Thermally-induced formation of hexagonal AlN in AlCrN hard coatings on sapphire: Orientation relationships and residual stresses

Cubic AlCrN coatings were epitaxially grown onto Al2O3(00.1) substrates by reactive magnetron sputtering at 500°C from Al/Cr targets with an atomic ratio of 70/30. The coatings were vacuum annealed at 1000°C for 2 hours in order to induce formation of wurtzite-type AlN. The as-deposited and annealed coatings were characterized using X-ray diffraction techniques. Pole figure measurements revealed orientation relationships of the cubic AlCrN phase with respect to the substrate. Residual stress characterization indicated compressive stresses of -1246 MPa in the as-deposited cubic AlCrN phase. After annealing, the residual stresses in the hexagonal wurtzite-type Al(Cr)N and the Al-depleted cubic Cr(Al)N phase are -132 and 346 MPa, respectively. The stress changes can be interpreted as a consequence of point defect recovery at temperatures above deposition temperature and Al(Cr)N formation in the annealed coating.