Practical adhesion of duplex coatings on AISI 4140 steel using HiPIMS and boriding processes

• Effect of substrate bias voltage on duplex (DLC + borided) coatings was evaluated. • Scratch tests revealed distinct cohesive and adhesive failure modes. • Critical loads ranged from ∼7 N to ∼108 N, showing strong system dependence. • A tungsten interlayer markedly improved adhesion in the DLC + borided system. This study investigates the effect of Substrate Bias Voltage (SBV) on the adhesion properties of duplex coatings comprising Diamond-Like Carbon (DLC) and borided layers on AISI 4140 steel. The coatings were deposited using High-Power Impulse Magnetron Sputtering (HiPIMS) and boriding techniques, with a DLC layer (∼800 nm) applied under varying SBV conditions (100 to 300 V). Additionally, the impact of a tungsten metallic interlayer on adhesion was examined. Mechanical and chemical characterizations were performed using optical profilometry, scanning electron microscopy (SEM), and Raman spectroscopy. Following ASTM G-1624 standards, Scratch tests employed a Rockwell C diamond tip with loads ranging from 0.5 to 120 N to assess the friction coefficient and residual penetration. Critical loads, indicating adhesive or cohesive failures, were determined by analyzing the wear track with SEM. Results revealed that the critical load for the DLC coating alone was approximately 7.0 N, whereas the DLC+borided system withstood up to ∼108 N, demonstrating significantly enhanced adhesion.