Plasma-spray forming has been used to fabricate thick-wall tubes of MoSi2 and MoSi2 containing concentric layers of Al2O3. This process is being investigated as a potential fabrication method for producing tubular components of MoSi2 and MoSi2 composites for use in high temperature fuel-burner applications. Results will be reported on the spray forming method used to produce tubes of various sizes. The room temperature strength of pure MoSi2 tubes in the as-deposited condition, and after heat-treating at 1500 °C for 2 hours in vacuum, will also be reported. The strength of plasma sprayed MoSi2 tubes were measured via diametral compression of O-ring and C-ring sections in air at room temperature. Qualification of the strength distribution was based on Weibull statistical theory.

Commercial polysiloxanes filled with alumina nano-particles have been employed for the preparation of β-SiAlON-based ceramics in the temperature range 1450–1550°C in nitrogen atmosphere. The formation of β-SiAlON was found to be preceded by the formation of intermediate alumino-silicate phases. The SiAlON yield was affected by the occurrence of phase separation in the oxycarbide ceramic residue (SiOC) derived from the silicones and by the partial vaporization of silica, by reduction into gaseous SiO, leading to products with an oxide contamination, consisting of corundum. Filled silicones finally found a promising application in the ceramic joining, sandwiched between two pre-existing α–β (Yb-)SiAlON pieces and treated at high temperature (1550°C): with a proper formulation, a significant inter-diffusion was observed between the joining layer and the SiAlON parts, causing the evolution of a homogeneous joint region, matching the microstructure and the mechanical properties of the parent ceramics. The pre-oxidation of the SiAlON, generally aiding the wetting of the joining media prior to thermal treatment, showed no significant benefit on the microstructure. On the contrary, the addition of a small load during the thermal treatment allowed the formation of strong joints, not exhibiting any significant difference in mechanical properties with the parent material.

Room temperature tensile strengths of sapphire fibers were evaluated in the as-received condition (Dow MethocelTM sizing applied by Saphikon), after flame cleaning, after cold water washing, and after a chemical cleaning procedure. Flame cleaning yielded an approximately 30% degradation in strength compared to the other surface treatments. Fractographic analysis revealed surface deposits at the failure origins of flame cleaned fibers; EDS analysis revealed the presence of Na, CI, Ca, Fe, and Ce impurities in the deposits. Tensile strengths were also evaluated for unsized fibers in the as-received condition, after flame cleaning, and after self-abrasion. Flame cleaning of unsized fibers yielded a less significant strength degradation than was observed for sized fibers and self-abrasion resulted in a 30% degradation in strength. Results indicate that use of sizing is imperative for preventing fiber damage during handling, but that improved procedures for sizing removal are required.

The float process produces flat glass with a tin-rich surface due to contact with the molten metal bath. The incorporation of tin into the glass network is expected to modify the mechanical properties of the surface and the relative durability of the two sides of the material. In this work nanoindentation was used to evaluate the elastic modulus and hardness of a 2mm thick commercial float glass. The near-surface elastic modulus (depths<400nm) of both sides of the glass was elevated by up to 10%, and could not be attributed solely to the presence of tin. However, slight differences in hardness (<10%) between the air and tin sides of the float glass were observed. These results suggest that tin may alter the flow properties of the glass, but the elastic modulus changes are masked by other structural and chemical differences between the air and tin sides of the float glass.

Reliable interpretation of nanoindentation data often requires that corrections be made for geometric effects such as material pile-up. These corrections are often made with the assumption that the reference glasses used to calibrate the instrument are not susceptible to such effects. This letter presents the pile-up behavior of four silicate reference glasses, (a proprietary fused silica supplied by Hysitron, Inc., Corning 1737f, Corning 2947 (soda-lime-silica), and Schott BK-7, respectively), and correlates their tendency to pile-up with the free volume of their network structures. Irrespective of the reference glass that is used, shallow penetration depths (i.e., < 150nm) should be corrected for the errors induced by pile-up in the reference glass to avoid anomalous trends in modulus and hardness during nanoindentation studies.

