This study is focused on the determination of the effects of processing time and temperature on the thickness, morphology, and hardness of boride layers grown on AISI 304L stainless steels. For boriding, a new molten salt electrolysis method called as CRTD-Bor (Cathodic Reduction and Thermal Diffusion-based boriding) was chosen due to its fast and green nature. CRTD-Bor of AISI 304L substrates was carried out in a borax-based molten electrolyte at temperatures ranging from 950 to 1050 °C for periods of 15 to 60 min at a constant current density of 200 mA/cm 2 . The x-ray diffraction analyses revealed the mixed iron boride phases including Fe 2 B, FeB. Moreover, cross-sectional scanning electron microscopy examinations confirmed the growth of these phases. Additionally, a phase homogenization (PH) step was adapted into CRTD-Bor to eliminate brittle FeB layer. It was founded that after 70 min of treatment at 1000 °C (15 min of CRTD-Bor + 55 min of PH) it is possible to grow % 40-lm-thick Fe 2 B layer exhibiting 1700 ± 100 HV on the surface with excellent adhesion to the substrate (HF1). Besides, kinetic calculations showed the activation energy (Q) of boride layer growth as 181.45 kJ/mol.
An alternative approach for producing a hard TiB2/TiC multilayer on M2 high-speed steel was introduced by combining cathodic arc physical vapor deposition (CA-PVD) and cathodic reduction and thermal diffusion-based boriding (CRTD-Bor). In this regard, the CRTD-Bor process was applied on CA-PVD Ti-deposited M2 steel and the effects of boriding parameters (i.e., temperatures and durations) on multilayer growth were examined. During boriding, Ti coating on the substrate was converted into Ti-borides on the top surface and a TiC layer was simultaneously formed at the interface of the Ti deposit and the steel matrix. The growth of boride and carbide phases was found to obey the parabolic law. The pre-exponential factors (K0) and the activation energy (Q) values were calculated as 7.50 × 10−9 m2/s and 146.10 kJ/mol for TiB2 growth and 1.81 × 10−7 m2/s and 187.31 kJ/mol for TiC formations, respectively. Additionally, empirical equations for estimating the thicknesses of TiB2 and TiC layers were derived. The penetration depth-dependent hardness measurements revealed the TiB2 layer hardness as 41 ± 5 Gpa, which decreased gradually toward the TiB region (24 ± 2 GPa) and fell to 13 ± 1 GPa in the Ti-rich area. The hardness then increased to 20 ± 1 GPa with the contribution of the TiC layer adjacent to the substrate. This multilayer coating exhibited −5.5 to −4.5 GPa compressive stress and good adhesions (HF1) to the substrates. Also, the results of tribological tests indicated a sevenfold increase in wear resistance under dry sliding conditions.
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