T. Scholz

Technische Universität Hamburg-Harburg, Hamburg, Hamburg, Germany

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Publications (12)26.59 Total impact

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    ABSTRACT: Objectives/Methods: From a materials scientist's perspective, dental materials used for tooth repair should exhibit compatible mechanical properties. Fulfillment of this criterion is complicated by the fact that teeth have a hierarchical structure with changing mechanical behavior at different length scales. In this study, nanoindentation with an 8 μm spherical indenter was used to determine the elastic/plastic transition under contact loading for enamel. Results: The indentation elastic/plastic transition of enamel at the length scale of several hundreds of hydroxyapatite crystallites, which are within one enamel rod, is revealed for the first time. The corresponding penetration depth at the determined indentation yield point of 1.6 GPa and 0.6 % strain is only 7 nm. As a consequence of the small depth it is decisive for the experiment to calibrate the indenter tip radius in this loading regime. The elastic modulus of 123 GPa was evaluated directly by the Hertzian penetration and not by the unloading part of the indentation curve. Significance: We believe these data are also a valuable contribution to understand the mechanical behavior of enamel and to develop nanoscale biomimetic materials.
    Dental materials: official publication of the Academy of Dental Materials 11/2009; · 2.88 Impact Factor
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    ABSTRACT: Using atomic force microscopy and piezoresponse force microscopy (PFM), as well as complementary dielectric measurements, the ferroelectric domain structure of single crystals of pure and ruthenium (Ru)-doped 0.9 Pb ( Zn <sub>1/3</sub> Nb <sub>2/3</sub>) O <sub>3</sub> ( PZN )-0.1 PbTiO <sub>3</sub> ( PT ) , and Ru /( Zn + Nb + Ti )∼0.002 was analyzed. The coexistence of tetragonal and nontetragonal ferroelectric domains in PZN-PT near the morphotropic phase boundary (MPB) was directly observed via PFM imaging. The incorporation of Ru in the perovskite structure substantially decreases the ferroelectric domain size, thus reducing the polar fraction distributed in the pseudocubic matrix. The polarization and strain hysteresis loops show that Ru doping leads to significant ferroelectric hardening, which is attractive for high-power applications. The local structure and valent state of the incorporated Ru cations were additionally analyzed by electron paramagnetic resonance spectroscopy. Our results demonstrate that doping of tetragonal PZN-PT near MPB is a promising route to design advanced multifunctional single-crystal materials.
    Journal of Applied Physics 11/2009; · 2.21 Impact Factor
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    ABSTRACT: The nanoindentation in out-of-plane domains of a 001 oriented BaTiO 3 single crystal was performed with a conical indenter of 0.9 m tip radius. The first pop-in occurs at 5 0.5 mN load. The surface deformation was studied with atomic force microscopy and piezoresponse force microscopy. In addition to dislocation structures inside the remanent indentation, a rosette arm pattern is observed. The four identified screw arms are due to created a-domains as well as to at the free surface emerging dislocations of the 11011 ¯ 0 glide system. For parts of the screw arms, a detailed analysis of the domain and dislocation structure is presented. © 2008 American Institute of Physics. The ferroelectric and ferroelastic hysteric electrome-chanical behavior of barium titanate BaTiO 3 and PbZr 1−x Ti x O 3 PZT under macroscopic bulk loading is due to domain switching. 1–4 Moreover, not astonishingly, the fracture behavior of these materials is determined by the movement of ferroelectric domain walls. 4–8 Although these results suggest that domain switching processes are dominant in the deformation of ferroelectric ceramics, there is growing evidence from nanoindentation experiments at BaTiO 3 that dislocations significantly contribute to the nonlinear deformation. 9,10 The existence of grown-in dislocations due to the sinter-ing stress in BaTiO 3 is known from transmission electron microscopy TEM studies. 11,12 However, only one 21 year old publication discusses the initiation of dislocations by a mechanical scratch test. 13 Recently, the existence of disloca-tions around nanoindents in BaTiO 3 was deduced from an-isotropic pileups 9,10 and its similarity with screw arm struc-tures known in MgO and ionic crystals. 14–16 A detailed atomic force microscopy AFM-surface topography study and oriented TEM cross sections of the indents showed paired parallel slip lines. 17 This evidence of dislocation nucleation and movement around nanoindents naturally ex-plains the hardness as in metal plasticity. In addition, the experimentally determined increasing hardness with smaller spherical tip radii allows an interpretation applying the con-cept of geometrically necessary dislocations. 10,18,19 However, besides these convincing results proving that the 110 11 ¯ 0 glide system determines decisively the deformation under contact loading in BaTiO 3 and, probably, also in PZT, there is a clear evidence that also domain wall motion and eventually nucleation takes place extensively. 20 For a better understanding of the micromechanics of contact deformation, the following investigation aims for de-termining the very first occurrence of dislocations and changed domain pattern under nanoindentation loading.
