Novel solid-lubricant materials for multifunctional applications
Polymers and their composites possess excellent friction and wear characteristics, corrosion resistance, and mechanical properties that make them potential replacements for metals in many applications.1–6 Self-lubricating composites are particularly attractive because of their simple design, easy operability, and low cost, among other things. Moreover, the use of additives such as kaolin,2 molybdenum disulfide,3, 7 multiwalled carbon nanotubes,5,8,9 calcium stearate,6 wollastonite,10 short fibers,11 boron trioxide,12 serpentine,13 and silica14 has been shown to further enhance the mechanical and tribological performance of polymers. In previous studies7–14 it has been hypothesized that the addition of hard, machinable, conductive, and lubricious particles, such as Ti3SiC2 (MAX phase) will significantly alter the mechanical and tribological behavior of polymer matrix composites (PMCs). Briefly, Mn+1AXn (MAX) phases (more than 60 phases) are thermodynamically stable nanolaminates where n is 1, 2, or 3, M is an early transition metal element such as titanium (Ti), A is an A-group element such as silicon (Si), and X is carbon (C) or nitrogen. MAX phases are layered hexagonal, with two formula units per cell. These solids are highly damage tolerant, thermal shock resistant, and readily machinable.15–19
In a series of studies, we have reported that the addition of Ti3SiC2 particulates, in both thermosets23 and thermoplastics20–22 enhances the mechanical performance and solid-lubrication behavior of PMCs. We have coined the term MAXPOL—composites of MAX phases and polymers—to designate this new generation of composites. Our detailed methods for fabrication and the characterization procedure are available elsewhere.19, 23 We have found that MAXPOL composites show solid-lubrication behavior during self-mating,20, 21 and here we review some of the important results from our recent studies.
Scanning electron microscopy (SEM) images of Ti3SiC2-UHMWPE (ultra-high-molecular-weight polyethylene) composites are shown in Figure 1.20 We find that the Ti3SiC2 particulates are well dispersed in the UHMWPE matrix when the concentration of Ti3SiC2 particulates is about 5vol.%, as shown in Figure 1(b) and (c). In contrast, the Ti3SiC2 particulates segregate to form Ti3SiC2- and UHMWPE-rich interfaces around the UHMWPE matrix at higher concentrations of Ti3SiC2, as shown in Figure 1(d) to (i). At this juncture, we are not sure of the exact mechanism causing this behavior, but most probably, dewetting of Ti3SiC2 particulates by the polymer causes the Ti3SiC2 particulates to segregate in Ti3SiC2-rich polymer regions.
The mean friction coefficient (μmean) and wear rate (WR) of our composites, as a function of the addition of Ti3SiC2 particulates, are summarized in Figure 2. UHMWPE sliding against itself exhibts a higher μmean compared with the Ti3SiC2-UHMWPE composite sliding against itself, as shown in Figure 2(a). Teflon-Ti3SiC2 and polyetheretherketone (PEEK)-Ti3SiC2 composites showed a similar trend, which suggests that Ti3SiC2 particulates may prove an effective solid-lubricant additive in a host of polymer matrices.
The WR results—see Figure 2(b)—are also very promising. During self-mating, both surfaces showed wear. The total WR of the UHMWPE surfaces was less than about 1.6 × 10−4mm3/N.m, whereas in 5vol.% Ti3SiC2-UHMWPE and 20vol.% Ti3SiC2-UHMWPE the WR was negligible (less than 4 × 10−7mm3/N.m). The WR then increased to about 2 × 10−6mm3/N.m in 35vol.% Ti3SiC2-UHMWPE. We could not detect wear in the 5vol.% Ti3SiC2-UHMWPE and 20vol.% Ti3SiC2-UHMWPE compositions after cycling for 10,000m. This further indicates the long-term stability of these composites during self-mating.20
A detailed investigation of the wear tracks showed that the tribology of Ti3SiC2-UHMWPE composites is driven by the formation of tribofilms.19, 23 We have previously shown that the tribological behavior of MAX phases and their composites is driven by tribofilm formation.19 The tribofilms formed during self-mating of Ti3SiC2-UHMWPE composites made thin layers over each tribosurface. Using a previously proposed classification,19 we find that the tribofilms correspond to type IIIa. A simple schematic diagram, in which lubricious tribofilms are formed at tribocontacts, is shown in Figure 3.
In summary, we have synthesized and characterized novel MAXPOL composites. The addition of Ti3SiC2 particulates enhances both the mechanical behavior and, especially, the tribological performance of the materials. Tribofilms that formed during self-mating of Ti3SiC2-UHMWPE composites were barely visible to the naked eye and formed thin layers over each tribosurface (as observed by SEM). We therefore classified these tribofilms as type IIIa.20 Our tribological studies showed that the addition of Ti3SiC2 in the UHMWPE matrix imparts self-lubricity to the composites and aids in decreasing adhesive wear during dry sliding of polymer-on-polymer tribocouples. In particular—when used in a polymer matrix to reduce adhesive wear—Ti3SiC2 particulates can lead to the development of polymer-on-polymer wear couples, which may provide engineers with novel approaches for designing devices.24 We are currently conducting studies to investigate the wetting behavior of Ti3SiC2 and polymers. We are also exploring novel polymer systems such as PEEK, Teflon, and polylactic acid. In addition, we are developing novel techniques for 3D printing of these composites.
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