Two mating materials sticking and slipping when sliding against each other is one of the main causes of noise in a vehicle interior. The noises generated by stick-slip motion predominantly occur at places where parts made of plastic, leather, rubber and glass rub against each other. Examples include door panels sliding against clear coatings or paint and armrests sliding against door panel inserts. The challenge for vehicle acoustic engineers is to track down points of contact that produce stick-slip motion and eliminate them early in the design stage.

Surface Coatings Help

One approach to solving interior noise issues has been to apply a surface treatment to the contact areas of the parts generating the noise or suspected of producing it. These dry, lubricious films have proven to be an effective way to avoid excessive noise, friction and wear by modifying component surface structure and chemistry.

One type of antifriction coating, based on binder systems, is composed of solvents and a high content of solid lubricants such as molybdenum disulfide or polytetrafluoroethylene. They can be readily applied to a broad range of substrates such as polymers, metals and other inorganic materials by either heat curing or air drying.

Antifriction coatings can eliminate noise from the sliding contact of a wide array of material combinations used in auto interiors. They operate under boundary lubrication and, sometimes after a short running-in period, provide effective lubrication in dry environments. The coatings have been used extensively in a variety of vehicle systems, and they can help improve service life as well as energy efficiency.

Why Solids?

Finished lubricants come in a myriad of forms to meet different application requirements. For example, a bearing requiring hydrodynamic lubrication needs a lubricating fluid to form the hydrodynamic film. In contrast, a low-speed, highly loaded gear sets require a solid lubricant to form adhesive boundary layers to protect gear teeth from wear. A component subject to start-stop conditions and shock loads may need a combination of lubricant forms.

The boundary regime normally requires solid lubricants, pastes and antifriction coatings. The mixed regime is best handled with greases and dispersions containing solid lubricants. And the requirements of the hydrodynamic regime can typically be met with oils and greases.

In general, certain lubricant forms perform best in the specific regimes, but there is some overlap. This is especially true for the mixed regime in which both boundary and hydrodynamic lubrication are present. Solid lubricants are the predominant component in antiseize pastes and antifriction coatings, explaining why these two lubricants are best for boundary lubrication films.

An advantage of the solid lubricants comprising the dominant portion of antifriction coatings is that they are relatively unaffected by temperature and pressure. As temperatures and pressures increase or decrease, boundary films formed by solid lubricants maintain steady thickness without changing as fluids do.

In addition, because these lubricant materials are in a solid state, they do not evaporate. Also, oxidation temperatures for the coatings significantly exceed those for oils and greases. Particle size and adhesive and cohesive properties enable solid lubricants to stay in place, even under high gravitational forces.

Perhaps the most important difference between solid lubricants
and fluid lubricants is that solid lubricants do not rely on relative surface speed to form tribological films or require a certain surface temperature to form tribolayers from chemically active additives. This can be an advantage under static and high-load conditions as well as at slow speeds.

Whats more, solid lubricants can provide benefits when used in combination with fluid lubricants to help protect surfaces during transient events, including start-up and shutdown. The same applies to shock loads.

How Solid Lubes Work

Solid lubricants consist of fine-particle powders that can fill in, smooth and cover surface asperity peaks and valleys on component surfaces. As relative motion and loads are applied, the solid particles adhere to the substrate, forming protective layers to control friction and reduce wear.

Solid lubricants adhere strongly to the surface and also cohere to each other to form lubricating layers. However, solids in powder form are relatively difficult to apply to a surface with much consistency, and keeping them on the surface can be difficult despite the intermolecular attraction. As a result, solid lubricants in high quantities normally are applied as a constituent of antiseize pastes and antifriction coatings. This discussion focuses on antifriction coatings.

Antifriction coatings are paint-like products in which solid lubricants in a solvent carrier are bound to a surface by a resin material. Like paint, the coating dries or cures to form a thin, dry layer of solid lubricants as the solvent evaporates. The solids provide lubrication while the resins and solid-film layers provide some degree of corrosion protection.

Typical formulations contain solid lubricants, resins, additives and solvents. Solids constitute about 30 percent of the formulation, resins 12 percent and additives 3 percent. Solvents make up the balance – about 55 percent – of the formulation and serve as agents to aid dispensing and dispersing the solids.

Some might question the use of solvents in antifriction coating formulations. Similar to paints that use water or solvents to facilitate the spread of pigments, antifriction coatings use solvents to dispense and disperse the solid lubricant and resin components onto the surface. The solvents evaporate and provide little or nothing to lubricant film formation.

