Tribological and mechanical properties of pine needle fiber reinforced friction composites under dry sliding conditions

Abstract

Pine needle fibers were pretreated with alkali, and then mixed with other raw materials to fabricate pine needle fiber reinforced friction composites through compression moulding. The effects of pine needle fiber content on the tribological properties of the friction composites were tested using a friction material tester at constant speed. Experimental results showed that the friction coefficient of the pine needle fiber reinforced friction composites was very stable and markedly fade was not obvious compared with specimen FC0 (containing 0 wt% pine needle fibers); the wear rates of the friction composites generally increased with the increase of temperature and were significantly influenced by the test temperature. The wear rate of specimen FC7 (containing 7 wt% of pine needle fibres) was the lowest compared with that of other specimens at each temperature, except for that when the temperatures were about 200 °C. Morphologies of wear surfaces of pine needle fiber reinforced friction composites were observed using scanning electron microscopy (SEM) and the friction characteristics were analyzed. The results showed that the worn surface of specimen FC7 was smoother compared with that of specimen FC0.

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1. Introduction

Friction composites are important parts of brakes, clutches and friction gearings for all types of vehicles.¹ They directly relate to the reliability and stability of the operation systems.² Therefore, they have to show stable friction coefficients and low wear rates over wide ranges of operating conditions, as well as stability at higher temperatures.^3,4 They should be noiseless and without vibrations during working, and be composed of ecologically friendly components. To meet these requirements, friction composites are multi-components and often composed of more than 10 components. There are fibres, abrasives materials, lubricants, space fillers, property modifiers, and polymer binders.^5–8 The reinforced fibres play a critical role in enhancing the mechanical strength, avoiding mechanical damage, i.e. crack and fracture, thermal stability, and friction and wear properties of the friction composites.^9–13

Natural fibres¹⁴ have found its way to the friction composites industry due to asbestos fibre has adverse effect on human health such as asbestosis, mesothelioma and lung cancer.¹⁵ Meanwhile, the consumption of petroleum resources, plastic disposal problems and emissions during incineration along with increasing environmental regulations also lead to increased interest in replacing the conventional synthetic fibre with natural fibre.^16,17 Natural fibers offer some distinctive advantages like low cost, light weight, nonirritant to skin and/or respiratory system, abundant, renewable and biodegradable nature, etc. They are the best choice to be used as components in different kind of friction composites.¹⁸ During the last few years, several research works have been done to study the effect of natural fiber type, content, length and orientation on the tribological performance¹⁹ of friction composites. Dwivedi et al.²⁰ studied the effect of fibre orientation and applied load on tribological behavior of jute fibre reinforced polyester composites under dry sliding condition. The study concludes that the normal oriented samples give higher wear resistance and the coefficient of friction decreased with the increase of applied load. Nirmal et al.²¹ investigated the effect of betelnut fibres treatment and contact conditions on adhesive wear and frictional performance of polyester composites under dry/wet contact conditions using a block-on-disc machine. The results showed that the wear and frictional performance of the composite were enhanced under wet contact conditions by about 54% and 95% compared to that under dry contact conditions, and the composite exhibited high wear performance in anti-parallel orientation under both dry/wet contact conditions. Fu et al.²² developed an eco-friendly brake friction composite containing flax fibers. The study concluded that the optimized amount of flax fibers in the composites is 5.6 vol%, and the flax fibers can stabilize the friction coefficient and improve the wear rate at high temperature in the composites. Ma et al.²³ studied the friction and wear characteristics of bamboo fiber reinforced friction materials with different contents. This work revealed that the wear rates of the bamboo fiber reinforced friction materials with 3 wt% fiber were lower than that of others, and the carbonized bamboo fiber can reduce the specific wear rate and the noise and provide stable friction coefficient. Bajpai²⁴ investigated the tribological behavior of natural fiber (nettle, grewia optiva and sisal) reinforced polylactide composites. The study concluded that experimental results indicated that incorporation of natural fiber mats into polylactide matrix significantly improved the wear behavior of neat polymer. There was 10–44% reduction in friction coefficient and more than 70% reduction in specific wear rate of developed composites as compared to neat polylactide.

