Exploring the Direction of Sparks from Steel-on-Steel Friction

[Skaperen]

TL;DR Summary

which direction should the sparks go when two powered steel railroad wheels are forced to rub against each other in opposite directions?

when a railroad engine wheel slips on a rail as it tries to move forward i can see sparks mostly going in the rear direction. while thinking about what might happen in other situations, such as a moving engine (being pushed by its massive 100 car load, perhaps), i came to a simplistic case that seemed to also be ambiguous. if everything is steel, then two wheels rubbing against each other (both turning in the same direction, with railroad flanges out of the way, rubbing in opposite direction where they contact) should also produce sparks. but it is not clear to me which direction(s) the sparks would go.

i am assuming that steel to steel sparks are still just rubbing off fine bits of iron, due to the friction and force of movement, or the steel used in railroads has some flint-like material in alloy.

maybe this is less of a question about the direction of sparks and more of a question about the direction of fine material being rubbed off (that burns quickly).

 

[Baluncore]

The sparks come from sand on the track that is dropped in front of the driving wheels to increase traction. Since the shear between the wheel and the rail is backwards, that is where the crushed sand, dust, and sparks are thrown.
https://en.wikipedia.org/wiki/Sandbox_(locomotive)

 

[Baluncore]

Sparks are generated when a carbon steel is struck or scratched by flint, SiO₂, which is quartz, as is sand. Pure iron is soft and dents, while high carbon steel is hard, so it flakes and will produce sparks.

Locomotive wheels are cast from steel, an alloy of iron and up to 2% carbon. The rail is a hard, medium carbon steel, with about 0.75% carbon and 0.85% manganese.

The carbon in steel is held in what is called cementite, iron carbide, Fe₃C, which is a hard ceramic. After being struck, the sparks come from the burning of the iron carbide chip or flake in the oxygen atmosphere. The oxygen in the SiO₂ does not seem to play a part as sparks are also generated when carbon steel is ground with carborundum, which is silicon carbide, SiC.

You can quickly identify the grade of steel by the colour and quantity of sparks produced when it is cut or ground with a hard abrasive. Are the sparks from railway wheels orange or white? Do they fly from the contact then fade, or do they fly before ending in a star-burst? Is there a mixed population produced by the different composition of the wheel and the rail?

Sand grains are a problem when handling explosives. Sparks are prevented by the use of bronze wire cutters, and wooden floors are built without steel nails.

 

[jrmichler]

A theory of friction between two surfaces of similar hardness is that actual contact is between asperities (high spots). Since initial contact is point contact, the contact stress at points of contact is very high, about three times the yield stress of the material. See Case (d) in the diagram below, but without the lubricant film.

The two surfaces are moving relative to each other, and they are not separated by a lubricant film. The asperities weld at the contact areas, and are sheared off as fast as they weld. The high strain rate in the shear zone generates enough heat that sheared off particles burn up. These are the sparks that you see.

 

[Lnewqban]

... maybe this is less of a question about the direction of sparks and more of a question about the direction of fine material being rubbed off (that burns quickly).

For two rotating wheels forced to rub against each other in opposite directions, I believe that any rubbed off material will go in both directions, depending on the relative tangential velocity of one wheel respect to the other.