V. M. Sglavo

Joining of reaction-bonded silicon carbide using a preceramic polymer

Ceramic joints between reaction-bonded silicon carbide (RBSiC) were produced using a preceramic polymer (GE SR350 silicone resin) as joining material; samples were heat treated in an argon flux at temperatures ranging from 800–1200°C without applying any pressure. The strength of the joints was determined by four-point bending, shear and indentation tests. Microstructural and microchemical analyses were performed by optical microscopy, SEM, TEM and AEM. The room-temperature strength of the joints increased with the joining temperature. Maximum values as high as 220 MPa in bending and 39 MPa in shear tests were reached for samples joined at 1200°C. No detectable residual stresses were observed both in the joining material and the joined parts, and the fracture mechanism was nearly always cohesive. The joint thickness was shown to depend on the processing temperature, and ranged from about 2–7 μm. The joining material was a silicon oxycarbide amorphous ceramic, with no oxygen diffusion occurring between this and the RBSiC joined parts. The lack of compositional gradients, precipitates or reaction layers indicate that the SiOC ceramic acted as an inorganic adhesive, and that the joining mechanism involved the direct formation of chemical bonds between the RBSiC parts and the joining material.

Microstructural and electrical investigation of flash-sintered Gd/Sm-doped ceria

Ceria-based ceramics can be considered among the most promising solid electrolytes for intermediate-temperature solid oxide fuel cells (IT-SOFC). In the present work, variously doped nanocrystalline ceria powders were flash-sintered, and the role of doping (Gd and Sm, 5–20xa0mol%) and sintering aid (Li and Co) on the final microstructure and the electrical behavior was investigated. Gd- or Sm-doped nanocrystalline ceria powders were synthesized by co-precipitation method using ammonia solution as precipitating agent. The synthesized nanopowders were characterized by DSC-TG, XRD, and nitrogen physisorption analysis. The nanopowders were isostatically pressed and flash-sintered. The relative density was measured by hydrostatic balance, and the corresponding microstructure was observed by SEM. The electrical behavior was studied by EIS. Flash-sintered powder pellets showed different behaviors depending on the dopant and sintering aid. The electrical conductivity of the samples increased by increasing the relative density. Fully dense Gd-doped ceria samples, synthesized by co-precipitation, were obtained by flash sintering in very short times at 700xa0°C. The total conductivity was comparable to that measured on samples sintered with conventional route at much higher temperatures such as 1500xa0°C.

Ni3Al intermetallic compound as second phase in Al2O3 ceramic composites

Abstract Several metals have been proposed as second phases in ceramic matrix composites in order to improve their fracture toughness. Unfortunately, the use of metals is limited by low melting temperature, as for Al and Ag, poor oxidation resistance, as for Ni, Mo and W, and decrease of mechanical strength as temperature increases. In these respects, high temperature structural intermetallics show better properties. This work presents the preparation and the characterization of a Ni 3 Al reinforced-alumina. A ceramic composite containing 10 vol% Ni 3 Al powder was prepared by hot-pressing at 1350°C for 1.5 h green compacts of the mixture of ceramic and intermetallic powder. Microstuctural features were investigated by scanning electron microscopy (SEM). Elastic modulus, flexural strength and fracture toughness were measured at room and high temperatures and correlated to the microstructural characteristics of the material. A toughening mechanism due to plastic deformation of the intermetallic particles during crack propagation was seen to operate both at room and at high temperature.

Indentation Method for Fracture Resistance Determination of Metal/Ceramic Interfaces in Thick TBCs

The indentation technique has been used to measure the adhesion of plasma- sprayed ceramic coatings on metals intended for thick thermal barrier coating ( TTBC) applications. This approach provides the adhesion value as the critical strain energy release rate,Gc, of the interface, which also takes into account any residual stresses. The theoretical background of the method is outlined, and specific examples are reported with respect to the effect of substrate temperature on the metal/ceramic adhesion of thick TBCs.

In situ formation and high-temperature reinforcing effect of MoSi2 and WSi2 on reaction-bonded Si3N4

Abstract A three-step procedure consisting of presintering, reaction-bonding and post-hot-pressing was employed to prepare Si3N4/MoSi2 and Si3N4/WSi2 composites, using as starting materials powder mixtures of Si–Mo and Si–W, respectively. A presintering step in an Ar-base atmosphere was performed for the in situ formation of intermetallic compounds—MoSi2 and WSi2; the nitridation in an N2-base atmosphere with a total cycle of 1350°C×20 h+1400°C×20 h+1450°C×2 h in order to realize the complete conversion of Si into Si3N4; post-hot-pressing was employed for the further densification of the reaction-bonded specimens. The 3- and 4-point bend tests were performed at 25, 1000, 1200 and 1400°C. Si3N4/MoSi2 composite shows a maximum value of bend strength at about 1200°C, whereas the bend strength of Si3N4/WSi2 composite slightly increases from room temperature up to 1000°C.

Effect of etch depth on strength of soda-lime glass rods by a statistical approach

Abstract Hydrofluoric acid solution etching has long been known as a useful method for strengthening glass. In this way surface flaws are at first reduced in length and rounded then completely removed. At this point glass strength depends only on defects emerging from the bulk or on damage produced by the etching reaction products. In this study more than a hundred values of strength of glass rods, etched with hydrofluoric acid, are elaborated by means of a statistical approach, and attention is focused on what have previously been called ‘volume defects’. The analysis allows some results about the stress concentration factor of this defects family to be obtained.

Fabrication and characterization of polymer-derived Si2N2O-ZrO2 nanocomposite ceramics

Amorphous Si-Zr-N-O powders, obtained by nitridation in an NH3 flow of zirconium modified polycarbosilane, have been sintered to full density by hot pressing at 1500‡C. The resulting ceramic shows an extremely fine-grained microstructure composed of Si2N2O and ZrO2 crystallites 20–30 nm in diameter. Thermal stability measured in air appears excellent up to 1300‡C for 48 h. Mechanical characterization pointed out good values of flexural strength (330 MPa), fracture toughness (4.1 MPam0.5) and Weibull modulus.

Fracture mechanics determination of stress profiles in NaK ion-exchanged glass optical waveguides

Abstract A method based on fracture mechanics rather than on the measurement of the refractive index and birefringence profiles, which determines stress profiles in Naue5f8K ion-exchanged soda-lime glass waveguides is presented and discussed. With this technique the stress profile is obtained by measuring the surface stress field by Vickers indentations on waveguides which are progressively HF leached, and the profile is subsequently corrected by considering successive reductions in the thickness.