Gurdial Blugan

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.

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.

Failure Behaviour of High Toughness Multi-Layer Si3N4 and Si3N4-TiN Based Laminates

Multi-layer laminates were produced using alternating layers of Si3N4 and Si3N4+TiN. The differences in the coefficient of thermal expansions between the alternating layers lead to residual stresses after cooling. These are compressive in the Si3N4 layers and tensile in the Si3N4+TiN layers. The existence of these stresses in the laminates effect the crack propagation behaviour during failure. Different designs of laminates were produced with external layers under compression and tension exhibiting different failure mechanisms. Facture toughness was measured by SEVNB method. In systems with external layers under compression the measured fracture toughness was up to three times that of Si3N4, i.e. up to 17 MPa m1/2. In systems with external layers under tension during failure the energy absorbing effects of crack deflection and crack bifurcation were obtained. High temperature tests were performed to determine the onset temperature for residual stresses in these laminates. Micro-laminates with compressive layers of only 30 µm thickness with high strength and fracture toughness and were manufactured.

Structural Micro-Layered Ceramics with Surfaces under Tension and Compression with Increasing Apparent Fracture Toughness

Two different designs of high fracture toughness micro-laminate ceramics were produced containing 50 μm thick Si3N4 layers and 100 μm thick Si3N4 + TiN layers. The first design with external tensile layers had a predicted maximum apparent fracture toughness of 10.5 MPa m1/2. The second design with external compressive layers had a predicted maximum apparent fracture toughness of 18.0 MPa m1/2. The fracture toughness of these micro-laminates was tested by the SEVNB method. A stiff testing machine was used to measure the R-curve behavior by observing crack growth in single notched specimens. A soft testing machine was used to measure the R-curve behavior using several specimens with notches at different depths.

Multi-Layer Silicon Nitride Laminates Exhibiting High Fracture Toughness and Crack Deflection

Si3N4-TiN based multi-layer ceramics laminates have been produced. With external compressive layers, laminates with a three-fold increase in KIc over the monolithic ceramics have been realised. When external tensile layers are used in conjunction with thin internal compressive layers, energy absorbing crack deflection and bifurcation processes are observed.

Ceramic Spheres—A Novel Solution to Deep Sea Buoyancy Modules

Ceramic-based hollow spheres are considered a great driving force for many applications such as offshore buoyancy modules due to their large diameter to wall thickness ratio and uniform wall thickness geometric features. We have developed such thin-walled hollow spheres made of alumina using slip casting and sintering processes. A diameter as large as 50 mm with a wall thickness of 0.5–1.0 mm has been successfully achieved in these spheres. Their material and structural properties were examined by a series of characterization tools. Particularly, the feasibility of these spheres was investigated with respect to its application for deep sea (>3000 m) buoyancy modules. These spheres, sintered at 1600 °C and with 1.0 mm of wall thickness, have achieved buoyancy of more than 54%. As the sphere’s wall thickness was reduced (e.g., 0.5 mm), their buoyancy reached 72%. The mechanical performance of such spheres has shown a hydrostatic failure pressure above 150 MPa, corresponding to a rating depth below sea level of 5000 m considering a safety factor of 3. The developed alumina-based ceramic spheres are feasible for low cost and scaled-up production and show great potential at depths greater than those achievable by the current deep-sea buoyancy module technologies.

Rapid carbon nanotubes suspension in organic solvents using organosilicon polymers

A strategy for a simple dispersion of commercial multi-walled carbon nanotubes (MWCNTs) using two organosilicones, polycarbosilane SMP10 and polysilazane Ceraset PSZ20, in organic solvents such as cyclohexane, tetrahydrofuran (THF), m-xylene and chloroform is presented. In just a few minutes the combined action of sonication and the presence of Pt(0) catalyst is sufficient to obtain a homogeneous suspension, thanks to the rapid hydrosilylation reaction between SiH groups of the polymer and the CNT sidewall. The as-produced suspensions have a particle size distribution <1μm and remain unchanged after several months. A maximum of 0.47 and 0.50mg/ml was achieved, respectively, for Ceraset in THF and SMP10 in chloroform. Possible applications as polymeric and ceramic thin films or aerogels are presented.

Production of improved SiC and SiCN ceramics from polycarbosilane and polysilazane composites

With the aim of modifying the composition and microstructure of SiC and SiCN ceramics, we have investigated different reactive additives for polymer derived ceramic (PDC) precursors as well as changes in processing conditions (polymerization conditions, sintering conditions, etc.). Different concentrations of additive were dispersed in commercial carbosilane and silazane resins to give preceramic composites, which were pyrolyzed to ceramics. By adjusting the ratio of additive, the microstructure of resultant ceramics was affected as are the electrical and mechanical properties.With the aim of modifying the composition and microstructure of SiC and SiCN ceramics, we have investigated different reactive additives for polymer derived ceramic (PDC) precursors as well as changes in processing conditions (polymerization conditions, sintering conditions, etc.). Different concentrations of additive were dispersed in commercial carbosilane and silazane resins to give preceramic composites, which were pyrolyzed to ceramics. By adjusting the ratio of additive, the microstructure of resultant ceramics was affected as are the electrical and mechanical properties.

The influence of spinel and magnesia powder bed on mechanical properties of alumina sintered under air and nitrogen atmosphere

ABSTRACT The aim of the present work was changing the surface properties of alumina ceramic via sintering of samples in spinel MgAl2O4 and magnesia MgO powder bed. During sintering nitrogen and air atmosphere were used; and the reference material was sintered at the same conditions but in an alumina powder bed. All samples were sintered to a density higher than 99.2% of theoretical density of alumina. In order to examine samples SEM, EDX and XRD (X-ray diffraction from 18 to 70 °2Theta) analysis were performed together with B3B flexural strength measurement. Results revealed that by sintering of alumina samples in a spinel powder bed pore-free surfaces were prepared, which had a favourable effect on B3B flexural strength. This was ascribed to the high solubility of Al3+ in spinel at sintering temperatures, which increases the sintering driving force. A further positive effect on B3B flexural strength was observed when samples were sintered in nitrogen instead of air.

Oxidation and Corrosion of Silicon Nitride at 1200°C and 1500°C

The effect of different sintering additives on the high temperature oxidation and corrosion behaviour of Si3N4 ceramics is investigated. A corrosion furnace was set up which allows testing of ceramic discs at test temperatures upto 1500°C. It was possible to conduct tests under ambient oxygen conditions, or with increased water vapour as well as testing with corrosive gases such as HCl or SO2. Si3N4 was prepared with MgO, Al2O3, Y2O3 and Al2O3+Y2O3 sintering additives. These discs were subjected to different oxidation and corrosion test conditions at temperatures of 1200°C and 1500°C for upto 128 h. The effects that the different corrosion enviroments have on the corrosion resistance of the Si3N4 materials are presented.