In this study, two CrAl/CrAlN multilayer coatings of 1.5 µm and 1.2 µm thicknesses were deposited from a (50:50) CrAl sectioned target by high-power impulse magnetron sputtering (HIPIMS) onto A11 tool steel to evaluate the effect over mechanical and tribological properties by the addition of an (a-CNx) top layer. XRD analysis showed a CrAlN FCC structure (111) and a Cr phase (110). A glass-like morphology was observed in the CrAl layer and in the (a-CNx) top layer, while the CrAlN layers exhibited a columnar morphology according to the FE-SEM analysis. Raman analysis results confirmed characteristic peaks G and D for the carbon amorphous structure of the (a-CNx) top layer. Nanoindentation results showed comparable hardnesses (23.08 GPa ± 2.8 and 20.58 GPa ± 1.8) in both multilayer coatings. The CrAl/CrAlN multilayer with an (a-CNx) top layer increased the absorption energy capacity and the mechanical potential energy stored. The nano scratch test showed the strongest crack resistance and highest adhesive energy values for the CrAl/CrAlN multilayer. The addition of an (a-CNx) top layer improved the tribological behavior when higher normal loads were applied during the fretting test. Raman analysis showed an increase in the I(D)/I(G) ratio and a formation of a carbon sp² structure (i.e., graphite-like structure) over the worn surfaces, as well as dissipated energy (J/m) during the fretting test.

Dual Phase (DP) steel, composed of a ferrite matrix with dispersed islands of martensite, has become popular in auto-body car construction due to its outstanding mechanical properties (i.e., high strength and good ductility). DP steel softens at the sub-critical heat-affected zone (SC-HAZ) when subjected to welding thermal cycles, owing to the tempering of the martensite phase. In this work, DP980 steel was subjected to varied thermal cycles: (a) furnace-tempering treatment, (b) gas metal arc welding (GMAW), and (c) resistance spot welding (RSW), in order to characterize the tempering of martensite below the Ac₁ critical temperature and at the sub-critical heat-affected zone (SC-HAZ) in the case of the welded specimens. The coarsening stage of cementite phase was characterized through microstructure observations and hardness measurements. As expected, the comparative results indicated an advanced stage of the martensite tempering in the furnace heat-treated specimens, followed by the GMAW and the RSW specimens. Further, developed softening kinetic models have been suitably employed and adjusted in order to predict the extent of softening along the SC-HAZ of the GMAW specimen. Finally, as the advanced stage of cementite coarsening is due to the influence of the arc welded thermal cycle, a reasonable estimation of the hardness profile was obtained, particularly for tempering temperatures above 400 °C.