Investigating the Extreme Pressure and Anti-Wear of Lubricants

Originally developed by Boerlage in 1933, the four ball tester, otherwise known as an extreme pressure (EP) tester, was originally utilized for the investigation of the EP behavior of lubricants. As technology continued to develop, the four ball tester has evolved into a multipurpose lubricant tester that is capable of testing various other properties of lubricants aside from EP.

In fact, the currently used four ball test methods are capable of evaluating the shear stability, rolling contact fatigue and anti-wear (AW) behavior of lubricants. Several lubricant manufacturers will also utilize the four ball tester for screening both AW and EP additives for their suitability for a wide range of applications. Some of the most commonly utilized AW and/or EP additives include sulphides, phosphates, esters, fatty acids, borates and chlorides, which is further explained in Figure 1.

List of extreme pressure (EP) or/and anti-wear (AW) additives.

Figure 1. List of extreme pressure (EP) or/and anti-wear (AW) additives.

Benefits of AW and EP Additives

The purpose of AW and EP additives is to reduce wear of lubricated surfaces through the interaction between the additives, which are polar molecules, and the metal surface. This interaction between the two materials transforms into a protective and durable mixed layer with metals and additives on the surface.

Although the AW and EP additives reduce wear, they are activated at different temperatures as shown in Figure 2. To this end, EP additives are only effective when present at a high temperature and pressure, such as in metal working fluids that are used in drilling operations that are required to operate at a flash temperature that exceeds 600 °C and a contact pressure that is higher than a giga Pascal (GPa). Under such extreme conditions, the EP additives that are based on chlorine, sulphur or phosphorous compounds are used as lubricants.

On the other hand, AW additives are often used for lower temperature and pressure applications as compared to EP additives. For example, the piston ring on the cylinder within an engine will typically require a temperature lower than 400 °C and a contact pressure that is lower than a GPa. Any temptation to combine AW and EP additives should be treated with caution, as this combination can weaken the wear resistance behavior of the other additive material and even ultimately cause both additives to become ineffective. For example, AW polar molecules can inhibit the interaction of EP additives with the metal surface.

Lubricant additives and their activation temperature.

Figure 2. Lubricant additives and their activation temperature.

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Testing the Ducom Four Ball Tester

The Ducom Four Ball Tester (FBT3) was recently utilized to compare differences between AW and EP additives. As shown in Figure 3, the FBT3 is a computerized bench top lubricant tester that is well known for its portability, aesthetics, high throughput and user convenience in testing both AW and EP additives according to the ASTM D4172 and D2783, respectively.

Recipes for all the four ball test methods can be easily selected using the touch screen interface to the computer system. Once the tester is initiated, a real-time display and comparison of the applied load, friction coefficient, speed, lubricant temperature and test duration is shown on the screen for analysis and test reports.

Image of Ducom four ball tester (FBT3) and the bar graphs representing the results of anti-wear (AW) and extreme pressure (EP) tests of chain lube A and lube B.

Figure 3. Image of Ducom four ball tester (FBT3) and the bar graphs representing the results of anti-wear (AW) and extreme pressure (EP) tests of chain lube A and lube B.

Results from a test of AW and EP materials are shown as bar graphs in Figure 3. According to AW test method D4172, the chain lube A is a lower wear resistant lubricant as compared to lube B, however, the chain lube A was shown to exhibit better resistance to extreme pressure as compared to lube B according to test method D2783. Note that the tests were repeated twice.

The results of the four ball test indicated that the additive package in lube A can be differentiated from lube B, and that the theoretical calculation of the flash temperature can provide a better understanding on this difference as demonstrated in Figure 4.

These calculations demonstrated that the AW test method generated a friction that heated the additive materials up to 100 °C. Whereas, the EP test method generated a friction that heated the additive materials up to 1500 °C. By referring to the activation temperature of the additives in Figure 2, it can be inferred that the behavior of chain lube A and lube B is dominated by the EP and AW additive, respectively.

The flash temperature at the pass load prior to the welding for lube A was at 756 °C, whereas lube B exhibited a flash temperature of 516 °C. The hypothesis of this experiment was that sulphur based additives with a known activation temperature above 600 °C were solely used in lube A, which could therefore provide an explanation as to its superior EP behavior.

Conclusion

The four ball tester method cannot simulate the field parameters. However, its ability to easily screen the EP and AW additives has delivered a reliable efficiency in the formulation of lubricants. Furthermore, with only five clicks, the Ducom four ball tester can initiate a test that will improve the effectiveness of the traditional four ball test methods.

An equation used to calculate the flash temperature in a four ball tester and the bar graphs representing the flash temperature during the AW and EP test of lubricants.

Figure 4. An equation used to calculate the flash temperature in a four ball tester and the bar graphs representing the flash temperature during the AW and EP test of lubricants.

This information has been sourced, reviewed and adapted from materials provided by Ducom.

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