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Synthesis and characterization of the cordierite ceramics from nonstandard raw materials for application in foundry
Materials Letters 57 (2003) 2651 – 2656 www.elsevier.com/locate/matlet
Synthesis and characterization of the cordierite ceramics from nonstandard raw materials for application in foundry Zagorka Acimovic a,*, Ljubica Pavlovic b, Ljiljana Trumbulovic c, Ljubisa Andric b,1, Milan Stamatovic b b
a Faculty of Technology and Metallurgy, 4 Karnegy, 11000 Belgrade, Yugoslavia The Institute for Technology of Nuclear and Other Mineral Raw Materials, Franchet dVEsperey, 11000 Belgrade, Yugoslavia c High Technical School, 34 Sv. Save, 31000 Uzice, Yugoslavia2
Received 10 May 2002; received in revised form 14 October 2002; accepted 25 October 2002
Abstract Cordierite electric ceramics have an important role in modern technology. So far, this material has been used in electrothermics for the production of electric heater supports. However, nowadays, due to its electrical, electromechanical, and especially thermal properties, this ceramic finds its application in electronics for the production of microelectronic components or in the machine-building industry for manufacture of internal combustion components. In this paper, the results of the production of the cordierite ceramics based on sepiolite, as a magnesium silicate component, are presented. For a realistic evaluation of the cordierite quality, which is obtained from nonstandard raw materials, parallel investigations were made with one commercial cordierite mass, which contains talc. The end goal of these investigations is to examine a possibility of application of cordierite ceramics in foundry and defining the technological parameters of production of refractory coatings for sand moulds and cores, as well as of production of refractory linings for application in Lost Foam process. It should be emphasized that cordierite ceramics so far has not been applied in foundry. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Cordierite ceramics; Refractory lining; Lost Foam process; Quality casting
1. Introduction The study of cordierite ceramics synthesis and its application was a subject of many papers after C.W. Parmelee and G.H. Boldwin showed that in the system * Corresponding author. Tel.: +381-11-33-70-419; fax: +38111-33-70-387. E-mail address: [email protected] (L. Andric). 1 Tel.: +381-11-36-91-722; fax: +381-11-36-91-583. 2 All Serbia and Montenegro.
clay –feldspat – talc, ceramic masses with good thermal and electric properties could be obtained [1 – 5]. Significant contribution to development of cordierite ceramics was made by Rankin and Mervin [4] by studying the condition diagram MgO – Al2O3 – SiO2. Thanks to them, a term cordierite appears in literature which designated a phase 2MgO2Al2O35SiO2 [6,7]. A wide application of cordierite ceramics is a consequence, first of all, of its features such as: low diaelectric constant, low coefficient of thermal expansion, high resistance to thermal shock, and good
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01345-9
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mechanical properties. Depending on the composition of original mixtures and applied technology, cordierite products of different densities (1.9 – 2.6 g/cm3) and microstructures are obtained [8 –10]. Cordierite melts incongruently at 1460 jC, when mullite and liquid phase are produced. It is characterized with a complex polymorphism, whereas the following is differentiated: 5 a-cordierite of hexagonal symmetry, which is produced by a fast crystallization in the interval 1000– 1300 jC, 5 h-cordierite of a rhombic structure, produced by crystallization under 950 jC, 5 A-cordierite, metastabile form produced by devitrification of cordierite glass under 925 jC [5]. Cordierite masses always have a very narrow sintering interval, which is one of basic problems in production of cordierite ceramics [6]. Cordierite mass, composed of kaolin, talc and technical silica has sintering interval only 10 – 20 jC, which makes products very sensitive to sintering, due to deformation of products, as well as to properties. The investigations [2,3] showed that, first of all, a coefficient of linear thermal expansion depends on the maximum sintering temperature. If sintering is performed under optimum temperature, enough quantity of cordierite will not be formed and if sintering is performed under optimum temperature a part of the formed cordierite will disintegrate into mullite and metasilicate of magnesium. It will negatively influence the basic technical characteristic on which the application of cordierite is based as electrotechnic and refractory material, and this is a low coefficient of linear thermal expansion. Cordierite is obtained by a high temperature reaction in solid state whereas the following components are used for synthesis: 5 talc, kaolin, silica 5 magnesium carbonate, quartz, silica 5 phosterite, silica, quartz. In this paper, for cordierite synthesis, instead of the usual components of magnesium oxide bearers, a nonstandard component is applied, sepiolite, hydrated magnesium – silicate.
