The effect of heating schedule on microstructure and fracture resistance has been investigated in single-phase Nd-, Y-, and Yb-α-SiAlON. Such effect is strongly system dependent, reflecting the strong influence of phase stability on α-SiAON nucleation and the amount of transient/residual liquid during processing. The addition of 1% of α-SiAlON seeds to the starting powders nearly completely obliterates such effect, while it simultaneously improves microstructure homogeneity and fracture resistance. SENB toughness of 7 MPa∙m1/2 and peak R-curve toughness of ~11 MPa∙m1/2 have been obtained for seeded Y-α-SiAlON ceramics using heating rates from 1oC/min to 25oC/min

Chen, I-Wei

Shuba, Roman

Zenotchkine, Misha

English

Kosmopolis

Effect of Heating Schedule on the Microstructure and Fracture Toughness of alpha-SiAlON--Cause and Solution

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University of PennsylvaniaScholarlyCommonsDepartmental Papers (MSE) Department of Materials Science & EngineeringJuly 2002Effect of Heating Schedule on the Microstructureand Fracture Toughness of alpha-SiAlON--Causeand SolutionMisha ZenotchkineUniversity of PennsylvaniaRoman ShubaUniversity of Pennsylvania, rshuba@seas.upenn.eduI-Wei ChenUniversity of Pennsylvania, iweichen@seas.upenn.eduFollow this and additional works at: http://repository.upenn.edu/mse_papersCopyright The American Ceramic Society. Reprinted from Journal of the American Ceramic Society, Volume 85, Issue 7, July 2002, pages 1882-1884.This paper is posted at ScholarlyCommons. http://repository.upenn.edu/mse_papers/39For more information, please contact libraryrepository@pobox.upenn.edu.Recommended CitationZenotchkine, M., Shuba, R., & Chen, I. (2002). Effect of Heating Schedule on the Microstructure and Fracture Toughness of alpha-SiAlON--Cause and Solution. Retrieved from http://repository.upenn.edu/mse_papers/39Effect of Heating Schedule on the Microstructure and Fracture Toughnessof alpha-SiAlON--Cause and SolutionAbstractThe effect of heating schedule on microstructure and fracture resistance has been investigated in single-phaseNd-, Y-, and Yb-α-SiAlON. Such effect is strongly system dependent, reflecting the strong influence of phasestability on α-SiAON nucleation and the amount of transient/residual liquid during processing. The additionof 1% of α-SiAlON seeds to the starting powders nearly completely obliterates such effect, while itsimultaneously improves microstructure homogeneity and fracture resistance. SENB toughness of 7 MPa∙m1/2 and peak R-curve toughness of ~11 MPa∙m1/2 have been obtained for seeded Y-α-SiAlON ceramics usingheating rates from 1oC/min to 25oC/min.CommentsCopyright The American Ceramic Society. Reprinted from Journal of the American Ceramic Society, Volume 85,Issue 7, July 2002, pages 1882-1884.This journal article is available at ScholarlyCommons: http://repository.upenn.edu/mse_papers/39Effect of Heating Schedule on the Microstructure andFracture Toughness of -SiAlON—Cause and SolutionMisha Zenotchkine, Roman Shuba, and I-Wei Chen*Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104–6272The effect of heating schedule on microstructure and fractureresistance has been investigated in single-phase Nd-, Y-, andYb--SiAlON. Such effect is strongly system dependent, re-flecting the strong influence of phase stability on -SiAONnucleation and the amount of transient/residual liquid duringprocessing. The addition of 1% of -SiAlON seeds to thestarting powders nearly completely obliterates such effect,while it simultaneously improves microstructure homogeneityand fracture resistance. SENB toughness of 7 MPam1/2 andpeak R-curve toughness of 11 MPam1/2 have been obtainedfor seeded Y--SiAlON ceramics using heating rates from1°C/min to 25°C/min.I. IntroductionSOLID solutions of -Si3N4, known as -SiAlONs, are veryattractive ceramics for applications where high hardness, weardurability, and oxidation resistance are needed. The recent devel-opment of in situ-toughened -SiAlON with whiskerlike micro-structure has further increased the appeal of these ceramics andgenerated much interest.1 A central focus of the developmenteffort of toughened -SiAlON is on nucleation control thatpromotes the formation of whiskerlike grains. A two-step heatingschedule, which