ournal Inm Ceran. Soc, 82[1]117-28(1999) Creep and Fatigue Behavior in Hi-Nicalon TM-Fiber-Reinforced Silicon Carbide Composites at High Temperatures Shijie Zhu, t, t Mineo Mizuno, "t Yutaka Kagawa, Jianwu Cao, .t Yasuo Nagano, ,t and Hiroshi Kayas Japan Fine Ceramics Center, Nagoya 456, Japan; Institute of Industrial Sciences, University of Tok kyo 106 Japan; and Petroleum Energy Center, Tokyo 106, Japan Monotonic tension, creep, and fatigue tests of a composite interfaces, which allows the intact fibers to bridge crack t the bon). The weak interface can cause a crack to deflect along the where a silicon carbide (SiC) matrix that contains glass- However although the use of weak interfaces can increase the forming, boron-based particulates has been reinforced with fracture toughness and thermal shock resistance, 2 it is not com- Hi-NicalonTM fiber(Hi-NicalonMSiC) were conducted atible with creep and fatigue resistance at high temperature, ir at 1300C, and creep tests also were conducted in argon which demands strong interfaces that resist the nucleation and t 1300C. The ultimate tensile strength (UTS)of the growth of cavities. 3,4 Hi-NicalonTM/SiC composite was similar to that of a Sic The carbon-coating layer in SiC/SiC composites leads to low composite where a pure Sic matrix is reinforced with oxidation resistance at high temperatures in air. a glass Nicalon'M fiber(standard Sic/Sic) and a Sic composite forming, boron-based particulate that reacts with oxygen to where a matrix of glass-forming, boron-based particulates roduce a sealant glass that inhibits oxidation can be added to is reinforced with Nicalon M fiber (enhanced sic/sic); the matrix. 6 This technology is applied to SiC/Sic composites however, the strains at UTS of the Hi-Nicalon/SiC com- The modified SiC/SiC is called an enhanced SiC/SiC compos- osite and the enhanced SiC/SiC composite were much ite. 6, II Enhanced SiC/SiC composites exhibi larger than that of the standard SiC/SiC composite. The temperature(up to 1300 C) properties in alr. abit good high- Youngs modulus of the Hi-Nicalon Sic composite was Because matrix microcracking occurs during the initial ap -140 GPa. which is higher than that of the enhanced Sic/ ication of a creep load, fiber bridging of matrix cracks oper SiC composite(90 GPa) and lower than that of the standard tes during the creep of standard SiC/Sic composites at high C/SiC composite(200 GPa) at a temperature of 1300 C. stresses, although the creep resistance of SiC fibers is lower The minimum strain rates of cyclic creep(fatigue)were than that of the lower than those of static creep. The time to rupture under for the environmental resistance of the composites, if reep loads was slightly shorter than that under fatigue xposed to air. Because the creep of fibers control loads at a given maximum stress. The creep strain rates of crack growth, 24,25 increasing the creep resistance of th the Hi-NicalonTMsiC composite in air were lower than te. moreover those in argon. Consequently, the time to rupture at a given decreasing the creep resistance of the matrix by adding oxides stress in air was longer than in argon. The creep and fa tigue resistance of the Hi-Nicalon Sic composite both also is expected to increase the creep resistance and simul- taneously improve the environmental resistance. We found that were similar to that of the enhanced Sic/SiC composite but the addition of glassy phases in the Sic matrix increased the were much better than that of the standard siC/SiC col creep and oxidation resistance in an enhanced SiC/SiC com- posite in air. However, in argon, the standard SiC/Sic cor osite at 1300.C, compared with that of the standard SiC/SiC osite had the creep rate whereas the enhanced con the highest creep rate. The time to Cyclic fatigue behavior of CMCs at high temperatures is not SiC/SiC composite was the shortes well understood Conditions such as environment factors. cree and the Hi-NicalonTMSiC composite had the longest life of constituents, thermally induced stresses at interfaces. and interfacial sliding resistance may cause the reduction of fatigue life at high temperatures dation of the interfacial sliding resistance have been considered INTHE HE recent decade, the creep and fatigue of continuous- to be the reasons for decreased fatigue resistance at high tem- T-reinforced ceramic-matrix composites(CMCs) have eratures in a standard SiC/SiC composite. 