general, if the original µ is high, fade µ tends to be high. The best way of comparison is % fade ratio. Based on this, performance order was as follows. •
% Fade ratio- For F&R I – U (78.8) > T (73.9)
•
% Recovery ratio- For F&R I – U (94.7) > T (89.2)
•
% Fade ratio- For F&R II – T (80.1) > U (77.2)
•
% Recovery ratio - For F&R II – U (97.2) > T (94.1)
Overall, treatment did not help for fade & recovery properties in all of the parameters.
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100
100 T
U
% Recovery Ratio
90
% Fade Ratio
T
(a)
80
70
60
(b)
U
90
80
70
60 F&R-I
F&R-II
F&R-I
Fade & Recovery cycles
F&R-II
Fade & Recovery cycles
Figure 9 % Fade ratio and % recovery ratio for F&R cycle I and II Figure 10 showed the rise in disc temperature during braking called counter-face friendliness of brake-pads. Rise in disc temperature should be as small as possible. One of the major factors responsible for the rise in disc temperature is the µ itself. Higher the µ, higher will be the frictional heat generation and higher will be the rise in temperature of the disc. Due to plasma treatment of SSS, rise in disc temperature also increased, which followed the trends in µ. 370
Max. Disc Temperature (oC)
360
T
U
350 340 330 320 310 300 290 280 F&R-I
F&R-II
Fade & Recovery cycles
Figure 10 Disc temperature rise for F&R cycle I and II
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Wear performance of brake-pads Wear was measured by weight loss method after complete JASO C 406 test. Figure 11 shows wear data on brake-pads as weight loss and volume loss. Pad T showed almost 10 % lower wear than that of pad U. Thus, plasma treatment of SSS proved successful in increasing its adhesion with the binder matrix, which led to increase in wear resistance. This was due to less amount of digging of SSS from the worn-out pads as evident from SEM analysis as discussed in next section.
4.0 Wear Volume (cc)
Weight Loss (g)
8 7 6 5 4
3.5 3.0 2.5 2.0
T
U
Brake-pads
T
Brake-pads
U
Figure 11 Wear behaviour of brake-pads Worn surface analysis Wear mechanisms are explained with the help of SEM analysis of worn surfaces of pads as shown in Fig. 12. The major feature of the micrographs is about more number of peeled off SSS from the pad surface of U, supporting higher wear than T. The de-bonding of swarf (marked) can be easily seen. The surface topography also continued to be smoother for T. Hardly any evidence could see about de-bonded particles in Fig. 12 for T.
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U
T
Figure 12 SEM micrographs of worn surfaces of pads of U and T Figures 13 show energy dispersive X-Ray analysis (EDAX) of worn pads of U and T which confirm the main ingredients of SSS (Fe, Cr) and also presence of glass fibers (Si dot maps). The increase in loosely bonded SSS on the contact surface of pad U than pad T, can be observed, which was the probable reason behind the decrease in wear resistance.
U
U
U
Cr
Fe T
T
Fe
T
Cr
Figure 13 SEM and EDAX micrographs of worn pad-surface of U and T
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4
CONCLUSIONS
For replacement of copper (threat to aquatic life), swarf of stainless steel (SSS) had proved significant potential except higher wear. To reduce the wear, SS had to have better adhesion with resin so that they cannot be easily dug out during sliding. However, thin layer of Cr2O3 on the surface of SSS creates problems in adhesion. Its conversion in elemental Cr would help to increase the adhesion. Hence in this work, plasma treatment was planned. Based on the studies on influence of plasma treatment of SSS on the tribo-performance of brake-pads, following conclusions were drawn. •
The treated coupons showed higher surface energy as supported by the decrease in contact angle. Lap shear strength (LSS) of the joints indicated that the treated surfaces showed higher adhesion with the resin.
•
Pads containing plasma treated SSS showed higher density than the untreated one supporting more adhesion with resin.
•
It also showed lower sensitivity of µ towards speed and pressure than the untreated one, which is a beneficial feature.
•
Treatment led to 10% decrease in wear. Worn surface analysis confirmed enhanced adhesion between SSS and resin. Thus, the maiden effort of plasma treatment to SSS in brake-pads proved successful for decreasing the wear.