Based on above advantages and opportunities of natural fibres, there is a need to further investigate the tribological properties of other kinds of natural fibres. Therefore, the present research work was planned for developing a new natural fibre (pine needle fiber) reinforced friction composite and studying the friction and wear performance of these composites under dry sliding conditions. The objective of this work aimed to investigate the effects of fiber content on tribological and mechanical properties of pine needle fiber reinforced friction composites. The wear mechanism of the friction composites was discussed based on the morphologies of the worn surfaces obtained using the scanning electron microscopy (SEM).

2. Experimental design and methods

2.1 Preparation of pine needle fiber

Mechanical properties of friction composites based on natural fibres strongly depend on the interface adhesion between the fibres and substrate.^16,25–28 In order to enhance interaction between fiber and substrate, many researchers reported that it is significantly beneficial to treat fibers with a proper technique such as bleaching, acetylation and alkali treatment prior to their use in composites.^27,29–34 In this study, we treated fibers with alkali treatment. The pine needle fibers are in abundance, in particular in the Northeast of China. Before using them as the friction composites, the pine needle fibers were treated in such way that they were firstly dipped into a mixture of HCHO and C₆H₆ (1 : 1) for 24 h at 25 °C, then steeped in NaOH (2 wt%) solution for 2 h at 25 °C, rinsed with distilled water and neutralized with H₂SO₄ (1 wt%) for 30 min, and steeped in distilled water for 10 min at 25 °C;³⁵ dried the fibres and make heat treatment in an oven (JF980B, Changchun, China) at 100 °C for 4 h. Finally, the length of pine needle fiber amounted to 10 mm with diameter of about 1 mm.

2.2 Preparation of the specimens

Raw materials and their relative mass fractions of ingredients used for preparation of friction composites specimen are shown in Table 1 and in Table 2 respectively. The reinforced fibers, friction modifiers and fillers were mixed carefully using a blender (JF805R, Changchun, China) for 6 min. Then the mixtures were molded by compression molder equipment (JFY50, Changchun, China) for 30 min at 160 °C under 40 Mpa. In order to ensure resin completely cured, eliminate the residual stress and remove a spot of residual volatiles, the mixed materials were dealt with heat treatment after hot pressing.³⁶ The friction composites were post-treated at 140 °C for 1 h, at 160 °C for 3 h, and at 180 °C for 6 h continually in this study (Fig. 1). The specimens with standard size of 2.5 cm × 2.5 cm × 0.6 cm were prepared for the friction and wear tests. Five kinds of friction composites containing 0 wt%, 3 wt%, 5 wt%, 7 wt%, 9 wt% pine needle fibers were designated as FC0, FC3, FC5, FC7, FC9, respectively.

Table 1 Raw materials used for experiments

Raw materials	Trademark	Manufacturer
Compound mineral fibre	2–15	Hennian Technology & Trade Co. Ltd.
Phenolic resin	6818	Jinan Shengquan Hepworth Chem Co. Ltd.
Vermiculite	200 mesh	Hebei Jinjian Mining Industry Co. Ltd.
Porous iron powder	60 mesh	Beijing Jinke Composite Materials Co. Ltd.
BaSO4	Industrial grade	Henshui Zhongcheng Friction Materials Co. Ltd.
Petroleum coke	150 mesh	Changzhou Wujin Special Fibers Co. Ltd.
Artificial graphite	Flake type	Shanghai Taizhi Carbon Co. Ltd.
Alumina	300 mesh	Tianjin Rgent Chemicals Co. Ltd.
Antimony sulfide	100 mesh	Shanghai Danxu Trade Co. Ltd.
Friction powder	Industrial grade	Haiyan Huaqiang Resin Co. Ltd.
Carbon black	N115	Hebei Longxing Co. Ltd.
Pine needle fibers	Self-made

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Table 2 The mass fractions of raw ingredients used for friction composites (wt%)

