The effect of chromizing time on the microstructure and properties of B–Cr duplex-alloyed coating prepared by a two-step pack-cementation process was investigated. The phases, microstructure, and element distribution of three coatings obtained were characterized by X-ray diffraction (XRD), secondary electron imaging (SEI), backscattering electron imaging (BSEI), and energy dispersive spectroscopy (EDS), respectively. The results show that as the chromizing time increases, the net-like Fe₂B and rod-like CrFeB phases in the coating gradually disappear, and finally completely transform into the block-like Cr₂B and Cr_xC_y (Cr₇C₃ and Cr₂₃C₆) phases. The growth kinetics analysis shows that interface reaction dominates the coating growth during the early stage of chromizing, while atomic diffusion gradually controls the coating growth at the later stage. The evolution mechanism of the B-Cr duplex-alloyed coating was also discussed.

In this study, a two-step pack cementation process (preboronizing and then chromizing) was employed to prepare the B-Cr duplex-alloyed coating on the steel. After the first step of preboronizing (PB sample), box-type furnace chromizing (BC-1 sample) and induction heating chromizing (BC-2 sample) were carried out, respectively. The phases and microstructure of the coatings were characterized by X-ray diffraction (XRD), backscattering electron imaging (BSEI), and energy dispersive spectroscopy (EDS). The results reveal that the heating mode of the second step of chromizing has a significant effect on the phase composition and microstructure of the B-Cr coating. The efficiency of induction heating is higher than that of the box furnace heating, resulting in a thicker, denser, flatter surface, and B-Cr coating with fully reacted B and Cr elements. The wear and corrosion resistance of the steel is found to be significantly improved by the formation of effective B-Cr coating. The formation mechanisms and properties of the two duplex-alloyed coatings are investigated and discussed.

Cr-coating was deposited on AISI 5140 steel by electro brush-plating, followed by annealing treatment at different temperatures, from 300 to 1100 °C. The microstructure evolution of the Cr-coating was characterized by backscattered electron imaging (BSEI) and energy dispersive spectrometry (EDS). The results show that the brush-plated sample has a nodular shaped microstructure, which is very stable at 300 °C of annealing. At 500 °C of annealing, the constitution of the microstructure changes from nodules to grains. As the annealing temperature further increases, the grains grow significantly. When the temperature reaches 1100 °C, a Cr-Fe solid-solution layer is formed within the original pure Cr-coatings. With increasing annealing temperature, the number of micro-cracks in the coating increases first and then decreases, reaching a maximum at 500 °C. The hardness and wear-resistance of the coating are improved when the annealing temperature increases to 1100 °C, owing to the decrease of micro-cracks that formed during brush-plating.

In this study, Cr–Mo duplex-alloyed coating was prepared on carbon steel by double glow plasma surface alloying (DGPSA). The effect of annealing and quenching and tempering (Q&T) treatments on the microstructure and performance of the coating was investigated by X-ray diffraction (XRD), energy dispersive spectrometry (EDS), backscattering electron imaging (BSEI) and electron backscattering diffraction (EBSD) techniques. The results show that a gradient structured coating composed of an Fe–Cr–Mo solid solution (Fe–Cr–Mo SS) layer and an alloyed pearlite layer was obtained on the steel surface. The affected layer was adjacent to the coating. After annealing or Q&T, more carbides precipitated in the Fe–Cr–Mo SS layer and alloyed pearlite layer. Most of the C atoms in the subsurface were dragged into the coating to form carbides in the Fe–Cr–Mo SS and alloyed pearlite layers of the coating, transforming the affected layers into a carbon-poor zone. Annealing and Q&T hardly modified the thickness of the coating, but greatly changed the hardness and corrosion resistance of the coating. The Q&T treated samples had higher hardness and better corrosion resistance than the as-DGPSA treated and the annealed samples.

The FeS solid lubricating coatings were prepared on the AISI 5140 steel by chemical bath deposition (CBD) coating technique at various temperatures from 30 to 90 °C. The influence of temperature on microstructure, microcracks, and tribological properties was characterized and studied by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectrometry (EDS). The results show that the coating mainly consists of Fe_xS_y (FeS, FeS₂, and Fe_1−xS), and has a chrysanthemum-shaped (CS) morphology composed of the FeS crystal petals. The CS particles nucleate at about 30 °C, rapidly grow and reach a peak thickness at about 50–70 °C, and finally disappear and are replaced by disordered thick petals at 90 °C. The wear resistance of the steel was improved obviously after introducing of FeS coatings, owing to that the coatings can provide better lubrication and improve the wear resistance.

Cr-Fe₂B composite coatings were prepared on carbon steels by pack-boronizing followed by electro brush-plating. The microstructure and properties of the coatings annealed at different temperatures were studied. The coatings show a gradient structure composed of a Cr-layer and a Fe₂B-layer and have excellent thermal stability, and no new layers and/or transition layers are formed in the coating during annealing up to 1000 °C. The Cr-layer has an amorphous structure and is transformed into nanosized grains when the annealing temperature increases to 700 °C. As the annealing temperature is further increased, the nanograins grow rapidly. The microcracks in the Cr-layer increase sharply after annealing at 550 °C and then decrease significantly with any further increase of the annealing temperature. The pre-deposited Fe₂B-layer prevents the formation of carbon-poor zones in the steel substrate during annealing. It is considered that high-temperature (>700 °C) annealing helps to eliminate coating defects, increase the coating density and obtain better wear resistance and corrosion resistance. Surprisingly, the as-plated and low temperature annealed samples also show good wear resistance and corrosion resistance, which may be related to their amorphous structure and nanocrystalline structure.