Research On Preparation And Performance Of Ni-Co-P/SiO₂Electroless Composite Coating On Al Alloys

The striking feature of aluminum alloy is low density, high strength, fine thermalconductance, easy to forming, cheap for price, etc, and aluminum alloy has been widelyapplied. But its shortcomings of low hardness, inferior corrosion resistance and wearresistance are increasingly prominent, therefore in the process of production practice, itis necessary to carry out surface treatment for aluminum alloy. Taking advantage ofsurface treatment method of electroless Ni-P alloy, which may enhance substrates’hardness and wear resistance. But its resistance to friction and resistance to sintering isweak, while Ni-Co-P alloy coatings that contain Co possesses nice resistance to frictionand resistance to sintering, and they can increase fatigue strength of the blank, and theyare more suitable for the requirements of sliding friction parts. In order to improvehardness and wear behaviors of Ni-Co-P alloy coatings, preparation and research workof Ni-Co-P/SiO₂composite coatings are carried out in this paper.In this paper, three kinds of different SiO₂sizes （40nm、150nm、800nm），five kindsof different SiO₂concentration （3、6、9、12、15g/L） of Ni-Co-P/SiO₂composite coatingson Al-Si alloy were prepared by electroless composite plating technology. And underthe condition that all of SiO₂concentration was9g/L, three kinds of different SiO₂sizesof prepared Ni-Co-P/SiO₂composite coatings were heat-treated for1h at varioustemperature （200℃、300℃、400℃、500℃）. The effects of SiO₂concentration, SiO₂sizes and heat treatment temperature on surface morphology, cross-sectionalmorphology, chemical composition, crystal structure, Vickers hardness, wear resistanceand corrosion resistance of the composite coatings have been investigated by scanningelectron microscope, energy dispersive spectroscopy, X-ray diffractometer, Vickershardness tester, high speed reciprocating friction testing machine, scanning3d surfaceprofile instrument, electrochemical workstation. The results demonstrate that:（1） The surface of Ni-Co-P/SiO₂（40nm、150nm、800nm） composite coatings arecoarser than that of Ni-Co-P alloy without particles, distributing spherical nodularprotrusion of various sizes, relatively uniform and relatively dense arrange. SiO₂particles aggregate to some extent, and some areas have certain holes. After plating inacidic （pH=4.7±0.1） plating bath about85±2℃for1hour, the average thickness ofthree kinds of composite coatings are all approximate12μm. The content ofphosphorus in composite coatings is0.71wt%-1.4wt%，which belong to the coatings of medium or low phosphorus content.（2） SiO₂particle concentration has little influence on the crystallinity of preparedNi-Co-P/SiO（240nm）composite coatings, but it makes an obvious difference to Vickershardness and wear resistance of Ni-Co-P/SiO₂（40nm,150nm,800nm） compositecoatings. All of three kinds of composite coatings attain the maxima and minima at amedium concentration, and particle concentration has something to do with particlesizes at this moment. Vickers hardness of800nm SiO₂particles with6g/L concentrationwhose composite coating can attain a maximum of538HV, and average frictioncoefficient and wear volume of that can attain a minimum of0.542and0.102mm³,respectively. Vickers hardness of40nm and150nm SiO₂particles with9g/Lconcentration whose composite coating can attain a maximum of452HV and430HV,respectively，and average friction coefficient of that can attain a minimum of0.507and0.513, respectively，and wear volume of that can attain a minimum of0.09602mm³and0.1222mm³, respectively.（3） Under the condition that all of SiO₂particle concentration is9g/L，SiO₂particle sizes（40nm、150nm、800nm）have little impact on the content of a variety ofelements in these composite coatings. Among other things, the size relation ofphosphorus’average weight percentage is40nm>800nm>150nm. the strong andweak relation of corrosion resistance is40nm>800nm>150nm. Among them, corrosionresistance of40nm SiO₂particles whose composite coating is best. In same way, underthe condition that SiO₂particle concentration is same，the size relation of Vickershardness is800nm>40nm>150nm. Among them, Vickers hardness of800nm SiO₂particles whose composite coating is largest，while wear resistance has own advantagesand disadvantages.（4） By the electrochemical test experiment, it is detected that by contrast, theopen circuit potential between aluminium alloy and its surface Ni-Co-P/SiO₂（40nm,150nm,800nm）（all concentration is9g/L） composite coatings are close, while the latterhas higher corrosion potential, lower corrosion current density, smaller electric doublelayer capacitance and lager charge transfer resistance, which reflects these compositecoatings can enhance corrosion resistance of the substrates.（5） Three kinds of composite coatings were heat-treated at （200℃、300℃、400℃、500℃） for1hour，the results demonstrate that: Grains may enlarge, and surfacemay become coarse with increasing the heat treatment temperature. After upper400℃，crystallization of amorphous organization is apparent. To contrast with plating state， both hardness and wear resistance of composite coatings can enhance after heattreatment. Among other things, both hardness and wear resistance of three kinds ofcomposite coatings attain best at400℃annealing. Among three, the maximum coatinghardness is that of800nm，which is599HV；Among three, the minimum coatingaverage friction coefficient and wear volume are that of40nm，which are0.436and0.0654mm³, respectively. The improvement of hardness and wear resistance is relatedwith phase transformation that result from heat treatment then the formation ofintermetallic compound Ni3P.