Abstract
Subsequent electron beam hardening (EBH) of hard coated steels results in a significant improvement of the load-bearing capacity of the base material. The success of this combined treatment is based on the coatings' resistance to crack formation. This paper deals with the cracking behaviour of magnetron-sputtered Ti1-xAlxN hard-coatings on low alloyed QT-steel (51CrV4) during hardening of the base material by means of an electron beam (EB) technology. Ti0.68Al0.32N and Ti0.37Al0.63N with thicknesses of 1μm and 3μm, respectively, were studied, enabling systematic investigation of crack formation, which is determined by the coating properties. Experiments included instrumented indentation hardness testing, X-Ray diffraction and examination of surfaces and cross-sections with light microscopy (LM) and scanning electron microscopy (SEM) as well. Results indicated a strong influence of residual compressive stresses (σr) in the hard coatings on failure during EB hardening. It could be detected that relaxation processes occurring at the elevated temperatures, reduced σr significantly. Two failure mechanisms were distinguished: one was associated with the thermal expansion of the base material, while the second was related to a severe surface deformation caused by martensitic transformation of the steel.
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