includes a low-temperature nucleation step, hasbeen shown to be effective for this purpose when either -Si3N41or -Si3N42 starting powders are used. Komeya and co-workers3also have reported rather different microstructures of Ca--SiAlON obtained under different heating schedules, finding exag-gerated whiskerlike microstructure at very high heating rates. Suchsensitivity to heating schedule is obviously an important issue thataffects the manufacturability of -SiAlON. Here we address thisissue by first investigating the heating schedule effect in various-SiAlONs. We further demonstrate that the mere addition of 1%-SiAlON seeds to the starting powder can obliterate such effectand improve microstructure homogeneity as well as fractureproperties.II. Experimental ProcedureThe compositions investigated here are represented by theformula: (Nd,Y,Yb)m/3Si12–(mn)Al(mn)OnN(16–n). Here, the in-terstitial cations are listed in the order of decreasing ionic radiusand (m, n) is set to be (1.5, 1.2), which was referred to as 1512composition in our earlier publications.1,2,4–6 For Y-1512 compo-sition, we also prepared small crystals (0.3–0.7 m in width and2–5 m in length) of the same composition, which were then usedas -SiAlON seeds in the starting powders. These seed crystalswere grown from a liquid phase in a nitrogen atmosphere and laterseparated from the residual phases by chemical washing. Thedetails of this procedure are available elsewhere.4,5Starting compositions were prepared from powder mixtures of-Si3N4 (SE-E-10, Ube Industries, Tokyo, Japan), AlN (Type F,Tokuyama Soda Co., Tokyo, Japan), Al2O3 (AKP50, SumitomoChemical America, New York), and rare-earth oxides Nd2O3,Y2O3, and Yb2O3 (all 99.9%, Alfa–Johnson Matthew Co., Dan-vers, MA). The oxygen content in -Si3N4 (1.24 wt%) and AlN(0.88 wt%) was taken into account in the formulation. Powdermixtures were attrition milled in isopropyl alcohol for 2 h withhigh-purity Si3N4 milling media in a Teflon-coated jar. They weresubsequently dried under a halogen lamp during stirring. Whendesired, charges of seeds were added to the powder slurries 10–15min before the end of milling.Powder mixtures were hot-pressed under a uniaxial pressure of30 MPa in a graphite resistance furnace following various firingschedules. Samples were typically heated with a constant heatingrate (from 1°C/min to 25°C/min) to 1900°C and held there for 1 h.In the two-step process, samples were first heated at 15°C/min to1500°C, held for 1 h, then heated at the same rate to 1900°C, andheld there again for 1 h. Full densification was observed in allcases unless otherwise noted.Phase analysis was performed using X-ray diffractometry(Rigaku Co., Tokyo, Japan) with CuK radiation. In all cases,single-phase -SiAlON was found. Microstructures of sinteredsamples were observed on polished and etched sections. EtchantsP. Becher—contributing editorManuscript No. 187149. Received February 7, 2002; approved April 16, 2002.Supported by the U.S. Air Force Office of Scientific Research under Grant No.F49620–98-1–0126. Facilities at the University of Pennsylvania are supported by theU.S. National Science Foundation under MRSEC Grant No. DMR00–79909.*Member, American Ceramic Society.Table I. Fracture Toughness of -SiAlONCeramics (Nd,Yb,Y)0.5Si9.3Al2.7O1.2N14.8Sample†SENB fracture toughness,KIC (MPam1/2)‡Nd-25 6.1Nd-15 6.1Nd-5 6.1Nd-1 6.1Nd-2st 6.3Yb-25 4.3Yb-15 3.6Y-25 5.3Y-15 5.9Y-5 6.1Y-1 5.9Y-2st 6.1YS-25 6.7YS-15 7.1YS-5 7.0YS-1 6.9†Designated by the cation and the heating rate. Thus,Nd-15 is an Nd--SiAlON heated at 15°C/min. Two-stepheating schedule is indicated by 2st; e.g., Nd-2st. YSindicates Y--SiAlON with 1% seeds. ‡0.1 MPam1/2.1882journalJ. Am. Ceram. Soc., 85 [7] 1882–84 (2002)used were boiling solutions of NH4F (20 g) in HNO3 (50 mL).Scanning electron microscopy (SEM; Model 6300-F, JEOL, To-kyo, Japan) was used for microstructure characterization.Single-edge notched-beam (SENB) fracture toughness wasmeasured in four-point bending using bars of 30 mm  4 mm 2 mm, with their tensile surfaces lying parallel to the hot-pressing plane. A notch 0.1– 0.2 mm long was introduced toone side using a thin blade, followed by wire sawing to achievea tip radius of 10 m. To measure the R-curve, crack length wascontinuously monitored during loading