Effects of film thickness on structural color of DLC films deposited by plasma CVD method

Diamond-like carbon (DLC) films are known as a functional material having low friction, high wear resistance, high hardness, and so on. Their main component is carbon, and they have both graphite (sp 2 bonding) and diamond (sp 3 bonding) structures. Therefore, DLC films are applied to tips of cutting tools, surface of molding dies, sliding surface of machine elements, etc. The thickness of typical DLC films is approximately 1–2 μm. And DLC films with multi-interlayers provide achromatic colors. Colors of DLC films are generally black or brown. If their colors could be controlled, various decorative products can be expected to be applied on them. This study aims to examine the structural color of DLC films and focus on the effects of film thickness on structural color. The thicknesses of DLC films varied from 20 to 80 nm using plasma CVD method. And the relationship between film thickness and the three attributes of color, namely hue, lightness, and saturation, was investigated. Based on the color system, the dominant wavelength “ λ d ”, the parameter Y , and the excitation purity “ p e ”, which correspond to hue, lightness, and saturation, respectively, were calculated. And the variations of these parameters were confirmed to be caused by the thickness of the films. As a result, the colors of the DLC films were able to be changed with the films having various thicknesses. And we discussed the reason for the structural color. We found that the dominant wavelength “ λ d ” increased with increasing the thicknesses of the films. On the other hand, parameter Y increased below 50 nm of the film thickness and decreased above 50 nm. The excitation purity “ p e ” decreased below 50 nm of the film thickness and increased above 50 nm. ● The colors of DLC films changed from blue to green, and green to yellow as increasing film thickness. ● The dominant wavelength “ λ d ”, parameter Y and the excitation purity “ p e ” varied with changing film thickness. ● Parameter Y , which corresponds to lightness, increased below 50 nm of the film thickness and decreased above 50 nm. ● The excitation purity “ p e ”, which corresponds to saturation, decreased below 50 nm of the film thickness and increased above 50 nm.