Diletta Sciti

Effect of annealing treatments on microstructure and mechanical properties of liquid-phase-sintered silicon carbide

Dense hot pressed SiC based materials were produced with alumina and yttria as sintering aids in different percentages. The microstructure and mechanical properties were evaluated on as-sintered materials and related to the starting compositions. Annealing treatments were carried out at different temperatures and holding times to evaluate the possibility of a further improvement of the material properties, above all indentation toughness and high temperature properties. The attention was focused on the microstructural modifications induced by thermal treatments, i.e. formation of elongated grains, reduction and modification of secondary amorphous and crystalline phases. Mechanical properties of the annealed materials were explained on the basis of microstructural changes and compared to those of the as-sintered materials. Annealing treatments were found to be highly beneficial for improving mechanical properties and grain boundary phase microstructural changes were found to be the main factor affecting such improvements.

Microstructure and properties of porous β-SiC templated from soft woods

Abstract Porous β-SiC with a multimodal porosity preferentially oriented along one direction is obtained by infiltration of pyrolyzed wood with Si at T > T Si Melting . Hard woods (obece, poplar and assembled poplar) are used as starting materials. The microstructure of the starting wood, of the wood after pyrolysis and after Si infiltration, is characterised in terms of overall porosity, pore size distribution and of crystallographic phases. The process is optimised for obtaining porous templates of only β-SiC with a microstructure that replicates the original wood microstructure. Features such as the presence of unreacted carbon, or conversely, of Si within the open pores of the infiltrated materials are minimized by a careful control of the amount of Si in contact with the carbon preform during the infiltration cycle. Compression tests on cubic samples are performed along the axial and longitudinal direction.

Assessment of the high temperature elastic and damping properties of silicon nitrides and carbides with the impulse excitation technique

The impulse excitation technique (IET), based on resonant vibration analysis, was used to determine the high temperature elastic and damping properties of hot-pressed silicon nitride (Si3N4) and silicon carbide (SiC) materials with Al- and Y-additives. The Si3N4 materials were doped with 2 wt.% of TiN to suppress the crystallization of intergranular glassy pockets. Near 1000 � C the investigated materials display a characteristic damping peak, which is essentially unaffected by temperature excursions up to 1400 � C. Two existing models which aim at linking elastic and damping properties with microstructural and micromechanical details, are considered. One of the models is used to provide an estimate of the amount of amorphous intergranular pocket phase. This type of information is of particular relevance since the high temperature deformation resistance of silicon based ceramics is severely dependent on the presence and amount of amorphous multiple grain pockets. # 2002 Elsevier Science Ltd. All rights reserved.

Nanoindentation characterisation of HfC-based composites

Two hot-pressed HfC-based composites with 10 and 20 vol% of MoSi2 were characterised by nanoindentation tests. The hardness and the Youngs modulus of the constituent phases were measured at low peak loads. They resulted independent on the considered composite. With higher peak loads, the same properties were measured and attributed to the composites. In this peak load range, the Youngs modulus and hardness decreased by increasing the peak load. Inserting the values of the constituent phases in simple composites models, the upper and lower bounds for the composites properties were calculated and compared to the experimental results.

On the thermal shock resistance and mechanical properties of novel unidirectional UHTCMCs for extreme environments

Aerospace provides a strong driving force for technological development. Recently a novel class of composites for harsh environments, based on ultra-high temperature ceramic composites reinforced with continuous fibers (UHTCMC), is being developed. The goal of this work is to overcome the current data patchwork about their microstructural optimization and structural behavior, by showing a consistent mechanical characterization of well-defined and developed UHTCMCs based on ZrB2-matrix. The obtained composites have a density of 3.7u2009g/cm3 and porosity of less than 10%. The flexural strength increased from 360 to 550u2009MPa from room temperature to 1500u2009°C, showing a non-brittle behaviour. The composites were able to sustain a thermal shock severity as high as 1500u2009°C. The maximum decrease of strength at 1400u2009°C was 16% of the initial value, indicating that the samples could be shocked at even higher temperature. Flexural strength, Young’s modulus and coefficient of thermal expansions (CTE) of the composites were measured both along transverse and longitudinal direction and correlated to the microstructural features. The presented microstructural and mechanical characterization well defines the potentiality of the UHTCMCs and can be used as reference for the design and development of novel thermal protection systems and other structural components for harsh environments.