Which materials have the highest coefficients of friction?

[ItDoesn'tMatter]

I am aware that there needs to be two materials for there to be a coefficient of friction, but I mean in general. For example, I know synthetic setae are very resistant to slipping on surfaces.

 

[phinds]

Low grit sandpaper is really good at not slipping over wood

 

[CWatters]

Interestingly metal on metal can be quite high..

http://www.engineeringtoolbox.com/friction-coefficients-d_778.html

Car tire on Asphalt = 0.72
Aluminum on Aluminum 1.05 - 1.35

 

[JBA]

Depending upon your application, non-skid tape sold for boat and running board applications is quite good for both wood and other mating soft materials and for ice as well; but, it wears out quickly against concrete.

 

[Baluncore]

I am aware that there needs to be two materials for there to be a coefficient of friction, but I mean in general.

The highest coefficients of static friction are for dry contact between identical metals.
Examples of coefficients ≥ 1.00 are;
1.00 Iron — self
1.00 Copper — self
1.05 Copper — Cast iron
1.10 Cast iron — self
1.10 to 1.35 Aluminium — self
1.15 Rubber — self
1.20 Platinum — self
1.40 Silver — self
1.46 Indium — self

 

[Andy SV]

The highest coefficients of static friction are for dry contact between identical metals.
Examples of coefficients ≥ 1.00 are;
1.00 Iron — self
1.00 Copper — self
1.05 Copper — Cast iron
1.10 Cast iron — self
1.10 to 1.35 Aluminium — self
1.15 Rubber — self
1.20 Platinum — self
1.40 Silver — self
1.46 Indium — self

That's interesting I did not know that about self stick of metals

 

[Baluncore]

For example, I know synthetic setae are very resistant to slipping on surfaces.

The setae on small animals feet conform to follow the surface so a high proportion of the area can have an attractive adhesion. By conforming to the surface there is never a high pressure exerted on a high point that would push the foot away from the surface and counter the adhesion.

The friction coefficient is I believe a different phenomenon. With friction, only the total force is important because some patches will carry higher contact force while others patches will have none. Friction is one sided. It does not allow you to walk across the ceiling, or even to climb a vertical wall.

That's interesting I did not know that about self stick of metals

Yes, it is peculiar since the dry surfaces do not immediately cold weld to each other. Maybe there is enough surface oxidation to prevent a weld forming.

The crystal structure in the two samples is extremely unlikely to be aligned on the sample interface so the contact surface must be the average of all the diagonal contact “dislocations”, hence the high coefficient without a weld forming.

The thing that at first glance surprised me was indium. It has the highest static coefficient against itself in the list, yet it is used as the surface layer on thin shell engine bearings. I believe that is because it is highly resistant to acids and runs only against cast iron, steel or chrome that is very well lubricated with oil.

The list also demonstrates another reason why copper sheet makes such a good head gasket for old tractors with cast iron blocks and heads.

 

[jack action]

I once asked a tire company about drag tire friction coefficient. Here what was their response:

You may wish to review SAE paper #942484 (Chuck Hallum) for some very interesting findings on how a Top Fuel drag tire works. Statically, the coefficient of friction is 3.0 but dynamically the 36.0x17.0-16 Top Fuel tire generates close to 5.0 . We have some of the data you have requested and hope you find it helpful. The coefficient of friction is speed dependent and declines with speed. In our Pro Stock tire (33.0X17.0-15 in D-6 tread compound) the coefficient of friction was measured in the laboratory (flat belt treated with VHT track prep tackier application) for various speeds:

0 MPH 3.0
40 MPH 2.9
80 MPH 2.0
120 MPH 1.8

 

[Andy SV]

Yes, it is peculiar since the dry surfaces do not immediately cold weld to each other. Maybe there is enough surface oxidation to prevent a weld forming.

The crystal structure in the two samples is extremely unlikely to be aligned on the sample interface so the contact surface must be the average of all the diagonal contact “dislocations”, hence the high coefficient without a weld forming.

The thing that at first glance surprised me was indium. It has the highest static coefficient against itself in the list, yet it is used as the surface layer on thin shell engine bearings. I believe that is because it is highly resistant to acids and runs only against cast iron, steel or chrome that is very well lubricated with oil.

The list also demonstrates another reason why copper sheet makes such a good head gasket for old tractors with cast iron blocks and heads.

This is pure speculation but maybe the similarities of field allow the two samples to get closer together ... The only way I can think to say it is, less zero distance?

 

[OriginalJames]

Low grit sandpaper is really good at not slipping over wood

"Over wood"... What kind of wood and how was the surface prepared? Was it polished or roughed up? Was the aluminum polished or roughed up? The highest friction is when the two surfaces are like Velcro, even at the micro level. So, can we ask "what is the friction coefficient of sand paper on sand paper?" Why not?

 

[OriginalJames]

The other factor is pressure. Are you assuming friction under low pressure or high? Low gravity conditions or high? Molecular structures can dramatically change their behavior under different conditions.

 

[cjl]

Interestingly metal on metal can be quite high..

http://www.engineeringtoolbox.com/friction-coefficients-d_778.html

Car tire on Asphalt = 0.72
Aluminum on Aluminum 1.05 - 1.35

It's worth mentioning that 0.72 is very low for a modern tire. All season tires these days are closer to 0.85, summer tires are around 1, and racing tires can be in excess of 1.2 (sometimes very far in excess, as demonstrated by Jack Action's post above).

 

[Nik_2213]

{ Packrat memory...}

You must be very careful not to damage ultra-flat 'Jo Blocks' aka Gage / Gauge blocks, as they are so smooth, they'll stick together...

https://en.wikipedia.org/wiki/Gauge_block

IIRC, although it is not strictly friction, some materials exhibit 'vacuum welding' under ambient conditions. Worse, some steel alloys may gall or seize under stress, so you must use slightly different grades for nuts & bolts...

 

[Baluncore]

Slip gauges are wrung together with a thin film of light oil between the blocks. That film prevents the blocks from contacting, wearing or welding. The wrung blocks are held together by air pressure. They can slide sideways but the force needed to shear the mono-molecular film is quite high. Strictly speaking it is not a frictional force but the shear of a very thin layer of viscous liquid.

 

[Nik_2213]

Too true, but if mishandled, 'wrung dry', they may be ruined.