This study examines the effect of spark plasma sintering (SPS) temperature on the phase composition, microstructure, densification, and microhardness of cobalt-free WC–Fe and WC–Fe–Ni hard alloys. Increasing the sintering temperature from 1250 °C to 1300 °C improved densification in both alloy systems. Residual porosity decreased from 13.2% to 12.7% in the WC–Fe–Ni alloy and from 13.8% to 12.0% in the WC–Fe alloy, while the density increased from 12.39 g/cm³ to 12.46 g/cm³ and from 12.15 g/cm³ to 12.40 g/cm³, respectively. The WC–Fe -Ni alloy exhibited a more homogeneous microstructure and higher microhardness, increasing from 708 $\pm$ 10 HV_0.2 to 823 ± 15 HV_0.2, whereas the WC–Fe alloy showed an increase from 630 ± 15 HV_0.2 to 770 ± 5 HV_0.2. X-ray diffraction analyses identified Fe₃W₃C, FeNi₃, and (Fe, Ni) solid-solution phases formed during SPS. The results indicate that both the sintering temperature and binder composition influence densification, phase evolution, and mechanical performance. Among the investigated materials, the WC–Fe–Ni alloy showed better microstructural stability and hardness than the WC–Fe alloy. These findings suggest that Fe–Ni binders are promising candidates for cobalt-free cemented carbides, although additional studies on fracture toughness and wear resistance are required before practical cutting-tool applications can be fully assessed.

This study investigates the effect of oxygen pressure during high-velocity oxy-fuel (HVOF) spraying on the formation and structural-phase characteristics of ZrCN coatings deposited on 65G steel. The coatings were produced using the Termika-3 system under fixed parameters of fuel, air flow, and spray distance, with oxygen pressure varied in the range of 4.0-4.5 bar. X-ray diffraction analysis revealed that the main coating matrix consists of a Zr(C,N) solid solution with a face-centered cubic lattice (Fm-3m). Increasing the O₂ pressure leads to partial thermal decomposition of ZrCN and the formation of ZrC, ZrN, and oxide phases such as t-ZrO ₂ . Oxygen enrichment of the coatings results in grain refinement and lattice parameter expansion. The microhardness of the coatings (a-c) reaches 1512-1857 HV, which corresponds to an approximately fivefold increase relative to the initial sample. Based on the obtained results, it was shown that regulating the oxygen pressure in the HVOF jet significantly affects the phase composition, morphology, and adhesion quality, providing an optimal combination of hardness, ductility, and thermal stability.

Al-Cu-Fe quasicrystalline coatings were deposited on AISI 321 stainless steel substrates by high-velocity oxy-fuel (HVOF) spraying at oxygen pressures of 3.0, 3.5, and 4.0 bar. The influence of oxygen pressure on the phase composition, microstructure, porosity, corrosion behavior, thermal stability, and microhardness of the coatings was investigated using X-ray diffraction (XRD), scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM/EDS), ImageJ porosity analysis, electrochemical corrosion testing in 3.5 wt.% NaCl solution, simultaneous thermal analysis (TGA/DSC), and microhardness measurements. XRD analysis revealed the formation of quasicrystalline-related intermetallic phases together with Al, Fe₃Al₁₃, FeAl, Fe₃O₄, CuFe₂O₄, Cu₂O, and CuO phases. The coating deposited at 3.5 bar exhibited the lowest porosity (5.37%), the most homogeneous microstructure, and the largest residual coating thickness after corrosion testing. SEM and EDS analyses indicated that corrosion preferentially initiated at pores, splat boundaries, and phase interfaces, while the coating produced at 3.5 bar demonstrated the most stable surface condition after exposure to a 3.5 wt.% NaCl solution. Thermal analysis showed that all coatings remained stable up to 900 °C. Sample (a) exhibited the lowest mass loss and the highest thermal stability, whereas sample (b) demonstrated the most favorable combination of structural integrity, phase ordering, coating density, corrosion-related performance, and thermal stability. Microhardness values of the coatings ranged from 754 to 778 HV, significantly exceeding that of the AISI 321 substrate. The results demonstrate that oxygen pressure is a critical parameter controlling the microstructure and functional properties of HVOF-sprayed Al-Cu-Fe coatings, with 3.5 bar providing the most balanced set of properties.