On this page you can learn about friction and how to reduce it.  We have covered the topic under the following headings:-

Definition of Friction Coefficient 

If the normal applied load between one body and another is L, and the resultant friction force required to slide the bodies is F, then:

 
Friction Coefficient µ = F/L
(µ is dimensionless)

Range of Friction Coefficients 

Dry sliding friction coefficients vary from 0.05 for PTFE under high loads to as high as 5.0 for metals like gold sliding in vacuum. Typical values for engineering steels are between 0.3 and 0.6.

Lubricated sliding friction coefficients vary from about 0.03 under hydrodynamic conditions (complete separation of the sliding surfaces by the lubricant film) to around 0.15 under boundary conditions (when there is surface contact through the lubricant film).

Rolling friction coefficients (with hard steel balls and raceways) vary from about 0.002 when fully lubricated to about 0.05 when running dry.

Typical Values of Friction Coefficient 

All friction values should be treated with caution. The above is a guide, but environment and operating conditions often conspire to produce wide variations.

Coatings to reduce dry sliding friction  

The lowest friction values against metallic counterfaces can be achieved with coatings which contain PTFE, MoS₂ or graphite. Besides PTFE, certain other fluorocarbons will provide low friction, but they tend to be softer and less wear resistant. They are better for non-stick.

Diamond-like carbon coatings can provide exceptionally low friction values, as can PVD applied coatings based on MoS₂. Poeton coatings to reduce dry sliding friction include:

1. Apticote 200 Polymer coatings
2. Apticote 450 Nickel/PTFE co-deposited coatings
3. Apticote 460 Nickel/Polymer composite coatings
4. Apticote 350 Composite Anodising treatments for aluminium alloys
5. Apticote 1000 PVD coatings

Non-stick and Mould Release 

In moulding, curing or cooking applications, the difference from a sliding situation is that the contact is static under load, giving time for chemical and physical interaction and surface bonding.

To provide a non-stick surface it is necessary to use a smooth, pore free coating which is chemically inert to the product. In general, this will be some form of polymer, although high temperature situations (eg glass) may require cermic layers.

Coatings to give mould release and non-stick  

For most effective non-stick and ease of release, for instance with food products in moulds or containers, the best coatings are those based on fluro-carbons. There is a large range of such coatings, some giving excellent release but being soft and vulnerable to damage, others being harder and more wear resistance but giving less good release properties.

1. Apticote 200 Polymer coatings
2. Apticote 450 Electroless Nickel/PTFE dispersion coatings
3. Apticote 460 Electroless Nickel + Polymer composite coatings
4. Apticote 350 Polymer Composite Andodising for aluminium alloys

Lubrication and its Effect on Friction 

A lubricant is usually either a grease or an oil which conforms to the following principles:

A lubricant:

• Separates the moving surfaces
• Has low shear strength
• Has high viscosity (resistance to shear)
• Acts to remove heat from the contacting surfaces
• Is usually a hydrocarbon or fluorocarbon

The viscosity:

• Decreases as temperature rises
• Increases as pressure increases
• Both are exponential relationships

The purpose of the lubricant is to separate the surfaces and to eliminate contact and wear. This is achieved by the generation of a wedge of oil as it is drawn into the contact region by the motion of the parts.

The film thickness generated in this way is dictated by:

h, film thickness, is proportional to (viscosity)^0.7, (Speed)^0.7, and (Load)^-0.13

In the context of Surface Engineering, the relevance of lubricated contact is primarily related to Boundary Lubrication, where the film thickness h is insufficient to properly separate the surface, so that a coating or treatment is required to complete the protection of the parts. At higher speeds, the lubricant film thickness increases so that separation begins (Mixed Lubrication) and, finally, until there is no contact between the two surfaces (Hydrodynamic Lubrication)

Typical friction coefficient values are:

Surface Coatings and Lubrication 

In general, if a rubbing surface is well lubricated with an oil or grease, further reductions in friction are difficult to achieve through the application of surface coatings. In fact, polymer coatings like PTFE are likely to perform less well in the presence of an oil; the normal mechanism of polymer film transfer to the mating surface is disrupted by the lubricant. Certain coatings have an affinity for oil and provide extra protection for parts operating under extreme conditions. This is so in arduous scuffing situations (look in Problem Solver/Wear for more details) in cams and tappets and in automotive cylinder bores.

Coatings for anti-scuffing include:

1. Apticote 100 Hard Chrome Plate
2. Apticote 1000 PVD coatings

Poeton Industries Ltd. Head Office, Eastern Avenue, Gloucester GL4 3DN, England, UK   Tel: 01452 300500  Fax: 01452 500400
Production facilities in Cardiff & Gloucester and in Wisconsin. USA.