INFLUENCE OF DIBENZYL ETHER (DBE) ADDITION ON THE STRUCTURE OF CHROMIUM CARBIDE COATING OBTAINED BY THE MOCVD METHOD FROM COL “BARKHOS”

Investigations of the effect of the addition of dibenzyl ether (DBE) to the chromium organic liquid (COL) “Barkhos” on the structure and performance properties of chromium carbide coatings obtained by chemical deposition of their gas phase have been carried out. It is shown that the use of the DBE additive expands the temperature range for the formation of chromium carbide coatings with a horizontally layered structure, which are more resistant to corrosion and erosion wear. In this case, there is an increase in the adhesive strength and cavitation resistance of the coatings. The use of the DBE additive reduces the through porosity of the coatings. Coatings obtained with DBE additives have an abnormally high resistance to electrochemical dissolution in comparison with other materials used for work in corrosive environments.

The Enhancement Effect of Salt Bath Chromizing for P20 Steel

The TD (Thermal Diffusion) salt bath process is used to obtain a super hard carbide coating on the material surface by utilizing the mechanism of metal thermal diffusion. In this paper, chromium carbide coating was prepared on P20 hot-pressing die steel by the TD salt bath chromizing process. Characterization of the modified surface layer was made by means of scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS), a micro-hardness tester and an automatic scratch tester. The influence rules of different salt bath times and temperatures on the growth thickness of the cladding layer were explored through experiments, and the optimum salt bath process scheme was determined as a temperature of 960 °C and time of 6 h. The chromium carbide coating with a thickness similar to that of chromium plating was prepared, and the average thickness of the coating was about 8–10 μm. The results showed that hardness and bonding strength of chromium carbide coating are higher than that of electroplated chromium coating. The combination of chromium carbide coating and matrix is metallurgical, while the electroplated chromium coating is physical. Immersion corrosion test results show that both coatings have good corrosion resistance in a 65% nitric acid solution.

MICROSTRUCTURE AND FRICTION PARAMETERS OF THE SURFACE LAYER OF SINTERED STAINLESS STEELS

Sintered stainless steel (SSS) is manufactured using the powder metallurgy technology (PM). SSSs are characterized by a two–phase structure which can be obtained by mixing different proportions of the main structural components (i.e. austenite and ferrite). Taking into account the improvement of functional properties of SSSs, a number of surface modifications have been proposed. This study proposes a method to improve functional properties by formation of chromium carbide coating and alloying the surface by the gas tungsten arc welding (GTAW) process. The results of light optical microscopy and scanning electron microscopy (SEM/EDX), roughness parameters, hardness, and the coefficient of friction are presented.

Wear and Corrosion Resistance of Chromium–Vanadium Carbide Coatings Produced via Thermo-Reactive Deposition

Chromium carbide, vanadium carbide, and chromium–vanadium mixture coatings were deposited on AISI D2 steel via the thermo-reactive deposition/diffusion (TRD) technique. The carbides were obtained from a salt bath composed of molten borax, ferro-chrome, ferro-vanadium, and aluminum at 1020 °C for 4 h. Analysis of the morphology and microstructure of the coatings was done via scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The hardness of the coatings was evaluated using nano-indentation, and the friction coefficient was determined via pin-on-disk (POD) testing. The electrochemical behavior was studied through potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS). The XRD results show evidence of the presence of V8C7 in the vanadium carbide coating and Cr23C6 and Cr7C3 in the chromium carbide coating. The hardness value for the vanadium–chromium carbide coating was 23 GPa, which was higher than the 6.70 ± 0.28 GPa for the uncoated steel. The wear and corrosion resistance obtained was higher for the niobium–chromium carbide coating, due to the nature of the ceramic carbide produced.

Effect of Flow Velocity and Impact Angle on Erosion–Corrosion Behavior of Chromium Carbide Coating

In this study, the effect of flow velocity (4–7.5 m s−1) and impact angle (30–90 deg) on erosion–corrosion behavior of chromium carbide coating was investigated under impingement by silica containing NaCl solution. Chromium carbide coating was deposited on low carbon steel by thermal reactive deposition/diffusion method at 1050 °C for 12 h in a molten salt bath. Mass loss measurement and potentiodynamic polarization tests were employed in order to determine coating performance under impingement. Polarization curves showed that the coated samples had less corrosion current density and high chemical stability. High mass loss at low impact angle indicated ductile behavior for the uncoated sample, while the mass loss for the coated sample changes less than 30% with impact angle up to 60 deg. Furthermore, the erosion–corrosion behavior of the coated sample was slightly dependent on flow velocity. Scanning electron micrographs showed that at lower impact angle, the Cr7C3 coating eroded with flake fragmentation mechanism, while at high impact angle, fatigue fracture is the main degradation mechanism.