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Microstructure and Tribological Properties of a Laser Clad NiCr-Based Composite Coating in High Vacuum at Elevated Temperature, Atomic Oxygen and Ultraviolet (UV) Irradiation Environments
C. Guo, F. Chen, B-L. Wei and H. Zhang

Ni80Cr20/Cr3C2/Ag/BaF2-CaF2 lubricating composite coatings were fabricated on commercially pure Ti (TA2) substrates by laser cladding with a CO2 laser. X-ray diffractometry (XRD), a scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and a transmission electron microscope (TEM) were utilized to study the phase composition and microstructure. The outer space tribological characteristics of the prepared NiCr-based composite coatings were systematically investigated in simulated outer space environments detection system, including high vacuum, atomic oxygen erosion and ultraviolet (UV) irradiation in comparison with those in atmospheric air environment. The results showed that the composite coating was mainly composed of Cr3Ni2, TiC, NiTi, Ag and Cr2Ti phase. The homogeneous cellular crystals promoted that microhardness of the coating has increased by four times compared with that of the Ti (TA2) substrate. The friction coefficient and wear rate of the NiCr-based composite coating in the atmospheric air environment were superior to those in the different outer space environments with elevated outer space temperature and vacuum. The adhesive wear with the character of plastic deformation accompanied with mild abrasive wear was the predominant wear mechanism in the atmospheric air environment; however, the shallow grooves and adhesion craters on all of the worn surfaces dramatically increased, which indicated that adhesive wear and transfer were significantly intensified in the atomic oxygen, UV and high vacuum outer space environments.

Keywords: CO2 laser, titanium, Ti, TA2, NiCr-based composite coating, dry friction, wear mechanism, surface treatment, outer space, low earth orbit (LOE)

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