I can't speak from a tibology point of view, but from a physics/chemistry slant I am comfortable. Viscosity is a physical property. It is the resistance to flow. That property holds up well on the macro scale. Intuitively, we can imagine how a viscous material could help separate to metal surfaces. And, as long as the two surfaces are hydroplaning, there is value in our imaginative veiw. However, on the micro scale viscosity is no longer a useful term. In fact, and especially when polymers are involved, the functional viscosity of a fluid changes on the very small scale.
Film strength is derived from phenomena at the micro scale. In macro terms, it is the resistance of a thin layer of liquid to be ruptured or evacuated. Film stength is an easy concept when discussing a simple inert liquid. You can compare the phenomenon to trying to pinch chewing gum between your fingers. However, since film strength is driven by chemical interactions you have to consider how well the molecules of the film interact with the solid substrate. Think about how difficult it is to clean greasy food from a plastic container verses glass or metal. The non-polar or neutral lipids in food oils adhere very well to the non-polar plastic, but less well to the metal or glass. So film strength is dependend on an interaction between the fluid and the solid substrate - viscosity is not. So one must consider if the fluid has stronger adhesive properties (sticks to suface) or coehesive properties (sticks to itself).
Also, the fluid may have components that change or react in particular microenvironments. Some of the friction modifiers such as zddp act in this way. When the film becomes very thin, and the local pressures become very high, a chemical reaction takes place that results in zddp being converted to zinc polyphospate that forms a matrix on the substrate. The Zinc polyphosphate is no longer in the oil, it becomes an elastic solid on the metal.
Another complication occurs when solids or liquid crystals are added to the mix. For example, if microcrystaline grapite or molybdenum disulfide are suspended in the mix, these crystals act like decks of cards. In this example, the deck of cards may prevent your mug (lifter) from touching the table(cam). However, since the cards are flat the mug can slide easily. The film was not broken due to the additive, not the viscosity.
I apologize for the rant. I will stop now.
Hirev, many factors go into the term "film strength" as it is used here. Viscosity is only one factor. And for many types of wear, viscosity is a minor factor.
Film strength is derived from phenomena at the micro scale. In macro terms, it is the resistance of a thin layer of liquid to be ruptured or evacuated. Film stength is an easy concept when discussing a simple inert liquid. You can compare the phenomenon to trying to pinch chewing gum between your fingers. However, since film strength is driven by chemical interactions you have to consider how well the molecules of the film interact with the solid substrate. Think about how difficult it is to clean greasy food from a plastic container verses glass or metal. The non-polar or neutral lipids in food oils adhere very well to the non-polar plastic, but less well to the metal or glass. So film strength is dependend on an interaction between the fluid and the solid substrate - viscosity is not. So one must consider if the fluid has stronger adhesive properties (sticks to suface) or coehesive properties (sticks to itself).
Also, the fluid may have components that change or react in particular microenvironments. Some of the friction modifiers such as zddp act in this way. When the film becomes very thin, and the local pressures become very high, a chemical reaction takes place that results in zddp being converted to zinc polyphospate that forms a matrix on the substrate. The Zinc polyphosphate is no longer in the oil, it becomes an elastic solid on the metal.
Another complication occurs when solids or liquid crystals are added to the mix. For example, if microcrystaline grapite or molybdenum disulfide are suspended in the mix, these crystals act like decks of cards. In this example, the deck of cards may prevent your mug (lifter) from touching the table(cam). However, since the cards are flat the mug can slide easily. The film was not broken due to the additive, not the viscosity.
I apologize for the rant. I will stop now.
Hirev, many factors go into the term "film strength" as it is used here. Viscosity is only one factor. And for many types of wear, viscosity is a minor factor.
