The recent development in the field of superhard materials with Vickers hardness of ⩾40 GPa is reviewed. Two basic approaches are outlined including the intrinsic superhard materials, such as diamond, cubic boron nitride, C3N4, carbonitrides, etc. and extrinsic, nanostructured materials for which superhardness is achieved by an appropriate design of their microstructure. The theoretically predicted high hardness of C3N4 has not been experimentally documented so far. Ceramics made of cubic boron nitride prepared at high pressure and temperature find many applications whereas thin films prepared by activated deposition from the gas phase are still in the stage of fundamental development. The greatest progress has been achieved in the field of nanostructured materials including superlattices and nanocomposites where superhardness of ⩾50 GPa was reported for several systems. More recently, nc-TiN/SiNx nanocomposites with hardness of 105 GPa were prepared, reaching the hardness of diamond. The principles of de...

https://mediatum.ub.tum.de/doc/958917/file.pdf

The search for novel, superhard materials

The influence of grain size and water content on the high-temperature plasticity of olivine aggregates was studied, using a gas-medium high-pressure deformation apparatus. The specimens used were hot-pressed, dense olivine aggregates with controlled grain size ranging from a few to 70 μm, with or without added water. Mechanical tests were made at 1573 K and 300 MPa confining pressure and at strain rates of 10−3 to 10−6 s−1. The results reveal two distinct mechanisms of deformation, depending on stress level and grain size. At relatively high stress and large grain size, the strain rate is proportional to about the cube power of the stress and is nearly independent of grain size. In this regime, microstructural observations gave evidence of intragranular deformation involving dislocation motion. At low stress and small grain size, the strain rate depends almost linearly on stress and decreases markedly with increase in grain size. In the latter regime, little evidence was found for intragranular deformation. These observations suggest that the deformation mechanism in the grain size insensitive regime is dislocation creep, while that in the grain size sensitive regime is diffusion creep. In both regimes, water was found to enhance the creep rate. The absence of grain size sensitivity in the dislocation creep regime and comparison with single-crystal data indicate that the water weakening effect is mainly an intragranular process. However, the existence also of a water weakening effect in the diffusion creep regime indicates that water also enhances diffusion. The extrapolation of the present results to coarser grain sizes indicates that the transition from dislocation to diffusion creep occurs at 0.1–1 MPa for 10-mm grain size. Therefore it is suggested that this transition may occur in the upper mantle and that, in both regimes, the presence of trace amounts of water will result in significantly lower creep strength than under strictly “dry” conditions.

Rheology of synthetic olivine aggregates: Influence of grain size and water

The uniaxial tensile deformation behavior of 3 molpercent Y/sub 2/O/sub 3/-tetragonal ZrO/sub 2/ polycrystals was studied at temperatures up to 1500 C in ambient atmosphere. The tensile specimens which were elongated at constant displacement rates from 0.00011/s to 0.00055/s at 1450 C exhibited unusually large deformation. The superplastically deformed specimens retained a bending strength of 976 MPa at room temperature. Steady creep rates measured at a constant load in the range of relatively small strains were described by a stress component, n, of 2 and by an activation energy of 586 + or - 40 kJ/mol. Large deformations during the accelerating creep were accompanied by extensive formation of cavities. 19 references.

Superplasticity of yttria-stabilized tetragonal ZrO2 polycrystals

Creep of ceramics: Part 1 Mechanical characteristics

The ability to achieve a high tensile ductility in a polycrystalline material is of interest both from a scientific point of view and also because of potential applications in the materials forming industry. The superplasticity of conventional metallic alloys is now well-documented and understood reasonably well. However, the field of superplasticity has expanded recently beyond the traditional metallic alloys to include evidence of superplastic-like behavior in a very wide range of new and advanced materials. To date, superplasticity has been reported in mechanically alloyed metals, metal matrix composites, ceramics, ceramic matrix composites and intermetallic compounds. This review presents an overview of these new developments using the established behavior of conventional metallic alloys as a standard for comparison with the mechanical properties of these new materials. As well be demonstrated, the new materials often exhibit significant differences in their flow characteristics in comparison with the traditional superplastic metallic alloys. The successful utilization of superplastic materials in forming applications requires an understanding of the failure processes occurring in the materials in terms of both the localization of external flow and the accumulation of internal damage through the nucleation and growth of cavities. These problems are also addressed in this review.

Superplasticity in advanced materials

Fine-grained polycrystalline MgO specimens were deformed in compression at constant strain rates of ∼6.7×10−6 s−1 to 6.7×10−5 s−1 at 1173 to 1423 K. Both mechanical data and microstructural observations are distinctive of a diffusion-accommodated flow, where the grain-size parameter is of prime importance in enhancing plasticity. Under suitable experimental conditions, superplastic strain was achieved in specimens; no change was detected in the equiaxed-grain shape configuration. Finally, the effect of grain growth during deformation was considered; specimens of initial grain size ∼0.1 μm showed an apparent hardening which was quasi-linear with time.

B. Escaig

Papers

Mechanical Properties of Fine‐Grained Magnesium Oxide at Large Compressive Strains