ABSTRACT
According to this research, Fe-based amorphous alloy coating (containing Cr, Mo,Ni, P, B, Si) was deposited by air plasma spraying(APS) technique. The properties of the coating was investigated by means of Microhardness Tester, Optical Metallographic Microscopy(OM), X-ray Diffraction(XRD), Scanning Electron Microscopr(SEM),Differential Scanning Calorimentry (DSC), Corrosion and Wear resistance experiments.
The correlations between spraying parameters and the performance of as-sprayed coating oriented in microhardness and porosity was studied. Through orthogonal experimental design, the optimized parameters were obtained. When the Ar flow rate was 40 L/min, current was 550 A, arc voltage was 60 V and stand-off distance was 100 mm, the porosity of coating was as low as 0.89%, the microhardness was as high as 831.9 HV0.1. A dense, homogeneous and lamellur coating with high fraction of amorphous phase was obtained, which was composed of deformed strip shaped particles, un-melted particles, some porosity and cracks. It was found that the substrate and the coating was mainly mechanical bonding.
The DSC showed that the crystallization temperature of coating began at 552℃ and ended at 620℃ .According to XRD patterns, the heat treated coatings with the temperatures lower than 550℃ had an amorphous structure. When the annealing temperature was among 550℃and 800℃, α -Fe phase began to crystallize, then Ni3P，Cr1.2Ni0.8P. With the extension of annealing temperature to 800℃, the fully crystallized sample consisted of FeNi2P、Fe3B0.8P0.18 and Fe-Ni solid solution with grain size growed was obtained. Through statistical analysis, the microhardness of coating under different annealing condition presented both normal and logarithmic distribution. The microhardness of coating increased with the raise of annealing temperature and the extension of duration time below annealing temperature of 500℃,and fluctuated above annealing temperature of 550℃.
It was found through electrochemical and immersion test that the amorphous coating underwent the course of active dissolution-passivation-repassivation in 1NH2SO4, 10% NaOH and3.5% NaCl. The corrosion current density icorr and passivation current density ip of coatings in the above three electrolyte were both lower than the corresponding stainless steel substrates, and the corrosion rate reduced sharply after polishing treatment. The amorphous coating exhibited the best corrosion resistant capacity in NaOH solution, the worst in HCl solution. The heat treatment didn’t have much effect on the electrochemical parameters of coating. The microhardness of coating decreased with the increase of test load, exhibited phenomena of indentation size effect. The fracture toughness of coating was superior to bulk amorphous alloy with average KIC estimated at 2.7084 MPa ⋅ m1/2. The coating also had better wear resistance.
All the analysis indicated that by APS technique a amorphous coating alloy with high microhardness, density, good corrosion and wear resistance based on optimized parameters can be achieved
KEY WORDS:Fe-based amorphous alloy coating, plasma spray, porosity,microhardness, corrosion, wear
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