and a glass phase containing СаО, МgO and SiO 2

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1 Journal Stoyan of Djambazov, Chemical Technology Albena Yoleva, and Metallurgy, Vasil Hristov, 50, Nicolay 4, 2015, Ivanov A STUDY ON THE TECHNOLOGICAL PARAMETERS INFLUENCE ON THE PROPERTIES, POLISHING AND MICROSTRUCTURE OF CORUNDUM CERAMIC DISCS FOR WATER TAPS Stoyan Djambazov 1, Albena Yoleva 1, Vasil Hristov 2, Nicolay Ivanov 2 1 University of Chemical Technology and Metallurgy, 8 Kl. Ohridski, 1756 Sofia, Bulgaria, djam@uctm.edu 2 Techceramic M Ltd., P.O. Box, Received 11 November 2014 Accepted 01 April Mezdra, Bulgaria ABSTRACT Some of corundum ceramics properties like high chemical resistance, hardness and mechanical strength make it an extremely suitable material for water tap gaskets and regulators. Solid fragments that normally get into the water, like sand, rust and limestone, cannot damage corundum ceramics. The latter stability allows these products to remain perfectly smooth even when exposed to varying levels of pressure and extreme water temperature fluctuations. The corundum ceramics properties depend mainly on the purity of the starting powder, the production method and the microstructure. This work investigates how the pressure applied during the semidry pressing, the firing temperature and the source materials composition (percentage of Аl 2 ) affect the apparent density, the coefficient of abrasion, the polished surface (the degree of polishing) and the microstructure of corundum ceramic water taps discs. Four types of a corundum powder were used as a source material for the process of semidry pressing. Five different pressures (100 МРа, 150 МРа, 200 МРа, 250 МРа, 350 МРа) and three firing temperature values (1600 о С, 1650 о С, 1680 о С) were applied in addition to the subsequent procedures of lapping and polishing in the course of production of corundum ceramic discs for water taps. The fired corundum samples underwent characterization aimed to determine the apparent density (using the method of Archimedes), the coefficient of abrasion (determined by the loss of mass in percentage points), as well as the degree of surface polishing. Using SEM and optical microscopy we proved that the technological parameters determine to a large extent the appearance of a microstructure which in turn affects the properties and the degree of polishing of corundum ceramics. Water taps of a homogeneous structure having the highest density, the smallest pores, the lowest coefficient of abrasion and the highest degree of polishing were made from a starting powder containing 98 % of Аl 2 and a glass phase containing СаО, МgO and SiO 2. A firing temperature of 1650 о С and pressing pressure 350 МРа were applied. Keywords: corundum ceramic, technological parameters, structure, properties. INTRODUCTION Al 2 based ceramics is characterized by high values of melting temperature, chemical resistance, electrical resistance, hardness and mechanical strength. Corundum ceramics has a wide range of applications, including refractories, ceramic structural elements, coxofemoral prosthesis, computer components, grinding materials for ball mills, discs for water taps, etc. [1, 2]. These properties of corundum ceramic make it a suitable material for water tap gaskets and regulators. Solid fragments that normally get into the water, like sand, rust and limestone, cannot damage corundum ceramics. The latter stability allows these products to remain perfectly smooth even when exposed to varying levels of pressure and extreme water temperature fluctuations. Innovations in corundum ceramics production provide maximum precision in reproduction and size accuracy, as well as a constantly smooth surface [2-7]. Corundum ceramics properties depend mainly on the purity of the starting ceramic powder, the production method and the microstructure. The careful selection and 525

