Its a question I have read in a book, that,
When two surfaces are polished, the frictional coefficient decreases. But if the surfaces are made highly smooth, the frictional coefficient increases and becomes very large. Explain why?
My thinking says that with excessive smoothness there is more ease of bond formation between the two surfaces and since friction (modern theory) is because of temporary bond formation, friction increases and hence its coefficient. Is this thinking right?
Friction depends partly on the smoothness of the contacting surfaces, a greater force being needed to move two surfaces past one another if they are rough than if they are smooth. However, friction decreases with smoothness only to a degree; friction actually increases between two extremely smooth surfaces because of increased attractive electrostatic forces between their atoms.
That is, if you make the surfaces very flat and smooth, and remove all surface contaminants in a vacuum, the smooth flat surfaces will actually adhere to each other, making what is called a "cold weld". Once you reach a certain degree of mechanical smoothness, the frictional resistance is found to depend on the nature of the molecular forces in the area of contact, so that substances of comparable "smoothness" can have significantly different coefficients of friction.
