„I-1” – A ceramic composite with extremal mechanical strength and thermal shock resistancy. László A. G ÖMZE, Milla GÖMZE University of Miskolc, Department of Ceramics and Silicate Engineering, Hungary femg omze @uni-miskolc.hu T el: +36 46 565 111/ 15-66, Fax: +36 46 565 103. ABSTRACT.
„I-1” – A ceramic composite with extremal
mechanical strength and thermal shock resistancy
László A. GÖMZE, Milla GÖMZE
University of Miskolc, Department of Ceramics and Silicate Engineering, Hungary
Tel: +36 46 565 111/15-66, Fax: +36 46 565 103
With the use of well-known and relatively not expensive raw materials, the authors have successfully developed a new composition of oxide ceramic, which shows not only a very high compressive and bending strength, but extreme high surface hardness and excellent thermal shock resistance. These new ceramic composite contains neither barium-oxide nor chromium-oxide nor nano-carbon, so harmful contaminants are not released into the environment during the manufacturing process.
The advances in science and technology make higher and higher mechanical and thermal demands on high performance technical ceramics and ceramic composites in these days. The purpose of Igrex Ltd. (Hungary) – which is specialized in technical innovation – with the cooperation of the Department of Ceramics and Silicate Engineering of University of Miskolc, was to develop a new, high performance oxide-ceramic composition, which not contain harmful contaminants, like BaCO3, Cr2O3, nano-powders and nano-fibres. At the same time there are expectations for the new ceramic composite products of compressive and bending strength, hardness, the resistance to wear, impact and thermal shock to exceed the currently produced and commercially available ceramics and ceramic composite products.
THE SELECTION AND DEVELOPMENT OF RAW MATERIALS
The authors have compiled and tried dozens of mixing formulas during the development of the new ceramic composite, called „I-1”, with extreme mechanical strength and thermal shock resistance. In the first phase of the development used fine atomizer powders behave excellent in general, not only in uni-axial directional, but also during isostatic pressing, and it was possible to make them suitable relatively easily for injection molding, used for the production of complex geometrical shape products. Despite the dozens of mixing combinations – we were not able to prepare - from these powders - ceramic composites with the required mechanical strength reached by others. The reason for this was, that the structure of atomized powder grains are destructed by compacting, and the destroyed structure remains even after sintering.
Microstructure of sintered ceramic items
A., made from atomizer-powder (before HV test)
B., made from „I-1”ceramic composite powder (after HV test)
THE REALIZED MECHANICAL STRENGTHS, WEAR AND THERMAL SHOCK RESISTANCY
In case of thermal shock resistance tests the specimens were heated up to 1200 °C, than were taken out directly from the kiln and dropped into water with temperature of 20°C. Bending strength was determined by three-point failure tests. Wear tests were executed on OMSIS instrument with the use of SiC powder. The loss of mass and the change in size caused by abrasive wear were determined as a function of covered distance.
The hardness of the “I-1” ceramic composite specimens was determined by ST-3001 instrument manufactured by Tear Coating Ltd.
To avoid the faults of grains in the compositions, which are shown in Fig. 1., the bought powders are only added to the self-produced raw material after further milling. Toughness and thermal shock resistance of the new BaO and Cr2O3 –free ceramic composite are provided by the high Al2O3 content, while hardness was achieved by adding ZrO2 content, mixed in in the appropriate ratio. To keep the sintering temperature relatively low (1650 °C), high purity silica flour – manufactured and sold by Üveg-Ásvány Ltd. (Hungary) – was added to the raw material system in a certain quantity.
The final composition of the raw materials mixture used for the new composite ceramic „I-1” includes 16 different oxides. During the process of mixture preparation the oxides were milled each for different times, so that of their BET specific surface – measured by TRISTAR 3000 instrument – will be nearly equal.
The measured wear resistancies (A.) and thermal shock resistancies (B.)
THE SELECTION OF COMPACTING TECHNOLOGY, DETERMINATION OF THE OPTIMAL PRESSURE
For the forming of the new „I-1” ceramic composite with extreme mechanical strength, hardness and thermal shock resistance, uni-axial pressing, isostatic pressing and injection molding was tested. Injection molding was necessary for the production of products with complex geometry.
The specimens with the best thermal shock resistance have damaged only over 24 cycles of heating. The hardness of the new “I-1”ceramic composites specimens in case of 200 N tensile load was: HV ≥ 6000 MPa, whereas the average amount of three-point bending strength was just Rb ≈ 350 MPa. But the amount of compressive strength fluctuated between 8 GPa ≤ Rn ≥ 26 GPa.
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From the point of the mechanical properties and thermal shock resistance of the sintered products, uni-axial pressing was the most effective technology out of all the applied compacting methods. Previous research, done at the Department of Ceramics and Silicate Engineering of University of Miskolc shows, that compressive strength, bending strength and thermal shock resistance of ceramic products and ceramic composites made by uni-axial pressing significantly depends on the size distribution of grains in the forming tool, and the distribution of the applied pressure in the compressed powder.