Rainer Telle

Direct Inkjet Printing of Dental Prostheses Made of Zirconia

CAD/CAM milling systems provide a rapid and individual method for the manufacturing of zirconia dental restorations. However, the disadvantages of these systems include limited accuracy, possible introduction of microscopic cracks, and a waste of material due to the principle of the ‘subtractive process’. The hypothesis of this study was that these issues can be overcome by a novel generative manufacturing technique, direct inkjet printing. A tailored zirconia-based ceramic suspension with 27 vol% solid content was synthesized. The suspension was printed on a conventional, but modified, drop-on-demand inkjet printer. A cleaning unit and a drying device allowed for the build-up of dense components of the size of a posterior crown. A characteristic strength of 763 MPa and a mean fracture toughness of 6.7 MPam0.5 were determined on 3D-printed and subsequently sintered specimens. The novel technique has great potential to produce, cost-efficiently, all-ceramic dental restorations at high accuracy and with a minimum of materials consumption.

Reactive formation of coatings at boron carbide interface with Ti and Cr powders

Abstract Boron carbide, B4C, is an attractive candidate material for reinforcement in metal matrix composites, whose application is severely hampered by its reactions with most engineering alloys at the high processing or service temperatures. The reactivity of B4C with some of the metals, however, may be made use of to create protective coatings on its surface. In the present research, the microstructure of coatings obtained by the interaction of B4C with Ti and Cr powders at 1000–1200 °C was investigated employing X-ray diffraction, scanning electron microscopy and Auger electron spectroscopy. Coatings obtained by treating B4C in Ti powder were found to contain Ti carbide, TiC1− x, and Ti borides (TiB2 and TiB). A relatively thin inner layer of the coating was carbide-free and contained only borides, while the major part of the coating was a mixture of TiC1− x and TiB. In contrast to this, coatings formed by reaction of B4C with Cr powder contained no carbides, and were shown to consist of Cr borides (CrB2, CrB, Cr5B3 and Cr2B) and amorphous carbon. A thick outer layer of the coating was carbon-free and consisted almost entirely of CrB. In both cases, the growth of the coatings was controlled by diffusion, the activation energy for the growth of B 4 C Ti coating being approximately 175 KJ mol . The phase composition, layer sequence and morphology of the coatings obtained were interpreted on the basis of kinetic and thermodynamic data of the ternary systems involved. A good agreement between the experimental results and theoretical predictions was obtained.

Manufacturing of individual biodegradable bone substitute implants using selective laser melting technique

The additive manufacturing technique selective laser melting (SLM) has been successfully proved to be suitable for applications in implant manufacturing. SLM is well known for metal parts and offers direct manufacturing of three-dimensional (3D) parts with high bulk density on the base of individual 3D data, including computer tomography models of anatomical structures. Furthermore, an interconnecting porous structure with defined and reproducible pore size can be integrated during the design of the 3D virtual model of the implant. The objective of this study was to develop the SLM processes for a biodegradable composite material made of β-tricalcium phosphate (β-TCP) and poly(D, L)-lactide (PDLLA). The development of a powder composite material (β-TCP/PDLLA) suitable for the SLM process was successfully performed. The microstructure of the manufactured samples exhibit a homogeneous arrangement of ceramic and polymer. The four-point bending strength was up to 23 MPa. The X-ray diffraction (XRD) analysis of the samples confirmed β-TCP as the only present crystalline phase and the gel permeations chromatography (GPC) analysis documented a degradation of the polymer caused by the laser process less than conventional manufacturing processes. We conclude that SLM presents a new possibility to manufacture individual biodegradable implants made of β-TCP/PDLLA.

Solid solutions of silicon in boron-carbide-type crystals

Abstract The optical absorption spectrum of silicon-doped boron carbide in the spectral range of the absorption edge and its low-energy tail, obtained from transmission measurements between 0.25 and 4 eV, is compared with that of undoped boron carbide (B 4.3 C) with comparably low distortions. Silicon is proved to be an effective dopant of boron carbide, because the edge absorption spectrum and the plasma edge are considerably changed. However, the conduction remains p-type for the sample investigated. The IR phonon spectrum of silicon-doped boron carbide is similar to that of boron-rich boron carbide, which contains a considerable amount of chainless unit cells. It is shown that the Si atoms occupy these cells, forming a two-atom chain. From a comparison of the Raman spectrum with those of similar structures and theoretical calculations, it can be estimated that the force field constant in such chains varies linearly with the distance between the atoms.

Cytocompatibility of high strength non-oxide ceramics.

Oxide ceramic materials like alumina (Al(2)O(3)) and zirconia (ZrO(2)) are frequently used for medical applications like implants and prostheses because of their excellent biocompatibility and high wear resistance. Unfortunately, oxide ceramics cannot be used for minimal invasive thin-walled implants like resurfacing hip prostheses because of their limited strength. The hypothesis of this study is that non-oxide ceramics like silicon nitride (Si(3)N(4)) and silicon carbide (SiC)-not previously used in the medical field-are not only high strength and mechanically reliable ceramic materials due to their high amount of covalent bonds, but also exhibit a suitable biocompatibility for use as medical implants and prostheses. Mechanical investigations and cell culture tests with mouse fibroblast cells (L929) and human mesenchymal stem cells (hMSC) were performed on the ceramics. An excellent cytocompatibility was demonstrated by live/dead stainings for both L929 cells and hMSC. HMSC were able to differentiate towards osteoblasts on all tested ceramics. The determined strength of silicon nitride and silicon carbide was shown as significantly higher than that of oxide ceramics. Our results indicate that the high strength non-oxide ceramics are material candidates in the future especially for highly loaded, thin-walled implants like ceramic resurfacing hip prostheses.

