UDK 621.762.5:669.018.95 Professional article/Strokovni članek
ISSN 1580-2949 MTAEC9, 47(5)669(2013)
OXIDATION OF THE Al2O3-TiB2 COMPOSITES PRODUCED WITH THE REDUCTION-COMBUSTION SYNTHESIS TECHNIQUE
OKSIDACIJA KOMPOZITA Al2O3-TiB2, IZDELANEGA S TEHNIKO REDUKCIJSKE ZGOREVNE SINTEZE
Nuri Ergin, Yigit Garip, Ozkan Ozdemir
Sakarya University, Technology Faculty, Department of Metallurgy and Materials Engineering, 54187 Esentepe Campus, Sakarya, Turkey
nergin@sakarya.edu.tr
Prejem rokopisa — received: 2012-08-31; sprejem za objavo - accepted for publication: 2013-03-04
In this study, Al2O3-TiB2 composites were synthesized in an electrical resistance furnace in open atmosphere under the uniaxial pressure of 150 MPa at 1200 °C for 4 h, using the reduction-combustion synthesis technique. The initial powder mixture used in this study is Al+TiO2+B2O3. TiB2, AhO and some trace phases were found in the produced composites using the X-ray diffraction analysis. The densities of the samples were measured using the Archimedes' technique. The relative density was determined as 94.2 % for the composites. The oxidation properties of the composites were examined in open atmosphere at 600 °C, 800 °C and 1000 °C after up to 64 h. The activation energy of the composite was calculated to be 90 kJ/mol.
Keywords: composite, sintering, reduction combustion synthesis, oxidation
V tej študiji je bil sintetiziran kompozit Al2O3-TiB2 v električni uporovni peči z normalno atmosfero pri enoosnem tlaku 150 MPa in 4 h pri 1200 °C z uporabo tehnike redukcijske zgorevne sinteze. Začetna mešanica prahov, uporabljena v tej študiji, je bila Al+TiO2+B2O3. Z rentgensko difrakcijo so bili v izdelanem kompozitu odkriti TiB2, AUO3 in nekaj faz v sledovih. Gostota vzorcev je bila izmerjena z Arhimedovo tehniko. Relativna gostota kompozita je bila 94,2-odstotna. Oksidacijske lastnosti kompozita so bile preiskane na zraku po 64 h na temperaturah 600 °C, 800 °C in 1000 °C. Izračunana aktivacijska energija kompozita je bila 90 kJ/mol.
Ključne besede: kompozit, sintranje, redukcijska zgorevna sinteza, oksidacija
1 INTRODUCTION
Reduction (thermite type) combustion synthesis (RCS) is one of the three main types of combustion synthesis from the viewpoint of chemical nature.1 The in-situ synthesis is used for fabricating the metal- or ceramic-matrix composites.2 As the reinforcements are generated directly from the chemical reaction within the matrix, the composites show many excellent advantages, such as a clean reinforcement-matrix interface, fine and thermodynamically stable reinforcement, good compatibility and high bonding strength between the reinforcement and the matrix, and low fabrication costs.3 When TiC or TiB2 are combined with ALO3, the composite, without a significant drop in the hardness, has a better oxidation resistance and possesses a superior mechanical strength and fracture toughness than TiC or TiB2 alone. Researchers have chosen reduction (thermite type) combustion synthesis systems like TiO2/Ti-B2O3/B-Al, Nb-B-Al-Nb2O5, ZrO2-B2O3-Al, TiO2-Al for fabricating the AI2O3 based in-situ composites.4-6 The main objective of the present study is to investigate the synthesis of the Al2O3-TiB2 in-situ composites produced from TiO2, B2O3 and Al precursors with the one-step, pressure-assisted, reduction-combustion technique, and the oxidation properties of the produced composites.
2 EXPERIMENTAL DETAILS
The Al2O3-TiB2 in-situ composite was densified by uniaxial loading during the reaction. TiO2 (98.8 % purity, 1 pm), B2O3 (99.99 % purity, less than 38 pm) and Al (99 % purity, 15 pm) were used in the powder mixtures to produce the TiB2-Al2O3 composites using the alumi-nothermic reduction. The mixed powders were pressed in a cylindrical mold, then the in-situ composite was formed in an electrical resistance furnace in open atmosphere under a uniaxial pressure of 150 MPa, at 1200 °C, with a heating rate of 20 °C/min and for 4 h. To examine the relative density, the Archimedes' method with a sensitive balance (0.0001 g) was applied. The specimens were polished with the emery papers (up to 1200 grit) and, finally, with a diamond paste up to 1 pm before the oxidation test. The oxidation properties of the samples were investigated at 600 °C, 800 °C and 1000 °C for (4, 16, 32 and 64) h in open atmosphere. Each sample was carefully weighed before and after the oxidation test to determine the weight changes. The morphology and nature of the oxide layer and the phases formed in the oxidized layers of the samples, tested at 600 °C, 800 °C and 1000 °C for 64 h, were characterized using the SEM and XRD analyses. In order to understand the kinetics of the oxidation, the data were analyzed using the parabolic law:
Aw
T
=k p î
(i)
where Aw is the change in the weigh, A is the surface area of the sample, t is the oxidation time and kp is the parabolic rate constant.7
3 RESULTS AND DISCUSSION
The composite produced with the pressure-assisted RCS is compact and dense. This technique is more advantageous than the classic two-step production methods. The aluminothermic reaction results in the Al2O3-TiB2 composite according to the following reaction:
3TiO2+3B2O3+10Al ^5Al2O3+3TiB2
(2)
TiB, TiO2 and the AIsBOq trace phases, along with the major Al2O3 and TiB2 peaks, were observed during the XRD analysis (Figure 1). The Al2O3-TiB2 composite with the calculated volume proportions of Al2O3 (71 %) and TiB2 (27 %) was obtained with a synthesizing system of 3TiO2-3B2O3-10Al.8 In the present study, the relative densities of the samples, synthesized under a pressure of 150 MPa and at 1200 °C for 4 h, were measured as 94.2 %.
