Simple Question: which metal property describes the wear resistance ?

[ChristianM]

Hello @ all,
Sorry about that simple/naive question but i think i can´t really answer it my self.

Ok, lets think about following situation:

We have 2 motorcycle rear sprockets:
No.1 is made of an high grade Aluminium alloy (7075-T6)
No.2 is made of standart steel (as used by the OEM Manufacturer of the bike)

The alloy Sprocket wears off twice as fast as the steel sprocket under normal conditions/usage.

So my question now:
Which Property makes the steel sprocket more resistant than the alu-alloy one?

Is it the Tensile Strength or Young's modulus or Vickers hardness or elastic modulus or yield /Ultimate strength?


Many thanks for your answers!
Christian

 

[Ron]

Hardness, but all are factors in the design

 

[desertfox]

Hi ChristianM

Yes hardness of the material is important in relation to wear.

Look at these links:-

http://www.cladtechnologies.com/Articles/Hardness vs. Wear/hardness.htm

http://www.roymech.co.uk/Useful_Tables/Tribology/Wear.html

http://books.google.co.uk/books?id=yU5TyJrOMF8C&pg=PT679&lpg=PT679&dq=material+hardness

+relative+to+wear&source=bl&ots=SAM37uWDqd&sig=veUyVnWwoy0AIlTd7hq4RXKJiBo&hl=en&ei=nfo0S9WUH4n44Aat8K2qCA&sa=X&oi=
book_result&ct=result&resnum=2&ved=0CA4Q6AEwATgy#v=onepage&q=material%20hardness%20relative%20to%20wear&f=false

desertfox

 

[Tmoose]

I don't think there is very often a great answer or an easy to that type of question.

If its a dirt bike sprocket then abrasive wear from the dirt might >seem< to dominate. Gee, then it should be as easy as Throw a hard anodized sprocket in there and it should last "long, long time."

But then, some of the dirt is being squeezed between the chain roller and the sprocket tooth. The hard dirt particles may be forced through the hard-but-thin anodize into the softer subsurface aluminum, like breaking thru the hard candy shell of an M&M candy or dipped ice cream. Now the armour is of little value.

This next bit is probably not applicable to sprocket wear, but Throw "hard" abrasive in an airstream at material to create erosive wear. Then varying the angle of impingement can yield some surprising results.

Then there is the pretty well documented wear resistance of polyurethane materials in some pretty difficult environments, compared to harder rubbers and much harder metals.

Dan T

 

[Tmoose]

missing erosion link

 

[unclesyd]

Here is a very good paper by Dr. Peter Min that covers all aspects of wear.

http://www.solorb.com/dat-heads/digests/V4.100/D100

 

[ChristianM]

Thank you all for your help. Especially the articel off Dr. Peter Min was very enlightening.

But i have two more questinons:

1. Would the hardeness of an aluminium alloy increase if you use an alluminum alloy with an higher tensile strenght (eg. weldalite with 710 mpa instead of the 573mpg of 7075T6)

2.What is the best coating available for aluminium alloys? In terms of increasing hardness and wear resistance because of the high number of load cycles for an motorbike sprocket.
I know the standart is hard anodizing. But what about DLC coating or Shot peening?

Thanks for your help!
regards
Christian

 

[desertfox]

Hi CristianM

Yes generally the higher the tensile strength the higher the hardness, look at the first link and go to page3 it gives alloys of aluminium along with tensile strength and hardness.
The second link gives info on DLC coatings

http://www.aalco.co.uk/technical/datasheets/Aalco_Datasheet_Al_Specifications.pdf

http://www.pvd-coatings.co.uk/Dymon-iC-coating.htm

desertfox

 

[racookpe1978]

Surface area and amount of rubbing surface (which come from the strength of the material being used) affect the erosion rate as well - though it's a secondary effect to what was addressed in greater detail above.

For example, if (after a little bit of wear) the steel has a larger surface exposed to the pressure point, then the psi (same force/large area) goes down, and the metal will continue to wear, but at a lower rate.