using a microscope witha resolution of 5 m. Further details of this procedure aredescribed elsewhere.6III. Results and DiscussionThe effect of heating rate on the microstructure varies greatlyfrom ceramic to ceramic. Nd-1512 and Yb-1512 offer two ex-tremes, because the stability of -SiAlON is the lowest inNd-SiAlON while the highest in Yb-SiAlON among all therare-earth-stabilized systems. Microstructures of Nd-1512, heatedat 25°C/min and 5°C/min (samples Nd-25 and Nd-5 in Table I),respectively, are shown in Figs. 1(a) and (b). Their microstructuresappear similar, with a few large grains, some quite elongated. Bothof these ceramics are dense, which is expected, because a rela-tively large amount of liquid is known to exist in this systembecause of the low stability of Nd--SiAlON.7 In contrast, at thelower heating rate, i.e., 5°C/min, the Yb-1512 ceramic remainsporous after hot-pressing. This is because of the higher stability ofYb--SiAlON, which forms much more readily and at a lowertemperature during slow heating,8 hence depriving the compact thetransient liquid required for densification at higher temperatures.The microstructure of such a ceramic is shown in Fig. 1(d). Athigher heating rates, the formation of Yb--SiAlON is delayed toa higher temperature so that more liquid is available for densifi-cation; thus, dense ceramics remain obtainable. Some large, blockygrains also form at the intermediate heating rate, 15°C/min, asshown in Fig. 1(c). However, at 25°C/min, the microstructure hasonly fine grains, similar to those shown in Figs. 1(c) and (d). TheSENB fracture toughness, tabulated in Table I, reflects the abovedifference in microstructure. Nd-SiAlON has generally higher butsimilar toughness regardless of heating rate. Yb-SiAlON has lowertoughness except at the intermediate heating rate (Yb-15 in TableI). Two-step hot-pressing of Nd-SiAlON leads to a microstructuresimilar to those of Figs. 1(a) and (b) and similar fracture toughness(Nd-2st in Table I).Y--SiAlON has an intermediate stability7,8 and can be densi-fied at all heating rates from 1°C/min to 25°C/min. However, themicrostructure varies systematically with the heating rate, as doesthe fracture toughness. As shown in Fig. 2, the fracture toughnessthroughout the R-curve is lower for the 25°C/min case than the5°C/min case. This difference can be attributed to the differentmicrostructure (see inset in Fig. 2): at 25°C/min it is comprised ofmostly fine, equiaxed grains, whereas, at 5°C/min, many large,elongated grains are present. Two-step heating also generates aFig. 1. SEM micrographs of Nd-1512 ceramics heated at (a) 25°C/min and (b) 5°C/min and of Yb-1512 ceramics heated at (c) 15°C/min and (d) 5°C/min.Micrographs correspond to Nd-25, Nd-5, Yb-15, and Yb-5 in Table I.July 2002 Communications of the American Ceramic Society 1883microstructure with many large, elongated grains and higher tough-ness, as shown in Fig. 2. The microstructure and the toughness for1°C/min heating are similar to those of two-step heating.The above results are consistent with the proposal of Chen,Rosenflanz, and Kim1,2,9 regarding -SiAlON nucleation, whichwas first demonstrated using  powders. Namely, the two-stepheating treatment (or slow heating) allows the formation of alimited number of nuclei at low temperatures; such nuclei laterdominate the growth at high temperature to develop a microstruc-ture of elongated grains. For Yb-SiAlON, this strategy works for-Si3N4 powders,1,9 but it breaks down when -Si3N4 powders areused. This is because -Si3N4 powders have a higher driving forceand reactivity than -Si3N4 powders;10 therefore, even at lowtemperature, -SiAlON formation is too fast. For Nd-SiAlON, thedriving force is so low that nucleation is always limited.1,2,8,9Therefore, elongated grains can always be obtained regardless ofheating schedule. (Our unpublished study also has found thatCa-SiAlON is similar to Nd-SiAlON, and always forms elongatedgrains. If a very high heating rate is used so that the  formationreaction is delayed to very high temperatures, then the largeamount of liquid present and the fast growth kinetics at hightemperatures are conducive to abnormal grain growth. This leadsto an exaggerated whiskerlike microstructure, as first reported byKomeya and co-workers.3) The case