I read in a book named Introduction To Tribology that when a soft material is in contact with hard material, the soft surface will undergo a plastic flow because it is pressed by the hard asperities of the hard materials. I didn't understand this - what is the meaning of plastic flow and does the stress play some role in the formation of the plastic flow? Please inform me if you know about this.
A plastic flow is a
deformation of a material that remains rigid under stresses of less than a certain intensity but that behaves under severer stresses approximately as a Newtonian fluid.
In other words, let's say you have a sheet of plastic (though plastic flow can happen to not-plastic items) and you put it across two blocks and put weight on it, and measured the stress on the plastic, sort of like in the picture below.
So you are putting weight on it, and the measured stress goes up and up, and you don't even see it bow a bit. Then, all of a sudden, the stress meter hits a certain point and the plastic doesn't snap, but sort of behaves like a liquid, flowing instead of bending or breaking like in the diagram below. That's plastic flow.
(Quote from http://www.dictionary.com/browse/plastic-flow). Hope this helps!