Elsevier

Wear

Volumes 456–457, 15 September 2020, 203383
Wear

Tribological behavior of glass fiber reinforced-PA66 in contact with carbon steel under high contact pressure, sliding and grease lubricated conditions

https://doi.org/10.1016/j.wear.2020.203383Get rights and content

Highlights

  • The tribological behavior of glass fiber reinforced PA66 in contact with steel under grease lubrication is studied.

  • Sliding induces damage of the surface, leading to an increase in wear and creep of the composite and friction.

  • The initial orientation of the fibers in the composite sliding surface has significant effects on the wear of the steel.

  • The damage to the composite surface and the increase in the temperature have great effects on the tribological properties.

  • Wear of the composite is reduced by using softer steels by the relation between the hardness of fibers and steel.

Abstract

Polyamide 66 is widely used for sliding parts, such as resin worm gear. Glass fibers are usually added to increase its strength. In this study, the tribological behavior of glass fiber reinforced-polyamide 66 composite in contact with carbon steel under high contact pressure, sliding, and grease lubricated conditions is studied. Measurement of mechanical properties and SEM observations of composite sliding surfaces after different sliding cycle numbers indicate that sliding induces characteristic damage of the surface (peeled off fibers and scratching of polyamide) and a degradation in mechanical properties. These lead to an increase in friction and creep of the composite. The wear effect increases with the increase in sliding cycles, and the initial orientation of the fibers in the composite sliding surface has significant effects on the wear of the steel counterpart. The contribution of each phenomenon to the tribological behavior is discussed. The damage to the composite surface and the increase in contact temperature due to sliding have great effects on the tribological properties compared to the presence of wear debris in grease and wear on the steel. The effects of the hardness of the steel on the tribological properties are investigated, and it is found that wear of the composite is reduced by using softer steels.

Introduction

The requirements for lighter automobile resin parts to achieve energy savings and to reduce carbon dioxide emissions are increasing owing to energy issues such as global warming. In addition, the use of resin-made sliding parts in the automotive industry is increasing in response to the demand for higher levels of quiet accompanying the computerization and hybridization of automobiles [[1], [2], [3], [4]]. Among various types of resin materials, polyamide 66 (hereinafter PA66) is known as an engineering plastic, and has useful properties, such as heat resistance, high strength, toughness, and high wear resistance [[5], [6], [7], [8]]. Therefore, PA66 is widely used for sliding parts in automobiles or industrial machines such as various types of gears [[9], [10], [11], [12], [13]], bearing retainers [[14], [15], [16]], and rollers [17]. Recently, the requirements for the downsizing of automobiles and industrial machines and growing concern for the environment have increased the demand for reducing the size and weight of these plastic sliding parts and their capability to withstand high stress [18,19].

The demand for the use of plastic worm gears is particularly important in the worm reducer of the automobile electric power steering because the gears are lightweight, reduce vibration and noise, and are corrosion resistant. The plastic worm gear of the reducer used in electric power steering experiences a significantly high contact pressure (over 100 MPa) under sliding conditions. Therefore, some form of grease lubrication is necessary to reduce friction between the plastic worm gear and the steel worm shaft [[20], [21], [22]]. In this application, the effect of sliding is greater than the effect of rolling. The recent downsizing of automotive components, coupled with the high demand for electric power steering in large vehicles owing to their impact on fuel economy, highlights the need for robust worm gear support. However, applying higher torque to worm gears will lead to large deformation of the teeth of the plastic gear (increase in the backlash), breakage at a much earlier stage, and increase of the sliding torque [23].

As a consequence, improvements in the mechanical properties of plastic materials are required. Adding reinforcement fibers, such as glass fibers (GF), carbon fibers (CF), or aramid fibers (AF), is a common means to improve the tribological properties of polyamide [[24], [25], [26], [27], [28], [29]]. Kim et al. [27] noted that adding GF can decrease the friction coefficient and wear amounts of polyamide 12 material by decreasing the adhesion between polyamide 12 and carbon steel under dry conditions; the lowest friction coefficient and a better wear resistance of polyamide 12 composite were achieved at 30 wt% addition of GF. In addition, they noted that fibers in a parallel orientation exhibited slightly higher friction levels than those in a perpendicular orientation. However, the fiber orientation had less effect on the friction and wear of the composite than the fiber content or the applied load. Shin et al. [28] found that the amount of GF in the composite and the molecular weight were strongly affected by the friction level and the wear rate because these altered the shear strength and adhesion of PA66 when performing block-steel ring sliding tests under dry conditions. They also stated that the temperature at the sliding interface was important in determining the morphology of the wear debris, which was closely related to the wear resistance. Kim et al. [29] demonstrated that the wear resistance of unreinforced PA66 and GF reinforced PA66 became worse with water absorption because of softening caused by absorbed water and increased interfacial adhesion on the steel counter surface. In addition, the degraded wear characteristics of unreinforced PA66 resulting from chain scission of the amide functional group caused by water molecules were substantially improved by using short-GF reinforcement.

