Holleck et al. fabricated multilayer coatings with different ceramic materials, demonstrating that the improvements in adhesion, indentation toughness, hardness, and wear-resistance performance can be achieved with optimized layer thickness [39]. The improved performance of multilayer coatings is believed to partially attribute to the stress relaxation and crack deflection, which exists in various contact conditions with cyclic loading and fatigue. Besides the influence of yield strength, the stacking sequence also has influence on the coating performance. It was found that the alternation of layers with high/low shear modulus can provide more benefits for multilayer DLC/metal carbide coatings [40] or TiN/Ti coatings [41,42].
Chameleon 0.9 Crack Mac Osx
The fracture pattern on the Ti-TiN-(Ti,Al,Cr)N multilayer coatings with nanolayer thickness of (a) 302 nm and (b) 10 nm after the cutting tests with different cutting speeds (vc). (a) With nanolayer thickness of 302 nm, the formation of longitudinal cracks and internanolayer delaminations is typical. (b) With nanolayer thickness of 10 nm, rarer internanolayer delaminations can be observed. Reprinted with permission from [59].
The influence of element concentration and microstructure on the performance of Cr/CrN/CrTiN coatings was also investigated through various characterization techniques [88]. It was found that the increased Ti content in the coatings led to increased hardness and elastic modulus, and increased H/E and H3/E2 ratios. However, the increased Ti content in the coatings led to a decreased scratching toughness, which was attributed to the conformal cracking at higher load. In addition, the wear-resistance performance of the coatings decreased with increased Ti content, which was attributed to the mismatch between the modulus of coating and substrate. In general, they found that the multilayer structure can effectively reduce the residual stress compared to the monolayer coating, leading to higher adhesion stress and better wear-resistance performance.
In nature, shooting mechanisms are used for a variety of purposes including reproduction, prey capture, and defense. Shooting mechanisms evolved multiple times in a diversity of plant and fungal taxa, from the catapulting mechanisms in ferns, the water jet mechanisms in ascomycetes, and the air pressure gun of peat mosses (launching spores with accelerations up to 36,000g [1]) (g = magnitude of the gravitational acceleration [9.81 m/s2]), to the exploding seeds, fruits, and flowers of angiosperms (i.e. flowering plants). Similarly, fast movements occur in disparate animal groups, including stomatopods (marine crustaceans) that use a fast appendage strike to ambush prey, cnidarians that shoot stinging organelles at prey or foe with accelerations reaching 5,413,000g [2], and small chameleons that shoot their tongues with accelerations up to 264g (in the smallest specimens) to capture elusive prey [3]. Each of these highly effective shooting mechanisms fulfills specific functional demands and has evolved under the influence of natural selection [4]. This has resulted in several unique adaptations that can be linked to a range of successful adaptive radiations (see [5,6] for an example of adaptive radiation in lungless salamanders, family Plethodontidae).
We only considered articles in the English language and focused the literature search on solid projectiles. Only shooting mechanisms in which the projectile is produced in the body (e.g. a spore or seed) or is part of the body (e.g. chameleon tongue) and undergoes a ballistic trajectory were included. We excluded the following systems: (1) Shooting mechanisms in which foreign objects are used as projectiles (occurring often in throwing actions). (2) Liquid and gas projectiles, such as the Archer fish that shoots down prey from overhanging foliage with a fast, forceful water shot [12] and the Bombardier beetle that uses a liquid venom for defense [13]. (3) Jumping and throwing actions, sometimes referred to as shooting (e.g. the catapult mechanisms used by froghopper to jump with accelerations of up to 408g [14]). (4) Mechanisms in which shooting is directly triggered by the environment, such as spore and seed launch by means of raindrop impact [15] and buzz-pollination in flowers, in which pollen are ejected by means of bumble bee vibrations [16,17]. (5) Single-cellular or subcellular shooting mechanisms. (6) Relatively slow (tongues in some frogs [18]) and fast (tentacle strike in squid [19]) extensions by muscular hydrostats that are not truly ballistic.
The stored energy is released and transformed to kinetic energy of the projectile. In plants, the elastic energy is released by the fracture of molecular bonds or cavitation inside the cytoplasm of the cells. Similar release mechanisms occur in fungi. An exception is found in the fungi genus Sphaerobolus, where the stored elastic energy is released by the eversion of a membrane [10]. In animals, the stored elastic energy for shooting is either released by eversion of the projectile itself in cnidarians [29], relaxation of collagen fibers in the ballistic tongues in chameleons [30] and lungless salamanders [31], or release of a latch in stomatopods [25]. Unfortunately, not all release mechanisms in animals are known or sufficiently understood, such as those of frogs that use a rapid jaw movement to project their tongue [16]. Finally, the stored energy is transferred to the projectile, which gains kinetic energy, using a specific catapult mechanism.
Depending on the main function of the shooting mechanism, e.g. food capture or seed dispersal, different characteristics of the shooting mechanism are of importance, such as the launch velocity, launch acceleration, dispersal distance, spread, and accuracy. In plants and fungi, the dispersal distance and spread are the most important parameters. The effectiveness of the shooting mechanisms in terms of improving survival of the spore or seed can be mainly deducted from the dispersal patterns [69]. Seed or spore dispersal over even a short distance from the plant (beyond the canopy) or fungus can already increase the probability of seed or spore survival. However, high dispersal distances and spread tend to increase the survival rate and reproductive success [77,86,94]. Furthermore, by projecting spores into an airstream (wind), fungi or mosses can increase the probability of encountering susceptible hosts or environments [10]. To negate the negative effects of viscous drag on the launch distance, plants and fungi often synchronize the discharge of thousands of (often small) pollen, seeds, and spores, such as in ascomycetes [46], Sphagnum [1], and S. martensii [96], or optimize the shape of the projectile [45]. In animals, on the other hand, high launch acceleration, launch velocity, and accuracy are needed, as the shooting mechanisms are critical for territory and self-defense, prey capture, substrate attachment, and locomotion. For example, in chameleons and lungless salamanders, the tongue is critical for catching elusive prey, and in cnidarians the cnidocysts are important for self-defense and locomotion (amongst others) [97]. A trade-off seems to be present between the launch velocity (and acceleration) and accuracy, with faster shooting mechanisms being less accurate. For example, in anurans the inertial elongation catapult mechanism has an accuracy of approximately 33%, whereas the non-elastically enhanced and much slower hydrostatic mechanism has an accuracy of over 99% [16]. This difference in accuracy can be mainly led back to the lack of feedback control (i.e. the inability to adjust the trajectory during the shooting action) of the inertial elongation catapult and muscle-powered squeeze catapult. 2ff7e9595c
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