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Creep Behavior of Multi-Cation • -Sialon Partially Stabilized

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Creep Behavior of Multi-Cation • -Sialon Partially Stabilized Creep behavior of multi-cation α-SiAlON partially stabilized, produced with an yttrium- rare earth oxide mixture (CRE2O3) C. Santos1; K. Strecker1; M.J. R. Barboza1; S. A. Baldacim2; F. Piorino Neto2; O. M.M.Silva2; C.R.M.Silva2 1 Departamento de Engenharia de Materiais DEMAR-FAENQUIL, Polo Urbo-industrial, gleba AI-6, s/n, cep 12600-000, Lorena-SP Brazil. 2 Centro Técnico Aeroespacial – Divisão de Materiais AMR-CTA, Pça. Marechal do Ar Eduardo Gomes, 50, cep. 12228-904, S. J. Campos –SP Brazil Key-words: α-SiAlON, Si3N4, sintering additives, CRE2O3, compressive creep. ABSTRACT α−SiAlON (α’) is a solid solution of α−Si3N4, where Si and N are substituted by Al and O, respectively. The principal stabilizers of the α’-phase are Mg, Ca, Y and rare earth cations. In this way, the possible use of the yttrium-rare earth oxide mixture, CRE2O3, produced at FAENQUIL, in obtaining these SiAlONs was investigated. Samples were sintered by hot- pressing at 17500C, for 30 minutes, using a sintering pressure of 20 MPa. Creep behavior of the hot-pressed CRE-α-SiAlON/β-Si3N4 ceramic was investigated, using compressive creep tests, in air, at 1280 to 13400C, under stresses of 200 to 350 MPa, for 70 hours. This type of ceramic exhibited high creep and oxidation resistance. Its improved high-temperature properties are mainly due to the absence or reduced amount of intergranular phases, because of the incorporation of the metallic cations from the liquid phase formed during sintering into the Si3N4 structure, forming a α’/β composite. 1. INTRODUCTION Silicon nitride (Si3N4) and its solid solutions (SiAlONs) are ceramics widely used for structural applications due to its physical and mechanical properties, such as high wear, hardness and creep resistance [1,2]. The liquid phase sintering of Si3N4 is governed by a solution-precipitation (α−β−Si3N4 transformation) mechanism, that occurs in temperatures 0 between 1400 and 1800 C and is related, among other factors, to the initial α−Si3N4 phase content of the starting powder, amount and type of additives used and sintering parameters such as temperature, time and atmosphere [2-3]. The utilization of the additives based on Al2O3, AlN demonstrate great potential for obtaining of SiAlONs [4,5]. The microstructural aspects and the intergranular phase content are preponderant factors for the high creep resistance of these materials [6,7]. Due to their characteristics previously mentioned, the technological applications of silicon nitride ceramics (Si3N4) are numerous, and in this context, the study and understanding of the mechanical behavior of these materials at high temperatures is of primary interest. It has been also demonstrated that the secondary glassy phases, which appear during sintering, have a strong influence on the creep resistance [8]. In these ceramics the remaining glassy phase softens at high temperatures and reduces the high temperature strength. The creep deformation of materials containing intergranular glassy phase is generally thought to occur as a combination of viscous flow, grain boundary sliding, solution-precipitation and cavitation, mechanisms [6-8]. Previous works [9,10] have shown that the yttrium-rare earth oxide mixture, CRE2O3, consisting mainly of Y2O3 (44 wt.%), Yb2O3 (17 wt.%), Er2O3 (14 wt.%) and Dy2O3 (10 wt.%) produced at DEMAR-FAENQUIL [11], is an effective and cheap substitute for pure Y2O3 as sintering additive for Si3N4 ceramics, presenting a similar sinterability and mechanical properties at room temperature behavior. The objective of the present work has been to characterize the creep behavior of the hot- pressed multi-cation α-SiAlON/β-Si3N4 ceramics produced with AlN and CRE2O3 as sintering additives. 2. EXPERIMENTAL PROCEDURE 2.1. Processing and creep tests The powder batches prepared consisted of a mixture of 95 vol.% of commercial α−Si3N4 (HCST-Germany containing 0.1wt% Si and 5wt.% β−Si3N4), and 5 vol.