Raw materials	Samples
	FC0	FC3	FC5	FC7	FC9
Compound mineral fibre	20	19.4	19	18.6	18.2
Phenolic resin	15	14.55	14.25	13.95	13.65
Vermiculite	5	4.85	4.75	4.65	4.55
Porous iron powder	12	11.64	11.4	11.16	10.92
BaSO4	20	19.4	19	18.6	18.2
Petroleum coke	6	5.82	5.7	5.58	5.46
Artificial graphite	8	7.76	7.6	7.44	7.28
Alumina	7	6.79	6.65	6.51	6.37
Antimony sulfide	3	2.91	2.85	2.79	2.73
Friction powder	1	0.97	0.95	0.93	0.91
Carbon black	3	2.91	2.85	2.79	2.73
Pine needle fibers	0	3	5	7	9

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Fig. 1 Heat treatment process for preparation of friction composites.

2.3 Friction and wear tests

Friction tests were performed on a friction material tester (JF150D-II, Changchun, China) with a constant speed (average linear sliding speed of 7.45 m s⁻¹) and pressure of 0.98 Mpa in accordance to China national standard GB5763-2008.³⁷ A cast iron (HT250) rotating disc with hardness of HB180 to HB220 was used as counterpart. The schematic of friction-wear mode is shown in Fig. 2. The disc rotation was kept fixed at 480 rpm. The rotating disc was rotated for 5000 revolution at each testing temperatures of 100, 150, 200, 250, 300 and 350 °C, and the friction coefficient (μ) and specific wear rate [V, cm³ (N m)⁻¹] were measured accordingly during the friction tests. The volume wear rate was defined and calculated as followed:where R is the horizon distance between the centers of specimen and the rotating disk (R = 0.15 m), N is the number of rotation of the disk during tests (N = 5000), A is the friction area of the specimen (A = 6.25 cm²), d₁ and d₂ are the average thickness of specimen before and after experiment (cm), respectively, and ƒ is mean value of the force due to sliding friction.

Fig. 2 Schematic of friction-wear mode.³⁸

Density of each testing sample was measured using an electronic balance (MP-5002, Shanghai, China). Rockwell hardness values of the friction composites were measured using a Rockwell hardness tester (HRSS-150, Shanghai, China) in accordance to the standard methods of China National Standards (CNS) 2114. Impact strength was measured using an impact testing machine (XJ-40A, Wuxi, China). The surface roughnesses of untreated and treated pine needle fiber were measured using a hommel roughness tester (MarSurf LD120, Mahr, German). The fiber surface and the worn surface morphology of tested friction composites reinforced with pine needle fiber were characterized using the scanning electron microscope (EVO-18, ZEISS, German). The SEM images were obtained using back-scattered electrons at operating voltage of 25 kV.

3. Results and discussion

3.1 Surface morphologies of pine needle fibers

The surface roughnesses of untreated and treated pine needle fiber were measured using a hommel roughness tester (MarSurf LD120, Mahr, German). The average roughness profile of the untreated pine needle fiber was R_a = 0.294 and that of treated pine needle fiber was R_a = 1.402. Surface morphologies of the pine needle fiber before and after the treatment are shown in Fig. 3(a) and (b). It can be seen from the Fig. 3 that the surface of fibre with alkali treatment is rougher than that of untreated fibre. Meanwhile, some squamous surfaces exist on the outer surface of treated fibre. These would be beneficial to interfacial adhesion strength between the pine needle fibers and the substrate.^39,40

Fig. 3 Micrographs of untreated and treated pine needle fiber: (a) untreated pine needle fibre and (b) treated pine needle fibre.

3.2 Physical and mechanical properties of friction composites

The properties of natural fiber reinforced friction composite extremely depend on the degree of its physical and mechanical properties. Thereby, physical and mechanical properties of the friction composites related to the reliability and stability of automobile operation.