1.1. Requirements of refractory coatings and linings production in foundry Foundry linings are integral part of casting production, since they provide a quality surface of castings without glued and baked sand. For sand moulds and cores refractory coatings are used with alcohol as solvent. During transportation and storing of coatings, processes of separation, sedimentation and alcohol loss occur due to evaporation. Alcohol coatings are mixtures of refractory materials in isopropyl alcohol with suspension agent and corresponding binding system. With development of casting engineering, a demand for quality coatings is increasing—for use of new refractory fillers, suspension agents and binders as well as improving manufacturing technology. Refractory linings for application in Lost Foam procedure must satisfy a number of specific demands as follows: 5 corresponding refractoriness of linings, 5 permeability should be compatible to that of sand which is used for mould making: highly permeable lining is used for rougher sand while medium and low permeable linings are used for finer sand, 5 quick drying, 5 dried layer should be visible on the model, 5 lining should easily stick to the model, 5 there should be a possibility of controlling and adjusting lining layer thickness, 5 appropriate strength, resistance to abrasion, resistance to cracks during storage, resistance to bending and deformation during mould making, 5 if rougher sand is used for mould making and a high casting temperature, then the refractory lining layer should be thicker [10,11]. Most modern refractory linings, depending on the purpose, represent complex mixtures of over 15 components. Four basic components are refractory powder, liquid carrier or solvent, binders, and agents for maintaining suspension. On the market, the linings are delivered under commercial names, and their composition and manufacturing technology are wellkept business secrets. There are various types of linings, especially made to satisfy numerous requirements of different casting procedures, type of mate-
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rial, which is cast and castings configuration. However, there is a permanent need for further research in order to achieve a choice of optimal type and lining thickness for materials and configuration of the castings obtained by the Lost Foam casting process.
2. Experimental 2.1. Material and methods Cordierite ceramics studied in this paper is synthesized from the following original elements: 5 sample C1: kaolin, quartz, technical silica sepiolite 5 sample C2: talc, kaolin, silica, feldspar. The accent in the paper is given to examination of sepiolite obtained from the local deposit Magura in order to estimate a possibility of its wider application in manufacturing cordierite ceramics. Sepiolite is a fibrous hydrated magnesium silicate of the general formula 2MgO3SiO2nH2O. In nature it appears in crystal and colloid states. The crystal form is called a-sepiolite and the colloid one h-sepiolite. The chemical composition of pure sepiolite is 54.02% SiO2, 23.95% MgO, 21.96% H2O. The origin of sepiolite is associated with serpentine disintegration, which is then transformed into magnesite and more or less quantities of sepiolite. Sepiolite is the youngest phase of serpentine transformation and due to that it appears in the form of wires together with magnesite. The chemical composition of sepiolite applied for cordierite synthesis is given in Table 1. As could be seen from the table, the sample of sepiolite has a great annealing loss, which is conditioned by magnesite presence in the original material. This mineral phase is confirmed by the X-ray diffraction methods and Differential Analysis (Figs. 1 and 2). In Fig. 2, the DTA curve of sepiolite is shown which points to existence of endothermic peaks at
Fig. 1. X-ray of Magura sepiolite.
120j, 310j, and 510j as well as of exothermic peak at 810 jC. The endothermic effects correspond to the three types of water: ‘‘zeolite’’ (120 jC), crystal (310 jC) and constitutional OH-group (510 jC). The exothermic effect at 810 jC corresponds to pre-crystallization of sepiolite into magnesium silicate. The original materials for cordierite masses, except for kaolin, are ground up to the grain size of 0.1 mm, and then mixed in the ratio 2 MgO:2 Al2O3:5 SiO2. After homogenisation, the mixture of powders is pressed under pressure of 1 MPa, and then sintered at temperatures of 1250, 1300 and 1350 jC in the time interval of 8 h in the laboratory furnace in oxidizing atmosphere. On the cordierite samples the examinations were made by the following methods: 5 Differential Thermal Analysis on the apparatus Netzsch STA-408 EP in the temperature interval of 20 –1200 jC at heating rate of 10j/min, with the aim of examining characteristic temperatures in
Table 1 The chemical composition of sepiolite Oxide Loss of SiO2 Al2O3 TiO2 Fe2O3 MgO CaO Na2O K2O ignition Mass 23.5 %
55.21 1.19
0.3
0.22
29.01 0.49 0.03 0.14 Fig. 2. Differential thermal sepiolite analysis.
