Laser beam surface hardening of CVD TiN-coated steels

Abstract

Properties of hard coatings deposited by chemical vapor deposition (CVD) processes are usually excellent. However, high deposition temperatures may influence the properties of steel substrates negatively. Therefore, a subsequent heat treatment is necessary to restore the properties of steel substrates that are ready for operation (e.g., sheet-forming tools in the automotive industry). By duplex treatment of CVD coating with laser beam hardening, a composite material with flat strength and hardness gradients from the interface to the core, together with a high surface hardness, can be implemented. The supporting effect of the laser beam-hardened substrate leads to an increase of the wear resistance under high surface contact pressures and, at the same time, to an increased material resistance against high mechanical load. Laser beam hardening is applied on CVD TiN-coated AISI 4140 (EN 42CrMo4), AISI A2 (EN X100CrMoV5-1), and AISI D2 (EN X153CrMoV12) substrates. For characterization of composite materials, the chemical composition of the coating and the substrate are analyzed by glow discharge optical spectroscopy (GDOS). Additionally, the microstructure and the substrate hardness are examined. To show the characteristic improvements in comparison with individual processes, scratch tests are executed. The results show great advantages of laser beam surface hardening because only low distortion occurs and the properties of the steel substrates are improved without losing good coating properties.