A. Gallardo-López

High-temperature compressive creep of liquid phase sintered silicon carbide

Creep of liquid phase sintered SiC has been studied at temperatures between 1,575 and 1,700 C in argon under nominal stresses from 90 to 500 MPa. Creep rates ranged from 3 {times} 10{sup {minus}8} to 10{sup {minus}6}/s, with an activation energy of 840 {+-} 100 kJ/mol (corresponding to carbon and silicon self-diffusion), and a stress exponent of 1.6 {+-} 0.2. The crept samples showed the presence of dislocation activity, generally forming glide bands and tangles. Degradation of the mechanical properties due to cavitation or reaction of the additives was not detected. SEM and TEM microstructural characterization and analysis of the creep parameters leads to the conclusion that the creep mechanisms operating are grain boundary sliding accommodated by lattice diffusion and climb-controlled dislocation glide operating in parallel. Other possible operating mechanisms are discussed and the data are compared with published data.

Contribution to the study of the transition to a superstructure in high yttria content YCSZ

Abstract This work deals with the interpretation of the diffuse spot array observed in experimental electron diffraction patterns (DPs) of 24–32 mol% yttria completely stabilized zirconia (YCSZ). DPs corresponding to the microdomains of Y 4 Zr 3 O 12 compound described as an hexagonal structure in a cubic fluorite matrix have been simulated, paying special attention to the relationship between the cubic and the hexagonal structures. The comparison between experimental and simulated electron DPs has been used to test the proposed model of microdomains of δ-phase forming in the cubic fluorite matrix as an explanation to the phenomenon of diffuse scattering.

Correlation between plasticity and dislocation dissociation in ceramics

The influence of dislocation core extension on plastic properties is discussed. Dislocation dissociation has been studied in ceramics. Such investigation has been carried out for the past decades, but in spite of new results, there are still pending questions. Here, we review cases of glide and climb dissociation. The former has been found in 4H-SiC with implications not only on the plasticity but also on the crystal structure changes. The latter has been found in several oxides (sapphire, spinel, strontium titanate, etc.). It is, sometimes, associated with a maximum in the critical resolved shear stress (CRSS) dependence on temperature. We discuss the reasons for such effects, emphasising factors that can reduce the mobility of dislocations to justify why dissociation can influence plasticity and, sometimes, has negligible effects.

High Temperature Mechanical Properties of Ti(C,N)-Co-Mo2C Cermets

The creep behavior of a TiCxN1-x-Co-Mo2C cermet has been investigated at temperatures between 1100-1200°C in an inert atmosphere to assess the one step mechanically induced self-sustaining reaction synthesis and pressureless sintering process, and the influence of the Mo2C additive in the high temperature mechanical properties of this cermet. The samples deform plastically at the chosen temperatures, and values of the stress exponent (n=1.70.6) and activation energy (Q= 4.30.5 eV) have been estimated from uniaxial compression tests. No significant grain growth has been detected after deformation. The reproducibility of the creep tests compared to other compositions indicates that the Mo2C addition contributes to increase notably the resistance to high temperature oxidation of the samples, so that the plastic behavior is not affected by oxidation when deformation experiments are performed in an inert atmosphere.

Influence of strain rate on the plastic deformation of high yttria content YCSZ single crystals at 1400–1500 °C

Single crystals of 24–32 mol% yttria fully stabilised cubic zirconia have been deformed in compression along 〈112〉 at T=1400–1500 °C with strain rates e ranging from ∼10−8 to ∼10−4 s−1 (nominal stresses 100–320 MPa). The deformation mechanisms are discussed in terms of macroscopical and microscopical parameters.

Plastic Behaviour of Nanostructured Yttria Tetragonal Zirconia Polycrystals: The Effect of Yttrium Segregation

Plastic deformation of yttria tetragonal zirconia polycrystals (YTZP) has been the objective of utmost interest owing to the exceptional properties such as superplasticity exhibited for grain sizes in the submicrometer range. In theory, this property is expected to improve with decrease in the average grain size. Therefore, the study of nanometer-sized YTZP ceramics is receiving an increasing attention in the ceramic scientific community. This paper is a critical analysis of the role played by yttrium segregation at the grain boundaries as a driving force to account for the peculiarities of the mechanical response of these ceramics.

Influence of defects on the high temperature mechanical behaviour of yttria fully-stabilized zirconia single crystals

Abstract This work is focused on the study of the influence of high temperature heat treatments given prior to deformation in yttria fully-stabilized zirconia single crystals (YFSZ). Experimental results show that the yield stress found in constant-strain tests performed in heat-treated samples is remarkably dependent on the annealing time when the treatment is given under a very low oxygen pressure atmosphere. This phenomenon has been explained through the analysis of the thermodynamics of complex defects found at high temperature, as well as the interaction between mobile dislocations with them during plastic deformation.

Obtention of Li3xLa2/3−xTiO3 ceramics from amorphous nanopowders by spark plasma sintering

ABSTRACT In this work, Li3xLa2/3−xTiO3 powder with nominal lithium content (x = 0.08) was synthesized by mechano synthesis method. Spark plasma sintering (SPS) was employed to prepare lithium lanthanum titanium oxide solid-state ceramic. The techniques of X-ray diffraction, high resolution scanning electron microscopy, and Raman spectroscopy were used to characterize the composition and microstructure of samples. The results showed that fine-grained ceramics with relative density of 95.5% were obtained by sintering the oxide powders at 1100°C for only 5 min.