Study On Microstructure,Properties And Cavitation Erosion Behavior Of Multi-dimensional Micro-nano Structured WC-CoCr Coatings

Cavitation erosion?CE?is a major failure form of overcurrent components in fluid machinery,which is widely found in ship rudder blade,propeller,a variety of pumps,turbine impeller,and other devices.With the implementation of the national marine engineering equipment and high technology ship development strategy,the cavitation protection of marine equipment has become a key research area.The ocean is a harsh corrosive environment,which accelerates the cavitation erosion of fluid machinery due to the combination of cavitation and corrosion.Therefore,the development of anti-cavitation materials and study the erosion mechanisms have great theoretical and engineering value.WC-CoCr cermet coating has been a focus of recent research on anti-cavitation materials,especially the micro-nanostructured WC-CoCr coatings prepared by High Velocity Oxygen Fuel Spray?HVOF?,which has received extensive attention on its potential anti-cavitation performance and applications.In this thesis,based on the kinetics and thermodynamic characteristics of nano,submicron and micro WC particles in high velocity oxy-fuel flame,the microstructure and cavitation failure mechanism of different structured WC-CoCr coatings,a novel multi-dimensional WC-10CoCr composite powder with analogous structure to concrete,was designed and fabricated,in which submicron and micro WC particles are similar to sand?fine aggregate?and gravel?coarse aggregate?,nano WC and CoCr alloy are analogous to cement in concrete.The multi-dimensional powder contains micro-nano structured WC particles,including nano WC?20 wt%?,submicro WC?30wt%?and micro WC?50 wt%?with size of 60-180 nm,0.4-0.6?m and 2.2-2.6?m,respectively.The powder was fabricated with agglomeration-sintering method.Only WC and Co phases were detected in X-ray diffraction,with no trace of other detrimental phases such as W₂C,Co_xW_yC and W.Prior to coating deposition,particle temperature and speed of different sized WC-10Co4Cr powder in various HVOF flame were monitored by Spray Watch-2i online system.After spray parameters optimization,multi-dimensional,bimodal and nano WC-10Co4Cr coatings were deposited by both High Velocity Oxygen Liquid Fuel Spray?HVOLF?and High Velocity Oxygen Gas Fuel Spray?HVOGF?techniques.The flattening characteristics of multi-dimensional micro-nano WC-10Co4Cr droplets and the deposition behavior of the coating were studied.In the cases of HVOLF and HVOGF deposit multi-dimensional WC-10Co4Cr particles,disk and splash state are present after droplets collide with substrate surface.This is because WC-CoCr particles have different kinetic and thermodynamic characteristics in HVOLF and HVOGF flame.The mechanism of coating formation prepared by HVOF was proposed based on above results.The microstructures of multi-dimensional WC-10Co4Cr coatings were characterized by scanning electron microscopy?SEM?,optical microscopy?OM?,transmission electron microscopy?TEM?and XRD.The mechanical,electrochemical properties and wear performances of the coatings were also analyzed.Results show that the coatings sprayed by HVOF are mainly composed of WC and a tiny amount of W₂C.Co disappears in XRD patterns due to formation of amorphous phase.In HVOLF sprayed multi-dimensional coating with analogous structure to concrete,nano,submicro and micro WC particles evenly distributed within CoCr binding phase,forming dense multi-dimensional coating with porosity of 0.31%.The coating only contains 1.4 wt%W2C and has excellent mechanical(microhardness of 1100 HV0.3and fracture thoughness of 5.16 MPa·M^1/2),electrochemical properties?potentiodynamic potential of-0.31V?,sand solid erosion and abrasive wear resistance.Cavitation erosion resistance of different WC-10Co4Cr coatings was studied via ultrasonic vibration method in fresh water and 3.5 wt%NaCl solution.Compared to bimodal and nanostructured WC-10Co4Cr coatings,multi-dimensional coating displays the best cavitation erosion resistance in both media,with 24%and 50%decrease of cavitation volume in fresh volume,and with 16%and 70%decrease in NaCl solution.The effects of microhardness,fracture toughness,porosity and corrosion electrode potential on the cavitation resistance of WC-10Co4Cr coatings were studied.In fresh water and NaCl solution,fracture toughness and porosity of coatings possess respectively the most significant effects on the coating cavitation resistance;the cavitation mathematical model of WC-10Co4Cr coating deposited by HVOF in fresh water and NaCl solution was established respectively.The cavitation erosion mechanisms,including the formation of cavitation sources,proparation of cracks and generation of craters of multi-dimensional WC-10Co4Cr coating,were explored by OM and SEM in-situ analysis method.In fresh water,HVOLF sprayed multi-dimensional WC-10Co4Cr coating has the slowest cavitation source formation and crack propagation speed.The propagation of cracks is generally parallel to the coating surface.The cavitation erosion in NaCl solution occurs much quicker than in fresh water.The cracks extend both parallel and perpendicular to the surface,leading to bigger and deeper craters.The HVOGF sprayed WC-10Co4Cr coatings show much rougher surface,with craters of larger amount and greater depth.The particles exfoliated from coatings in fresh water and NaCl solution display different shapes.In fresh water,the particles are well separated.In NaCl solution,the cavitation products are clusters bonded by small and dotted particles.Further investigation shows the catitation erosion of multi-dimensional WC-10Co4Cr coating in NaCl solution is mainly caused by combined effect of mechanical force,interaction of mechanical force and corrosion,while mere chemical effect is negligible.In NaCl solution cavitation,the surface of the coating will show many large and deep craters,which are mainly the result of mechanical force,self-catalyzing of corrosive medium,interaction of mechanical force and electrochemical corrosion.The cracks tend to extend along the CoCr binding phase in the regions which consist of nano and submicro WC,but can sometimes cut through micro WC particles.The coexistence of micro,submicro and nano WC grains and excellent fracture toughness of multi-dimensional WC-10Co4Cr coating make cracks difficulty to propagate,thus leading to the improvement of anti-cavitation performance of the coating.