83 S. Zhu et al./ Composites Science and Technology 59(1999)833-857 stress[ 19,20]. The gradual damage growth is accompanied have become attractive ceramic-matrix composites and by a modulus decrease in CMCs under fatigue loading, have already been applied in some fields which has been studied in detail [12, 13, 16, 19, 22]. At high TEM observation and electron diffraction exhibited a temperature, the fatigue limit coincides with the propor- high degree of polycrystallinity, a relatively small crystal tional limit for unidirectional SiC/Si3 N4, tested under the size (3-4 nm)and a lack of preferred orientation for the conditions for which creep occurs [14] Sic component of the fibers in SiC/SiC [33]. The For creep behavior of CMCs, a classification was microstructure of Sic fibers does not change sig roposed [31, 32] in terms of creep mismatch ratio nificantly during fabrication of the composite. There are (CMR), defined as a ratio of the creep rate of the fiber two kinds of Sic grains in the matrix, polyhedral ones to that of the matrix. When CMr l, the main near to the fibers with a size ranging from 10 to 100 damage mechanism occurring during creep is periodic and columnar grains, further away from the fibers and fiber fracture and the creep behavior is governed by up to 500 nm in length. The interfacial layer between the embedded fibers. When CMR> 1, matrix microcrack- matrix and fibers is a carbon layer ing is the dominant mode of damage and creep behavior The carbon layer in Sic/Sic leads to low oxidation is governed by bridging fibers. However, the creep mis- resistance at intermediate and high temperatures [34-39 match ratio provides only a starting point for consider- A glass-forming, boron-based particulate material can ing creep behavior and damage mechanisms, since it be added to the matrix that reacts with oxygen to pro- only depends on the uniaxial creep behavior of the duce a sealant glass that inhibits oxidation of the carbon constituents. Because of local stress concentrations and layer [40]. This technology has been applied to SiC/SiC the development of triaxial stresses, the in-situ creep composites. The SiC/SiC modified in this way is referred behavior of the constituents in a composite can differ to as an enhanced Sic/Sic composite [40] significantly from the creep behavior measured during Since matrix microcracking may occur during the unconstrained uniaxial loading [31, 32 initial application of p load, fiber bridging of he incorporation of SiC fiber into Si3 N4 results in matrix cracks operates during the creep of standard substantial improvements in creep resistance [23-25]. SiC/SiC at high stresses, although the creep resistance of Multiple fiber fracture rather than multiple matrix SiC fibers is lower than that of the Sic matrix [41-491 cracking was observed and the creep mechanism was a This is undesirable for environmental resistance of the repetitive matrix stress relaxation-fiber rupture- load composites if they are exposed to air. Because creep of transfer and distribution scheme [23-25. Moreover, a the fibers controls matrix crack growth, increasing the threshold stress was found for the tensile creep of a creep resistance of the fibers should improve the creep unidirectional SiC/HPSN(hot-pressed silicon nitride) behavior of the composite. Hi-Nicalon'M fiber is one of composite, which was much higher than the propor- the improved SiC fibers [46, 49), which is used to rein- tional limit [23-25 force a SiC matrix(Hi-Nicalon/ SiC composite Different mechanisms were found in creep of SiC/ In this paper, monotonic tensile behavior, fracture lass-ceramics at 1200C at different tensile stress levels toughness and thermal shock resistance fatigue and [26]. At low stresses, cavities formed in the matrix with creep behavior of standard SiC/SiC, enhanced SiC/SiC no significant fiber or matrix damage [26]. At moderate and Hi-Nicalon TM SiC composites is reviewed stresses,periodic fiber rupture occurred, and at high stresses matrix fracture and rupture of the highly stres sed bridging fibers limited creep life [26]. Since grain 2. Monotonic Tension growth in Nicalon fibers enhanced creep resistance creep deformation was found to be transient in nature 2. 1. Monotonic tensile behavior in standard SiC/Sic at1200°CD27 Chemical vapor infiltration (CVI) is an important A characteristic in the stress versus strain curves of technique for manufacturing long-fiber-reinforced cera- SiC/SiC is the existence of inelastic deformation [50-60 mic-matrix composite, in which a porous preform of Fig. I shows the stress versus strain of a plain-weave fibers is infiltrated by a gaseous precursor which then 2DSiC/SiC composite at both room temperature and deposits a ceramic matrix. Although the feasibility of 1000oC in argon [55]. The room temperature curve CVI process has already been established for a number indicates a linear elastic behavior up to the proportiona of ceramic matrices including carbides(SiC, B4C, TiC), limit of 80 MPa, and this stress is about 40% of the nitrides(Bn, Si3 N4) and oxides(Al,O3, ZrO,), only Sic ultimate tensile strength (UTS). The modulus calculated matrix CVI composites are currently produced on an from the linear portion of the curve is 250 GPa. The industrial scale [2]. SiC (NicalonTM) fiber is one of the average values of UTS are 209 MPa at room tempera- most successful of the fine ceramic fibers, commercially ture and 251 MPa at 1000 C. It is noted that the UTS produced by Nippon Carbon. Therefore, NicalonTM. and the strain at UTS at 1000 C are higher than those fiber-reinforced silicon-carbide composites (SiC/SiC) at room temperature. The proportional limit and