% of AlN (HCST- Germany) and CRE2O3 (FAENQUIL-Brazil) as sintering additive, at a molar ratio of 9:1, respectively, in order to form a certain content of α-SiAlON with β-Si3N4 as matrix of this ceramic. The powders were mixed by planetary ball milling during 2 h using ethanol as milling media. After mixing, the powder batches were dried first in a rotary evaporator and subsequently in an oven at 120oC during 12 h. The powder mixtures were then compacted and o sintered by uniaxial hot-pressing under 20 MPa pressure in 0.1 MPa N2 atmosphere at 1750 C for 30 minutes, with a heating rate of 15 0C/minute. From the as received materials, samples of approximate 6 x 3 x 3 mm3 were cut and grounded for tests. Compressive creep tests were carried in air, at temperatures ranging between 1280 and 1340oC and under nominal stresses of 200, 300 and 350 MPa. The details of the experimental device can be found elsewhere [12]. Strain x time curves were obtained and the results were analyzed in terms of Norton‘s equation [13] as shown in Eq. (1): • n ε ss = Aσ exp(-Q/RT) (1) • Where, ε ss is the true steady-state strain rate, A is a constant, σ is the applied stress, T the absolute temperature and R the gas constant, n and Q are, respectively, the stress exponent and the apparent activation energy for creep in the steady-state region. The results were performed: at constant temperature and different stress, in order to evaluate the stress exponent n; at constant load and different temperatures, to evaluate the apparent activation energy Q. All tests were finished after 70 hours, without evidence of macroscopic failure. 2.2. Sample Characterization The samples were characterized by relative density using the immersion method. Phase analysis was done by X-ray diffraction (XRD), in both samples before and after creep tests comparing the diffraction patterns with the JCPDS files. Quantitative α’/β-phase contents were evaluated using the procedure proposed by Gazzara et al [14]. XRD analysis was conducted on a plane parallel to the hot pressing direction in a bulk specimen. The ratio of β- phase (101)/(210) peak area in the XRD patterns was used as an indication of the orientation degree of elongated grains after creep tests [15,16]. For microstructural analysis, using scanning electron microscopy (SEM), the samples were chemically etched by a 1:1 mixture of NaOH and KOH at 500 oC, for 4 minutes. By SEM analysis on a plane parallel to the hot pressing direction in the crept sample, the reorientation of the grains was observed. 3. RESULTS AND DISCUSSION 3.1. Samples Characterization The sintered samples presented average relative density of 96%. This result is related, mainly, to the reduction of the liquid phase amount in the sintering, during the solution- precipitation stage, where atoms of Al and O, besides cations Y+3 (and other rare earth cations) incorporate in the Si3N4 structure forming solid solution, substitutional and intersticial, respectively [4,5]. This reduction of the additive content hinders the obtaining of full density, due to the difficulty of performance of the densification mechanisms, in systems with little or any amount of liquid phase. XRD analysis identify the presence of α-SiAlON and β-Si3N4 as crystalline phases, in relative amounts of 10% and 90%, respectively. 3.2. Creep behavior Table 1 presents steady-state creep rates in the appraised conditions in this work. Typical creep curve obtained in this work is presented in the Fig. 1. Table 1– Creep parameters of samples tested between 1280 and 1340oC, under stress of 200, 300 and 350 MPa. Tests conditions • Creep Rates (ε ss ) o Temperature ( C) Stress (MPa) • -1 • -1 ε ss (h ) ε ss (s ) 1280 300 4.2 x 10-4 1.2 x 10-7 1300 200 2.1 x 10-4 5.9 x 10-8 1300 300 6.1 x 10-4 1.7 x 10-7 1300 350 7.5 x 10-4 2.1 x 10-7 1340 300 1.7 x 10-3 4.6 x 10-7 0,12 1300oC / 300 MPa 0,10 . -4 0 0,08 εSS= 6.1 x 10 (1300 C - 300 MPa) (mm/mm) 0,06 Strain 0,04 0,02 0,00 0 10203040506070 Time (h) Fig. 1 - Typical creep curve obtained by compressive test, at 1300oC and 300 MPa. Silicon nitride based ceramics (Si3N4), usually suffer creep deformation due to the viscous flow, with sliding of grain boundary, solution-precipitation and cavitation [6-8,16]. The creep results showed in Table 1, indicate that the α-SiAlON/β-Si3N4 (α‘/β) composites, produced in this work, present lower steady-state creep rates that β-Si3N4 ceramics produced with the same additive, CRE2O3, with the same additive content, and Al2O3 substituting AlN. The α‘/β composites present steady-state creep rates of 2.1 x 10-4 h-1 and 6.1 x 10-4 h-1 0 for stresses of 200 and 300 MPa, respectively, while β-Si3N4 ceramics, at 1300 C, the creep rates were 7.94 x 10-4 h-1 (200 MPa) and 1.19 x 10-3 h-1(300 MPa) [12]. Comparatively, these composites, in the same stresses and temperatures, are much more creep resistant for compression. The results indicated reductions of the creep rates among 50% (1300oC / 300 o MPa) and 75% (1300 C / 200 MPa) when AlN was used as additive, in substitution to Al2O3. The justification for such behavior can be in the microstructural characteristics and in the crystalline phases presents in this composite, that possesses a lower amount of phase intergranular. The values of steady-state creep rates of the α-SiAlON/β-Si3N4 ceramics are plotted logarithmically as a function applied stress in Fig.
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