The lubricant solids can include molybdenum disulfide, graphite and polytetrafluoroethylene. Graphite and MoS₂ typically provide higher load-carrying capacity (up to 1,000 Newtons per square millimeter), while PTFE and other resin waxes provide lower load-carrying capacity (up to 250 N/mm²) but are typically good at providing a low coefficient of friction in sliding conditions.

The resin or binder system helps the solid lubricants adhere to the substrate. Resins often provide chemical and corrosion resistance that complements the surface protection of the solid-lubricant layers. In general, the higher the concentration of resin in the formulation, the better the corrosion protection.

Resins can be epoxy, polyamide, phenolic, acrylic or titanate, each offering different cure conditions as well as different adhesion and robustness. Organic resins are best for temperatures of 250 degrees C and below; inorganic resins are needed for temperatures up to 600 degrees C.

Solvents help keep the antifriction coating in fluid form to aid application and substrate coverage, and they help regulate viscosity during application, much like paint thinners promote smooth, even coverage. Solvents may be organic or water-based. Additives play a role similar to that in oils, greases and pastes, imparting additional properties to the coating or substrate.

Antifriction coatings are typically applied as a wet film about 30-micrometers thick. As the solvent evaporates, the resin matrix cures to bind the lubricants to the substrate in a dry film approximately 15-m thick.

Curing temperatures vary depending on the resins or binder systems used, ranging from ambient to as high as 250 degrees C. Cure times also can vary from as short as 5 minutes to as long as 2 hours. Drying time for water-based formulations can be reduced to 2 minutes with 60 degrees C hot air.

Coatings typically are fully cured after 2 hours. The lubricating film is transparent if applied at recommended film thickness. However, the film changes the gloss of the coated surface, so it typically is not suitable for visible areas. Black pigments or ultraviolet tracers can be added to the formulation without affecting physical properties or noise reduction.

Application

Antifriction coatings can be applied similarly to how paints are applied, such as spraying, brushing or dipping. Additional methods – like dip-spinning (in which a centrifuge spins off excess material) or screen printing – can help promote even film thickness and uniform appearance.

Antifriction coatings offer many advantages for controlling friction and wear. Cured coatings are dry, will not attract dust and dirt, and they work in the presence of dust and dirt. They are not susceptible to aging and evaporation like oil and grease lubricants.

Applications for antifriction coatings generally involve low-speed and high-load conditions that require boundary lubrication. The coatings also are good for dusty or dirty environments where oil and grease can attract contaminants that may accelerate abrasive wear.

Applications where oscillation and vibration can cause fretting corrosion also are good candidates for antifriction coatings. In addition, they can help reduce premature wear from initial start-up and run-in operations. They also can provide good corrosion protection, replacing heavy metal coatings to prolong component life and enhance environmental friendliness. Applications with sliding friction and high wear potential – such as cams, slides, ways and springs – also can take advantage of antifriction coatings.

When applied to automotive interior components, antifriction coatings can eliminate annoying squeaks, provide long-term noise protection and produce low coefficients of friction to enhance the feel of parts. Transparent water-based coatings are compatible with typical plastic materials used in auto interiors.

Finally, application processes are easy to integrate into existing production lines and can be fully automated (robot spraying). This is an advantage for parts with complicated geometries.

Noise Testing

The German Automotive Association has issued standard VDA 230-206 for evaluating the noise generation and coefficient of friction of two materials sliding against each other. It is commonly used to analyze plastic materials and leather used in cars interiors, enabling designers to build a database of how much noise various material pairings produce.

In the test, one material is attached to an oscillating linear guide; the other is fixed to a stationary holder. The samples are pushed together, and then sliding at a specified stroke and speed is started.

A strain gage continuously records the friction force so that the static and dynamic coefficients of friction can be calculated. An accelerometer measures whether sliding speed is constant or variable. Any speed variation is directly related to the formation of stick-slip or intermittent movement that generates an acoustic wave that can be detected by the human ear.

The machine is equipped with software suitable for running durability trials to verify that antinoise properties remain effective over time, simulating the life of two mating materials in actual service.

Dow Corning has modified the test setup by replacing the plastic with a section of a vehicle door panel. This arrangement is more similar to real applications and provides higher flexibility between the machine holder and the tribological contact, enhancing sensitivity to stick-slip formation.

Manfred Jungk is Associate Industry Scientist, Automotive Solutions, at Dow Corning GmbH, Wiesbaden, Germany. Contact him at manfred.jungk@dowcorning.com.

Vittorio Clerici is Senior Technical Service Specialist, Automotive Solutions, at Dow Corning GmbH, Wiesbaden, Germany. Contact him at vittorio.clerici@dowcorning.com