I take your terms ‘tribolayer’ and ‘glaze layer’ to refer respectively to diffusely and mirror-like reflecting surfaces. In effect the surface layer of the sample being tested is being smoothed at the micro level by the rub test and is becoming more mirror-like. In consequence light reflectance is more diffuse before and more directionally oriented after rubbing. In practice the surface reflectance is typically a proportionate combination of both modes. The light grey shiny wear-track after rubbing thus reflects more light directionally, and may appear slightly lighter and the rougher un-rubbed surface may appear slightly darker due to light scattering.

thank you, Dr David Oulton .