Study On The Tribological And Protective Properties Of Nanocrystalline Cobalt Based Alloy/Composite Film Produced By Electrodeposition

The deposition of film materials with good wear and corrosion resistance on the surface of mechanical parts is of great significance for reducing the wear and corrosion as well as prolonging the service life of the mechanical parts.Nanocrystalline Cobalt?Co?based alloy/composite film with excellent properties that has the potential to replace the hexavalent chromium?Cr?with serious pollution of the environment and endanger human health.In this thesis,nanocrystalline Co based alloy/composite film materials were fabricated on metal surface by electrodeposition.The effect of electrodeposition techniques,electrolyte agitation techniques,introduction of supercritical carbon dioxide?Sc-CO₂?,introduction of nano-carbon material and heat treatment technique on the microstructure and properties of the Co based alloy/composite films were studied.The wear and corrosion mechanism of the nanocrystalline Co based alloy/composite film under different test condition were also investigated systematically.The main conclusions are listed as follows:1.The surface roughness,grain size,hardness,wear and corrosion properties of the nanocrystalline Co and Co-W alloy films are significantly affected by different electrodeposition techniques?direct current?DC?,unipolar pulse,reverse pulse,and bipolar pulse?.Compared to DC electrodeposition,pulse electrodeposition process can provide intermittent time for metal ions rapidly replenish to the cathode and reduce the the thickness of the diffusion layer and concentration polarization.The higher cathod overpotential of pulse electrodeposition can refine the grain size and improve the microstructure and properties of the as-deposited films.The film prepared by bipolar pulse electrodeposition displays smooth surface,compact structure,small grain size,high microhardness as well as excellent wear and corrosion resistance.2.The introduction of Multi-walled CNTs?MWCNTs?and graphene oxide?GO?favors the refinment of grain size and the improvement of microhardness,friction reduction and corrosion resistance of the nanocrystalline Co/MWCNTs,GO/Co and Co-W/MWCNTs composite films.The functionalization of MWCNTs,the use of bipolar pulse and ultrasound agitation favor the co-deposition of MWCNTs and Co ions,and the uniform dispersion of MWCNTs in the composite film.In additon,the ultrasonic power greatly affects the dispersion of MWCNTs.As the ultrasonic power of 400 W was applied for agitation,the as-prepared nanocomposite film is incorporated with a large number of evenly distributed MWCNTs,which leads to the highest hardness and best friction-reducing and anti-wear ability of the as-prepared composite film.3.The morphology,interface with the substrate,grain size,crystal orientation,wear and corrosion resistance of the nanocrystalline Co,Co-Ni and Co-Ni-P alloy films are significantly affected by the introduction of Sc-CO₂.The low viscosity,high hydrogen dissolution capacity of Sc-CO₂ and the periodic electrodeposition characteristics during the Sc-CO₂ electrodeposition process change the crystal orientation of Co-Ni-P alloy film,elminate the pinhole defects caused by hydrogen evolution,improve the microhardness and the combination of the film with the substrate.The mainly wear mechanism is slight abrasive wear during the rubbing process.4.The heat treatment temperature significantly affects the microstructure and properties of the Co-Ni-P alloy films prepared by Sc-CO₂ electrodeposition.The as-deposited Co-Ni-P alloy film transforms from the amorphous to nanocrystalline structure,accompanied with great microhardness improvement after annealed at 400�C.The Co-Ni-P alloy film annealed at 300�C displays the lowest wear rate,which is attributed to the combination of its relative high microhardness and appropriate surface roughness.The Co-Ni-P alloy film annealed at 400�C displays the best corrosion resistance,which is because of its smooth surface,compact structure and small grain size.