Study On Preparation,Properties And Mechanism Of Chemically Bonded Ceramic Coatings Reinforced By Nanomaterials Cured With Low Curing Temperature

The coating on metal surface has attracted more and more attention due to its high heat resistance,wear resistance and corrosion resistance.However,the chemically bonded phosphate ceramic coatings?CBPCs?prepared by solid reaction method is basically based on high temperature curing at present,so it is difficult to be applied to large-scale industrial equipment,and the curing process is complicated and time-consuming.This paper investigated the effects of curing temperature on coating corrosion properties at first.Besides,the effects of inorganic curing agent and nano-titanium dioxide on the thermal property of coating was researched.And the effects of preparation technology on coating corrosion and wear properties were studied so as to optimize the low temperature curing scheme of coating.Firstly,the effects of curing temperature and nano-Ti O₂ on the corrosion resistance of coating were researched.The results of electrochemical impedance spectroscopy show that with the increase of curing temperature?150 ^oC,200 ^oC and 250 ^oC?,the impedance magnitude at the lowest frequency(f_min=0.01 Hz)increases.In addition,the f_min=0.01 Hz value of coating without nano-Ti O₂ is lower than that of coating containing 3 wt%,6 wt%and 9 wt%nano-Ti O₂.The results indicate that the corrosion protection property of coating under low curing temperature?below 250??is mainly associated with the mass ratio of Al H₃?PO₄?₂.3H₂O to Al PO₄?CAA?,which decreases with the increase of CAA.The increase of nano-Ti O₂ and curing temperature can effectively enhance the corrosion resistance of coating.With the addition of nano-Ti O₂,more bonded phase?Al PO₄?can be formed,which can help the coating to get more compact microstructure.That is because bonded phase?Al PO₄?can provide a strong bond between ceramic particles and improve the strength of the coating.Moreover,nano-Ti O₂ and bonded phase?Al PO₄?possess the low density and more likely distribute on the surface of coating,which can efficiently protect the metal surface from chloride ions destruction.Secondly,the effect of inorganic curing agent on the corrosion and wear property of coating was studied.According to the electrochemical and wear test results,the corrosion and wear behavior of coating with different inorganic cuing agent?Mg O,Ca O,Zn O and Al₂O₃?correspond to the mass ratio of Al PO₄to Al₂O₃?RAA?.The results show that there is an optimal point for the effect of RAA value on the corrosion and wear properties of coating.Before the optimum point,the wear and corrosion resistance of coating increase with the increase of RAA value.After the optimum point,the corrosion and wear resistance of coating reduce again.That is because with the increase of RAA,ceramic particles are surrounded by an appropriate quantity of the bonded phase?Al PO₄?.Therefore,ceramic particles have high bonding strength and fewer defects,leading to the well performance of coating.However,the mechanical properties and corrosion resistance of Al PO₄ are lower than that of Al₂O₃,so the performance of coating with high RAA value is poor.Thirdly,the corrosion and wear behavior of coating with different thickness were analyzed and the influence mechanism of coating thickness on coating performance was revealed.According to the electrochemical and wear test results,with the increase of coating thickness,the coating resistance increases and the wear rate decreases.In addition,the protection efficiency of coating greatly increases with the increase of etching time,indicating coating can effectively protect the mild steel from corrosion damage even with long-term service.Therefore,coating with appropriate thickness is conducive to improving coating performance,which not only can block the channels for corrosive medium to invade the interior of coating and prevent corrosive medium from contacting with the surface of the substrate,but also the hardness and fracture toughness of coating can be guaranteed,which is beneficial to improve the wear resistance of coating.The failure mechanism for CBPCs in Na Cl solution:due to the intrusion of the electrolyte,some porosities appear on the surface and cross?section of CBPCs.As the etching time increases,coating starts to delaminate,which forms direct paths between corrosive environment and substrate leading to the decrease in the corrosion resistance of CBPCs.Lastly,the stress and strain of coating with different thickness in the wear process was investigated.The reliability of the model was verified by J.H.Zhao's elastic-plastic theory.The results show that the maximum pressure of the contact position between ball and coating decreases with the increase of coating thickness during the wear process.In addition,with the increase of coating thickness,the equivalent stress of coating decreases in the wear process,which effectively reducing the generation and extension of cracks and making the coating not easy to be damaged,which further verifies the wear test results.In particular,with the increase of coating thickness,the equivalent plastic strain of coating decreases significantly during the wear process.The reason is that the equivalent hardness and fracture toughness of coating-substrate increase with the increase of coating thickness,which leads to the enhanced ability of coating to resist external pressure.