Papers by Author: Péter Arató

Abstract: The deformation of ceramic matrix composites (CMC) is controlled by several processes occurring in the matrix, in the enforcing phase, at the interfaces between different phases and at the surface of the body. The main tool to study morphology features of CMC-s is high resolution scanning electron microscopy (SEM). In this study destructive chemical methods such as chemical etching and burning of combustible phases were used to prepare samples for SEM investigations from three types of materials. I. SiC skeletons of C/C-SiC structures prepared of 2D woven fabric and chopped fiber bodies were produced by chemical elimination of the constituents: Si, reinforcing C- fibers and amorphous carbon. II. Silicon-nitride reaction bonded silicon-carbide samples with finishing surface oxide films – as produced and aged - were handled by HF etchants. Etching revealed cristobalite crystallites at the interface between ceramics and oxide film, and cracks in the glassy surface layer. III. Microstructure and pore structure of Si3N4 ceramics, carbon nanotube reinforced Si3N4 (CNT/Si3N4), and C/Si3N4 with graphite and carbon black addition were examined by HF etching.

Abstract: A comprehensive experimental and theoretical work aiming at studying the dynamic failure process of silicon nitride ceramics has been recently started. The main goal of this research programme consists in characterizing the mechanical behaviour of the material under dynamic loading as well as investigating the dynamic failure process using micro- and macro-fractography. The current paper deals with the phenomenon of special rate dependence of KId dynamic fracture toughness of Si3N4 based ceramics. The KId values have been determined during instrumented impact test on the one hand based on the dynamic key curve method using notched specimens, on the other hand based on fractographic analyses of fracture surface of impacted unnotched samples.

Abstract: Silicon nitride based composites with 3wt% of single wall carbon nanotubes and exfoliated graphite have been prepared. Optimisation of the manufacturing processes has been conducted to preserve the carbon nanotubes and exfoliated graphite in composites and to avoid damaging during high temperature processing. The first results show that carbon nanotubes and exfoliated graphite have a good contact to the surface of silicon nitride grains. In the case of increase of sintering pressure an increase of bending strength was achieved. It was found that microstructure features achieved by properly designed sintering parameters are the main responsible factors for the strength improvements.

Abstract: The Si3N4 ceramics are usually known as strongly refractory and enduring materials and they have typical insulating material properties on room temperature. If reinforcing phase of the Si3N4 matrix composite is a good electrical conductor, in that case it is worth to investigate the composite in electrical aspect. In our case carbon nanotubes, black-carbon and graphite with good electrical conductivity were mixed in the base ceramic. During our electrical investigations DC resistivity measurements were used to determine the percolation threshold and the conductivity of the composites. In case of high resistance samples AC impedance spectroscopy was applied. As result of the impedance spectroscopy capacitive properties were found. In some cases of conductor samples combined mechanical – electrical measurements were done to study the integrity of the additions in the matrix.

Abstract: Multiwall carbon nanotube reinforced silicon nitride composites have been prepared by hot isostatic pressing. A manufacturing process has been worked out to avoid the damage of nanotubes during sintering. This method provides their preservation even in severe circumstances at temperature 1700°C and gas pressure 20 MPa. As shown by scanning and transmission electron microscopy after low and high pressure processing, carbon nanotubes have good adherence to the silicon nitride grains. Moreover, carbon nanotubes have been found to be located not only at grain surfaces, but in several cases they are well integrated with the silicon nitride grains. Composites with higher strengths can be obtained by increasing the nitrogen gas pressure.

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Abstract: This work is focusing on exploring preparing processes to tailor the microstructure of carbon nanotube (CNT) reinforced silicon nitride-based ceramic composites. Samples with different porosity’s and different amount (1, 3 or 5 wt%) of carbon nanotubes have been prepared by using gas pressure sintering or hot isostatic pressing. In comparison, composites with 1wt%, 5wt% or 10wt% carbon black and graphite have been manufactured. We measured the room temperature mechanical and electrical properties, examined the micro and nano structure by X-ray diffraction and electron microscopy. It was found that it is possible to develop CNT-silicon nitride composite for applications where a decent electric conductivity and good mechanical properties are required.

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: In this work partially and fully densified silicon nitride based composites have been prepared using carbon black and graphite additions. Alumina and yttria sintering additives were also added to silicon nitride starting powders. Sinter-HIP and pressure-less sintering (PLS) have been employed for composite processing. The effects of carbon nano- and micro-grains on the microstructure, bending strength and hardness have been investigated. Higher strengths could be achieved by HIP, whereas more homogeneous properties can be assured by PLS.

Abstract: This study focused on the effect of ion implantation on the mechanical properties of silicon nitride based ceramics. The examined materials contained 4% Al2O3, 6% Y2O3 and, in some cases, graphite or black carbon additives. Fully densified bodies were manufactured by two-step hot isostatic pressing, in other experiments pressureless sintering gave porous ceramics. Nitrogen or carbon ions with 190 keV or 350 keV were implanted, a part of samples obtained laser treatments for comparison. Measurements of elastic modulus, four point and three point bending strength were carried out at room temperature on sintered samples, partially sintered samples and sintered and oxidised samples. Carbon or nitrogen implantation influenced the strength of partially sintered samples only if the density was over 80%. Both the implantation of carbon or nitrogen ions and the laser treatment affected the bend strength of sintered samples, enlarging the power introduced its value first increased than decreased. Implantation slightly decreased the strength of samples oxidised at 1400 °C. Comparison of different mechanical characteristics may support a better understanding of phenomena during ion beam implantation or laser irradiation.