Stribeck curves is a common name used to described friction vs entrainment speed results, measured on a tribometer. These are very useful as they give a good overview of the frictional performance of a lubricant.
The variation in speed allows friction to be measured across the main lubrication regimes, as shown in the diagram below:
In the boundary regime the load between the two surfaces is supported at the surface asperities. Friction is dominated by the properties of the surface and any surface active additives. In the elastohydrodynamic (EHL) regime the load is supported by the fluid film. Friction is controlled by the physical properties of the lubricant. In the mixed regime the load is supported by a mixture of the surface asperities and the fluid film.
Stribeck curves can be used to quickly assess the lubricants ability to reduce friction in the boundary, mixed and EHD lubrication regime.
An example Stribeck curve taken for 3 very different fluids is shown below:
In this example the boundary and mixed regime can be clearly seen for the classic motor oil (blue) and the heady duty engine oil (orange). The heavy duty engine oil showing higher overall friction in the boundary and mixed regime. The axle fluid shows very low friction in the boundary and mixed regime. At higher speeds ~ 3000 mm/s, the friction in the EHD regime for the three oils is similar.
Common effects in MTM type Stribeck curves:
Stribeck curves can be used to measure and investigate a large range of different effects. These include surface active additives, which will control friction in the boundary regime and the position of the mixed regime. Base oil viscosity and viscosity index improvers can vary the position of the curve (at a constant speed). Some of the more common effects studied using Stribeck curves are shown here…
Additives such as organic friction modifiers reduce friction in the boundary and mixed regime:
Antiwear additives / ZnDDP
Thick tribofilm forming additives, such as ZnDDP can increase the roughness of the surface and extent the speed at which the contact remains in the boundary and mixed regime. Pushing the curve to the right.
Increasing the viscosity of the oil has the effect of keeping the contact in full film lubrication to lower speeds. Pushing the curve to the left:
Using Stribeck curves to compare lubricant formulations
Stribeck curves are commonly used to study fully formulated products. This can give an indication of how the combination of different additives effects the friction in the boundary and mixed regime, along with the position of the mixed regime.
The Stribeck curves formed by 6 different 5W30 engine oils are shown below:
This type of result can be used by lubricant formulators and chemists to directly compare oils. For this particular result – it can be seen that lubricants MTMD003, 16,17 and 19 have a very similar frictional performance in the boundary regime. Lubricants MTMD003 and 16 have a similar response in the mixed regime (200 – 3000 mm/s) suggesting a similar thickness and morphology tribofilm. The same can be said for MTMD003 and 18.
Lubricant MTMD018 has a slightly lower boundary friction coefficient – and interestingly the friction increases with speed (an effect sometime seen with friction modifier type additives).
Lubricants MTMD017 and 20 have a lower overall friction coefficient in the mixed regime – an effect which is likely due to the formation of a thinner tribofilm – or very little tribofilm. This is commonly observed with ashless type antiwear additives.