Design and preparation of gradient graphite/cermets self-lubricating composites

doi:10.1016/j.jmst.2017.09.018

Abstract

Abstract:

Based on the functionally graded materials (FGMs) design concept, the laminated-graded graphite/cermets self-lubricating composite was prepared to achieve the integration of mechanical properties and lubrication performance of the cermet. The effects of the layer number and thickness of graded structure on residual stresses in the gradient composites were investigated by finite element method (FEM). From the FEM analyses, the optimal gradient structure design was obtained corresponding to the following parameters: the number of graded layers n = 2 and the thickness of graded structure t = 1 mm. According to the optimum design, a graded graphite/cermets self-lubricating material with two layers was fabricated by a typical powder metallurgy technique. Compared with the homogenous graphite/cermets composite, the surface hardness and indentation fracture toughness of graded composite were increased by approximately 15.9% and 6.3%, respectively. The results of X-ray diffraction (XRD) stress measurement identified the existence of residual compressive stress on the surface of graded composite. Additionally, the friction and wear tests revealed that the wear resistance of the graphite/cermets self-lubricating composite was improved significantly via the graded structural design, whereas the coefficient of friction changed slightly.

Key words: Graphite/cermets self-lubricating material, Graded structure, Finite element analysis, Mechanical properties, Tribological performance

, Ji Xiong, Zhixing Guo, Junliu Ye. Design and preparation of gradient graphite/cermets self-lubricating composites[J]. J. Mater. Sci. Technol., 2018, 34(8): 1378-1386.

Figures/Tables 15

Fig. 1. Physical model of gradient graphite/cermets self-lubricating composites.

Fig. 2. Finite element analysis model.

Table 1 Thermo-physical properties of materials.

Materials	Young’s modulus, E (GPa)	Poisson’s ratio, ν	Thermal expansion coefficient, α (10^-6 K^-1)	Thermal conductivity, k (W/(m K))	Density, ρ (kg/m³)
Cermets	430	0.19	9.03	17.6	5797
Graphite	14	0.25	3.9	129	2150

Table 2 Details of the starting powers.

Powders	FSSS^* (μm)	Purity (%)	Manufacturer
Ti(C_0.7N_0.3)	1.80	>99.5	Changsha Wing high New Materials Co., Ltd., China
Mo₂C	1.55	>99.2	Changsha Wing high New Materials Co., Ltd., China
Ni	2.65	>99.9	Chengdu Nuclear 857 New Materials Co., Ltd., China
Graphite	3.80	>98.5	Qingdao Shuofeng Graphite Products Co., Ltd., China

Fig. 3. Variations of the residual stresses with the number of gradient layers: (a) radial stresses on the surface, (b) shear stresses at the interface, (c) axial stresses at the edge, (d) Von Mises stresses.

Fig. 4. Variations of the residual stresses with thickness of gradient layers: (a) radial stresses on the surface, (b) shear stresses at the interface, (c) axial stresses at the edge, (d) Von Mises stresses.

Fig. 5. Typical contour maps of stress distribution of gradient graphite/cermets composite: (a) radial stress, (b) shear stress, (c) axial stress, (d) Von Mises stress.

Fig. 6. Micrographs of the graphite/cermets graded self-lubricating composite with the number of gradient layers n = 2 and thickness of graded structure t = 1.0 mm.

Table 3 Comparison of mechanical properties of non-graded and graded composites.

Samples	Hardness (GPa)	Fracture toughness (MPa m^1/2)	TRS (MPa)
Non-graded	11.67 ± 0.2	7.29 ± 0.08	1569 ± 50
Graded (n = 2)	13.53 ± 0.3	7.75 ± 0.13	1880 ± 61

Fig. 7. Macroscopical crack propagation path of (a) the homogeneous graphite/cermets composites and (b) the graded graphite/cermets self-lubricating composite with two layers.

Table 4 Residual stress measured by XRD method.

Ψ (°)	sin²ψ	2θ_?,ψ (°)
		t = 0.5 mm	t = 1.0 mm	t = 1.5 mm	t = 2.0 mm
0	0.0000000	122.697	122.705	122.713	122.712
10	0.1745329	122.726	122.728	122.718	122.728
20	0.3490658	122.738	122.733	122.738	122.730
30	0.5235988	122.744	122.735	122.743	122.741
40	0.6981317	122.761	122.760	122.739	122.738
σ_? (MPa)		-121.1	-95.6	-73.9	-54.0

Fig. 8. Comparison between the experimental results and the FEM results.

Fig. 9. Volume wear rates and friction coefficients of non-graded and graded graphite/cermet composite under dry sliding condition.

Fig. 10. SEM images of worn surface of (a) homogenous graphite/cermets composite and (b) graded graphite/cermets composite.

Table 5 EDS results of worn surface of different samples (at.%).

Samples	C	O	Ti	Ni	Mo
Non-graded	22.82	48.36	23.65	3.41	1.76
Graded	40.76	20.32	31.25	4.93	2.74

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