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[13] Mahale V, Bijwe J, Sinha S. A step towards replacing copper in brake-pads by using stainless steel swarf. Wear 2019; 424–425: 133–42. [14] Peden C. H. F, Kidd K. B, Shinn N. D. Metal/metal‐oxide interfaces: A surface science approach to the study of adhesion. Journal of Vacuum Science & Technology A 1991; 9 (3): 1518-24. [15] Mahale V, Bijwe J, Sinha S. Efforts towards green friction materials. Tribol. Int. 2019; 136: 196–206. [16] Kesavan S, Shao X. Copper-free non-asbestos organic friction material. Google Patents US 2006/0151268 A1, 2006. [17] Chen H, Paul H G. Copper-free friction material for brake pads. Google Patents WO2011131227A1, 2011. [18] Zhang J Z. Copper-free friction material composition for brake pads. Google Patents CN102191016A, 2014. [19] Subramanian V. Friction material for brakes. Google Patents WO2013048627A1, 2013. [20] Han M H, Jegal J P, Park K W, Choi J H, Baik H K, Noh J H, Song K M, Lim Y S. Surface modification for adhesion enhancement of PET-laminated steel using atmospheric pressure plasma. Surf. Coatings Technol. 2007; 201(9–11): 4948–52. [21] Abdul Samad M, Satyanarayana N, Sinha S K. Tribology of UHMWPE film on airplasma treated tool steel and the effect of PFPE overcoat. Surf. Coatings Technol. 2010; 204(9–10): 1330–38. [22] Samad M A, Satyanarayana N, Sinha S K. Effect of Air–Plasma Pre-treatment of Si Substrate on Adhesion Strength and Tribological Properties of a UHMWPE Film. J. Adhes. Sci. Technol. 2010; 24(15–16): 2557–70. [23] Hutchings I M. Tribology: Friction and Wear of Engineering Materials. Edward Arnold 1992. [24] Kadiyala A K, Bijwe J. Investigations on performance and failure mechanisms of high temperature thermoplastic polymers as adhesives. Int. J. Adhes. Adhes. 2016; 70: 90– 101.
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Research Highlights: •
Exploration of plasma treatment on Stainless-steel swarfs (SSS) for application in Copper free friction material was first time reported.
•
Comparative study of influence of untreated and plasma treated SSS on triboperformance of brake-pads was also first time reported.
•
Plasma treatment of SSS proved beneficial for reduction in wear of brake pads.
Declaration of Interest I here by certify that, we do not have any conflict of interest Jayashree Bijwe
S0301-679X(19)30625-5
DOI:
https://doi.org/10.1016/j.triboint.2019.106111
Reference:
JTRI 106111
To appear in:
Tribology International
Received Date: 15 August 2019 Revised Date:
5 December 2019
Accepted Date: 7 December 2019
Please cite this article as: Mahale V, Bijwe J, Exploration of plasma treated stainless steel swarf to reduce the wear of copper-free brake-pads, Tribology International (2020), doi: https://doi.org/10.1016/ j.triboint.2019.106111. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
CRediT author statement
Vishal Mahale: Conceptualization, Methodology, Experimentation, Writing- Original draft preparation, Visualization, Investigation. Jayashree Bijwe: Conceptualization, Supervision, Investigation, Reviewing and Editing.
Exploration of Plasma Treated Stainless Steel Swarf to reduce the wear of Copper-free brake-pads Vishal Mahale*; Jayashree Bijwe1 Industrial Tribology Machine Dynamics and Maintenance Engineering Centre, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India. *Current address- Rane Brake-lining Limited, Chennai 600 058, India.