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5 phenol of formaldehyde resin—in powder form of density q = 1.17 g/cm3, viscosity 76 s.
Table 2 The chemical compositions of cordierite samples Designation
C1 C2
The chemical composition [%] SiO2
Al2O3
Fe2O3
CaO
MgO
55.63 46.2
28.59 28.0
1.44 2.4
4.06 6.18
9.88 15.1
which reactions in solid state occur in the threecomponent system MgO – Al2O3 – SiO2. 5 Dilatometric examination at the dilatometer Netzsch up to 1250 jC at heating rate of 10j/ min and holding the maximum temperature at 120 min with the aim of determining the coefficient of thermal expansion a. 5 X-ray analysis was done in the powder diffractometer Philips PW 1710, whereas copper anticathode radiation was applied as well as graphite monochromator, work tension at tube 40 kV, currency power I = 30 mA and the samples were tested within the interval 2h 10– 50j. 5 The density of the sintered part is determined by the conventional method of peak meter by measuring mass and volume. From the obtained cordierite samples C1 and C2, refractory coatings and linings for application in sand casting and Lost Foam process were produced. Control of quality of coatings and linings was performed in accordance with quality requirements and technical conditions for application of these products in foundries, by using examining bodies made of sand and polystyrene. Also, with the aim of determining distribution of refractory fillers—cordierite and binders, preparations of suspension of coatings and linings were observed on polarization microscope. The refractory coatings for sand moulds and cores, based on the cordierite samples C1 and C2 were made on alcohol base with isopropyl alcohol of density 0.785 g/cm3, purity 98%. In the contents of the coating, there was a suspension binding concentrate based on the following: 5 benton SD—organic derivate of montmorilonite 5 resin—organofilic binder, density q = 1.03 g/cm3, iodine number 160, acid number 150 5 yellow dextrin—hydrofil binder in powder form
The refractory linings for the Lost Foam process were made on water base, and within their composition they had: 5 refractory material—cordierite of granulation 40 Am; 5 agent for maintaining suspension—carboxymethylcellululose; 5 binder—bentonite; 5 Na5P3O10. In the lining a higher quantity of binder was not used, since the role of binder was overtaken by clay from the composition of cordierite mass. The density of suspensions of coatings and linings was 2 g/cm3.
3. Results and discussion In Table 2, the chemical compositions of cordierite samples applied for manufacture of coatings and linings are shown. In Table 3, the characteristic temperatures are shown of endothermic and exothermic effects during heating of the samples examined at apparatus for DTA. The endothermic effect corresponds to the phase transformation a-tridimit-a ! quartz, while exothermic effect corresponds to the reaction between MgO and SiO2, when magnesium/metasilicate is obtained [6]. The results of linear shrinkage, density and coefficient of thermal expansion within the interval 20– 1000 jC, obtained by dilatometric study of the cordierite masses, are shown in Table 4. The phase composition of cordierite samples C1 and C2 sintered at three different temperatures as a Table 3 The characteristic temperatures of the examined samples of the cordierite masses based on sepiolite and talc Sample
Endothermic effect (jC)
Exothermic effect (jC)
Sample C1 Sample C2
595 598
1098 1100
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Table 4 The results of dilatometric examinations Sample
Relative shrinkage, Dl/l (%)
Density (g/cm3)
Coefficient of thermal expansion, a jC 1 (20 – 1000)
C1 C2
5.02 4.85
1.64 1.53
2.7 10 6 2.65 10 6
result of Semi-Quantitative X-ray Analysis is shown in Table 5. The detailed analysis of the obtained X-ray photos have shown the following: 5 The sample of the mass C-1/1, sintered at 1250 jC, except for quartz as the most present phase, contains crystobalite, a certain quantity of spinel (MgOAl2O4), corundum (a-Al2O3), while the presence of cordierite is not confirmed. 5 In the sample C-1/2, sintered at 1300 jC, the presence of cordierite is observed (2MgO 2Al2O35SiO2); the contents of quartz and corundum is decreased; the contents of spinel decreases and crystobalite increases. Phosterite (Mg2SiO4) and periclass (MgO) are present in smaller quantities. 5 In the sample C-1/3, sintered at 1350 jC, the presence of cordierite is observed, as well as decreased contents of quartz and corundum, the contents of spinel decreases, of crystobalite increases while phosterite and periclass are present in smaller quantities. From the table, it is seen that the mass sample C2 has the greatest presence of cordierite after annealing at 1350 jC.
