The microstructure evolution, elements diffusion and fracture behavior of the Stellite 6 weld overlay, deposited on 10Cr9Mo1VNbN (F91) steel by the tungsten inert gas (TIG) cladding process, were investigated after long-time service. Obvious diffusion of Fe occurred from the steel and fusion zone to the Stellite overlay, resulting in the microstructure evolution and hardness increase in the coating, where hard Co–Fe phases, σ phases (Fe–Cr metallic compounds) and Cr-rich carbides (Cr_18.93Fe_4.07C₆) were formed. Besides, the width of the light zone, combined with the fusion zone and diffusion zone, increased significantly to a maximum value of 2.5 mm. The fracture of the Stellite coating samples mainly occurred in the light zone, which was caused by the formation and growth of circumferential crack and radial crack under high temperature and pressure conditions. Moreover, the micro-hardness values in the light zone increased to the maximum (470–680 HV) due to the formation and growth of brittle Co–Fe phases. The formation of these cracks might be caused by formed brittle phases and changes of micro-hardness during service. Full article

The electrochemical corrosion behaviour of trivalent Cr-C-coated steel samples with Ni and Cu undercoatings was studied in the 0.1 M H₂SO₄ + 1 g/L NaCl solution. The corrosion resistance of Cr–C-coated samples depends strongly on the undercoating material and the polarised anodic potential. With a soft undercoating, the cracks and crack width in the Cr–C coating can be obviously reduced. Different corrosion potentials were measured from Cr–C-coated steel samples with Ni and Cu-undercoatings. Better electrochemical corrosion resistance of a Cr–C/Cu-coated steel sample was detected at 0 V, but it easily corroded at 0.5 V. On the contrary, the Cr–C/Ni-coated steel sample had better electrochemical corrosion resistance at 0.5 V, and poor at 0 V. The through-coating cracks in the Cr–C coating could provide active corrosion paths for dissolution of the Ni or Cu undercoating during potentiostatic tests polarised at 0 and 0.5 V. The reduction behaviour of Cu²⁺ ions dissolved from the Cu undercoating of the heat-hardened Cr–C-coated sample was studied and recognised by means of the immersion test in a solution composed of 200 g/L CuSO₄·5H₂O and 70 g/L H₂SO₄. Full article

Nanocomposite Si₃N₄/PEEK 708 coatings were successfully fabricated on the Ti-6Al-4V alloy substrate by electrophoretic deposition (EPD) and post-EPD heat treatment. The addition of chitosan polyelectrolyte into ethanolic-based suspensions enabled the cathodic co-deposition of ceramic and polymeric particles. Zeta potential measurements allowed the elaboration of stable suspensions. The selection of the optimal EPD voltage and time enabled uniform coatings to be obtained. Heating above the PEEK melting point and cooling with a furnace or in water resulted in the formation of dense coatings with semi-crystalline or amorphous polymer structures, respectively. Both coatings with a thickness in the range of 90–105 µm had good adhesion and scratch resistance to the substrates, despite the presence of relatively high degrees of open porosity. The coatings improved the tribological properties of the titanium alloy. However, a strong relationship between the polymeric matrix structure and wear resistance was observed. Semi-crystalline coatings proved to be significantly more wear resistant than amorphous ones. Full article

In this study, a two-step pack cementation process (preboronizing and then chromizing) was employed to prepare the B-Cr duplex-alloyed coating on the steel. After the first step of preboronizing (PB sample), box-type furnace chromizing (BC-1 sample) and induction heating chromizing (BC-2 sample) were carried out, respectively. The phases and microstructure of the coatings were characterized by X-ray diffraction (XRD), backscattering electron imaging (BSEI), and energy dispersive spectroscopy (EDS). The results reveal that the heating mode of the second step of chromizing has a significant effect on the phase composition and microstructure of the B-Cr coating. The efficiency of induction heating is higher than that of the box furnace heating, resulting in a thicker, denser, flatter surface, and B-Cr coating with fully reacted B and Cr elements. The wear and corrosion resistance of the steel is found to be significantly improved by the formation of effective B-Cr coating. The formation mechanisms and properties of the two duplex-alloyed coatings are investigated and discussed. Full article