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Effect of Seeding on the Microstructure and Mechanical Properties of Alpha-Sialon: II, Ca-Alpha-Sialon

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Effect of Seeding on the Microstructure and Mechanical Properties of Alpha-Sialon: II, Ca-Alpha-Sialon University of Pennsylvania ScholarlyCommons Departmental Papers (MSE) Department of Materials Science & Engineering May 2002 Effect of Seeding on the Microstructure and Mechanical Properties of alpha-SiAlON: II, Ca-alpha-SiAlON Roman Shuba University of Pennsylvania, [email protected] I-Wei Chen University of Pennsylvania, [email protected] Follow this and additional works at: https://repository.upenn.edu/mse_papers Recommended Citation Shuba, R., & Chen, I. (2002). Effect of Seeding on the Microstructure and Mechanical Properties of alpha- SiAlON: II, Ca-alpha-SiAlON. Retrieved from https://repository.upenn.edu/mse_papers/36 Copyright The American Ceramic Society. Reprinted from Journal of the American Ceramic Society, Volume 85, Issue 5, May 2002, pages 1260-1267. This paper is posted at ScholarlyCommons. https://repository.upenn.edu/mse_papers/36 For more information, please contact [email protected]. Effect of Seeding on the Microstructure and Mechanical Properties of alpha- SiAlON: II, Ca-alpha-SiAlON Abstract Seeding effects on the microstructure and mechanical properties of single-phase Ca-α-SiAlON ceramics have been investigated. Whereas a small amount of seeds can transform the microstructure from one of fine equiaxed grains to one consisting of many needle-like grains, the highest fracture toughness of 8 MPa . m1/2 is not reached until 8% seeding. This contrasts with the much higher seed efficiency in- Y SiAlON, where the peak toughness is reached at 1% seeding. The difference and the general trend of mechanical properties of seeded α-SiAlONs are discussed in terms of α-SiAlON formation and toughening mechanisms. Comments Copyright The American Ceramic Society. Reprinted from Journal of the American Ceramic Society, Volume 85, Issue 5, May 2002, pages 1260-1267. This journal article is available at ScholarlyCommons: https://repository.upenn.edu/mse_papers/36 journal J. Am. Ceram. Soc., 85 [5] 1260–67 (2002) Effect of Seeding on the Microstructure and Mechanical Properties of ␣-SiAlON: II, Ca-␣-SiAlON Roman Shuba and I-Wei Chen* Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6272 Seeding effects on the microstructure and mechanical proper- paper.2 Through this comparison, we hope to understand the ties of single-phase Ca-␣-SiAlON ceramics have been investi- mechanism of seeding and microstructure development in gated. Whereas a small amount of seeds can transform the ␣-SiAlONs and to provide a basis for further microstructure microstructure from one of fine equiaxed grains to one con- optimization for future applications. sisting of many needle-like grains, the highest fracture tough- ness of 8 MPa⅐m1/2 is not reached until 8% seeding. This contrasts with the much higher seed efficiency in Y-SiAlON, II. Experimental Procedure where the peak toughness is reached at 1% seeding. The Experimental procedures used in this study were similar to difference and the general trend of mechanical properties of 2 seeded ␣-SiAlONs are discussed in terms of ␣-SiAlON forma- those described in the companion paper. The main difference here tion and toughening mechanisms. is the composition, which is Ca0.75Si9.3Al2.7O1.2N14.8. This has been referred to as Ca-1512 in our previous studies,9 indicating m ϭ 1.5 and n ϭ 1.2 in the conventional designation of ␣-SiAlON, I. Introduction Mm/zSi12Ϫ(mϩn)Al(mϩn)OnN16Ϫn. SiAlON of this composition is known to be rather stable and can be prepared using Si3N4, AlN, OMPARED to yttrium- and rare-earth-containing silicon nitride CaCO3 (CS-3NA, Pred Materials, Inc., New York) and Al2O3,as Cand SiAlONs, calcium-containing products have a cost advan- shown in Table I. Samples were hot pressed in nitrogen at either tage by using less expensive raw materials and requiring lower 1900° or 1950°C for 1 h. Full densification was achieved in all ␤ sintering temperatures. In manufacturing self-reinforced -Si3N4 cases. The phase purity and the composition were assessed using ceramics, calcium is also known to promote whisker growth, X-ray diffraction (XRD), which showed reflections that all could resulting in needle-like grains of a high aspect ratio.1 This effect is be identified with the ␣-SiAlON structure with the lattice param- apparently quite dramatic, because the amount of calcium used for eters listed in Table I. such a purpose is typically in the range of only 0.1–1 wt%. For seeding, we selected seed crystals that have a similar ␤ ␣ Fracture surfaces of calcium-added self-reinforced -Si3N4 show a average area/volume as those used for seeding Y- -SiAlON much higher calcium content than that in the background glasses, ceramic in the companion paper. These seeds were prepared using indicating a strong tendency for calcium segregation that weakens the method described elsewhere.10 A scanning electron micro- grain boundaries.1 In ␣-SiAlON, there is also