Designing the color of AlTiN hard coating through interference effect

Hard coatings are commonly used in decorative applications due to their attractive colors and high wear resistance. In most cases only intrinsic colors of the coatings are exploited. Such colors are determined by the stoichiometry of the coating and can be varied only in a limited range. We demonstrate a more flexible concept of color design by utilizing the interference effect in semi-transparent and reflective materials. In this design a thin film of partially transparent hard coating material is deposited over a material with specific reflectivity. Such construction enables wider color control since the interference-based colors can be tuned by the thickness of the interference layer and by the optical constants of the thin film and the reflective substrate. In this work such principle is demonstrated with AlTiN. A single-layer of AlTiN was deposited by DC magnetron sputtering on substrates with different reflectivities. Colors of high saturation, ranging from yellow, pink, violet to blue, were prepared by varying the thickness of AlTiN in the range of 20-55 nm. Optical properties were studied by spectrophotometry and ellipsometry. Only small color shifts were measured with the angle of observation. Interference-based color design is further demonstrated on the tools with complex geometry combining protective and decorative functionality of the coating. A uniform blue color was prepared in an industrial deposition system on drills of different sizes.