2.1. Friction
Frictional force is not always intuitive. This is apparent when one considers two blocks on a plate as shown in
Figure 2.1.
The blocks are of equal mass and surface finish. The block on the right has twice the surface contact area of the other. An equal vertical force (
Fn) is applied to each block. Both blocks are made to slide by the application of an equal horizontal force (
F). A frictional force (
Fs) resists the sliding motion. What most people will find surprising is that the frictional force (
Fs) will be the same for both blocks even though the surface contact area is different. The frictional force (
Fs) depends only on the vertical applied force (
Fn) and is described by
Equation 2.1.
where
μ = the coefficient of friction.
The coefficient of friction is a parameter that depends on the combination of block and plate materials. It is approximately 0.5 for many material combinations, but fortunately not for all materials. As it turns out, the coefficient of friction is constant only under a given set of conditions. It can vary with velocity and temperature. There are actually two coefficients of friction for each material pair. The static coefficient of friction (μs) is determined from the force that is just enough to start the block moving. Once the block is moving, the dynamic coefficient of friction (μd) is determined from the force that is just enough to keep the block moving. Dynamic coefficient of friction is sometimes called kinetic coefficient of friction.
The sliding surfaces do not contact completely over the expected contact area. Even the smoothest surface is “rough” at a microscopic scale as shown in
Figure 2.2. At the junction between the two surfaces, the materials only touch over small patches, called “asperities.” The asperities support the load and deform (especially for plastics, elastically or plastically) to reach an equilibrium. When the apparent contact area is measured or calculated, it is not the real contact area in tribological terms. The apparent contact area is much larger than the true contact area.
When movement of the block occurs, the asperities rub against one another creating a natural resistance
to movement as they slide over and deform one another. This resistance to the movement is the frictional force (
Fs) as defined by
Equation 2.1.
Frictional properties of plastics differ markedly from those of metals. The coefficients of friction vary with applied load, velocity, and temperature.
Figure 2.3 shows an example of the temperature dependence of the coefficient of friction for a polyimide, Vespel® SP-21.
The rigidity of even the highly reinforced resins is low compared to that of metals; therefore, plastics do not exactly behave according to the classic laws of friction. Metal to plastic friction is characterized by adhesion and deformation of the plastic, resulting in frictional forces that are more dependent on velocity rather than load. In thermoplastics, friction generally decreases as load increases.
Figure 2.4 shows the dependence of the coefficient of friction of Teflon® PTFE as a function of both velocity (sliding speed) and load (pressure).
A unique characteristic of most thermoplastics is that the static coefficient of friction is typically less than the dynamic coefficient of friction. This accounts for the slip/stick sliding motion associated with many plastics on metal and with plastics on plastics.
2.5. Wear-Resistant Additives
Even among plastic materials with excellent natural lubricity, wear characteristics between two thermoplastics differ greatly. When an application calls for plastic on plastic, dissimilar polymers should be used and incorporated with one or more wear-resistant additives. Reinforcements such as glass, carbon, and aramid fibers enhance wear resistance by increasing the thermal conductivity and creep resistance, thus improving the LPV and working PV of the part.
PTFE has the lowest coefficient of friction of any internal lubricant. Its particles shear during operation to form a lubricous film on the part surface. Often referred to as the best lubricant for metal mating surfaces, PTFE modifies the mating surface after an initial break-in period. PTFE goes an extra step in lessening wear and fatigue failure by actually cushioning shock. What is most important about PTFE is its distribution throughout the thermoplastic compound. PTFE has a typical optimum loading of 15% in amorphous thermoplastic resins and 20% in crystalline resins. However, there is a price performance limit at which PTFE can actually begin to demonstrate diminishing returns.
MoS2, otherwise known as moly, is a solid lubricant usually used in nylon and other composites to reduce wear rates and increase PV limits. Acting as a nucleating agent, MoS2 creates a better wearing surface by changing the structure of nylons to become more crystalline, creating a harder and more wear-resistant surface. MoS2 will not lower the coefficients of friction like other modifiers, and its use is therefore confined to nylons where it has this crystallizing effect on the nylon molecular structure.