ABSTRACT In recent few years, the environmental issues especially related to the aquatic life are pushing the Friction Industry to develop NAO FMs (non-asbestos organic friction materials) without Cu. Stainless-steel swarf (SSS) having good corrosion resistance was tried as one of the replacing ingredients for Cu in FMs in Authors’ laboratory in earlier work. It proved to have a great potential for Cu replacement almost in all the performance properties but at the cost of slightly lower wear resistance and hence the theme of current research was to enhance the wear resistance capability of SSS. Hence, in the present study SSS were treated with air-plasma and used in brake-pads. Thus, two types of brake-pads with the identical composition but differing in type of SSS 10 wt. % (treated and untreated) were tribo-evaluated on a full-scale brake inertia dynamometer and treated SSS containing pads exhibited higher wear resistance (approx. 10 %) confirming success of the treatment, first time explored for the friction materials. Enhanced adhesion with resin was the main cause for the improved tribo-performance. KEYWORDS: - Non-asbestos organic (NAO); Copper-free; Plasma treatment; Stainless steel swarf. 1. INTRODUCTION Friction material (FM) is a heart of braking system which comprise of four major classes of ingredients viz. resin as binder, reinforcing fibers, functional fillers and space fillers. All these ingredients are added in FMs to achieve amalgam of desired performance properties. Some of the major desirable properties of FMs are; moderate and stable value of µ which will be least sensitive to the various operating conditions, resistance (as high as possible) to fade (FR), wear (WR), squeal and judder along with good counter-face friendliness and recovery performance. The WR due to economical point of view is a real concern to the Industries. Consistently a lot of
1
Corresponding author. Tel.: +91 11 26591280. E-mail address: [email protected] (J. Bijwe)
1
research is being focused on the wear reduction of FMs either by adding appropriate ingredients or deleting those, which are contribute more to the wear of FMs [1-14]. Another major concern about the current non-asbestos organic (NAO) FMs is related to the copper as an important ingredient. The environmental issues especially related to the aquatic life in recent few years are pushing the Industry to develop NAO FMs without Cu. A lot of research is done on this aspect [7-13] and few patents are also available [16-19]. Authors have first time reported on the successful exploration of stainless steel swarf (SSS) as a substitution for Copper [13]. It proved to be quite competitive with Cu except wear resistance, which was lower by almost 10 %. Extracts of the finding are shown in Table 1. Table 1. % increase/ decrease in performance properties of brake-pads containing SSS/ Cu powder (10 wt. %) compared to the metal-free brake pad [13]. Brake pads → Performance Properties ↓ Density Hardness Thermal conductivity µ @ 80 kmph, 0.5 g % fade ratio Disc temperature Wear volume
S10
C10
%↑
%↓
%↑
%↓
14 2 42 3.7 17 8 -
0.5
27 6 68 5 17 8 -
9
The worn surface analysis of brake-pads indicated easy removal of SSS. It is well known that very high resistance of SS to the corrosion is due to its instantaneously generated Cr2O3 layer on the surface [13]. It is also known that metal oxides are formed consistent with minimizing surface energy [14] and hence do not have high adhesion to other surfaces or phenolic resin in the case of FMs. Hence, the particles get dug out very easily during wear- process. In case of Cu particles, such strong and homogeneous layer of CuO does not exist. It has hence better adhesion with resin and hence these particles offer higher wear resistance. If by any way, the adhesion of SSS to the resin is increased, the wear resistance may also increase. Such efforts, however, are not reported for FMs in the literature. Some researchers tried plasma treatment for increasing the surface adhesion of the stainless steel with the selected matrix such as Polyethylene terephthalate (PET) polymer [20]. Samad et al.
