Conventional electroless nickel–boron deposits are produced using solutions that contain lead or thallium, which must be eliminated due to their toxicity. In this research, electroless nickel–boron deposits were produced in a stabilizer-free bath that does not include any toxic heavy metal. During processing, the plating rate increased from 10 to 14.5 µm/h by decreasing the concentration of the reducing agent, leading to increased bath stability. The thickness, composition, roughness, morphology, hardness, wear, and corrosion resistance of the deposits were characterized. The new deposit presents an excellent hardness of 933 ± 56 hv₅₀, 866 ± 30 hk₅₀, and 12 GPa from the instrumented indentation test (IIT), respectively, which are similar to that of hexavalent hard chromium coating. Moreover, by using both potentiodynamic polarization and salt spray tests it was shown that the coating presents higher corrosion resistance as compared to standard nickel-boron coatings. The new deposit exhibits properties close to those of the conventional electroless nickel–boron deposits. Therefore, it could replace them in any industrial applications.

Cemented carbide tools suffer from many issues due to the use of tungsten and cobalt as raw materials. Indeed, those are listed by the European Commission as “critical raw materials” since 2011 and by the US Department of Interior as “critical minerals” in 2018. To remain competitive with the conventional high-speed steels, less performant but cheaper, WC–Co tools can be recycled. In the present paper, a WC–7.5Co powder, recycled by the “Coldstream” process, has been sintered with vacuum sintering. As preliminary experiments have shown that the sinterability of the powder is low, the sintering temperature was set at 1500 °C to achieve full density. In parallel, the influence of ball milling conditions (rotation speed and milling medium) on the reactivity of the recycled powder has been studied in terms of grain size distribution, hardness, and fracture toughness. The optimized milling conditions were found to be 6 h wet milling, leading to a hardness of about 1870 HV₃₀ and a toughness of about 10.5 MPa√m after densification. The recycled powder can thus totally compete with conventional powders, opening avenues for the recycling of cemented carbide tools. Graphical Abstract

Electroless nickel-boron coatings were deposited from a novel deposition bath that was stabilizer-free. Those coatings were analyzed by DSC to determine the crystallization temperature of nickel-boride phases, and then the best heat treatment conditions for the new coatings were selected using the Knoop hardness test and XRD. The results of DSC analysis and XRD were coherent with the findings of previous studies, which shows that the new coating has a boron content lower than 4% wt. The maximum hardness was obtained after heat treatment at 300 °C for 4 h and reached 1196 ± 120 hk₅₀, which is much higher than the as-deposited coating. The heat-treated coatings were then fully characterized using optical and scanning electron microscopy, as well tribological and corrosion tests. Various sliding tests (ball-on-disk and ball-on-flat configurations) were conducted to analyze the coefficient of friction (COF) and wear behavior of the coatings. The maximum von Mises stress was calculated, and values of 624 MPa and 728 MPa were obtained for the ball-on-disk and ball-on flat, respectively, at a depth close to 14 µm from the surface, confirming the suitability of the applied load. Abrasive, adhesive, and fatigue wear mechanisms were observed on the worn sample morphology using SEM. It has been determined that during the corrosion test the OCP and corrosion potential values for the heat-treated coating increase as compared with the as-deposited one, whereas its corrosion resistance decreases.

In automotive and aerospace fields, the use of lightweight materials is required. Magnesium and its alloys combine a low density with high mechanical properties and excellent thermal conductivity. However, those materials suffer from low corrosion and wear resistances. The combined action of corrosion and wear is also critical for these materials. Tribocorrosion of magnesium alloy AZ31 has been investigated with reciprocal sliding wear as a function of the applied load in dry and wet (NaCl) conditions. The study shows that the main wear mechanisms were adhesion, abrasion, and oxidation in dry sliding wear while no adhesion was found in wet sliding wear. Corrosion of the worn surface occurs also in wet sliding wear. It is interesting to notice that wear is less pronounced in wet sliding wear than in dry sliding wear due to the natural lubrication of the NaCl electrolyte. Only severe conditions, high normal load, and wet conditions bring magnesium AZ31 alloy in filiform corrosion.