Fig. 3. The microstructure of suspension of the C1 sample lining.
Ceramic materials contain phases of crystal materials and pore phases. During sintering dependant on temperature, the evolution of microstructural constituents starts (grains of crystal phases and pores). With increase of temperature and sintering time the processes of grain size growth occur, i.e. decrease of pore size. With the samples sintered at 1250 jC, a growth of neck between certain pores is observed as well as beginning of pores closing, and with the samples sintered at 1300 jC beginning of grain growth and further pores closing is observed; while with the samples sintered at 1350 jC, further grain growth and rounding of closed pores is observed. By examining quality of coatings and linings it is concluded that suspensions, made with the samples C1 and C2 on alcohol and water base, uniquely pour down and during drying they neither crack, nor peel, or form bubbles. The dried layer does not wipe from the mould, core and model.
Table 5 Phase contents of cordierite samples Phase
Cordierite Spinel Phosterite Quartz Crystobalite Corundum Periclass
Designation of the sample and sintering temperatures C-1/1 (1250 jC)
C-1/2 (1300 jC)
C-1/3 (1350 jC)
C-2/1 (1250 jC)
C-2/2 (1300 jC)
C-2/3 (1350 jC)
0 10 0 60 5 20 5
21 4 2 5 50 13 5
56 1 1 0 35 5 2
0 15 2 50 10 20 3
10 12 5 25 20 15 13
50 5 2 20 15 5 3
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Fig. 4. The microstructure of suspension of the C2 sample lining of non-homogenous composition.
The results of examination of the ready lining preparations on polarization microscope show that with both the samples C1 and C2 in suspension, very small irregular cordierite peels are observed which are at average homogenously distributed in the lining mass and are connected with the water solution of sodium thiosulphate (Fig. 3). With coatings, the cordierite peels are also homogenously distributed in the coating mass and are connected with the binder based on rosin, bentonite and dextrin. If refractory coatings and linings of inadequate composition are applied and are inadequately prepared prior to usage, it will have a negative influence on their quality as well as on the quality of castings for which production they are applied. In Fig. 4, a microphotography is given of the suspension of the lining based on C2 sample, which is of inadequate composition and preparation.
4. Conclusions Based on the results of the examinations as presented in this paper, the following can be concluded: 5 Sepiolite of Maguara (YU) deposit, which represents a mixture of minerals of sepiolite and
magnesite, can in the masses of cordierite type replace talc; whereas electrothermic mass is obtained, the properties of which fully satisfy parameters, as prescribed by DIN 40685, group 510. 5 Application of cordierite samples based on sepiolite (C1) and talc (C2) as refractory fillers during manufacture of refractory coatings for sand moulds and cores as well as of refractory linings for the Lost Foam foundry process showed positive effects. Further investigations should be continued towards production of more efficient suspension binder concentrates with a new suspension binder. Development of the cordierite based refractory lining can contribute to the development of Lost Foam process, which represents one of the newer foundry technologies intended to the production of responsible parts in automotive and air craft industries.
