Surface Modification Of Stainless Steel Ultra-Thin Strips And Its Bonding Performance With Epoxy

Fiber metal laminates（FMLs）hybrid materials with the high strength and stiffness,low deadweight,and excellent fatigue resistance are highly favored in the aerospace industry equipment manufacturing.However,the poor adhesion properties between of stainless steel ultra-thin strips and epoxy resin in fiber reinforcement greatly limit the overall performance of FMLs.Although the metal surface modification strategy can effectively improve the bonding characteristics between metal and epoxy resin,the existing modification processes applied to stainless steel plate may no longer be applicable to stainless steel ultra-thin strips with a thickness below 0.1 mm due to its exceeded flexibility.Therefore,it is urgent to explore and innovate surface modification processes suitable for stainless steel ultra-thin strips,to provide scientific theoretical basis for the efficient preparation of high-performance stainless steel ultra-thin strip based FMLs composite materials.Based on the above background,to solve the weak connection problem between 304 austenitic stainless steel ultra-thin strip with a thickness of 0.1 mm and E-51 bisphenol A type epoxy resin,this work conducted a relatively mild non macroscopic contact physical and chemical surface modification on stainless steel ultra-thin strip,and investigated the influence of different modification processes on the tensile shear strength of stainless steel ultra-thin strip/epoxy resin single lap joints.This study also preliminarily explored the mild surface modification processes suitable for stainless steel ultra-thin strips,and systematically revealed the bonding strength enhancement mechanisms introduced by different surface modification processes,further providing the theoretical basis and process guidance for improving the bonding strength between stainless steel ultra-thin strips and epoxy resin.The main research work was as follows:（1）An irregular and high-density microstructure was constructed on the surface of stainless steel ultra-thin strips using laser surface modification technology,and the influence of laser power on the surface microstructure,chemical composition,and physicochemical properties of stainless steel ultra-thin strips was investigated.It was found that,expect for the construction of the circular microtextur,the thermal input effect of laser surface microtexturing modification process can also generate-O-C=O polar groups on the surface of stainless steel ultra-thin strips,thus effectively improving the infiltration degree of epoxy resin on it,thereby improving the bonding performance between them.When the laser power is 40 W,the depth and density of the modified surface ring-shaped microtexture are greatly increased with the intensification of the austenite melting solidification process.The number and strength of the mechanical interlocking reinforcement units formed with the epoxy resin are both increased,as well as the number and strength of the mechanical interlocking reinforcement units also increased.Compared to sandpaper polishing process joints,the tensile shear strength of the modified joint can reach 14.43 MPa under this power condition.（2）The strategy of using acid etching to remove the passivation layer on the surface of stainless steel ultra-thin strips and improving the dissolution effect during anodizing has successfully overcome the weak connection problem between anodized modified surfaces and stainless steel ultra-thin strips.And a detailed exploration was conducted on the specific formation process of the surface modified by anodizing of stainless steel ultra-thin strips.It was found that,when the dissolution effect is much higher than the formation effect during the anodic oxidation process,a nano array type of anodizing-annealing modified surface can be formed,which is closely bond to the stainless steel ultra-thin strip substrate.On the contrary,a nano porous modified surface is formed,and there are still some unconnected areas between it and the stainless steel ultra-thin strip substrate.Under the condition of 50 V anodizing voltage,when the anodizing time is 2 h,the concentration of H₂O in the electrolyte is 0.3 mol/L,and the concentration of NH₄F is 0.1 mol/L,the nano array pore structure on the modified surface is the most abandunt due to appropriate formation and dissolution effects during the anodization process.The tensile shear strength of the joint constructed by the modified stainless steel ultra-thin strip can reach to16.04 MPa.（3）A comparative study was conducted on the surface chemical composition and microstructure of stainless steel ultra-thin strips treated with HCl acid etching,anodizing,and anodizing annealing modification processes.The improvement effects of different modification processes on the bonding performance of stainless steel ultra-thin strips/epoxy resin were investigated.It was found that,there is a strong capillary interaction between high viscosity epoxy resin and the nano pore structure formed by acid etching.Therefore,the tensile shear strength of the joint is just increased to 7.05 MPa comparing to mechanical polishing modified joints.The rich micro scale annular pits on the anodizing modified surface are beneficial for increasing the contact area between epoxy resin and stainless steel ultra-thin strips,thereby providing more active sites for mechanical interlocking reinforcement units,and further improving the tensile shear strength of the joint to 9.96 MPa.And for the nano array type of anodizing-annealing modified surface,the micro-nano coexisting structure greatly enhances the physical and chemical activity of the modified surface,greatly improves the surface uniformity,and the formed oxidized modified surface can also react chemically with the amino groups in the curing agent to form metal organic complexes,providing chemical bonding for the interface.Consequently,the anodized-annealed modified joint thus showed the highest value of 16.04 MPa.（4）On the basis of the above research,a silane coupling agent coating process was further applied to the modified surface to introduce an organic-inorganic bridging transition layer between the epoxy resin and the stainless steel ultra-thin strip,further improving the bonding characteristics between the stainless steel ultra-thin strip and the epoxy resin.It was found that,acid etching and anodizing annealing modified surfaces have the best response to KH550 silane coupling agent,which can be mainly attributed to their rich micro/nano scale porous structure on the modified surface.After the application of the KH550,the tensile shear strength of the joint constructed from the acid etched modified sample increased from 7.05 MPa to 10.38 MPa.In addition,the tensile shear strength of the joint constructed by the anodized annealing modified sample can be increased from 16.04 MPa to 20.36 MPa after the KH550 coating modification with a concentration of 4 wt.%.Moreover,due to the water molecule isolation effect provided by KH550 molecular layerin the interfaces,the aging resistance of joints has also been greatly improved.The tensile shear strength of the joint only decreased by 8.7%after two weeks,while the decline rate of the joint without coupling agent coating was 28.6%.