Bodo Wolf

Nanoindentation of diamond, graphite and fullerene films

Abstract The recently developed method of nanoindentation is applied to various forms of carbon materials with different mechanical properties, namely diamond, graphite and fullerite films. A diamond indenter was used and its actual shape determined by scanning force microscopy with a calibration grid. Nanoindentation performed on different surfaces of synthetic diamond turned out to be completely elastic with no plastic contributions. From the slope of the force–depth curve the Youngs modulus as well as the hardness were obtained reflecting a very large hardness of 95 GPa and 117 GPa for the {100} and {111} crystal surfaces, respectively. Investigation of a layered material such as highly oriented pyrolytic graphite again showed elastic deformation for small indentation depths but as the load increased, the induced stress became sufficient to break the layers after which again an elastic deformation occurred. The Young’s modulus was calculated to be 10.5 GPa for indentation in a direction perpendicular to the layers. Plastic deformation of a thin fullerite film during the indentation process takes place in the softer material of a molecular crystalline solid formed by C 60 molecules. The hardness values of 0.24 GPa and 0.21 GPa for these films grown by layer epitaxy and island growth on mica and glass, respectively, vary with the morphology of the C 60 films. In addition to the experimental work, molecular dynamics simulations of the indentation process have been performed to see how the tip–crystal interaction turns into an elastic deformation of atomic layers, the creation of defects and nanocracks. The simulations are performed for both graphite and diamond but, because of computing power limitations, for indentation depths an order of magnitude smaller than the experiment and over indentation times several orders of magnitude smaller. The simulations capture the main experimental features of the nanoindentation process showing the elastic deformation that takes place in both materials. However, if the speed of indentation is increased, the simulations indicate that permanent displacements of atoms are possible and permanent deformation of the material takes place.

Nanoscale mechanical properties of polymers irradiated by UV

Abstract The application of depth sensing nanoindentation to determine mechanical properties of three different polymers is described in this work using three different techniques to calibrate the measurement system. The nano-hardness and the elastic indentation modulus of polyvinyl chloride, polyethylene oxide and polyacrylic acid were inferred from nanomechanical tests, and the influence of ultraviolet irradiation on the mechanical properties of measured polymers is studied. A multicycling test—a sequence of several loading and unloading procedures—allowed the measurement of changes in the sample viscoelasticity. The nano-hardness of the polymers is shown to increase with radiation dose while the viscoelasticity decreases.

Ion-beam induced chemical and structural modification in polymers

In order to increase the sensitivity to moisture uptake of polyimide (PI) and polyethersulfone films applied in bimorphic humidity sensors 50, 130 and 180 keV boron ions with irradiation doses between 1013 and 1016 B+/cm2 were implanted. A complex investigation of the following features has been carried out: chemical changes in the surface regions by attenuated total reflection–FTIR spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS); optical properties by spectroscopic ellipsometry; hardness and elastic modulus by depth-sensing low-load indentation technique; conductivity of modified polymer films. It could be shown, that the partial destruction of chemical bonding under ion bombardment leads to the creation of new amorphous and graphite-like structures, which increase the surface film conductivity by several orders of magnitude, and enhances the sensitivity of these polymer films to moisture uptake. The ion-beam irradiation destroys the anisotropic features of the refractive index of PI layers leading to its isotropization. Radiation-induced changes in the layer structure result in an increase of the hardness and elastic modulus of the modified layers up to ten and six times, respectively. The hardness and refractive index depth profiles were determined. The detectable effective modification depth estimated from the depth profiles is 250–300 nm at an ion energy of 50 keV and 400–450 nm at an ion energy of 180 keV.

Atomistic modelling of ploughing friction in silver, iron and silicon

Molecular dynamics (MD) simulations of atomic-scale stick–slip have been performed for a diamond tip in contact with the (100) surface of fcc Ag, bcc Fe, Si and H-terminated Si, at a temperature of 300 K. Simulations were carried out at different support displacements between 5 and 15 A. The simulations illustrate the important mechanisms that take place during stick–slip. In particular, for the case of the metals they show a direct link between tip slip events and the emission of dislocations from the point of contact of the tip with the substrate. This occurs both during indentation and scratching. For the case of silicon, no slip events were observed and no subsurface dislocations were generated underneath the scratch groove. At the deeper support displacement of 15 A the silicon atoms undergo some local phase transformations and the atom coordination number varies between 5 and 8, with the majority being five-fold or six-fold coordinated. Both the dynamic and the static friction coefficients were found to be higher for Si compared to the corresponding values for H-terminated Si. Comparisons were made between the MD simulations and experimental measurements for indentation on the (100) surface of Si and Al. A good qualitative agreement was observed between the experimental and theoretical results. However, in both the cases of Si and metals the MD simulations give a contact pressure under load that is depth dependent and values that are higher than experimental nanohardness values.