References [1] G.W. Parmelee, G.H. Baldwin, Anvendung von Talk in Porzellanmassen, Trans. Am. Soc. 15 (1913) 606. [2] F. Singer, Veber nanartige Steinzeugmassen, Teil I Ber, DKG 10 (1929) 269. [3] G.W. Parmelee, H. Thurnauer, Some effects of additions to a talk body, Bull. Am. Ceram. Soc. 14 (1935) 69. [4] G.A. Rankin, H.E. Mervin, Ternary system MgO – Al2O3 – SiO2, J. Am. Ceram. Soc. 45 (1918) 30. [5] V.I. Baruschkun, G.M. Motveyer, O.P. Mcheldov, Termodinamika Silikatov, Stroizdat, Moscow, 1973, p. 256. [6] G.H. Gibbs, Polymorphisam of cordierite, A1992m, Mineral 51 (1996) 1068. [7] H. Rasch, Cordieritkeramik, 1992, pp. 1 – 5, Chapter 4.1.30. [8] Lj. Pavlovic, Z. Acimovic, Hem. Ind. 53 (4 – 5) (1999) 119 – 122 (in Serbian). [9] Z. Marinkovic, N. Nikolic, Lj. Pavlovic, M. Ristic, J. Min. Metall. 37 (3 – 4 B) (2001) 57 – 67. [10] Lj. Trumbulovic, The Influence of the Type and Thickness of Refractory Lining on the Quality of Castings obtained by Casting with Evaporable Models, Master paper, Facult. Of Techn. and Metall., Belgrade (1997). [11] Lj. Trumbulovic, Z. Acimovic, Lj. Pavlovic´, V. Djordjevic, B. Jordovic, International Symposium—Light Metals and Composite Materials, Belgrade, 26 – 27 October, 1999, pp. 234 – 238, Proceedings.
Synthesis and characterization of the cordierite ceramics from nonstandard raw materials for application in foundry Zagorka Acimovic a,*, Ljubica Pavlovic b, Ljiljana Trumbulovic c, Ljubisa Andric b,1, Milan Stamatovic b b
a Faculty of Technology and Metallurgy, 4 Karnegy, 11000 Belgrade, Yugoslavia The Institute for Technology of Nuclear and Other Mineral Raw Materials, Franchet dVEsperey, 11000 Belgrade, Yugoslavia c High Technical School, 34 Sv. Save, 31000 Uzice, Yugoslavia2
Received 10 May 2002; received in revised form 14 October 2002; accepted 25 October 2002
Abstract Cordierite electric ceramics have an important role in modern technology. So far, this material has been used in electrothermics for the production of electric heater supports. However, nowadays, due to its electrical, electromechanical, and especially thermal properties, this ceramic finds its application in electronics for the production of microelectronic components or in the machine-building industry for manufacture of internal combustion components. In this paper, the results of the production of the cordierite ceramics based on sepiolite, as a magnesium silicate component, are presented. For a realistic evaluation of the cordierite quality, which is obtained from nonstandard raw materials, parallel investigations were made with one commercial cordierite mass, which contains talc. The end goal of these investigations is to examine a possibility of application of cordierite ceramics in foundry and defining the technological parameters of production of refractory coatings for sand moulds and cores, as well as of production of refractory linings for application in Lost Foam process. It should be emphasized that cordierite ceramics so far has not been applied in foundry. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Cordierite ceramics; Refractory lining; Lost Foam process; Quality casting
1. Introduction The study of cordierite ceramics synthesis and its application was a subject of many papers after C.W. Parmelee and G.H. Boldwin showed that in the system * Corresponding author. Tel.: +381-11-33-70-419; fax: +38111-33-70-387. E-mail address: [email protected] (L. Andric). 1 Tel.: +381-11-36-91-722; fax: +381-11-36-91-583. 2 All Serbia and Montenegro.
clay –feldspat – talc, ceramic masses with good thermal and electric properties could be obtained [1 – 5]. Significant contribution to development of cordierite ceramics was made by Rankin and Mervin [4] by studying the condition diagram MgO – Al2O3 – SiO2. Thanks to them, a term cordierite appears in literature which designated a phase 2MgO2Al2O35SiO2 [6,7]. A wide application of cordierite ceramics is a consequence, first of all, of its features such as: low diaelectric constant, low coefficient of thermal expansion, high resistance to thermal shock, and good
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01345-9
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mechanical properties. Depending on the composition of original mixtures and applied technology, cordierite products of different densities (1.9 – 2.6 g/cm3) and microstructures are obtained [8 –10]. Cordierite melts incongruently at 1460 jC, when mullite and liquid phase are produced. It is characterized with a complex polymorphism, whereas the following is differentiated: 5 a-cordierite of hexagonal symmetry, which is produced by a fast crystallization in the interval 1000– 1300 jC, 5 h-cordierite of a rhombic structure, produced by crystallization under 950 jC, 5 A-cordierite, metastabile form produced by devitrification of cordierite glass under 925 jC [5]. Cordierite masses always have a very narrow sintering interval, which is one of basic problems in production of cordierite ceramics [6]. Cordierite mass, composed of kaolin, talc and technical silica has sintering interval only 10 – 20 jC, which makes products very sensitive to sintering, due to deformation of products, as well as to properties. The investigations [2,3] showed that, first of all, a coefficient of linear thermal expansion depends on the maximum sintering temperature. If sintering is performed under optimum temperature, enough quantity of cordierite will not be formed and if sintering is performed under optimum temperature a part of the formed cordierite will disintegrate into mullite and metasilicate of magnesium. It will negatively influence the basic technical characteristic on which the application of cordierite is based as electrotechnic and refractory material, and this is a low coefficient of linear thermal expansion. Cordierite is obtained by a high temperature reaction in solid state whereas the following components are used for synthesis: 5 talc, kaolin, silica 5 magnesium carbonate, quartz, silica 5 phosterite, silica, quartz. In this paper, for cordierite synthesis, instead of the usual components of magnesium oxide bearers, a nonstandard component is applied, sepiolite, hydrated magnesium – silicate.