Concurrent impression and uniaxial compression creep studies were performed on three Yb‐SiAlON materials. Stress exponents were approximately 1 in compression and 2 in impression. The higher stress exponents were due to the complex stress field in the impression creep test, which caused microstructural dilation. The dilated multi‐grain junctions also became filled with additional intergranular glassy phase. Focused ion beam milling and in situ lift‐out specimen preparation combined with transmission electron microscopy was successful in identifying microstructural changes after creep testing. These observations have important implications in the design of creep‐resistant materials in complex stress fields.

The goal of the study was to fabricate laminated WC-Co cutting tools with residual thermoelastic stress states tailored to counteract the thermal and mechanical stresses imposed by machining. Cutting tools were fabricated using tape casting and bulk powder compaction of submicrometre and nanograin WC-Co powders. The weight fraction of cobalt in the tool was graded to produce residual compressive stresses in the tool surface. Spark plasma sintering (SPS) was used to fully densify the laminates while suppressing cobalt redistribution via sub-eutectic (solid state) sintering. Microstructural analysis showed that the cobalt binder was not well distributed around the WC grains after sintering. Contiguity of the WC grains was high, and segregation of cobalt was apparent. These factors are expected to hinder machining performance. However, the laminated tools performed similarly to commercially-available, monolithic tools in turning Ti-6Al-4V alloy. This demonstrates that the incorporation of residual stresses into the surface of the tool is beneficial for wear resistance. Refinement of powder processing methods to produce laminated WC-Co tools with a better distribution of cobalt should yield a further increase in machining performance.

Laminated WC-Co cutting tools were fabricated with residual thermoelastic stress states tailored to counteract the thermal and mechanical stresses imposed by machining. The weight fraction of cobalt in the tool was graded to produce compressive stresses in the tool surface. Spark plasma sintering (SPS) was used to densify the laminates. Submicron and nanograin powders were used, though the nanograin tools had sintered grain sizes that were similar to the submicron tools. Microstructural analysis showed that the cobalt binder was not well distributed around the WC grains. Contiguity of the WC grains was higher than that of tools sintered by other methods, and pools of cobalt were apparent. The laminated tools performed similarly to commercially available tools in taming Ti-6Al-4V alloy. Performance of the laminated tools may have been hindered by the poor distribution of the cobalt binder. Refinements in powder processing methods are underway to produce tools sintered by SPS that outperform those currently available.

Different biaxial flexure test geometries were investigated to determine the most reliable test for evaluating the strength of float glass specimens before and after enameling. The enameled and unenameled samples were tested using the ring-on-ring (ROR) and ball-on-ring (BOR) strength tests. The strengths of each sample set were analyzed using a conventional two-parameter Weibull analysis. For direct comparison of the data, combined Weibull moduli were calculated for the unenarneled tin side, unenameled air side, and enameled tin side data sets. The principle of independent action (PIA) was assumed to be the appropriate fracture criterion and the data were scaled to determine if discrepancies existed between the different testing methodologies. The scale parameters (sigma(o)) were also calculated for the different test geometries. The results showed that there were no statistical differences between the scaled data. Based on the results it was concluded that the ROR test geometry was the better choice compared to the BOR test geometry because of the larger stressed area. The use of a concentric support ring was also found to be more desirable than a ring on equally spaced balls because of the stress concentrations at the support balls, which can lead to problems.

The effect of a glass enamel coating on the strength and fatigue behavior of float glass was investigated. Commercially available enamel that was comprised of Cu2Cr2O4 pigment particles in a bismuth‐zinc borosilicate glass matrix was applied to a soda–lime–silica float glass via screen printing, followed by fusion at elevated temperature. Strengths of the enameled specimens were evaluated in biaxial flexure using a ring‐on‐ring (ROR) test geometry, and the data were analyzed using a conventional two‐parameter Weibull distribution. Enameling was found to significantly degrade the strength of the float glass. There was no statistical difference in the characteristic strengths of samples enameled on the air side (66 MPa) compared with samples enameled on the tin side (61 MPa) of the float glass. Fractographic analysis revealed that the failures in the enameled float glass samples initiated at pores and pigment aggregates in the enamel, whereas failures in float glass samples initiated solely from surface flaws. Dynamic fatigue tests were performed on enameled float glass and indented float glass samples to determine the effect of the enamel on the stress corrosion behavior of the enameled components. There was no statistically significant difference between the stress corrosion exponents for the float glass and enameled float glass specimens.

