Improvement of scratch resistance in transparent hard surfaces through layer-by-layer coating

Layer-by-layer (LbL) coating can be a promising method to ameliorate the mechanical robustness of soft surfaces. To improve scratch resistance, we prepared composites in which a polymer and montmorillonite (MMT) were alternately deposited on hard-coating surfaces. The alternating deposition of each LbL layer was confirmed using the water contact angle and surface morphology, and the resulting LbL-coated hard surface had excellent optical transparency of over 95 % in the entire visible region. In addition, nanoindentation confirmed that the mechanical properties of the hard-coating surface were not considerably impaired even after the LbL coating. When the scratch creation characteristics were comparatively estimated using common mechanical analysis tools—such as nanoindentation, Vickers indentation, and dynamic-scratch tests—it was clearly noted that the MMT-coated surface exhibited superior scratch resistance, by >12 %, compared to other films in the dynamic-scratch test. Indentation methods only reflect the surface response to normal stress, but dynamic-scratch evaluations can identify the response to oblique-directional stress commonly working for scratch creation. Because planar MMTs reduce the actual friction against oblique stress, MMT-coated surfaces exhibit lower friction in dynamic-scratching test, ultimately providing improved scratch resistance.