Film strength is one of the most critical lubricant properties for protecting a machine’s internal components from wear and degradation. It is heavily influenced by the lubricant’s base stock and additives. This article will discuss the importance of a lubricant’s film strength and what impacts its effectiveness.
When you think of lubrication, what comes to mind? It might be the base oil creating a film thickness to separate two metal surfaces. After all, the primary intent is to avoid metal-on-metal surface contact. For the base oil to provide separation in this situation, there must be a balance of three contributing factors: the relative velocities, the base oil viscosity and the amount of load. These three factors are also influenced by other elements such as temperature and contamination. When the film thickness is the result of a balance between these factors, it is called hydrodynamic lubrication.
In applications with rolling contact (and thus negligible relative sliding motion), film thickness between the metal surfaces can still occur, even with greater localized pressure points. In fact, these pressure points play an important role. The base oil’s pressure-viscosity relationship allows the oil’s viscosity to increase temporarily due to the higher pressures. This is called elastohydrodynamic lubrication. A full film separation remains, albeit a very thin one.
In practice, it is best to keep machine surfaces separated, with the film thickness providing the best opportunity for reduced friction and wear. But what happens if these film thickness conditions are not met, such as when there is insufficient relative velocity, inadequate viscosity or too much load? Most machine designs and operating parameters will require instances when insufficient velocity exists, such as starts and stops or changes in directional motion. There may also be concerns when the temperature increases too much, causing the viscosity to decrease, or excessive contamination contributes to abrasive contact in the film gap.
When the hydrodynamic or elastohydrodynamic lubrication prerequisites are not met, the base oil will require support during what is called boundary contact conditions. This support involves wear and friction-control additives. The base oil and additives are carefully blended together to produce the specific lubricant product (either oil or grease), which is formulated to mitigate the anticipated boundary conditions. The lubricant then possesses film strength and boundary lubrication properties.
Film strength can be described as the lubricant’s ability to lessen the effects of friction and control wear by means other than the film thickness. As mentioned, the viscosity is the primary contributor to film thickness during hydrodynamic and elastohydrodynamic lubrication. When the base oil viscosity is insufficient to overcome metal-to-metal surface contact, the base oil and additive chemistry work together to create a surface protection mechanism. During these boundary conditions, boundary lubrication is also influenced by the chemical and physical properties of the mechanical surfaces and any contributing environmental factors. Even when loads and temperatures are higher and relative surface velocities are lower, the film strength is improved.
If you were to observe contacting mechanical surfaces on a molecular level, you would see that they can be relatively rough, even if they are machined to be very smooth and appear that way to the unaided eye. This could be compared to how the earth looks like a perfect sphere from the perspective of an astronaut in space but is brimming with mountains and valleys of all heights and depths when viewed by someone standing on the earth’s surface.
This is relevant because when two unlubricated metal surfaces come in contact, the actual contact area will be substantially less than the apparent contact area. The surfaces will only come in contact where these “microscopic mountains” called asperities are the tallest and reach to the other surface, preventing lower asperities from making contact. These asperity surfaces can then elastically deform based on the corresponding shear strength of the metals. Thus, the real contact area will increase proportionally with an increase in load because the initial contact points will elastically deform first and more contact points will connect.
Friction, the resistance to sliding motion of interacting surfaces, is subject to several influencing parameters or processes. Most people consider the roughness of the surface as the primary contributing parameter for friction. However, when considering that the real contact area may be less than 1 percent that of the apparent contact area, the actual roughness becomes much less relevant. The significant process contributing to friction is a result of the adhesive bonds occurring at the atomic level of asperity contact.
In conditions where there is inadequate lubricant film thickness between the metal surfaces, the asperity contact points can lead to cold welding, which is the prerequisite for adhesive wear. The adhesion at these asperity points undergoes a work-hardening process, which strengthens the material. Thus, the shear point happens at layers below the asperity contact point where the metal has not been strengthened. As the metal shears, the asperity tip is then either transferred to the other surface or broken off as an abrasive particle.
Adhesion is often seen as the initial form of mechanical wear, but as abrasive particles present themselves (either from wear or from an external source), abrasive wear can become more destructive. This form of abrasion is called three-body abrasion, whereas two-body abrasion is caused by cutting or gouging of sharp surface contact points.
During rolling contact, surface fatigue can occur. Fatigue mechanisms stem from cracks propagated at the surfaces or up from layers under the surface that contain inclusions or other impurities. The high stresses from rolling conditions at these surfaces will lead to fatigue wear.
Friction and wear-control additives are formulated in small quantities within the base oil and have polar properties that foster metal surface attraction. These attractions are then further encouraged to chemically react with the surface as a result of the interacting conditions, which are inversely associated to the conditions leading to sufficient film thickness: higher pressure and higher temperature.
When machine surfaces interact with higher pressures and temperatures, the additives mitigate the typical effects of metal-to-metal contact (wear) by creating initial molecular layers on the machine surface that are more ductile. These friction-control layers directly reduce the shear strength during contact and become sacrificial. The initial layers can mitigate friction by allowing the lubricant’s weaker molecular bonds to release with less force compared to that of the strong bonds that result from the metal-to-metal asperity boundary conditions. The formation of low-shear-strength films is also influenced by the base stock type and the metallurgy of the mechanical surfaces.
There are three types of lubricant additives that help reduce this friction and control wear formation: friction modifiers, anti-wear additives and extreme-pressure additives.
Polar compounds, such as a fatty acid added to the base oil, decrease friction at low sliding speeds by forming a soap film. They typically are used in components that are sensitive to fuel economy to reduce friction and stick-slip at low speeds, such as in an engine or transmission. They act like anti-wear additives but are more effective with lighter loads and do not require high temperatures. The resulting low-shear-strength soap breaks down at higher temperatures. However, when the surface metal is more reactive to the fatty acid, creating a metal soap, the breakdown temperature is higher.
These polar compounds are typically sulfur- or phosphorus-based, such as a zinc dialkyldithiophosphate (ZDDP) type of additive. They are designed to chemically react with the metal surface only at boundary conditions. However, anti-wear additives are more effective at higher temperatures, where they become more activated and create a barrier film. ZDDP-type additives have been widely used for wear protection and are also beneficial as antioxidants in the oil.
Friction modifiers and even anti-wear additives become less useful and break down when surface temperatures get too high. Extreme-pressure additives, which are also sulfur- and phosphorus-based, are the best choice when high surface temperatures are expected. These additives form a low-shear-strength, soap-like film with metal surface reactions and can withstand fairly high temperatures. While the reaction is beneficial for the film to be developed, it is important to take caution when the reaction has the potential to result in chemical corrosion of more reactive metals.
The physical molecular interactions of asperities at the actual contact pressure points are the main concern when unlubricated or poorly lubricated machine surfaces come into sliding contact. At this molecular scale of the machine surfaces, boundary conditions are subject to numerous principles of physics and chemistry. The role of oxidation, corrosion, chemisorption and other chemical reactions at the machine surfaces must be carefully balanced when additive compounds are selected for film strength protection.
These friction and wear-control additive films on the metal surfaces reduce the shear strength at the contact points. The low-shear-strength films are sacrificial during physical interactions and protect the surface from the effects of adhesive, abrasive and fatigue wear. These submicron films have a gradation of liquid to solid properties as they get closer to the metal surface. While the base oil is preferred to protect the machine surfaces with hydrodynamic and elastohydrodynamic lubrication, boundary conditions will exist. Therefore, to protect against boundary conditions, a properly formulated lubricant with friction and wear-control additives should be used to provide a film strength that is proportional to the exhibiting mechanical interactions within reasonable limitations.
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