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Additives in a lubricant can resist changes in oxidation and oil viscosity, reduce friction and wear, and thereby enhance the service life of any lubricated system. The synergy between the oil molecules and additive is vital for their role as a lubricant.

Activation and adsorption of additives should be selectively enabled by synergistic effects depending on the lubricated system's demands. For example, antifriction additive adsorption is triggered by systems demand to stabilize friction, anti-wear additives to reduce wear, and extreme pressure (EP) additive activation to resist seizure.

It is also a critical requirement from this synergy that it can prevent the depletion of additives for lubricant longevity. It is artful work to achieve this synergy in a completely formulated lubricant. Further, it is a function of “polarity” that is controlled by the surface energy, dielectric constant functional group, molecular weight, and molecular structure.

While molecular models are utilized to study the competition between molecular species contributing to protective layers by way of their physical and chemical adsorption on the surface, these models are too weak for predictions when there is the involvement of mechanical shearing (or mechanochemistry).

In order to overcome this limitation in molecular modeling, we depend on the supplementary experimental observations via laboratory-scale tests in a tribometer.

 Ducom Four-Ball Tester (FBT-3) is a benchtop tribometer powered by automation and a precision measurement system for lubricant characterization. FBT-3 enables data reproducibility that delivers productivity and efficiency in scientific research. 

Four ball Tester (FBT-3, contact between a top ball and three bottom balls) is a benchtop tribometer that is often used in tribology labs for investigating the competition and synergy between the molecular species within a lubricant.

Only 5 clicks to start a test in Ducom FBT-3.

In this study, FBT-3 was used for the testing of three different lubricants with mineral base oils.

Lubricant type A was a mineral base oil without additives. Type B has Zinc dialkyl dithiophosphate- ZDDP additive (5 to 10%) in a mineral base oil and type C has polysulfide additive (10 to 25%) in a mineral base oil.

ZDDP is used as an anti-wear additive, corrosion inhibitor, and antioxidant in hydraulic and motor oils (as well as in grease). Polysulfide is utilized as an extreme pressure additive within metalworking fluids for extrusion, stamping, cutting, etc.

From the list of test methods in FBT-3, ASTM D2783 test method was chosen. Operating parameters like temperature, test duration, speed, and load, were auto set as required by D2783. Each test had a duration of 10 seconds. For these 10 seconds, the FBT-3 software stored and displayed the real-time changes in friction, temperature, speed, and load.

After each test, an image acquisition system was used to measure the wear scar diameter on the bottom three balls in its natural state. Later, the mean wear scar diameter (MWSD) was calculated.

The D2783 graph with the MWSD vs. load (kg) plot was used to identify the weld load, load wear index, and incipient seizure load.

Incipient seizure load region characterizes the load range that triggers the breakdown of protective layers formed due to chemical reaction during mechanical shearing. Weld load signifies the load at which the surface layer neglected to protect the system at extreme pressure.

The load wear index determines the capability of the surface protective layer to minimize wear because of applied loads. Load wear index is the corrected load average, in other words: (Hertzian diameter/MWSD) x Applied Load.

Figure 1. FBT-3 results for base oil (without additive) according to ASTM D2783 test method. The mean ball wear scar diameter (MWSD, micrometer) and friction coefficient (CoF) is plotted against the test load (Kg). For each test load, there is an MWSD value, wear image, and friction coefficient.

 Image Credit: Ducom

Figure 1 shows the friction coefficient, MWSD, and ball wear topography changes during an increase in applied loads for a mineral-based oil.

The incipient seizure region (load range of 40 kg to 63 kg) is represented by an abrupt rise in friction coefficient and MWSD. The load region above 63 kg is referred to as immediate seizure, and at an applied load of 126 kg, the lubricant failed as a result of extreme pressure, as determined by welded balls.

During the test, the maximum Hertzian shear stress and contact pressure were 350 MPa and 2.5 GPa, respectively. For mineral base oil, the load wear index was 25 kg.

In the instance of base oil formulated with ZDDP additive (see Figure 2), the incipient seizure was delayed (100 kg to 163 kg load range), and at 400 kg, the lubricant failed. The load wear index was 80 kg, which is 3.5 times the base oil. The maximum shear stress and Hertzian stress was 600 MPa and 2.75 GPa, respectively.

Figure 2. FBT-3 results for ZDDP in base oil according to the ASTM D2783 test method. The mean ball wear scar diameter (MWSD, micrometer) and friction coefficient (CoF) is plotted against the test load (Kg). For each test load, there is an MWSD value, wear image, and friction coefficient.

 Image Credit: Ducom

For polysulfide as an additive in the base oil (see Figure 3), the incipient seizure was delayed further (load range of 250 kg to 400 kg) and, even at an extreme load of 900 kg, the lubricant did not fail. The maximum shear stress and Hertzian stress was 400 MPa and 4 GPa, respectively. The load wear index was 202 kg (2.5 times the base oil with ZDDP).

Figure 3. FBT-3 results for polysulfide in base oil according to the ASTM D2783 test method. The mean ball wear scar diameter (MWSD, micrometer) and friction coefficient (CoF) is plotted against the test load (Kg). For each test load, there is an MWSD value, wear image, and friction coefficient.

 Image Credit: Ducom

In this FBT-3 study, the load-carrying capacity and wear resistance of the base oil was improved by the protective layers formed by the additives.

In polysulfide, sulfur activation at extreme pressure chemically reacted with the steel surface, forming iron sulfide that was able to protect the contacts from immediate seizure and welding. In ZDDP, the phosphate-based films (Ex. zinc phosphate and iron phosphate) are formed due to the chemical reaction of ZDDP at high pressure.

However, these phosphate films are unsuccessful in resisting seizure at extreme pressure. Zinc cations allow a higher film formation rate with respect to the rate of wear. Hence, ZDDP has greater suitability as an anti-wear additive.

Note: As revealed in this FBT-3 friction study, the ZDDP friction coefficient is greater than polysulfide. This is because of patchy and rough phosphate films, which have been shown in other tribological studies on ZDDP using Ball on Disk, AFM, and MTM.

In order to study the performance of protective layers formed as a result of physical and chemical adsorption during mechanical shearing at extreme pressure, it takes 600 seconds. FBT-3 is an economical and quick test method to supplement the theoretical molecular models utilized for predicting competition and synergy between molecular species within a lubricant.

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

For more information on this source, please visit Ducom.

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