*2.6. Frictional Properties Measured by the Progressive Load Scratch Test*

A microtribometer (model CETR-UMT-2; Bruker, Billerica, MA, USA) was used to characterize the frictional properties by the progressive load scratch test. A diamond stylus having a 12.5-μm tip radius was moved 5 mm over a specimen surface with linearly increasing normal load (0.5 to 20 gf) at a constant speed of 0.016 mm/s, and the value of the friction coefficient was obtained (*n* = 5).
