Investigation of Friction Coefficient Evolution and Lubricant Breakdown Behaviour at Elevated Temperatures in a Pin-on-Disc Sliding System ^†

Abstract

The lubricant behaviour at elevated temperatures was investigated by conducting pin-on-disc tests between P20 tool steel and AA7075 aluminium alloy. The evolutions of coefficient of friction (COF) at elevated temperatures showed three distinct stages: stage I (low friction stage), in which boundary lubrication was prevalent and the coefficient of friction was low; stage II (transient stage), in which the lubricant film thickness decreased to a critical value and the coefficient of friction increased rapidly; and stage III (breakdown stage), in which the lubricant was completely removed from the interface and the coefficient of friction was equal to its value under dry sliding conditions. In the present work, 2 types of water-graphite based lubricants were studied by pin-on-disc tests under different contact conditions. The effects of tooling and workpiece temperature determined from the experimental results were investigated in this paper and a comparison with an oil-graphite based lubricant was conducted.

1. Introduction

Two-phase lubricants are observed to exhibit transient tribological behaviour due to a transformation in lubrication mechanisms and contact conditions [1]. Studies show that different contact conditions, such as workpiece temperature, tool temperature, sliding speed, and load pressure could change the viscosity and mechanochemical behaviour of lubricants [2,3]. This change could cause lubricant breakdown during metal forming processes and lead to failure at the tool–workpiece interface [4]. Two-phase lubricants are used in the hot metal forming industry due to their advanced performance at elevated temperatures [5]. In the previous research regarding oil-based two-phase lubricants, the behaviours at elevated temperatures were studied under complex contacting conditions [3]. The lubricant was found to provide adequate lubrication for long sliding distances in severe contact conditions before premature lubricant breakdown took place [4].

The utilisation of water-based lubricants in metal forming industries is increasing due to their good cooling effects, easy-to-clean characteristic, fire retardancy and stability. However, the use remains limited because of the low viscosity, low evaporation temperature and lack of lubricity at an elevated temperature [6]. The combination of effective solid lubricant additives and application under elevated tooling temperatures are potential methods to improve the performance of water-based lubricants.

In the present work, 2 water-graphite based lubricants were studied by pin-on-disc friction tests between P20 tool steel and AA7075 aluminium alloy. The effects of workpiece temperature and tooling temperature on the transient lubricant behaviours and breakdown phenomenon were discussed. In addition, a comparison of performance with an oil-graphite based lubricant was carried out. This research aims to assist the industry in finding a solution for lubrication during the hot stamping and in-die quenching technology for the manufacture of lightweight aluminium components.

2. Methodology

A novel autonomous tribology testing system—Tribo-Mate—was utilised to conduct friction tests with the water-graphite based lubricant applied at the contact interface [3]. Figure 1 shows the hardware components of the Tribo-Mate. In this project, 2 water-graphite two-phase lubricants (A and B) were studied. Detailed compositions are not disclosed due to the confidential requirements from the industrial partner. The specific gravity at 20 °C was 1.01 and 1.25 for Lubricant A and B respectively, and the viscosity at 20 °C was 400 and 1500 cSt. The experiment conducted in this study involved sliding the flat P20 steel pin over a heated stationary AA7075 aluminium blank under different testing conditions to reproduce the contact conditions during the hot stamping process. The testing matrix of the friction test is shown in

Table 1. The testing matrix for water and oil-based lubricants.

No.	Workpiece Temperature (°C)	Contact Load (N)	Sliding Speed (mm/s)	Tool Temperature (°C)
1	300	6	30	20
2	250	6	30	20
3	350	6	30	20
4	350	6	30	100
5	300	6	30	100

. At least 3 repeating tests were conducted for each testing condition. The average and standard deviation were calculated to obtain reliable results.

3. Results and Discussion

3.1. Lubricant Breakdown of Water-Graphite Two-Phase Lubricants

The transient behaviour of two-phase lubricant at elevated temperatures was due to the mechanism transition from boundary lubrication and dry sliding [1,4,7]. The change in the lubrication mechanism was due to the reduction in lubricant thickness which was caused by physical diminution and chemical decomposition. The physical diminution includes the lubricant transfer onto the wear track and evaporation of the liquid agent during sliding, which results in the decrease of entrapped liquid lubricant thickness. In the meantime, the removal of liquid agents (both evaporation and decomposition) leads to the deposition of solid additives at the interface as a solid tribo-layer, which would be worn off during sliding offering reduced protection for the two contacting surfaces.

At the beginning of the sliding process, the COF was low and the main mechanism for this stage was boundary lubrication. The principal source of friction is correlated to the shear stress of lubricant and the contact of asperities. As sliding continues, the entrapped lubricant thickness decreases to a critical value when the premature lubricant breakdown occurs, and the COF value begins to increase.

Figure 2 shows the performance of the 2 water-graphite lubricants under different workpiece temperatures. As provided in Figure 2b, it can be observed that the lubricant breakdown occurred earlier at a higher temperature. The initial COF value increased from 0.15 to 0.3 when the workpiece temperature increased from 250 °C to 350 °C. The breakdown distances also showed a decreasing trend: 5 mm at 250 °C, 2 mm at 300 °C and 0 mm at 350 °C. This was because high temperatures led to low viscosity and a high liquid evaporation rate, hence encouraging the lubricant thickness reduction and resulting in the earlier lubricant breakdown.

