Tribological behaviors of NiAl based self-lubricating composites containing different solid lubricants at elevated temperatures
Graphical abstract
Introduction
NiAl is attractive because it can present a good combination of properties, such as high melting point (1638 °C), low density (5.86 g/cm3), high Young's modulus (294 GPa), large thermal conductivity (76 W/(mK)) and excellent oxidation resistance at high temperatures (~1000 °C) [1], [2], [3]. Tribological performance is an important property need to be characterized for NiAl as the candidate for structural components.
On one hand, some researches have been carried out to study the tribological behaviors of NiAl at RT, indicating that the NiAl possesses good tribological properties [4], [5]. Wear testing of three NiAl alloys containing 45 at%, 48 at%, 50 at% aluminum indicated that all of them had low friction coefficient (0.25–0.35) and wear rate (1.5–2.4×10−5 mm3/N/m) at RT. On the other hand, in order to further improve the wear resistance of NiAl intermetallic materials at high temperatures, many efforts have been made in recent years. NiAl–31BaF2–19CaF2 (mass %) exhibited the low friction coefficients (0.3–0.4) and wear rates (3–4×10−5 mm3/N/m) [6]. Zhu et al. [7] selected ZnO and CuO as lubricants in NiAl matrix composites due to their high melting point and favorable wear resistance at high temperatures. It was found that NiAl matrix composites with addition of ZnO showed the lowest wear rate (7×10−6 mm3/N/m) at 1000 °C, while CuO addition into NiAl matrix composites exhibited the self-lubricating performance and the best tribological properties at 800 °C. From the tribology point of view, the addition of effective solid lubricants becomes an excellent solution to promote the application of NiAl matrix materials at different temperatures.
However, combined with the former research results, the NiAl matrix composites containing independent solid lubricant could not possess excellent tribological properties over a wide temperature range. Solid lubrication over a wide temperature has been a long challenge for the tribologists as it is difficult or impossible for a bulk monolithic material to possess all the surface properties of low friction, good wear and oxidation resistance over a wide temperature [8]. Consequently, there is a great need to design a tribological system possessing low friction coefficients and wear rates over a wide temperature range for NiAl matrix composites.
Conventional solid lubricants like MoS2 and WS2 can work well at low temperatures because of their special structure [9], [10], [11], [12], [13], [14], while they can be easily oxidized at high temperatures. Ti3SiC2 ceramics have been proved to be the promising tribological materials at high temperatures [15], [16]. It is known that the lubricity of PbO usually is very poor at room temperature (RT), while effective at high temperatures [17]. These materials only possess good lubricious properties in a restricted temperature range. It is expected that when two or more solid lubricants are incorporated, an unexpected synergetic lubricating effect can be observed, which is superior to any one of the single lubricants. In order to widen operating temperature range of lubricating, one approach is to combine low temperature lubricants with high temperature lubricants into a composite structure.
Gupta et al. [18] investigated the tribological performances of Ta2AlC and Cr2AlC composites with 20 vol% Ag over a temperature range from RT to 550 °C during sliding against Ni-based SA and Al2O3. It was found that the MAX/Ag composites demonstrated much improvement in wear resistance at low temperatures as compared to the pure MAX phases, while their performance at elevated temperatures was slightly worse.
Li et al. [19] prepared nickel-base composites containing silver, MoS2 and CeO2 by powder metallurgy method. It was found that the self-lubrication over a wide temperature range was reached. The friction coefficients of silver and CeO2 containing Ni-based composite from room temperature to 600 °C were in the range of 0.16–0.26. The wear rates were reduced by more than one order of magnitude after adding silver and CeO2.
The objective of this study was to explore the concept of synergetic lubricating action of MoS2 with Ti3SiC2, as well as WS2 with Ti3SiC2 from RT to 800 °C. The NiAl based self-lubricating composites were prepared through the spark plasma sintering (SPS), in which MoS2, WS2 and Ti3SiC2 were used as lubricants to improve the tribological properties of the composites. The wear mechanisms at different temperatures were also discussed by analyzing the morphologies and compositions of worn surfaces of the composites. NiAl based alloy without lubricant as well as NiAl containing PbO have also been prepared and tested under the same conditions for the purpose of comparison.
Section snippets
Experimental details
NiAl based self-lubricating composites with different lubricants were prepared by SPS. Commercially available PbO, MoS2, Ti3SiC2 and WS2 powders were used as the lubricants. The composite powders of NiAl based alloy were composed of commercially available Ni, Al, Mo, Nb and B powders (30–50 μm in average size, 99.9 wt% in purity) by atomic ratio of 48: 50: 1: 1: 0.02. In our previous study [20], the NiAl matrix self-lubricating composites exhibited low friction coefficient and wear rate when the
Results and discussion
Fig. 1 was the XRD patterns of NiAl based self-lubricating composites prepared by SPS. It could be found that the diffraction peaks primarily belonged to the NiAl phase in all samples. After adding PbO, PbO and Pb3O4 phases could be found in NiAl–PbO, and Pb3O4 should be attributed to the oxidation of PbO. Lubricating phases of Ti3SiC2 and MoS2 could be found in NiAl–Ti3SiC2–MoS2. Owing to the decomposition reaction of Ti3SiC2, the new phase of TiC was formed in NiAl–Ti3SiC2–MoS2. Similarly,
Conclusions
NiAl based self-lubricating composites were prepared by SPS at 1100 °C under a pressure of 30 MPa for 5 min in pure Ar atmosphere protection. The dry friction and wear behaviors of NiAl based self-lubricating composites against Si3N4 ceramic ball pair at different temperatures from RT to 800 °C were emphatically researched through the determination of friction coefficients and wear rates and the analysis of the morphologies and compositions of worn surfaces of NiAl based self-lubricating
Acknowledgments
This work was supported by the Project for Science and Technology Plan of Wuhan City (2013010501010139); the National Natural Science Foundation of China (51275370); the Nature Science Foundation of Hubei Province (2012FFB05104); the Fundamental Research Funds for the Central Universities (2013-ZY-049); the Program for New Century Excellent Talents in University; the Academic Leader Program of Wuhan City (201150530146); and the Project for Teaching and Research project of Wuhan University of
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