Preparation,Wear And Corrosion Resistance Of HVAF Sprayed Fe-based Composite Coating

Compared to Ni-or Co-based alloy coatings,tungsten carbide or chromium carbide coatings,Fe-based coatings possesses advantages,such as low cost and environmental friendness,which render it one of the major protective coatings in the field of surface engineering.However,to achieve alternative wear and corrosion resistance of traditional coatings and overcome their shortcomings in hardness,toughness and sprayed porosity,the demands to develop composite feedstocks and design matched preparation technologies for powders and coatings are urgent.This dissertation based on the characteristics of Fe-based metallic alloy and amorphous（FA）alloy,and doping reinforcements for significantly improving their wear and corrosion resistance,respectively.In details,TiB₂ and CNTs were doped in metallic alloy powders via combined method,and prepared partially clad FeCrMoCBSi/Mo composite powders by ball milling.Then,high velocity air fuel（HVAF）spraying was employed to deposit coatings on carbon steel and stainless steel（SS）substrates.Based on the experimental results about powders and coatings,the microstructure,wear and corrosion properties and mechanisms were systematically investigated.It shows that both the composite coatings exhibited high enhancement on tribological and corrosion properties than sole alloy coatings and showed popularize value,which provides new strategy and scientifical evidences to fabricate and develop high performance Fe-based composite coatings.The results imply that combined procedures of ball milling,spray drying and spheroidizing could successfully achieve the even incorporation of 50 vol.%TiB₂ and 2.5 vol.%CNTs,which helps to attain feedstocks satisfying the spraying requirements in powder size and flowability.The as-deposited coatings possessed a dense microstructure with well-bonded interfaces,and the minimum porosity could reach to 0.71%.This is mainly due to the improvement on the microstructure and melting state about molten drops brought by the fabrication procedures and doped phases.Simultaneously doping high hardness TiB₂ and superhigh elasticity modulus CNTs rendered the composite coating great comprehensive properties with highest microhardness of 874 HV_0.3 and fracture toughness of 6.15 MPa·m^1/2.The wear groove,wear loss volume and specific wear rate of Fe/（TiB₂-CNT）composite coating were better than those of metallic coating and Fe/TiB₂ cermet coating.In particular,the specific wear rate could decrease to 2.47×10^-6 mm³/N·m.The wear loss of metallic coating mainly caused by severe abrasive wear and mild adhesive wear.After doping TiB₂,the loss degree of abrasive wear and adhesive was reduced,and the wear mechanism was dominanted by oxidative wear.While doping TiB₂ and CNTs,the comprehensive increase on microhardness and fracture toughness of coating could impel the disappearance of abrasive wear and adhesive wear,which replaced by oxidative wear.The abrasive wear and adhesive wear occurred again when doubled the wear load to 32 N.In the 3.5 wt.%Na Cl corrosion solution,the corrosion resisted features such as corrosion potential,corrosion current density,passivation region and impedance of Fe/（TiB₂-CNT）composite coating were best.Its major corrosion failure mode was pitting,which was different from the etching pore produced between the intersplats and phase boundary of metallic coating.This is ascribed to the restriction on permeatation of corrosion media by the uniform and well-bonded microstructure with lowest porosity and the super-hydrophobic surface.The partially clad FeCrMoCBSi/Mo composite feedstock suited for spraying was initially prepared via ball milling,and the Mo was tightly attached to the FA powder.Later,the spraying distance and feeding rate were optimized to 250 mm and 25 kg/h,respectively.The results demonstrate that the FeCrMoCBSi amorphous coating can effectively protect the SS components from high wear load（30N）and marine environment.FeCrMoCBSi/Mo composite coating was deposited by HVAF technique,and relatively homogeneous Mo phases and in-situ formed Fe₇Mo₃ phases were introduced,which reduced the porosity to 0.46%and simultaneously improved the interlaminar and coating-substrate bond.After doping Mo,the microhardness of the coating was slightly decrease by 4%,while the fracture toughness was enhanced by 35%.Moreover,its surface hydrophobicity was improved too.Therefore,in this work,the Mo addition rendered the composite coating significant enhancement on tribological and corrosion properties.In details,the specific wear rate was reduced by 40.8%.Doping Mo helped to decrease the abrasive wear but increase the oxidative wear.The Mo oxides acted as the major lubricative phase could effectively reduce the wear loss.Furthermore,adhesive wear was both found in these two amorphous coatings.In term of corrosion mechanism,thanks to the enhanced hydrophobicity and remained amorphous microstructure with dramatically decreasing porosity,pitting other than severe corrosion holes was the failure mode of FeCrMoCBSi/Mo composite coating.