Overview
Friction is a force that resists the movement of one surface over another. In some instances it can be desirable; but more often is not desirable.
It is caused by surface rough spots that lock together. These spots can be microscopically small, which is why even surfaces that seem to be smooth can experience friction. Friction can be reduced but never eliminated.
Friction is always measured for pairs of surfaces, using what is called a coefficient of friction.
Coefficient of Friction
The coefficient of friction (also known as the frictional coefficient or the friction coefficient) is a scalar value used to calculate the force of friction between two bodies. The coefficient of friction depends on the materials used -- for example, ice on metal has a very low coefficient of friction (they rub together very easily), while rubber on pavement has a very high coefficient of friction (they do not rub together easily). It is interesting to note that, contrary to common belief, the force of friction is invariant to the size of the contact area between the two objects. This means that friction does not depend on the size of the objects.
The force of friction is always exerted in a direction that opposes movement. For example, a chair sliding to the right across a floor experiences the force of friction in the left direction.
Saying that rougher surfaces experience more friction sounds safe enough - two pieces of coarse sandpaper will obviously be harder to move relative to each other than two pieces of fine sandpaper. But if two pieces of flat metal are made progressively smoother, you will reach a point where the resistance to relative movement increases.
If you make them very flat and smooth, and remove all surface contaminants in a vacuum, the smooth flat surfaces will actually adhere to each other, making what is called a "cold weld". Once you reach a certain degree of mechanical smoothness, the frictional resistance is found to depend on the nature of the molecular forces in the area of contact, so that substances of comparable "smoothness" can have significantly different coefficients of friction.
Types of Friction
Static Fricton
Static friction occurs when the two objects are not moving relative to each other (like a desk on the ground). The coefficient of static friction is typically denoted as μs. The initial force to get an object moving is often dominated by static friction.
Kinetic Friction
Kinetic friction occurs when the two objects are moving relative to each other and rub together (like a sled on the ground). The coefficient of kinetic friction is typically denoted as μk, and is usually less than the coefficient of static friction.
Examples of kinetic friction:
When an object is pushed along a surface with coefficient of friction μk and a perpendicular (normal) force acting on that object directed towards the surface of magnitude N, then the energy loss of the object is given by:
U = N μk d
Where d is the distance travelled by the object whilst in contact with the surface. This equation is identical to Energy Loss = Force x Distance as the frictional force is a non-conservative force.
Note: this equation only applies to kinetic friction, not rolling friction.
Physical deformation is associated with friction. While this can be beneficial, as in polishing, it is often a problem, as the materials are worn away, and may no longer hold the specified tolerances.
The work done by friction can translate into deformation and heat that in the long run may affect the surface's specification and the coefficient of friction itself. Friction can in some cases cause solid materials to melt.
Friction may occur between solids, gases and fluids or any combination thereof. See aeroscentics and hydroathletics.
Reducing Friction
A common way to reduce friction is by using a lubricant such as oil that is placed between the two surfaces, often dramatically lessening the coefficient of friction. The science of friction and lubrication is called tribology. Superlubricity, a recently-discovered effect, has been observed in graphite: it is the substantial decrease of friction between two sliding objects, approaching zero levels - a very small amount of frictional energy would be dissipated due to electronic and/or atomic vibrations.
Lubricants to overcome friction need not always be thin, turbulent fluids; acoustic lubrication occurs when sound (measurable in vacuum by placing a microphone on one element of the sliding system) permits vibration to introduce separation between the sliding faces. World War II Panzer tank treads lubricated by their own squeak provide the most famous, if serendipitous, example.
Anti-Friction Technology
AF coatings (anti-friction coatings) have been successfully used for years as an element of heavy-duty lubrication. Typically used for applications where a permanent lubricating film is needed for metal-to-plastic or plastic-to-plastic lubrication, AF coating technology offers an economic solution to a wide range of engineering problems.
The usage of AF coatings, such as Molykote® brand or other prominent anti-friction coating brand, is most successful when requirements for wear and corrosion protection and optimal coefficient of friction are properly met. A low, high, or even constant coefficient of friction is achievable, if the appropriate application and type of AF coating is utilised.
A firm, completely dry, and non-contaminating lubricating film results once it is properly prepared and applied. The AF coating generally consists of the resin (epoxy, phenolic, and silicone) - a base material, which adheres well to the surface. Solid lubricants such as MoS2, PTFE, polyamide, polyethylene, and Graphite are set in this base material, passing on the anti-friction properties of an AF coating.
Water-dilutable AF coatings, coatings low in solvents, as well as non-combustible or electrostactically sprayable AF coatings, are now being offered to help save energy and meet environmental protection regulations.
Many products using AF technology offer corrosion protection in excess of normal industrial requirements, while some are unaffected by fuels, solvents, or oils.
Application is typically simple: preferably by spraying, dipping, or brushing on thoroughly degreased metal surfaces. The drying and curing times are short (between three minutes for air-drying and sixty minutes) for oven cured coatings.
Source: CDX Global & Wikipedia - en.wikipedia.org
