Preparation And Properties Of Co-WC Composite Coatings

Composite electroplating is a sort of burgeoning composite surface technology which isadding particles into electrolyte or plate fluid, depositting the partials to the based metalbounding to cathode, and creating a new type of coating with superior properties. Thistechnique is gradually becoming the plating research centre. In this paper, WC was chosen asreinforced phase and Co was chosen as based metal and Co-WC composite coatings weredeposited on W18Cr4V substrates with direct current and pulse technologies. The connectionamong properties of composite coating and WC concentration in bath, stirring rate, currentdensity, pulse duty cycle and pulse frequency were studied. The optimal parameters ofelectrodeposited Co-WC composite coatings were confirmed. The cross-sectionalmicrographs of the composite coatings were observed by optical microscope and scanningelectron microscopy （SEM）.The chemical composition and phase structure of compositecoatings were analyzed by energy dispersive spectroscopy （EDS） and X-ray diffraction（XRD）.The results demonstrated that a relatively uniform concentration of WC particlesdisperse Co based mental, and the continuous and uniform composite coatings which show aclear bounding between composite coating and based metal with about120μm of thicknessindicate tight adhesion to substrate. The deposited structure of Co-WC composite coatingswas crystalline. When the WC concentration in solution ascend to35g/L, particles content incoating and deposition rate of DC composite coating reached the maximum,17.4wt.%and56.3μm/h, respectively. When the duty cycle was50%, frequency was500Hz, particlescontent in coating and deposition rate reached18.78wt%and60.7μm/h, respectively withusing pulsed electrodeposition. When the Re （CeO₂） concentration in solution ascend to6g/L,particles content in coating and deposition rate were19.9wt%and78.2μm/h, respectively.Meanwhile, the study showed that the presence of WC particles can significantly enhancemicro-hardness and wear resistance performances of composite coating. However, thecorrosion resistance of the composite coating decreased. The Co-WC composite coatings were prepared by DC and pulsed electrodepositionrespectively.The microstructure of the coating surface were observed by the scanning electronmicroscopy （SEM）.The results revealed that the microstructure of the coating prepared bypulsed electrodeposition got better refinement, the grain size was decreased, the crystal sizewas uniform, and the accumulation was relatively tight compared with the surfaceMorphologies of DC plating. The microcosmic crystal-structure of the coating was analysedby XRD and the effect of the different deposition ways on the coating deposition and growthwere studied. The results showed that the preferred orientation of the Co-WC compositecoatings didn’t change by different deposition methods, the grain refined, and lattice constanthas increased, and crystal lattice distortion increase, making diffraction peaks of the coatingcomposition reduce.The matrix metal deposited continuously, the particles embedded coatingon the electrode surface uninterruptedly which prepared by DC electrodeposition. whenprepared by pulsed electrodeposition, the size of the composite particles decreased in thecoating as the larger-volume particles were desorbed and return to the solution owing to theoff-time.The grain prepared by pulsed electrodeposition refined which demonstrated that thepulse deposition prevented the growth of grain and develop nucleation rate in depositionprocess.The influences of microstructure, microhardness, tribological properties which RE CeO₂and LaCl₃have effected on composite coating with direct current electrode-position werestudied. The result shows that WC will distribute more homogeneous in coating if appropriateRE were added to promote WC deposition. The average microhardness of composite coatingwill be enhanced15.68%with6g/L CeO₂than the one without CeO₂.Meanwhile, The averagemicrohardness of composite coating will be enhanced13.47%with0.2g/L LaCl₃than the onewithout LaCl₃. The result of friction-wear test shows that RE can improve the dry frictionfeature, prevent from sheet drop of coating efficiently, improved wear resistance of coatingobviously. Grinding cracks of composite coating were slight and narrow and abrasion losswas reduced by70%when6g/L CeO₂were added. The friction and wearing properties of thecomposite coating under400℃shows that abrasion loss in high-temperature is reducedcompared with room temperature with no matter direct current or pulsed electrodeposition. Itdemonstrates that wear resistance of Co-WC composite coating is improved under400℃.