Cr(Al)N/Al₂O₃ Superhard Coatings Prepared by Differential Pumping Cosputtering: Structure and Mechanical Properties

Various Cr(Al)N/Al₂O₃ coatings were fabricated in a differential pumping cosputtering system, which has two chambers for radio frequency magneton sputtering of different materials and a substrate holder rotating above the target materials. The main product of Cr(Al)N/Al₂O₃ layers that were sputtering deposited from CrAl and Al₂O₃ targets on transition buffer layers prepared on the Si substrate had a columnar structure normal to the substrate surface. The buffer layers were prepared successively in the same chamber so that their compositions were to gradually change from metal (Cr) to nitride (CrN), and appropriate as adhesion between the metal substrate and the composite nitride layer. In a Cr(Al)N/17 vol% Al₂O₃ coating prepared at a substrate rotational speed of ω = 12 rpm, each column comprised NaCl-type Cr(Al)N crystallites including homogeneously dispersed fine particles of amorphous (a-) Al₂O₃. The nanoindentation hardness (H_IT) and Young’s modulus (E*) of the Cr(Al)N/Al₂O₃ coatings increased with increasing ω within a measured range of ω = 1–12 rpm. With increasing Al₂O₃ fraction, H_IT and E* increased until maximum values and then decreased, showing that the hardness of the nitride coatings is improved by fabricating the nanocomposite layer. The maxima of H_IT and E* were obtained at 17 vol% Al₂O₃ for the coatings prepared at ω = 12 rpm. The fine a-Al₂O₃ particles in the Cr(Al)N/17 vol% Al₂O₃ layer worked as obstacles against the deformation of the Cr(Al)N lattices, which yielded the superhard coating of H_IT = ~43 GPa.