Solid solubility was examined in Zr-doped sapphire and Al-doped yttria-stabilized zirconia (YSZ) single crystals from 1200degrees to 1600degreesC. Specimens were fabricated via ion implantation of single crystals, followed by annealing in air. Secondary ion mass spectroscopy (SIMS) was used to quantify solute redistribution during annealing. Comparison of SIMS results with analytical electron microscopy (AEM) revealed an alumina solubility of 0.2-0.3 wt% in zirconia, and a zirconia solubility of 0.004-0.027 wt% in alumina. Direct imaging of zirconia precipitates revealed that tetragonal zirconia precipitates from supersaturated sapphire with the following orientation relationship: (100)(tetragonal) parallel to (0001)(sapphire) and [0(1) over bar 1 ]tetragonal parallel to [1(2) over bar 10](sapphire).

Silicon oxycarbide (SiOC) ceramic foams, obtained from the pyrolysis of a preceramic polymer, were subjected to thermal multiple cycles from 800°–1200°C to room temperature in a water bath. Flexural and compression strengths, as well as elastic modulus, were characterized before and after quenching. Excellent thermal shock and cycling resistance behavior was observed, with only moderate strength and stiffness degradation. The phase assemblage of the foam remained unchanged, and no crack formation in the foams was observed. However, microstructural characterization revealed the development of porosity in the struts and cell walls due to the oxidation of residual carbon in the amorphous SiOC material, thereby contributing to a small decrease in stiffness after quenching.

The strength and dynamic fatigue behavior of float glass was investigated using biaxial flexure tests. The samples were tested using the ring‐on‐ring (ROR) biaxial flexure test geometry, and the data analyzed using a conventional two‐parameter Weibull distribution. The as‐received samples revealed that the air side exhibits a higher characteristic strength (243 MPa) compared with the tin side (114 MPa); fractographic analysis confirmed the presence of significantly larger flaws on the tin side of the specimens, presumably due to contact damage by the rollers in the float glass process. Dynamic fatigue results for as‐received and indented samples were performed to assess whether differences in the stress corrosion behavior of float glass exist because of tin penetration. No statistical difference in the stress corrosion exponent was found between the air (n= 21.7) and tin (n= 21.6) sides of the float glass. This indicates either that the tin penetration (which extends ∼25 μm) plays no role in altering the stress corrosion susceptibility of float glasses because the native flaw size is larger than the tin penetration depth or that the tests do not have the required sensitivity to distinguish the effect of the tin. Alternative test methods for direct observation of slow crack growth in tin‐doped bulk glasses are planned to investigate this in the future.

The mechanical properties of ceramic foams obtained through a novel process that uses the direct foaming and pyrolysis of preceramic polymer/polyurethane solutions were investigated. The elastic modulus, flexural strength, and compressive strengths were obtained for foams in the as‐pyrolyzed condition; values up to 7.1 GPa, 13 MPa, and 11 MPa, respectively, were obtained. The strength of the foam was virtually unchanged at temperatures up to 1200°C in air; however, long‐term exposure at 1200°C led to a moderate degradation in strength, which was attributed to the evolution of intrastrut porosity during the oxidation of residual free carbon, as well as devitrification of the foams struts.

