Stainless on Stainless - relationship between galling and friction 1

[loktinl]

Hello all,

I have a question regarding a biomedical design project I'm working on that I'd love to hear people's opinion on. I work in a small team here, so I'd love to get some exposure from others who have more experience with mechanisms design in the biomedical tools field, specifically with surgical tool development. I'm fairly green to this forum, so I apologize for any 'dumb questions' or poor posting etiquette in advance!

I'm designing a mechanical device that requires sliding between two stainless steel parts. Friction needs to be kept at a minimum (I have a quantitative idea on how low but we'll leave numbers out for now as I'm not 100% confident with my estimates yet). I've experienced galling before (I noticed excessive scoring on the softer of the two mating surfaces), so I referenced some materials and wear properties literature ("R.G. Bayer, Mechanical Wear Prediction and prevention, Marcel Dekker, Inc., 1994" - I've uploaded the page for reference) and I changed material choice from 440C SS (55-58 HRC) on 17-4SS (H900) to 440C on 316L, thinking I would reduce wear particulates and reduce sliding friction forces. What I noticed was that friction forces increased when I changed to 316L from 17-4. Galling did not occur in this new situation though.

So the question is, does increasing threshold galling stress by changing SS material couples mean a reduction in sliding friction as well? That was my assumption, but perhaps I'm wrong. Also, does a larger difference in hardness also mean reduction in friction and chances of galling? I noticed that the chart says the 440C is at 560 Brinell = 55HRC - my specimen was tested at around 55-58. Finally, I don't believe there were any gross differences in surface roughness of the 316 and 17-4.

Thank you in advance, maybe someone out there has some insight on this?

loktinl

 

[desertfox]

Hi loktinl

I don't think increasing the galling threshold stress will give you lower friction because friction isn't dependant on surface area but on the force clamping the two components together.
I read the attachment as saying that the galling stress is higher or lower for a particular combination of materials.
If we have 440C and 410 combined together then the threshold for galling is 21Mpa, change the 410 for 416 and the threshold increases to 145 Mpa.
I can increase the galling threshold stress by simply changing material grade, however the same two components still have the same surface area and the same clamping force.

Changing the surface finish on the components in contact will change the friction though, the smoother the finish, the less friction you will encounter.

 

[desertfox]

Note highly polished surfaces can reduce galling threshold, have a look at these references below:-

http://www.mddionline.com/article/understanding-wear-and-friction-medical-grade-stainless-steels

http://www.assda.asn.au/images/PDFs/FAQs/faq5.pdf

 

[EdStainless]

SS grades with high Mn and high work hardening rates are less prone to gall.
Using Nitronic 60 on 44C would give much higher galling resistance.
Friction comes down to surfaces. Getting them flat and smooth (<10 microinch) but not polished will be critical.

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P.E. Metallurgy, Plymouth Tube

 

[loktinl]

Thanks for everybody's input! All the links provided have been very useful.

About this comment:

Changing the surface finish on the components in contact will change the friction though, the smoother the finish, the less friction you will encounter.

and this:

Friction comes down to surfaces. Getting them flat and smooth (<10 microinch) but not polished will be critical.

Does this mean that the friction coefficient between two materials, mu, are predominantly dependent on surface finish over other things such as hardness and Alloy composition? As a thought experiment, if I had 6061 T6 Aluminum on 316SS, at a clamping force and surface roughness for the two surfaces, and compared the friction forces to another material combination like 316SS on 440CSS with the same clamping force and surface roughness, can I expect very similar friction forces? My guess is no, but I'm intrigued as both the comments largely mentioned surface roughness as the main determining factor to the friction coefficient between two materials.

Maybe I should repost this on the tribology section...once again, I really appreciate it everybody, its great tapping into such a wealth of knowledge!

loktinl

 

[desertfox]

Hi

I don't know enough about friction to quantify that surface finish dictates sliding friction anymore importantly than alloy composition or hardness but I believe surface finish does have a major role to play.

Have a look at this link:-

https://pantherfile.uwm.edu/menezesp/

http://www/IJT.pdf

Some alloy composition materials have lead or some other material like carbon flakes in grey cast iron, in which both lead and the carbon in these materials act to give lubrication to mating parts and reduce friction.

Lubrication can also reduce friction in a major way, take for example a nut and bolt without lubrication and torque test it to failure, repeat the exercise with a oil lubricated nut and bolt of the same size and you will find the latter might well fail at only a quarter of the torque value the unlubricated fasteners achieved

 

[racookpe1978]

You're flipping back and forth between two very similar, and very much related problems - but they are still different problems in metal-metal contact of two surfaces under pressure.

Consider galling as low-temperature welding between contacting surface under high pressure and modest or low temperatures.
Galling is worst under high pressure. Increase the surface area and reduce teh pressure, reduce the forces if you can.
Galling gets much worse when both surfaces are the same (or are very compatible), have a lot of areas that touch intimately (very smooth surfaces that are very flat. Very smooth but corrugated have little touching areas for example).
Galling gets worse when both surfaces are very clean. ( try lubrication or deliberate "contamination" with a safe coating. Unfortunately, you need that cleanliness for most bio-medical devices most of the time. In space, for example, the vacuum and surface outgassing of two bare metal surfaces means many contacts can "weld shut" even under light pressure that would not be a problem in the regular atmosphere and regular contamination.)
Galling seldom happens when two surfaces are constantly moving: Galling is worse when the two surfaces have a lot of time being squished together so the surfaces can fuse with each other. For example, a high-strength coupling bolt between a turbine and a generator shaft will often gall when that bolt is left in place for several years under very high stresses.

So keep the two parts moving, lubricated, and under low pressure with one another.

Friction is the resistance to motion between two parts: And galling can be a part of the static friction that must be overcome before motion begins. But once motion begins, a galled part may drag or gouge a deep scar in the other part, but that's impromptu machining using galling to detroy the parts, not really a part of the dynamic friction between the two assemblies. the galled weld spot already happened before the movement started.

 

[EdStainless]

Thanks Racookpe, good statement.
Friction is not a function of surface area, if you measure it and it is a function of surface area than you are measuring traction, and the surfaces are mechanically interacting with each other. This can lead to galling.
My suggestion, one surface as hard as you can set it, the other medium hard and in a gall resistant alloy (Nitronic), grind reasonably smooth (but not polished), and coat with PVD or CVD friction reduction coating (WS2).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[loktinl]

Great! Thanks for the suggestions, I'm augmenting my next prototype based on everyone's input. Thank you!

As a junior engineer working in a small team with little supervision, I don't get a chance to work with many other engineers. I truly value the input I'm getting from everyone, makes me feel like there's people investing in my success.

loktinl

 

[3DDave]

Another factor to consider is that in stainless steel you are not often contacting between metals, but the oxide film on the metal. Breakdown of the oxide film is what leads to galling as the bare, active, chromium forms welds across the pieces. Often galling can be limited by reduced roughness of the harder part which decreases the chances of high loads breaking the oxide film.