1.1. Requirements of refractory coatings and linings production in foundry Foundry linings are integral part of casting production, since they provide a quality surface of castings without glued and baked sand. For sand moulds and cores refractory coatings are used with alcohol as solvent. During transportation and storing of coatings, processes of separation, sedimentation and alcohol loss occur due to evaporation. Alcohol coatings are mixtures of refractory materials in isopropyl alcohol with suspension agent and corresponding binding system. With development of casting engineering, a demand for quality coatings is increasing—for use of new refractory fillers, suspension agents and binders as well as improving manufacturing technology. Refractory linings for application in Lost Foam procedure must satisfy a number of specific demands as follows: 5 corresponding refractoriness of linings, 5 permeability should be compatible to that of sand which is used for mould making: highly permeable lining is used for rougher sand while medium and low permeable linings are used for finer sand, 5 quick drying, 5 dried layer should be visible on the model, 5 lining should easily stick to the model, 5 there should be a possibility of controlling and adjusting lining layer thickness, 5 appropriate strength, resistance to abrasion, resistance to cracks during storage, resistance to bending and deformation during mould making, 5 if rougher sand is used for mould making and a high casting temperature, then the refractory lining layer should be thicker [10,11]. Most modern refractory linings, depending on the purpose, represent complex mixtures of over 15 components. Four basic components are refractory powder, liquid carrier or solvent, binders, and agents for maintaining suspension. On the market, the linings are delivered under commercial names, and their composition and manufacturing technology are wellkept business secrets. There are various types of linings, especially made to satisfy numerous requirements of different casting procedures, type of mate-
Z. Acimovic et al. / Materials Letters 57 (2003) 2651–2656
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rial, which is cast and castings configuration. However, there is a permanent need for further research in order to achieve a choice of optimal type and lining thickness for materials and configuration of the castings obtained by the Lost Foam casting process.
2. Experimental 2.1. Material and methods Cordierite ceramics studied in this paper is synthesized from the following original elements: 5 sample C1: kaolin, quartz, technical silica sepiolite 5 sample C2: talc, kaolin, silica, feldspar. The accent in the paper is given to examination of sepiolite obtained from the local deposit Magura in order to estimate a possibility of its wider application in manufacturing cordierite ceramics. Sepiolite is a fibrous hydrated magnesium silicate of the general formula 2MgO3SiO2nH2O. In nature it appears in crystal and colloid states. The crystal form is called a-sepiolite and the colloid one h-sepiolite. The chemical composition of pure sepiolite is 54.02% SiO2, 23.95% MgO, 21.96% H2O. The origin of sepiolite is associated with serpentine disintegration, which is then transformed into magnesite and more or less quantities of sepiolite. Sepiolite is the youngest phase of serpentine transformation and due to that it appears in the form of wires together with magnesite. The chemical composition of sepiolite applied for cordierite synthesis is given in Table 1. As could be seen from the table, the sample of sepiolite has a great annealing loss, which is conditioned by magnesite presence in the original material. This mineral phase is confirmed by the X-ray diffraction methods and Differential Analysis (Figs. 1 and 2). In Fig. 2, the DTA curve of sepiolite is shown which points to existence of endothermic peaks at
Fig. 1. X-ray of Magura sepiolite.
