Study On Microstructure And Interface Bonding Mechanism Of WC-Ni/Ni Composite Coating Prepared By Vacuum Brazing

The development of industrial modernization has higher and higher requirements on the surface performance of equipment and parts.In the fields of thermal power generation,petroleum exploitation,mining machinery,metallurgy and chemical industry,mechanical parts and components work in harsh environments such as high temperature,high pressure,high speed,and heavy load for a long time,and the surface will be prone to wear,cracking,oxidation and corrosion.Thus,it is of great scientific significance to study new coating material systems and preparation methods,and to explore new surface engineering theories.In this paper,the self-developed forming technology was used to prepare the WC-10Ni/NiCrBSi composite coated cloth,and the coated cloth on the steel surface was prepared into a composite coating by vacuum brazing.The main research contents are as follows:（1）The micro-interface metallurgical mechanism and microstructure evolution of vacuum brazing composite coating/substrate interface,WC-10Ni/WC-10 Ni and WC-10Ni/NiCrBSi were studied.The interface metallurgy of WC-10Ni/NiCrBSi composite coating was divided into three stages: liquid phase formation,particle rearrangement,element diffusion and liquid phase solidification.Its microstructure was divided into surface layer,hard layer and interface layer.γ-Ni（Fe,Cr）,γ-Ni（Si）and α-Ni（Si）solid solutions were formed in the hard layer,which support the hard particles,and the Ni₃Si and Ni₃B formed in the hard layer played the role of dispersion strengthening;the fine structures in the hard layer were mainly: coherent dual-phase structure of Ni₃Si and Cr_0.19Fe_0.7Ni_0.11,interstitial carbides of Ni₃C and Ni₃B,Ni₃₁Si₁₂,CrB and Cr₈O₂₁;γ-Ni（Fe,Cr）,γ-Ni（Si,Cr）and α-Ni（Si）solid solutions were formed in the interface layer,the WC near the hard layer was dissolved,and a brittle complex compound composed of chromium-rich phase,carbides and borides was formed.（2）The experimental and mathematical model of coating components-joining processcoating properties were established.When the content of WC-10 Ni was 75% and the brazing temperature was 1095℃,the maximum hardness of the surface was 64.7HRC;when the content of WC-10 Ni was 60% and the brazing temperature was 1095℃,the maximum interface shear strength was 362.9MPa;The shear fracture was brittle fracture.With the increase of brazing temperature,the fracture form changes from a mixture of cleavage and quasi-cleavage to quasi-cleavage.Mathematical models of shear strength,surface Rockwell hardness and cross-section Vickers hardness were established,and the models had good prediction precision and high reliability.The error range between mathematical model value and actual value was 2.1-2.3%,and the minimum error was 2.1%;the error range between mathematical model value and actual value of surface Rockwell hardness was 0.2-0.5%;the error range between mathematical model value and actual value of section Vickers hardness was 0.2-8.6%.（3）The dry sand wear characteristics of the composite coating were studied.The dry sand wear mechanism of the composite coating was that the WC particles shielded and protected the surrounding composite coating substrate,effectively reducing and preventing the cutting effect of the abrasive particles.The dry sand wear failure mode of the composite coating was that the hard WC particles were exfoliated due to fatigue.（4）The thermal fatigue properties of composite coatings were studied.The reason for thermal fatigue cracks in composite coatings was the thermal stress caused by differences in thermal expansion coefficients between materials.When the upper limit temperature was500℃,the main cause of thermal fatigue cracks was thermal stress.The cracks experienced an initiation period,in which the growth rate was slow,and the coating didn’t change significantly with the number of cycles increasing;when the upper limit temperature was above 600℃,the oxidation behaviors acted together,then thermal fatigue cracks occurred.Under different process parameters,the main crack perpendicular to the interface between the coating and the substrate was initiated first and then the transverse crack near the diffusion layer was initiated.Under the action of alternating cycle thermal stress,secondary cracks were initiated and expanded,and finally the cracks were staggered and connected to each other to form macroscopic cracks.The propagation mode of thermal fatigue cracks in WC-10Ni/NiCrBSi composite coating was: a)propagation through the WC particle aggregation area;b)crack growth connecting the bulk carbide itself;c)propagation along the boundary between WC particles and the matrix;d)cracks continued to initiate and expand at the voids formed by the detachment of WC particles.The research in this paper provides data and theoretical support for the component design and properties control of composite coatings,which can be used to solve the existing problems in surface engineering technology.