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What is Which of the following alloys has the highest coeﬃcient of friction? and explain in detail?
The surface of NiTi is rougher (because of surface defects, not the quality of polishing) than that of beta-Ti, which in turn is rougher than steel. However, there is little or no correlation for orthodontic wires between the coefficients of friction and surface roughness. (i.e., interlocking and plowing are not significant components of the total frictional resistance). 
Although NiTi has greater surface roughness, beta-Ti has greater frictional resistance, it turns out that as the titanium content of an alloy increases, its surface reactivity increases, and the surface chemistry is a major influence on frictional behavior. Thus, beta-Ti, at 80% titanium, has a higher coefficient of friction than NiTi at 50% titanium, and there is greater frictional resistance to sliding with either than with steel.
Key Concept
Although NiTi has greater surface roughness, beta-Ti has greater frictional resistance.
Ref: Proffit 4th edition page 378