Effect of Shot Content on the Microstructure and Corrosion Performance of Cold-Sprayed Zn-6Cu Alloy Coatings

Zinc is a biocompatible and biodegradable metallic material that is used in orthopedic repair and oral fixation applications.Because the mechanical properties of pure Zn are lower than the requirements for biomedical implants，the addition of alloying elements (e.g.，Mg，Li，Fe，Cu，and Al) can effectively improve the strength and toughness of Zn alloys.In particular，copper (Cu) not only provides antibacterial activity but also markedly enhances the mechanical strength of Zn-Cu alloys.Previous studies have extensively examined how deformation processes such as casting，drawing，and hot rolling affect the corrosion resistance of bulk Zn-Cu alloys；however，studies on the fabrication of Zn-Cu alloy coatings and their corrosion performance remain limited.Among various coating preparation technologies，cold spraying is a low-temperature and environmentally friendly process.During cold spraying，metal particles undergo plastic deformation upon high-velocity impact with the substrate and the already-deposited coating，and dense coatings are formed via successive particle accumulation.In this study，cold-sprayed Zn-Cu alloy coatings were prepared using stainless steel shots and Zn-6Cu particles as feedstock powders，and the effects of shot addition on the phase composition，microstructure，microhardness，and corrosion resistance of Zn-6Cu coatings were investigated.Zn-6Cu alloy coatings were fabricated via cold spraying using 1Cr18 stainless steel shot particles and Zn-6Cu alloy powder，with shot volume fractions of 0，30%，50%，and 70%.The phase composition of the cold-sprayed Zn-6Cu coatings was characterized by X-ray diffraction (XRD).The results showed that the coatings were mainly composed of the Zn phase and the CuZn5 phase，with a small amount of ZnO.During high-velocity continuous impacts between the shot particles and Zn-6Cu particles，significant internal stresses were induced within the coatings，which promoted an increased dislocation density and/or grain refinement of the Zn-6Cu particles.The full width at half maximum (FWHM) of the Zn(101) diffraction peak was measured as (0.062±0.002) rad，(0.110±0.002) rad，(0.116±0.003) rad，and (0.128±0.003) rad for coatings with 0，30%，50%，and 70%shot addition，respectively.The microstructure and the deposition morphology of individual particles were examined by scanning electron microscopy (SEM).The coating surfaces were found to exhibit a rough morphology and impact pits formed by shot collision and rebound，and the pit size increased with increasing shot content.Good interfacial bonding between the Zn-6Cu coating and the substrate was observed，and neither interfacial delamination nor internal cracking was detected.Except for a small number of tiny pores，dense interparticle bonding was achieved within the coatings.A mechanical mixing phenomenon was observed near the coating/substrate interface，particularly at shot volume fractions of 50%and 70%.From the deposition morphology of individual particles，some Zn-6Cu particles were shown to have been embedded into the polished substrate，whereas others rebounded after impact and produced pits on the substrate surface.Microhardness was measured on polished cross-sections using a Vickers microhardness tester.The microhardness values of the Zn-6Cu coatings were determined to be (82.7±4.1) MPa，(95.2±4.9) MPa，(104.3±8.5) MPa，and (108.9±24.3) MPa for shot additions of 0，30%，50%，and 70%，respectively.The corrosion performance of the Zn-6Cu coatings as a function of shot content was evaluated by linear sweep voltammetry and electrochemical impedance spectroscopy.Both the corrosion current density and the corrosion rate were found to decrease first and then increase with increasing shot content.Specifically，the corrosion current density was measured as (5.54±0.34)×10－5 A/cm2，(3.37±0.56)×10－5 A/cm2，(2.17±0.65)×10－5 A/cm2，and (2.93±0.73)×10－5 A/cm2，and the corrosion rate was calculated as (3.19±0.41) mm/a，(1.94±0.34) mm/a，(1.25±0.95) mm/a，and (1.69±0.89) mm/a for shot additions of 0，30%，50%，and 70%，respectively.With increasing shot addition from 30%to 70%，the charge-transfer resistance at the substrate/electrolyte interface (R1) was observed to decrease first and then increase，whereas the charge-transfer resistance at the coating/electrolyte interface (R2) was observed to increase first and then decrease；in all cases，both R1 and R2 were higher than those of the coating prepared without shot addition.Overall，the cold-sprayed Zn-Cu alloy coatings were mainly composed of Zn and CuZn5 phases with a small amount of ZnO.With increasing shot addition，the FWHM of the Zn(101) diffraction peak increased.The Zn-6Cu coatings exhibited rough surfaces and dense interiors，and the deposition of Zn-6Cu coatings was promoted by adding an appropriate amount of shots.Owing to the work-hardening effect，the microhardness of the Zn-6Cu coatings increased with increasing shot addition.The corrosion resistance of the Zn-6Cu coatings was improved when an appropriate amount of shots was introduced during cold spraying，and the best corrosion performance was achieved at a shot addition of 50%.