Creep of ceramics: Part 1 Mechanical characteristics

This review analyses a wide range of experimental data on the creep of ceramic materials and reveals many similarities with the creep of metals. It is demonstrated that there are two important differences in the creep behaviour of ceramics: (1) there is an enhanced role of diffusion creep, and (2) in the power-law regime, ceramics divide into two categories with stress exponents of ∼ 5 and ∼ 3, respectively. It is concluded that the behaviour with an exponent of ∼ 5 represents fully ductile behaviour as in f cc metals, whereas the behaviour with an exponent of ∼ 3 is due to dislocation climb from Bardeen-Herring sources under conditions where there is either a lack of five independent slip systems or, if five independent slip systems are available, a lack of interpenetration of these systems.

Creep of ceramics

Densification maps for spark plasma sintering (SPS) of nanocrystalline MgO powders were constructed using hot-isostatic pressing (HIP) model. Effects of the grain size, applied pressure, vacuum level, SPS temperature, and duration on densification were determined. Plastic flow and diffusion were the dominating mechanisms at the first and the final stages of densification, respectively, whereas power-law creep did not contribute to densification. Most impressive was the effect of the grain size on the SPS duration needed for full densification. At 800 °C and 150 MPa, the 20 nm particle size powder compact may be fully densified within 1 min. Significant densification is expected by the plastic yield if a threshold pressure is applied. Small temperature decrease of 50 °C may increase the densification duration from tens of seconds to tens of minutes. Relative densities as high as 75% may be reached within 1 min, irrespective of the initial particle size. However, threshold particle size exists (i.e. 50 nm at 800 °C and 100 MPa) for full densification at workable SPS durations. The densification maps highlight the presence of a temperature window between 750 °C and 850 °C within which fast densification of nanocrystalline MgO may be achieved by SPS. Reasonable agreement between the calculated and the experimental SPS results confirms the validity of the HIP model for description of the SPS process.

Densification maps for spark plasma sintering of nanocrystalline MgO ceramics

The creep mechanisms in ceramics are reviewed briefly and then compared with experimental data. It is shown that there are three major types of creep behaviour: a stress exponent close to 5 due to control by dislocation climb and fully ductile behaviour; a stress exponent close to 3 due to control by climb from Bardeen–Herring sources and less than jive interpenetrating independent slip systems; and a stress exponent close to 1 due to diffusion creep. The role of interface reaction control and the transitions from diffusion to power law creep are also examined.MST/1389

Characteristics of creep deformation in ceramics

Resume Nous presentons une revue sur la deformation plastique a haute temperature mettant en jeu le deplacement des dislocations dans les oxydes. Les liaisons chimiques et les structures cristallines sont plus complexes que dans les metaux; aussi, les dislocations ne sont pas encore bien decrites ni dans les oxydes binaires (NiO, Cu2O …) ni dans les oxydes ternaires (MgAl2O4…). Ces composes, meˆme purs, contiennent de grandes quantites de defauts ponctuels pouvant provoquer des ecarts a la stoechiometrie; une bonne connaissance des concentrations et de la mobilite de defauts est indispensable pour expliquer les proprietes plastiques. Les essais mecaniques habituels sont la flexion et surtout la compression; ils presentent de nombreux inconvenients, en particulier ils conduisent a une deformation heterogene des eprouvettes. Le but de ces tests est d'etablir la loi qui relie la vitesse de deformation aux autres parametres (contrainte, temperature …). Une grande quantite de resultats existe et est rassemblee...

Mouvement des dislocations et plasticité à haute température des oxydes binaires et ternaires

Creep deformation in ultrafine-grained (0.1 to 1μm) Fe-doped magnesia polycrystals is studied in compression, at temperatures of 700 to 1050° C, and constant loads of 50 to 140 MPa. The stress exponent observed to be nearly unity and the strong grain size sensitivity (ė∼d−2.85) suggest that diffusional creep mechanisms dominate the deformation. In the grain size range of the present study the grain boundary diffusion contribution is significantly more important than lattice diffusion. Magnesium is tentatively identified as the rate-controlling species along grain boundaries from an analysis of the diffusivities inferred from the present work and from other authors for Fe-doped magnesia.

J. Crampon

Papers

High temperature creep of ultrafine-grained Fe-doped MgO polycrystals