Emőke Rudnayová

Fracture Characterization of Silicon Nitride Based Layered Composites

Silicon nitride based structural ceramics are a family of advanced materials that exhibit a combination of high hardness, high strength, good corrosion and erosion behaviour, high elastic modulus and dimensional stability. Major application of these ceramics includes wear components, cutting tools and parts of engines (turbochargers, bearings, etc.). Their wide application is, however, still limited mainly due to their brittleness, low flaw tolerance and low reliability1,2. In recent years nitride based ceramics have been very intensively investigated all over the world with the aim to improve their mechanical properties and make them suitable for structural applications. The main ways of improving the room temperature mechanical properties of silicon nitride based ceramics can be summarized as follows: improving the strength level and reducing the strength values scatter, i.e., enhancing the reliability by reduction of the critical defect size (improved properties of powders, clean room manufacturing, etc.) — the flaw diminution approach3,4; promoting the localized bridging behind the crack tip (in the form of frictional and mechanical interlocking, or pull out) by which the flaw tolerance of the material can be improved — the flaw tolerance approach5−7; improving the strength values by incorporating into the matrix the nano-sized, second-phase particles with different expansion coefficients — the nano-particle dispersion strengthening8; improving the structural reliability by designing novel laminar composites with a promoted crack deflection at the interlayer boundaries and utilizing the compressive residual stresses arisen during cooling down from the sintering temperature because of the differences in the thermal expansions between the layers which have different compositions — the laminar structure approach9−12.

Mechanical and Fracture Properties of R-Glass Reinforced Composites with Pyrolysed Polysiloxane Resin as a Matrix

Mechanical and fracture properties of unidirectional composites reinforced with R-glass fibres and utilizing various commercially available polysiloxane resins as matrix precursors were investigated. As the matrix becomes more brittle after the pyrolysis the impact toughness and flexural strength of the composites fall. On the other hand, the shear modulus rises after the pyrolysis as the matrix becomes stiffer in shear. The appearance of fracture surfaces generated during the flexural strength at room temperature (RT) and elevated temperatures is discussed.

A Contribution of Fractal Geometry to Characterization of Wear Process

Irregular evolution of friction coefficient, recorded during the ball-on-disc test on Si3N4 based ceramic materials, was analysed by means of fractal geometry methods. Tests were carried out at room temperature, in air and without any lubricant. It was proven that the friction coefficient trace, considered as a geometric object, has the property of a fractal curve. The fractal dimension of this curve increased with increasing wear rate measured in a corresponding wear test. This could indicate the possible correlation between the wear rate and the fractal dimension of friction coefficient as a function of sliding distance (time).

Comparative Measurements of Mechanical Properties of α and β SiAIONS

SiAlONs are mostly taken for a low cost alternative for silicon nitride ceramics due to their simple processing. In spite of this, their properties in some aspects can be superior to the pure silicon nitride. SiAlON can be taken for silicon nitride with an extended lattice in which a part of the silicon is replaced by aluminium and a part of nitrogen by oxygen. α-SiAl0N is isostructural with α-Si3N4 and has the formula MexSi12−(m+n)Alm+nOnN(16−n) where Me is Li, Ca, Mg, Y and rare-earth elements except La, Ce, Pr, and Eu [1,2] and m = 3x where 3 represents the valence of the used Me atom. β-SiAl0N is isostructural with β-Si3N4 and has the formula Si6−zAlzOzN8−z