    Applied Physics Letters 01/2008; 92. · 3.79 Impact Factor
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    ABSTRACT: Nanoindentation measurements in tetragonal lead zirconate–titanate (PZT) ceramics doped with 0.8 mol% of La 2 O 3 with a composition close to the morphotropic phase boundary (MPB) were performed using indenters with different tip radii. The elastic modulus was independent of indenter size but varied as expected with the Zr/Ti ratio of the samples. On the other hand, the calculated mean pressure remained constant for each indenter above the first pop-in, also varying with the Zr/Ti ratio, but was in all samples clearly dependent on the indenter radius. Indentation size effect (ISE) in tetragonal PZT was partially explained by the concept of 'geometrically necessary dislocations'. However, the experimental data also indicate that the actual ISE model needs to be slightly extended to include the ferroelastic domain switching contribution.
    Journal of Physics D Applied Physics 01/2008; 41. · 2.53 Impact Factor
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    ABSTRACT: Plastic behavior during loading of ferroelectric bulk materials is commonly explained by domain switching processes. In this study the authors show that dislocation induced deformation also occurs during nanoindentation of BaTiO3. Studies were conducted using a spheroconical shaped indenter together with observations using atomic force microscopy (AFM) and focussed ion beam cross sections for transmission electron microscopy (TEM) of the contact areas. Force/displacement measurements showed the sudden appearance of “pop-in” events, which AFM/TEM observations revealed were associated with plastic deformation and dislocation arrays at the surface. TEM cross sections of the indents showed paired parallel slip lines with 45° to the original surface.
    Applied Physics Letters 08/2007; 91(6):062903-062903-3. · 3.79 Impact Factor
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    ABSTRACT: Atomic Force Acoustic Microscopy has been proven to be a powerful technique for materials characterization with nanoscale lateral resolution. This technique allows one to obtain images of elastic properties of materials. By means of spectroscopic measurements of the tip-sample contact-resonance frequencies, it is possible to obtain quantitative values of the mechanical stiffness of the sample surface. For quantitative analysis a reliable relation between the spectroscopic data and the contact stiffness is required based on a correct geometrical model of the cantilever vibrations. This model must be precise enough for predicting the resonance frequencies of the tip-sample interaction when excited over a wide range of frequencies. Analytical models have served as a good reference for understanding the vibrational behavior of the AFM cantilever. They have certain limits, however, for reproducing the tip-sample contact-resonances due to the cantilever geometries used. For obtaining the local elastic modulus of samples, it is necessary to know the tip-sample contact area which is usually obtained by a calibration procedure with a reference sample. In this work we show that finiteelement modeling may be used to replace the analytical inversion procedure for AFAM data. First, the three first bending modes of cantilever resonances were used for finding the geometrical dimension of the cantilever employed. Then the normal and in-plane stiffness of the sample were obtained for each measurement on the surface to be measured. A calibration was needed to obtain the tip position of the cantilever by making measurements on a sample with known surface elasticity, here crystalline silicon. The method developed in this work was applied to AFAM measurements on silicon, zerodur, and strontium titanate.
    Journal of Physics Conference Series 04/2007; 61(1):293.
  • Applied Physics Letters 01/2007; 91(6):062903-062903-3. · 3.79 Impact Factor
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    ABSTRACT: Nanoindentation tests in an 90°-ac-domain area of an 001 orientated barium titanate single crystal were performed using four different indenters two with cube corner and two with spherical shape with tip radii from 61 nm to 1.9 m. Extensive calibrations of the tips on fused quartz and sapphire defined the penetration depth range for approximately spherical contact prior to indentation of barium titanate BaTiO 3 . The measured elastic modulus is independent of the different indenters. The measurements showed plastic deformation after "pop-in". The calculated mean pressure remained constant for each indenter, but clearly depends upon the indenter radius. The indenter radius dependence of the hardness support the concept of "geometrically necessary dislocations", proposed by W. 2002. The results show this concept fits the data generated with indenter radii which are at least an order of magnitude lower than investigated by Swadener. Furthermore, the results agree with estimates of the statistically stored dislocation density determined for BaTiO 3 . © 2006 American Institute of Physics. Nanoindentation is widely used for measuring the elastic modulus, E, and the hardness, H, of small volumes of material and thin films. 1 A more critical appraisal of the force-displacement response of the samples during nanoin-dentation experiments can however provide far more signifi-cant insights into the mode and onset of plastic deformation or fracture of a material. Recently, it is has become possible to perform indenta-tion tests at dimensions of tens to hundreds of nanometers using nano-and microindentation methods. At these small indentation depths classic plasticity theory predicts constant hardness using a geometrically self-similar indenter on a ho-mogenous material. Nevertheless a strong size dependent indentation hardness result is well known for metallic materials. This so called "indentation size effect" ISE characterized by an increasing hardness up to a multiple of the macroscopic hardness as the indentation depth is re-duced to the order of microns or submicrons. This phenom-enon was interpreted by Nix and Gao, 2 based on the works of Fleck et al., 3 and Ma and Clarke. 