The role of lubricants is critical in virtually every type of machinery by influencing the reliability, speed, and service life of the machinery. Candidate lubricants need to be evaluated for their performance under various conditions by lubricant manufacturers as part of the development process. Similarly, machine manufacturers have to evaluate the performance of other options of lubrications to determine the right formulation delivering the best performance and service life.
Lubricant evaluation was previously carried out based on experience and intuition. Today however, products are required to run more efficiently, longer, and faster. This requires a scientific approach in the development and application of lubricants. This article discusses an overview of lubrication science and engineering and covers the latest techniques for comprehensive characterization of the lubricant performance in virtually all applications.
Functions of Lubricants
Reducing wear and friction is the major role of lubricants. However, they also have an important role in:
- Sealing gases
- Eliminating oxidative and corrosion damage
- Lowering surface fatigue, vibration, and shock
- Cooling moving parts
- Removing debris and contaminants, and preventing them from entering the system
Types of Lubricants
Several categories of lubricants are available based on their formulation:
- Aqueous lubricants, including brush polymers (polyethylene glycol)
- Mineral oils obtained from petroleum
- Solid lubricants, such as graphite, polytetrafluoroethylene (PTFE), hexagonal boron nitride (HBN), WS2, MoS2, Au, Cd, Sn, Pb, Cd, Zn, and bronze
- Synthetic oils, including poly-alpha olefin; alkylated naphthalene and ionic fluids; and phosphate-, silicate-, synthetic-esters
- Bio-based lubricants, such as triglyceride esters obtained from plants such as palm oil, soybean, castor, rapeseed, and canola; and from animal-based products like lanolin that is derived from the wool of sheep and animal fats
Lubrication processes exhibit boundary, mixed (elastohydrodynamic) and fluid film (hydrodynamic and hydrostatic) regimes. The surface asperities carry the load instead of the lubricant in the boundary film lubrication, where the ratio between the effective shear stress and the contact materials’ plastic flow stress is referred to as the coefficient of friction, with a typical range of ≥0.1. Alternatively, the two mating surfaces’ properties play a far more important role when compared to the lubricants. A low-shear-strength interface is formed between hard metal contacts by lubricant additives in order to minimize friction.
It is possible to minimize friction by applying the adsorbed mono-molecular compounds, like Silane or fatty acids, on the contacting surfaces, at pressures of up to 1 GPa and relatively low temperatures of 100-150°C. Here the formation of a low shear-strength layer reduces friction. Sacrificial films of inorganic materials are formed at higher temperatures following the reactions between the metal surface and lubricant additives consisting of sulfur like phosphorous, chlorine, and dibenzyl disulfide, preventing metal-to-metal contact and eventually reducing wear. Under such conditions lubrication is possible by attaining a working temperature required to form the film. The formation of sacrificial films can also be influenced when water and oxygen are present.
The lubricating film carries the load completely in both hydrostatic and hydrodynamic lubrication regimes. A lubricant film separates the two contacting surfaces completely in the case of hydrodynamic lubrication, lowing wear and friction and maintaining a very low level of the coefficient of friction of roughly 0.005 owing to the shearing of the fluid. The probability of failure caused by friction is very low under these conditions. Two conditions are necessary to achieve hydrodynamic lubrication.
It is essential to maintain relative motion between the two surfaces in contact with sufficient velocity so that a load-carrying lubricating film can be generated. The second condition is the formation of a converging gap between the surfaces that draws the fluid to generate a pressure field for separating the solid surfaces and supporting the load. In this regime, it is possible to use any gas or liquid as the lubricant, but the fluid must have a viscosity suitable for the speed and load, and be chemically inert to the bearing surfaces. Hydrodynamic lubrication is preferred in most piston/linear assemblies, bearing systems, etc., but vibration during operation and damage during start and stop are potential risks.
