Abstract
We report the synthesis of SiC/Si3N4 coatings on carbon/carbon composites via pack cementation and heat treatment with nitrogen gas, the latter of which improves the coating wear resistance. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) are used to analyse the microstructure, surface morphology, chemical states and elemental distribution, respectively, of SiC and SiC/Si3N4 coatings. In addition, we analyse the tribological behaviour of the SiC and SiC/Si3N4 coatings and the related microstructure and wear mechanisms. The results show that SiC/Si3N4 coatings are compact and contain the three phases: Si3N4, SiC and C. Additionally, specimens with the SiC/Si3N4 coating have smaller, more stable friction coefficients and less weight loss than specimens with only the SiC coating. Adhesive wear and abrasive wear are the main wear mechanisms contributing to the higher friction coefficient of the SiC coating. Furthermore, with the SiC/Si3N4 coating, adhesive wear is the main wear mechanism causing a high friction coefficient at the initial stage of frictional wear. In the subsequent stages of frictional wear, the graphite in the SiC/Si3N4 coating generates a thin lubricating film that decreases the friction coefficient.
Highlights
Mechanical seals are extensively used in industry to prevent losses of lubricant or gases and to prevent the entry of contaminants into hydraulic or lubrication circuits
Carbon/carbon composite (C/C composites) have drawn great interest owing to their excellent mechanical properties such as light weight, high module, high specific strength, low thermal expansion, high temperature resistance, outstanding corrosion resistance and low friction coefficient [2,3,4,5]
For conventional C/C composite-based sealing materials, the stability of the friction coefficient is relatively low and the wear resistance is poor when applied as mechanical seals
Summary
Mechanical seals are extensively used in industry to prevent losses of lubricant or gases and to prevent the entry of contaminants into hydraulic or lubrication circuits. To address this issue, conventional C/C composites may be coated with wear-resistant materials to improve their tribological properties. Liu reported that the SiC and ZrSi2 coatings were prepared in two steps: pack cementation and supersonic atmospheric plasma spraying, respectively. The SiC coating was prepared via the pack cementation process. Silicon nitride (Si3 N4 ), a typical non-oxide ceramic material, has been used in applications requiring high tribological performance owing to its low chemical activity, high wear resistance and satisfactory mechanical properties [22,23]. SiC coatings suffer from defects that reduce the adhesion between the SiC coating and substrate To address these problems, we applied nitriding and low-temperature packing cementation to form SiC/Si3 N4 recombination coatings on C/C composites.
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