Natural fibers, such as bamboo, banana, flax, hemp, jute, kenaf, ramie, and sisal fibers, used for reinforcing polymer composites, are attracting considerable attention from industry [1
]. These natural fibers have unique ecological advantages over inorganic fibers such as glass and carbon fibers because they are renewable, have relatively high strength, stiffness, low density, are low cost, biodegradable, and can be incinerated [8
]. Recently these natural fiber-reinforced polymer composites have been widely used as materials in various applications [1
]. Until now, little attention had been given to the use of such composites in the field of polymeric tribomaterials [9
] (for mechanical sliding parts such as bearing, cum, gear, and seal). However studies in this field are gradually increasing in recent years [13
]. These studies report that tribological properties such as the frictional and wear properties of such composites basically improve when filled with natural fibers, but study results show different tendencies according to the type of natural fiber, matrix polymer, tribo-testing mode, various test conditions, etc. However, it should be noted that most of these studies use the polymer made from petroleum as the matrix polymer of such composites.
To further enhance the eco-friendliness of materials, there is a strong need to use not only reinforcement fibers but also biopolymers obtained from plant-derived materials as the matrix polymer of these composites. Biomass polymer composites, which are used in both reinforcement fiber and matrix polymer based on renewable raw materials, are environmentally friendly to a large extent and have unique performances. Studies have been conducted on the processing, mechanical, chemical, and thermal properties of these biomass polymer composites over the last two decades [4
]. In particular, the mechanical properties of natural fiber-reinforced polylactide acid (PLA) composites have been studied extensively. On the contrary, only a few studies have been published on the tribological properties of biomass polymer composites used for reinforcement fibers and matrix polymers based on renewable raw materials. Bajpai et al. reported the tribological properties of three different types of natural fiber (nettle, grewia optiva and sisal) reinforced polylactic acid (PLA) biomass composites [28
]. This study concluded that the incorporation of natural fiber mats into a PLA matrix significantly improves the wear behavior of neat polymer. Therefore, to further enhance the eco-friendliness of materials in the field of tribology, it is crucial to investigate the tribological properties of various natural fiber-reinforced biopolymers, other than PLA, biomass composites.
Moreover, since the main disadvantage of such composites is the poor interfacial adhesion between natural fiber and matrix polymer, these composites exhibit poor mechanical properties [4
]. Interfacial adhesion and the mechanical properties of these biomass composites can be considerably improved by suitable surface treatments. Most natural fibers are pretreated before they are used as secondary phases in composite materials. Physical and chemical methods for the surface modification of natural fibers can be used to optimize the interface between fiber and polymer. For example, physical methods such as the corona treatment and plasma treatment and chemical methods such as alkali treatment (mercerization), silane treatment, and graft copolymerization have been investigated in this field. In particular, surface treatment by the silane coupling agent is used widely for various purposes [30
]. Over the last two decades, various investigations have been conducted on the effects of the surface treatment on the mechanical properties of these biomass composites [3
]. These studies have demonstrated that the suitable surface treatment of natural fiber improves the mechanical properties of these biomass composites.
However, to date, little interest has been paid to the surface treatment of natural fibers in polymeric tribomaterials based on such composites. It is well known that the surface treatment of natural fibers improves the interface adhesion between natural fibers and matrix polymer as mentioned earlier [3
]. As a result, this improvement of the interface adhesion not only improves mechanical properties but also tribological properties such as frictional and wear properties of such composites. In other words, this interfacial adhesion plays a substantial role in controlling the tribological properties of these biomass composites [20
]. Few studies have been reported on the effect of the surface treatment of fiber on the tribological properties of such composites [13
]. Most of these studies have investigated the effects of alkali treatment using sodium hydroxide (NaOH) solution. These studies also report the results of the research on oil palm fiber-reinforced polyester composites by Yousif et al. [16
], that of betelnut fiber-reinforced polyester composites by Nirmal et al. [21
], that of sugar palm fiber-filled phenolic composites by Rashid [22
] etc. In other studies by Chand and Dwivedi, the influence of the coupling agent, maleic anhydride grafted polypropylene (PP-g-MA), in this case on the abrasive wear behavior of chopped jute [35
] and sisal fiber [36
] polypropylene composites, was investigated. Chand and Dwivedi [37
] also studied the effects of the surface treatment of fiber by the silane coupling agent, which was directly added to polyester resin, on the tribological properties of sisal fiber-reinforced polyester composites. On the other hand, Siva et al. [38