Reactions between zirconyl nitrate hydrate and condensed phosphates can be used to produce castable low CTE sodium zirconium phosphate (NZP) monoliths. Reaction between sodium nitrate, zirconyl nitrate hydrate and condensed phosphoric acid at room temperature (alkali nitrate method) produces monoliths having a heterogeneous microstructure, which are multiphasic in appearance. Except for the presence of crystalline sodium nitrate, they are X-ray amorphous. Differential thermal analysis revealed two distinct exothermic crystallization events when these materials are heated. The first event, with an onset temperature of 650°C, is the result of NZP and ZrO 2 crystallization. The second is the result of ZrP 2 O 7 crystallization. Reaction between zirconyl nitrate hydrate and condensed sodium phosphate (condensed alkali phosphate method) results in a more homogeneous microstructure in which crystalline zirconium hydrogen phosphate hydrate and sodium nitrate are present. Two exothermic peaks, with onset temperatures of approximately 570 and 860°C, are observed. The first exotherm is the result of NZP, ZrO 2 and ZrP 2 O 7 crystallization; the second exotherm is the result of a further NZP formation. After heating materials made by these two methods at 940°C for 24 h, the condensed-alkali-phosphate-method-derived material converted to phase-pure NZP, while the alkali-nitrate-method-derived material contained ZrP 2 O 7 . The differences in phase evolution between the materials prepared by these two methods are attributable to the differences in chemical and microstructural homogeneity that result from the reactants used.

The effect of Si-O-C coatings on the tensile strength of Saphikon(TM) single crystal sapphire filaments and ceramic grade Nicalon(TM) fibers was examined. A moderate strengthening of Saphikon(TM) was observed for fibers in the as-coated form, and was attributed to suppression of environmentally-assisted crack growth. Pyrolysis of the coating at 1000 degrees C in argon, and subsequent oxidation at 1000 degrees C for 24 hours in air, yielded no strength degradation relative to the as-received Saphikon(TM) strength. No strength modification was observed for the Nicalon(TM) fiber in the as-coated form. However, a 13% strength degradation was observed after coating pyrolysis at 1200 degrees C in argon, and was attributed to microstructural modification and CO evolution in the fiber during coating pyrolysis.

A variety of open cell ceramic foams were subjected to rapid thermal cycles by infrared heating and forced air cooling to study the thermal fatigue behaviour of these materials. After cycling, the extent of damage in the samples was determined by measuring the elastic modulus using dynamic resonance (non-destructive test) and the retained strength in three-point bending (destructive test). It was found that the retained elastic modulus and strength gradually decreased with an increase in the number of cycles, followed by a saturation behaviour indicating a damage accumulation mechanism. The extent of damage was found to depend on the cellular structure parameters (i.e. cell size and density), composition, as well as thermal cycling variables, such as maximum temperature, cooling rate, etc.

An experimental methodology is proposed to evaluate the thermal shock resistance of ceramics. A technique based on infrared heating has been developed to perform systematic and well controlled thermal shock experiments. This novel technique was used to evaluate the resistance of yttria-stabilized zirconia–alumina foams to thermal loads. Foams of varying cell sizes were subjected to thermal shock and the damage was evaluated using retained strength and non-destructive elastic modulus measurements. The transient thermal gradients and the resulting thermoelastic stresses in the foams were predicted using finite element analysis and the extent of damage was correlated to the maximum thermal strains generated in foams.

There is a growing interest in understanding the residual stress state in particulate composites that contain a thin interfacial phase. In this work, new analytical solutions were obtained for the residual stresses in composites with (1) a coated particle in an infinite matrix and (2) finite concentration of coated particles. The explicit expressions provided an easy evaluation of the influence of constituent properties on residual stresses in such systems. The interactions amongst the constituents are complicated but are demonstrated graphically in an example.

Sapphire fibre surface damage caused by a polycrystalline zirconia coating has been analysed using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). “Pitting” of the sapphire surface was partially attributed to formation of a transient liquid phase (interphase) capable of local dissolution of alumina at zirconia grain contacts. Chemical etching was used to verify that the interphase material was silicate-based and resided at triple points between zirconia grains and the fibre surface. An additional crystalline calcium hexaluminate phase (hibonite) was found on some fibres. Origin of these impurities and resulting consequences of their presence are rationalized in view of observed surface modification.