of Y-SiAlON is intermediatebetween the two limits and is most subject to influences caused bythe heating schedule.We have also investigated the combined effect of seeding (1%seeds) and heating rate on the microstructure and fracture tough-ness of Y-SiAlON. As shown in Fig. 3, at 5°C/min and 25°C/min,the microstructures are relatively uniform and similar, containingmany elongated grains that are tightly packed. The observedR-curves are also similar. The fracture toughness on the R-curve isconsiderably higher than that without seeding (see Fig. 2). Table Ishows that all the seeded Y-1512 ceramics have similar SENBfracture toughness higher than the values of unseeded ceramics.Thus, the large influence of the heating schedule on the micro-structure and fracture properties of Y-SiAlON can be removed bythe addition of only 1% seeds. This indicates that these -SiAlONseeds are so thermodynamically stable at all temperatures that theyare always able to grow and consume the driving force, renderingthose potential nucleation sites among the starting powders inac-tive. The microstructure, therefore, is dominated by these seeds,which have a one-to-one correspondence to the elongated grains inthe final microstructure, according to our recent study of seedingstatistics.11,12IV. Conclusions(1) Firing schedule has a profound influence on the micro-structure of -SiAlON. The influence systematically varies withthe relative phase stability of -SiAlON, which is compositiondependent. To optimize densification and mechanical properties, ittherefore is necessary to tailor the heating schedule for individual-SiAlON systems.(2) Seeding has a remarkable effect of removing the micro-structure/property sensitivity to the firing schedule. As a result,higher toughness and more uniform microstructure can be readilyobtained for seeded ceramics under a wide range of heatingconditions.References1I-W. Chen and A. Rosenflanz, “A Tough SiAlON Ceramic Based on -Si3N4 witha Whisker-like Microstructure,” Nature (London), 389, 701–704 (1997).2J. Kim, A. Rosenflanz, and I-W. Chen, “Microstructure Control of in Situ-Toughened -SiAlON Ceramics,” J. Am. Ceram. Soc., 83 [7] 1819–21 (2000).3C. Zhang, E. Narimatsu, K. Komeya, J. Tatami, and T. Meguro, “Control of GrainMorphology in Ca--SiAlON by Changing the Heating Rate,” J. Mater. Lett., 43,315–19 (2000).4M. Zenotchkine, R. Shuba, J-S. Kim, and I-W. Chen, “Synthesis of -SiAlONSeed Crystals,” J. Am. Ceram. Soc., 84 [7] 1651–53 (2001).5M. Zenotchkine, R. Shuba, and I-W. Chen, “Liquid-Phase Growth of SmallCrystals for Seeding -SiAlON Ceramics,” J. Am. Ceram. Soc., in review.6M. Zenotchkine, R. Shuba, J-S. Kim, and I-W. Chen, “R-Curve Behavior of inSitu-Toughened -SiAlON Ceramics,” J. Am. Ceram. Soc., 84 [4] 884–86 (2001).7A. Rosenflanz and I-W. Chen, “Phase Relationships and Stability of -SiAlON,”J. Am. Ceram. Soc., 82 [4] 1025–36 (1999).8A. Rosenflanz and I-W. Chen, “Kinetics of Phase Transformations in SiAlONCeramics: I. Effects of Cation Size, Composition, and Temperature,” J. Eur. Ceram.Soc., 19, 2337–48 (1999).9A. Z. Rosenflanz, “-SiAlON: Phase Stability, Phase Transformations, andMicrostructural Evolutions”; Ph.D. Dissertation. University of Michigan, Ann Arbor,MI, 1997.10A Rosenflanz and I-W. Chen, “Kinetics of Phase Transformations in SiAlONCeramics: II. Reaction Paths,” J. Eur. Ceram. Soc., 19, 2337–48 (1999).11M. Zenotchkine, R. Shuba, and I-W. Chen, “Effect of Seeding on the Micro-structure and Mechanical Properties of -SiAlON: I, Y-SiAlON,” J. Am. Ceram. Soc.,85 [5] 1254–59 (2002).12R. Shuba and I-W. Chen, “Effect of Seeding on the Microstructure andMechanical Properties of -SiAlON: II, Ca-SiAlON,” J. Am. Ceram. Soc., 85 [5]1260–67 (2002). Fig. 2. R-curves and SEM micrographs of unseeded Y-1512 followingthree heating schedules as shown. Curves and micrographs correspond toY-25, Y-5, and Y-2st in Table I.Fig. 3. R-curves and SEM micrographs of seeded Y-1512 heated at tworates. Curves and micrographs correspond to YS-25 and YS-5 in Table I.1884 Communications of the American Ceramic Society Vol. 85, No. 7

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Effect of Heating Schedule on the Microstructure and Fracture Toughness of alpha-SiAlON--Cause and Solution

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