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 (HIT) 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, HIT 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 HIT 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 HIT = ~43 GPa. © 2015, Springer Science+Business Media New York and ASM International.