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Table 1 Hardness of the common materials versus TiN, AlN and TiAlN for surface coloring. The Bragg coating is a commercially available mirror (EKSMA Optics, 042-5135)

From: Wear-resistant surface coloring by ultrathin optical coatings

Materials Hardness [49] Evaluated thin-film Mohs hardness by scratch tests.
Metallic coatings angle-independent Yes
Au 2.5-3 <1.5 (30-nm-thick on c-Si, <10B pencil)
Al 2.5-3 <3 (30-nm-thick on c-Si, 5H pencil)
Cu 3 <3 (30-nm-thick on c-Si, 4H pencil)
Pt 3.5 3 (30-nm-thick on c-Si, 10 H pencil)
TiN 9 7-9 (50-nm-thick on sapphire, >quartz; <sapphire)
Semiconductor coatings angle-independent Yes
Ge 6 <3 (30-nm-thick on sapphire, 8H pencil)
Si 6-7 <3 (30-nm-thick on sapphire, 8H pencil)
TiAlN 9 7-9 (50-nm-thick on sapphire, >quartz; <sapphire)
Dielectric coatings angle-independent No
AlN 9 7-9 (100-nm-thick on glass, >quartz; <sapphire)
Bragg coatings angle-independent No
Multilayer dielectrics 5.5-6.3 (6.5 μm on glass, <stainless steel)
SM-type coating angle-independent Yes
Si(50nm)-Au(50nm) <2 (substrate: Si wafer, 6B pencil)
Ge(50nm)-Au(50nm) <2 (substrate: Si wafer, 6B pencil)
Si(50nm)-Cu(50nm) <2.5 (substrate: Si wafer, 3H pencil)
Si(50nm)-Al(50nm) <2.5 (substrate: Si wafer, 4H pencil)
Si(50nm)-Ag(50nm) <2.5 (substrate: Si wafer, 4H pencil)
Si(50nm)-Pt(50nm) <3 (substrate: Si wafer, 8H pencil)
Si(30nm)-TiN(50nm) <3 (substrate: Si wafer, 9H pencil)
TiAlN(30nm)-TiN(50nm) 7-9 (substrate: Si wafer, >quartz; <sapphire)
FROC-type coating angle-independent Yes
TiAlN-TiN-AlN-TiN 5.5-6.5 (130-nm-thick on sapphire, >glass; <c-Si)