Coatings

Mechanical properties and fatigue performance are highly dependent on the microstructure formed during heat treatment. C75 steel is widely used in mechanically loaded components subjected to cyclic stresses, where fatigue resistance plays a critical role in ensuring operational reliability and service life. In this context, the present study investigates the influence of bainitic and martensitic microstructures, obtained through salt-bath quenching and oil quenching, respectively, on the fatigue behaviour of C75 steel. To isolate the effect of microstructure, both heat treatment processes were designed to achieve an identical final hardness of 52 HRC, enabling a direct comparison of the resulting microstructures and their influence on the evolution of dynamic stiffness during cyclic loading. Salt-bath quenching resulted in a fine and homogeneous bainitic microstructure, whereas oil quenching produced a predominantly martensitic structure. The treated samples were characterised using hardness measurements, optical microscopy and scanning electron microscopy (SEM). To evaluate the influence of microstructure on fatigue resistance, Wöhler (S-N) curves were established using high-frequency four-point bending fatigue tests performed in accordance with ISO 22407:2007: Metallic Materials—Fatigue Testing—Axial Plane Bending Method. The progression of fatigue damage was assessed through frequency monitoring during cyclic loading, enabling the identification of the onset of frequency decrease and the subsequent evolution of dynamic stiffness during cyclic loading. The results revealed noticeable differences in fatigue behaviour, expressed as descriptive trends, despite similar hardness levels. The two microstructures exhibited different crack-initiation and propagation characteristics, while the overall fatigue performance remained comparable. Furthermore, the results demonstrated that fatigue life is governed not only by hardness but also by microstructural morphology and phase distribution. These findings highlight the critical role of the quenching medium in controlling fatigue damage mechanisms and provide valuable insights for optimising the fatigue performance of C75 steel components subjected to cyclic loading.

To investigate the effects of laser energy density on the microstructure, phase constitution and distribution, wear resistance, and corrosion resistance of AlCoCrFeNi high-entropy alloy coatings, three coatings were fabricated on ATI 718Plus substrates by laser cladding at different laser energy densities. Their microstructure, phase constitution, microhardness, tribological behavior, and electrochemical corrosion properties were systematically characterized. The results showed that all coatings exhibited good metallurgical bonding with the substrate. With increasing laser energy density, the coating microstructure initially became finer and more homogeneous and subsequently coarsened. All coatings were primarily composed of BCC/B2 phases with a small fraction of the FCC phase. The coating produced at the medium laser energy density (MHI) exhibited the most homogeneous phase distribution and the highest average microhardness of 576.1 HV_0.3. It also showed the lowest coefficient of friction and specific wear rate, with values of 0.52 and 6.94 × 10⁻⁴ mm³/(N·m), respectively. The dominant wear mechanism was abrasive wear, accompanied by localized delamination and tribo-oxidation. Furthermore, the MHI coating exhibited the highest charge-transfer resistance of 18,880 Ω·cm² and the lowest corrosion current density of 96.8 μA·cm⁻², indicating superior corrosion resistance. These results demonstrate that an appropriate laser energy density promotes a finer and more homogeneous solidification microstructure and a more favorable phase distribution, thereby achieving a synergistic improvement in the hardness, wear resistance, and corrosion resistance of AlCoCrFeNi high-entropy alloy coatings.

Soft Magnetic Composites (SMCs)—powdered ferromagnetic materials characterized by high macroscopic electrical resistivity—constitute an excellent resource for electrical machines with non-traditional magnetic flux configurations, such as axial flux machines. Their adoption is currently expanding within the mass production of small components, particularly in the automotive sector. Consequently, numerous research activities focus on performance optimization to maximize and harmonize the mechanical, energetic, and magnetic properties of SMCs. In this context, the layer-by-layer (LbL) chemical deposition process offers multiple degrees of freedom regarding the types, quantities, and arrangements of materials used to construct insulating and binding layers. This study presents a specific system comprising a polymer and nanometric silica, investigating the influence of several process parameters on the overall performance of the final material. The resulting samples were characterized using a hysteresigraph to determine their magnetization curves, relative magnetic permeability, and specific losses across different frequencies. Additionally, mechanical properties were evaluated through transverse rupture strength tests.