] applied the silane coupling agent to the fiber as pre-treatment before for the surface treatment of coconut fiber in polyester composites. Moreover, Goriparthi et al. studied the effects of the type of various surface treatments, including two types of silane coupling agent used for pre-treatment, on the abrasive wear performance of jute fiber-reinforced PLA biomass composites [39
]. These results showed that the tribological properties of these surface-treated natural fiber-reinforced polymer composites significantly improved as compared to those of the untreated ones. In particular, the most effective method for controlling interface adhesion between natural fiber and matrix polymer in these biomass composites was found to be the combination of alkali treatment and silane coupling agent. This is because proper surface treatment of natural fiber by alkali treatment and silane coupling agent can increase the interfacial adhesion between natural fiber and matrix polymer, and improve the mechanical and tribological properties. However, there are only few published studies on the influence of the surface treatment, other than alkali treatment by NaOH, on the tribological properties of these biomass composites [20
In previous studies, we investigated the thermal, rheological, mechanical and tribological properties of hemp fiber (HF) reinforced plant-derived polyamide 1010 (PA1010) biomass composites (HF/PA1010) to develop new engineering materials and tribomaterials made of 100% inedible plant-derived materials [7
]. Hemp fiber is a bast fiber crop and an annual plant that grows in temperate climates [4
]. The surface of HF is pre-treated by alkali treatment by NaOH and by aminosilane coupling agent (3-(2-aminoethylamino) propyltrimethoxy silane, A-1120). On the other hand, PA1010 is made from sebacic acid and decamethylenediamine, which are obtained from plant-derived castor oil [48
]. As castor oil is not used for food, there is no competition with human food production. It was found that the addition of HF, the surface treatment of HF such as alkali treatment by NaOH, and the surface treatment by aminosilane coupling agent, have strong influences on the thermal, rheological, mechanical and tribological properties of HF/PA1010 biomass composites [7
]. In particular, the effect of surface treatment on these properties of HF/PA1010 biomass composites differs according to whether alkali treatment by NaOH is performed on the aminosilane coupling agent uses or not [8
]. However, to further enhance the mechanical and tribological properties of all plant-derived polymer-based biomass composites, it is necessary to clarify of the effects of the type of silane coupling agent and volume fraction of natural fiber such as HF on the tribological properties of these biomass composites.
To improve the performance of all inedible plant-derived materials for new polymeric tribomaterials, this study aimed to experimentally investigate the effects of silane coupling agent on the tribological properties, namely the frictional coefficient, specific wear rate, and limiting pv (pressure p × velocity v) values, of hemp fiber-reinforced plant-derived polyamide 1010 biomass composites. In this study, three types of silane coupling agents such as aminosilane, epoxysilane and ureidosilane were used for the surface treatment of hemp fibers.
We studied the effects of silane coupling agent on the tribological properties of hemp fiber reinforced plant-derived polyamide 1010 biomass composites. The following results were obtained:
Tribological properties by the constant normal load and constant sliding velocity test for HF/PA1010 biomass composites improved with the surface treatment by the silane coupling agent. In particular, the surface treatment effect on the specific wear rate of HF/PA1010 biomass composites was better than that on the frictional coefficient. These improvements may be attributed to the change in the mode of friction and wear mechanism by the type of silane coupling agent caused by the interaction and interphase adhesion between the hemp fiber and the matrix polymer of plant-derived PA1010. The influence of volume fraction on the tribological properties of ureidosilane treated HF/PA1010 biomass composites differed for each tribological property. This may be due to the change in the mode of wear mechanism by fiber dispersion and orientation in the composites.
In the SEM observations after the sliding wear test, the differences in the morphology of the worn surface of treated HF/PA1010 biomass composites were more conspicuous than the formation of transfer films and the shape and size of wear debris. In particular, the effects of surface treatment and volume fraction of fiber were found to significantly influence the wear mechanism. Surface treatment by silane coupling agent prevents the debonding, breakage, and detachment of fibers on the worn surface.
The limiting pv value improved when filled with hemp fiber, surface-treated by silane coupling agent, and increased volume fraction of fiber. These tendencies are similar to the mechanical properties of various surface-treated HF/PA1010 biomass composites in this study. In particular, the ureidosilane coupling agent (S3, A-1160) had the best improvement effect for tribological properties such as frictional coefficient, specific wear rate, and limiting pv value of HF/PA1010 biomass composites in this study.
However, there are very few studies on the comparisons of the tribological properties of hemp fiber (natural fiber) reinforced PA1010 biomass composites and those of glass fiber-reinforced PA1010 composites. Since this study focused on the effect of the silane coupling agent on the tribological properties of HF/PA1010 biomass composites, it will be necessary to conduct these comparative studies in the future.