2
[21] used air-plasma treatment on steel substrate to increase the adhesion with Ultra-High Molecular Weight Polyethylene (UHMWPE) and reported that wear life was enhanced by 10-12 times for plasma treated steel sample compared to the untreated sample. In further studies Samad et al. [22] compared the effect of piranha and air-plasma treatment on Si substrate to increase the adhesion with UHMWPE coating and reported that air-plasma treatment led to 25 times higher wear life than piranha treated samples. The claimed benefits of air-plasma treatment were about its simplicity, cost effectiveness and it does not use harmful chemicals such as phosphorous and sulphur unlike other pre-treatment procedures, and it can be easily adapted in industrial applications. Hence, in the present study SS was treated with air-plasma and used in FM for brake-pad. The SSS were exposed to plasma under vacuum for 500 seconds using a RF power of 18W as discussed in subsequent section. Thus, two multi-ingredient brake-pads with the identical composition, one with 10 wt. % plasma treated SSS and other with same amount of untreated SSS were developed to explore the influence plasma treatment on the performance of pads and tribo-evaluated on a full-scale brake inertia dynamometer and the results are presented in subsequent section. 2. EXPERIMENTAL DETAILS 2.1. Selection of Materials The theme ingredient SS swarf-SSS (K434) were procured from Kasturi Metals Pvt. Ltd. Amravati, India. Stainless steel (K434) coupons of dimensions 65 mm X 15 mm X 2 mm with 0.1 mm Ra value were used as substrates for measuring Lap shear strength (LSS). Two Cu-free FMs were developed in the form of brake-pads with identical composition and differing only in the plasma treated and untreated SS swarf. Composite designation details are given in Table 2. Table 2 Compositions and designations of FMs Ingredients by Wt.% Identical Composition* SS Swarf (SSS) Barite
Designations of FMs U- Untreated SSS
T- Plasma treated SSS
60 10 30
60 10 30
*Identical composition – binder: fibres: friction modifiers: functional fillers – 6:16:12:26
3
Brake-pad composition consisted of several ingredients viz. 6 wt.% straight phenolic resin as a binder, 10 wt. % SSS, 16 wt.% fibers (rockwool, aramid, Polyacrylonitrile), 12 wt.% friction modifiers (alumina and thermo-graphite), 26 wt.% functional fillers (cashew dust, promaxon, hydrated lime, crumb rubber, vermiculite) and 30 wt.% BaSO4. U and T denote the brake-pads with untreated and plasma treated SSS (10 wt. %) respectively. Brake-pads were prepared in the authors’ laboratory as per procedure discussed in the earlier research papers [12-13]. Standard mixing schedule [13] as shown in Table 3 was used for homogeneous mixing of ingredients. Mixture was cold pressed in the pre-former with the shape of brake-pads. Then hot pressing was done at 140 bars and 160o C for 9 minutes. Finally, pads were post cured for 120 °C for 2 hours and 160 °C for 5 hours. Table 3 Schedule of mixing of ingredients in plough shear mixer Steps Ingredients
Mixing duration (minutes)
A
Aramid
7
B
PAN + Rockwool
4
1. Aramid + BaSO4 2. Previous + (PAN + Rockwool)
5 (Batch-I)
3. Previous + SS swarf 4. Mixing of remaining ingredients 5. (Batch I) + (Batch-II) Total Duration
5 3
(Batch-II)
4 2 30 minutes
2.2. Plasma Treatment Procedure The plasma cleaning procedure included the following steps. SSS were cleaned with acetone and dried in the oven. The samples were then air-plasma treated using Harrick Plasma Cleaner (PDC32G-2). The SSS were exposed to plasma under vacuum for 500 seconds using a RF power of 18 W. Treatment time and power was chosen to keep total 9000 J energy constant as mentioned in the literature [21]. It was expected that the plasma treatment in vacuum would disintegrate Cr2O3 in elemental Cr on the surface. The metals have significantly higher surface energy compared to their oxides [23]. Higher surface energy of SSS with Cr on surface would lead to higher adhesion with the resin. However, exposure of treated SSS to the environment would instantaneously 4
convert the Cr on surface to Cr2O3. Hence, air contact had to be avoided. To avoid re-formation of Cr2O3, plasma treated SSS from the chamber were instantaneously dipped into the phenolic resin solution (4 g resin in 100 ml acetone) so that a thin film of resin would cover the treated surfaces efficiently restricting to oxidation. Then acetone was allowed to evaporate in the oven and resin coated SSS were used for preparing the brake-pads. Care was taken to keep the total 6 wt. % of resin in both types of pads (U and T). 2.3. Sample Preparation for Lap Shear Strength (LSS) Analysis It was necessary to prove if this treatment increases adhesion of resin with the SSS. Hence, another experiment was designed which would justify the motive of treatment. Stainless-steel coupons of the grade identical to SSS (K434) with dimensions 65 mm X 15 mm X 2 mm were selected to investigate the adhesion between steel and resin. The small area 15 mm X 15 mm at the tip of each coupon (as per standard procedure ASTM D1002) was used for preparing LSS joints. The coupons were cleaned with acetone and dried followed by the treatment with airplasma. The treatment to coupons and SSS was identical in every respect. It was anticipated that the treated surface would have interacted with air leading to Cr2O3. To avoid this, the tip area (15 mm x 15 mm) of strips from one end was immediately immersed in phenolic resin solution to avoid oxidation of Cr layer. The LSS joints were then prepared in the specially designed mould [23]. As shown in Fig. 1 (a), uniform layer of phenolic resin powder was kept between the two SS coupons. After that these two coupons were placed in the mould (Fig. 1 b) and then in compression moulding machine. Gauge Length 15 mm
Gauge Width 15 mm
Phenolic resin
Grip distance 85 mm
Overall length 115 mm
5
Figure 1 (a) Dimensions of LSS specimen as per ASTM D1002 Then it was heated up to temperature of 160 °C. After reaching the desired temperature 10 bar pressure was applied and allowed it to cure. It was expected that LSS (lap shear strength) joints when tested on an INSTRON 5582, 100 kN UTM machine as per standard procedure ASTM D1002, adhesion strength of treated SSS joints would be higher compared the joints containing untreated SSS.