8, 10, 20 en investigated, because these properties are very impor- The presence of a silicon oxycarbide(SiC, O, ) amorphous for the application of CMCs. To obtain high fracture phase in Nicalon TM fibers(Nippon Carbon Co., Tokyo, Ja toughness and thermal shock resistance. CMCs have been de is responsible for the low creep resistance, because of a viscous signed with a weak interface between the fibers and the matrix flow at temperatures as low as 1000%-1200C 6 The SiC,O (e.g, the interface in a SiC/SiC composite is coated with car- phase decomposes, forming SiC and gaseous species such as CO and SiO, whose diffusion through the fiber and reaction with the free carbon are believed to create pores and other defects in the fiber structure. 27 Such a decomposition cause R. Naslain--contributing editor degradations in the strength and the Youngs modulus and affects the fiber creep behavior, even in inert atmospheres. 28-30 The elimination of Sic, O from the fibers by electron irradia- tion under vacuum, instead of curing in air, can improve the of 12 Rtcmiotr ceramic g as ugine approed erroe imgs a. creep resistance I-35 The Youngs modulus also is creased 31 The modified Nicalon TM fibers are called merican Ceramic Ceramics Center NicalonTM fibers. To increase the mechanical properties of SiC/ Industrial Sciences, University of Tokyo SiC composites, Hi-Nicalon M fibers have been used to pEtroleum Energy Center reinforce the enhanced SiC matrix. This paper will present 117Creep and Fatigue Behavior in Hi-Nicalon™-Fiber-Reinforced Silicon Carbide Composites at High Temperatures Shijie Zhu,†,‡ Mineo Mizuno,*,† Yutaka Kagawa,*,‡ Jianwu Cao,*,† Yasuo Nagano,*,† and Hiroshi Kaya§ Japan Fine Ceramics Center, Nagoya 456, Japan; Institute of Industrial Sciences, University of Tokyo, Tokyo 106, Japan; and Petroleum Energy Center, Tokyo 106, Japan Monotonic tension, creep, and fatigue tests of a composite where a silicon carbide (SiC) matrix that contains glass￾forming, boron-based particulates has been reinforced with Hi-Nicalon™ fiber (Hi-Nicalon™/SiC) were conducted in air at 1300°C, and creep tests also were conducted in argon at 1300°C. The ultimate tensile strength (UTS) of the Hi-Nicalon™/SiC composite was similar to that of a SiC composite where a pure SiC matrix is reinforced with Nicalon™ fiber (standard SiC/SiC) and a SiC composite where a matrix of glass-forming, boron-based particulates is reinforced with Nicalon™ fiber (enhanced SiC/SiC); however, the strains at UTS of the Hi-Nicalon™/SiC com￾posite and the enhanced SiC/SiC composite were much larger than that of the standard SiC/SiC composite. The Young’s modulus of the Hi-Nicalon™/SiC composite was ∼140 GPa, which is higher than that of the enhanced SiC/ SiC composite (90 GPa) and lower than that of the standard SiC/SiC composite (200 GPa) at a temperature of 1300°C. The minimum strain rates of cyclic creep (fatigue) were lower than those of static creep. The time to rupture under creep loads was slightly shorter than that under fatigue loads at a given maximum stress. The creep strain rates of the Hi-Nicalon™/SiC composite in air were lower than those in argon. Consequently, the time to rupture at a given stress in air was longer than in argon. The creep and fa￾tigue resistance of the Hi-Nicalon™/SiC composite both were similar to that of the enhanced SiC/SiC composite but were much better than that of the standard SiC/SiC com￾posite in air. However, in argon, the standard SiC/SiC com￾posite had the lowest creep rate, whereas the enhanced SiC/SiC composite had the highest creep rate. The time to rupture of the standard SiC/SiC composite was the shortest and the Hi-Nicalon™/SiC composite had the longest life. I. Introduction I N THE recent decade, the creep and