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Mammalian grinding dentitions are composed of four major tissues that wear differentially, creating coarse surfaces for pulverizing tough plants and liberating nutrients. Although such dentition evolved repeatedly in mammals (such as horses, bison, and elephants), a similar innovation occurred much earlier (~85 million years ago) within the duck-billed dinosaur group Hadrosauridae, fueling their 35-million-year occupation of Laurasian megaherbivorous niches. How this complexity was achieved is unknown, as reptilian teeth are generally two-tissue structures presumably lacking biomechanical attributes for grinding. Here we show that hadrosaurids broke from the primitive reptilian archetype and evolved a six-tissue dental composition that is among the most sophisticated known. Three-dimensional wear models incorporating fossilized wear properties reveal how these tissues interacted for grinding and ecological specialization.

Composites in TribologyPolymer composites are well known for offering engineers high strength-to-weight ratios and flexibility in material design. [1,2] The physical properties of a composite can be tuned to satisfy various functional requirements of a target application, including stiffness and strength, thermal and electrical transport, and wear resistance to name a few. Often, composites are designed to fulfill several functions simultaneously.One area of engineering that is particularly invested in the development and design of high performance polymer composites is tribology, the science related to interacting surfaces in relative motion. Bearings are systems that contain sliding interfaces, and are relied upon by nearly all moving mechanical systems. Though rarely recognized, Feature ArticlePolymer nanocomposites operate in applications where fluid and grease lubricants fail, and have superior tribological performance to traditional polymer composites. Nanoparticle fillers have been a part of notable reductions in the wear rate of the polymer matrix at very low loadings. Despite instances of remarkable wear reductions at unprecedented loadings (3 000 times at 0.5% loading in one case), there is a lack of general agreement within the literature on the mechanisms of wear resistance in these nanocomposites. In addition, results appear to vary widely from study to study with only subtle changes of the filler material or blending technique. The apparent wide variation in tribological results is likely a result of processing and experimental differences. Tribology is inherently complex with no governing laws for dry sliding friction or wear, and the state of the art in polymeric nanocomposites tribology includes many qualitative descriptors of important system parameters, such as particle dispersion, bulk mechanical properties, debris morphology, and transfer film adhesion, morphology, composition, and chemistry. The coupling of inherent tribological complexities with the complicated mechanics of poorly characterized nanocomposites makes interpretation of experimental results and the state of the field extremely difficult. This paper reviews the state of the art in polymeric nanocomposites tribology and highlights the need for more quantitative studies. Examples of such quantitative measurements are given from recent studies, which mostly involve investigation of polytetrafluoroethylene matrix nanocomposites.

Measured friction coefficients of carbon nanotubes vary widely from l < 0.1-l > 1.0 [1][2][3][4][5][6], while theoretical studies suggest intrinsically high friction coefficients, approaching unity [7]. Here we report that measured friction coefficients of MWNT films are strong functions of surface chemistry and temperature, but are not dependent on the presence of water vapor. We hypothesize that the origin of the temperature dependence arises from the interaction of the surface chemical groups on the nanotubes [8][9][10][11][12] and rubbing counterface. The friction coefficient of individual films can be easily tuned by changing the surface temperature and chemistry of either the countersurface or the nanotubes, we have demonstrated the ability to create and control high and low friction pairs through plasma treatments of the nanotube films with argon, hydrogen, nitrogen, and oxygen. This behavior is completely reversible, and when coupled with the superior strength, thermal, and electrical properties of nanotubes, provides a versatile tunable, multifunctional tribological system. KEY WORDS: carbon nanotubes, coefficient of friction, micro-tribology, engineered surfaces A schematic of the MWNT film grown with a vertical orientation is shown in figure 1 (a). Scanning electron microscopy images of a free edge of the vertical film are shown in figures 1(b)-1(d). The vertically aligned film was grown by a chemical vapor deposition (CVD) process using ferrocene and xylene precursors [13]. Following CVD growth, the MWNT films are cleaned using an oxygen plasma treatment. The vertically aligned MWNT films are approximately 65 lm thick and 5% dense. The MWNTs were vertically aligned, with the last few micrometers from the top surface the films entangled and intertwined as shown in figures 1(b)-1(d). A sample of transversely orientated nanotubes was also prepared by mechanically removing the vertical MWNTs, sonicating in acetone and dispersing onto an identical quartz substrate. After drying, this transversely oriented nanotube film was found to be approximately 5 lm thick and was comprised of a distributed ensemble of entangled nanotubes oriented in plane with the quartz substrate (figure 1(e) and 1(f)).The mechanical, electrical, and thermal properties of individual nanotubes are highly anisotropic [14][15][16][17][18], and the frictional behavior was also recently found to be anisotropic [6]. These friction experiments used a countersurface made from a borosilicate glass pin (schematically shown in figure 2 (a)) and were run in a regime where wear was not observed. Under both inert gas and ambient conditions, the transversely distributed films were repeatedly found to have friction coefficients l $ 0.1, while the vertically aligned sample had l $ 0.9. Molecular dynamics studies report nanotube films can accommodate relative motions or slip by rolling, sliding, or a combination of rolling and sliding at the interface [19]. However, this mobility related hypothesis seems unlikely considering the degree of nanotube ent...

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