Jakob Kuebler

Mechanical properties of ZrB2–SiC ceramic composites: room temperature instantaneous behaviour

Abstract Abstract The mechanical properties of ZrB2–30 wt-%SiC ultra high temperature ceramic composites have been studied. The composite was processed by hot pressing at 2100°C, 30 MPa and 45 min dwell time to achieve a good densification. Young’s modulus, single edge V notch beam fracture toughness, hardness, stress–strain deformation, four-point bending strength and Weibull parameters were measured. Fractography and microstructure analyses of ZrB2–30 wt-%SiC ceramic composite were also performed.

Macrostructural engineering of ceramic-matrix layered composites

Abstract A method of computer simulation of the fracture of layered composites during a 3-point bending test is developed. A model of failure is considered which can be applied to two-component brittle layered composites (in particular ceramic-matrix composites). This particular model is executed for composites with the number of layers N=3, 7 and 15. The model is applied for the description of mechanical behaviour of two-component ceramic-matrix layered composites. The trends of the theoretical calculations agree with data obtained from 3-point bending test of real ceramic-matrix layered composites.

Enhanced fracture toughness by ceramic laminate design

Abstract A review of the potential toughening and failure mechanisms for ceramic laminate materials is presented. An integrated approach to the design of ceramic laminates incorporating biaxial residual stresses for specific applications is outlined. Restrictions placed on the laminate architecture to avoid spontaneous transverse cracking of the tensile layer are discussed. The phenomena of edge cracking and crack bifurcation are considered with reference to elastic moduli, Poissons ratio, mismatch in thermal expansion coefficients, temperature gradient and laminate architecture. The use of compressive layers to produce a material that exhibits a threshold strength and criteria for increasing the critical applied stress below which failure will not occur are reported. A single edge V-notched beam (SEVNB) test geometry was used to measure crack growth resistance (R curve) behaviour of multilayer Si3N4/Si3N4–TiN composites. Fracture mechanics weight function analysis was applied to predict the R curve behaviour of multilayer composites having a stepwise change in composition. A conservative, non-optimised laminate design exhibiting apparent fracture toughness in excess of 17 MPa m1/2 is reported, in excellent agreement with the weight function analysis.

Microstructural engineering of ceramic-matrix layered composites: Effect of grain-size dispersion on single-phase ceramic strength

Abstract A failure model is considered which can be applied to n-phase brittle materials (in particular to ceramics). The authors attempt to solve a physical problem of the description of failure of a micro-inhomogeneous solid as stochastic process of the cracking of separate structural elements. The particular model is executed for n=1 (case of single-phase ceramics) and is applied for the description of the mechanical behaviour of single-phase ceramic layers with various statistical distributions of the grains sizes. An effective continuum and a statistical description of the failure process is used. There are four stages in the process. The first stage is loading without microcracking and the second stage is a stable non-localized microcracking before maximum stress. The third stage is a stable localized microcracking after stress maximum and the fourth stage is an unstable (catastrophic) fracture. The trends of the theoretical calculations agree with data obtained from acoustic emission studies.

High Toughness Ceramic Laminates by Design of Residual Stresses

Multilayered ceramic composites are very promising materials for different engineering applications. Laminates with strong interfaces can provide high apparent fracture toughness and damage tolerance along with the high strength and reliability. The control over the mechanical behavior of laminates can be obtained through design of residual stresses in separate layers. Here we report a development of tough silicon nitride based layered ceramics with controlled compressive and tensile stresses in separate layers. We design laminates in a way to achieve high compressive residual stresses in thin (100-150 micron) Si 3 N 4 layers and low tensile residual stresses in thick (600-700 micron) Si 3 N 4 -TiN layers. The residual stresses are controlled by the amount of TiN in layers with residual tensile stresses and the layers thickness. The fracture toughness of pure Si 3 N 4 (5wt%Y 2 O 3 +2wt%Al 2 O 3 ) ceramics was measured to be of 5 MPa m 1/2 , while the apparent fracture toughness of Si 3 N 4 /Si 3 N 4 -TiN laminates was in the range of 7-8 MPa m 1/2 depending on the composition and thickness of the layers.

Cytotoxicity evaluation of polymer-derived ceramics for pacemaker electrode applications

Ceramics are known to be chemically stable, and the possibility to electrically dope polymer-derived ceramics makes it a material of interest for implantable electrode applications. We investigated cytotoxic characteristics of four polymer-derived ceramic candidates with either electrically conductive or insulating properties. Cytotoxicity was assessed by culturing C2C12 myoblast cells under two conditions: by exposing them to material extracts and by putting them directly in contact with material samples. Cell spreading was optically evaluated by comparing microscope observations immediately after the materials insertion and after 24 h culturing. Cell viability (MTT) and mortality (LDH) were quantified after 24-h incubation in contact with the materials. Comparison was made with biocompatible positive references (alumina, platinum, biocompatible stainless steel 1.4435), negative references (latex, stainless steel 1.4301) and controls (no material present in the culture wells). We found that the cytotoxic properties of tested ceramics are comparable to established reference materials. These ceramics, which are reported to be very stable, can be microstructured and electrically doped to a wide range of conductivity and are thus excellent candidates for implantable electrode applications including pacemakers.

