Letter www.acsami.org
Novel Approach to the Fabrication of an Alumina-MoS2 SelfLubricating Composite via the In Situ Synthesis of Nanosized MoS2 Yunfeng Su,†,‡ Yongsheng Zhang,*,† Junjie Song,†,‡ and Litian Hu*,† †
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: The fabrication of a self-lubricating ceramic composite with MoS2 as the solid lubricant is extremely difficult given the high temperature sensitivity of MoS2. In this study, a hydrothermal method was utilized for the in situ synthesis of nanosized-MoS2 in Al2O3 ceramic to fabricate an Al2O3−MoS2 self-lubricating composite. The composite exhibited excellent self-lubricating properties with low friction coefficient and wear rate in a high-vacuum environment because of the efficient formation of self-lubricating films from the finely structured MoS2 in the Al2O3 matrix. The results of this study establish a new approach to the fabrication of self-lubricating ceramic composites with temperaturesensitive solid lubricants. KEYWORDS: ceramic matrix composite, alumina, in situ synthesis, friction and wear, hydrothermal method
C
excellent lubricating performance. To keep MoS2 intact, Ouyang et al.8 employed spark plasma sintering at a low temperature for a short duration to sinter a ceramic selflubricating composite with MoS2 as the solid lubricant. In the present study, we designed a novel approach to introduce the lubricant MoS2 into a sintered Al2O3 ceramic matrix via the hydrothermal method.9 In this method, MoS2 was protected from the effects of high temperature through the in situ synthesis of MoS2 in the pores of the Al2O3 ceramic. The schematic of the fabrication processes is presented in Scheme 1. Al 2O 3 powder (80−120 nm) and 10 vol % graphite (approximately 5 um) were mixed via the ball-milling method. The mixed powders were then placed in a steel mold to prepare the green body. The green body was sintered in a muffle furnace at 1,500 °C for 4 h under air atmosphere. Graphite, which acted as the pores former, was oxidized and removed while the green body was sintered, thus introducing pores in the matrix. The polished sintered sample was immersed in the homogeneous reactant solution that contained sodium molybdate, thiourea, and deionized water. Ultrasonic and vacuum infusing processes were then conducted to ensure the pores in the samples were filled with the reactant solution. Finally, the solution with the samples was transferred into a 150 mL Teflon-lined stainless steel autoclave, heated at 220 °C for 56 h, and then naturally cooled to room temperature. Samples
eramic materials are promising wear-resistance components because of their superior strength, hardness, corrosion resistance, and antiwear. However, the industrial applications of ceramic materials are limited by their poor lubricating property, as indicated by their high friction coefficient under dry sliding conditions. Solid lubricants such as graphite, metal molybdenum, and CaF2 are incorporated in ceramic matrixes to improve the self-lubricating properties and tribological performances of ceramic materials. The incorporation of solid lubricants in ceramic matrixes enables ceramics with favorable friction coefficients because these lubricants can form lubricating and transferring films that completely cover ceramic surfaces and that pair during sliding.1,2 Lamellar-like MoS2 is a highly promising solid lubricant3,4 that has been widely used to improve the tribological characteristics of aviation, air craft and electronic consumer products. However, few ceramic self-lubricating composites are produced with MoS2 as the solid lubricant given the lower melting point (1,185 °C) and decomposition temperature (approximately 1,300 °C) of MoS2 than those of ceramic materials.5,6 The incorporation of MoS2 in ceramic matrixes remains challenging because high temperatures during ceramic sintering cause MoS2 melt or even decomposition. To address this problem, researchers have developed some methods for the fabrication of ceramic materials with solid lubricant MoS2. By combining laser surface texture and burnishing to protect MoS2 from the effects of a high-temperature process, Fang et al. fabricated a three-dimensional lubricating layer on the surface of Al2O3 composites;7 the modified composites exhibited © XXXX American Chemical Society
Received: June 22, 2017 Accepted: August 28, 2017 Published: August 28, 2017 A
DOI: 10.1021/acsami.7b09000 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces Scheme 1. Schematic Diagram and Fabrication Procedures of the Composite
Figure 1. (a, b) SEM and (c) TEM of the MoS2 synthesized via hydrothermal reaction; (d) BSE and (e) SEM micrographs of the composite; (f) XRD spectrum of the composite.
high-magnification SEM image (Figure 1e) shows that the flower-liked MoS2 with thin lamellae existed in the pores between Al2O3 grains. The X-ray diffraction (XRD) analysis of the composite showed that only the α-Al2O3 peaks and three MoS2 peaks existed at the (002), (100), and (110) crystal planes, indicating that Al2O3−MoS2 composites were successfully fabricated without any other phases. As shown in Figure S1, friction tests were performed using a standard vacuum tribometer (Anton Paar, Austria) with reciprocating motion. A Si3N4 ceramic ball (Ø 6 mm) with a surface roughness of ∼0.1 μm (Ra) was utilized as the pairing sample. The tests were conducted at a sliding speed of 5 cm/s for a total time of 7,200 s at an amplitude of 2.5 mm, and under a load of 5 N in a high vacuum environment (
