HIGH TEMPERATURE ELASTIC STRAIN EVOLUTION IN Si 3 N 4 -BASED CERAMICS

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1 Copyright JCPDS - International Centre for Diffraction Data 23, Advances in X-ray Analysis, Volume HIGH TEMPERATURE ELASTIC STRAIN EVOLUTION IN Si 3 N 4 -BASED CERAMICS G.A. Swift 1, E. Ustundag 1, B. Clausen 1, M.A.M. Bourke 2, H.-T. Lin 3, C.-W. Li 4 1 California Institute of Technology, Pasadena, CA 2 Los Alamos National Laboratory, Los Alamos, NM 3 Oak Ridge National Laboratory, Oak Ridge, TN 4 Honeywell Corporation, Morristown, NJ ABSTRACT Tension experiments in vacuum were performed on a monolithic Si 3 N 4 sample at 1375 C and a 2Vol% SiC p -Si 3 N 4 composite sample at 14 C using the new SMARTS diffractometer at the Los Alamos Neutron Science Center. The deep penetration of neutrons facilitated these in-situ studies. In particular, the hkl-dependent strains were measured and the results interpreted by Eshelby-based modeling for the Si 3 N 4 -SiC p sample. The diffraction data also provided information about thermal expansion coefficients for both specimens and the elastic constants at high temperature for the monolithic sample. INTRODUCTION Structural ceramics have been gaining more use worldwide as their properties are enhanced and verified. For structural applications an understanding of the mechanical properties at service temperatures is necessary before they are employed. One structural ceramic that holds considerable promise is AS8 silicon nitride (Si 3 N 4, Honeywell Ceramic Components, Torrance, CA). It has an acicular grain structure providing in-situ reinforcement (ISR) and is envisioned for use as components in hostile environments, such as turbines. ISR Si 3 N 4 generally has good creep resistance, good strength retention at high temperature, and high fracture toughness [1]. Addition of SiC particles to Si 3 N 4 is believed to improve the high-temperature properties, including creep resistance, though the actual effect seems somewhat inconclusive [2-4]. This study was motivated by the technological importance of ISR Si 3 N 4 and SiC p -Si 3 N 4 composites. Previous traditional ex-situ tensile creep studies have been performed on Si 3 N 4 [5,6], but there have been no in-situ creep studies. Thus neutron diffraction (ND) was employed in order to measure the average microstructural response to applied stress at temperature. This research is a verification of capability for a new instrument using tensile load. High-temperature ND experiments are not prevalent in the literature, due to lack of suitable instrumentation. Among the few that have been performed are composite creep studies using ND [7,8], but they used lower temperatures than are relevant for Si 3 N 4 materials. This report details the characterization of the high-temperature properties of AS8 and 2Vol% SiC p -Si 3 N 4, including the single crystal elastic constants and coefficients of thermal expansion (CTE). To the authors knowledge, this is the first ND investigation at temperatures this high for quantification of mechanical properties. EXPERIMENTAL PROCEDURE

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright JCPDS - International Centre for Diffraction Data 23, Advances in X-ray Analysis, Volume Sample Details Two different samples were tested at elevated temperatures. First, a monolithic AS8 ISR Si 3 N 4 sample was tested at 1375 C. The ISR nature of these materials is derived from the processing method, and serves to strengthen the material in the direction of grain alignment. The second sample was a particulate composite of Si 3 N 4 containing 2Vol% SiC tested at 14 C. SiC particles generally reside in the grain boundary phase and at grain triple junctions, serving, perhaps, to inhibit grain reorientation and thus improve creep resistance. Aside from the different test temperatures, 51mm gage length dog-bone samples of each material were tested identically in regards to heating rate and applied loads. The AS8 sample had a cross-section of 6.3 x 5. mm 2, while the composite sample was 3 x 3 mm 2. Relevant data used in fitting diffraction patterns were space group, p6 3 /m, and lattice parameters, a = 7.68Å, c = 2.911Å for hexagonal (β) Si 3 N 4, and F43m, a = 4.361Å for SiC [1]. Thermo-Mechanical Testing Constant tensile stresses were applied in vacuum at elevated temperature using a horizontally aligned stress fixture on pin-loaded samples. The loading fixture (Instron Corp., Canton, MA) and furnace (MRF Inc., Suncook, NH) were custom-made for use with ND, employing hightemperature W-1%Ta