Design, Construction and Performance of Silicon Nitride Tool Parts in Steel Thixoforming

The design and performance of silicon nitride (Si3N4) dies for the semi-solid processing of steels is studied by the example of a punch and a lower swage in upset forging. The observed failure mechanisms and degradation effects are related to short-term effects resulting from mechanical and thermal loads and long-term effects owing to chemical and tribological attack. Results show that well-defined process conditions (cycle time, solidification time in the die) and a ceramic-suitable design are a pre-requisite for reliability and sufficient service life of ceramic dies.

Self-diffusion of boron in TiB2

Self-diffusion studies of boron in polycrystalline TiB2 were carried out as a function of temperature, using a specially designed experiment with stable 10B tracers, 11B−enriched TiB2 samples, and secondary ion mass spectrometry for depth profiling. The diffusivities were extracted from the isotope depth profiles in the range between 950 and 1600 °C. They obey an Arrhenius behavior with an activation enthalpy of about ΔH=2.2 eV and a preexponential factor of D0=4×10−12 m2/s. Interpolation of the diffusivities to the melting point of 3225 °C reveals a very low value of about D(Tm)≈10−15 m2/s, which reflects the covalent bonds present in the material. A possible explanation for the low values obtained for D0 and ΔH is the assumption of a diffusion mechanism via vacancies, where in addition to thermal vacancies a substantial concentration of structural vacancies are present. The possible influence of grain boundaries and of the anisotropic crystal structure on the results is discussed together with crystallo...

Calcium phosphate scaffolds mimicking the gradient architecture of native long bones

The synthesis of beta-tricalcium phosphate (β-TCP) scaffolds offering both the macroporous inner structure required for proper in vivo degradation and a non-macroporous outer structure for the enhancement of mechanical properties continues to be a challenge. The hypothesis of this study was to realize biomimetic β-TCP scaffolds with a macroporous inner structure and a compact outer structure using a lost wax casting technique. The porosity, macropore size, interconnectivity of the inner porous structure, and diameter of the outer compact structure were adjusted to specific values using a three-dimensional wax printer to manufacture the wax molds for the casting process. After the slip casting, the wax was pyrolyzed and the specimens were sintered. The resulting graded β-TCP scaffolds (porous + compact) were characterized and compared with β-TCP scaffolds with overall apparent macropores (only porous) and samples without macropores (only compact). The porosity and the compressive strength of the only compact, porous + compact, and only porous β-TCP samples were 31.4 ± 0.4 vol %, 55.6 ± 0.9 vol %, and 66.9 ± 0.4 vol % and 192 ± 7 MPa, 36 ± 2 MPa, and 9 ± 1 MPa, respectively. The macropore size was 500 µm and the micropore size was up to 10 µm, both featuring a completely open porous structure. From these results, we conclude that the lost wax casting technique offers an excellent method for the fabrication of β-TCP scaffolds with an inner macroporous structure and compact outer structure which mimics the cancellous and cortical structure of natural bone.

Improved mechanical long-term reliability of hip resurfacing prostheses by using silicon nitride

Although ceramic prostheses have been successfully used in conventional total hip arthroplasty (THA) for many decades, ceramic materials have not yet been applied for hip resurfacing (HR) surgeries. The objective of this study is to investigate the mechanical reliability of silicon nitride as a new ceramic material in HR prostheses. A finite element analysis (FEA) was performed to study the effects of two different designs of prostheses on the stress distribution in the femur–neck area. A metallic (cobalt–chromium-alloy) Birmingham hip resurfacing (BHR) prosthesis and our newly designed ceramic (silicon nitride) HR prosthesis were hereby compared. The stresses induced by physiologically loading the femur bone with an implant were calculated and compared with the corresponding stresses for the healthy, intact femur bone. Here, we found stress distributions in the femur bone with the implanted silicon nitride HR prosthesis which were similar to those of healthy, intact femur bone. The lifetime predictions showed that silicon nitride is indeed mechanically reliable and, thus, is ideal for HR prostheses. Moreover, we conclude that the FEA and corresponded post-processing can help us to evaluate a new ceramic material and a specific new implant design with respect to the mechanical reliability before clinical application.

Development of a self-heating ceramic tool for the thixoextrusion of high melting point alloys

A novel ceramic tool concept allows near-isothermal steel thixoextrusion experiments. Thermal shock impacts are successfully eliminated from the load profile of conventional semi-solid processing technologies of high melting point alloys. Thus, the application of thermal shock sensitive oxide ceramics exhibiting excellent corrosion resistance as forming dies is feasible. Extruded steel parts show high shape accuracy at very low extrusion forces.