Daniele Dalle Fabbriche

Densification, Microstructure Evolution and Mechanical Properties of Ultrafine SiC Particle-Dispersed ZrB2 Matrix Composites

The densification behavior along with the microstructure evolution and some mechanical properties of four ultrafine SiC particle-dispersed ZrB2 matrix composites were studied. The SiC–ZrB2 composites, with a SiC content of 5, 10, 15 and 20 vol%, were densified to near full density by vacuum hot pressing at 1,900°C under a maximum uniaxial pressure of 45 MPa. The presence of SiC greatly improved the sinterability of ZrB2. Grain growth of the diboride matrix was increasingly inhibited for larger amounts of SiC added. Elastic modulus, Poisson ratio, microhardness, flexural strength and fracture toughness were measured at room temperature. Unexpectedly, no obvious effect of the increasing amount of SiC on flexural strength and fracture toughness was found. The former property ranged from 650 to 715 MPa but was actually affected by the exaggerated size of several tenths of micrometers of sintered SiC clusters which acted as dominant critical defects. Also fracture toughness did not receive a marked contribution from the increase of the SiC content. As for the matrix, the prevailing fracture mode of the composites was intragranular, regardless of the SiC content.

Microstructure and Compressive Strength of Porous SiC Derived from Wood

Porous SiC derived from wood is studied. The production process includes a pyrolysis step, where a carbon preform is obtained, and an infiltration cycle, where liquid silicon penetrates into the preform by capillarity and simultaneously reacts with carbon, forming SiC. With this process, microstructures mimicking those of original woods are obtained. Due to the wide variety of woods existing in nature, porous SiC with different microstructures can be produced. In the present study, specific woods are selected in function of their microstructure. In particular, the results obtained with a selection of Softwoods and Hardwoods, characterized by a different pore size distribution along the axial direction, are compared. The effects of the different porosity and microstructure on the mechanical strength are investigated in the three principal directions.

Microstructure Design of Liquid Phase Sintered Silicon Carbide in Function of the Powders Characteristics

Different LPS-SiC were produced by pressureless sintering in argon atmosphere, starting from different powders: either α-SiC or β-SiC or mixtures of α and β-SiC, using the same sintering aid system (Y2O3+Al2O3). The green densities and the densities after various sintering tests (Temperature range from 1850 to 1900° and holding time 1-2 hours) were related to the powder characteristics and to the processing conditions. The microstructure of materials from α-SiC powder is generally equiaxed and fine (grain size from 0.5 to 3.5 μm), while the use of β-SiC powder favours the development of elongated grains (up to 7 μm) with a mean aspect ratio of about 4. It is possible to tailor morphology and grain size with a proper selection of powder mixtures α/β-SiC. Mechanical properties were evaluated on selected samples.

Bioactive Hydroxyapatite/Calcium Silicate Composites Obtained by Fast Hot Pressing: Structure and Flexural Strength

The present work deals with the preparation and characterization of ceramic composites for the substitution of load-bearing bone portions, made of hydroxyapatite (HA) and bioactive β- calcium silicate (β-Ca2SiO4) as a reinforcing phase. The composite materials were prepared by Fast Hot-Pressing technique (FHP), which allowed the rapid sintering of monolithic ceramics at temperatures up to 1500 °C, well above the commonly adopted temperatures for sintering of hydroxyapatite (1200-1300 °C), in order to achieve the densification of the reinforcing phase also. XRD analysis reported no formation of secondary phases other than HA and β-Ca2SiO4, after FHP cycles. Flexural strength tests were performed on selected samples sintered at different temperatures: the composite materials exhibited increased mechanical resistance compared to samples constituted of HA only. These preliminary results confirmed that composites of HA and β- Ca2SiO4 are promising for the development of bioactive load-bearing ceramic bone substitutes.