grips and Al windows. Samples were gripped in the furnace hot-zone center, also the center of diffraction. A constant stress of 3 MPa was maintained during heating and cooling. ND spectra were recorded at 25MPa increments up to 175MPa. Each load was held constant for 45-6 min while diffraction patterns were recorded, to check for creep effects. Each sample was then unloaded and cooled to room temperature. An extensometer measured bulk strains in all tests save the AS8 mechanical testing, due to a data acquisition error. Neutron Diffraction This study made use of the SMARTS diffractometer at the Los Alamos Neutron Science Center [9]. SMARTS employs time-of-flight ND to acquire entire diffraction patterns quickly. The neutron beam illuminated the entire gage width (centered at the midpoint of the gage length) of the dog-bone tensile samples. The load axis was aligned at a 45 angle to the incident beam. Diffraction data were collected at Bragg angles of 2θ = ± 9, corresponding to transverse (+9 ) and longitudinal (-9 ) diffraction strains. Diffraction data were analyzed using the Rietveld method [11] via the GSAS program [12]. This least-squares method accounts for various parameters affecting the diffraction pattern in order to fit the experimental data. Refined parameters included lattice and thermal parameters, and preferred orientation. Typical fitting errors were ~ 6%. Single peaks were fit as Voigt functions, with error ~ 8%, for hkl-dependent calculations. Patterns were obtained for the samples at room temperature, the test temperature, and intermediate temperatures. Samples were heated at 2 C/min. Intermediate temperature diffraction patterns were not taken until the extensometer indicated thermal expansion was complete. These patterns allowed computation of CTEs. Diffraction patterns were recorded in 15 min (AS8) to 3 min (SiC p -Si 3 N 4 ); the minimum times necessary for high quality diffraction patterns. The AS8 patterns show only β-si 3 N 4, thus the grain boundary phase does not appear, despite a pre-test crystallizing heat treatment [7]. The SiC p -Si 3 N 4 diffraction patterns were primarily β-si 3 N 4 with only one independent SiC peak.

4 Copyright JCPDS - International Centre for Diffraction Data 23, Advances in X-ray Analysis, Volume RESULTS AND DISCUSSION Thermal Expansion Since diffraction patterns were recorded at several temperatures during the heating process, it was possible to determine the CTE of the samples. The average a and c lattice parameters from full pattern fits are shown in Figure 1 for longitudinal strains in both samples. These data are the averages of multiple scans at each temperature, relative to each sample s initial room temperature scans. There is close agreement with the extensometer, also averaged for each temperature. Note that the Si 3 N 4 in the composite and its extensometer data show greater strain than in AS8, which correlates with the high thermal strain in the SiC particles in the composite. Thermal expansion is linear over the entire temperature range for the 3MPa applied nominal stress. Using peak positions from single peak fits (for non-overlapping peaks in the composite sample), the CTE tensors were determined for Si 3 N 4 in both samples using the program ALPHA [13]. Results were, for AS8: α 11 = 3.5 (±.3) x 1-6 K -1, α 33 = 4.6 (±.4) x 1-6 K -1 ; for the composite: α 11 = 3.38 (±.7) x 1-6 K -1 and α 33 = 4.21 (±.1) x 1-6 K -1. There are only two independent terms, since Si 3 N 4 is hexagonal [14]. Using α = (2α 11 + α 33 )/3, gives α AS8 = 3.69 x 1-6 K -1, α compsn = 3.66 x 1-6 K -1. Compare these, respectively, to the literature value of 3.6 x 1-6 K -1 [15]. Lattice parameter changes gave α SiC = 4.33 x 1-6 K -1, compared to 4.7 x 1-6 K -1 in literature [15]. Thermal Expansion, d/d (*1 6 ) = µε AS8 a AS8 c AS8 extensometer Composite Si3N4 a Composite Si3N4 c Composite SiC Composite extensometer Temperature (K) Figure 1. Thermal expansion in AS8 and SiC p -Si 3 N 4 ( Composite ) at 3MPa. Shown are the data for the Si 3 N 4 lattice constants in both samples, extensometer data for both, and SiC lattice constant in the composite. Phase Strain under Applied Stress The a and c lattice parameters from GSAS fits for AS8 in these applied stress tests were averaged over the many peaks appearing in the patterns and used to compute the strain in the A A 2a + c material according to ε =, where A =, the polycrystalline average for hexagonal A 3