Male part Female part
Fixture Figure 1 (b) Molding die for LSS sample preparation 2.4 Characterization of Brake-pads Developed brake-pads were characterized for density, porosity, hardness and acetone extraction. These characterizations tests were repeated for 3 times and average results are shown in Table 4. The details of tribological pair considered for brake inertia dynamometer testing are shown in Fig 2 (a). Tribological performance of brake-pads was evaluated on full scale brake inertia dynamometer, description was already given in our earlier work [12] as per JASO C 406 testing standards [13] shown in Table 5. The schematic diagram of brake inertia dynamometer is shown in Fig. 2 (b). Brake callipers and rotor disc of Maruti® Alto car were used for tribo-testing on dynamometer.
6
Table 4 Characterization of developed brake-pads Properties Density (g/cc) Oil porosity (%) (JIS D 4418) Water porosity (%) (JIS D 4418) Acetone extraction (%) (ASTM D 494) Hardness (HRR) (ASTM D 785)
U 2.135 12.84 14.16 0.212 83
T 2.183 12.67 14.84 0.391 82
Figure 2 (a) Schematic diagram of Brake pads and disc Table 5 JASO C-406 test standard
Bedding
Speed (kmph) 65
Pre-effect Effectiveness-I Re-Burnish-I Emer. Brake test
50 50,80,100 65 80
Baseline check Fade-I Recovery-I Re-Burnish-II
50 80 50 65
Baseline check Fade-II Recovery-II
50 80 50
Description
Deceleration Disc initial (g) Temp. (ºC) 0.35 120 ºC Effectiveness-I 0.3 80 ºC 0.1 to 0.8 80 ºC 0.35 120 ºC 0.1 to 0.25 80 ºC Fade & Recovery- I 0.3 80 ºC 0.45 60(first time) 0.3 80 ºC 0.35 120 ºC Fade & Recovery- II 0.3 80 ºC 0.45 60(first time) 0.3 80 ºC
Air Blower Off
No. of brake applications 200
Off Off Off Off
10 72* 35 8
Off Off On Off
3 10 12 35
Off Off On
3 15 12 7
*Three brake applications at each speed and deceleration value.
It consists of bedding of brake pads by 200 stops at 65 kmph and 0.35 g followed by effectiveness and Fade & Recovery section. In effectiveness section temperature was kept constant and pressure-speed were varied to emerge out influence of pressure and speed on the performance of brake pads. On the other hand, in Fade & Recovery section pressure-speed were kept constant and temperature allowed to increase to emerge out the influence of temperature on performance of brake pads. In this test schedule, three brakes were applied at each pressure and speed and average of three µ values was considered for presenting the final results.
1. 2. 3. 4. 5. 6. 7. 8.
A.C Power Supply Frequency Drive D.C. Motor Main Shaft Encoder Inertia Wheels Emergency Brake Rotor Disc
9. 10. 11. 12. 13. 14. 15. 16.
