Residual stresses in thermally cycled CrN coatings on steel

CrN coatings were deposited on polycrystalline ferritic steel substrates at 350deg. C by magnetron sputtering using Cr targets in Ar + N2 atmosphere. In order to simulate the thermal fatigue, the samples were repeatedly irradiated using a laser beam of 6mm in diameter. The thermal cycling was performed in the range of 50-650 deg. C with up to 100 000 cycles. Subsequently, the structures were characterized using high-energy synchrotron and high-temperature laboratory X-ray diffraction. The structures exhibit complex changes in the morphology and in residual stress state in the heated spot. The annealing results in the relaxation of compressive stresses in the coating and in the formation of high tensile stresses in the steel substrate. This effect decisively depends on the number of applied cycles. The reduction of compressive stress in the coating is caused by the annealing of point defects and by dimensional changes of the substrate due to its plastic deformation in the center of the irradiated spot. The plastic deformation of the substrate is also the probable reason for the ripples observed for samples cycled more than 3000 times. The presented approach allows a complex characterization of thermo-mechanical processes in coating-substrate composites and opens the possibility to understand phenomena related to the thermal fatigue of coated tools