An investigation of an influence of loading parameters on fractal dimension of electrotechnical ceramics fracture has been presented. Investigation objects were samples made of C110 and C130 ceramics. Specimens were breaking under cyclic and static loading conditions. To estimate fractal characteristics a method used for metals fracture had been adapted. Fractal dimensions were estimated for profiles obtained by the vertical cross section method. Obtained results show that fractal dimension does not characterises the material. That dimension does not characterises loading conditions, as well.

The absence of a plastic phase in boron carbide and its failure at shock impact velocities just above the Hugoniot elastic limit (HEL) has been a puzzle for a long time. In the present work, using self-consistent field density functional simulations we are able to account for many experimental observations by noticing that several boron carbide polytypes [(B11C)C2B, (B12)C3, etc …] coexist without significant lattice distortions. Our analysis also indicates that above a threshold pressure all the candidate microstructures are less stable than a phase involving segregated boron (B12) and amorphous carbon (a-C) but the energetic barrier between boron carbide and B12 + 3C, is by far lower for the B12(CCC) microstructure, requiring the lowest atomic displacement for a transformation B4C→3B+a-C, occurring at pressures of 6 GPa = P(HEL). For such a configuration, segregation of free carbon occurs in layers orthogonal to the (113) lattice directions, in excellent agreement with recent transmission electron microscopy (TEM) analysis

It has been known for about 15 years that when a Vickers indenter is loaded on to a crystalline semiconductor, such as silicon, a semiconductor to metallic phase transition occurs during indenter loading and on removal of the indenter the material within the residual indentation becomes amorphous. Here we report on a completely opposite effect: when a Berkovich or Vickers diamond indenter is loaded on to a submicrometre thick film of amorphous germanium, it densifies, crystallizes and undergoes structural phase transitions. These observations are based on transmission electron microscopy and Raman scattering investigations. It has also been shown that the indentation-induced crystallization and phase transitions occur close to the indenter tip, where the plastic strains are the highest.

A method for prediction of deformation twin modes from analysis of the necessary atomic shuffles was developed for monazite (monoclinic LaPO4). This method is applied to scheelite (tetragonal CaWO4), xenotime (tetragonal YPO4), and zircon (tetragonal ZrSiO4). The predictions are compared to observations in these materials. Merit of the predictive method is discussed.

Static microindentation as well as dynamic displacement-sensitive indentation (DSI) tests have been performed between RT and 450°C on the (001) surface of undoped GaAs. In the static tests, the 4-fold symmetry reduces to 2-fold symmetry in some studied range of temperatures. The temperature dependence of the indentation diagonal and the crack length has been measured and, from disappearance of indentation cracks, the indentation brittle-to-ductile transition (IBDT) temperature TIBDT has been estimated. It is found that the temperature at which a transition in the energy density in DSI tests occurs nearly coincides with TIBDT as measured by static indentation tests; for undoped GaAs, both these temperatures are in the range of 200-225°C. This is about 100°C lower than the true brittle-to-ductile temperature TBDT, as measured by 4-point bending tests. The difference between TIBDT and TBDT is explained in terms of the superimposed hydrostatic stress component in the indentation experiments, and the crack asymmetry around the indentations is interpreted in terms of the different mobilities of α and β dislocations in GaAs.

Plasma-enhanced chemical vapor deposited (PECVD) silicon oxide (SiOx) thin films have been widely used in MEMS to form electrical and mechanical components. In this paper, both the time-independent and the time-dependent plastic responses of the PECVD SiOx films were studied by the instrumented nanoindentation experiments. Our experiments found an enhanced rate-sensitivity and size-effect in the plastic responses of the PECVD SiOx thin films. In addition, the plastic flow behavior is more homogeneous compared with most inorganic glasses and many metallic glasses. The deformation mechanism in the PECVD SiOx thin films is depicted by the shear transformation zone (STZ) based amorphous plasticity theory. The physical origin of the STZ is elucidated and linked with the plastic deformation dynamics.