Multi-cycling nanoindentation in MgO single crystals before and after ion irradiation

The paper presents a nanoindentation study of MgO single crystals before and after ion irradiation up to a fluence of 1020 Ar+ m−2. It is confirmed that crystalline MgO is a brittle material of comparatively high nanohardness, ranging from H = 12.5 to 14.5 GPa depending on surface orientation. The plastic deformation is based on a dislocation glide with formation of slip bands giving rise to piling up around the indent which is strongly related to the sample crystallography. Repeated loading–unloading cycles (multi-cycling) revealed the appearance of hysteresis loops that are related to nanofracturing. Irradiation with 100 keV Ar+ ions resulted in a reproducible hardness increase to a value of H = 19 GPa independent of surface orientation. The indentation modulus E = 285 GPa remained unaffected by ion irradiation. Furthermore, no hysteresis loop in the force–displacement curve was formed with multi-cycling after implantation. This is explained in terms of point defect assisted plasticity: defect pinning will decrease the dislocation mobility and hence increase the hardness and also cause the sample to become less brittle resulting in a large decrease in nanocrack formations. In contrast to the dislocation glide, the point defect contribution to plastic deformation is almost orientation independent. This picture is supported by the orientation independent hardness as well as by the observation that ion irradiated samples exhibit a reduction in, and more homogeneously distributed, piling up around the indent.

Mechanical properties of quasicrystals investigated by indentation and scanning probe microscopes

Hardness, elastic modulus and crack propagation were studied using micro and nanoindentation, atomic force microscopy and scanning acoustic microscopy. We present measurements performed on icosahedral AlPdMn (three-dimensional quasicrystal) and decagonal AlCuCoSi (two-dimensional quasicrystal) from room temperature to 550 °C. Additionally, YMgZn was impressed at room temperature. The surface of icosahedral specimens became fractured into segments exhibiting steps in height along shear cracks. Quantity of piling up as well as number and extension of cracks are smaller for the two-dimensional quasicrystalline material, which also displays a hardness anisotropy between different surface orientations. Quantitative hardness measurements revealed a strong indentation size effect exhibiting a hardness increase with decreasing load at room temperature, and the inverse behaviour for higher temperatures.

On the orientation dependence of crack-like failure formation near indentations on an icosahedral Al-Pd-Mn quasicrystal

Abstract Crack propagation near microhardness indentations on surfaces of an icosahedral Al70Pd23Mn7 single quasicrystal exhibiting fivefold and twofold symmetry has been studied under ambient conditions at room temperature using optical microscopy, atomic force microscopy and laser scan microscopy. Cracks exhibit a shear-like habit and prefer propagation along planes of symmetry 2, 3 and 5 rather than following the plane of maximum stress. A fracture toughness of 1.25 MPam½ has been derived from indentations on a fivefold plane at room temperature.

On the Impact of Light on Nanoindentations in ZnSe

The (111Zn) surface of single-crystalline ZnSe was subject to nanoindentations in darkness and under illumination using white light. A positive photoplastic effect was observed in the load range from 200 μN to 2 mN: a reduction of the penetration depth by about 5%, resulting in a reversible hardness increase by 10% under illumination. The effect was found to saturate already at 30 mW/ cm 2 , comparable to daylight intensity. There is also a photoplastic after-effect for some seconds, i.e. a reduced hardness increase was found after having turned off the light some seconds before performing the indent.

Investigation of Semiconductors by Nanoindentation

Si and InSb were subject to depth sensing multicycling nanoindentation. T he load-depth curves exhibited hysteresis loops which are explained in terms of pres sur induced phase transformations. In order to study the impact of crystal distortions on phase trans formation, the specimens were subject to boron implantation (ion energy 180keV) of different implanta tion doses (10 14 to 10 ions/cm) and indented without annealing. In InSb, the hysteresis loops disappea red after implantation of 10 ions/cm, and for Si with its stronger bonds a dose of 3*10 /cm is required for the same effect. In GaAs the contact pressure is not high enough to cause phase transitions. Characteristic pile-up patterns for the crystal symmetry are observe d inst ad.

Comparative morphology study of indentations in quasicrystalline AlPdMn and single-crystalline NiAl investigated by atomic force microscopy

Atomic force microscopy (AFM) was used to characterize surface modifications that developed from Vickers indentations into single-crystalline NiAl(100) and single-quasicrystalline AlPdMn (surface of fivefold symmetry). Indenter rotation was used to track the competition between crystal and indenter anisotropy. Distinct differences between the two model substances were found with respect to the lateral extension of the indentation-induced deformation zone, to crack formation, size, shape and radial extension of created structures, and volume balance between the impression and piled-up elevations.