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Physcal ano Chemcal Processes During Firing Of ZrSi0₄-AljO, Powoers

Dispersed zirconia particles, added as the second phase to mullite materials. enhance their thermo-mechanical properties. Zirconia particles toughen the materiał mainly as a consequence of the tetragonal-monodinic phase trans-formation [6, 7] that can occur during the fracture process or during sintering cooling. In the former case, the stress induced phase transformation toughening mechanism is activated. while in the latter, the micro cracks produced in the matrix as a result of the phase transformation that occurs during the sintering cooling are responsible for the increase of crack propagation energy (8,9]. If mullite ery sta Is appear in a needle shape, then an additional toughening mechanism is present (bridging). There are many ways to produce mullite-zirconia composites, e g.. conventional or Chemical Processing of alumina. zirconia and silica powders as well as sol-gel can be used. The easiest and the most economi-cal way to obtain homogenous mullite-zirconia ceramics is reaction sintering. in which alumina and zircon (ZrSi0₄) [10] are used as the initial powders. Without sintering aids or sta-bilizing oxides. the densification is achieved at 1450-1500°C and the complete mullitization occurs at temperatures near 1600°C (11].The mechanism of high temperaturę reactions proceeding in the mentroned mixture, however. has not been determined so far. The results of investigation concerning the time evolution of phase composition during the reaction between zircon sand and corundum are presented in this paper. The influence of the grain size distribution on the process is also considered.

2. Experimental

Starting powders were constituted by commerdally avail-able alumina CTC20. CTC50 (Alcoa Inc.. USA) and zircon (ZrSi0₄, Australian Zircon Sand. Australia). The character-istics of the raw materials are described in Tabłe 1. Fig. 1 shows particie size distribution of starting materials.

The mixture composition was selected according to stoichiometry of the following reaction:

3 A1A+2 ZrSi0₄ - AI*SiA, ♦ ZrO_z.    (1)

It contained 45.54 % by weight of AIjOj (type 1 or 2) and 54.35 % by weight of ZrSi0₄ (type A or B). Homogenization of the mixture proceeded for 5 hours in acetone. Cylinder-shaped samples having diameter of 1 cm and height of 3 cm were uniaxially compressed under a pressure of 10 MPa. The samples were fi red in an electrical fu mace Nabertherm RHT08/16 at various temperatures ranging from 1300®Cto 1600°C. with furnace heating ratę of 10°C/min. Firing time at the given temperaturę ranged from 30 minutes to 60 hours. The samples were allowed to cod from firing temperaturę in the furnace. The Philips diffractometer system was used for phase Identification and crystallinity analysis of the powders. The quantitative crystalline phase composition was

Fig. 1. Particie size distribotions of starting materials. Zircon type A is not includod. Is 0100 < 30 pm.

calculated by XRD using the Riełveld method. Finał micro-structures were characterized by SEM-EOAX aft er polishing.

3. Results and discussion

The sintering process of ZrSi0₄-AIjOj mixture involves two reactions. the first is de composition of ZrSI0₄, the second. formation of mullite. The total reaction can be expressed as the reaction above. According to the thermodynamic data from the literaturę (12). the free energy of the reaction in the rangę of 1200-1400 K can be described by this linear func-tion of temperaturę, T:

AG = 0.515 T ♦ 72.19 (kJ/mol).    (2)

The calculation shows that the reaction should start when temperatura T > 1400 K (about 1127*C) which is a relatively Iow temperatura. This equation matches to the equation calculated by other authors (13).

Fig. 2 shows the results obtained in case of the samples fired for 5 h at a temperaturę from 1300°C to 1600°C, it is evident that the reaction starts at a much higher temperaturę than the temperatura calculated using thermodynamic data.

The reaction products are detected after firing at 1500*C. The reaction starts at the surface of a zircon grain. as it can be seen in Fig. 3 in which there are favourable places. Then the decomposition reaction goes faster and there are places where it even does not co mm en ce. This indicates that a very

Tablo 1. Chemical composition of starting matortats [wt%].

	Na.O	MgO	AIjOj	SiOj	K.O	CaO	TO,	FełOJ	ZrO,	HfOj
Alumina ty po 1 (CTC20)	0.11	0.06	99.69	0.01	0.01	0.02	0.09	0.01	0.00	0.00
Alumina type 2 (CTC50)	0.10	-	99.8	0.03	-	0.02	-	0.02	-	-
Zircon type A (fine)	0.02	0.02	0.18	31.89	0.01	0.02	0.11	0.04	66.35	1.35
Zircon type B (coarsc)	0.00	0.01	0.27	32.76	0.01	0.02	0.14	0.05	65.40	1.35

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MAIkRIAlYCtRAMIC/NE ,<_t KAMIC MAItMALV. 6 J. I. (2011)