Investigation Of The CeO₂ Based Hydrophobic Coatings Prepared By Air Plasma Spraying And Their Properties

Hydrophobic coatings have attracted much attention for their unique advantages（such as self-clean）.However,the traditional hydrophobic coatings are usually fabricated by polymers.These materials have poor wear resistance and cannot survive after high temperature annealing,which restrict their application field.To overcome these restrictions,a series of CeO₂based coatings were investigsted in this research.These coatings were fabricated by air plasma spraying（APS）method which is suitable for engineering application and has high efficiency.The pure CeO₂ coating with good hydrophobic property,the black Ti O₂/CeO₂ coating with self-clean property and the CeO₂/Ni Co Cr Al Y coating with better mechanical properties and thermal stability have been successfully fabricated.The main research content and conclusions are described as follows:（1）The CeO₂ coating was fabricated using APS and the bluk was sintered using the same raw powders as the coating for comparison.The research indicated that the water contact angle of the coating（139^o）was apparently higher than that of the bulk（95^o）.This phenomon was caused by two reason:on the one hand,the dual scale roughness structure of the coating enhances the hydrophobicity of the coating.On the other hand,the surface chemistry of the coating changed druing the APS processing.The Ar⁺decreased the content of the non-lattice oxygen of the coating.The coating exhibited less tendency to adsorb water moleculars and hydroxyl on the air and has better hydrophobicity.After bombing by the Ar⁺,the bulk exhibited higher water contact angle than before.（2）To solve the problem that the degeneration of the hydrophobicity of the hydrophobic materials caused by hydrophilic adsorptions exposed in outdoor,the black Ti O₂ powders which have photocatalysis activity under visible light were mix with the CeO₂ powders.Then the mixed powders were used to prepared coatings with different CeO₂ content using APS.The balck Ti O₂/CeO₂ coatings with CeO₂ volume percentage from 40%to 90%have been fabricated and the self-clean property were tested.The results indicated these coatings all have a higher water contact angles than 110^o as prepared and the water contact angle increased with the increasing of the CeO₂ content.Then the water contact angles of the coatings decreased to 63^oafter polluted.After irradiation by visiable light,the pollutions were decomposed,and the water contact angles decreased compared with those before polluted.After irradiation,election-hole pairs were generated on the surface of the black Ti O₂.These pairs can compose the polllutions and bring defects on the surface.Thus the black Ti O₂ tranformed to be hydrophilic after irradiation while the CeO₂ maintian its hydrophobicity.When the CeO₂volume percentage is higher than 60%,the coating can exhibite hydrophobic after irradiation,or the coatings will show hydrophilic.However,the coatings will recover their hydrophobicity after storage in dark environment for a while.Thus,coating with different water contact angles could be fabricated by adjusting the content of the coating and meet different demands.（3）To enhance the wear resistance and thermal stability of the CeO₂coating under harsh environments,Ni Co Cr Al Y alloy powders were added into the CeO₂ powders.Then the mixed powders were used to fabricate coatings using APS.The stability of the coating under high intensitive wear resistance test（under 10 k Pa for 4 m distance）and long time high temperature annealing（500℃for 8 hours）were measured.The results showed the addition of the alloy powders reamrkbly increased the wear resistance of the coating even the volume percentage of the alloy is 20%.After abrasion test,the roughness of the pure CeO₂ coating decreased from7.87μm to 0.89μm,while the roughness of the coating of with 20%addition of the alloy decreased from 7.55μm to 3.23μm.The reason for the improvement of the wear resistance of the coating is that the alloy has a relative high hardness and low melting point（～1400℃）than the CeO₂（2200℃）which resulted in a better molten condition and improve the bonding force between the coating.And the coatings will not spall from the substrate when the volume percentage of the alloy is higher than 20%after annealing and the hydrophobicity is also maintained after annealing.（4）In order to investigate the application prospect of the hydrophobic CeO₂ coating.The icing behaviors of the CeO₂ coatings and the stainless steel with different roughness were observed.The results indicated that the icing time of the droplet on the CeO₂ coating was obviously longer than that on the stainless steel.The nucleation time occupies the majorty of the icing time.For droplets on hydrophobic surface,the nucleation time increased with increasing of the roughness of the surface and the water contact angle,which was caused by the air cell on the solid-liquid interface.These cell decreased the subcool temperature and increased the nucleation time.But for the droplet on hydrophilic stainless steel surface with different roughness,the nucleation time was very closed.