Self-Lubricating PEO Coating on an Al Alloy Produced by Vacuum Impregnation Post-treatment

doi:10.1007/s40195-015-0280-2

Abstract

Abstract:

The objective of this research was to develop a novel self-lubricating coating on an AA6061 aluminum alloy. Three coatings were prepared by the plasma electrolytic oxidation (PEO) process using 50-, 500-, and 1000-Hz pulsed direct current, respectively. The as-deposited coatings were then post-treated using two different methods, viz., ultrasonic vibration-aided vacuum oil impregnation (UVOI) and oil impregnation under ambient pressure (OIAP). After post-treatment, an oil-containing, self-lubricating top layer was formed on the coatings. The effects of the coatings’ surface morphologies and structures on their oil-holding capabilities were discussed. The results revealed that coatings prepared with higher frequency had a greater oil-holding capacity using OIAP post-treatment, while those prepared with lower frequency had a greater oil-containing capability using UVOI post-treatment. These phenomena are related to the morphologies of the coatings produced with various current modes. The tribological properties of the coatings before and after post-treatments were investigated by pin-on-disc sliding wear tests. Due to the formation of a lubricant-containing top layer, the post-treatment coatings had a lower friction coefficient and improved wear resistance compared with the as-deposited coatings. In addition, the coatings after UVOI treatment had better wear performance than those post-treated using the OIAP process. Among all coatings, the coating produced with a 50-Hz pulsed current followed by UVOI post-treatment achieved the lowest friction coefficient (0.03) and best wear resistance when sliding against a Si₃N₄ ceramic counterface. This study indicates that a novel self-lubricating coating can be prepared by a PEO process combined with vacuum oil impregnation post-treatment.

Key words: Tribological properties, Self-lubricating coating, Plasma electrolytic oxidation process, Oil impregnation, Vacuum

Ling Hong, Dong Wang, Guang-Dong Bian, Lin-Lin Wang, Shu-Gen Hu. Self-Lubricating PEO Coating on an Al Alloy Produced by Vacuum Impregnation Post-treatment[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(8): 965-.

Figures/Tables 11

Fig. 1 Schematic diagram of the setup of ultrasonic vibration-aided vacuum oil impregnation (UVOI)

Fig. 2 SEM micrographs of surface morphology a-c and cross-sectional structure d-f of 50-N a, d, 500-N b, e, 1000-N c, f

Fig. 3 Surface roughness of coatings before and after post-treatments

Fig. 4 Mass increase rate of the coatings produced with 50 Hz a, 500 Hz b, 1000 Hz c pulsed current after post-treatments

Fig. 5 Coefficient of friction (COF) evolution during the sliding tests on sample: a 50-N, b 500-N, c 1000-N

Fig. 6 3D images and cross-sectional profiles of wear tracks on 50-N a, 500-N b, 1000-N c

Fig. 7 Micrographs of optical microscope a-c and 3D microscope images d-f of worn Si3N4 ceramic balls after sliding tests against 50-N a, d, 500-N b, e, 1000-N c, f

Fig. 8 Coefficients of friction (COF) of the coatings: a 50-AP, b 50-UV, c 500-AP, d 500-UV, e 1000-AP, f 1000-UV

Fig. 9 3D images a, c and cross-sectional profiles b, d of wear tracks on 50-AP a, b, 50-UV c, d

Fig. 10 3D images a, c and cross-sectional profiles b, d of wear tracks on 500-AP a, b, 500-UV c, d

Fig. 11 3D images a, c and cross-sectional profiles b, d of wear tracks on 1000-AP a, b, 1000-UV c, d

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