High Temperature Oxidation and Tribological Behaviors of NiCrAl-Graphite Self-Lubricating Composites

doi:10.1007/s40195-020-01188-9

Abstract

Abstract:

Nickel-graphite self-lubricating composites are a promising candidate to be used in turbine constructions that are usually exposed to high temperature oxidation and wear. However, the high-temperature stability of graphite as well as the effect that the oxide scale will play on the following wear process are still yet in debate. In this work, oxidation behavior of a NiCrAl-graphite composite and the subsequent friction and wear performances were studied. Results indicate that graphite is stable in the composites after oxidation at T ≤ 400 °C for 300 h, which contributes synergistically with the thin oxide film to self-lubrication. The friction coefficient is below 0.20 and the wear rate is ~ 1.43 × 10 ^-5 mm ³ N ^-1 m ^-1. The composite has the highest friction coefficient and wear rate when it was suffered from the high temperature oxidation at 500 °C. Once it was oxidized at 600 °C, a glaze layer would develop during the subsequent sliding. It plays a positive role in improving tribological properties though in the absence of lubricant phase of graphite, with to be exactly the friction coefficient and wear rate reduced by 13% and 21%, respectively, in comparison with the case of oxidation at 500 °C.

Key words: Oxidation, Self-lubricating composites, Wear, Graphite

Xuan Kong, Yang Liu, Minghui Chen, Tao Zhang, Fuhui Wang. High Temperature Oxidation and Tribological Behaviors of NiCrAl-Graphite Self-Lubricating Composites[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(7): 900-912.

Figures/Tables 15

Fig. 1 Macro morphologies of the Ni-coated graphite at a low magnification and b high magnification

Fig. 2 Oxidation kinetics of NiCrAl-graphite self-lubricating composites at different temperatures

Fig. 3 XRD patterns of the NiCrAl-graphite composites after oxidation for 300 h at different temperatures

Fig. 4 Microstructures and EDS mappings of the NiCrAl-graphite composites: a RT, b 400C, c 500C and d 600C

Table 1 EDS results for the tagged spots as shown in Fig. 4a (at%)

Spot	Ni	Cr	Al	C
1	2.0	54.4	-	43.6
2	74.2	7.0	7.2	11.6
3	2.6	53.1	-	44.3

Table 2 Density and porosity of composites before and after oxidation

	Density (g/cm³)	Porosity
RT	5.15	1.3
400C	5.12	1.9
500C	4.45	14.7
600C	4.44	14.9

Fig. 5 Surface roughness and micro-hardness of the NiCrAl-graphite composites before and after oxidation for 300 h3.3 Friction Coefficient and Wear Rate

Fig. 6 Tribological properties of NiCrAl-graphite composites before and after oxidation at different temperatures: a friction coefficient, b wear rate of composites and coupled balls and c profiles of wear track3.4 Worn Scar Characterization

Fig. 7 Worn surface morphologies of the NiCrAl-graphite composites before and after oxidation at different temperatures: a RT, b 400C, c 500C and d 600C

Fig. 8 Microstructures of the lubricating film and glaze layer formed after sliding on the NiCrAl-graphite composites of a 400C and b 600C

Table 3 EDS results for the tagged spots as shown in Fig. 8a, b (at%)

Spot	Ni	Cr	Al	C	O	Fe
1	43.3	3.7	4.3	37.8	10.8	-
2	14.9	1.3	1.4	69.6	12.5	0.1
3	12.7	4.9	1.3	11.4	61.5	8.0

Fig. 9 Raman spectroscopy at worn scar of the composites RT, 400C and 600C

Fig. 10 Cross-sectional microstructures at worn scar (a, b) along with the surface morphologies of the coupled balls (c, S) against composites 500C and 600C

Fig. 11 Surface morphologies of the composite 600C after rubbing for different times: a 5 min, b 10 min, c 15 min and d 20 min

Fig. 12 Schematic diagram showing oxidation-friction mechanisms of the composites: a 400C, b 500C and c 600C

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