Study On Cavitation Erosion Behaviour And Mechanism Of Different Structure WC-based Composite Coatings Sprayed By HVOF

Cavitation exists widely in many fluid machineries such as ship propeller,turbine blades,turbine impeller,water pump and etc.,especially the components in the marine engineering equipment,and cavitation erosion damage is one of the major causes of failure of fluid mechanical parts.Along with the advancement of Chinese ocean strategy,the cavitation protection of marine engineering equipment has become one of the important scientific researches.So far,surface coating techniques are the most economic and effective cavitation erosion protection methods.The WC-10Co-4Cr coatings deposited by high velocity oxy-fuel with excellent cavitation erosion resistance,have been successfully used in many fluid mechanical.But in the face of increasingly severe using environment,the limitations of conventional WC–10Co-4Cr coating is more obvious.In order to further improve the cavitation erosion resistance of WC-10Co-4Cr coatings,and design the more excellent coating system,the cavitation erosion behavior and the mechanism of the coatings are needed to be further studied.Based on the optimized parameters of HVOF process by SprayWatch monitoring system,the micron,nano and bimodal-structured WC-10Co-4Cr composite coatings with different WC scale were respectively deposited by the fuel type and gas type two kinds of HVOF process.The deposition behaviour of WC-10Co-4Cr particles and the microstructures of the coatings were analysed by SEM,OM and XRD;the porosity,microhardness,crack toughness and electrochemical properties of six kinds of WC-10Co-4Cr coatings were tested.The surface morphology change characteristics and the cavitation erosion particle morphology of WC-10Co-4Cr coatings during different cavitation erosion period were analysed,the cavitation crack characteristics of cross section was also analysed.Finally,on the basis of experimental study and theoretical analysis,the cavitation erosion mechanisms of different structured WC-10Co-4Cr coatings were proposed and the cavitation erosion physical model was established.The results show that the microstructure,mechanical properties and electrochemical properties of the WC-10Co-4Cr coatings deposited by the fuel type HVOF are better than those of the gas type.The cavitation erosion behaviour and microscopic mechanisms of the coating by different HVOF processes and different WC-10Co-4Cr powders are not exactly the same.Although the cavitation erosion process of six kinds of coatings is the same,that is the cavitation erosion crack extension lead the particles on the surface to peel off and produce corrosion pits,but the predominantly cavitation erosion behaviour of the coatings by fuel type HVOF is the horizontal development of shallow pits,the predominantly cavitation erosion behaviour of the coatings by gas type HVOF is the longitudinal development of deep pits.The more secondary cracks produce in the process of cavitation erosion of the coatings deposited by the gas type HVOF,and the micro short crack of different structured WC-10Co-4Cr coatings extend in a different way.The micro short cracks of the nanostructured coatings extend along the grain boundary,but the micro short cracks of the micro and bimodal coatings extend in intergranular,transgranular and mixed crystal three ways.The main reason for the different cavitation erosion behaviour and mechanism is that various WC-10Co-4Cr coatings possess the different microstructures,and the microstructures are influenced by spraying process and powder material characteristics.More structure defects exist in the coatings by gas type HVOF due to the inadequate deformation of powder particles.WC-10Co-4Cr powder with nano WC particles requires more strict spraying process because of easy oxidation and decarbonization.In the six kinds of coatings,the structure of bimodal coating deposited by the fuel type HVOF is dense,the corrosion pit is shallowest and the crack extend more difficultly,these all illustrate that it possesses the most excellent cavitation erosion resistance.That means the composite coating with different size of WC particle is a feasible research direction to improve the cavitation erosion resistance performance.