The majority of the previous studies on the tribology of fiber reinforced polyamide were performed in dry conditions, and there are very few reports on the tribological properties of fiber reinforced PA66 under grease lubrication. The reason is that polymer materials have self-lubricating properties, and it is possible to use them as sliding parts without lubrication in many cases. Kurokawa et al. [30] reported that the tribological properties of spur gear made using CF-reinforced polyamide 12 material improved under grease lubrication, and that the load bearing characteristic was higher with increasing molecular weight of polyamide 12. However, the counterpart gear was made of the same polyamide 12 material. In addition, they reported that increasing the molecular weight of polyamide 12 can improve the fatigue properties of resin gear. However, few explanations were provided regarding the wear mechanism of the composite material. Kurokawa et al. [31] further reported on the tribological properties of spur gear made from CF-reinforced PEEK (poly-ether-ether-ketone) under lithium grease lubrication, with a counterpart of CF-reinforced PEEK or steel. They stated that the affinity between PEEK and CF, the difference in the intervention of CF worn debris at the engagement region, and the characteristics of CF (strength and modulus of fibers) play a significant role in determining the wear resistance of the composite. Kunishima et al. [32] reported on the tribological properties of carbodiimide-added GF and AF-reinforced PA66 in contact with steel in the presence of Ba complex grease. They stated that the toughness and wear resistance of the PA66 composite were improved by the increase of the molecular weight of PA66 through the reaction extrusion between PA66 and poly-carbodiimide compounds. However, the detailed tribological mechanism of the composite and steel counterpart was not discussed.

It is necessary to consider not only the wear resistance of PA66 but also the wear resistance of the metallic counterpart, especially when using hard-fiber reinforced PA66 material, which may have the most aggressive effects on the metallic counterpart because the wear on both materials will lead to much earlier breakage of actual parts and to an increase of the sliding torque. However, the wear mechanism has not been clarified sufficiently. Particularly, considering an actual worm gear, the machining of the teeth and the heat treatment of the steel worm shaft affect the performance of the products and their production costs. For example, applying a heat treatment to the steel worm shaft counterpart before the formation of the teeth will make the formation of the teeth much more difficult, leading to increased production costs. On the other hand, if the heat treatment of the worm shaft is applied after the formation of the teeth, the accuracy of the dimensions of the worm shaft decreases because of the strain induced by the heat treatment, and the gear meshing accuracy will decrease, leading to undesirable effects on the product's performance. Therefore, it is necessary to understand the tribological phenomena to properly design the contact surface of the composite and to select the proper steel and grease. A previous study on the effects of the metallic counterpart on the tribological properties of fiber reinforced-PA66 by Chen et al. [33] found that the type of metallic counterpart (such as aluminum, brass, or steel with different surface treatment or heat treatment) has notable effects on the wear resistance of GF-reinforced PA66 and on the friction coefficient during the rolling-sliding contact. They indicated that this difference was attributed to the capability of the metal surface to form a stable polymer transfer film, and that the surface treatment for steel (such as Tufftride coating) has a significant effect on the wear rate of GF-reinforced PA66. However, all sliding tests were performed in dry conditions, and the effect of the hardness of steel was not investigated in their study.

Given these points, the present study focuses on the typical tribological properties during sliding of GF-reinforced PA66 and a steel counterpart under high contact pressure and grease lubricated conditions (i.e., the wear resistance of GF-reinforced PA66 and steel, and the associated friction properties). First, the change over time of the tribological properties is investigated, and the contribution of each parameter such as the damage in the sliding surface, the contamination by wear debris in the grease, and the increase of surface temperature on the tribological properties are elucidated. The wear resistance of the steel counterpart is discussed, considering the fiber orientation of the composite sliding surface. Furthermore, the effects of the mechanical properties of the steel counterpart on the tribological behavior are evaluated.

Section snippets

Experimental set-up, measurement, and observation

Tribological properties were evaluated through sliding tests under grease-lubricated conditions using a rotating composite ring in contact with four fixed steel cylinders [23,32]. Fig. 1 presents the schematic view of the test specimens used for the sliding test as well as the tribometer setup. With this test setup, it is possible to perform sliding tests under high-contact pressure and sliding conditions, similar to those encountered by commercial worm gears. Intermittent sliding, in which

Detailed tribological behavior of the contact and damage of the composite

Fig. 3 shows the evolutions of the vertical displacement, the temperature, and the average friction coefficient (μ) during 61,500 cycles for a normal load of 350 N and a stopping time of 1 s (after every 10 s of sliding), with the steel cylinders (hardness: 4.50 GPa). The temperature of the sliding surface was increased from 25 °C to 102 °C. The displacement did not increase in the initial stage of the test (indicating that neither wear nor creep occurred) and the average friction coefficient

Conclusion

The tribological mechanisms of glass fiber-reinforced polyamide 66 material sliding in contact with a steel counterpart under high contact pressure in grease-lubricated conditions were clarified. The following points should be highlighted.

  • (1)

    Detailed tribological behavior of the contact area and damage to the composite.

An initial inflection point of the vertical displacement during sliding tests was observed and it was related to the creep of the composite ring. After this inflection point, the

CRediT authorship contribution statement

Takeshi Kunishima: Conceptualization, Validation, Investigation, Investigation, Writing - original draft. Yasuharu Nagai: Validation, Investigation. Takanori Kurokawa: Conceptualization, Project administration, Supervision. Gaëtan Bouvard: Methodology, Software, Writing - review & editing. Jean-Christophe Abry: Writing - review & editing. Vincent Fridrici: Conceptualization, Writing - review & editing, Project administration, Supervision. Phillippe Kapsa: Conceptualization, Writing - review &

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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