Tribological Properties Of D-gun WC-12Co Coatings In Dry Sliding And Molten Salt Media

With the rapid development of the new energy industry,nuclear energy and solar energy have become important research objects in the field of new energy for many scholars.As a functional material,fluoride molten salt has high thermal conductivity,large specific heat capacity,low viscosity,low saturated vapor pressure,small neutron absorption cross section and high thermal stability.It has a series of superior physical and chemical properties,such as resistance,and has broad application prospects in the fields of nuclear energy and solar energy industry.Fluoride salts are extremely corrosive at high temperatures,so the development of molten salt corrosion and wear-resistant materials is an important guarantee for the safe service of key friction parts such as pumps,valves,and bearings in molten salt systems.And D-gun spraying can well ensure the consistency of tungsten carbide（WC）-based ceramic coating and powder composition,so D-gun spraying WC based coating may have good corrosion resistance and wear resistance in the fluoride molten salt environment.In this paper,WC-12Co and WC-12Co-5%（Ni-Mo-Cr-Fe）coatings were prepared on S31008 steel by D-gun spraying technology.The evolution of microstructure and phase structure of the coatings in FLi Na K molten salt environment were investigated.The tribological properties of the two coating materials at 600℃,dry sliding and FLi Na K molten salt medium were investigated.The effects of Al2O3 ball,Si3N4 ball,SiC ball and T91 pin on the tribological properties of coating were investigated.The main research results are as follows:1.The WC-12Co and WC-12Co-5%（Ni-Mo-Cr-Fe）coatings were prepared by explosive spraying.The two coatings have dense structure,thickness of about 250μm,the porosity of the two coatings is 0.71%and 0.23%,respectively.The results indicate that the addition of5%（Ni-Mo-Cr-Fe）powder can promote the densification of WC-12Co coating.The two kinds of coating materials designed and prepared have very excellent mechanical properties.The binding strength of WC-12Co and WC-12Co-5%（Ni-Mo-Cr-Fe）coating and matrix is 44 MPa and 39 MPa,and the surface hardness is 1388.5 HV and 1197 HV,respectively.2.Under the condition of high temperature 600℃and dry sliding,the friction coefficient between D-gun spraying WC-12Co coating and four different mating materials ranges from0.152～0.426,and the wear rate ranges from 0.54 to 1.46×10^-6 mm³N^-1m^-1.In FLi Na K molten salt environment,the friction coefficient of WC-12Co coating ranges from 0.097～0.167,and the wear rate ranges from（4.95～8.4）×10^-7 mm³N^-1m^-1.Oxidative wear of the WC-12Co coating in dry sliding resulted in high wear even though it was tested in a low oxygen environment.In FLi Na K molten salt environment,oxidation of the WC-12Co coating was inhibited and the molten salt provided good lubricity,so the friction and wear of the coating were reduced.3.The addition of 5%（Ni-Mo-Cr-Fe）powder increases the dry sliding friction coefficient between WC-12Co coating and Al2O3 ball,Si3N4 ball,SiC ball and T91 pin pair,and the friction coefficient is in the range of 0.23～0.5.The wear rate is（5.42～7.98）×10^-7 mm³N^-1m^-1.The friction coefficient of FLi Na K molten salt at 600℃ranges from 0.08 to 0.14,and the wear rate is（5.12 to 8.57）×10^-7 mm³N^-1m^-1.The oxidation wear of WC-12Co-5%（Ni-Mo-Cr-Fe）coating occurs during dry sliding.In FLi Na K molten salt medium,the wear mechanism of WC-12Co-5%（Ni-Mo-Cr-Fe）coating is oxidation wear and corrosion wear.4.When the WC-12Co-5%（Ni-Mo-Cr-Fe）coating slides with Al2O3,Si3N4,SiC and T91pairs in FLi Na K molten salt medium under vacuum environment at 600℃,Ni-based alloy on the coating surface reacts with fluoride molten salt.The Fe element in the coating reacts with molten salt to produce Fe F2,and the Ni and Mo elements are enriched on the surface of WC-12Co-5%（Ni-Mo-Cr-Fe）coating.During the friction process,Ni O and Mo OX are generated by oxidation reaction.5.The thickness of WC-12Co coating and WC-12Co-5%（Ni-Mo-Cr-Fe）coating remained basically unchanged after being corroded in FLi Na K molten salt environment at600℃for 2 h,and obvious corrosion pits appeared on the surface of WC-12Co coating.The addition of 5%（Ni-Mo-Cr-Fe）enhances the corrosion resistance of the coating,and there is no obvious corrosion pit on the surface of the corroded coating.