Figure 1: XRD diffraction patterns of the synthesizing at 1200 °C for 4 h
Slika 1: Rentgenska difrakcija sintetiziranega kompozita po 4 h na 1200 °C
Table 1: Variation in the weight gain as a function of the process time and temperature
Tabela 1: Spreminjanje prirastka mase v odvisnosti od časa in temperature procesa
Time (h)	Weight change (g/cm2)		
	600 °C	800 °C	1000 °C
4	0.587	1.493	2.322
16	0.618	2.899	3.277
32	0.709	3.123	3.738
64	0.782	3.322	4.371
The mass change of the oxidized samples during the oxidation treatment at 600 °C, 800 °C and 1000 °C as a function of the process time for the Al2O3-TiB2 composites occurred parabolically with the process time. The mass changes of the composites during the oxidation test at 600 °C, 800 °C and 1000 °C, lasting for 4-64 h, are listed in Table 1.
The parabolic rate constant was calculated from the slope of the plots that was drawn from the square of the mass change versus the treatment time of the composites. The parabolic rate constants (kp) of the composites during the oxidation tests at 600 °C, 800 °C and 1000 °C are 0.322-10-5 g2/(cm4 s), 5.98-10-5 g2/(cm4 s), and 9.59-10-5 g2/(cm4 s), respectively. The temperature dependence of the parabolic rate constant (kp) follows an Arrhenius-type expression, kp = ko exp(-Q/RT). The slope of the plots that was drawn from the LKp-values versus 1/T is to give the Q/R-value. In the present study, the calculated value of the activation energy was approximately 90 kJ/mol in the temperature range of 600-1000 °C, for the Al2O3-TiB2 composite.
Tampieri and Bellosi9 have reported the activation energy of 230 kJ/mol (T = 400-900 °C) and 40 kJ/mol (T = 900-1100 °C) for the monolithic TiB2. An activation energy of 110.56 kJ/mol was reported for the temperature range of 750-950 °C by Murthy et al.10 Murthy et al. explained that the vast difference in the activation energy for the TiB2 oxidation with the temperature is due to the change in the mechanism caused by the evaporation of B2O3 at higher temperatures. The SEM images of the oxidized surfaces of the composites at 1000 °C for 64 h are shown in Figure 2. The cracks, along with the coarsening of the oxide, were observed on the surface of the sample oxidized at 1000 °C. The results presented are consistent with the study of Murthy et al.7 A large volume expansion was seen during the oxidation of the composites. During the oxidation process, the TiB2 phase oxidized and, subsequently, changed to the TiO2 phase. During the oxidation, the transformation of the TiB2 phase to TiO2 causes a cracking of the oxide layer, resulting in an increase in the active area for further oxidation.7 Because of the active area formed by the cracks, the oxide layer was thought to accelerate this mechanism. The oxide layer was charac-
Figure 2: SEMs of the oxidized surfaces of the composites at 1000 °C after 64 h
Slika 2: SEM-posnetka oksidirane površine kompozita po 64 h na 1000 °C
2
Figure 3: XRD patterns of the oxidized surfaces of the composite specimens at 1000 °C after 64 h
Slika 3: Rentgenska difrakcija oksidirane površine vzorca po 64 h na 1000 °C
terized by the presence of the surface cracks, probably caused by either the large volume expansion of B2O3 or the thermal stresses generated during the cooling.11 The X-ray diffraction patterns of the oxidized undoped and doped composites at 1000 °C for 64 h are given in Figure 3.
The phases formed in the oxidized layer of the composite materials at 1000 °C after 64 h were the TiO2, B2O, ALBO9 and Ti3O5 phases, besides the ALO3 and TiB2 phases. Possible reactions of TiB2, B2O3 and ALO3 are:
TiB2+5/2O2^TiO2+B2O3 (solid if T < 450 °C, and liquid if T > 450 °C), (3)
5/2Al2O3+1/2B2O3^Al5BO9	(4)
TiO2 is a semi-protective oxide formed at a high temperature. Depending on the defect concentration, the growth of TiO2 is governed by either an outward diffusion of the interstitial Ti ions or an inward diffusion of the oxygen ions via the vacancies.11 It is possible that B2O3 has the dominant effect on the oxidation of the TiO2 phase.
4 CONCLUSION
This study reports on the oxidation properties of an Al2O3-TiB2 in-situ composite obtained with the pressure-assisted, reduction-combustion synthesis of the thermite mixtures. The mass (the weight gain) of the oxidized samples during the oxidation treatment in open atmosphere at 600 °C, 800 °C and 1000 °C as a function of the process time (up to 64 h) for the A^O3-TiB2 composites was changing parabolically with the process time. The activation energy of the composites was calculated to be 90 kJ/mol.
Acknowledgement
This work was supported by SAU Scientific Research Foundation (Project No: 2009-50-01-043).
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