We report the observation, by transmission electron microscopy, that single-crystal silicon and germanium are converted to an amorphous state at room temperature directly under both Vickers and Knoop indentations. The effect is seen for crystal orientations of [001], [Oil], and [111], and with applied loads between 0.1 and 0.5 N. We also observe that the materials become electrically conducting under load and that the process is reversible on subsequent unloading and reloading. Furthermore, the transformed phase is found to make Ohmic contact to the surrounding, untransformed, semiconductor.PACS numbers: 64.70.Dv, 64.70.Kb, 72.60.+g It has long been recognized that extremely high (hydrostatic and deviatoric) stresses are generated when a sharp indenter is loaded onto a flat surface of a solid. The exact details of the stress fields depend on the shape of the indenter and on the constitutive equations describing flow in the particular material, a subject that has now become the province of the theories of the hardness of materials and its measurement by indentation methods.' The existence of high hydrostatic stress has led investigators to suggest that the indentation experiment could be used to study high-pressure phase transformations, although it is recognized that the large deviatoric stresses make calculations difficult. Indeed Eremenko and Nikitenko, 2 and more recently Pirouz, Chaim, and Samuels, 3 report the formation of the hexagonal polymorph (Si IV) when silicon is indented at temperatures of 450-700 °C. Similarly, there is the suggestion by Gerk and Tabor 4 that silicon, germanium, and diamond may undergo a semiconductor-to-metal transition, as the pressures under an indenter are similar to those predicted for such a transition. We report here our findings of a rather different phase transformation in both silicon and germanium, the transition, through an electrically conducting state under load, to a metastable amorphous form after unloading, directly under an indentation. Such a crystailine-to-amorphous transition has not been observed in conventional high-pressure experiments.As part of our studies of cracks in brittle solids we have prepared samples containing arrays of small indentations together with their attendant cracks for transmission electron microscopy. Single crystals, mechanically ground and polished to a thickness of -100 /zm, were patterned with arrays of regularly spaced indentations with typically 100 indentations in a 3-mm-diam disk. The indentation impressions, produced by pyramidal diamond indenters having either the Vickers (148° included edge angle) or Knoop (asymmetric pyramid with included edge angles of 130° and 172.5°) configuration, were formed with loads in the range 0.1 and 0.5 N. In all cases the constant loading and unloading rates were 16.67 Ns" 1 . With the top, indented, surface protected by a glass cover slip, the samples were then ion thinned from the back until suitably thin for viewing in the transmission electron microscope. Figure 1 is a transmission electro...

The effects of indentation on the electrical resistance of rectifying gold-chromium contacts on silicon and germanium have been studied using nanoindentation techniques. The DC resistance of circuits consisting of positively and negatively biased contacts with silicon and germanium in the intervening gap was measured while indenting either directly in the gap or on the contacts. Previous experiments showed that a large decrease in resistance occurs when an indentation bridges a gap, which was used to support the notion that a transformation from the semiconducting to the metallic state occurs beneath the indenter. The experimental results reported here, however, show that a large portion of the resistance drop is due to decreases in the resistance of the metal-to-semiconductor interface rather than the bulk semiconductor. Experimental evidence supporting this is presented, and a simple explanation for the physical processes involved is developed which still relies on the concept of an indentation-induced, semiconducting-to-metallic phase transformation.

The toughness behavior of 30 wt % glass fiber reinforced PA6/PA66 blends colored with different masterbatches containing carbon black (CB) was characterized by the instrumented Charpy impact test. Two different CB types with different particle diameters as well as two different polymers, PE and PA6, were used to prepare the masterbatches. The CB concentration was varied from 0 to 1.2 wt % in the compounds and all materials were examined dry and after water absorption. The toughness of the compounds significantly decreased when CB was incorporated. Moisture conditioning of the materials led to increased toughness and ductility but did not compensate for the negative influence of CB. Using PE as a masterbatch polymer succeeded in limiting the influence of CB on toughness whereas the largest particle diameter led to the highest reduction in toughness. By taking into account crack resistance curves, it could be shown that there is a significant change in crack propagation behavior when the concentration of the larger particle CB exceeds a certain level; this was ascribed to the existence of complex CB structures at this concentration.

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To investigate the influence of moisture and EPR-g-MA content on the fracture behavior of glass-fiber reinforced PA6 materials, brittle-to-tough transition temperatures (T btt ) were determined. Water absorption was taken into account by conditioning the analyzed materials. Tensile tests could reveal the temperature range of the largest moisture dependence of mechanical properties between 10 and 50 C. J-integral values were used to describe the fracture behavior under conditions of impact load as a function of temperature. The brittle-to-tough transition of reinforced polyamides was found to be less approximate than in unreinforced materials. Two different characteristic temperature points T s and T e were identified, which were the intercept between elastic and elastic-plastic deformation on the one hand and the starting point of dominating stable crack propagation with strong plastic deformation on the other hand. Characteristic brittle-to-tough transition temperatures T btt could be calculated as the arithmetic average of these two points.