A lubricant’s film strength plays a part in how well the lubricant film thickness between moving parts protects and performs against friction and wear. Lubrication film strength is the ability for a lubricant film separating moving parts to not break under pressure, which is a critical property in avoiding metal to metal surface contact. This is especially important for thin film lubrication under high pressure situations.
Contributing factors that result in a proper lubrication film as it relates to film strength are viscosity, relative speed of the moving parts, amount of load and pressure, and operating temperature. When in balance these factors allow hydrodynamic lubrication. Hydrodynamic Lubrication is full separation of moving parts and is created at a point of high pressure in which the oil viscosity will temporarily increase, allowing for a thin full film separation under pressure. Film strength determines the performance of the lubrication film under demands of the operating conditions without the lubrication film breaking.
Boundary Lubrication situations, where the oil film thickness is smaller than the average clearance of surface roughness asperities between moving surfaces, may require improved film strength through additives to enhance the lubricity and film strength beyond what can be achieved by the base oil properties alone. This is especially applicable during startup and shutdown activities. Improved film strength under Boundary Lubrication conditions requires a carefully balanced additive formulation that is relative to the mechanical stresses experienced by working components.
Additive formulation plays and important role in lubrication film strength. Zinc/Phosphorus based anti-wear additives (AW) are designed to chemically react with the metal surface at boundary conditions. When temperatures get too high anti-wear additives can breakdown. Under these conditions Sulfur/Phosphorus extreme pressure (EP) additives are required which create a sacrificial low-shear strength surface film under high pressure. Lubricity additives are effective under lighter loads and lower temperature conditions. Fatty acids are commonly used as a friction modifier for lubricity additives.
When testing in-service lubricants the viscosity may not always show much change from the new oil product. Contaminants, oxidation, and entrained air will affect the lubrication film strength negatively and may not always be indicated by the viscosity measurement. Additive effectiveness gets used up over time, contaminants can accumulate, oxidation can occur, and varnish can develop. All of these areas that affect long term in-service lubricants may require additional testing, such as for lubricity, or oil degradation testing such as varnish potential, acid number, and oxidation tests. Water contamination and particulate contamination are also detrimental to film strength, which routine testing can monitor.
David Doyle, CLS, OMA I, OMA II
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