| Chemical conversion treatment can produce a conversion coating on metal surface by the chemical reaction through the surface atoms of metal and anion in the medium,the coating is widely used in painting and has anticorrosive,wear-resisting,anti-friction and other features as well.Nanomaterials are functional materials with manifold excellent performances and have broad application prospects because of their special structure.Recent research about the application of nanoparticles in the chemical conversion treatments were reported,such as nano-SiO2 in molybdate conversion coating,nano-Al2O3,TiO2 and SiO2 in phosphate conversion coating.The nanoparticles can increase the coating thickness,decerease the crystal size,improve the coating desity and then enhance the corrosion resistance.However,there is no in-depth research concerned with the whole influence of nanoparticles on the chemical coating,and the aggregation of nanoparticles in the treatment solution also needs to be addressed urgently.In this research,the phosphate chemical convrsion(PCC)coating on 35CrMnSi steel was prepared by the method of chemical conversion process,and then the lab-made nano-SiO2 paticles and sol were introduced into PCC technology.The effects of the curing time of iron powder,conversion time,liquidity of conversion bath on the morphology and phase of PCC coating were studied to determine appropriate conversion conditions;The influence of the different heat treatments of 35CrMnSi steel on the microstructure and corrosion resistant of PCC coating were also investigated to explore the influence rule of steel microstructure on the PCC coating as well;Meanwhile,the effect of nano-SiO2 form and content on the morphology,phase,surface roughness,bond strength,wear resistance,corrosion resistance and other properties were conducted as a systematic study.On this basis,the formation mechanism of PCC coating and the effect mechanism of nanopartices on PCC coating were preliminary discussed and analysed.Results show that the optimal conversion process parameters for 35CrMnSi steel at room temperature(25±5?)as follows:pH value of 2.75±0.5,iron powder curing time of 12 h,conversion time for 20 min.The different heat treatments of 35CrMnSi steel substrate yield an important effect on the coating formation rate,crystal size and corrosion resistance of PCC coating,the coating formation rate of heat treated substrates was as follows in descending order:quenching,tempering,low temperature tempering,normalizing and rolling.Meanwhile the heat treatments could affect the crystal size within the coating,especially the tempered sample presented the finest crystals.By means of electrochemical analysis,the successively descending order of corrosion resistance was tempering,low temperature tempering,quenching,normalizing and rolling.And the PCC coated tempered sample presented the best corrosion resistance arising from the homogeneous and fine-crystalline microstructure of the coating on tempered substrate.Nano-SiO2 hard phase was prepared by sol-gel process,which was spherical with the average particle size of 100 nm and it was amorphous structure;The adding forms were divided into two kinds of particles with 0.5-2.0 g/L and sol with 50-200 mL/L;The dispersive way of first 20 min ultrasound and then 30 min stir was confirmed by repeating the experiments for improving the uniformity and stability of the composite convesion bath.Under the two addition ways,the addition content played an significant effect on morphology,coating mass and phase of PCC coating.Nano-SiO2 improved the coating compactness,refined the coating crystals and then decreased coating thickness.Nano-SiO2 did not affect the phase composition of coating and the preferential growth direction of crystals,but made the crystal diffraction peak intensity decreased.Under the two addition ways,the addition content significantly affected the surface roughness,bond strength,wear resistance and corrosion resistance of PCC coating.Nano-SiO2 decreased the surface roughness of PCC coating.The surface roughness first decreased and then increased with increasing SiO2 content,the minimum value with 0.63 ?m of patrticles and 0.59 ?m of sol were corresponding to the addition of 1.0 g/L and 100 mL/L,respectively.Nano-SiO2 did not decreased the bond strength of PCC coating.Nano-SiO2 increased the friction coefficient of PCC coating.The friction coefficient first increased and then decreased with increasing SiO2 content,the maximum value with 0.643 ?m of patrticles and 0.659 ?m of sol corresponding to the addition of 1.0 g/L and 100 mL/L,respectively.Nano-SiO2 decreased the wear scar width of PCC coating.The wear scar width first decreased and then increased with increasing SiO2 content,the miniimum value with 890 ?m of patrticles and 900?m of sol corresponding to the addition of 1.0 g/L and 100 mL/L,respectively.By means of electrochemical analysis,the PCC-SiO2 coating with additon of 1.0 g/L patrticles and 100 mL/L sol presented the excellent corrosion resistance.By the analysis of PCC coating fabricated with a short time,the coating consisted of a double-layer structure,namely an amorphous base layer in contact with the steel substrate,and an outer crystalline phosphate layer.The crystalline layer was mainly composed of hopeite(Zn3(PO4)2·4H2O)with phosphophyllite(Zn2Fe(PO4)2·4H2O),and the amorphous base layer contained Fe2O3 and FePO4.The formation process of the PCC coating could be depicted as follows.Substrate surface was dissolved in acid PCC bath,accompanying with the deposition of amorphous phase,and then phosphate crystals were formed on amorphous base layer.Finally PCC coating was gradually improved until the dynamic balance of the coating dissolution and formation was established.The suspension in conversion bath and then adsorption on the substrate surface of nano-SiO2 promoted the formation of phosphate crystals and covered by continuous crystallization phosphates,and eventually nano-SiO2 enhanced PCC-SiO2 composite coating was formed.Compared to two addition ways,the sol addition could not only increase the nano-SiO2 into the coating,but also have no effect on the formation,crystal shape and size of the coating. |
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