120j, 310j, and 510j as well as of exothermic peak at 810 jC. The endothermic effects correspond to the three types of water: ‘‘zeolite’’ (120 jC), crystal (310 jC) and constitutional OH-group (510 jC). The exothermic effect at 810 jC corresponds to pre-crystallization of sepiolite into magnesium silicate. The original materials for cordierite masses, except for kaolin, are ground up to the grain size of 0.1 mm, and then mixed in the ratio 2 MgO:2 Al2O3:5 SiO2. After homogenisation, the mixture of powders is pressed under pressure of 1 MPa, and then sintered at temperatures of 1250, 1300 and 1350 jC in the time interval of 8 h in the laboratory furnace in oxidizing atmosphere. On the cordierite samples the examinations were made by the following methods: 5 Differential Thermal Analysis on the apparatus Netzsch STA-408 EP in the temperature interval of 20 –1200 jC at heating rate of 10j/min, with the aim of examining characteristic temperatures in
Table 1 The chemical composition of sepiolite Oxide Loss of SiO2 Al2O3 TiO2 Fe2O3 MgO CaO Na2O K2O ignition Mass 23.5 %
55.21 1.19
0.3
0.22
29.01 0.49 0.03 0.14 Fig. 2. Differential thermal sepiolite analysis.
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5 phenol of formaldehyde resin—in powder form of density q = 1.17 g/cm3, viscosity 76 s.
Table 2 The chemical compositions of cordierite samples Designation
C1 C2
The chemical composition [%] SiO2
Al2O3
Fe2O3
CaO
MgO
55.63 46.2
28.59 28.0
1.44 2.4
4.06 6.18
9.88 15.1
which reactions in solid state occur in the threecomponent system MgO – Al2O3 – SiO2. 5 Dilatometric examination at the dilatometer Netzsch up to 1250 jC at heating rate of 10j/ min and holding the maximum temperature at 120 min with the aim of determining the coefficient of thermal expansion a. 5 X-ray analysis was done in the powder diffractometer Philips PW 1710, whereas copper anticathode radiation was applied as well as graphite monochromator, work tension at tube 40 kV, currency power I = 30 mA and the samples were tested within the interval 2h 10– 50j. 5 The density of the sintered part is determined by the conventional method of peak meter by measuring mass and volume. From the obtained cordierite samples C1 and C2, refractory coatings and linings for application in sand casting and Lost Foam process were produced. Control of quality of coatings and linings was performed in accordance with quality requirements and technical conditions for application of these products in foundries, by using examining bodies made of sand and polystyrene. Also, with the aim of determining distribution of refractory fillers—cordierite and binders, preparations of suspension of coatings and linings were observed on polarization microscope. The refractory coatings for sand moulds and cores, based on the cordierite samples C1 and C2 were made on alcohol base with isopropyl alcohol of density 0.785 g/cm3, purity 98%. In the contents of the coating, there was a suspension binding concentrate based on the following: 5 benton SD—organic derivate of montmorilonite 5 resin—organofilic binder, density q = 1.03 g/cm3, iodine number 160, acid number 150 5 yellow dextrin—hydrofil binder in powder form
The refractory linings for the Lost Foam process were made on water base, and within their composition they had: 5 refractory material—cordierite of granulation 40 Am; 5 agent for maintaining suspension—carboxymethylcellululose; 5 binder—bentonite; 5 Na5P3O10. In the lining a higher quantity of binder was not used, since the role of binder was overtaken by clay from the composition of cordierite mass. The density of suspensions of coatings and linings was 2 g/cm3.
