Effects Of Annealing And Surface Nanocrystallization On Bonding Properties Between Cold-sprayed Zn-Al Coating And If Steel Substrate

Interstitial-free (IF) steel is a kind of very important structural material due to its eminent deep-pressing capability. It is widely used in making shells of automobiles and electrical appliances. However, the erosion resistance of it is not satisfying for long term service. Zn-Al alloy is a commonly used material for making protective coatings on carbon steels owing to their excellent wear and corrosion resistance. As a new type of coating preparing technology, cold spray can deposit coatings at low temperature, and it has a broad application prospects in metal coating preparation. How to prepare dense and stable protective coatings and increase the interfacial bonding strength of coating/substrate are two key issues for the developing of cold spray technology.In the present work, a dense Zn-Al alloy coating was successfully deposited on IF steel by means of cold spray technology. Effects of post annealing and surface nanocrystallization (of IF steel substrate) on the bonding property between coating and substrate have been studied. Optical microscopy, scanning electron microscopy,transmission electron microscopy and X-ray diffraction were used to investigate the microstructural characteristics of Zn-Al coating, IF steel substrate and the coating/substrate interface. The mechanical and corrosive properties of Zn-Al coating were studied by using micro-hardness tester, nano-indenter, ball-on-plate sliding wear tester and three-electrode cell. The bonding strength between coating and substrate was estimated by a 3 points-bending tester and a stud pull-off tester.The main results are followed:1. A dense Zn-Al coating with a porosity ratio less than 0.5% was deposited on IF steel by cold spray technique. There was almost no oxidation and phase transition during the preparing process. After annealing at temperature from 200 ℃ to 350 ℃,the hardness of coating increased gradually with temperature rising. However, the wear resistance showed a decreasing trend with temperature rising at the same condition. Post-annealing treatment had no significant effect on the corrosion resistance of cold-sprayed Zn-Al coating.2. After diffusion annealing at 400 ℃, the stripping resistance of cold sprayed Zn-Al coating on initial IF steel (Zn-AI/IF-CG) sample increased slightly with a short annealing duration (< 60min), and then decreased drastically when holding time was longer than 60 min because of the formation of Fe11Zn40、FeZn10和Fe2Al5-xZnx compounds layer.3. A gradient nanostructured layer was produced on the IF steel substrate by means of surface mechanical attrition treatment (SMAT). The average grain size was～10 nm on the topmost surface layer,and it showed a good thermal stability when temperature was lower than 400 ℃ . In comparison with the original IF steel, the hardness of surface layer increased -33%.4. The bonding property of coating/substrate was improved significantly by SMAT of IF steel substrate. The bonding strength of the as-sprayed Zn-Al coating on the SMAT sample (Zn-Al/IF-SMAT) reached 58.1 MPa. This value was ～30% higher than that of the Zn-Al/IF-CG sample.5. The bonding strength of Zn-Al/IF-SMAT sample wasn’t further improved by post annealing treatment. During annealing at 400 ℃, it decreased first when duration was less than 30 min; and then increased to a stable value of ～50.2 MPa with the prolonging duration; finally, it decreased again when duration was more than 90 min.6. Under a same annealing condition, the bonding strength of the Zn-Al coating on the IF-SMAT substrate was always no less than that on the IF-CG substrate.