CrN and diamond-like carbon (DLC) coatings are deposited on the surface of 431 stainless steel by the direct current magnetron sputtering technique. The surface morphology, micro-structure, hardness, friction, and wear properties of CrN, CrN/DLC and multi-layer composite DLC coatings are investigated by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, nanoindentation tester, scratch tester, and friction and wear tester. The results show that the surface of the single CrN coating is very rough for the columnar crystal structure with preferred orientation. When it serves as inner transition layers to form the composite DLC coatings, the surface gets much smoother, with reduced defects. The friction and wear results indicate that the composite DLC coatings exhibit lower coefficients of friction, and better wear and corrosion resistance in dry friction, deionized water, and seawater. In the dry wear and friction process, the single CrN coating is easily worn out, and severe friction oxidation and furrow wear both appear with a friction coefficient of 0.48. But the friction coefficient of a CrN coating in seawater is reduced to 0.16, and friction oxidation and wear loss are further reduced with water lubrication. The CrN/DLC coating has excellent tribological performance in three test concoctions and has the lowest friction coefficient of 0.08 in seawater, which is related to the higher sp³ bond content, density (1.907 g/cm³) and high degree of amorphization, contributing to high hardness and a self-lubrication effect. However, due to the limited thickness of CrN/DLC (1.14 µm), it easily peels off and fails during friction and wear in different testing conditions. In multi-layer composite DLC coatings, there are more sp² bonds with decreased amorphization, high enough thickness (4.02 µm), and increased bonding strength for the formation of different carbides and nitrides of chromium as transition layers, which gives rise to the further decreased average friction coefficient and the lowest wear loss. Therefore, the CrN coating alone has good wear resistance, and, as with the inner transition layer with a DLC coating, it can effectively improve the overall thickness and the bonding strength of the multi-layer films by optimizing the chemical compounds of DLC coatings. These results provide experimental support and reference for the design and selection of surface coatings for 431 stainless steels in different working conditions.

Ti/TiN/(Ti,N)-DLC/Ti-DLC/DLC coatings were deposited on 431 stainless steel using direct current magnetron sputtering technology under different bias voltages(0 V, −100 V, −200 V and −300 V). The microstructure and tribocorrosion performance of these DLC coatings in seawater was investigated. The results indicated that under the bias voltages, a denser and smoother surface of DLC coatings with a higher bonding strength between coatings and substrates was observed related to the increased incident kinetic energy of deposited ionized atoms. When the bias voltage was −200 V, the surface roughness reduced from 9.81 nm to 7.03 nm, and the bonding strength enhanced from 8.23 N to 8.86 N. What is more, the sp³ bond proportion and the disorder degree in DLC coatings both increased, which resulted in improved hardness and deformation resistance. However, when the bias voltage was −300 V, the increase of the amorphization was associated with a simultaneous rise in internal stress, which reduced the hardness and bond strength a little (8.72 N). DLC coatings can effectively improve the tribocorrosion properties of 431 stainless steel in seawater. When the voltage was −200 V, the average friction coefficient decreased from 0.35 to 0.07, with shallower wear traces and the wear loss of the DLC coating also being the smallest. The abrasive wear caused by metal oxides falling off the grinding ball, and the plastic deformation of the DLC coatings are the main wear forms. The high-density structure of DLC coatings under bias voltages can not only prevent the rapid expansion of cracks during deformation, but also provides a physical barrier to the erosion, which improves the corrosion and friction resistance in seawater. The optimization of bias voltage can improve the tribological performance of DLC coatings by regulating the carbon chain bond and microstructure. These results provide reference for DLC preparation and their potential engineering applications in stainless steel.

Diamond-like carbon (DLC) coatings doped with Ti and Ni elements were deposited on 316 L stainless steel substrate using magnetron sputtering technology. The morphology, microstructures, and performances of the coatings were detected using scanning electron microscopy, a Raman spectrometer, nanoindenter, scratch tester, and a tribological machine. The effects of element doping on the microstructures, friction and wear properties of DLC coatings were analyzed. The results revealed that Ti/Ni doping ensures the uniform cover and tight fit of DLC coatings on the substrate. Additionally, Ni-doped DLC coatings have a much smoother surface and denser texture with higher bonding strength and enhanced hardness (7.5 GPa) though Ti doping also can improve the bond strength to some extent. The presence of Ni both in the 316 L substrate and in Ni-doped DLC coatings improves interface matching, decreases structural differences, and increases bonding strength. Moreover, the presence of Ni effectively inhibits oxidation and corrosion in friction interfaces, stabilizes the friction coefficient, and enhances wear resistance. Therefore, based on this study, it was concluded that reasonable matching between the dopant elements and the substrates can effectively improve the performance of DLC coatings.

