| It is necessary to improve key properties（such as corrosion resistance,electrical insulation and thermal management）of light alloys（Ti,Mg,Al）in the harsh service environment of high-tech equipment.Traditional mairoarc oxidation（MAO）ceramic coatings have some structural defects,such as porous surface,thin interface layer and limited thickness,which seriously restricts the improvement of key properties.Meanwhile,in the micro-nanoparticles modified MAO process,only a small amount of particles exist on the coating surface or in the limited depth.In view of the bottlenecks mentioned above,in this paper,a microarc oxidation-depositing nanoparticles technique was proposed to construct the large-thickness composite coatings consisting of MAO bottom layer+PTFE deposition outer layer on various alloys（TA15,6061Al,AZ31B）by matching specific electrolyte system and polytetrafluoroethylene（PTFE）particles.The incorporation process and chemical change of PTFE particles,as well as the formation mechanism of MAO-depositing PTFE composite coatings on the different alloy surfaces during micro-arc discharge were comprehensively analyzed.Differential structural tailoring of composite coatings on various alloy surfaces,and the relationship between microstructure and hydrophobicity,corrosion resistance,electrical insulation and radiation heat dissipation of the composite coatings were investigated.The action mechanism of composite coatings on the performance improvement was revealed.The technological conditions of MAO-depositing PTFE composite coating on the different alloy surfaces were obtained:through tailoring electrolyte environment（basic composition design,PTFE emulsion content≥8 vol.%,electrolyte temperature:50-70°C）and the threshold voltage,the Ti O2+PTFE,Mg O+PTFE and Al2O3+PTFE composite coatings with adjustable thickness（?100μm）are prepared on Ti,Mg and Al alloy surfaces,respectively,where the PTFE outer layer is about 1/3～1/2 of the total coating thickness.The formation mechanism of MAO-depositing PTFE composite coating is that when the temperature,voltage and PTFE concentration of the electrolyte reach the threshold simultaneously,a small amount of PTFE in the discharge microregion is ionized to produce C and F plasma,enhancing the instantaneous intensive discharge,which promotes the migration and deposition of a large number of PTFE nanoparticles in the discharge micro-area and combines with the metal in-situ oxidation layer to form a self-sealing PTFE outer layer.The instantaneous plasma reaction leads to the formation of a small amount of carbide such as PTFE amorphous and unsaturated F-C-H in the PTFE outer layer.The‘bionic’superhydrophobic composite coatings with micro/nano structures are designed and constructed on the Ti,Mg and Al alloy surfaces,respectively,by synergistic tailoring of electrolyte compositions and electrical parameters:A Ti O2layer with micro/nano mastoid array is constructed on the of titanium alloy surface in Na2Si O3+（Na PO3）6+KOH solutions,assisted further by PTFE deposition.Meanwhile,the‘transplant’micro/nano mastoid array Ti O2 and the PTFE outer layer are connected to each other to form an‘lotus leaf’Ti O2+PTFE coating framework.Among them,the Ti O2 bottom layer with mastoid array plays the role of wear-resistant‘armor’protection.During the growth of Mg O+PTFE composite coating on magnesium alloy surface,PTFE particles can be inert incorporated to the Mg O bottom surface at low voltage（low energy input）,so it is easy to prepare thick PTFE layer;Meanwhile,to construct‘ridge-like’structures is easy,due to the long-lived microbubbles at the coating/electrolyte interface.The Al2O3+PTFE composite coating composed of Al2O3 ceramic bottom skeleton and PTFE outer layer rich in nanoparticles is constructed on aluminum alloy surface.The‘lotus leaf-like’Ti O2+PTFE composite coating of mastoid array Ti O2‘armor’constructed on titanium alloy surface by‘bionic’micro/nanostructure tailoring,the‘ridge-like’Mg O+PTFE composite coating on magnesium alloy surface,and the Al2O3+PTFE composite coating on aluminium alloy surface,all have superhydrophobicity with contact angles up to 160°and sliding angle less than 8°.The contact angle of composite coating is still more than 150°,after undergoing repeated mechanical wear（Wear distance>30m）and long-term corrosion immersion in Na Cl,Na OH and aqua regia solutions,exhibiting excellent wear resistance and chemical immersion stability.The composite coating still has superhydrophobicity after exposure at 500℃-1.5h or 400℃-18h,and remains superhydrophobic for a long time below 300℃,exhibiting a high-temperature endurance.Compared with a single MAO coating,the composite coating significantly improves the corrosion resistance and electrical insulation properties of the substrates,which is attributed to the fact that the thick and relatively dense superhydrophobic PTFE outer layer fromed on ceramic bottom layer shows strong‘pinning’and sealing effects.The composite coating of different alloy surfaces shifts the corrosion potential positively and reduces the corrosion current density by 1-2 orders of magnitude.During the long-term immersion process,the impedance of composite coating at the low-frequency region first decreases and then decreases slowly,and is always higher than that of the substrate,indicating a long-term corrosion protection.The Al2O3+PTFE composite coating on aluminium alloy has high dielectric strength（>20V/μm）,high resistivity(>1010Ω·cm),low dielectric constant（6.5）and dielectric loss（0.05-0.6）.The average emissivity values of the composite coatings are up to 0.8-0.9 in the range of 3～20μm,depending on the synergy of the enhanced radiation at the micro-nano scale and the multi-chemical bonding matched absorption vibrations in the wide spectral range.The stretching and bending vibrations of Ti-O,Al-O and Mg-O bonds in the MAO ceramic layer correspond to the high emissivity（0.8）in the wavelength range of 8～20μm,while the values are about 0.3-0.5 in 3～8μm.The vibrational rotation of the C-F functional group and the asymmetric bond formed by the chemical change of PTFE in the PTFE outer layer can enhance the strong vibration absorption band in the range of 3～8μm,increasing the emissivity to 0.8-0.9 in the range of3～20μm and achieving wide-band radiation heat dissipation. |
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