The overall trend in behaviour of lubricant A at the 3 testing temperatures was similar, where the COF under the lowest temperature (250 °C) showed the best lubricity among the 3 testing conditions. The initial COF was close to 0 and it generally increased to 1 at a sliding distance of 20 mm. However, the performance at 350 °C was better than that at 300 °C. The initial COF at 350 °C was 0.5 but increased to 0.9 when the workpiece temperature decreased to 300 °C. The friction test at 300 °C reached the friction value of 1 at a 5 mm sliding distance while the 350 °C results survived until 10 mm. The reason behind this may be because of the pressure difference. Although the same load was applied in all the experiments, the pressure may decrease due to the wider wear track which was caused by softening of the workpiece material when the temperature increased. This led to a reduction of lubricant transfer onto the wear track and hence decreased the thickness reduction by physical diminution.

3.2. Comparison with Oil-Graphite Two-Phase Lubricant

The test results of an oil-graphite based lubricant, Omega 35, were used as a reference to compare with the lubricant performance of the water-graphite based lubricant investigated in the present study [3]. Viscosity at 40 °C was 35 and specific gravity at 20 °C was 1.33. Figure 3 shows the comparison of behaviours of both types of lubricant at 300 °C with the same contacting material pairs.

Based on Figure 3, it can be found that the initial value of COF was 0.15 for Omega 35. The initial COF of the water-graphite lubricants were higher: 0.9 and 0.25 respectively for lubricants A and B. The COF for oil-graphite lubricant remained at a low level and the breakdown distance was over 30 mm. On the other hand, the COF values for all water-graphite lubricants increased soon after sliding began, and breakdown occurred within 5 mm.

It should be noted that although the same load was applied in both studies, the test with Omega 35 was performed with a spherical pin instead of a flat-ended pin, which was used in this study, resulting in much higher contact pressure. Considering these more severe contact conditions and elongated breakdown distance, the oil-graphite based lubricant demonstrates superior performance to the water-based one.

The reasons for these could be due to the difference in viscosity and evaporation point. The water-graphite lubricants provide an excellent cooling effect in metal manufacturing by evaporating the liquid agent, but it also causes poor lubricity in the friction tests at elevated temperatures. When the lubricant is in contact with the hot metal workpiece, physical diminution takes place: the liquid agent in the two-phase lubricant evaporates and transfers to the wear track. Since the evaporation point for the water-graphite agent is relatively low, it evaporated quickly after contact with the hot workpiece. Moreover, the low viscosity made the water-graphite lubricant more easily squeezed out and transferred to the wear blank. In the meanwhile, as this occurred rapidly, there was not enough time for the solid tribo-layer to form before entering the dry sliding stage.

3.3. Effect of Tooling Temperatures on the Lubricant Behaviours

The friction test with different tooling temperatures, e.g., 20 °C and 100 °C, was carried out under workpiece temperatures of 300 °C and 350 °C as shown in Figure 4 and Figure 5 respectively. According to the plots, it can be observed that lubricants A and B both show improvement in lubricity when the tooling temperature is increased from 20 °C to 100 °C. The improvement is more significant with the 300 °C workpieces (Figure 4). The lubricants reach the dry sliding stage at 5 mm and 7 mm with the room-temperature tool, and these distances improved to 30 mm and 50 mm respectively with the heated tool. For the blank temperature of 350 °C, the improvement was less significant than the 300 °C results. The breakdown distances with the heated tool were similar to that with the tool under room temperature (20 °C). The sliding distances to reach the friction value of 1 increased for both lubricants. The distance increased from 10 mm to 20 mm for lubricant A and 5 mm to 10 mm for lubricant B.

The increase in the tooling temperature could encourage the evaporation and chemical decomposition of the water-graphite lubricants during the dwelling time before the sliding process began. Since there was no sliding during the dwelling time, the physical diminution did not take place and there was sufficient time for the deposition of dry matter, hence forming a more coherent solid tribo-layer at the tool–workpiece interface. This led to a significant increase in the hardness of solid tribo-layer, hence slower the worn-off process. This could cause an improvement in lubricity with the heated tool.

4. Conclusions

In the present research, 2 types of water-graphite lubricants were studied with the friction test to investigate the lubricant behaviour during hot stamping of AA7075 aluminium blank by using a P20 tool. The effects of tool and workpiece temperature observed from the experimental results were presented in this paper as well as the comparison with an oil-graphite lubricant. Based on the experimental results, the following conclusions can be made:

• The transient behaviours and breakdown phenomenon of the water-graphite based lubricants are due to the transformation of lubrication mechanisms, which is caused by both the physical diminution and chemical decomposition. The water–graphite lubricants performed better at lower workpiece temperatures.
• By comparing the 2 water-graphite lubricants to oil-graphite lubricants, both the initial COF value and lubricant breakdown distance indicated that the oil-graphite based lubricant has superior performance compared to the water–graphite lubricants. It may be because of the high viscosity and slow liquid agent evaporation for the oil-graphite lubricant.
• It was found that the increase of tooling temperature and dwelling time after the lubricant was applied on the warm tool would increase the breakdown distance and decrease the initial COF, which was likely due to the deposition of solid additives and formation of tribo-layer at the contact interface.