MoS2 also has a high affinity for metal. Once attracted to the metal, it fills the metal’s microscopic pores, making the metal surface slippery. This makes MoS2 the ideal lubricant for applications in which nylon wears against metal, such as industrial bushings, cam components, and ball joints. Two added benefits occur during molding: fast injection molding times which lower per part costs; and less and more uniform shrinkage.
Graphite’s unique chemical lattice structure allows its molecules to slide easily over one another with little friction. This is especially true in an aqueous environment and makes graphite powder an ideal lubricant for many underwater applications such as water meter housings, impellers, and valve seals.
Silicone or polysiloxane fluid is a migratory lubricant. A particular silicone fluid is chosen that is compatible enough with the base resin to allow compounding, yet incompatible enough to migrate to the surface of the compound to continuously regenerate the wear surface.
Perfluoropolyether (PFPE) synthetic oil marketed by DuPont under the trademark Fluoroguard® is an internal lubricant that imparts improved wear and low friction properties like silicone or polysiloxane fluids.
Silicone resin offers engineers several unique advantages based on its ability to be both a boundary lubricant and an alloying partner with the base resin. Silicone acts as a boundary lubricant because silicone moves or migrates to the surface of a part over time, by both diffusion as a result of random molecular movement, and by its exclusion from the resin matrix which is a result of migration. As a partial alloying material with the base resin, silicone remains in the component over its service lifetime, but because silicone is incompatible enough, the silicone is constantly moving from the matrix to the surface. This continuous secretion eases friction and wear at start-up and when high-speed lubricity is necessary. Silicone is excellent for start-up, high-speed, and low-pressure wear applications such as keyboard keycap receptacles and high-speed printer components.
Silicone fluid is available in a wide range of viscosities. The lower the viscosity, the more fluid the additive is, and the quicker it will migrate to the surface and provide lubrication. This is particularly important in wear applications that require numerous start and stop actions. However, if the additive’s viscosity is too low, the silicone can vaporize during processing, or migrate too quickly from the molded part.
Silicone and PTFE will work together to create a high-temperature grease which will create better wearing characteristics and lower friction, particularly at high speeds and during start-ups. When used together, PTFE acts as a thickening agent as well as
an extreme pressure additive to make the grease at the surface. Because the silicone is constantly moving to the surface, this provides the added lubricity necessary during start-ups and at high speeds. Since failure at high speeds is more dependent on wear than failure at low speeds, and because the benefits of the silicone/PTFE synergy are most evident at these higher speeds, this combination should not be considered for low-speed components. In these cases, usually a PTFE-only compound is needed.
Glass fibers are mainly added to resins to improve both short-term mechanical and thermal performance properties, particularly strength, creep resistance, hardness, and heat distortion. Wear resistance can also be improved with the addition of glass fibers, but the improvement is directly correlated to the efficiency of the glass sizing system which bonds the resins and fibers together. Glass reinforcement results in a marked improvement of the resins limiting PV by enhancing creep resistance, thermal conductivity, and heat distortion.
Glass fiber reinforcement often leads to increased coefficient of friction and mating surface wear. This can be counteracted with the addition of an internal lubricant.
Carbon fibers are added to engineering resins to produce high-strength, heat distortion temperatures, and modulus as well as creep and fatigue resistance. Often referred to as the perfect additive for wear and friction resins, carbon fibers also greatly increase thermal conductivity and lower coefficients of friction and wear rates. In fact, the strengthened compound may have lower friction coefficients than the base resin.
Carbon fibers should be considered as replacements or alternatives for glass fiber when wear and friction are not sufficiently addressed in glass fiber reinforced components. Unlike glass, carbon is a softer and less abrasive fiber. It will not score the surface of iron or steel. Most resins which are reinforced with 10% or more of carbon fibers will dissipate static electricity and overcome problems with static buildup on moving parts. This can be extremely important for business machines, textile equipment, and other electronic components.
Aromatic polyamide fiber, commonly known as aramid fiber or Kevlar®, is one of the latest wear-resistant additives to be used in thermoplastic composites. Unlike the traditional fiber reinforcements of glass and carbon, aramid is the softest and least abrasive fiber. This is a major advantage in wear applications, particularly if the mating surface is sensitive to abrasion.