2 Journal of Chemical Technology and Metallurgy, 50, 4, 2015 processing of the starting ceramic powders allow well controlled microstructure and properties. The latter are also greatly influenced by the presence of admixtures and additives in the source material and hence they can be controlled by selecting the right additives. The glass phase of corundum ceramics, when present in small quantities, decreases the grains size and hence increases its strength [8-17]. This work aims to study how the pressure applied during the semidry pressing, the firing temperature and the source materials affect the properties, the polishing and the microstructure of corundum ceramic discs to be used for water taps. EXPERIMENTAL Source materials There are four types of starting powders of varying corundum content which are used in semidry pressing to produce corundum ceramic discs for water taps: А. Al 2 (Nabaltec, Granalox NM 9212, D 50 = 180 μm μm, purity of 92 %) В. Al 2 (Nabaltec, Granalox NM 9620, D 50 = 80 μm μm, purity of 96 %) С. Al 2 (Nabaltec, Granalox NM 9816, D 50 = 180 μm μm, purity of 98 %) D. Al 2 (Martinswerk, MZS-3, D 50 = 5 μm, purity of 96 %) modified with addition of TiO 2 (rutile, Kronos International INC, D μm, purity of 99,00 %) and MnO 2 (pyrolusite, Teokom OOD, D μm, purity of 90,00 %). Production technology of corundum ceramic discs for water taps We applied five different levels of pressure in the course of semidry pressing (100 МРа, 150 МРа, 200 МРа, 250 МРа, 350 МРа) and three firing temperature values (1600 о С, 1650 о С, 1680 о С) with an arrest for 1 hour at the maximum temperature aiming to produce corundum ceramic discs for water taps of a varying microstructure. The technological scheme consisted of the following stages: resource materials preparation, semidry pressing with the application of a hydraulic press at a pressure of 100 МРа to 350 МPа, firing the ceramic items in an electric furnace at different temperatures of 1600 о С, 1650 о С and 1680 о С and, finally, machining lapping and polishing in particular. The corundum samples were pressed using a powder compacting press 526 model EPM C160 R. We made series of corundum ceramic discs for water taps (300 in every series) for each of the four types of starting press-powders applying different levels of pressure ( МРа), The corundum ceramic discs were then fired in a laboratory chamber furnace Naberterm HT at three different temperatures (1600 о С, 1650 о С, 1680 о С) with one-hour arrest at the maximum temperature. Then they underwent lapping to reduce their thickness and prepare the working surface for polishing. The lapping was performed on a burnishing machine using diamond suspension of grains of a 28/20 size. The processing time was 6 min. The goal in this phase was to polish the surface to a certain degree. The degree of polishing required for ceramic discs is in the 58 % - 75 % range. Values below that threshold result in poor adhesion between components, while values above the 75 % mark make sliding difficult. The polishing was performed with a burnishing machine with diamond suspension of a 7/5 grain size. The processing time was 9 min. Then the polished ceramic discs were thoroughly washed in an ultrasound tub and dried. All discs prepared underwent strict screening for visible defects, surface evenness and thickness. They are shown in Fig. 1. Samples properties determination The fired samples were characterized on the basis of the following physico-mechanical properties: apparent density (determined using the method of Archimedes), a coefficient of abrasion (determined by the loss of mass in percentages) and a degree of surface polishing. Fig. 1. Corundum ceramic discs for water taps.

3 Stoyan Djambazov, Albena Yoleva, Vasil Hristov, Nicolay Ivanov Methods of corundum samples analysis Optic microscopy was performed using a Shanghai Changtang Optical Instrument Co. Ltd. and a Reflected Metallurgical Microscope CMM 30 E. The main requirement referred to a preliminary thermal exposure which provided clearer, distinct and well visible surface in case of using a moderate magnification. This was achieved through heating at a temperature value of 200 о С о С lower than that applied for materials firing. SEM was performed by a scanning electron microscope SEM 525 M, Phillips with an energy-dispersive spectrometer EDS ЕDAX RESULTS AND DISCUSSION Figs. 2-4 show the results referring to the apparent density, the coefficient of abrasion and the degree of surface polishing. The corundum samples made from press-powder type A and fired at 1680 о С got stuck to the fireproofing, i.e. there was no data for their behavior at that temperature. The pressure increase results in increase of the corundum discs density. The temperature increase influences also the consolidation in a positive way. The increased percentage of corundum in the starting powder results in increased density of the product. At 1600 о С о С and high pressure during pressing, the degree of surface polishing increases. Higher density and temperature lead to a lower coefficient of abrasion. The pressing at a lower pressure results in a lower degree of surface polishing, while the increase of the corundum content of the starting powder increases it. No correlation with the temperature variation is established. The coefficient of abrasion shows a tendency of decrease after polishing at higher temperature and pressure levels. There is no correlation between the corundum content of the starting powder and the change of the product s size after lapping and polishing. The optical microscopy results after the thermal exposure of the polished corundum samples are shown in Fig. 5. The microscopic analyses shows that corundum ceramic discs for water taps made from starting powders A, B and C have a finer-grained structure compared to that of discs made from powder D. It is found that the temperature increase results in increase of the grains size. As far as the influence of pressure is concerned, no change is observed. SEM is used to analyze discs from powder B subjected to pressure of 100 МРа and 350 МPа during the pressing stage and firing temperature of 1600 о С and 1650 о С as well as discs from powder C treated at pressure of 350 Мpа at a firing temperature of 1600 о С and 1650 о С. The microscopic images are shown in Fig. 6. It is seen that the pressure applied during pressing and the firing temperature affect the ceramics microstructure after polishing, and more specifically, the pores quantity, size and Fig. 2. Apparent density of corundum samples from resource materials type A, B, C and D pressed applying varying levels of pressure and fired at different temperatures. 527

4 Journal of Chemical Technology and Metallurgy, 50, 4, 2015 Fig. 3. Coefficient of abrasion of corundum samples made from resource materials type A, B, C and D pressed applying varying levels of pressure and fired at different temperatures. Fig. 4. Polishing surface (degree of polishing) of corundum samples from resource materials type A, B, C and D pressed applying varying levels of pressure and fired at different temperatures. type. The SEM analysis also shows that compositions containing 96% of Аl 2 (В) have larger pores compared to those of 98 % content of Аl 2 (C) at 1600 о С and 1650 о С and an equal level of applied pressure during pressing. Fig. 7 shows the SEM image of the corundum ceramics structure viewed from the inside of a pore of ca 50 microns. It is seen that the ceramic sample is very dense, fine-grained, with round grains of a size less than 10 microns. The grains roundedness is an indication of the presence of a glass phase around the edges of the grains. Its composition is determined by an EDS analysis and is as follows: < 1 % СаО, < 1 % МgO, < 1% SiO 2. CONCLUSIONS This work reports data from an investigation on the effect of the pressure in the course of semidry pressing, the firing temperature and the initial composition (percentage of Аl 2 ) on the apparent density, the coefficient 528