5 Copyright JCPDS - International Centre for Diffraction Data 23, Advances in X-ray Analysis, Volume systems. Figure 2 shows the diffraction strains due to applied stress. The reference for strain is the average A of the 3MPa, 1375 C patterns. The Young's modulus (from a linear fit to the longitudinal data) is 339GPa, while the Poisson's ratio is.32. Both directions indicated only a slight residual strain upon unloading Applied Stress (MPa) E = 339 GPa Transverse Longitudinal Lattice strain (*1 6 ) Figure 2. Applied stress-diffraction strain plotted for AS8 at 1375 C. Using single peak fits (of the same reflections as the CTE calculation) from both directions, strains were calculated with the same reference noted above. The largest strains were realized due to thermal expansion, over 5µε for the ( 2) reflection of Si 3 N 4. After reaching the test temperature and applying higher stresses, a maximum of 575µε was realized, again for the ( 2) reflection, with significant variation of strains dependent upon the hkl of the reflection. Unloading to 3MPa returned the peaks to their original positions. After cooling down (still at 3MPa) there was only slight residual strain noted, with a maximum of 19µε for the ( 2) reflection. Nor was there was any change in peak breadth, thus the deformation was almost entirely elastic. Calculation of the single crystal elastic constants of this material at this temperature was justified by this fact and by the lack of any notable texture change due to stress. To calculate the 1375 C elastic stiffness tensor of AS8, an elastic-plastic self-consistent (EPSC) polycrystal deformation model [16] was applied. EPSC models accurately predict the diffraction elastic constants measured by neutron diffraction [16-19]. A reverse process was used, in which the measured diffraction elastic constants were input and a least squares fitting routine refined the single crystal stiffnesses, rather than inputting the single crystal stiffnesses. The starting point for the refinement was the isotropic stiffness tensor calculated from the measured macroscopic Young s modulus and Poisson s ratio (Fig. 2). The multiple patterns from each stress were summed into a single pattern, and nine single peak fits were used to refine the 1375 C stiffness tensor for AS8, with values shown in Table I. Comparing the present values to those of Vogelgesang et al. [2] shows some agreement. Their test was at room temperature and used a single crystal of Si 3 N 4 manufactured by nitridation of Si, so discrepancy

6 Copyright JCPDS - International Centre for Diffraction Data 23, Advances in X-ray Analysis, Volume is unsurprising. AS8 is known to have elongated grains (c-axis), which could account for the marked change in the C 33 value. The tensor values yield the Young's modulus and Poisson's ratio as 313GPa and.31, respectively, close to the values from diffraction. Honeywell reports these values at 12 C as 293GPa and.28. A possible explanation for these higher values is that this test was in vacuum, which protected the sample from oxidation. Table I. Single crystal stiffness tensor values for AS8 at 1375 C, compared with literature. C 11 C 33 C 44 C 66 C 12 C 13 AS8 456 ± ± ± ± ± ± 21 Ref [2] Figure 3 shows the polycrystal average diffraction strains for the composite as a function of applied stress, relative to the lattice spacing measured for 3MPa at 14 C. The extensometer data exhibited the same behavior as the c-axis for the Si 3 N 4, increasing much more than the a- axis which leveled off. This might indicate creep. The SiC peaks were almost all obscured, thus there is unusual strain behavior, especially in the transverse direction.. Extreme scatter for the various reflections prevented a stiffness tensor refinement. Instead, an Eshelby inclusion model [21] was used to compare with the strain data. Literature values were used for the Young's moduli and Poisson's ratios for each phase. For Si 3 N 4, the room temperature literature values [22] were multiplied by.85, to account for high temperature softening, as estimated from Figure 2 in [23]. The volume fraction of the SiC particles, confirmed by GSAS as 2 Vol%, and an arbitrary thermal misfit (1 C), were used for calculation. The model calculations for the composite are shown in Fig. 3. The model agrees with the data for the Si 3 N 4 polycrystalline average. This indicates that the model is successful for predicting the behavior of this system under stress at 14 C. 2 Applied Stress (MPa) Lattice strain (*1 6 ) Si3N4 Longitudinal SiC Longitudinal Si3N4 transverse SiC transverse Eshelby composite Figure 3. Applied stress-diffraction strain plotted for SiC p -Si 3 N 4 at 14 C.

