A lubricant contains zinc oxide nano particles and carbon FMs, the coefficiet of friction is high while the wear scar diameter is less for the samples. The load was 392N @1200rpm @75degree C.
This is a high Hertzian pressure loading as I understand from your question, Thus friction modifier will affect the properties of the film specially at the beginning of the test and therefore the friction is higher, but as you move on zinc oxide will start forming solid tribofilm with the additives of the lubricant which is a wear resistant film and you will be able to see a decrease in the wear scar.
The agglomeration or clusters of the nano-additives are probable to occur in the lubricant owing to increase of the concentrations, which can be larger than the film thickness in the contact area. In this case, the nanoparticles play a role as an abrasive body (third body) in the direct contact area and lead also to increase the friction coefficient, especially under high loads (392N). Besides, the nano-additives (carbon and ZnO) become mechanically unstable causing the broken and exfoliation to form the third body and adhesion on the worn surfaces. These conditions with high temperature lead to the tribochemical reaction between the nano-additives and tribopair causing the formation of a self-tribofilm on the frictional surfaces to separate the asperities. Hence, the wear scar will be reduced.
I would agree to Dr. Ali. I have also experienced similar results in one of my works. Here the additional frictional effect may be attributed to the friction between separated particles and the pin (if it is a pin on disc tribometer).
i also know that friction modifier will cause irregular activity with the lubricant additives at the onset causing higher friction.
Increase in friction may be attributed to the interference of nanoparticle aggregation in film region, but in contrary decrease in wear due to formation of sacrificial tribolayer.
As I said earlier modifier will increase friction at the onset till the temperature settle
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