Why static coefficient of friction dry contact between "Ag-Ag" is highest among materials? Cu-CI,Pt-Pt, Al-Al,st-rub, Rub-Rub are comparatively lower? By applying formula, however we can see applied normal force for "silver- silver" is not less than paper-plexiglass, paper- cast iron, rubber- rubber.......what manufacturing criticality or metallurgy makes Silver- Silver a highest dry contact coefficient of friction material.
This is a false question, because materials do not have ‘friction coefficients’ static or dynamic. Friction is a system effect, depending on system variables (environment, geometry of contact, load, speed, . . ) Tables of friction coefficients are merely numbers generated at a specific set of variables by someone, somewhere, at one time.
Hi Irwin,
The static coefficient dry contact between the Ag-Ag is 1.40 is highest among all, compared to Cu-Cu, Pt-Pt, St-St and all. Static coefficient between Steel-Steel is 0.6, Aluminium-Aluminium is 1.10-1.15, Aluminium- Steel- .61, Cd-Cd-0.5, Hard steel- Hard steel- 0.78, rubber-concrete- 0.45-0.78, Platinum-Platinum 1.2, Cu-Cu-0.2 is smaller than Ag-Ag-1.40. Try to look at the table chart value, friction coefficient in air is always less than friction in vacuum.
Generally, static coefficient of friction can be calculated ration of resistive force of friction(Fr) divided by perpendicular normal force(N). If the coefficient of the friction between metal-metal greater than 1 , it means that force is greater than the object weight, that means friction is greater than normal force, like silicon-rubber is greater than 1. This is the measure of the roughness of the surface , the rougher the surface the friction is high, it depends upon the nature of the surface in contact and on their roughness, and it also depends on the components of the gravitational surface. Basically on the nature of the contact forces, now conceive for the Ag- Ag , restive friction act to the normal is higher, however normal force is lower due to malleability property of material made highest among al the materials.
Materials Indeed have static friction coefficient. I asked why silver-silver higher ???? why not Pt-Pt? Cu-Cu? For dynamic friction, lowest amount of motion played here, your answer reasonably good, but I didn't ask dynamic coefficient. Come to environment, wet condition, temperature, roughness , dry condition, vacuum condition coffictiont of fiction varies absolutely true and it sensing a interpretation on molecular and atomic level spinning science behind it.
Regards,
Ashish
Hi Ashish,
I can give a hypothesis as to why the maximum friction values for Ag-Ag and Pt-Pt are in your series. Only these two metals do not have any oxide layer on the surface. This means that when Ag-Ag and Pt-Pt contact, micro-welding (diffusion welding) in air is possible. This diffusion welding also depends on the plasticity and the melting point of the metal, so the degree of welding in the Ag-Ag couple is higher than in the Pt-Pt couple.
The hypothesis can be verified by the Cu-Cu pair: if the copper surface is made completely free of oxides (e.g. after etching under protective gas), then the coefficient of friction for the oxide-free Cu-Cu pair should also rise above 1.2.
Dear Vadim, please don't mind. Hypothetically also media interface does not matter much in these two materials, infact force matters a lot. One way you can assume there is no oxide layer on Ag-Ag, Pt-Pt, than one can ask very simple question why static friction dry is highest???? if no oxide level, than static friction should be less theoretically but here its very high value demands. Minimum force required to initiate the motion. It's a complicated question, I think only science could give the real answer, why and how it happens?we have to think why it is so?????tangential forces will be required lower here , normal reaction forces also intake lower with the frictional moment, if it is so than how static friction is high.
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