evidence that scope (SEM) image of such seed crystals is shown in Fig. 1. Their calcium promotes the formation of needle-like grains. For exam- essential structures and geometric parameters are summarized in ple, whereas Ca-␣-SiAlONs with 4–7 wt% CaO (such as Ca-0809 Table II. The lattice parameters of seeds determined using XRD, and Ca-1512; for designation, see the companion paper2) have also listed in Table II, are very similar to those of the ceramics of mostly equiaxed grains in the microstructure, compositions with the overall composition of Ca-1512 (Table I). Various amounts of higher calcium contents, up to 14 wt% CaO (Ca-3020), show many seeds from 1–16 wt% were used to control the microstructure. The elongated grains.3–6 Unfortunately, however, ceramics of high maximum amount here is much larger than that used in the calcium and aluminum content also tend to have a large amount of companion paper (5%) because peak toughening was achieved at residual glasses, often with AlN polytypes.4,5,7 This in turn results a larger amount, as will become clear later. in disappointing properties, such as low hardness (not exceeding Microstructures were studied by SEM using polished sections 16 GPa), inferior toughness, weak creep strength, and poor normal to the hot-pressing axis. Molten KOH was used for etching. chemical resistance.8 Further details of the characterization and mechanical testing Recently, we have shown that ␣-SiAlONs of a low calcium methods were described in the companion paper. Strength was content can impart a microstructure with elongated grains when measured in three-point bending using samples of dimensions 30 seed crystals are introduced during processing.9 The resultant mm ϫ 2.5 mm ϫ 2.5 mm finished with 400 grit diamond wheel ceramics are both hard and tough, with properties roughly com- grinding. An outer span of 13 mm and a crosshead speed of 0.5 parable to rare-earth containing ␣-SiAlONs.9 In this paper we report in detail the seeding effects on the microstructure and mechanical properties of Ca-␣-SiAlON. These results are com- pared with ones on Y-␣-SiAlON reported in the companion Table I. Composition and Lattice Parameters of Ca-1512 Ceramic Parameter Value P. F. Becher—contributing editor Composition of starting powder (wt%) Si3N4 (1.24% oxygen) 74.02 AlN (0.88% oxygen) 18.57 Al2O3 0.29 CaCO 12.71 Manuscript No. 187583. Received July 16, 2001; approved February 7, 2002. 3 Supported by the U.S. Air Force Office of Scientific Research, under Grant No. Lattice parameters of ceramic F49620-01-1-0150. Facilities at the University of Pennsylvania are supported by the a (Å) 7.837 (2) U.S. National Science Foundation under MRSEC Grant No. DMR00-79909. c (Å) 5.702 (2) *Member, American Ceramic Society. 1260 May 2002 Effect of Seeding on the Microstructure and Mechanical Properties of ␣-SiAlON: II 1261 Table II. Lattice Parameters and Average Dimensions of Seed Crystals Parameter Value Lattice parameter a (Å) 7.8374 (8) c (Å) 5.7009 (9) Average dimensions Width (␮m) 0.33 Ϯ 0.10 Length (␮m) 0.90 Ϯ 0.73 Aspect ratio 2.57 Ϯ 1.59 Area (␮m2) 0.34 Ϯ 0.43 Volume (␮m3) 0.12 Table III for comparison. Also shown is the estimated areal number density of seed crystals in the ceramics, assuming an average area of 0.34 ␮m2 per seed (see Table II). We can see that the number density of (needle-like) grains within the tail is initially (1% and 2% seeding) comparable to that of seed crystals. This ␣ Fig. 1. Seed crystals of Ca-1512 -SiAlON. would suggest an approximate one-to-one correspondence be- tween seed crystals and elongated grains. As the amount of seed crystals increases (e.g., 4% and 8%), the number of seeds begins to mm/min were used. Both resistance (R-curve) and single-edge- exceed that of large grains. This indicates that not every seed notched-beam (SENB) fracture toughness were measured in four- crystal could nucleate a needle-like grain. At the largest amount of point bending. Bars of 30 mm ϫ 3.5 mm ϫ 2 mm, with their seed crystals used (16%), the number density of needle-like grains tensile surfaces lying parallel to the hot-pressing plane, were actually decreases. This is accompanied by an increase of their used for both measurements. The initial crack of ϳ40%– 60% average grain area and an abrupt increase of the scatter in width, of total bar height was introduced using a thin blade, then which results from the coalescence of some grains in this ceramic followed by wire sawing to achieve a tip radius of 10 ␮m. In the (Fig. 2(d)). Overall, the total area fraction of needle-like grains in ϳ R-curve measurement, the load was increased in small incre- the tail distribution remains nearly constant at 0.42, but the ments to allow in situ monitoring of crack extension.9 In the relative scatter of their dimensions (grain area, width, length, and SENB toughness measurement, the load was increased until the AR), defined by the ratio of standard deviation to the mean, is the specimen failed and the initial crack length was used for smallest at 4% and 8% seeding and markedly higher at lower and toughness determination.
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