Christoph Berthold

Direct evidence for continuous linear kinetics in the low-temperature degradation of Y-TZP

The kinetics of the tetragonal to monoclinic (t-m) transformation of zirconia in a hydrous environment at 134°C and 3 bar pressure was studied. As surface X-ray diffraction, which is conventionally used to explore the progress, has a very limited depth of information, it distorts the quantitative results in a layer-on-layer situation and by itself is ill suited for this reason. Analyzing cross sections is more suitable; therefore, focused ion beam techniques were used to prepare artifact-free cuts. The material was subsequently investigated by scanning electron microscopy, electron backscatter diffraction and Raman spectroscopy. Only the combination of methods makes it possible to resolve the quantifiable details of the process. The transformation starts in the near-surface areas, forms a layer, and the growth of this layer proceeds into the bulk material following a simple linear time law (0.0624 μm h(-1) for material in the chosen condition), without apparent retardation or limit. The progress yields a gradientless layer with a fixed amount of residual tetragonal zirconia (~27% for 3Y-TZP in the present conditions) separated from unaffected material by a boundary, which has a roughness only in the grain size range. The kinetics indicates a reaction rate control, where the hydration reaction is the key factor, but is modified by the stepwise access of water to the reaction front opened by the autocatalytic transformation of zirconia with a critical hydration level.

XRD2 micro-diffraction analysis of the interface between Y-TZP and veneering porcelain: Role of application methods

OBJECTIVES The metastability of the tetragonal crystal structure of yttria partial stabilized zirconia polycrystalline (Y-TZP) ceramics is a basis of concern for dental restorations. Reactions between the porcelain and the Y-TZP framework may result in a reduction of the stability of the zirconia and interface bonding caused by a transformation from tetragonal to monoclinic crystalline structure during veneering. METHODS XRD(2) micro-diffraction measurements were carried out on tapered veneered cross-sections of the interface area to generate locally resolved information of the phase content in this region. To get a high intensity X-ray beam for short measurement times a focussing polycapillary with a spot size of app. 50 microm was used to evaluate the porcelain zirconia interface. RESULTS Under almost all conditions the phase composition of zirconia grains at the interface revealed both the monoclinic and tetragonal structure. These observations indicate that destabilization of the tetragonal phase of zirconia occurs at the interface during veneering with porcelain. SIGNIFICANCE These results and their relevance to the long-term stability of the interface adhesion between zirconia and veneering porcelain as well as the tetragonal to monoclinic crystal transformations at the interface are discussed.

Structural characterization of nanocrystalline hydroxyapatite and adhesion of pre-osteoblast cells

Nanocrystalline hydroxyapatite (Nano HA), a prototype of minerals of bones and teeth, attracts increasing interest in medicine and dentistry. Different parameters for synthesis and post-treatment were investigated to determine their effects on crystallinity of nano HA, and in vitro cell responses to nano HA were studied. XRD and TEM analyses indicate that the crystallinity of nano HA synthesized by a chemical method was within the range of 15–50 nm, which is adapted to natural minerals of hard tissues. Increasing the ageing temperature significantly increased the crystallinity of nano HA, while lengthening the ageing time or varying the post-ageing drying process did not have any influence on its crystallinity. Nano HA annealed between 300 and 900 °C showed a small increase in crystallinity with increasing annealing temperature due to the long-range ordering effect. Cell attachment and spreading on nano HA were lower than those on pure titanium, and decreased as the crystallinity of nano HA increased. However, cells on nano HA demonstrated well-developed filopodia and lamelliopodia, which facilitate migration of the cells on it. This may benefit osteogenesis at the interface between bone and nano HA in vivo.

Long-time aging in 3 mol.% yttria-stabilized tetragonal zirconia polycrystals at human body temperature.

We present new findings on the low-temperature degradation of yttria-stabilized zirconia at 37°C over several years and at high and low partial pressures of water. With the aid of focused ion beam cross-section confirmation studies we are able to show an extensive linear, continuous degradation without retardation, even at low temperatures and low water pressures. The characteristic layer growth and its inferred rate constant imply a lifetime of tens of years under simple tension and open the possibility of studying the longevity of these ceramics more rigorously. In addition, we show reproducibility complications of accelerated aging tests by the use of different autoclaves and possible implications for standardized procedures.