5 Stoyan Djambazov, Albena Yoleva, Vasil Hristov, Nicolay Ivanov Fig. 5. Optical micrography of polished corundum ceramic discs for water taps produced at different temperatures and levels of pressure during pressing. Fig. 7. SEM of the structure of corundum ceramic products from type C powder pressed at 350 МРа of pressure and fired at 1650оС - viewed from the inside at an area of a large pore. Fig. 6. SEM of ceramic discs for water taps pressed from powder type B using 100 МРа and 350 МРа and from powder type C МРа of pressure and fired at 1600оС and 1650оС of temperature. of abrasion, the degree of surface polishing and the microstructure of corundum ceramic discs to be used for water taps. It is found that the pressure increase during semidry pressing results in an increased apparent density and a higher degree of surface polishing - a trend that corresponds to a decreased coefficient of abrasion of the samples. Also, the coefficient of abrasion decreases, the 529

6 Journal of Chemical Technology and Metallurgy, 50, 4, 2015 apparent density increases, while the polished surface improves with increase of the firing temperature. The increase of Аl 2 content of the resource material leads to increase of the apparent density, to increase of the degree of surface polishing and decrease of the coefficient of abrasion. Using SEM and optical microscopy we find that the size of the crystals grows, the size and the quantity of the closed pores decreases, while the density of the ceramic materials increases with the increase of the pressure in the course of semidry pressing, with the increase of the firing temperature and the increase of Аl 2 percentage. Corundum ceramic discs for water taps of a homogeneous structure having the highest density, the smallest pores, the lowest coefficient of abrasion and the highest degree of polishing are made from a starting powder containing 98 % of Аl 2 and a glass phase containing СаО, МgO and SiO 2. A firing temperature of 1650 о С and pressure of 350 МРа have to be applied. REFERENCES 1. R. Morrell, Handbook of properties of Technical and Engineering Ceramics. Part 2. Data reviews, Section I. High-alumina ceramics. London: Her Majesty s Stationery Office, 1987, p S.J. Bennison, M.P. Harmer, A history of the role of MgO in the sintering of α-al 2, in Sintering of Advanced Ceramics, Ceramic Transactions 7, American Ceramic Society, Ohio, 1990, p C.A. Handwerker, P.A. Morris, R.L. Coble, Effects of chemical inhomogeneities on grain growth and microstructure in Al 2, J. American Ceramic Society, 71, 1, 1989, S.J. Bennison, M.P. Harmer, Grain-growth kinetics for alumina in the absence of a liquid phase, J. Amer. Cer. Soc., 68, 1, 1985, J. Rodel, A.M. Glaeser, Anisotropy of grain growth in alumina. J. Amer. Cer. Soc., 73, 1990, C.H. Hsueh, A.G. Evans, Microstructure evolution during sintering: the role of evaporation /condensation, Acta Metall., 12, 1983, S. Krueger, G.G. Long, Evolution of the pore size in final-stage sintering of alumina Measured by smallangle X-ray scattering, J. Amer. Cer. Soc., 74, 10, 1991, S. Baik, J.H. Moon, Effects of magnesium oxide on grain-boundary segregation of calcium during sintering of alumina. J. Amer. Cer. Soc., 74, 4, 1991, M. Kumagai, G.L. Messing, Enhaced densification Boehrmte by α-alumina seeding. J. Amer. Cer. Soc., 67, 11, 1984, H. Song, R.L. Coble, Origin and Growth Kinetics of Platelike Abnormal Grains in Liquid-Phase- Sintered Alumina, J. Amer. Cer. Soc., 73, 7, 1990, W.A. Kaysser, M. Sprissler, C.A. Handwerker, J.E. Blendell, Effect of liquid phase on the morphology of grain growth in alumina, J. Amer. Cer. Soc., 70, 5, 1987, B.J. Hockey, Plastic Deformation of Aluminum Oxide by Indentation and Abrasion. J. Amer. Cer. Soc., 54, 5, 1971, C.A. Powell-Dogan, A.H. Heuer, Microstructure of 96 % Alumina Ceramics: I, Characterization of the As-Sintered Materials, J. Amer. Cer. Soc., 73, 12, 1990, G. Elssner, G. Petzow, Modern ceramodraphic preparation and etching methods for incident light and scanning electron microscopy, Microstructural Science, 9, 1981, W.E. Lee, Mark Rainforth, Ceramic Microstructures: Property control by processing, London: Chapman & Hall.,