Study On Adhesion Strength Of Epoxy Composite Coating With C/SiO₂ Fillers On Metal Surface

Coating is one of the important methods to prevent the metal substrates from environmental erosion in the aerospace industry.Epoxy resin is widely adopted in the coating industry as a common resin to coating film-forming substance in corrosion protection.In the application process of the coating,the peeling failure of the coating often occurs due to the insufficient adhesion strength of the coating.The adhesion strength is a prerequisite for the coating to obtain other properties,and it is also one of the main indexes for judging the performance of the coating.For the application in the aerospace field,this thesis studies the adhesion strength of the epoxy-based coatings to metal substrates.Some techniques in enhancing the adhesion strength related to resin,substrateand filler are explored in this thesis,the main research results are as follows:(1)The thesis had analyzed the effects of epoxy molecular structure on the adhesion strength of the coatings.An epoxy polymer tensile model based on molecular dynamics principle was established,which could be used to evaluate the adhesion properties of polymers;The influences of curing agents,cross-linking degree and polymerization degree on adhesion strength has been analyzed,and the predictions were made about the adhesion performance of polymers to different metal substrates.The thesis made a comparison about the curing system consisted of DGEBA and curing agent MPD,DDS and TETA respectively.Based on the Van der Waals interaction,it was found that the DGEBA×TETA curing system which with a high degree of crosslinking and a low degree of polymerization had the highest adhesion strength between the iron substrate.Modifying the curing agent based on theses conditions,the adhesion strength of the cured system after screening was as high as 973 MPa.(2)The thesis studies the improvement of coating adhesion strength to metal substrate by surface treatment process.The relationship between the grinding process,substrate roughness and adhesion strength are established.The maximum adhesion was 7.54 MPa and 10.17 Mpa,when the surface roughness of iron and aluminum substrates was 0.832?m and 1.332?m respectively.A surface treatment process of hydrochloric acid etching after abrasive was proposed,which had an improvement effect on both substrates.The adhesion strength was 7.67 MPa and 10.63 MPa,compared with the sample mechanically polished iron and aluminum substrates had increased by 1.7%and 4.5%respectively,and it was predicted that there was still room for further improvement.Compared with the adhesion strength to two substrates with the same surface treatment,it was found that the adhesion strength was not only affected by the surface roughness,but also related to the uniformity of the peaks and valleys of the rough surface that can be observed from the difference in three-dimensional morphology,which is the reason for the generally higher adhesion strength of the aluminum substrate.(3)The thesis has carried on the enhancement of coating adhesion strength to metal substrate by filler filling.Innovative selection of natural plant“Chaenomeles sinensis(Thouin)Koehne”as carbon source and a method in the preparation of controllable quality carbon material fillers by using high-temperature pyrolysis and grinding process were raised in this thesis.The successful preparation of carbon material fillers and C/SiO₂ composite filler which provided engineering guidances for the subsequent preparation of natural plant pyrolytic carbon materials.An enhancement method based on filling the prepared C/SiO₂ composite filler was proposed,and the adhesion strength could be further increased to 15.5 MPa,which was more than doubled compared with the adhesion strength of pure epoxy coatings.The relationship between the fillers'parameters and adhesion strength was established in this thesis.What's more,the epoxy-based C/SiO₂ composite coatings perfect formula was developed through orthogonal experiments,and the highest adhesion strength reached 18.3 MPa.(4)The thesis focused on the mechanisms of improving the adhesion strength of coatings by filler filling.By observing the tensile failure interfaces,it was found that there was a difference in the mechanism of increasing the adhesion strength of coatings by carbon and C/SiO₂ fillers.The results shown that the filling of carbon filler mainly enhance the cohesive strength of the coating for the introduced particle reinforcing the coating and reducing the possibility of cohesive failure.While the introduction of SiO₂ in the C/SiO₂ fillers not only enhanced the cohesive strength of the coating,but also its adhesive strength.The polar groups will increase the coating adhesive strength,and thus promote the overall adhesion strength of the coating,which can be detected by the contact angle test.It was found that the existence of certain polar groups contributing to the enhancement of the coating adhesion strength,which was confirmed by infrared spectrum.