4 Nix and Gao showed that the ISE for crystalline materials can be explained using the concept of "geometrically necessary dislocations", which leads to a strain gradient plasticity law. Swadener et al. 5 extended this model for the case of spherical indenters. Recently Feng and Nix 6 showed for small indentations that the ISE model overestimates the hardness of MgO single crystals. The present work focuses on the nanoindentation behav-ior of a 001 oriented BaTiO 3 giant grain using four differ-ent indenters with tip radii from 61 nm up to 1.9 m. This material displays classic dislocation associated with plastic deformation. For loads up to and exceeding that to initiate pop-in at the beginning of the elastic/plastic part of the load-ing curve all indents can be described by an approximately spherical contact. They were analyzed using the approach of Swadener et al., 5 which proposes that spherical indenters show a dependence of hardness on the indenter radius rather than on the depth of the penetration. In this work, we report on nanoindentation tests with two spheroconical CON1, CON2 and two nominally cube corner CC1, CC2 shaped indenters with different effective tip radii. The full loading range of our testing device Triboscope, Hysitron, Minneapolis, USA, from 0.25 mN to 10 mN was used. More than 80 unloading curves on a fused quartz sample Hysitron, Minneapolis, USA were fitted to calibrate every indenter tip area function using the method of Oliver and Pharr. 1 Furthermore, the calibration of the sharpest cube corner CC1 was improved by additional indentations on sapphire, especially for small penetration depths. For the investigation, 001 oriented BaTiO 3 giant grains with typical dimensions of 500 m were prepared in house, described elsewhere. 7 A series of indents were carried out in a 90°-ac-domain area for which the force-displacement curves were acquired. The slopes of the unloading curves lead to a measured stiffness S versus indentation depth h function.
    Applied Physics Letters 03/2006; 46(50). · 3.79 Impact Factor
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    ABSTRACT: TiSiN coatings have been prepared by reactive DC magnetron co-sputtering on Coming glass and carbon steel substrates, using Ti-Si targets, with a constant Si:Ti area ratio of 0.2. The flow rate of nitrogen has been varied from 1.6 to 7.0 sccm, for a fixed argon flow rate of 25.0 sccm. We present a study of structure (texture, crystallite size and microstrain), chemical composition, and mechanical properties of the coatings and their dependence on the argon/nitrogen ratio. Moreover, a study of the thermal stability of the coatings has been performed by means of thermal annealing under oxidizing conditions (air atmosphere) at 500 and 600 degrees C. Coatings with the smallest crystallite size (similar to 2 nm) present the highest hardness (26 GPa) and the best thermal stability.
    01/2006: pages 93-98;
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    ABSTRACT: Nanoindentation tests in an aa-in-plane domain area of an {001} oriented barium titanate single crystal were performed using a conical indenter with a tip radius of 800 nm. The topography and the polarization vectors of the area after indentation were imaged afterwards by both atomic force and piezoresponse force microscopy (PFM), respectively. Two perpendicular oriented cracks in the {110} planes were identified in the topographic image. An unexpected considerable uplift occurs inside the residual impression, which was correlated with a sharp pop-out-like behavior observed in the force-displacement curve just prior to unloading. Furthermore, PFM revealed an almost a twofold symmetric arrangement of the domains around the indent, which can be explained by residual circumferential tensile stresses around a residual impression and was unambiguously correlated to the crystal orientation.
    Applied Physics Letters. 01/2005; 86(19):192903.
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    ABSTRACT: Indentation tests with loads between 0.5 and 10 mN were performed on fused quartz, (0001) oriented sapphire and (001) oriented barium titanate. The resulting submicron cracks were used to determine the fracture toughness KIC of the tested samples. The indentation crack length method was applicable, but a c/a dependency of the constant of proportionality was found. In addition, a very effective and simple approach—using the extra penetration of the indenter, due to the formation of cracks, so called pop-in—was used to determine KIC. © 2004 American Institute of Physics.
    Applied Physics Letters 04/2004; 84(16):3055-3057. · 3.79 Impact Factor
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    ABSTRACT: The mechanical properties of thin amorphous tungsten-carbon films (W-X C-Y) deposited on unheated substrates by non-reactive DC-cosputtering were characterized by nanoindentation as a function of the penetration depth of the Berkovich indenter. The results were analyzed using the work-of-indentation model. Films with Y in the range of 65-70at% show maximum hardness of about 22GPa, value comparable to the measured for a bulk WC reference sample. Chemical composition and structure were determined using other techniques such as X-ray diffraction, scanning electron microscopy as well as Raman and Auger electron spectroscopies. (C) 2004 Elsevier Ltd. All rights reserved.
    Vacuum. 01/2004; 76(2-3):173-176.