Effects of Y2O3 Content on Microstructure and Tribological Properties of WC-reinforced Ti-based Coatings on TC4 Surfaces
To enhance the surface mechanical properties and extend the safe service life of aerospace TC4 alloy,a composite coating of TC4,Ni-MoS2,WC,and Y2O3 powders was prepared on the surface of TC4 alloy using laser cladding,which improved the microhardness and wear resistance of the substrate surface. This study investigated the effect of Y2O3 content on the microstructure,hardness,and tribological properties of the coating. Using a Laser TruDisk4002 laser under argon protection,wear-resistant composite coatings of TC4+Ni-MoS2+WC+xY2O3 (x=0wt.％,1wt.％,2wt.％,3wt.％,4wt.％) were prepared on a TC4 alloy substrate. The coating was observed with a scanning electron microscope (SEM). Phase identification was conducted by X-ray diffraction (XRD),energy dispersive spectroscopy (EDS),electron backscatter diffraction (EBSD),and transmission electron microscopy (TEM). The microhardness distribution of the coating was studied with a Vickers microhardness tester. The friction and wear properties of the coating were tested with a dry friction tester,and the wear morphology was analyzed with a non-contact white light interferometer profilometer. Additionally,the coherent relationships between the phases in the coating were calculated according to the Bramfitt's two-dimensional lattice misfit theory.Results showed that the 3wt.％ Y2O3 coating exhibited the best forming quality. As the Y2O3 content increased,the coating thickness increased from 0.975 mm to 1.447 mm. The cladding layer mainly consisted of TiC,Ti2Ni,Ti2S,matrix β-Ti,and unmolten WC. The addition of Y2O3 did not change the types of phases formed in the coating. According to the Bramfitt two-dimensional lattice misfit calculations,the misfit percentages between Y23O(001) and 2TiNi(100),Y23O(100) and 2TiS(010),Y23O(001) and TiC(100) . Were 4.273％,3.671％,and 1.974％,respectively. Therefore,a coherent attachment growth relationship was formed between Y2O3 and TiC,Ti2S and Ti2Ni,and coherent composite structures of TiC-Ti2S,TiC-Ti2Ni,and Ti2S-Ti2Ni were effectively synthesized in all coatings. With the increase of Y2O3 content,the exposed area of the matrix increased,and the remaining phases gradually refined,aggregated,and grew continuously along the grain boundaries. The microhardness of all coatings was higher than that of the TC4 substrate,with the 3％ Y2O3 content coating having a hardness of 563.14HV0.5,which was a 39.89％ improvement compared with the substrate. All coatings underwent dry friction tests under load of 10 N,duration of 60 min,speed of 200 r/min,and room temperature conditions. Among them,the 3％ Y2O3-containing coating exhibited the lowest friction coefficient (μ=0.4813),the best wear resistance (3.55×104 mm3/(N·m)),and the optimal surface roughness (Ra=21.24 μm).Research finds that an appropriate amount of Y2O3 helps improve the forming quality of the coating,and its addition does not affect the types of phases in the coating. Moreover,it can serve as a heterogeneous nucleation substrate,promoting the mutual growth of TiC-Ti2S-Ti2Ni coherent structural phases. Besides the matrix phase,Y2O3 also attracts other phases at the grain boundaries of the coating. However,an increase in Y2O3 content does not increase the microhardness of the coating,and all coatings lack friction reduction performance,but their wear resistance is superior to that of the TC4 substrate. Proper addition of Y2O3 can optimize the overall tribological performance of the coating.
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