As an important joint material, tantalum (Ta) needs to be studied intensively in terms of wear and corrosion resistance. Two films of Ta/Ti/DLC (diamond-like carbon) (referred to as TD film) and Ta/Ti/TiN/Ti/DLC (referred to as TTTD film) were prepared by magnetron sputtering technology, and their electrochemical corrosion performance in lactic acid and sodium bicarbonate solutions was also investigated. The morphology observation, electrochemical corrosion experiment, and static contact angle measurement were carried out using a scanning electron microscope, electrochemical workstation, and a contact angle meter. This paper mainly discusses and analyzes the corrosion influencing factors and corrosion mechanisms of these two films under two different environments. The 20-day immersion experiments show that the TD film in both solutions results in severe surface corrosion due to its loose structure and many internal defects. In contrast, the surface of the TTTD film with a dense structure, a multilayer structure, and a thicker thickness is not corroded under the same conditions. The electrochemical corrosion experiments show that the corrosion resistance of the TD film is worse than that of the Ta substrate, while the TTTD film has the strongest corrosion resistance, improving the corrosion resistance of the Ta substrate. Although the TD film had a higher sp³ content than the TTTD film, it does not provide protection to the Ta substrate due to increased structural defects in the TD film. The results of the contact angle show that the contact angle of the TTTD film is smaller than that of the TD film in both solutions. However, the TTTD film has the lowest roughness and the densest structure, and thus the TTTD film is more resistant to corrosion. It is finally concluded that the surface morphology and internal structure of the films are the main factors affecting corrosion resistance. This provides an important reference for the application of DLC film in weak alkaline and weak acid environments.

For shaft parts, 45 steel has been widely used due to its favorable mechanical properties and low cost. However, the relatively low wear resistance of 45 steel limits its application. In this work, high-entropy alloy of FeCrCoMnSi_x (x = 0, 0.3, 0.6, 0.9, 1) coatings were prepared on the surface of a 45 steel substrate using laser cladding technology to improve the wear performance of 45 steel. The effect of the Si element on the microstructure and tribological property of these coatings is investigated. The results show that the structure of FeCrCoMn coatings is mainly an FCC + HCP dual-phase solid solution, grown in equiaxial crystals. When a small amount of Si (x = 0.3) is added, the BCC phase is generated in the coating; meanwhile, the microstructure is transformed into the divorced eutectic character. When the content of Si is x = 0.6, the eutectic structure is promoted, and the microstructure is refined and becomes denser. When the content of Si increases to x = 0.9 and 1.0, the metal silicate phase containing Mn and Cr is formed due to the precipitation of supersaturated solid solution. At the same time, the microstructure is transformed into dendritic crystals due to the composition super-cooling effect by the excessive Si element, inducing serious element segregation. The hardness of FeCrCoMnSi_x high-entropy alloy coatings increases to 425.8 HV when the Si content is 0.6 under the synergistic effect of the solid-solution and dense eutectic structure. The friction and wear analysis shows that the friction and wear mechanisms of the coating are mainly abrasive wear and oxidative wear. The coefficient of friction and the wear rate of the FeCrCoMnSi_x high-entropy alloy coating decreases to 0.202 and 4.06 × 10⁻⁵ mm³/N·m, respectively, when the content of Si is 0.6 due to the dense microstructure and high hardness. The above studies prove that the presence of Si in the FeCrCoMnSi_0.6 high-entropy alloy coating induces a refined eutectic microstructure and improves the coating’s anti-wear properties by increasing hardness and decreasing the coefficient of friction.

The giant magneto-impedance (GMI) effect of Co_83.2Fe_5.2Si_8.8B_2.8 ribbons at frequencies of <1 MHz was analyzed. To improve the GMI response, a Joule annealing treatment was conducted with a direct current, and the domain structure of the ribbon surface was investigated via magneto-optical Kerr effect microscopy. The annealed ribbons show larger impedance changes under external magnetic fields, and higher field sensitivity is obtained by certain current annealing treatments. The field sensitivity of 418 and 782%/(kA/m) at 0.2 MHz and 0.8 MHz are achieved after annealing at 0.8 A for 20 min. The annealing treatment under direct electric current induces stress relaxation, and domain rearrangement, and the crystallization process gradually increases with the increasing current density, which gives rise to anisotropic reformation. The release of stresses due to Joule heating below the crystallization temperature causes the homogenous distribution of stress induced by rapid solidification and influences the elastic anisotropy, causing the domain structures to become much more regular. The crystallization, along with the precipitation of hard magnetic phases, increases the crystal anisotropy and induces the intense magnetic coupling action. Consequently, the magnetic domains in the annealed ribbons are rearranged with reformed anisotropy by Joule annealing heat and by the transverse magnetic field induced by the current. The irregular domains, with complex anisotropy in the as-cast ribbons corresponding to the weak GMI response, are transformed into regular and strip-like domains, with transverse easy magnetization after annealing at 0.4 A. After annealing at 0.8 A, the domains are further transformed into fine axial fingerprint-like domains, which are much more sensitive to the change in the axial external magnetic field, allowing for the best GMI response. These results indicate that the Joule annealing treatment is an optional method to optimize the soft magnetic properties and the GMI effect of these Co-rich ribbons at low frequencies.