Investigation Of The Fabrication And Properties Of Conductive And Corrosion Resistant C/CrN Graded Multilayer

Proton exchange membrane fuel cell（PEMFC）is a promising ideal device for vehicle power sources and distributed power stations due to its advantages of high energy conversion rate,zero emissions,fast start up and high specific power.Bipolar plate is an important key component and major part in PEMFC stack.However,the insufficient corrosion resistance and surface conductivity of stainless steel are two important drawbacks hindering the performance of bipolar plate materials in PEMFC.In this research,we applied physical vapor deposition technique to fabricate corrosion resistant and surface conductive CrN and graphite-like carbon coating on stainless steel.The key deposition parameters and corresponding microstructure,corrosion resistance and interfacial contact resistance（ICR）were systematically investigated.Based on the above results,we proposed and designed a novel C/CrN multilayer coating as corrosion resistant and surface conductive coating for stainless steel bipolar plate.The relationship between deposition parameters,chemical composition,microstructure,corrosion resistance and ICR were revealed.The main results are as follows:1.The effects of deposition parameters on microstructure,corrosion resistance and surface conductivity of CrN coatingThe bias voltage has significant effects on the crystal preferred orientation growth of CrN coating.The N atom concentration in CrN coating increases with increasing bias voltage due to the higher probability of nucleation on the surface and the decreased diffusion distance of Cr atom on（2 0 0）crystal surface.In the meantime,（2 0 0）crystal surface has lower chemical potential as compared to（1 1 1）crystal surface.N₂ gas flow rate greatly affects the surface morphology and microstructure of CrN coating.When the N₂ gas flow rate ranges from 10 10 sccm to 35 sccm,N atom concentration in CrN coating changes from18.83 at.%to 57.29 at.%.The phase composition of CrN coatings is Cr+Cr₂N（N₂ gas flow rate 10 sccm）,Cr₂N（N₂ gas flow rate 15 sccm）,Cr₂N+CrN（N₂ gas flow rate 20 sccm）,and pure CrN（N₂ gas flow rate 35 sccm）.The size of CrN granules change slightly with the increasing of the deposition time due to the“V”type growth mode.As the electrochemical test and ICR measurement results show,the microstructure composing of both Cr₂N and CrN phase in CrN coating has higher corrosion resistance and surface conductivity.When the Cr₂N and CrN phase ratio is 1:1,the CrN coating has the best corrosion resistance and surface conductivity.The optimized deposition parameter is bias voltage of-60V,N₂ gas flow rate of 20 sccm,Cr target current of 5 A and deposition time of 60 min.The corrosion current density and ICR value of CrN coating with optimized deposition parameter are 6.23μAcm^-2 and 9.12 mΩcm².2.The effects of deposition parameters on microstructure,corrosion resistance and surface conductivity of GLCThe bias voltage has significant effects on the bonding type and microstructure of GLC.With increasing of bias voltage,the granule size of GLC becomes finer,and the content of sp² increases first and reaches a maximum value.The electrochemical test and ICR measurement results reveal that ICR and corrosion resistance of GLC is proportional to the sp² content.The carbon target current affect the microstructure slightly.The deposition rate increases while the content of sp² decreases,when the carbon target current increases,due to the increasing carbon ion density.The optimized deposition parameter is bias voltage of-60V,carbon target current density of 3 A,deposition time of 5 hours.The corresponding GLC has the best corrosion resistance and surface conductivity of 0.34μAcm^-2 and 5.35mΩcm².The effects of W incorporation on the microstructure,ICR,and corrosion resistance of carbon films are investigated systematically.W exists in primarily the metallic state in C_W0.2（A）,C_W0.5（A）and C_W1（A）,but a considerable amount of WC with an amorphous microstructure is observed from C_W2（A）and C_W3（A）.The ICR value of the W-doped carbon films increases marginally with W concentrations in the range between 6.25 and 7.21mΩcm² at compaction force of 150 Ncm^-2.The electrochemical tests results indicate that carbon films doped with smaller W concentrations have better corrosion resistance as reflected by the nobler corrosion potential and smaller current density.These samples also exhibit self-passivating ability as a result of the gradually increasing OCP.The self-passivating ability is attributed to oxidation of metallic W in the carbon films.The C_W1（A）coating with W content of 17.3 at.%has the best corrosion resistance of 0.07μAcm^-2 and surface conductivity of 6.53 mΩcm².3.The fabrication,corrosion resistance and surface conductivity of C/CrN filmThe C/CrN multilayer coatings are composed of carbon granules,compact,and dense.The ICR results disclose superior surface conductivity due to high percentage of sp2 bond and coexistence of the CrN coating.The values range from 2.6 to2.9 mΩcm² at a compaction force of 150 Ncm^-2.Increasing the thickness of the carbon coatings over the threshold value only improves the ICR marginally.The potentiodynamic,potentiostatic,and ICP results provide evidence that the corrosion resistance is significantly improved by the C/CrN multilayer coatings as a result of the high chemical inertness.Our results also disclose that the corrosion resistance is closely related to the thickness of the carbon coating.A carbon coating that is too thin,such as those on C/CrN-1 and C/CrN-2,is prone to pitting corrosion as confirmed by SEM and more severe release of metal ions during the potentiostatic tests.A C/CrN multilayer coating with a minimum thickness of 800 nm（C/CrN-3）is required to provide adequate functionality and corrosion resistance.The corrosion resistance of C/CrN graded multilayer changes slightly with different deposition time of CrN coating,ranging from 15 nAcm^-2 to 140 nAcm^-2.The sample with CrN deposition time of 15 min has the lowest corrosion current density of 15 nAcm^-2.The ICR results shows that the ICR value of C/CrN graded multilayer increase gradually with the increase of the deposition time of CrC transform layer,ranging from 2.35 mΩcm² to 2.88 mΩcm².Two kinds of transition layer,namely CrC and CrNC,with different deposition times are investigated.The potentiodynamic and potentiostatic shows that the corrosion resistance of C/CrN graded multilayer increase at the first and then decrease with the increase of the deposition time for both the CrC and CrNC transition layers.The ICR results shows that the ICR value of C/CrN graded multilayer increase gradually with the increase of the deposition time for both the CrC and CrNC transition layers.In compare,the corrosion resistance of C/CrN graded multilayer with CrNC transition layer is higher than that with CrC transition layer and the corrosion current density decreases by 11.5%,being 10.6 nAcm^-2.In the meantime,the ICR value of C/CrN graded multilayer with CrNC transition layer is decreased by 63%,being 2.2 mΩcm²,revealing better surface conductivity than that with CrC transition layer.