Effects of La_2O_3 on microstructure and wear properties of laser clad γ/Cr_7C_3/TiC composite coatings on TiAl intermatallic alloy

摘要

The effects of La_2O_3 addition on the microstructure and wear properties of laser clad γ/Cr_7C_3/TiC composite coatings on γ-TiAl intermetallic alloy substrates with NiCr-Cr_3C_2 precursor mixed powders have been investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive spectrometer (EDS) and block-on-ring wear tests. The responding wear mechanisms are discussed in detail. The results are compared with that for composite coating without La_2O_3. The comparison indicates that no evident new crystallographic phases are formed except a rapidly solidified microstructure consisting of the primary hard Cr_7C_3 and TiC carbides and the γ/Cr_7C_3 eutectics distributed in the tough γ nickel solid solution matrix. Good finishing coatings can be achieved under a proper amount of La_2O_3-addition and a suitable laser processing parameters. The additions of rare-earth oxide La_2O_3 can refine and purify the microstructure of coatings, relatively decrease the volume fraction of primary blocky Cr_7C_3 to Cr_7C_3/γ eutectics, reduce the dilution of clad material from base alloy and increase the microhardness of the coatings. When the addition of La_2O_3 is approximately 4 wt.%, the laser clad composite coating possesses the highest hardness and toughness. The composite coating with 4 wt.%La_2O_3 addition can result the best enhancement of wear resistance of about 30%. However, too less or excessive addition amount of La_2O_3 have no better influence on wear resistance of the composite coating.