Posted by Hong Wei Wang (220.127.116.11) on 15:02:02 06/05/05
In Reply to: one more question: friction coefficient of WC-12Co coating posted by Arturas
: I would like to know how friction coefficient of WC-12Co coating depends on sliding speed, load and temperature. Is it constant value or not?
: : :
: : :
: : The coefficientnof friction is dependent on many factors, including those you mention plus lubrication (oil, water, dirt etc.), surface finish, counter surface material and properties etc. So the value is not constant unless you apply the exact same test conditions every time.
: : Regards Gordon
: Sorry if my question wasn t full. I would like to get more exact information about friction coefficient. If WC-12Co coating surface finish, counter surface material and properties etc. are constant in all cases (surfaces are cleaned by alcohol) and I am changing only load. How coefficient of friction will change, and after that how does it change after increasing sliding speed (in my cases from 0.3m/s to 1.5m/s) .
: "I got that friction coefficient of WC-12Co coating with increasing load decrease from 1 up to 0.5 (I am increasing load from 30N uo to 150N). And with increasing sliding speed - 1 (speed 0.3m/s) up to 0.7 (speed 1.5m/s)" Is it normal or not ?
: Thank you
THE EFFECT OF LOAD ON COEFFICIENT OF FRICTION (COF):
If every factors are kept constant, the effect of changing load or sliding speed on COF can also be twofold, depending the mating material. The reason being -
(1) If the chosen mating material is metallurgically "incompatible" with the WC-CoCr, the adhesion wear mechanism will be excluded. I would suspect this has been so in your case since otherwise an increase of load should have increased the adhesive friction contribution, hence the COF ( this is opposite to your findings).
(2) The microscopic wear (hence the friction force) would be mostly the abrasive type, which is governed by the relative hardness of the pairs:
(2.1) If the relative hardness is sufficiently high, the plastic deformation and ploughing would be confined to one mating surface mainly and under this condition, an increase of loading would see the friction force, hense COF, increasing, too.
(2.2) If the relative hardness difference is not high enough, the micro-ploughing deformation will occur to both surfaces, blunting the sharpness of asperities, hence the abrasive friction (cutting) will be reduced as the contact load increases.
Therefore, for this reasoning, the COF can really go either ways depending the governing wear (or rather frcition) mechanisms - the mechanisms being determined by really the relative chemstry and simple mechanical properties such as hardness.
The respose of COF to speed change - that again has to be discussed in the context of fixed material pairs and material properties such as strain hardening rates.
Hong-Wei Wang, ERA Technology Ltd, UK
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