HVOF Sprayed WC-10Ni Cemented Carbide Coatings And Their Wear Performance Under Water Lubrication

WC-Ni cemented carbide coatings developed by substituting Ni for Co as binder phase in WC-Co system,are promising for applications as wear-resistant protective coatings in harsh working environment such as high temperature and high pressure,high humidity,corrosive media and radioactive condition etc,due to their superior resistance to high temperature oxidation,corrosion and radiation.As a relatively new coating material,the investigations on WC-Ni cemented carbide coatings are still not systematic as compared to that of WC-Co system,both in spraying process and wear performance in working environment mentioned above.High velocity oxy-fuel spraying(HVOF)is one of advanced thermal spray process mainly developed for WC-based cemented carbide coatings,and in this work is employed to spray WC-10Ni coatings on 17-4PH precipitation hardening martensite stainless steel substrates.Here,aiming at wear performance under water environment,the HOVF spraying process is developed to prepare wear-resistant cemented carbide coatings of WC-Ni system,where the influence of oxygen-to-fuel ratio(O/F),spray angle and inert gas protection is emphasized on coating porosity,microstructure,and phase structure,along with characterizations of coating mechanical properties including microhardness,elastic modulus,fracture toughness,interface fracture toughness etc.Subsequently,the water-lubricated tribological behavior of HVOF sprayed WC-10Ni coatings is studied to explore its wear mechanism,in order to establish correlations between from process parameters to coatings performance which may provide basic principles as well as technology for manufacturing of wear and corrosion resistant cemented carbides coatings in water lubrication working conditions.The major results obtained in this work are summarized as the following four parts.(1)Effect of O/F value.The O/F value is the essential parameter of HVOF spraying process,determining the temperature and velocity of spray flame as well as spray particles accelerated and heated in it,adjustable by different combinations of oxyen flow rate and kerosene flow rate.As the kerosene flow rate was kept at 22.7 L/min,enhancing the oxygen flow rate from 811 L/min to 946 L/min makes O/F value increase from 1.1 to 1.3,and the correspondingly the particle velocity is obviously increased and its temperature is slightly reduced,leading to a coating porosity changed from 0.86%to 0.78%.If the oxygen flow rate is fixed at 811 L/min to reduce the kerosene flow,the O/F value can be also adjusted from 1.1 to 1.3,but both the velocity and temperature of the particles apparently decreased,resulting in a significant increase in coating porosity up to 4.5%.It is shown that,the temperature of particles could be more or less reduced with increasing the O/F value,causing mitigation of WC oxidation,i.e.increase of WC retention.Moreover,if the particle velocity could be increased at the same time,it is beneficial to a further enhancement in WC retention.The coating microhardness is affected by both the porosity and phase composition in the coating,where porosity mainly contributes negatively to the microhardness value tested under relatively higher load,and WC retention improve more significantly the value under lower load.The coating fracture toughness is mainly influenced by coating microhardness and elastic modulus as well as microstructure morphology,i.e.,higher ratio of elastic modulus to microhardness(E/H)and denser microstructure will enhance the coating fracture toughness.(2)Effect of spray angle.Spray angle is one of necessarily controlled major paraemters for spraying components of complicated shape.As the spray angle is reduced,the normal component of kinetic energy of impacting particles onto substrates can be lowered,leading to a coating with higher porosity,i.e.from 0.86%at a spray angle of 90° to 1.68%at 30°.Morover,the impacting area of spray flame onto substrates is also greatly increased at the smaller spray angle,causing excess heating of deposited splats and larger surface exposed to oxyen containing ambient enviorment.Consequently,the decomposition of WC phase is enhanced due to reaction with oxyen at the high temperature during coating deposition,with WC retention of 82%at 90° spraying down to 69%at 30°.As a result of higher porosity and lower WC retention,coatings hardness varied from 10.8 GPa at 90° to 9.9 GPa at 30°,accompanied with a reduction in E/H value.Splats cohesion of deposited particles decreases at the smaller spray angle,promoting the crack propagation that reduced the fracture toughness.Moreover,interface fracture toughness of coatings has different changing trend as compared to that of fracture toughness,due to a more significant influence from both residual tensile stress and microhardness.(3)Effect of protective N2 gas shielding.N2 shielding is introduced to reduce the WC phase decomposition in WC-10Ni due to oxidation,simultanesouly its influence on the coating microstructure is also observed,i.e.porosity and splats morphology.The WC retention is improved by 5%under direct N2 shielding onto substrate in open air,by which many pores occurred especially concentrating at interfacial region between deposited particles/splats due to a strong cooling effect of N2 on the sprayed particles,with a porosity nearly twice of that for coatings sprayed in air without N2 shielding.Higher WC retention under N2 shielding compensates the negative effect of higher porosity on microhardness,and a comparable microhardness is obtained for two types of coatings.However,lower fracture toughness is observed for the coatings sprayed with N2 shielding in open air,due to crack propagation along between the deposited particles.In order to mitigate cooling effect of N2 shielding in open air as well as control the shielding effect,an improved shielding scheme in casing is developed by which the N2 flow is entrained in the casing.A lower coating porosity sprayed with N2 shielding in casing is obtaind similar to that of sprayed in air.(4)Wear performance of WC-10Ni coatings obtained at different process parameters is tested under water lubrication.Effect of coating microstructure and mechanical properties is evaluated on the tribological behaviors.The wear mechanism of coatings with low porosity is mainly adhesive wear accompanied with mild abrasive wear due to pull-off and fragmentation of hard WC grains around the pores.As the porosity of coatings increases,the mild abrasive wear becomes dominant process due to weakening of bond strength between WC phase and Ni binder phase near the larger pores.The porosity has a positive effect in decreasing friction coefficient.The porosity and microhardness has a combined effect on the wear performance of WC-10Ni coatings.The coatings sprayed with the improved N2 shielding in casing have a low porosity between 0.9-1.9%close to that of 0.86-1.68%of coatings sprayed in air,and due to a higher WC retention,the former has a lower specific wear rate by 10%reduction.It is indicated that,the higher bonding strength between hard phase and binder phase of cemented carbide coatings can lead to a higher wear resistance at similar microhardness,presenting a mild combined adhensive and abrasive wear.