To improve the relatively low hardness and unsatisfactory wear resistance of 304 stainless steel, corresponding surface coatings with high hardness and wear resistance are prepared in this study using the laser cladding surface modification technology. Accordingly, the service life of 304 stainless steel is extended. Medium-entropy alloys (MEAs) are derived from high-entropy alloys (HEAs) and primarily comprise three to four major elements. Compared with traditional alloys, MEAs have unique phase structures and excellent mechanical properties. Based on the design concept of HEAs with unequal atomic and eutectic structures, Ta is incorporated into FeCrNi MEAs and its content is adjusted to realize FeCrNiTa_x eutectic medium-entropy alloy coatings (EMEACs). Using this in-situ nanoscale laminate structure which combines both soft and hard phases, the overall strength and wear resistance of the alloy are greatly improved, while considerable plasticity is maintained, achieving a balance between strength and plasticity. In addition, the rapid laser cladding solidification technology is beneficial for refining the characteristic structure and improving the mechanical properties of the coating.

In this study, five FeCrNiTa_x (atomic fraction x=0, 0.2, 0.4, 0.6, and 0.8) alloy coatings with varying Ta contents are designed as research samples. The 304 stainless steel is chosen as the substrate for laser cladding. Elemental powders with different components are uniformly mixed and preplaced on a polished substrate using a planetary ball mill. The coatings are prepared using a laser cladding device with the laser power of 1400 W, scanning speed of 8 mm/s, spot diameter of 3.6 mm, and overlap rate of 35%. The crystal structures of the coatings with different Ta contents are analyzed using X-ray diffractometer (XRD), and their microstructures and chemical compositions are characterized via scanning electron microscope (SEM). Subsequently, the Vickers hardness values of the coatings from the cladding layer to the substrate are measured using a Vickers hardness tester and the change curves are recorded. Finally, a friction and wear tester is employed to measure the friction coefficients of the coatings, and an electronic balance is used to measure the mass loss. The wear morphologies and elemental compositions of the coatings are then characterized using SEM and XRD .

From the XRD patterns, only the face center cubic (FCC) phase diffraction peak is detected for the FeCrNi MEAs. After the addition of Ta, the Laves phase corresponding to the hexagonal structure appears when x=0.2, and the respective diffraction peak becomes clearer with increasing Ta content. Meanwhile, the FCC phase diffraction peak continuously weakens, indicating that the volume fraction of the Laves phase increases from x=0.2 to x=0.8 (Fig. 2). A clear boundary line between the coating and substrate is observed in the cross section of the laser-melted alloy coating (Fig. 3), indicating good metallurgical bonding of the alloy. Moreover, the microstructure of the coating cross section (Fig. 4) shows that the primary phase is the FCC phase when x=0.2, accompanied by a small amount of the Laves phase, which is a typical hypoeutectic structure. With increasing Ta content (x=0.4), the Laves phase increases significantly and a completely layered eutectic structure forms. When the Ta content further increases, the primary phase also changes from the FCC phase to the Laves phase, and large Laves phase blocks precipitate in the structure, forming a peritectic structure. From the hardness measurement results of the coatings (Fig. 7), the corresponding curves generally show a downward stepwise distribution, and the higher the Ta content, the higher the hardness value. The alloy with x=0.2 exhibits the highest hardness value of 705.3 HV, approximately 3.7 times that (190 HV) of the 304 stainless steel substrate. In addition, the improvement in hardness significantly influences the wear resistance (Fig. 9). The wear surface of the substrate exhibits obvious plastic deformation and severe adhesive wear. After the addition of Ta, the wear of the coating decreases. The coating with x=0.8 is generally smoother and flatter with the best wear performance, and the wear mechanism is slight adhesive and oxidative wear.

In this study, FeCrNiTa_x EMEACs with different Ta contents are successfully prepared on the surface of 304 stainless steel using the laser-cladding technology. The coating exhibits good metallurgical bonding with the substrate and no obvious defects appear on the surface. The microstructural analysis shows that an increase in the Ta content results in the transition of the coating structure from the sub-eutectic to fully eutectic, and finally to super-eutectic structure. In addition, the coating hardness gradually increases with increasing Ta content, and the hardness value of 705.3 HV is the highest when x=0.8, which is approximately 3.7 times that of the 304 stainless steel substrate. The solid-solution strengthening and precipitation strengthening caused by the addition of Ta are important factors in improving the coating hardness. Regarding the wear resistance of the coating, the Laves phase generated by the addition of Ta has an inhibitory effect on the coating wear, and the friction coefficient is the lowest and the wear resistance is the best when x=0.8. The wear mechanisms mainly include adhesive and oxidative wear.