3. Results and discussion In Table 2, the chemical compositions of cordierite samples applied for manufacture of coatings and linings are shown. In Table 3, the characteristic temperatures are shown of endothermic and exothermic effects during heating of the samples examined at apparatus for DTA. The endothermic effect corresponds to the phase transformation a-tridimit-a ! quartz, while exothermic effect corresponds to the reaction between MgO and SiO2, when magnesium/metasilicate is obtained [6]. The results of linear shrinkage, density and coefficient of thermal expansion within the interval 20– 1000 jC, obtained by dilatometric study of the cordierite masses, are shown in Table 4. The phase composition of cordierite samples C1 and C2 sintered at three different temperatures as a Table 3 The characteristic temperatures of the examined samples of the cordierite masses based on sepiolite and talc Sample
Endothermic effect (jC)
Exothermic effect (jC)
Sample C1 Sample C2
595 598
1098 1100
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Table 4 The results of dilatometric examinations Sample
Relative shrinkage, Dl/l (%)
Density (g/cm3)
Coefficient of thermal expansion, a jC 1 (20 – 1000)
C1 C2
5.02 4.85
1.64 1.53
2.7 10 6 2.65 10 6
result of Semi-Quantitative X-ray Analysis is shown in Table 5. The detailed analysis of the obtained X-ray photos have shown the following: 5 The sample of the mass C-1/1, sintered at 1250 jC, except for quartz as the most present phase, contains crystobalite, a certain quantity of spinel (MgOAl2O4), corundum (a-Al2O3), while the presence of cordierite is not confirmed. 5 In the sample C-1/2, sintered at 1300 jC, the presence of cordierite is observed (2MgO 2Al2O35SiO2); the contents of quartz and corundum is decreased; the contents of spinel decreases and crystobalite increases. Phosterite (Mg2SiO4) and periclass (MgO) are present in smaller quantities. 5 In the sample C-1/3, sintered at 1350 jC, the presence of cordierite is observed, as well as decreased contents of quartz and corundum, the contents of spinel decreases, of crystobalite increases while phosterite and periclass are present in smaller quantities. From the table, it is seen that the mass sample C2 has the greatest presence of cordierite after annealing at 1350 jC.
Fig. 3. The microstructure of suspension of the C1 sample lining.
Ceramic materials contain phases of crystal materials and pore phases. During sintering dependant on temperature, the evolution of microstructural constituents starts (grains of crystal phases and pores). With increase of temperature and sintering time the processes of grain size growth occur, i.e. decrease of pore size. With the samples sintered at 1250 jC, a growth of neck between certain pores is observed as well as beginning of pores closing, and with the samples sintered at 1300 jC beginning of grain growth and further pores closing is observed; while with the samples sintered at 1350 jC, further grain growth and rounding of closed pores is observed. By examining quality of coatings and linings it is concluded that suspensions, made with the samples C1 and C2 on alcohol and water base, uniquely pour down and during drying they neither crack, nor peel, or form bubbles. The dried layer does not wipe from the mould, core and model.
Table 5 Phase contents of cordierite samples Phase
Cordierite Spinel Phosterite Quartz Crystobalite Corundum Periclass
Designation of the sample and sintering temperatures C-1/1 (1250 jC)
C-1/2 (1300 jC)
C-1/3 (1350 jC)
C-2/1 (1250 jC)
C-2/2 (1300 jC)
C-2/3 (1350 jC)
0 10 0 60 5 20 5
21 4 2 5 50 13 5
56 1 1 0 35 5 2
0 15 2 50 10 20 3
10 12 5 25 20 15 13
50 5 2 20 15 5 3
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Fig. 4. The microstructure of suspension of the C2 sample lining of non-homogenous composition.
The results of examination of the ready lining preparations on polarization microscope show that with both the samples C1 and C2 in suspension, very small irregular cordierite peels are observed which are at average homogenously distributed in the lining mass and are connected with the water solution of sodium thiosulphate (Fig. 3). With coatings, the cordierite peels are also homogenously distributed in the coating mass and are connected with the binder based on rosin, bentonite and dextrin. If refractory coatings and linings of inadequate composition are applied and are inadequately prepared prior to usage, it will have a negative influence on their quality as well as on the quality of castings for which production they are applied. In Fig. 4, a microphotography is given of the suspension of the lining based on C2 sample, which is of inadequate composition and preparation.
4. Conclusions Based on the results of the examinations as presented in this paper, the following can be concluded: 5 Sepiolite of Maguara (YU) deposit, which represents a mixture of minerals of sepiolite and
magnesite, can in the masses of cordierite type replace talc; whereas electrothermic mass is obtained, the properties of which fully satisfy parameters, as prescribed by DIN 40685, group 510. 5 Application of cordierite samples based on sepiolite (C1) and talc (C2) as refractory fillers during manufacture of refractory coatings for sand moulds and cores as well as of refractory linings for the Lost Foam foundry process showed positive effects. Further investigations should be continued towards production of more efficient suspension binder concentrates with a new suspension binder. Development of the cordierite based refractory lining can contribute to the development of Lost Foam process, which represents one of the newer foundry technologies intended to the production of responsible parts in automotive and air craft industries.
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