Abstract: 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).
Abstract: Laser scanning confocal microscopy (LSCM) is a microscopic technique which allows for height discrimination. The ability to gather 3D data, along with adequate resolution (around 400 nm), makes the technique suitable for fractography; however, its applications in this area are not sufficiently explored. In this work, LSCM and SEM are applied to the study of fracture surfaces in sapphire and ruby fibers submitted to tensile stress in high-temperature conditions. The obtained
qualitative and quantitative information demonstrates the validity of LSCM as a fractographical technique, allowing for clear identification of fractographical features and providing novel insight in the phenomenon of subcritical crack growth (SCG).
Abstract: Metal/ceramic composite materials were produced with core/shell structure by traditional pressing and the influence of the compacting parameters on the material properties has been studied. Different quantities of ceramic have been introduced to coat the base stainless steel powder. The use of a new generation of coated powders gives the opportunity to achieve near net shape massive composite exhibiting a composite microstructure, with a uniform dispersion of ceramic hard particles embedded in the metal matrix. Sintering was carried out in vacuum and at temperatures typical of metal sintering process (1250 °C). Microstructures of produced samples have been investigated through the use of LOM and SEM. Hardness as well as tensile and bending tests have been performed.
Abstract: Indentation methods have been used for the study of the hardness and deformation
characteristics MoSi2. Micro-nanoindentation tests at loads from 10 mN to 2000 mN were carried out using the depth-sensing method. Measurements of the microhardness using conventional Vickers method was carried out at loads of 500 mN, 1000 mN and 2000 mN. The Universal (Martens), Plastic and conventional Vickers hardness values were calculated at different indentation loads. Evident indentation load - size effect was found in both materials. According to the results,
the pre-strain reduces the micro-nano hardness values, probably due to the activation of slip systems during the high-temperature deformation.
Abstract: Four-point bending strength and Weibull distribution of the strength values of a carbon derived Si3N4-SiC nanocomposite have been investigated. The fracture origins were identified and characterized and a “labeled Weibull plot” was constructed. The fracture origins are surface, subsurface and volume located technological defects with a dimension from 10 µm to 180 µm, mainly in the form of cluster of pores and large SiC grains.
Abstract: Thermal shock resistance of the RE-Si-Mg-O-N glasses (RE = La, Nd, Yb, Lu) with 0 and 20 eq.% of nitrogen was investigated by the indentation-quench method based on propagation of Vickers cracks. Crack growth was measured on the same sample for a test series of different quenching temperatures. Thermal shock resistance of the studied materials was determined as a temperature difference resulting in 10 % growth of the initial cracks (∆T10) and by the thermal shock parameter R calculated from the material properties. Although the comparison of ∆T10 and R values as a function of glass composition revealed some differences between these two approaches, also a common trend was observed. Thermal shock resistance increased with the fractional glass compactness resulting from RE type and N content increase.
Abstract: Biaxial testing was used to get insight into the incidence of the surface flaw properties (size, shape) on the strength of float glass specimens. Grinding grooves, Vickers' indentations and scratches as defects were introduced at the surface of annealed float glass specimens. The machined and fractured surfaces were observed using optical and confocal microscopes. The influence of the flaw characteristics on the strength of glass was evaluated and analyzed in the light of the fracture mechanics.
Abstract: The current paper give a brief overview over the principle and information content of the instrumented impact test completed by electro-emission measurement. Introducing over experiences taken by applying these test on a high-tech SiAlON ceramic a novel method for the determination of the dynamic fracture toughness of silicon nitride ceramics is suggested.
Abstract: The Hertzian indentation technique has been evaluated as a possible means of assessing the surface fracture behaviour of a set of qualitatively similar liquid phase sintered polycrystalline aluminas with systematically varied composition, and a range of grain sizes. The method provides a better correlation between microstructure characteristics and surface fracture behaviour than does Vickers indentation. The fracture toughness determined from Hertzian indentations was found to be a function of grain size, irrespective of the composition of the material. The cumulative probabilities of formation of ring cracks at specific loads during indentation with a spherical indenter were determined for each material from the set of 25 measurements, and the parameters of distribution of the probability of fracture were calculated. A statistical correlation between the failure probability distribution and the size distribution of alumina grains was detected.
Abstract: Bulk ceramic materials on the base of β-sialon were prepared from hydrosilicates. Except of some mechanical properties (HV, KIC) also the corrosion resistance of β-sialon against Cl-, iron and steel was investigated. The results were compared with β-sialon of similar composition z = 3.8, which was prepared from commercial powders Si3N4, AlN and Al2O3. The differences in mechanical properties of β-sialons prepared from natural hydrosilicates and synthetic powders are discussed.