J.Am. Cera.Soe.,8951652-165802006 DOI:10.111.1551-2916.200600939x C 2006 The American Ceramic Society urna Creep and Stress-Strain Behavior after Creep for SiC Fiber Reinforced, Melt-Infiltrated SiC matrix Composites Gregory N Morschen Ohio Aerospace Institute, NASA Glenn Research Center, MS 106-5, Cleveland, Ohio 44135 Vijay v. Pujar Materials and Simulation Center, Goodrich Corporation, Brecksville, Ohio 44141 Silicon carbide fiber(Hi-Nicalon Type S, Nippon Carbon) re- combining CVI-SiC with Sic particulate slurry and molten sil- inforced silicon carbide matrix composites containing melt-in- icon(Si) infiltration(referred to hereafter as the MI matrix)has filtrated silicon were subjected to creep at 1315C at three resulted in the best combination of thermo-mechanical proper different stress conditions. for the lies for many targeted applications. In particular, the improved ture after 100 h of tensile creep, fast-fracture experiments were densification in these MI composites give improved matrix performed immediately following the creep test at the creep cracking properties relative to composites consisting of the con- temperature(1315 C)or after cooling to room temperature. All entional all-cvI matrix' There are two reasons for the im- specimens demonstrated excellent creep resistance and com- proved matrix cracking properties. First, the Si-infiltrant fills in pared well to the creep behavior published in the literature on most of the macro-porosity of the CVi-SiC matrix, thereby re- similar composite systems. Tensile results on the after-creep noving the stress concentrators in the cvi-sic matrix at cross- specimens showed that the matrix cracking stress actually in- over points in the woven structure, which are observed to be the creased. which is attributed to stress redistribution between site of matrix crack initiation at lower stresses 7.8 Second. the mposite constituents during tensile creep. volume expansion of Si upon cooling below its melting point and the lower thermal expansion coefficient of Si compared with SiC places the matrix under residual compression in the ma- L. Introduction trix relative to the fiber. However the mi matrix itself is ex pected to be less creepresistant than an all-CVI-SiC matrix ILICON carbide fiber reinforced silicon carbide matrix ceramic because of its reduced CVI-SiC content and the low creep re- matrix composites( CMCs), commonly referred to as SiC/ sistance of the Si infiltrant at temperatures near SiC CMCs, have been studied extensively over the last few dec-(1410.C) des for their potential as structural materials in different fields This study was aimed at understanding the tensile be. including advanced turbine engines for aerospace and pow ha avior of the Mi composite system at 1315.C in air, including generation applications. A primary reason for use of SiC fiber the effect of creep on the resulting stress-strain response and reinforced SiC CMCs is their ability to retain a high degree of ultimate properties of the system. There have been only limited structural performance at high temperatures because of their studies reported on inherent creep-resistance. Much work on SiC/SiC composite no studies on after eep behavior in SiC-SiC MI CMCS, and reep behavior of these materials. There creep has been performed for systems with lower-use tempera- have been. however, some studies on creep and after-creep be- ture fibers, e.g. NicalonM and Hi-NicalonM (Nippon Carbon havior on other ceramic composite, namely glass ceramic and Co., Tokyo, Japan)and with chemical vapor infiltrated(CvI) Si3N systems. Specifically, composite systems SiC matrices. In these studies, woven 2D 0/90 lay-up com which the fiber is more creep resistant than the matrix, it has were crept at temperatures ranging from 1000 to 1300C been shown that at creep stresses lower than the matrix cracking and argon,and creep stresses ranging from 30-180 stresses, the matrix relaxes during creep and transfers the load to the primary fiber direction. At the higher temperatures, the fiber It has been shown in these materials that the stress to 300C, and at applied tensile creep stresses above 100 cause matrix cracking in the after-creep specimens is actually MPa, rupture usually occurred after a few hours in these sam- higher, relative to that in the as-produced material. As de- ples with rupture strains upwards of 1%, thereby limiting their scribed in this paper, a similar phenomenon appears to be op use to lower temperatures. erative in the MI system studied he With the availability of more creep-resistant polycrystalline SiC fibers, e.g., Hi-Nicalon Type