Average values of physical and mechanical properties of the friction composites, such as density, hardness and impact strength are listed in Table 3. As can be seen from Table 3, the densities of the friction composites decreased with the increase of the content of pine needle fibre. The density of the friction composite without the pine needle fibre was largest, and that of the specimen FC9 was the lowest. The hardness of the friction composite increased at first and then decreased with the increase of the pine needle fibre content. The hardness of the specimen FC3 which has a value of 107.4 is the largest than that of the other friction composites. The impact strength of the specimen FC5 is the largest, and that of the specimen FC3 and specimen FC9 are the lowest, because there are excellent interface adhesion between the fibres and substrate to some extent. The result showed that the existence of the pine needle fiber can improve the physical and mechanical properties of the friction composites.

Table 3 Mechanical properties of the friction composites

	Density (g cm^−3)	Hardness (HRR)	Impact strength (MPa)
FC0	2.32	103.4 ± 2.3	0.472
FC3	2.29	107.4 ± 1.6	0.433
FC5	2.25	106.8 ± 3.2	0.496
FC7	2.19	105.4 ± 2.1	0.481
FC9	2.16	103.3 ± 2.8	0.431

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3.3 Effect of the pine needle fibers on the friction and wear properties of friction composites

The effects of temperature on the friction coefficients for these five specimens were shown in Fig. 4. It can be seen that the coefficients of specimen FC0 fluctuate significantly, the maximum value is 0.44 at 150 °C and the minimum value is 0.33 at 350 °C, showing markedly fade at high temperature. For specimen FC5, the rangeability of the friction coefficient is relatively low and very stable, and the difference between the minimum and maximum values is only 0.06. The specimen FC3, FC7 and FC9 also showed fade at high temperature, but the extent of the fade is smaller than that of specimen FC0. This is because the phenolic resin and some mineral fillers began to decompose at high temperature, and the adding of the pine needle fibers can improve the stability of friction coefficients in some extent. The friction coefficient of the pine needle fiber reinforced friction composites have some variation at 250 °C, because the phenolic resin began to decompose and the interface adhesion between the pine needle fibre and substrate decreased when surface temperature of friction composites was higher than 250 °C.

Fig. 4 Effects of temperature on the friction coefficients for the pine needle fiber reinforced friction composites.

Fig. 5 shows the variation of the wear rates of the pine needle fiber reinforced friction composites with temperature. It can be seen that the wear rates of the friction composites generally increased with the increase of temperature, and significantly influenced by the test temperature. This is consistent with the research results of Wang et al.¹⁰ Compared with specimen FC0, the wear rates of specimen FC3 and FC9 are larger and that of specimen FC5 and FC7 are lower. The wear rate of specimen FC7 is the lowest among that of other specimens at each temperature, except for that when the temperatures were about 200 °C. This is because specimen FC7 has preferable mechanical properties and interface adhesion between the pine needle fibre and substrate may have a good effect on the worn property of specimen FC7 among the friction composites. In a word, the wear property of friction composite containing 7 wt% of pine needle fibers was superior, and the presence of 7 wt% pine needle fibers in the friction composites leads to significant reduction of disc wear as indicated by the reduction of disc thickness variation.⁴¹

Fig. 5 Effects of temperature on the wear rates for the pine needle fiber reinforced friction composites.

3.4 Effect of the pine needle fibers on worn surface morphology

The worn surface morphology of the friction composites can provide important information to reveal wear mechanism.³⁸ The worn surface morphology of tested pine needle fibers reinforced friction composites were characterized using SEM operated at 20 kV. The typical worn surfaces of specimens with temperature of 350 °C are shown in Fig. 6. It can be clearly seen that the worn surface of specimen FC7 was much smoother than other specimens corresponding to the smallest wear rate (Fig. 5). This is because pine needle fibers are carbonized when surface temperature was higher than 250 °C and much carbon powder appeared on the friction surface. What's more, dark and smooth friction film on the friction surface led to the best wear resistance of specimen FC7. Some wear debris and surface scratches were found in the worn surfaces of specimen FC0 and FC5. This surface topography appeared to be rough and further increased the wear rate compared with that of specimen FC7.

