In-situ formation of Ni-Al intermetallics-coated graphite/Al composite in a cold-sprayed coating and its high temperature tribological behaviors

doi:10.1016/j.jmst.2017.01.026

Abstract

Abstract:

In this study, cold spraying (CS) was used to deposit a mixture of nickel-coated graphite and 40 vol.% Al powder (Ni-Gr/Al) on a steel substrate aiming to effectively preserve a certain volume fraction of graphite in the deposited Ni-Gr/Al composite coating. The microstructure of the as-sprayed coating and the effect of post-spray heat-treatment (PSHT) temperatures on the in-situ formation of Ni-Al intermetallic phases in coating were studied. The tribological behaviors of the as-sprayed coating and the PSHTed coating under 450 °C were tested at 25 °C, while the as-sprayed coating was tested at 450 °C for comparison. As a result, the Ni-Gr particles showed a homogenous distribution in the coating. The multilayer Ni-Al intermetallics-coated graphite/Al composite coating was achieved in situ after the PSHT of 450 °C, where the graphite did decompose at 550 °C leaving big pores in the coating. The coefficients of friction (COF) of the CSed coating and the PSHTed coating were measured at 450 °C as well as 25 °C, which showed a similar tendency, much higher than that of the CSed coating tested at 25 °C. The lubrication phase (graphite) improved the formation of a graphite film during sliding friction and decreased the COF, while the hard Ni-Al intermetallic phases contributed to the increase of COF.

Key words: Cold spraying, Ni-coated graphite powder, Post-spray heat-treatment, Intermetallics, Tribological performance

Huang Chunjie, Li Wenya, Planche Marie-Pierre, Liao Hanlin, Montavon Ghislain. In-situ formation of Ni-Al intermetallics-coated graphite/Al composite in a cold-sprayed coating and its high temperature tribological behaviors[J]. J. Mater. Sci. Technol., 2017, 33(6): 507-515.

Figures/Tables 12

Table 1 Typical metal-intermetallic of Ni/Al coatings prepared by post-spray heat-treatment.

Coating	Author	Ref.	Feedstock	Powder preparation method	Heat treatment			Phase
					Temperature	Time	Atmosphere
Ni-Al	Lee at al.	^[4,5]	Weight ratio 25:75, 10:90	Dry blended with V-mill	450-550 °C	4 h	N₂	Al₃Ni, Al₃Ni₂
Ni-Al	Spencer and Zhang	[6]	Atomic ratio 1:1	Mixed in a Turbula mixer	420 °C	4-20 h	Ar	Al₃Ni₂, Al₃Ni
Ni-Al	Zhang et al.	[7]	Atomic ratio 1:1	Ball-mixed in liquid N₂ atmosphere	500 °C	3 h	Ar	Al₃Ni₂ → AlNi
					600 °C	2 h
					850 °C	2 h
					1050 °C	1 h

Table 2 Composition of Ni-Gr and Al composite powders.

Type	Size (μm)	Shape	Supplier
Ni-Gr particles (75 wt% Ni + 25 wt% graphite)	d(0.1) = 56.7	Irregular morphology	Beijing General Research Institute of Mining and Metallurgy, China
	d(0.5) = 82.1
	d(0.9) = 117.7
Al (99.5 wt%)	d(0.1) = 26.5	Spherical morphology	Youxinglian Nonferrous Metals Ltd., Beijing, China
	d(0.5) = 49.5
	d(0.9) = 86.8

Fig. 1. Secondary electron-scanning electron microscope (SE-SEM) morphologies of the used (a) Ni-Gr powder and (c) Al powder, (b) the inset in (a) showing the back scattered electron-scanning electron microscope (BSE-SEM) micrograph of the cross-section of a Ni-Gr particle.

Table 3 Coatings under different friction conditions.

No.	Sample	Friction temperature
1	As-sprayed coating	25 °C
2	Heat-treated coating at 450 °C	25 °C
3	As-sprayed coating	450 °C

Fig. 2. Cross-sectional SE-SEM morphologies of the as-sprayed Ni-Gr/Al composite coating at 200× (a), and the higher magnification at 500 × (b) and 2000 × (c). (d) Showing a distorted Ni-Gr particle at the interface, and (e) XRD pattern of as-sprayed coating.

Fig. 3. BSE-SEM micrographs of the annealed coating under the temperature of 450 °C at 200× (a), 500× (b), and 2000× (c and d). (e) Showing the composition of the annealed coating detected with EDS.

Fig. 4. BSE-SEM micrographs of the annealed coating under the temperature of 550 °C at 200× (a), 500× (b), and 2000× (c). (d) Showing the composition of the annealed coating detected with EDS.

Fig. 5. XRD patterns of as-sprayed coatings at annealing temperatures of 450 °C and 550 °C.

Fig. 6. Changes of COF of the Ni-Gr/Al composite coating with sliding time: as-sprayed coating tested at 25 °C and 450 °C, and heat-treated coating at 450 °C tested at 25 °C.

Fig. 7. Worn surfaces of the as-sprayed Ni-Gr/Al composite coating tested at 25 °C (a and b) and (c) elemental mapping of the marked zone in (b).

Fig. 8. Worn surfaces of the heat-treated Ni-Gr/Al composite coating tested at 25 °C (a and b) and (c) elemental mapping of the marked zone in (b).

Fig. 9. Worn surfaces of the as-sprayed Ni-Gr/Al composite coating tested at 450 °C (a and b) and (c) elemental compositions of points in (b).

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