Thermal Fatigue Properties Of Vacuum Brazing WC-10Ni/NiCrBSi Composite Coating

In this paper,WC-10Ni/NiCrBSi coating consisting of WC-10 Ni particles with different manufacturing processes was prepared on the surface of Q235 substrate by using flexible metal cloth fabrication technology and vacuum brazing technology.The macroscopic morphology,microstructure and phase composition of the coating were studied by means of scanning electron microscopy,energy dispersive spectroscopy,X-ray diffractometry and metallographic microscope.The hardness distribution of the coating section was analyzed by a fully automatic microhardness tester.The bonding strength between the coating and the substrate was measured by an electronic universal testing machine.The thermal fatigue behavior of the composite coating and the mechanism of thermal fatigue crack initiation and propagation at different temperatures were studied by self-cooling-thermal fatigue equipment.The results indicated that:The distribution ratio and type of flexible metal cloth had a significant influence on the forming of coating.When the proportion of WC-10 Ni and NiCrBSi in flexible metal cloth was 50%,the coating was best formed;WC-10 Ni particles prepared by agglomeration sintering in flexible metal cloth were too high to crack easily,while WC-10 Ni particles prepared by mechanical alloying were too low to form the coating.With the increase of brazing temperature,the coating of WC-10 Ni particles prepared by agglomeration sintering became more and more obvious.The structure was divided into surface layer,hard layer and diffusion layer.However,the coating of WC-10 Ni particles prepared by mechanical alloying only contained hard layer and diffusion layer with the increase of brazing temperature,and there was no surface layer.The solubility of WC-10 Ni particles prepared by agglomeration sintering was less than that of WC-10 Ni particles prepared by mechanical alloying.The process parameters of vacuum brazing coating had obvious influence on the mechanical properties of the coating.The microhardness of WC-10 Ni coatings prepared by agglomeration sintering was higher than that of WC-10 Ni coatings prepared by mechanical alloying.The bonding strength of WC-10 Ni coatings prepared by agglomeration sintering method increased with the increase of brazing temperature.The bonding strength of WC-10 Ni coatings prepared by mechanical alloying decreased with the increase of brazing temperature,and the highest bonding strength was 351.5Mpa when the brazing temperature was 1080℃.For WC-10 Ni coatings prepared by mechanical alloying,the bonding strength of coatings decreased with the increase of brazing temperature,while the brazing temperature was 1030℃.The highest bonding strength was 365.1Mpa.The experimental upper limit temperature had a significant effect on the initiation and propagation of thermal fatigue cracks.Under the test conditions,the crack initiation period was long and the crack propagation speed was slow when the test temperature was 500℃,and there was no crack in the coating macroscopically when the cycle number was increased.When the test temperature was 600℃,the crack initiation period was shortened and the crack propagation speed was accelerated.The oxidation of WC-10 Ni particles on the coating surface will affect the thermal fatigue properties of the coating,and the coating cracks about 200 times during the test temperature was 800℃.Cracks hardly have incubation period and crack propagation speed was very fast.After 100 cycles,cracks appear on the coating surface and the joint between coating and matrix.Microscopic analysis of thermal fatigue cracks in WC-10Ni/NiCrBSi coatings showed that the main ways of crack propagation were as follows: 1)Propagation through the aggregation region of WC particles;2)crack propagation along the boundary of WC particles and matrix;4)crack initiation and propagation at the voids formed by the shedding of WC particles.