7 Copyright JCPDS - International Centre for Diffraction Data 23, Advances in X-ray Analysis, Volume CONCLUSION This investigation demonstrated the capability of SMARTS to obtain microstructural information from tensile tests on Si 3 N 4 -based materials at service temperature in vacuum. The CTE of AS8 Si 3 N 4 was measured as 3.69 x 1-6 K -1, and 3.66 x 1-6 K -1 for Si 3 N 4 in the 2Vol% SiC composite, from the anisotropic CTE tensor components. The AS8 sample was mechanically loaded at 1375 C. ND indicated the deformation was mostly elastic with no texture evolution. Thus, the high-temperature elastic tensor was calculated using single peak fits from hightemperature applied stress diffraction patterns. The tensor values are comparable to room temperature single crystal values, with softening in the c-direction. A 2Vol% SiC p -Si 3 N 4 sample was tested at 14 C. Mechanical anisotropy was evident in the diffraction strains, and stiffness constants were unattainable, but Eshelby modeling compares well with the data. REFERENCES [1] H. T. Lin, et al., Cer. Engr. and Sci. Proc., 22 [3], Edited by M. Singh and E. Üstündag, pp , The American Ceramic Society, Westerville, OH, 21 [2] C.-W. Li, et al., 6 th Int l Symp. on Ceram. Mat. and Comp. for Engines, pp (1997) [3] H. Klemm, et al., Cer. Engr. and Sci. Proc., 21, [3], Edited by T. Jessen and E. Üstündag, pp , The American Ceramic Society, Westerville, OH, 2 [4] A. Rendtel and H. Hübner, Ceramic Engr. and Sci. Proc., 21, [4], Edited by T. Jessen and E. Üstündag, pp , The American Ceramic Society, Westerville, OH, 2 [5] C. J. Gasdaska, J. Am. Cer. Soc., 77 [9] (1994) [6] H. M. A. Winand, et al., Mat. Sci. and Engineering, A284, pp (2) [7] C.-W. Li and F. Reidinger, Acta Mater., 45 [1], (1997) [8] M. R. Daymond, et al., Metall. and Mater. Trans. A, 3, pp (1999) [9] M.A.M. Bourke, et al., Appl. Phys. A, In press (22) [1] P. Villars and L.D. Calvert, Pearson's Handbook of Crystallographic Data for Intermetallic Phases I, Vol. 3, pg 279, American Society for Metals, Metals Park, OH, 1985 [11] H. M. Rietveld, J. Appl. Cryst., 2, pp (1969) [12] A. C. Larson, R. B. von Dreele, GSAS - General Structure Analysis System, LAUR , Los Alamos National Laboratory, 1986 [13] S. M. Jessen and H. Küppers, J. Appl. Cryst., 24, pp (1991) [14] D. Sands, Vectors and Tensors in Crystallography, pp , Dover Publications Inc., New York, 1995 [15] W. D. Callister, Jr., Materials science and Engineering: An Introduction, 3 rd ed., pg. 768, John Wiley and Sons, Inc., New York, 1994 [16] P. A. Turner, C. N. Tomé, Acta Metall. Mater., 42, (1994) [17] B. Clausen, et al., Acta Mater., 46, (1998) [18] B. Clausen, et al., Mat. Sci. & Eng. A, 259, (1998) [19] T. M. Holden, et al., Mat. Sci. & Eng. A, 282, , (2) [2] R. Vogelgesang et al., Appl. Phys. Lett., 76 [8], pp (2) [21] T. Clyne and P. Withers, An Introduction to Metal Matrix Composites, Cambridge University Press, 1993 [22] D. Richerson, Modern Ceramic Engineering, 2 nd ed., Marcel-Dekker, Inc., New York, 1992 [23] T. Rouxel, et al., Acta Mat., 5, pp (22)