| Underwater moving parts of hydraulic machineries such as hydraulic turbine,water pump etc.suffer from slurry erosion,cavitation erosion and cavitation-corrosion during operation process,which accelerates the surface damage and reduces the operation efficiency of hydraulic system Considering that the slurry erosion,cavitation erosion and corrosion behavior first act on the surface of materials,it is an effective method to prepare coatings on stainless steel substrate by suitable materials and processes.Slurry erosion resistance of the coating is mainly related to the microhardness,cavitation erosion resistance is closely related to the fracture toughness and elastic modulus,and the corrosion resistance is concerned with the distribution of the phases and the microstructure of the coating.Cemented carbide coating materials are composed of hard phase and bonded phase,in which the existence of hard phase improves the wear resistance of the coating,and single metal or alloy acts as bonded phase with high fracture toughness,hence compared with other kinds of coating materials,cemented carbide coating materials have the combination of excellent slurry erosion,cavitation erosion and electrochemical corrosion resistance.Tungsten carbide(WC)has hexagonal crystal structure,high microhardness and elastic modulus,and Co,Ni and Cr metals are widely used in WC coating design due to their good adhesion,toughness and corrosion resistance.The cemented carbide coating prepared by high velocity oxy-fuel(HVOF)process is significantly better than other thermal spraying processes in terms of compactness and bonding strength.In this paper,the WC based cemented carbide coatings with different material systems were prepared by HVOF,and the related properties of the coatings were studied through slurry erosion,cavitation erosion and cavitation-corrosion tests.The mechanisms of slurry erosion,cavitation erosion and cavitation-corrosion were clarified to provide theoretical and technical support for surface protection of hydraulic machinery.The main research work and conclusions are as follows:(1)A new pot-type of slurry erosion tester was designed and manufactured.The frame was designed with triangular welding structure to improve the rigidity of the whole machine.A new sample holder was designed.The sample holder and positioning sleeve were designed by revolute pairs,which can adjust the sample at any angle.Programmable Logic controller(PLC)and frequency converter were adopted to achieve automatic control of equipment and regulation of motor speed.The motor power is estimated by using the surface pressure analysis method of steady flow impacting rotating disc.The slurry erosion tests of cemented carbide coatings and stainless steel substrate were carried out by home-made slurry erosion tester.(2)WC-12Co and Cr3C2-25NiCr cemented carbide coatings were prepared on 16Cr5Ni stainless steel substrate by HVOF.Orthogonal experiments were designed based on three slurry erosion parameters,namely rotational speed,particle size and slurry concentration.The influence of rotational speed,particle size and slurry concentration on slurry erosion performance of the cemented carbide coatings and substrate was studied.The results show that the WC-12Co coating exhibits the lowest erosion rate and the highest for 16Cr5Ni stainless steel under all the test conditions.The variation of erosion rate for WC-12Co coating is the least during the test,reflecting that the slurry erosion resistance of WC-12Co coating is the most stable.The exponents of velocity,particle size as well as slurry concentration of all samples were calculated by the equation of slurry erosion rate,it is found that the effect of rotational speed on the erosion rate of WC-12Co coating is the most significant,the influence of particle size on Cr3C2-25NiCr coating is the most remarkable,and 16Cr5Ni stainless steel is most sensitive to the change of slurry concentration.The slurry erosion mechanism of WC-12Co coating and 16Cr5Ni stainless steel is brittle and ductile respectively,Cr3C2-25NiCr coating shows a composite mechanism of ductile and brittle,dominantly ductile(3)The WC-12Co coatings were heat treated at temperatures of 650,800,950 and 1100℃respectively with nitrogen atmosphere.The cavitation erosion test was carried out with an ultrasonic cavitation erosion equipment.The effects of heat-treated temperature on phase change,microstructure,mechanical properties and cavitation erosion resistance of WC-12Co coatings were investigated.The results show that with the increase of heat treatment temperature,the content of η phase(Co6W6C)increases in the coating.The microhardness of the coating is closely related to the porosity and phase composition,proper heat treatment temperature can improve the microstructure and mechanical properties of the coating.The coating heat-treated at 800℃exhibits the best cavitation erosion resistance,followed by 650℃,950℃,as-sprayed coatings,and the worst is 1100℃ heat-treated coating.The cavitation erosion rate of the coatings is closely related to the micro-defects and phase transition produced in the heat treatment process.The value of contour mean deviation is positively correlated with cavitation erosion rate,which means surface roughness parameters can be used to quantitatively evaluate cavitation erosion behavior of materials.The cavitation erosion model based on secondary erosion was constructed,which reasonably explains the characteristics of stepped morphology for the eroded coatings.(4)Cavitation erosion and cavitation-corrosion experiments were carried out for HVOF sprayed WC-25WB-10Co-5NiCr and MOB-25NiCr,WC-10Co-4Cr,Cr3C2-25NiCr coatings in deionized water and 3.5 wt.%NaCl solution,respectively.Two equivalent circuit models are proposed to fit the electrochemical impedance spectra(EIS)of four cemented carbide coatings.The results show that the values of the micro-pore resistance Rp and the charge transfer resistance Rct of the coatings reveal that the corrosion resistance of the four cemented carbide coatings after 8 h of cavitation-corrosion tests from high to low is ranged in the order:WC-25WB-10Co-5NiCr>WC-10Co-4Cr>MoB-25NiCr>Cr3C2-25NiCr.Due to the impact energy generated from cavitation erosion hinders the formation of corrosion products and passivation films on the coating surface,there is no obvious passivation reaction in the direction of anodic corrosion for each coating,the electrochemical corrosion of the four cemented carbide coatings is caused by the galvanic effect.The mass loss of all cemented carbide coatings under both pure cavitation erosion and cavitation-corrosion conditions increases linearly with the test time,and there is no cavitation incubation period,acceleration period and stability period.Cavitation is the primary factor causing the material removal for all the coatings in the cavitation-corrosion tests.The damage degree of each coating under the synergistic action of cavitation and corrosion is higher than that of pure cavitation.Cavitation erosion resistance and cavitation-corrosion resistance are positively correlated with microhardness.The spalling of the hard phase and secondary phase as well as the dissolution of bonded phase are the main cavitation-corrosion mechanisms for WC based coatings,the peeling of coating caused by the extension of cracks is the dominant mechanism for Cr3C2-25NiCr and MoB-25NiCr coatings. |
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