Mechanical properties of three-dimensional interconnected alumina/steel metal matrix composites

Three-dimensional interconnected alumina/steel metal matrix composites (MMCs) were produced by pressureless Ti-activated melt infiltration method using three types of Al2O3 powder with different sizes and shapes. By partial sintering during infiltration an interpenetrating ceramic network was realised. The effect of the ceramic particle size and shape on the resulting ceramic network, volume % fraction and the MMC properties is presented. The MMCs were characterised for mechanical properties at room temperature and elevated temperature. An increase in flexural strength and Young’s modulus with decreasing particle size has been observed. In addition, the effect of the volume of ceramic content and the surface finish of the MMCs on the wear behaviour is shown.

In-situ neutron diffraction of LaCoO3 perovskite under uniaxial compression. II. Elastic properties

Calculations of elastic constants and development of elastic anisotropy under uniaxial compression in originally isotropic polycrystalline LaCoO3 perovskite are reported. The lattice strains in individual (hkl) planes as well as average lattice strain were determined both for planes oriented perpendicular and parallel to the loading direction using in-situ neutron diffraction. Utilizing average lattice strains as well as lattice strains along the a and c crystallographic directions, an attempt was made to determine Poissons ratio of LaCoO3, which was then compared with that measured using an impulse excitation technique. The elastic constants were calculated and Youngs moduli of LaCoO3 single crystal in different crystallographic directions were estimated.

Are asymmetric metal markings on the cone surface of ceramic femoral heads an indication of entrapped debris

BackgroundThe probability of in vivo failure of ceramic hip joint implants is very low (0.004-0.05%). In addition to material flaws and overloading, improper handling during implantation can induce fractures of the ceramic ball head in the long term. Identifying the causes of an in vivo fracture contributes to improved understanding and potentially to further reduction of the fracture probability for patients. Asymmetric metal markings on the cone surface of in vivo ball head fractures have been reported. The question, therefore, is whether asymmetric loading is the sole cause or whether additional factors, specifically contamination entrapped in the taper fit, also contribute or are even the main cause.MethodsThe influence of the asymmetric physiological load configuration on resulting metal markings in the cone surface of an alumina femoral ball head with and without biological contaminants was investigated. Static and cyclic tests on ball heads were carried out in a load configuration of 0° (axisymmetric) and 40° in a physiological environment. The analysis of the metal marking was carried out to gain a better understanding of the processes that contribute to the generation of metal marking. Fractography was carried out to determine the fracture initiation of failed ball heads.ResultsDifferent types and sizes of residuals entrapped in the conical surface are shown to yield strongly asymmetric metal marking patterns. All heads tested without contaminants exhibited an almost homogenous distribution of residual metal markings around the circumference of the ceramic cone surface at the proximal end of the bore hole. The failure of ball heads that contained entrapped contaminants revealed a common fracture pattern. The site of fracture initiation on two of the failed heads was in the entrance region of the bore hole on the superior half of the head.ConclusionAsymmetric metal markings observed on the ball heads tested in this investigation are most probably caused by the presence of contaminants entrapped in the taper fit. Homogenous metal mark distributions around the circumference indicate proper assembly of the ball head without entrapped contaminants. It should, however, be noted that different taper designs may possibly result in different marking patterns.

Sintering Behavior and Phase Characterisation of Composite Perovskite/Fluorite Ceramics for Intermediate Temperature SOFCs and Oxygen Separation Membranes

The sintering behavior and structural changes of fluorite Gd 0.2 Ce 0.8 O 2-δ (GDC) with one of three perovskites: LaMnO 3 (LMO), (La 0.7 Sr 0.3 ) 0.98 MnO 3 (LSM) and La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3 (LSFC) composite ceramics were studied. Sintering was carried out for two hours at five different temperatures: 1100°C, 1200°C, 1300°C, 1400°C, and 1500°C. The highest sinterability has been found in LSFC + GDC. LSM + GDC appear to undergo two-step sintering mechanisms. Of all types of ceramics, only sintered LMO experiences a phase change, from rhombohedral R C to orthorhombic Pnma structure, with respect to its powder phase at sintering temperatures ≥1200 °C. The transition is notably suppressed when LMO is part of an LMO + GDC composite. GDC in a fluorite and/perovskite composite, when sintered, undergoes a temperature-dependent expansion in its unit cell that is not observed in pure GDC ceramics. This structural change will impact the function of composite ceramics as a fuel cell cathode, or oxygen separation membranes.