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.

The present work has been conducted in order to assess the mechanical and tribological performance of a ZrN/Mo₂N coating deposited onto a H13 steel by means of a closed field unbalanced magnetron-sputtering ion-plating process. The hardness and elastic modulus of the coated system have been determined employing nanoindentation techniques. Dry and wet (3.5 wt-% NaCl solution) sliding wear tests were carried out making use of an alumina ball as counterpart. It has been determined that, during testing under the corrosive solution, the coating experiences a severe abrasive wear mechanism, due to the combined action of the alumina ball, the hard ‘debris’ and a coating localised corrosion attack. On the other hand, it has been shown that the coated system is able to increase the wear resistance of the substrate by more than one order of magnitude, if the wear tests are carried out in air under the same conditions.

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.

Electroless nickel-boron coatings present outstanding properties such as high hardness, excellent wear resistance and uniform coating, and thus they are considered to be alternative to toxic hard chrome coatings. However, they contain lead that is toxic and used as stabilizer in the plating bath. This study aims to investigate the tribocorrosion behaviour of lead-free electroless nickel-boron coatings. In the present research, several tests were carried out to investigate the behaviour of these coatings under both dry and tribocorrosion reciprocating sliding wear against alumina balls, at room temperature. The open circuit potential (OCP) method was used to determine the degradation mechanism of the coatings. The results of the tribocorrosion and dry wear tests showed that the performance of coatings was very different from each other. A steady state for the coefficient of friction (COF) is achieved during the tribocorrosion test, whereas the constant production of debris and their presence in the contact implied an increase in COF with distance during the dry wear test. The wear mechanisms of these coatings also presented variations in these tests. It was found that the wear area calculated from tribocorrosion is lower (56 µm²) than the one from dry sliding test (86 µm²).

Coatings based on WC–12Co, with and without the reinforcement of carbon nanotubes (CNTs) have been deposited by means of the HVOF thermal spraying technique on a SAE 1020 steel substrate. The aim of the present study is to evaluate in detail the influence of the 36 h milling time on both feedstock powders, particularly on the mechanical properties of WC-12Co coatings, with (WC_CNT) and without (WC_CONV) CNTs addition. Properties such as hardness, apparent yield stress and fracture toughness were observed to exhibit an improvement due to CNTs addition, whereas the elastic modulus was found to be insensitive to the CNTs content. These findings could explain the enhancement of the wear resistance of the CNTs-reinforced coatings (WC_CNT) reported in the literature.

WC-12Co HVOF-sprayed coatings (~400 µm in thickness) reinforced with multiwalled carbon nanotubes (MWCNTs), were deposited on a steel substrate. The aim of this work is to provide and analyze data from HVOF WC-12Co sprayed coatings, concerning the influence of the addition of MWCNTs on their tribocorrosion performance, in a 3.5% NaCl electrolyte. Electrochemical data (current density and corrosion potential from potentiodynamic polarization curves) and wear data (coefficients of friction, coating volume losses and wear constants) are reported for the coatings, with and without the addition of MWCNTs (labeled WC_CNT and WC_AS-RECEIVED, respectively), considering the synergistic effects of wear and corrosion coupling. Scanning electron microscopy (SEM) and profilometry are used to explain both the wear and corrosion mechanisms that account for each coating’s performance in this environment. During the tribocorrosion tests, similar wear constants, of the order of approximately 10⁻¹² m³/Nm, were found for all samples, with an increase of ~20% due to the MWCNTs presence. However, for the coatings reinforced with MWCNTs, a remarkable increase in $i_{\mathrm{corr}}$, representing almost 3 times the $i_{\mathrm{corr}}$ of the WC_AS-RECEIVED coating, was determined. The above results illustrate the complex mechanisms that occur when these coatings are tested under tribocorrosion conditions, which give rise to concurrent interacting phenomena, involving both electrochemical and mechanical responses.