Brake Calliper holding brake pads Brake fluid hose pipe Blower Compressor Filter Pneumatic pressure Sensor Hydraulic piston Cylinder Brake Fluid Receptacle
17. 18. 19. 20. 21. 22. 23. 24.
Hydraulic pressure Sensor Disc Temperature sensor Brake Pad Temp. thermocouple Torque Sensor NI 6008 Card Control Unit Electronic EPB control LabVIEW based Computer Display
Figure 2 (b) Schematic diagram of brake inertia dynamometer
8
2.5 Contact angle measurement The wettability property of the SSS with the resin is an important parameter for attaining good adhesion with the binder. This was indirectly done with coupons of the same grade of SSS. The DI water contact angle measurements were carried out for untreated and plasma treated SS coupons and the contact angle images are shown in Fig 3.
T
U
Figure 3 Contact angle for untreated and plasma treated SS coupons 3. RESULTS AND DISCUSSION The results of LSS test and contact angle measurement along with physical, chemical, mechanical and tribological characterization of the pads are discussed in Figs. 3-11 while worn surface analysis is shown in Figs. 12-13.
3.1 Wettability Studies As seen in Fig. 3, contact angles for untreated and treated coupons were 75° and 45°. The remarkable decrease by 30° indicated increased hydrophilicity, which in turn indicates increase in the surface energy and hence possibility of increased adhesion with the resin. This was further confirmed from LSS studies.
3.2 LSS Results Table 6 shows the significant increase in LSS (62 %) due to plasma treatment of SS coupons as compared to untreated coupons which confirmed increase in the adhesion between SS coupons and phenolic resin due to plasma treatment. 9
Table 6 Results of LSS test on coupons SS Coupons
Lap shear strength (MPa)
Untreated
2.32
Treated
3.76
3.3 Characterization of Brake-pads Table 3 shows the results of characterization of brake-pads. There was no remarkable difference in most of the properties of U and T barring density. Density of treated SS –pads was higher (2.2 %) supporting increased bonding between SSS and resin.
3.4 Tribo-Characterization of Brake-pads Results of effectiveness performance i.e. sensitivity of friction towards pressure and speed are shown in Figs. 4 and 5 respectively. Figures. 6-10 show the results of fade and recovery (F&R) test. Wear behaviour is shown in Fig. 11. Effectiveness performance Sensitivity of µ towards pressure Figure 4 is the effectiveness graph showing changes in coefficient of friction (µ) with deceleration (0.1g–0.8g) at 3 speeds (50, 80 and 100 kmph). Salient features are as follows. •
Overall µ varied from 0.42 to 0.34 for both the brake-pads. Treated SS pads showed higher µ in general, especially when deceleration was high (beyond 0.3) than the untreated one. It could be because of stronger hold of resin on SSS due to treatment.
10
0.42
µ
0.39 (a) 50 kmph 0.36
T
U
0.42
µ
0.39 (b) 80 kmph 0.36
0.42 (c) 100 kmph
µ
0.39 0.36 0.33 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Deceleration (g) Figure 4 Variation in µ as a function of deceleration for (a) 50 kmph (b) 80 kmph and (c) 100 kmph •
Pressure fade i.e. decline in µ with pressure (deceleration in this case) mainly depended on the type of pad and speed. Both the brake-pads showed almost similar sensitivity for pressure fade at lowest speed. For higher speeds T showed superior performance.
•
When speed sensitivity at different pressure levels was investigated (moving from lowest speed to highest speed for a selected g level- as marked for 0.5g), it was observed that plasma treated SS-pad proved better than untreated SS-pad.
•
Speed fade was always higher than pressure fade.
11
Sensitivity of µ towards speed Sensitivity of speed was represented in the form of speed spread (% SS). Figure 5 shows the changes in speed spread with deceleration and salient features are as follows. Speed spread is the percentage ratio of friction coefficient at higher speed to lower speed. Ideally, the slope of % SS curve should be lowest with minimum undulations. Higher value of % SS represents the better performance of FMs.
105
Speed Spread (%)
(a) Mild 100
95
90
T
Speed Spread (%)
85 105
U
(b) Severe
100
95
90
85 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Deceleration (g) Figure 5 Variation in speed spread as a function of deceleration for (a) mild conditiontransition from 50 to 80 kmph and (b) severe condition- transition from 50 to 100 kmph
12
•
Pad T showed significantly higher speed-spread (lower tendency to speed) than pad U in both mild and severe conditions.