If the Al alloy wears to a sharp eroded "point" (by fretting at the edges perhaps) then its pressure goes up, and it will wear away faster. (Same force/smaller net area = greater psi on the remaining metal.)

 

[metman]

Quote: The alloy Sprocket wears off twice as fast as the steel sprocket under normal conditions/usage.

So why bother fooling with Aluminum alloy? Apparently you are thinking that some combination of High strength Al alloy and a special coating of same can outperform Steel in this particular wear application.

Quote: 1. Would the hardeness of an aluminium alloy increase if you use an alluminum alloy with an higher tensile strenght (eg. weldalite with 710 mpa instead of the 573mpg of 7075T6)

Probably but I am not familiar with weldalite so cannot comment. But you will most likely be able to find a steel alloy/surface-treatment/heat treat combination that will outperform any Al alloy/surface-treatment/heat treat combination.

2.What is the best coating available for aluminium alloys? In terms of increasing hardness and wear resistance because of the high number of load cycles for an motorbike sprocket.
I know the standart is hard anodizing. But what about DLC coating or Shot peening?

I like Tmoose's comments especially wr2 M&M candy analagous to anodized Al. Another analogy is a ball or roller bearing race made of carburized steel where the core hardness of the race is improperly heat treated so that the compressive strength of the core is insufficient to support the stress applied to the carburized case and thereby the case cracks under load.

Another factor against the anodized surface is because it is so thin that even though it is very hard, the asbrasive dirt/sand will wear thru this skin relatively quick.

Under bearing load type of compressive stress such as you have with a roller chain against the sprocket, Al is not a good choice because as mentioned above the core strength must be sufficient to support the stress applied thru the hardened surface. Whereas there are steel alloys that can be heat treated to much higher strength than any Al alloy. Also their are a number of surface hardening methods for steel that will provide a thick enough layer to substantially outperform anodized Al under this type of abrasive condition.

Quote: No.2 is made of standart steel (as used by the OEM Manufacturer of the bike)

First you need to determine what is this so-called standard steel. Is it mild steel -- could be but probably not -- or direct hardened carbon steel or direct hardened alloy steel or carburized steel, or flame hardened carbon-or-alloy-steel or carbonitried or nitrided steel, etc, etc.

If the stock Steel sprocket is only outlasting the Al alloy by 2:1, I will guess that the steel sprocket can be much improved from the stock model by upgrading the steel alloy/surface-treatment/heat treat combination.

If you have an anodized Al part sliding against a steel part with lubrication and minimal "contact stress" (bearing stress or compressive stress)and no abrasive materials, the anodized surface will likely last indefenitely. But alas, you have sliding, abrasive friction, with high contact stress.

 

[ChristianM]

@metman

Thank you for your detailed answer.
The reason why I want to use an aluminium alloy is weight. My main goal is to reduce the weight of this rear sprocket as much as possible but keep the wear resistance of an steel one.
If there wouldn´t by the challenge to reduce the weigt, of course I would have planned to use an appropriate steel alloy as you described above.

The properties of the Weldalite alloy sound very promising to me (because of the tensile strength of 710mpa, but a density of 2,6 g/m³)

My company wil thy to machine some sample pieces and I will test them rigorously.

Thanks to all how wote an answer to this thread. This place here is very helfull!

 

[metman]

Yes one of the main advantages of Aluminum alloy is good strength to weight ratio and if this small advantage in weight makes enough difference then by all means persue it and if it makes that much difference I should think that even if you have to replace the sprocket after every race it must be worth the effort and cost.

Maybe there is process for adding a thicker layer of hardfacing on aluminum which of course is what you are asking.

How about steel sprocket teeth with large bored hole to accept an inner housing of Aluminum with HEAVY shrink fit btween the Al and steel? Or another means of attaching the two pieces?

 

[racookpe1978]

You'll get some marketing "words" (key words and tricky phrases) that you can use also: "case-hardened steel sprockets" or "our unique heat-treated steel rear drive sprockets last longer, resist wear better than the competitors' softer aluminum parts" ... Either would conceal/replace/make up for the few tenths of an oz of greater weight.