Structure and stress of TiAlN/CrN superlattice coatings as a function of CrN layer thickness

Abstract

TiAlN/CrN superlattice coatings have been grown in an industrial sized multiple target PVD coater by a combined steered arc/unbalanced magnetron process. The coatings were deposited using three Ti 0.5 Al 0.5 alloy targets and one Cr target. The power to each of the Ti 0.5 Al 0.5 targets was maintained at 8 kW in all the processes, whilst the target power to the Cr target was varied between 1 kW and 12 kW to produce different superlattice period thickness. The structural characteristics were determined by X-ray diffraction using both Bragg–Brentano and glancing angle parallel beam (sin2ψ method) geometries. Superlattice period thickness was determined using low angle X-ray diffraction and was found to increase from 2.2 nm at 1 kW Cr power to 4.8 nm at 12 kW Cr target power. An approximately linear dependence of Cr power on superlattice period thickness was also observed.A maximum Knoop hardness of HK0.025 3500 was observed at 8 kW Cr power when the thickness of the individual layers was equal. The residual state of stress was found to be compressive in all cases rising from 3.25 GPa at 2 kW Cr power to 6.5 GPa at 10 kW Cr power. Lattice parameters were determined using the Cohen–Wagner method and were observed to increase from 0.4183 nm for coating deposited at 2 kW Cr power to a maximum of 0.4197 nm at 10 kW Cr power.