Particles with different surface friction segregate under shear, but the underlying micro-mechanisms and their dependence on basic flow properties remain largely unexplored. In this study we use discrete element method simulations to investigate the vertical segregation that occurs when highly frictional (rough) and less frictional (smooth) particles are mixed in dense granular flows. We systematically vary the interparticle friction coefficients and flow parameters-including shear rate, confining pressure, gravity, particle size, and density-to explore the conditions under which segregation occurs. Rough particles rise to the top while the smooth particles move downward. The efficiency of segregation strongly depends on the ratio of the smooth over the rough particle surface friction, driven by asymmetries in granular temperature, rotational velocity, contact connectivity, diffusion, and contact force distributions between particle types. At low friction ratios, segregation is weakly dependent to friction differences but remains sensitive to flow conditions and dynamic asymmetries. As the friction ratio is increased beyond a threshold value, these asymmetries diminish, leading to uniformly reduced segregation across flow regimes.