Sea urchin spines as a model-system for permeable, light-weight ceramics with graceful failure behavior. Part I. Mechanical behavior of sea urchin spines under compression

The spines of pencil and lance urchins Heterocentrotus mammillatus and Phyllacanthus imperialis were studied as a model of light-weight material with high impact resistance. The complex and variable skeleton construction (“stereom”) of body and spines of sea urchins consists of highly porous Mg-bearing calcium carbonate. This basically brittle material with pronounced single-crystal cleavage does not fracture by spontaneous catastrophic device failure but by graceful failure over the range of tens of millimeter of bulk compression instead. This was observed in bulk compression tests and blunt indentation experiments on regular, infiltrated and latex coated sea urchin spine segments. Microstructural characterization was carried out using X-ray computer tomography, optical and scanning electron microscopy. The behavior is interpreted to result from the hierarchic structure of sea urchin spines from the macroscale down to the nanoscale. Guidelines derived from this study see ceramics with layered porosity as a possible biomimetic construction for appropriate applications.

Characterization of Fibres and Fibre Collectives with Common Laser Diffractometers

The method described utilises the effect that in many commercially available laser diffractometers a laminary flow of the suspension medium in the measurement cell exists. However, data analysis carried out using commercially available laser diffractometers is normally based upon the assumption that there is a statistical orientation of the particles in the measurement volume. The resulting diffraction patterns are, therefore, assumed to be centrosymmetric and ring-shaped. As a consequence, the detectors commonly used only record parts of the diffraction patterns. Based upon these assumptions, it is accepted that grain size analysis of fibrous particles gives an equivalent diameter between length and diameter. First experiments carried out using a Malvern Mastersizer X showed that fibres align in the flow direction. Analysis of the entire diffraction pattern should, therefore, provide information about the length and diameter of the fibres.

Sea Urchin Spines as a Model-System for Permeable, Light-Weight Ceramics with Graceful Failure Behavior. Part II. Mechanical Behavior of Sea Urchin Spine Inspired Porous Aluminum Oxide Ceramics under Compression

Sea urchin spines were chosen as a model system for biomimetic ceramics obtained using starch-blended slip casting. Porous alumina ceramics with cap-shaped layers with different alternating porosities were found to have superior fracture behavior under bulk compression compared to ceramics with uniform porosity. They fail in a cascading manner, absorbing high amounts of energy during extended compression paths. The porosity variation in an otherwise single phase material mimicks the architectural microstructure design of sea urchin spines of Heterocentrotus mammillatus, which are promising model materials for impact protection.

Particle Size and Shape Characterization of Kaolins-Comparison of Settling Methods and Laser Diffraction

It is well known that grain size distributions of clayey raw materials determined by sedimentation methods differ significantly from grain size distributions measured with laser diffractometers due to the anisometric shapes of the particles. Investigation on commercial Czech kaolins by XRD and SEM revealed that the differences of the grain size distributions can be attributed to different mineral contents of the fractions and varying habiti of the kaolinite. It is shown that a numerical analysis of the development of shape with grain size (“SSD-curve”) can translate between the grain size determination methods. The shape of those SSD-curves is regular and offers the possibility to evaluate the phase content of the kaolins.

Developing the Experimental Basis for an Evaluation of Scaling Properties of Brittle and ‘Quasi-Brittle’ Biological Materials

The development of lightweight structures exhibiting a high energy dissipation capacity and a locally adapted puncture resistance is of increasing interest in building construction. As discussed in Chap. 7, inspiration can be found in biology, as numerous examples exist that have evolved one or even several of these properties. Major challenges in this interdisciplinary approach, i.e. the transfer of biological principles to building constructional elements, are scaling (different dimensions) and (at least for the botanic examples) the fact that different material classes constitute the structural basis for the functions of interest. Therefore, a mathematical description of the mechanical properties and the scalability is required that is applicable for both biological and technical materials. A basic requisite for the establishment of mathematical descriptions are well-defined test setups rendering a reliable data basis. In the following, two biological role models from the animal and plant kingdoms are presented, namely, sea urchin spines and coconut endocarp, and two experimental setups for quasi-static and dynamic testing of biological and bio-inspired technical materials are discussed.

Plants and Animals as Source of Inspiration for Energy Dissipation in Load Bearing Systems and Facades

From the manifold strategies that nature offers to materials under overload conditions, we describe two: the fibrous and multi-layered system of the bark of the Giant Sequoia, which possesses an impressive damping mechanism, and the spines of pencil and lance sea urchins. The latter introduce a new concept to energy dissipation in brittle construction materials, namely quasi-ductility by multiple local fracturing. The potential for transfer as bioinspired technical solutions is high as the biological role models combine several advantages such as lightweight, recyclability and high protective efficiency. We demonstrate that, in principle, the concepts found in the biological role models can be transferred to concrete-based building materials.