Considerations on the Wear Behavior of Vacuum-Remelted ZrO2-Reinforced Self-Fluxing Ni-Based Thermally Sprayed Alloys.

Without proper post-processing (often using flame, furnace, laser remelting, and induction) or reinforcements' addition, Ni-based flame-sprayed coatings generally manifest moderate adhesion to the substrate, high porosity, unmelted particles, undesirable oxides, or weak wear resistance and mechanical properties. The current research aimed to investigate the addition of ZrO2 as reinforcement to the self-fluxing alloy coatings. Mechanically mixed NiCrBSi-ZrO2 powders were thermally sprayed onto an industrially relevant high-grade steel. After thermal spraying, the samples were differently post-processed with a flame gun and with a vacuum furnace, respectively. Scanning electron microscopy showed a porosity reduction for the vacuum-heat-treated samples compared to that of the flame-post-processed ones. X-ray diffraction measurements showed differences in the main peaks of the patterns for the thermal processed samples compared to the as-sprayed ones, these having a direct influence on the mechanical behavior of the coatings. Although a slight microhardness decrease was observed in the case of vacuum-remelted samples, the overall low porosity and the phase differences helped the coating to perform better during wear-resistance testing, realized using a ball-on-disk arrangement, compared to the as-sprayed reference samples.

Characteristics of Hydroxyapatite-Modified Coatings Based on TiO2 Obtained by Plasma Electrolytic Oxidation and Electrophoretic Deposition.

In order to modify the surface of light metals and alloys, plasma electrolytic oxidation (PEO) is a useful electrochemical technique. During the oxidation process, by applying a positive high voltage greater than the dielectric breakdown value of the oxide layer, the formation of a ceramic film onto the substrate material is enabled. The resulting surface presents hardness, chemical stability, biocompatibility, and increased corrosion wear resistance. The current study aims to investigate the corrosion resistance and tribological properties of PEO-modified coatings on titanium substrates produced by applying either direct or pulsed current in a silicate-alkaline electrolyte. In this way, a uniform TiO2 layer is formed, and subsequently, electrophoretic deposition of hydroxyapatite particles (HAP) is performed. The morpho-structural characteristics and chemical composition of the resulting coatings are investigated using scanning electron microscopy combined with energy dispersive spectroscopy analysis and X-ray diffraction. Dry sliding wear testing of the TiO2 and HAP-modified TiO2 coatings were carried out using a ball-on-disc configuration, while the corrosion resistance was electrochemically evaluated at 37 °C in a Ringer's solution. The corrosion rates of the investigated samples decreased significantly, up to two orders of magnitude, when the PEO treatment was applied, while the wear rate was 50% lower compared to the untreated titanium substrate.

Comparison of Ni-Based Self-Fluxing Remelted Coatings for Wear and Corrosion Applications.

The present study investigates the possibility to apply a vacuum furnace thermal post-treatment as an alternative solution for flame sprayed NiCrBSi wear and corrosion-resistant coatings, deposited on a low alloyed structural steel. The controlled atmosphere offers advantages regarding the fusion of the coating, porosity reduction, and degassing. An improvement of the applied heating-cooling cycle was performed through the variation of time and temperature. The best performing samples were selected by comparing their porosity and roughness values. The chosen samples were subsequently characterized regarding their microstructure, microhardness, sliding wear, and corrosion behavior. The experimental work confirms that the use of a vacuum remelting post-process reduces the porosity below 1% and leads to the formation of a larger quantity of hard boron-containing phases, promoting a significant decrease of the wear rate, while maintaining a good corrosion behavior.

Sliding Wear Behavior of High-Temperature Vacuum-Brazed WC-Co-NiP Functional Composite Coatings.

The present study aimed to investigate the tribological behavior of high-temperature vacuum-brazed WC-Co-NiP functional coatings deposited on 16MnCr5 case hardening steel. Dry sliding wear resistance was evaluated using a non-conformal ball-on-disk arrangement, at room temperature against 100Cr6 and WC-Co static partners, respectively. Morphological, microstructural, and chemical composition analyses showed a complex, phased structure composed of tungsten carbide, nickel, and hard cobalt-based η-structure. In the testing conditions, the coefficient of friction against 100Cr6 and WC-Co counterparts entered a steady-state value after approximately 1000 m and 400 m, respectively. The wear track analysis revealed phenomena of particles trapped between the sliding bodies, as well as gradual removal of asperities. The calculations of the wear rates proved that the values were strongly influenced by properties of the sliding system, such as crystal structure, stress discontinuities, hardness, and material homogeneity.