thestress [19,20]. The gradual damage growth is accompanied by a modulus decrease in CMCs under fatigue loading, which has been studied in detail [12,13,16,19,22]. At high temperature, the fatigue limit coincides with the propor￾tional limit for unidirectional SiCf/Si3N4, tested under the conditions for which creep occurs [14]. For creep behavior of CMCs, a classi®cation was proposed [31,32] in terms of creep mismatch ratio (CMR), de®ned as a ratio of the creep rate of the ®ber to that of the matrix. When CMR < 1, the main damage mechanism occurring during creep is periodic ®ber fracture and the creep behavior is governed by embedded ®bers. When CMR > 1, matrix microcrack￾ing is the dominant mode of damage and creep behavior is governed by bridging ®bers. However, the creep mis￾match ratio provides only a starting point for consider￾ing creep behavior and damage mechanisms, since it only depends on the uniaxial creep behavior of the constituents. Because of local stress concentrations and the development of triaxial stresses, the in-situ creep behavior of the constituents in a composite can di€er signi®cantly from the creep behavior measured during unconstrained uniaxial loading [31,32]. The incorporation of SiC ®ber into Si3N4 results in substantial improvements in creep resistance [23±25]. Multiple ®ber fracture rather than multiple matrix cracking was observed and the creep mechanism was a repetitive matrix stress relaxation!®ber rupture! load transfer and distribution scheme [23±25]. Moreover, a threshold stress was found for the tensile creep of a unidirectional SiCf/HPSN (hot-pressed silicon nitride) composite, which was much higher than the propor￾tional limit [23±25]. Di€erent mechanisms were found in creep of SiCf/ glass-ceramics at 1200C at di€erent tensile stress levels [26]. At low stresses, cavities formed in the matrix with no signi®cant ®ber or matrix damage [26]. At moderate stresses, periodic ®ber rupture occurred, and at high stresses matrix fracture and rupture of the highly stres￾sed bridging ®bers limited creep life [26]. Since grain growth in NicalonTM ®bers enhanced creep resistance, creep deformation was found to be transient in nature at 1200C [27]. Chemical vapor in®ltration (CVI) is an important technique for manufacturing long-®ber-reinforced cera￾mic-matrix composite, in which a porous preform of ®bers is in®ltrated by a gaseous precursor which then deposits a ceramic matrix. Although the feasibility of CVI process has already been established for a number of ceramic matrices including carbides (SiC, B4C, TiC), nitrides (BN, Si3N4) and oxides (Al2O3, ZrO2), only SiC matrix CVI composites are currently produced on an industrial scale [2]. SiC (NicalonTM) ®ber is one of the most successful of the ®ne ceramic ®bers, commercially produced by Nippon Carbon. Therefore, NicalonTM- ®ber-reinforced silicon-carbide composites (SiC/SiC) have become attractive ceramic-matrix composites and have already been applied in some ®elds. TEM observation and electron di€raction exhibited a high degree of polycrystallinity, a relatively small crystal size (3±4 nm) and a lack of preferred orientation for the SiC component of the ®bers in SiC/SiC [33]. The microstructure of SiC ®bers does not change sig￾ni®cantly during fabrication of the composite. There are two kinds of SiC grains in the matrix, polyhedral ones near to the ®bers with a size ranging from 10 to 100 nm, and columnar grains, further away from the ®bers and up to 500 nm in length. The interfacial layer between the matrix and ®bers is a carbon layer. The carbon layer in SiC/SiC leads to low oxidation resistance at intermediate and high temperatures [34±39]. A glass-forming, boron-based particulate material can be added to the matrix that reacts with oxygen to pro￾duce a sealant glass that inhibits oxidation of the carbon layer [40]. This technology has been applied to SiC/SiC composites. The SiC/SiC modi®ed in this way is referred to as an enhanced SiC/SiC composite [40]. Since matrix microcracking may occur during the initial application of a creep load, ®ber bridging of matrix cracks operates during the creep of standard SiC/SiC at high stresses, although the creep resistance of SiC ®bers is lower than that of the SiC matrix [41±49]. This is undesirable for environmental resistance of the composites if they are exposed to air. Because creep of the ®bers controls matrix crack growth, increasing the creep resistance of the ®bers should improve the creep behavior of the composite. Hi-NicalonTM ®ber is one of the improved SiC ®bers [46,49], which is used to rein￾force a SiC matrix (Hi-NicalonTM/SiC composite). In this paper, monotonic tensile behavior, fracture toughness and thermal shock resistance, fatigue and creep behavior of standard SiC/SiC, enhanced SiC/SiC and Hi-NicalonTM/SiC composites is reviewed. 2. Monotonic Tension 2.1. Monotonic tensile behavior in standard SiC/SiC A characteristic in the stress versus strain curves of SiC/SiC is the existence of inelastic deformation [50±60]. Fig. 1 shows the stress versus strain of a plain-weave 2DSiC/SiC composite at both room temperature and 1000C in argon [55]. The room temperature curve indicates a linear elastic behavior up to the proportional limit of 80 MPa, and this stress is about 40% of the ultimate tensile strength (UTS). The modulus calculated from the linear portion of the curve is 250 GPa. The average values of UTS are 209 MPa at room tempera￾ture and 251 MPa at 1000C. It is noted that the UTS and the strain at UTS at 1000C are higher than those at room temperature. The proportional limit and the 834 S. Zhu et al. / Composites Science and Technology 59 (1999) 833±851