Enhancement Mechanism for Carbohydrate Polymer Green Lubricant

Abstract

The objective of this study was to investigate a mechanism for enhancing the tribological properties of a green polymer material using MoS₂, which is a common material with good tribological qualities. In this study, MoS₂ nanoparticles were added to the biopolymer hydroxypropyl methylcellulose, obtaining an evident enhancement of the tribological behavior. The suitable content of MoS₂ can provide the best structure of the composite film and optimize the transfer layer, and further enhance the tribological properties. The addition of 5–10% MoS₂ particles to the polymer can improve the surface roughness up to 60%, reduce the friction coefficient of the thin-film material by more than 40% and reduce the wear rate of the substrate by 65%. The mechanism by which MoS₂ additives enhance the tribological properties of the biopolymer material lies in the reduction of the run-in time in the initial stage of wear by modifying the surface roughness. This effect accelerates the formation of a transfer layer of good quality and provides good tribological properties.

1. Introduction

In recent years, sustainable manufacturing has become a widely discussed topic^1,2. Among the various orientations aimed at sustainable manufacturing, the most important involve energy saving, recycling, and attention to the environment. In particular, in the field of machining, the use of lubricant is indispensable and therefore the development of green lubricants is becoming increasingly important ³ . Some natural biopolymer materials, such as hydroxypropyl methylcellulose (HPMC), are a good option as functional thin-film material^4,5. HPMC is biologically friendly, decomposes into natural components and is environmentally friendly. It is usually employed for drug delivery and in the food industry because of its chemical inertness and property of blocking light. Moreover, its production process is easy and fast. In addition, it has the characteristics required for being considered as a green lubricant ⁶ . There are some differences between biopolymer materials and petrochemical products with respect to their functional performances. However, previous studies showed that through appropriate enhancement mechanisms, the properties of functional thin film (machinability, corrosion resistance and tribology) can be substantially improved^7–9. Thus, understanding the enhancement mechanism is helpful for future design of thin films. The tribological and anti-wear properties of functional thin film and functional thin-film materials are determined by several factors: 1. material, structure, tribological property, content, proportion, and composition of additive particles; 2. morphology and material of anti-abrasive particles; 3. selection of suitable environment and parameters; 4. choice of nano material and lubricant base material for the requirements of green manufacturing^10,11. It has been proposed in previous studies that given a proper enhancement mechanism, HPMC can be used for the production of composite functional materials^12,13. Enhancement can be achieved with respect to mechanical properties, corrosion properties, and tribology^14,15. HPMC meets the requirements of sustainable manufacturing, also applicable to different environments as well. In this study, MoS₂ was added to the substrate of biopolymer HPMC, with the purpose of investigating its tribological enhancement mechanism for different composition proportions.

2. Experimental

2.1 Preparation of MoS₂/HPMC Composite Films

The MoS₂/HPMC composite films were prepared by adding 5 g HPMC (Pharmacoat 606–2910, Shin-Etsu, Tokyo, Japan) to 30 mL water and 130 mL alcoholic solution, which was then heated to 60°C. Five different quantities of MoS₂ powder (2, 4, 6.75, 10.1, and 13.5 g) were added to the solution. After ultrasonic stirring for 20 min, a micropipette was used to transfer 150 ml of solution to a silicon substrate. The thin films were then formed by resting for 1 h at 25±2°C and 60 RH±5%. Average film thickness of films is 50±5 μm.

2.2 Tribological Properties Analysis of MoS₂/HPMC Composite Films

A pin-on-disk test was used to measure the tribological properties of the thin films. The rotating radius was 2 mm, sliding speed was 0.03 m/s, load was 2 N, and the grinding ball used was a DIN 17350 chrome steel ball.

2.3 Surface Roughness Measurement of Composite Thin Films

A 3D scanner (VK9710, Keyence, Osaka, Japan) was used for the measurement of surface roughness and for observing the surface profile and topography of the MoS₂/HPMC.

