Formation and Function of Low-Friction Tribofilms
The use of low-friction coatings on machine element is steadily Increasing, And they are expected to play an Important role in the reduction of fuel consumption of Future motorized vehicles. Many low-friction coatings function by transformation of the outermost coating layer into tribo-film, Which then cover the coating surface and Its counter surface. It is within These tribo-film That sliding takes place, and Their Properties largely DETERMINE the performance. The role of the coating is then Not to Provide Low Friction, but to supply support and Constituents for the tribofilm.
In this thesis, the formation of Such tribo-film HAS BEEN studied for a Number of Different low-friction coatings. The sensitivity of the tribofilm formation towards changes in the tribological system, Such as Increased surface roughness, varied surrounding atmosphere and the reduced availability of the tribofilm Constituents HAS BEEN given special attention.
For TaC / aC coatings, the formation of a Functioning tribofilm was found to Be a multi-step process where wear fragments are formed, agglomerated, compacted and stabilized Eventually into a dense film of fine grains. This information is delayed by a moderate roughening of the coated surface.
Coatings based on tungsten disulphide (WS 2) are thwart Able to Provide Exceptionally Low Friction, helpfull Their use is restricted by Their poor mechanical properties and sensitivity to humidity. Large improvements in the mechanical properties Can Be Achieved by the addition of for example carbon, but the achievable hardness is still limited. When titanium was added to the WSC coatings, a carbidic hard phase was formed, Causing drastically Increased hardness, with Retained Low Friction. Titanium oxides in the tribo-film however caused the friction to Be High INITIALLY and unstable in the long term. In a study of WSN coatings, The effects of humidity and oxygen were studied separately, and it was found That the detrimental role of oxygen is larger than thwart assumed.
Low Friction tribo-film apr form by rearrangement of coating materials, but overpriced by tribo-chemical reactions between Constituents of the coating and Its counter surface. This was observed for the Ti-CS coatings, Which formed WS 2 tribo-film When sliding against heavy stone counter Surfaces, leading to dramatic friction Reductions.