COF - Case Hardened Steel to Case Hardened Steel

[mewhg]

I have a small mechanism to design with the sharp edge of one part rubbing on the flat surface of another. I am assuming a small break on the edge of the first part and this edge will rest on the second part over about 1/8' to 5/32". The distance the first part will rub over the second part is about .125" until it falls off edge of the second part (kind of a pawl-like mechanism).

Materials are identical case hardened steel, 8620 or 1018 or similar steel hardened to .010" deep at 60 Rc (~90 R15-N). Finish on the second part is about a 32 ground finish.

I am trying to minimize the effort to operate the mechanism and have the parts return to rest under a light spring pressure (~12 lb normal through the edge of the first part).

Coatings or surface treatments are out of the question, different materials (brass, bronze) are not an option and operational conditions preclude the assumption of consistent good lubrication (a moly grease).

How can I modify or change the composition of the base steel to get a smooth, low COF rubbing action? I notice that different steel parts in this application have very different friction properties in relation to their composition.

Some steels exhibit a stick-slip action, others are glassy smooth and others just seem to drag and scrape their way across each other. Can alloy content affect this behavior? From my limited work I am finding that the 1018 steel will have a lower COF than the steels with low amounts of nickle and chromium.

Any assistance is most welcome

Bill

 

[Carburize]

One of the nitride surface treatments like QPQ may suit this application.

 

[JTreglio]

Why are coatings and surface treatments out of the question?

Jim Treglio
Molecular Metallurgy, Inc.

 

[EdStainless]

Nitriding is a good option. It is slicker than carbarized material.
Plaiting one part with electroless NiP would also help. the NiP plating will age harden and it is very smooth and slick.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[mewhg]

There is a sharp edge that must be retained on both parts. If any coating would flake or degrade near the sharp edge it would cause the mechanism to become non-functional (it is in a critical application). I also need at least a case depth of .010-.015 in order for the part to survive a battering force that occurs on a periodic basis. I looked at QPQ but it has a finishing step with abrasive media which would destroy the sharp edge.

I would be comfortable trying a coating if it could be applied over the case hardened skin, would not flake and would keep the sharp edge.

thanks
Bill

 

[unclesyd]

A good one to try is TiN as used by the tool industry. This is the least expensive and most readily available.

There are several others that will not hurt the edge.

Any coating that is used by the razor blade people would be a candidate, even the old Pt coating.

 

[mewhg]

Unclesyd,

What is the old Pt coating??????

 

[unclesyd]

That was a vapor deposition, not sure of the process, coating of Pt that was used by one of the major razor blade manufacturers several year ago. It was put forward a stay sharp slick coating for razors blades. We tested it and from what I remember it did improve the friction coefficient of the coated component but not the wear resistance. Our applications were highly loaded. I think the razor blade also benefitted by the Pt lowering the corrosion on the edge.
I don’t know if I still have any information but will look around.

I think the TiN with all it’s variations is probably the way to go.

 

[JTreglio]

TiN could work, but it depends on the heat treat of the metal. Standard PVD requires the parts to be heated to 450 for 1-2 hours. If temperature is an issue, CrN might be a better bet. CrN deposited at 350 C or so is still a pretty good coating, and CrN has a lower elastic modulus than TiN. You can also consider solid lubricant coatings, also deposited by PVD but at much lower temperatures. There are many competing versions.

One thing to be careful of is that TiN is not TiN is not TiN. By that I mean TiN is the composition of the coating, but the composition is just one factor, and a minor one at that, in determining the suitability of the coating. Other factors include adhesion to the substrate -- the coating isn't worth much if it doesn't stick to the part -- stress in the coating, coating structure, et al. The amazing thing is that engineers familiar with all of the variations of chrome plating -- and all of the things that can go wrong -- assume that all TiN coatings are the same. It just ain't so.

Jim Treglio
Molecular Metallurgy, Inc.

 

[mewhg]

Jim,

I think heating the parts to 450 for another hour would lower the hardness of the case to a point below 90 R15N. I might be able to get away with that temp for 15 min or so...... How long is the heating time for CrN and could you suggest a supplier?

many thanks
Bill

 

[rnd2]

Consider NiHard

 

[mewhg]

rnd2,

Is the NiHard you are referring to a cast alloy (from what I gather on an Internet search)? If so I don't think it would be a substitute for the low alloy steel I am using.