3. Results and Discussion

Figure 1 shows the comparison of thin films with different amounts of MoS₂ and biopolymer HPMC with respect to the tribological behavior. The friction coefficient of HPMC alone was high and unstable, especially during the first 1200 cycles. Although showing similarly unstable conditions, the composite film with 1.5% MoS₂ showed initially a similar instability, and then attained stable tribological behavior after 120 cycles. For films with 5% and 10% MoS₂, the coefficient of friction (COF) reached the stability conditions faster, and remained constant during the whole wear process (∼0.11). The results suggest that adding a certain amount of MoS₂ is beneficial for reducing COF and maintaining stability, i.e., keeping stable lubricant behavior. According to the previous study, the wear process can be divided into three stages. The first is the run in stage, in which the wear rate strongly increases, the friction coefficient is unstable, and the protective transfer layer is removed from the contact area. Transition stage, in which the transfer layer is successfully transferred to the abrading piece, thereby forming a complete protective layer and resulting in a low wear rate and a stable friction coefficient. Steady state stage, in which an extremely low wear rate will only result from a third body ¹⁶ . Figure 1 shows these three stages, indicating that the speed of formation and the stability of the transfer layer and the coating conditions of the contact area have a substantial impact on the tribological behavior of thin-film materials.

Figure 1

Relationship between friction coefficient and cycles for different MoS₂ contents

MoS₂ content, average COF of the thin film material, and the surface roughness are shown in Figure 2 .

Figure 2

Effect of MoS₂ content on average COF and surface roughness

It is clear that the average COF of the thin-film material decreases for increasing contents of MoS₂. It suggests that higher MoS₂ contents will result in a better tribological behavior. The results are comparable to those of the previous studies in that the effects of additives are mutually validated^12,17,18.

Moreover, Figure 2 indicates that the surface roughness of the thin film increases for increasing contents of MoS₂, which directly affects the formation of the transfer layer.

The number of cycles required to reach a stable COF for different MoS₂ contents is shown in Figure 3 . For pure HPMC, COF becomes stable after about 1200 cycles. With the addition of MoS₂, the number of cycles required to achieve a stable COF is substantially reduced to about 100 cycles. Therefore, it can be concluded that a stable transfer layer can be produced more rapidly with the addition of MoS₂, with positive benefits on the tribological behavior.

Figure 3

Relationship between cycles needed to reach stable COF and MoS₂ content

From Figures 2 and 3, a schematic of the friction surface was proposed, as shown in Figure 4 . As previously illustrated in Figure 2 , the surface roughness of the coating layer was low under the conditions of pure HPMC and 1.5% MoS₂. Because of the small contact area, it is difficult to form a uniform and stable transfer layer ( Figure 4(a) ). However, under the condition of 5%–10% MoS₂, the surface roughness is large, resulting in more mechanical contact during the wear process; therefore, it is easy to form a large and evenly distributed transfer layer, which can effectively improve the tribological behavior.

Figure 4

Friction surface (a) Low surface roughness (b) High surface roughness

The wear depth of the baseplate under different coating conditions was further measured, as shown in Figure 5 . It can be seen that the silicon substrate alone shows a large wear depth. For either HPMC or MoS₂/HPMC, the wear depth is substantially reduced (60%). The reason is that the silicon substrate alone requires a prolonged high-friction running-in stage, whereas for HPMC and MoS₂/HPMC, much less time is needed to form the transfer layer that protects the baseplate. HPMC is soft; adding hard MoS₂ to soft HPMC will effectively improve the load capacity and transfer efficiency because when wear occurs, MoS₂-MoS₂ self-lubricating occur, achieving substrate protection.

Figure 5

Influence of MoS₂ content on wear depth

4. Conclusions

The objective of this study was to discuss the tribological enhancement mechanism of biopolymer HPMC with added MoS₂. The results show that the addition of MoS₂ can effectively improve the tribological behavior of the biopolymer by providing a low friction coefficient and achieving protection of the baseplate. The tribological enhancement mechanism operates through the addition of a correct content of MoS₂, which can help achieve the proper surface roughness, and therefore increase the uniformity and coverage of the transfer layer. The conclusions of this study are as follows:

1. 1.

   The addition of MoS₂ additives can effectively reduce the friction coefficient by 40%.

2. 2.

   The addition of MoS₂ nanoparticles can effectively increase the surface roughness by over 60%.

3. 3.

   MoS₂/HPMC composite thin film materials can effectively reduce the wear of Si.

4. 4.

   The formation speed, stability and evenness of the transfer layer are the key factors of the tribological behavior.