Temperature-Dependent Microstructure and Tribological Performance of Boride Layers Formed on 40 Kh Steel Using Boric Acid-Based Boriding

Boriding is widely used in various industries due to the unique combination of high mechanical, corrosion, and tribological properties of boride layers formed on the surface of steel components. In this work, the powder boriding of 40 Kh steel was investigated in a closed capsule using a specially prepared powder mixture containing boric acid as the boron source. Boriding was carried out in a furnace at 850, 900, and 950 °C for 10 h. The resulting boride layers were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD), which confirmed that all three coatings consist exclusively of the Fe2B phase. It was found that with increasing temperature, the thickness of the boride layer increased from 68 μm to 160 μm. The tribological properties were evaluated using the pin-on-disk method, followed by analysis of the wear surfaces using optical profilometry and SEM. The most significant reduction in wear rate was observed at 850 °C, where the wear decreased by a factor of 4.2—from 8.471 × 10−5 to 1.999 × 10−5 mm3·N−1·m−1. In addition, the hardness increased fivefold compared to the untreated material. These results demonstrate the high potential of diffusion boriding for enhancing the operational performance of parts subjected to severe wear conditions.