Characterisation of the growth of a carbonaceous film on silicon

In this paper we present the first characterisation of growth of a carbonaceous film on a silicon substrate exposed to a metastable atom beam using an in situ rotating polariser ellipsometer. The initial deposition of oil due to a background partial pressure in vacuum is investigated. Subsequent exposure of the deposited oil to a high flux metastable neon (Ne*) beam results in cross-linking of the oil film, creating a polymerised carbonaceous layer. Values for the mean residence time, polymerisation cross-section, and desorption cross-section are calculated and compared to similar studies performed for ion bombardment. Simple estimates can provide reasonable values for application of the theory to other systems.     

Fiber-reinforced plastic composites for energy absorption purposes

The energy-absorbing characteristics in axial collapse of structures made from fiber-reinforced plastic (FRP) composites are reviewed from both the materials and the structural viewpoints. The type and nature of the fiber and resin, the geometry of the structure and the fiber arrangement affect significantly the energy-absorbing capability, but the data currently available are not yet sufficient to give a clearly unambiguous answer for a particular situation. An effective trigger mechanism is necessary to initiate stable collapse since thin-wall structures in particular are prone to unstable collapse. Methods are described to estimate the energy-absorbing potential of an FRP composite structure.     

Early stages of bump formation on the surface of ion-bombarded graphite

We investigate possible mechanisms for the formation of bumps, of atomic dimensions, in the surface layer of graphite under low-energy, single ion bombardment. We perform a Molecular Dynamics simulation and analyse the results in view of recent experimental data obtained with the Scanning Tunnelling Microscope. Only the early stages of bump formation are described. The depth profile of momentum deposited in the cascade is evaluated and used, in conjunction with the atomic density profile, to characterise interlayer distortions. The surface bumps appear under the presence of stresses developed in the near-surface layers of the target, due to defects created by the collision cascade. The bump originates around the positions of vacancies and interstitials, and is stabilised by the presence of interlayer interstitials. We have also produced a video film that helps in the visualisation of the time evolution of topographical features.     

Craze formation and growth in anisotropic polymers

Craze formation and craze growth in anisotropic polymers has been studied as a function of the degree of anisotropy and the relation between the testing direction and the primary orientation direction. Tests on PMMA and PC indicate that both the morphology and orientation of the crazes are senstivie functions of testing direction. Crazes form in directions which arenot orthogonal to the principal tensile stress, and the data clearly show that craze growth occurs in directions governed by the major principal strain. The fracture process is identical in nature to that in isotropic polymers, i.e. craze formation, crack nucleation within the craze and subsequent crack propagation through the craze. Thus, the angle of fracture coincides with the craze angle rather than occurring perpendicular to the principal tensile stress.     

Testing the optimality properties of a dual antibiotic treatment in a two-locus,two-allele model

Mathematically speaking, it is self-evident that the optimal control of complex, dynamical systems with many interacting components cannot be achieved with ‘non-responsive’ control strategies that are constant through time. Although there are notable exceptions, this is usually how we design treatments with antimicrobial drugs when we give the same dose and the same antibiotic combination each day. Here, we use a frequency- and density-dependent pharmacogenetics mathematical model based on a standard, two-locus, two-allele representation of how bacteria resist antibiotics to probe the question of whether optimal antibiotic treatments might, in fact, be constant through time. The model describes the ecological and evolutionary dynamics of different sub-populations of the bacterium Escherichia coli that compete for a single limiting resource in a two-drug environment. We use in vitro evolutionary experiments to calibrate and test the model and show that antibiotic environments can support dynamically changing and heterogeneous population structures. We then demonstrate, theoretically and empirically, that the best treatment strategies should adapt through time and constant strategies are not optimal.     

Phenomenology of fracture in hot-pressed silicon nitride

Fracture phenomenology in hot-pressed silicon nitride has been studied fractographically as a function of flaw size, temperature and loading rate. Surface cracks of controlled size were introduced using the microhardness indentation technique. At room temperature, the fracture stress was found to depend on initial crack size according to the Griffith relationship and extrapolation of the data indicated that inherent processing flaws of the order of 12 to 24 m are strength-controlling in virgin material. Using a simplified Griffith approach, the fracture surface energy, , at 20° C for hot-pressed Si₃ N₄ is about 22 000 erg cm^–2. Two mechanistic regimes were manifest in the temperature dependence of the fracture stress. A mixed mode of fracture consisting of transcrystalline and intergranular crack propagation occurred up to 1100° C; at 1200° C and above, subcritical crack growth (SCG) occurred intergranularly and the extent of SCG increased with increasing temperature. Similarly, the extent of SCG decreased with increasing loading rate.     

Fracture phenomenology of a lithium-aluminium-silicate glass-ceramic

Crack propagation mechanisms in a lithium-aluminium-silicate glass-ceramic have been studied as a function of both initial flaw size and temperature. Using controlled surface cracks, the fracture stress at room temperature was found to conform to the Griffith flaw size dependence. Extrapolation suggests that processing flaws of the order of 6 to 8 m are strength-controlling in the material investigated. Two mechanistic regimes were manifest in the temperature dependence of the fracture stress. Up to 900° C, catastrophic transgranular crack propagation occurred from emplaced cracks. At 1000° C and above, subcritical crack growth occurred intergranularly and the extent of slow crack growth prior to catastrophic failure increased with increasing temperature. The influence of loading rate on slow crack growth and fracture stress was explored at 950 and 1100° C. Generally, the extent of slow crack growth decreased with increased loading rate until at a sufficiently high rate, catastrophic fracture occurred directly with no slow crack extension. These results are discussed in terms of the role of plastic accommodation in the crack extension process, a phenomenon which seems mechanistically dependent upon remnant amorphous (uncrystallized) phase at grain boundaries.     

The relationship between discontinuous yielding and cyclic behavior in polycrystalline molybdenum

The fatigue properties of polycrystalline molybdenum at room temperature have been determined by axial tension-compression tests in both stress and strain control modes to give lives of up to 10⁶ cycles. Under the appropriate conditions, the endurance limit can be considerably reduced,e.g., the stress to cause failure in 10⁶ cycles decreases from ～42,000 psi at 5 cps to ～28,000 psi at 0.5 cps. The fatigue resistance as assessed by either strain control or by stress control is discussed in relation to the tensile and creep behavior, and it is concluded that the reduction in the long life fatigue resistance with decrease in the cyclic frequency is due to the time dependent generation of a mobile dislocation population. The previously accepted value of 0.6 for the ratioFL/UTS for bcc metals can be attributed to this frequency effect. A lower value for this ratio, 0.4, can be derived from the high frequency data by the use of an extrapolation technique. This lower value is confirmed by long life, low frequency tests, in agreement with the value for fcc metals. The importance of obtaining fatigue data under comparable states of mobile dislocation density is emphasized, particularly for materials exhibiting pronounced tensile yield points, in order that meaningful comparisons can be made between different materials.