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topic 4179

Nickel Teflon Coating On Nitrided Steel Concerns


(2000)

I require technical assistance and information relating to issues with Electroless Nickel plating of Nitrided Steels. Our company is currently investigating the viability of utilizing the NicoTef process, in an attempt to reduce friction, on plain bearing, shaft journals.

However, we have a number of reservations with the process due to insufficient data which, is not allowing us to reach a conclusive position.

We are hoping that some of the expertise in this forum is able to shed some light on our questions and/or allay our concerns.

Process - Nickel-Teflon (Nico-Tef)

Material - Molybdenum Manganese Steel (Previously Nitrided)

1. Would a chemical reaction be expected by immersing the Molybdenum Manganese Steel material, that has been Nitrided, into any of the solutions used in the Nico-Tef process and or any of the cleaning processes?

2. If a reaction did occur would this cause surface distress/damage to the metal surface?

3. What effect will the Nitrided condition of the material have in the processes?

4. Has any long term detrimental damage been done to the crankshaft as a result of the chemical reaction? e.g. has the Ammonia / Nitriding / Chemical reaction compromised the integrity of the material? Embrittlement?

Thank you

Kind Regards

Peter Vucinic -
computerized bearing analysis - Bendigo, Victoria, Australia


(2000)

Peter,

I can't speak specifically about the Nickel Teflon coating process, but can speak about the nitriding process.

It sounds like you are dealing with a gas nitrided steel by your reference to ammonia. This is a thermal treatment process in which nascent hydrogen atoms diffuse into the steel and form nitride compounds with many of the alloys in solution in the steel. Molybdenum, vanadium and especially aluminum are the best nitride formers. This is not a coating at all but a change in the structure to a depth of .010-.015" or so. You don't specifically state depth or the alloy type so that's hard to determine.

Bottom line is that the nitrogen in the nitrided component is not free nor likely to react with a coating. It is present as a compounded aluminum or molybdenum nitride. It's also present in very minute amounts, and present in the diffused layer.

A natural byproduct of gas nitriding is a white layer, which is a surface phenomena, about .0003-.0005 thick. This is pure iron nitride, which acts as a nitrogen "feeder" layer for the diffusion. This is brittle but is typically removed post nitride by mechanical means, although there are commercially available acid salts for removing it.

I would expect no more chemical reaction with the nitride in the material than with the carbides typically present in tool steels or high carbon alloy steels. Nitriding is commonly used on crankshafts and bearing surfaces, and since it's done around 975 F, embrittlement is not a typical concern.

I hope this information is helpful in allaying some of your concerns for the nitriding process.

John Pease
flight systems - Shelby, North Carolina



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