Which of the following alloys has the highest surface roughness?
(A) Stainless steel (B) Elgiloy (C) NiTi (D) TMA
(C) 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).

Ref: Profitt 4th edition page 378