Coefficient of friction of different steels

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Discussion Overview

The discussion revolves around the coefficient of friction (COF) of different types of steel, specifically comparing austenitic steel and 9%Cr steel. Participants explore whether the differences in tribological properties among these steels are significant enough to influence the design of components that will slide against each other.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant states that the steel-steel COF is about 0.8 and questions if the differences between types of steel are significant for design considerations.
  • Another participant suggests conducting measurements to determine the COF directly.
  • A third participant notes that the COF can vary widely due to factors such as temperature, surface finish, and contamination, indicating that the value of 0.8 may not be precise and could range from 0.3 to 1.3.
  • A later reply expresses skepticism about the 0.8 value, suggesting that the kinetic COF for dry steel on steel might be closer to 0.18, and for stainless steel, around 0.20, while also mentioning the relationship between static and kinetic COF.
  • Participants agree that testing the COF is relatively straightforward, implying that empirical measurement could clarify uncertainties.

Areas of Agreement / Disagreement

Participants express differing views on the accuracy of the COF value of 0.8 and its relevance to design. There is no consensus on the exact values or the significance of the differences in COF among various steels.

Contextual Notes

Limitations include the variability of COF based on numerous factors such as surface conditions and environmental influences, which are not fully addressed in the discussion.

bill nye scienceguy!
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I know the steel-steel COF is about 0.8. Are the tribological properties of different steels, eg austenitic and 9%Cr, sufficiently different that this difference should be taken into account in the design of a component that will slide over two different steels, or sufficiently similar that the difference is negligible?

I hope this question makes sense.
 
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Hi

i would do a measurement.
 
The friction coefficient is highly dependent on many things like temperature, surface finish, surface contamination, relative velocity, normal pressure, amplitude of relative motion if it is oscillating, etc. From my own experience I would take "about 0.8" to mean "anything between about 0.3 and 1.3, except for situations when it is outside of that range".

As Shockman said, if this is important to your design, measure it. Even better, change the design to make it less important, so it doesn't matter.
 
bill nye scienceguy: I know some sources list static coefficient of friction (COF) for steel on steel as 0.80, but I would be skeptical about that value. Are you referring to static COF, or kinetic COF? I would think a better estimate of kinetic COF for dry steel on steel is ~0.18. And I imagine an estimate of kinetic COF for dry stainless steel on stainless steel would perhaps be about 0.20 (?). Also, static COF is generally roughly 1.33 times kinetic COF.

But fortunately, COF is relatively easy to test.
 

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