•
The speed spread (sensitivity of µ towards speed) depended on speed transitions also. It is shown in Table 7. Table 7 Trends of speed spread % at selected g levels
Selected value of g 0.1 0.5 0.8
Performance % SS (Mild condition) trend Transition from 50-80 kmph U T 95 101 96 102 97 97
% SS (Severe condition) Transition from 50-100 kmph U T 91 95 94 94 91 92
Fade and recovery performance Figures 6 and 7 show the results of F&R test at first and second F&R cycle respectively. Deterioration in µ during fade is also visible. The smaller circles show the changes in µ in most severe conditions. With help of thermocouple located in the centre of the pad, increase in the bulk temperature was monitored and presented in these Figs. Salient observations of F&R results (Figs. 6 and 7) are shown below. 0.40
400 Fade I
T(µ) U(µ) T(Pad temp.) U(Pad temp)
Recovery I
350
µ
250 200 0.30
Pad Temp.(°C)
300 0.35
150 100 0.25
50 2
4
6
8
10
12
14
16
18
20
22
Brakings Figure 6 Sensitivity of µ for temperature for F&R I 13
400
0.40 Fade II
Recovery II
T(µ) U(µ) T(Pad temp.) U(Pad temp)
350
µ
250 200 0.30
Pad Temp.(°C)
300 0.35
150 100 0.25
50 2
4
6
8
10 12 14 16 18 20 22 24 26
Brakings Figure 7 Sensitivity of µ for temperature for F&R II •
Tendency of fading depended on fade cycle. In second cycle, it was lower than the first one, which is as per general trend (13, 15). During first cycle the µ decreased because of degradation of organic contents in the top layer. Although this layer was removed before initiation of second cycle (in built in testing schedule), thermal stresses had percolated in sub-surfaces and metallic ingredients could have been work hardened. These started contributing abrasive action during brakings in II F&R cycle, thereby compensating for the loss in µ due to the degradation of organic contents to some extent. Hence fading tendency was lower in second F&R cycle as compared to first.
•
The small circles focus on performance when temperature was highest. In first F&R cycle, pad U proved better than pad T, while in second F&R cycle the performance trend was opposite.
Figures 8 - 10 show the extracts of the data from the F&R test. Figure 8 shows fade µ and recovery µ for both cycles while Fig. 9 shows % fade ratio and % recovery ratios. Figure 10 shows rise in disc temperature during fade cycle. Lower rise in the disc temperature is desirable 14
for better performance of brake-pads. As seen from Fig. 8 the fade µ and recovery µ for pads showed following order. •
Fade µ- For F&R I – U (0.282) > T (0.276) ; For F&R II – T (0.299) > U (0.286)
•
Recovery µ- For F&R I – T (0.366) > U (0.339); For F&R II – T (0.347) > U (0.321)
Fade µ should be as equal as performance µ. Generally higher the µ (not exceeding the desired value), better is the rating of the friction-material.
0.40
0.40 T
T
(a)
Fade µ
Recovery µ
0.35
U
0.30
0.25
U
(b)
0.35
0.30
0.25 F&R-I
F&R-II
Fade & Recovery cycles
F&R-I
F&R-II
Fade & Recovery cycles
Figure 8 Fade µ and recovery µ for F&R cycle I and II Figure 9 summarizes % fade and recovery ratios (percent ratio of minimum friction coefficient to maximum friction coefficient in the corresponding cycle) of the pads and it should be higher for better performance. In general, if the original µ is high, fade µ tends to be high. The best way of comparison is % fade ratio. Based on this, performance order was as follows. •
% Fade ratio- For F&R I – U (78.8) > T (73.9)
•
% Recovery ratio- For F&R I – U (94.7) > T (89.2)
•
% Fade ratio- For F&R II – T (80.1) > U (77.2)
•
% Recovery ratio - For F&R II – U (97.2) > T (94.1)
Overall, treatment did not help for fade & recovery properties in all of the parameters.
