The main use of lubricants is to minimize tribiological wear and friction between surfaces in contact. Lubricants remove the debris from the contact zones, thus minimizing further erosion of bearing surfaces due to abrasion.
The value of the global lubricant market is poised to cross $160 billion in a few years based on the current growth rate of 2.5%, with automotive and transportation sectors being the dominant areas of growth.
Manufacturers of lubricants who are looking to cash in on the market rate, need to concentrate on quality control, research and development activities. Customers would prefer dealing with lubricant suppliers who provide reliable products which improve the performance and bring down the overall costs at the same time.
Evaluating and testing lubricants for suitable applications based on the Stribeck curve is a reliable and straightforward procedure. The main challenge in using this procedure is finding a single tool which can cover all areas of the curve. The UMT TriboLab™ from Bruker is a universal tribometer that overcomes this challenge.
The Stribeck Curve
The Stribeck curve is obtained by plotting the coefficient of friction (COF) as a function of the Stribeck parameter – ηV/Fz , where V is the sliding velocity, η is the viscosity of the lubricant, and Fz us the normal load.
Figure 1 depicts the schematic of a Stribeck curve. There are three well-defined regimes in the figure-boundary, intermediate or mixed, and hydrodynamic. The surface asperities in the boundary lubrication condition carry the load. In the hydrodynamic lubrication regime, the load is completely supported by the lubricating film between the two surfaces in contact. In the intermediate lubrication regime, the elastic deformation of the asperity and viscous resistance of the lubricant together support the load.
Figure 1. Schematic of a lubricant Stribeck curve showing three lubrication regimes
The direct asperities contact existing between the two surfaces is based on the boundary and extreme pressure lubrication. At this condition, the COF is the ratio between the effective shear stress and the plastic flow stress of the contact materials.
Friction can be minimized by using additives with the lubricant that help in generating a low-shear strength interface on hard metal contacts. By using adsorbed mono-molecular and low-shear layer to cover the surfaces in contact, friction can be reduced at low temperatures ranging from 100-150oC and high pressures up to 1GPa.
At high temperatures, the inorganic sacrificial films that are formed due to reactions between the metal surface and lubricant additives that contain chlorine, sulphur or phosphorus protect the metallic contact from extreme wear. Under such conditions, the lubrication is determined by a working temperature at which such protective films are rapidly formed for protecting the contacts from wear.
The mixed and boundary lubrication regimes are caused when the lubricant film does not separate the bearing surfaces completely, and a certain amount of solid-to-solid contact is experienced, particularly in low velocity and/or high load conditions.
By subjecting a lubricant to different force, temperature and velocity conditions, the Stribeck curve that spans all the three lubrication regimes is obtained. A useful Stribeck curve can be plotted with a tribometer that can operate over a wide range of test parameters.
UMT TriboLab from Bruker
The UMT TriboLab system is based on the Universal Mechanical Test (UMT) platform and its precise control of speed, load, and positioning. The test capabilities of the TriboLab span a wide range of speed, force, and temperatures due to its modular design. The TriboLab comes with numerous innovative features that facilitate fast and easy configuration for any type of tribiological testing.
The TriboLab can be made user- friendly, productive and versatile by using intuitive integrated hardware and software interfaces like Tribo ID™ and TriboScript™. Tribo ID automatically detects and configures the components attached to the main system to ensure that the system works properly.
TriboScript provides a secure, advanced scripting interface that facilitates easy compilation of test sequences taken from previously created test blocks. The TriboLab system delivers high repeatability and accuracy with the help of the real-time control and data analysis software.
Evaluating Lubricants for Potential Applications
Stribeck testing was done on SAE 52100 balls and disks with the TriboLab. A break-in step was introduced at the start of each test set. Testing was done on four different lubricants (A, B, C and D) to plot the Stribeck curve. Based on the Stribeck Curve, suitable applications of the lubricants and their performance-based ranking at each lubrication regime were determined.
With the TriboLab, the velocity and normal force can be altered simultaneously in order to maintain a specific V/Fz ratio. Stribeck tests were performed on the four lubricants by varying the V/Fz from 0.01 to 20,000, covering all three lubrication regimes. The normal force and the friction force (Fx) were also measured during these tests. The value of electrical contact resistance (ECR) between the disk and the ball were determined and recorded. The ECR data that is measured gives a qualitative indication of the lubricant film thickness between the disk and the ball.
Figure 2 represents the Stribeck curve of Lube-A, and the ECR plot. The boundary, mixed and hydrodynamic regions are also shown in the figure. The COF in the hydrodynamic region was considerably high due to the viscosity of the lubricant. The COF tends to increase at the boundary zone compared to the mixed zone due to the direct metal-to-metal contact. At the beginning of the hydrodynamic zone, the value of COF was very low.
Figure 2. Stribeck curve of Lube-A along with an ECR plot showing three lubrication regimes.
The electrical contact resistance data, shown in Figure 2, is also segregated into three regions, confirming the friction results. The ECR signal in the hydrodynamic regime was considerably high, and showed a declining trend at the initiation of the mixed region corresponding to a V/Fz of approximately 200.
At the initiation of the boundary regime, and at a V/Fz ratio of 1.3, a steep decrease in ECR value was observed, which can be attributed to the metal-to-metal contact. Out of the four lubricants, Lube-A exhibited best performance at the mixed and boundary regimes, and hence, it was used as a reference for other lubricants.
The comparison of the Stribeck curves for Lube-A and Lube-B is shown in Figure 3. It can be seen that Lube-B exhibits higher COF values than Lube-A at the mixed and boundary regions.
Figure 3. Comparative Stribeck curves of Lube-A and Lube-B.
Figure 4 shows the comparison of the Lube-A and Lube-C Stribeck curves. It can be seen that Lube-C exhibited higher COF value in the intermediate lubrication regime. The friction behaviour of both lubricants was similar in the boundary region, but in the hydrodynamic region, the COF value of Lube-C was much lower than Lube-A due to the differences of viscosity contributions.
Figure 4. Comparative Stribeck curves of Lube-A and Lube-C
The Stribeck curves of Lube-A and–D are compared in Figure 5. It is seen that Lube-D exhibited lower COF values than Lube-A only in the hydrodynamic regions.
Figure 5. Comparative Stribeck curves of Lube-A and Lube-D
It is a general notion that lubricants with lower COF exhibit better performance. The four lubricants were ranked from 1-4 based on their COF data in each regime, where 1 represents the highest performance and 4 represents the lowest. Table 1 shows the ranking of the lubricants.
Table 1. Ranking of the lubes in three lubrication regimes
As per the data in the table, Lube-A and Lube-C showed the best performance in the boundary zone, while Lube-D was ranked least in this zone. Lube-A showed the best performance in the mixed zone, whereas Lube-D showed the worst. Lube-B was ranked second in the mixed and boundary regions.
Lube-D was ranked first in the hydrodynamic region, and Lube-C was ranked second. Both Lube-A and Lube-B were ranked 4 (poor) in the hydrodynamic region. The measured values of ECR were in agreement with the measured friction values during the Stribeck test.
For complete tribiological characterization of lubricants, the tests and the comparisons have to be conducted in all the three regimes, over a broad range of testing parameters, such as velocity, force, and temperature.
The UMT TriboLab enables comprehensive Stribeck testing for identifying the specific applications of lubricants and ranking them according to their performance in all the three regimes. For in-depth exploration of the application potential of lubricants, it is critical that it is tested under conditions that cover all the three lubrication regimes.
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.