fatigue of continuous￾fiber-reinforced ceramic-matrix composites (CMCs) have been investigated,1–25 because these properties are very impor￾tant for the application of CMCs. To obtain high fracture toughness and thermal shock resistance, CMCs have been de￾signed with a weak interface between the fibers and the matrix (e.g., the interface in a SiC/SiC composite is coated with car￾bon). The weak interface can cause a crack to deflect along the interfaces, which allows the intact fibers to bridge crack faces.1 However, although the use of weak interfaces can increase the fracture toughness and thermal shock resistance,2 it is not com￾patible with creep and fatigue resistance at high temperature, which demands strong interfaces that resist the nucleation and growth of cavities.3,4 The carbon-coating layer in SiC/SiC composites leads to low oxidation resistance at high temperatures in air. A glass￾forming, boron-based particulate that reacts with oxygen to produce a sealant glass that inhibits oxidation can be added to the matrix.6 This technology is applied to SiC/SiC composites. The modified SiC/SiC is called an enhanced SiC/SiC compos￾ite.6,11 Enhanced SiC/SiC composites exhibit good high￾temperature (up to 1300°C) properties in air.11 Because matrix microcracking occurs during the initial ap￾plication of a creep load, fiber bridging of matrix cracks oper￾ates during the creep of standard SiC/SiC composites at high stresses, although the creep resistance of SiC fibers is lower than that of the SiC matrix.7 This phenomenon is undesirable for the environmental resistance of the composites, if they are exposed to air. Because the creep of fibers controls matrix crack growth,24,25 increasing the creep resistance of the fibers can improve the creep behavior of the composite. Moreover, decreasing the creep resistance of the matrix by adding oxides also is expected to increase the creep resistance25 and simul￾taneously improve the environmental resistance. We found that the addition of glassy phases in the SiC matrix increased the creep and oxidation resistance in an enhanced SiC/SiC com￾posite at 1300°C, compared with that of the standard SiC/SiC composite.11 Cyclic fatigue behavior of CMCs at high temperatures is not well understood. Conditions such as environment factors, creep of constituents, thermally induced stresses at interfaces, and interfacial sliding resistance may cause the reduction of fatigue life at high temperatures.8,20–22 Creep of the fibers and degra￾dation of the interfacial sliding resistance have been considered to be the reasons for decreased fatigue resistance at high tem￾peratures in a standard SiC/SiC composite.8,10,20 The presence of a silicon oxycarbide (SiCxOy) amorphous phase in Nicalon™ fibers (Nippon Carbon Co., Tokyo, Japan) is responsible for the low creep resistance, because of a viscous flow at temperatures as low as 1000°–1200°C.26 The SiCxOy phase decomposes, forming SiC and gaseous species such as CO and SiO, whose diffusion through the fiber and reaction with the free carbon are believed to create pores and other defects in the fiber structure.27 Such a decomposition causes degradations in the strength and the Young’s modulus and affects the fiber creep behavior, even in inert atmospheres.28–30 The elimination of SiCxOy from the fibers by electron irradia￾tion under vacuum, instead of curing in air, can improve the creep resistance.31–35 The Young’s modulus also is in￾creased.31 The modified Nicalon™ fibers are called Hi￾Nicalon™ fibers. To increase the mechanical properties of SiC/ SiC composites, Hi-Nicalon™ fibers have been used to reinforce the enhanced SiC matrix. This paper will present R. Naslain—contributing editor Manuscript No. 191122. Received December 19, 1997; approved April 10, 1998. This work is part of the automotive ceramic gas turbine development programs at the Petroleum Energy Center. *Member, American Ceramic Society. † Japan Fine Ceramics Center. ‡ Institute of Industrial Sciences, University of Tokyo. § Petroleum Energy Center. J. Am. Ceram. Soc., 82 [1] 117–28 (1999) Journal 117