A lubrication film completely separates the two contacting surfaces even in the case of hydrostatic lubrication, but in this lubrication regime an external pump is used to generate the required pressure to continuously supply the pressurized lubricant. This type of lubrication has minimal friction force at a very slow sliding speed, making it suitable for mechanisms including precision control systems with low operating speeds. The system dependency on the reliability of the pump is a major drawback of this type of lubrication regime. The bearing surfaces may be damaged in the case of pump failure.
In the elastohydrodynamic regime, the load-bearing area is enlarged by the asperity contacts’ elastic deformation to the point that the lubricant’s viscous resistance helps to support the load. A lubricating film with a thickness of 0.1-1 µm reduces the wear and friction in the contacting surfaces by separating them. The hydrodynamic pressure present in the film causes elastic deformation of the contacting surfaces. In elastohydrodynamic lubrication, the lubricant properties are rapidly changed between nearly an ideal liquid state outside the contact zone and an extremely viscous solid-like state within the contact zone. This type of lubrication is achieved by using synthetic oils and minerals as they show piezo-viscosity or pressure-dependent viscosity.
Mixed lubrication has conditions between boundary and hydrodynamic, either because the load is too high, or the velocity or viscosity is too low to allow for complete separation of the surfaces. The Stribeck curve (Figure 1) depicts friction as a function of load, speed, and viscosity. The horizontal axis represents a dimensionless parameter that combines the load, speed, and viscosity and the vertical axis plots the coefficient of friction (COF).
Figure 1. The Stribeck curve
Where η = the lubricant viscosity; V = the sliding velocity; and Fz = the normal load.
The optimal speed for a lubricant contact that reduces the COF can be determined using the curve. It is possible to perform lubricant testing on various tribometers, each addressing a specific lubrication regime. Bruker's UMT TriboLab (Figure 2) helps to perform comprehensive testing of materials for determining their mechanical and tribological properties at a load range of 1 mN to 200 N. The same tool can be used to perform rotary, reciprocating, and block-on-ring tests, thanks to the interchangeable lower and upper drives. The independently programmable motions allow for custom wear monitoring. The tribological finger print can be obtained by generating automatic COF versus load and velocity curves. The UMT can perform ISO, ASTM and DIN tests, is equipped with environmental chambers, and can monitor and record in situ electrical data, acoustic emission, down force, friction, wear, temperature, humidity and so on.
Figure 2. The UMT TriboLab System
The rotary test (Figure 3) involves the installation of a ball or pin under the load sensor and the installation of a standard disk within a liquid holder. The COF is obtained by measuring Fx and Fz during the test. ASTM standard tests, like the test for low-temperature torque of ball-bearing grease, can be performed using this test setup. A Stribeck curve can also be generated using this test setup. Besides monitoring acoustic emissions and electrical contact resistance, the test can be carried out at both ambient and elevated temperatures.
Figure 3. Rotary test module
On the test machine, a plate can be installed within the liquid holder to perform reciprocating tests (Figure 4) such as pin-on-plate and ball-on-plate. This setup can also be used to perform the piston ring-on-cylinder test (ASTM G181). The COF value is obtained by measuring the Fx and Fz data. It is possible to use the reciprocating test to perform ASTM standard tests, including D2981: Test Method for Solid Lubricants in Oscillating Motion; D5707: Friction and Wear Properties of Lubricating Grease; and D5706: Extreme Pressure Properties of Lubricating Greases.
Figure 4. Reciprocating test module
A clutch operation with rotary motion can be simulated using disk-on-disk tests (Figure 5) by installing a disk within the liquid holder. The COF can be obtained by measuring the Fx (converted from torque) and Fz data. This module also allows performing the ring-on-disk (thrust washer) test.
Figure 5. Disk-on-disk test module
The block-on-ring test (Figure 6) involves loading a block from the top and pressing it against a rotating ring. The block can be held using a block holder. A standard ring is mounted on the arbor that is rotating on the horizontal axis. The COF can be obtained by measuring the Fx and Fz data. A 3D microscope can be used to measure the wear scar of the block (Figure 7).
