Structure And Regulation Of (Nb,M)C Multiple Carbide In Fe-based Hardfacing Coating

With high hardness,excellent wear resistance and good toughness,Niobium carbide?NbC?reinforced Fe-based composite coating has been applied for strengthening the surface of Fe-based workpieces.However,with the increasing demand for the service life of Fe-based workpieces,the higher wear resistance of its coating is also put forward.The wear resistance of the Fe-based coating is closely related to the hardness,morphology,size,distribution of the NbC carbide and its combination with the Fe matrix.By adding alloying elements into the coating,?Nb,M?C multiple carbides will form in it,which can change the above characteristics of the NbC carbide,and improve the wear resistance of the coating.In order to explore the mechanism of NbC reinforced Fe-based composite coatings,the coatings with different C,Ti and V contents were prepared by hardfacing method.The hardness and wear resistance of the coatings were determined.The hardness,distribution,morphology of?Nb,M?C multiple carbides and its combination with the Fe matrix were investigated.Finally,from the perspective of optimizing the morphology of carbides,the effectiveness of MgAl₂O₄ as the core of NbC heterogeneous nucleation were discussed.With the increase of C contents,the XRD diffraction peaks of matrix and MC carbide in the coating shift to the left.The MC carbides in the coating are changed from reticulated shape to short rod eutectic and granular primary ones.The hardness of the coating with 1.41 wt.%C content is the highest.The flaky M₇C₃ carbides can be observed in the coating,and holes are formed in this area during wear process,which reduce the wear resistance of the coating.Segregation structure regions are observed in the coatings with 1.24 wt.%and 1.41 wt.%C,and the segregation region is composed of?-Fe and?Fe,Cr?₃C carbide.The stability of the?Fe,Cr?₃C carbide is increased with the increase of Cr content.With the increase of Ti contents,the lattice constants of the?Nb,Ti?C carbides are decreased.The formation energy of the carbides are decreased.The hardness of the?Nb,Ti?C carbides is significantly improved compared to the NbC carbide,and the hardness of the Nb_0.25Ti_0.75C carbide even reaches 29 GPa.The lattice mismatch between Nb_0.25Ti_0.75C with?-Fe is decreased to 7.3%.The interface stability between?Nb,Ti?C carbide and?-Fe are increased.The interfacial combination of CNb-Fe,CTiNb-Fe and CNbTi-Fe interfaces are mainly benefit from metal bonds and weak covalent bonds.Metal bonds,ionic bonds and covalent bonds exist at the interfaces of NbC-Fe,TiNbC-Fe and NbTiC-Fe.With the increase of Ti contents in the coatings,the carbides in the coating change from eutectic carbide to primary carbide.The average diameter of the primary carbides is decreased,and the amount of primary carbides is increased.After the addition of the element Ti,the wear loss weight of the coating is significantly decreased.During the wear process in the coating without Ti addition,the eutectic carbides slide and leaves deep furrows.In the coating with 0.15wt.%Ti,the granular primary carbides break and prevent the wear path.The size of primary carbides are decreased with the increase of Ti content,and the primary carbides appear to slip.Among all the Nb_0.75M_0.25C?M=V,Mo,W and Cr?carbides,the lattice constants and formation energy of Nb_0.75V_0.25C carbide are the smallest.The hardness of Nb_0.75V_0.25C and Nb_0.75W_0.25C are 27.55 GPa and 26.26 GPa,respectively,which are higher than the hardness of NbC.The high hardness of the Nb_0.75V_0.25C carbide is mainly due to the strong covalent bond of Nb-C,and the high hardness of the Nb_0.75W_0.25C carbide is contributed by the strong covalent bond of W-C.The lattice mismatch between Nb_0.75V_0.25C and?-Fe is reduced to 8.1%.The stability order of the NbMC-Fe interface model is as follows:NbCrC-Fe>NbVC-Fe>NbMoC-Fe>NbC-Fe>NbWC-Fe.And the stability order of the CNbM-Fe interfaces is CNbCr-Fe>CNbV-Fe>CNbW-Fe>CNb-Fe>CNbMo-Fe.There are metal bonds,ionic bonds and covalent bonds at the interface of CNbM-Fe and NbMC-Fe.The covalent bonds are also formed between the V,Mo,W,and Cr atoms of the next nearest neighbor and the Fe atoms.Based on the elemental distribution of the carbides and the diffraction spot analysis,it is confirmed that the element V can be introduced into the NbC carbide to form a uniform structure,rather than the mixed structure of VC and NbC.The lattice mismatch between MgAl₂O₄ and NbC is 8.7%,less than 12%,which indicates that MgAl₂O₄ is structurally effective as a nucleation core of NbC.The stability relationship of MgAl₂O₄?111?/NbC?111?interfacial structure is as following:O?Mg?Nb>Al?Mg?C>Al?Mg?Nb>Mg?O?C.There are mixed bonding structure of metal bond,ionic bond and covalent bonds of O-Nb,Al-Nb,Al-C,and Mg-C at the O?Mg?Nb,Al?Mg?Nb,Al?Mg?C,and Mg?O?C interfaces.Core-shell carbides can be observed in the coating with 0.5 wt.%AlMg powder addition.The core of the core-shell structure is determined to be MgAl₂O₄ by TEM diffraction and EDS elemental analysis.