Effect of CrAIN coating properties on impact fatigue of tool steel

Abstract

The tools in forming processes are subjected to cyclic impact loads. Physical vapor deposition (PVD) coatings can potentially improve the service life of such tools, given the effect of coating properties on impact fatigue of coated substrate is well-understood. Previous investigations on the cyclic impact loading of PVD coated tool steels mainly correlate the plastic deformation of the substrate to the resulting coating cracks. Hereby, the effect of underlying elastic-plastic coating deformation mechanism in combination with coating properties needs further clarification. Therefore, the current work aims to investigate the combined effect of thickness, morphology, elastic-plastic deformation and residual stress state of the coating on the impact fatigue behavior of coated tool steel substrates. For this purpose, (Cr90Al10)N and (Cr66Al34)N coatings, each with a thickness of s = ∼ 1.7 μm and s = ∼3.5 μm, were deposited on HS6–5-2C substrates. In addition to coating characterization, the residual stresses were measured by focused ion beam-digital image correlation (FIB-DIC) ring-core method. The elastic-plastic deformation of the coatings was studied by nanoindentation. The coated samples were subjected to cyclic impact loading with an initial hertzian contact pressure pH = ∼ 9.7 GPa and frequency f = 50 Hz for N = 0.1, 0.5 and 1 million impacts. The impact imprints were analyzed using high resolution electron microscopy to study the coating deformation mechanism in combination with fatigue cracks initiation. Columnar inclination and shear gliding along the column boundaries resulted in the initiation of fatigue cracks in the coating around the boundary region of impact imprint. The chemical composition, morphology, residual stress state and elastic-plastic deformation behavior of the coating cumulatively affected the impact fatigue as thick coatings showed reduced resistance against initiation of fatigue cracks. The investigation contributes to adjusting the PVD coating thickness and resulting coating properties for higher tool service life in applications involving cyclic impact loading.