Figure 6. Block-on-Ring test module
Figure 7. Depth profile of wear scar in the block-on-ring test
Pin and Vee Test
In the pin-on-vee block test, a pin is rotated and loaded by forcing it down against a vee groove. A number of ASTM standard tests can be performed using this setup. The ASTM D3233 test starts with a run-in at 290 rpm and a load of 1174 N for five minutes. The next step is raising the load to 1832 N and performing the test at 290 rpm for one minute. The test is run by progressively raising the load with a one minute holding time for each load (2828 N for one minute, to 382 N for one minute, and so on) until the occurrence of the lubricant failure, which is characterized by a sharp increase of friction (Figure 8).
Figure 8. ASTM D3233 test results using pin-and-vee method
Piston Ring-on-Cylinder Liner Test
The friction of piston ring and cylinder liner materials can be measured under lubricated conditions as outlined in the ASTM G181 standard. The load is raised from 20 N to 200 N at 20 N increments with one minute holding times for each load, followed by reducing the load from 200 N to 20 N at increments of 20 N with one second holding times at each step. The test is performed at a temperature of 100 ± 2°C, frequency of 10 Hz and stroke of 10 mm. The average friction value during loading and unloading in correspondence with the same load is recorded as a function of load.
Figure 9 shows the results of a piston ring-on-cylinder liner friction test carried out on two different lubricants as per the ASTM G181 standard. A higher COF was observed for Lubricant A (red) at every load; however, the increase in COF is lower over the load range. Conversely, a lower COF was observed for Lubricant B (blue). The friction is raised with increasing load because a boundary film regime changes at higher loads.
Figure 9. Piston ring-on-cylinder linear test results.
The UMT TriboLab is used to perform this test using a 4-ball test setup equipped with a rotary drive in a heating chamber at a constant temperature of 75°C, as outlined in the ASTM D4172 standard test method. Three of the SAE 52100 balls of 0.5-inch diameter are assembled within a ball holder with a liquid reservoir. The ball holder is loaded with the test lubricant. The fourth ball is introduced from the top at the center of the three balls with 392 N load. With three balls, the liquid holder is rotated for one hour at a speed of 1200 rpm. The friction is measured using a torque sensor, and the COF is calculated using normal force. Once the test is completed, all three balls are measured for their wear scars to report the average wear scar diameter value. The performance of the lubricant is considered as better when the wear scar diameter is smaller.
Twist Compression Test
Sheet metal lubricants as well as cutting fluids that are utilized in metal cutting operations can be evaluated using the twist compression test, where the action of the flute of the tool is simulated on the workpiece.
Four distinct stages can be observed on the friction torque plot: initial break-in (I), effective lubrication (II), depletion of lubricant (III), and failure of lubricant (IV). The time taken for Tz and acoustic emissions (AE) to rise because of lubricant failure is dubbed the durability of the lubricant. It is possible to perform rotary and reciprocating tests at different sliding-to-rolling ratios. The tribometer can be configured for sliding-to-rolling ratios in rotary motion as follows (Figure 10):
- 0% sliding when using three balls
- 15% sliding when using three short rollers
- 35% sliding when using three long rollers
- 65% sliding when installing three long rollers on three slanted slots
Figure 10. Configuring sliding to rolling ratios for rotary motion
Testing and comparison of lubricants under various lubrication regimes, speeds, loads, temperatures, humidity levels, and sliding-to-rolling ratios are essential steps for comprehensive tribological characterization. An ideal platform is provided by the UMT to perform standard and customized tests for efficient and accurate characterization of the tribological properties of lubricants for development, QA, and applications processes. It is possible to use the UMT to conduct screening tests on the candidate materials even in the case of in situ testing of actual machine components, facilitating the elimination of unsuitable materials or lubricants rapidly. With comprehensive data collection and rapid screening, superior lubricants can be efficiently developed by the users.
This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.
For more information on this source, please visit Bruker Nano Surfaces.