Phase and Residual Stress Evaluation of Dual-Phase Al70Cr30N and Al80Cr20N PVD Films

Abstract

We investigated arc ion-plated Al70Cr30N and Al80Cr20N thin films deposited with three different bias voltages (50 V, 100 V, and 150 V) to study crystal phase stabilities, residual stresses, and mechanical properties. Commercial compositions of AlxCr100–xN coatings typically range from x = 50 to 70 where the cubic face centered crystal phase occurs. The present study focuses on films near the solubility limit of Al in the cubic Cr(Al)N lattice around 70 at.%, above which hexagonal AlN (h–AlN) starts to form in significant amounts. Residual stress values are obtained by two methods: grazing incidence diffraction with the wholepattern fitting and the conventional side inclination method (sin2Ψ method). When multiple phases are present in the film, wholepattern fitting turns out to be particularly effective and a comparison of both measurement methods will be discussed. The Al70Cr30N films consist of the cubic phase with crystallite sizes of about 70 nm for all bias voltages. Compressive stress increased with bias voltage from about 3 to almost 6 GPa and coatings become brittle. Al80Cr20N films showed a different dependence on bias voltage. Using 50 V bias voltage in deposition, the major phase is h–AlN phase with a crystallite grain size of < 30 nm and (0002) preferred orientation. With increasing bias the cubic phase is stabilized also reaching about 70 nm crystallite size. In general, the compressive residual stress was significantly lower than for Al70Cr30N films for the same bias voltages which may be a result of the presence of the hexagonal phase. Wear and scratch tests confirmed higher ductility of the Al80Cr20N variants but reduced resistance of the films in impact wear testing.