Wall-patterned GaAs surfaces have been elaborated by photolithography and dry etching. Different surfaces were produced in order to change the aspect ratio of the walls formed at the substrate surface. The mechanical behaviour of individual walls was investigated by nanoindentation and the responses were compared to that of a standard bulk reference (flat surface). Deviation from the bulk response is detected in a load range of 0.1-25 mN depending on the aspect ratio of the walls. A central-plastic-zone criterion is proposed in view of TEM images of indented walls and allows predicting the response deviation of a given wall knowing its width. The application of substrate patterning for optoelectronic devices is proposed in the perspective of eliminating residual dislocations appearing in mismatched structures.

Porous SiO2 low-dielectric-constant films containing different porosities and sizes of pores were prepared in this study. Their mechanical properties were analyzed by a nanoindentation test. The hardness and elastic modulus of the films prepared with an ethanol molar ratio of 3 and an aging time of 16 hours reached maximum values of 2.4 and 40 GPa, respectively. With a higher ethanol ratio, the porosity increased, and the mechanical properties consequently decreased. With increasing aging time, the mechanical properties increased and then dropped due to enlarged pore sizes. The porous SiO2 films were found to yield at an ultimate stress of 3.1 GPa, and the maximum fracture energy release rate was calculated as 3.4 J/m2. The deformation and fracture behavior was observed through crack initiation and propagation along the large amount of pores.

In this paper we summarize recent progress in applying large-scale atomistic studies of crack dynamics along interfaces of dissimilar materials. We consider two linear-elastic material strips in which atoms interact with harmonic potentials, with a different spring constant in each layer leading to a soft and a stiff strip. The two strips are bound together with a weak potential whose bonds snap early upon a critical atomic separation. An initial crack serves as initiation point for the failure. We will focus on the maximum speed of mode I loaded cracks along such a biomaterial interface. Upon nucleation of the crack, it quickly approaches a velocity a few percent larger than the Rayleigh-wave speed of the soft material. After a critical time, we observe that a secondary crack is nucleated a few atomic spacings ahead of the crack. This secondary crack propagates at the Rayleigh-wave speed of the stiff material. If the elastic mismatch is sufficiently large, the secondary crack can be faster than the longitudinal wave speed of the soft material, thus propagating supersonically. Supersonic crack motion is clearly identified by two Mach cones in the soft material. This suggests that mother-daughter mechanisms, formerly only reported in mode II cracks in homogeneous materials, play an important role in interfacial mode I crack dynamics. The studies reported in this paper exemplify the usage of large-scale atomistic simulation to investigate the physics of fracture.

The response of a material to a sharp contact loading, as in the case of Vickers indentation for instance, provides a unique insight into the material constitutive law, including elastic and irreversible deformation parameters as well. However, under such peculiar thermodynamical and mechanical conditions (the mean contact pressure on the contact area reaches values typically higher than 1 GPa, corresponding to the hardness of the material) the deformation processes are complex and the matter located just beneath and around the contact area may experience some structural changes and behave in a way different to the expected - or known - macroscopic behaviour. It is showed in this study by means of detailed topological investigations of the residual indentations by Atomic Force Microscopy (AFM) that the elastic recovery typically represents 50 to 70 % of the indentation volume at maximum load and that the densification contribution may reach 90 % of the residual deformation volume. Besides, most glasses exhibit indentation-creep phenomena, which become significant over time scale of few minutes because of a pronounced shear-thinning behavior..

We report a study of dynamic cracking in a silicon single crystal in which the ReaxFF reactive force field is used for ∼3,000 atoms near the crack tip while the other 100,000 atoms of the model system are described with a simple nonreactive force field. The ReaxFF is completely derived from quantum mechanical calculations of simple silicon systems without any empirical parameters. Our results reproduce experimental observations of fracture in silicon including details of crack dynamics for loading in the [110] orientations, such as dynamical instabilities with increasing crack velocity. We also observe formation of secondary microcracks ahead of the moving mother crack. We conclude with a study of Si(bulk)-O2 systems, showing that Si becomes more brittle in oxygen environments, as known from experiment.

Plasticity in amorphous silicon (a-Si) as modeled by the Stillinger-Weber (SW) potential was investigated using structure relaxation by potential energy minimization. Irreversible stress drops in the observed mechanical response are the source of plastic deformation in this model directionally bonded material. Every such stress drop was found to be accompanied by atomic rearrangements that give evidence of the existence of unit inelastic shearing events that are characteristic of a-Si and account for plasticity in this material.