Coatings

During the preparation of polycrystalline diamond compact (PDC), Co is often added as a binder, which reduces the material’s heat resistance and catalyzes the transformation of diamond to graphite at high temperatures, thereby reducing the material’s mechanical properties. After the removal of Co, the exposure of holes beside the diamond grains could also affect the wear resistance of PDC. This article provides a new approach for the surface enhancement of PDC, which involves depositing a layer of diamond coating using the hot filament chemical vapor deposition (HFCVD) method after the removal of Co and investigating the effect of the pretreatment time of PDC on the deposition of diamond. The optimal pretreatment was obtained using the surface morphology, composition, and content of Co for a cross-section of the diamond coatings. A H₂SO₄-H₂O₂ solution was mixed in a ratio of 1:10 as the acid corrosion reagent, and the best etching time was 5 h. When the time was short, the residual amount of Co was high, which affected the quality of the diamond coatings. When the time was long, the area fraction of the substrate was high, and the deposited diamond was not able to easily fill the holes caused by corrosion.

This study investigates the effect of the mass ratio of aluminum to aluminum oxide on the structure and properties of Al–Al₂O₃ composite coatings deposited by cold spray deposition onto AISI 316L stainless steel substrates. Powder mixtures containing 70, 35, 30, and 20 wt.% Al₂O₃ were used to form the coatings, and the phase composition, microstructure, porosity, hardness, surface roughness, and tribological characteristics were studied using XRD, SEM/EDS, instrumental indentation, profilometry, and ball-on-disk testing. X-ray phase analysis confirmed the presence of metallic Al and α-Al₂O₃ without the formation of additional phases, while SEM analysis revealed continuous coatings with mean thicknesses ranging from 68.08 ± 1.16 to 103.39 ± 5.64 μm and an apparent porosity not exceeding 2%. The best structural and mechanical properties were observed for coating B3, prepared from a mixture of 30 wt.% Al₂O₃ and 70 wt.% Al: the porosity was 0.48%, the Martens hardness (HM) was 1757.6±155.8 N/mm², and the Vickers hardness was 211.8 ± 20.7 HV. Coating B3 was also characterized by a minimum surface roughness of Ra = 0.955 μm and a relatively stable coefficient of friction in the range of 0.55–0.60. The results obtained show that the composition of 30 wt.% Al₂O₃-70 wt.% Al provides the favorable combination of low apparent porosity, high mean hardness, and relatively stable friction behaviour among the specimens tested.

Ti60 is a type of high-temperature titanium alloy that not only withstands high temperatures during long-term service, but also has excellent wear resistance and corrosion resistance. To enhance the wear and corrosion resistance of a Ti60 substrate, a Ti-Hf-Mo-Ta-Nb-B coating was fabricated onto a Ti60 substrate by the laser-cladding method. Based on the principle of the pin-on-disk friction-wear test and using an electrochemical workstation platform, the wear resistance and corrosion resistance of both the laser-clad coating and the Ti60 substrate were tested, respectively. The results show that the wear resistance of the coating is significantly improved compared to the Ti60 substrate, with the wear rate being only one-tenth of that of the substrate. Among them, the wear rate of the former is 6.28 × 10⁻⁴ mm³/(N·m), while that of the latter is 6.57 × 10⁻³ mm³/(N·m). Additionally, the corrosion resistance of the coating in 3.5 wt.% NaCl solution is superior to that of the as-received Ti60 substrate, exhibiting a higher corrosion potential (E_corr) of −0.3275 V and a lower corrosion current density (i_corr) of 4.1801 × 10⁻⁶ A·cm⁻², compared to the substrate’s values of −0.4516 V and 7.4039 × 10⁻⁶ A·cm⁻², respectively. The enhanced wear and corrosion resistance of the coating is primarily attributed to the formation of a hard and corrosion-resistant high-entropy diboride phase (Ti_0.2Hf_0.2Mo_0.2Ta_0.2Nb_0.2)B₂.