| Nickel and cobalt based alloys have been widely used in the surface modification of components, which can effectively overcome the conditions of high temperature, friction and wear, heavy load and corrosive medium. After adding WC particles acting as strengthening phases, the wear resistance and oxidation resistance of the alloy can be improved and the friction coefficient and wear rate of the coating can be further reduced by adding the graphite(G) as solid lubricant. In this paper, a series of Ni-Co based coatings were prepared on ZG45 substrate by vacuum cladding. The microstructure characteristics and properties were analyzed and studied.The results show that the Ni-Co/WC composite cladding layers have well formation and compact structure, and the WC particles well-distributed in the nickel-cobalt-based alloy coating and formed a special 3D reticular microstructure. The main phases are WC, Cr7C3, Cr23C6, Ni3 Si, FeNi3 and γ-Ni-Co based solid solution. The microstructure of the sample cross section can be divided into substrate zone, transition zone and composite zone. Moreover, where there is 25μm fusion zone between the transition zone and the substrate zone. After adding graphite, a large amount of pore with different size is produced in the cladding layers. The density and strength of the cladding layers are reduced by adding excess graphite. The Ni-Co based alloy cladding layers with the W element are easy to form a diamond or square M6 C phase with Fe, Cr, Ni and other elements in near the region next to the interface.Three point bending tests were carried out on the Ni-Co cladding samples with different compositions. The study show that the mechanical coordination of the Ni-40%Co alloy cladding layer contraposing the ZG45 matrix is better. When the Ni-40%Co alloy cladding layer on the top the fracture apparences show that the ductile facture happens for the substrate because of some dimple patterns and the fracture of the cladding layer has dual characteristics of quasi cleavage and ductile crack. When the cladding layer at the bottom the fracture mode of substrate is cleavage fracture and the fracture mode of cladding layer is intergranular and transgranular fracture. In the three point bending tests of Ni-Co/WC cladding specimen the analysis of fracture morphology show that ductile fracture happens at the substrate side. In the cladding layer, the intergranular fracture happens in the transition zone and accompanied with the transgranular fracture of hard phases such as Cr7C3. The fracture mechanism of composite zone is mainly the intergranular frcture that microcracks extend along WC plane. When the cladding specimens added graphite were to be bend, graphite particles acted as cracks source in the cladding layers, which occurred by transgranular and intergranular also. And the fractures of substrate haave characteristics of cleavage steps and dimple patterns.The polarization curves show that the corrosion resistance can be effectively improved by the preparation of Ni-Co based cladding layer on metal surface. The corrosion resistance of the Ni-Co based alloys cladding layer can be enchanced with the increase of the proportion of Co based alloy powder. The corrosion resistance of Ni-Co/WC composite cladding layer increases firstly with the increase of WC content, and then decreases. The corrosion resistance of Ni-Co/40%WC composite cladding layer is best. After the addition of graphite in the composite cladding layer of Ni-Co/WC, the corrosion resistance firstly increases with the increase of graphite content, and then decreases. The corrosion resistance of the composite cladding layer with 6%G is best.The tribological properties of Ni based alloy coatings were improved by adding Co. The wear mechanism of Ni-Co based alloy cladding layers is mainly adhesion and fatigue. And the wear resistance of Ni-30%Co based alloy cladding layer is optimal. After adding WC the wear mechanism of the composite cladding layers is mainly oxidation, and accompanied slightly with abrasion and adhesive. The friction and wear performance of Ni-Co/WC cladding layers added graphite is improved. The composite cladding layers with 4%~6%G have the best friction and wear performance. Due to excessive graphite producing a great number of holes the strength of the composite cladding layer with 8%G is reduced, which leads to fatigue wear and microcracks in the process of fiction and wear. |
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