Fig. 6 Surface morphologies of the pine needle fibers reinforced friction composites with different fibre content: (a) FC0; (b) FC3; (c) FC5; (d) FC7 and (e) FC9.

Many adhesive wear and spalling pits were observed on the worn surface of specimens FC3 and FC9 as shown in Fig. 6(b) and (e). This is chiefly because poor effect of interface adhesion between the pine needle fibre and substrate, and bared fibre fracture and fall off. Meanwhile, some hard particles ruptured and acted as abrasive of third body were embedded on the surface of friction materials, and then compact and destroy the worn surface of the specimen. This is also the main reason for quick increase of wear rates of specimens FC3 and FC9. In conclusion, the wear resistance of friction composites was affected both by the mechanical properties and the interfacial adhesion between fibres and substrate.⁴² The friction composite containing 7 wt% pine needle fibers has superior mechanical properties (Table 3) and good effect of interface adhesion between fibres and substrate. A certain amount of pine needle fiber can improve the tribological and mechanical properties. It has been reported earlier that the improved tribological property of bamboo fibers reinforced friction materials was highly controlled by the presence of a certain amount of fiber.²³

3.5 Morphology of the pine needle fibers of the friction composites

A certain amount of fiber was essential for the tribological and mechanical properties and interface adhesion between fibres and substrate. The morphologies of the pine needle fibers of friction material are shown in Fig. 7 and Fig. 6(e). As can be seen from Fig. 7(a), the bared fibre was observed on the worn surface of specimen FC7. This is because that some hard particles (such as iron powder) come loose into the surface of friction composites during the friction and wear process, and acted as abrasive of third body resulting in more material removal. At the same time, preferable interface adhesion between fibres and substrate of specimen FC7 made fibres difficultly pull out, and then the fibre appeared fatigue fracture phenomenon under friction force. The morphology is shown in Fig. 7(b).

Fig. 7 Morphology of pine needle fiber of the friction materials: (a) and (b) FC7 and (c) FC9.

For the Fig. 7(c), the pine needle fibers were pulled out from the substrate for the interfacial adhesion between fibres and substrate is weak at the temperature of 350 °C, which could then ultimately lead to grooves. Some wear debris existed in the grooves, which can reduce the incidence of abrasive wear phenomenon. At the same time, the pine needle fibers were gradually carbonized and also formed the grooves [Fig. 6(e)]. The grooves existing on the friction surface can absorb the braking noise in certain degree. And then the carbonized fibres can increase the carbon content on the surface of friction composites, so the adhesive wear was decreased with the decrease of the friction coefficient, which can have lubricant effect.³⁸

4. Conclusions

The effects of pine needle fiber with different contents on the friction performance of friction composites were investigated in this study. The results can be summarized as followed:

(1) Fibre consent has significant influence on the physical and mechanical properties of the friction composites. The densities of the friction composites decreased with the increase of the pine needle fibre content. The hardness of the specimen FC3 and the impact strength of the specimen FC5 are the largest compared with that of the other friction composites.

(2) Compared with specimen FC0, the friction coefficient of the pine needle fibers reinforced friction composites is very stable and markedly fade is not obvious.

(3) The wear rates of the friction composites generally increased with the increase of temperature, and significantly influenced by the test temperature. The wear rate of specimen FC7 is the lowest compared with that of other specimens at each temperature, except for that when the temperatures were about 200 °C.

(4) Compared with specimen FC0, a certain amount of pine needle fiber can obviously improve the interface adhesion between fibres and substrate; the worn surface of specimen FC7 containing 7 wt% pine needle fibres was smoother.

Acknowledgements

This project was supported by National Natural Science Foundation of China (Grant no.51075177), by Jilin Province Science and Technology Development Plan Item (Grant no. 20120716 and 20130101043JC), by Changchun Science and Technology Support Project Plan (Grant no. 11KZ43), by the Transformation Fund for Agricultural Science and Technology Achievements (2012GB23600635), by Jilin Province Overseas Students Technology Innovation and by National Science and Technology Support Project Plan of China (2014BAD06B03).

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