15
100
100 T
U
% Recovery Ratio
90
% Fade Ratio
T
(a)
80
70
60
(b)
U
90
80
70
60 F&R-I
F&R-II
F&R-I
Fade & Recovery cycles
F&R-II
Fade & Recovery cycles
Figure 9 % Fade ratio and % recovery ratio for F&R cycle I and II Figure 10 showed the rise in disc temperature during braking called counter-face friendliness of brake-pads. Rise in disc temperature should be as small as possible. One of the major factors responsible for the rise in disc temperature is the µ itself. Higher the µ, higher will be the frictional heat generation and higher will be the rise in temperature of the disc. Due to plasma treatment of SSS, rise in disc temperature also increased, which followed the trends in µ. 370
Max. Disc Temperature (oC)
360
T
U
350 340 330 320 310 300 290 280 F&R-I
F&R-II
Fade & Recovery cycles
Figure 10 Disc temperature rise for F&R cycle I and II
16
Wear performance of brake-pads Wear was measured by weight loss method after complete JASO C 406 test. Figure 11 shows wear data on brake-pads as weight loss and volume loss. Pad T showed almost 10 % lower wear than that of pad U. Thus, plasma treatment of SSS proved successful in increasing its adhesion with the binder matrix, which led to increase in wear resistance. This was due to less amount of digging of SSS from the worn-out pads as evident from SEM analysis as discussed in next section.
4.0 Wear Volume (cc)
Weight Loss (g)
8 7 6 5 4
3.5 3.0 2.5 2.0
T
U
Brake-pads
T
Brake-pads
U
Figure 11 Wear behaviour of brake-pads Worn surface analysis Wear mechanisms are explained with the help of SEM analysis of worn surfaces of pads as shown in Fig. 12. The major feature of the micrographs is about more number of peeled off SSS from the pad surface of U, supporting higher wear than T. The de-bonding of swarf (marked) can be easily seen. The surface topography also continued to be smoother for T. Hardly any evidence could see about de-bonded particles in Fig. 12 for T.
17
U
T
Figure 12 SEM micrographs of worn surfaces of pads of U and T Figures 13 show energy dispersive X-Ray analysis (EDAX) of worn pads of U and T which confirm the main ingredients of SSS (Fe, Cr) and also presence of glass fibers (Si dot maps). The increase in loosely bonded SSS on the contact surface of pad U than pad T, can be observed, which was the probable reason behind the decrease in wear resistance.
U
U
U
Cr
Fe T
T
Fe
T
Cr
Figure 13 SEM and EDAX micrographs of worn pad-surface of U and T
18
4
CONCLUSIONS
For replacement of copper (threat to aquatic life), swarf of stainless steel (SSS) had proved significant potential except higher wear. To reduce the wear, SS had to have better adhesion with resin so that they cannot be easily dug out during sliding. However, thin layer of Cr2O3 on the surface of SSS creates problems in adhesion. Its conversion in elemental Cr would help to increase the adhesion. Hence in this work, plasma treatment was planned. Based on the studies on influence of plasma treatment of SSS on the tribo-performance of brake-pads, following conclusions were drawn. •
The treated coupons showed higher surface energy as supported by the decrease in contact angle. Lap shear strength (LSS) of the joints indicated that the treated surfaces showed higher adhesion with the resin.
•
Pads containing plasma treated SSS showed higher density than the untreated one supporting more adhesion with resin.
•
It also showed lower sensitivity of µ towards speed and pressure than the untreated one, which is a beneficial feature.
•
Treatment led to 10% decrease in wear. Worn surface analysis confirmed enhanced adhesion between SSS and resin. Thus, the maiden effort of plasma treatment to SSS in brake-pads proved successful for decreasing the wear.
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Research Highlights: •
Exploration of plasma treatment on Stainless-steel swarfs (SSS) for application in Copper free friction material was first time reported.
•
Comparative study of influence of untreated and plasma treated SSS on triboperformance of brake-pads was also first time reported.
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Plasma treatment of SSS proved beneficial for reduction in wear of brake pads.
Declaration of Interest I here by certify that, we do not have any conflict of interest Jayashree Bijwe