s(made by Nippon Carbon referred to here as HNS). .Tyranno SA(Ube Industries,To- kyo, Japan), Sylramic(formerly Dow Corning, and now made Il. Experimental Procedure by Col Ceramics, San Diego. CA), greater creep resistance Details about composite processing, constituent content, room can be achieved. This enables higher use-temperatures for Sic and some elevated temperature properties can be found in Mo- Sic composites based on these fibers. At the same time, signif- scher and Pujar for the three different panels that tensile spec icant progress has been made in matrix development to achieve imens for this study came from. The melt-infiltrated composites nearly full densification. The matrix consolidation approach all consisted of eight plies of stacked five harness satin (7.1 tow ends/cm)2D woven 0/90 HNS fabric, a CVI BN interphase, a editor VI-SiC matrix layer, followed by Sic slurry infiltration and molten Si infiltration. The three panels varied in their relative constituent contents, as shown in Table I. Note that panels labeled Al and a2 consisted of similar fractions of cvi SiC and luscript No. 21068 Received October 11, 2005: approved December 19, 2005. Si whereas panel B had a significantly lower fraction of CVI SiC thor to whom correspondence should be addressed. e-mail: vijay pujar(a goodrich. content and consequently a significantly greater fraction of si isenior Research Scientist residing at NASA Glenn Research Center. contentCreep and Stress–Strain Behavior after Creep for SiC Fiber Reinforced, Melt-Infiltrated SiC Matrix Composites Gregory N. Morscherz Ohio Aerospace Institute, NASA Glenn Research Center, MS 106-5, Cleveland, Ohio 44135 Vijay V. Pujarw Materials and Simulation Center, Goodrich Corporation, Brecksville, Ohio 44141 Silicon carbide fiber (Hi-Nicalon Type S, Nippon Carbon) re￾inforced silicon carbide matrix composites containing melt-in- filtrated silicon were subjected to creep at 13151C at three different stress conditions. For the specimens that did not rup￾ture after 100 h of tensile creep, fast-fracture experiments were performed immediately following the creep test at the creep temperature (13151C) or after cooling to room temperature. All specimens demonstrated excellent creep resistance and com￾pared well to the creep behavior published in the literature on similar composite systems. Tensile results on the after-creep specimens showed that the matrix cracking stress actually in￾creased, which is attributed to stress redistribution between composite constituents during tensile creep. I. Introduction SILICON carbide fiber reinforced silicon carbide matrix ceramic matrix composites (CMCs), commonly referred to as SiC/ SiC CMCs, have been studied extensively over the last few dec￾ades for their potential as structural materials in different fields including advanced turbine engines for aerospace and power generation applications. A primary reason for use of SiC fiber reinforced SiC CMCs is their ability to retain a high degree of structural performance at high temperatures because of their inherent creep-resistance. Much work on SiC/SiC composite creep has been performed for systems with lower-use tempera￾ture fibers, e.g., NicalonTM and Hi-NicalonTM (Nippon Carbon Co., Tokyo, Japan) and with chemical vapor infiltrated (CVI) SiC matrices.1–3 In these studies, woven 2D 0/90 lay-up com￾posites were crept at temperatures ranging from 10001 to 13001C in air2,3 and argon,1–3 and creep stresses ranging from 30–180 MPa in the primary fiber direction. At the higher temperatures, 12001–13001C, and at applied tensile creep stresses above 100 MPa, rupture usually occurred after a few hours in these sam￾ples with rupture strains upwards of 1%, thereby limiting their use to lower temperatures. With the availability of more creep-resistant polycrystalline SiC fibers, e.g., Hi-Nicalon Type S (made by Nippon Carbon, referred to here as HNS),4,5 Tyranno SA (Ube Industries, To￾kyo, Japan),5 Sylramic (formerly Dow Corning, and now made by COI Ceramics, San Diego, CA),5,6 greater creep resistance can be achieved. This enables higher use-temperatures for SiC/ SiC composites based on these fibers. At the same time, signif￾icant progress has been made in matrix development to achieve nearly full densification. The matrix consolidation approach combining CVI–SiC with SiC particulate slurry and molten sil￾icon (Si) infiltration (referred to hereafter as the MI matrix) has resulted in the best combination of thermo-mechanical proper￾ties for many targeted applications. In particular, the improved densification in these MI composites give improved matrix cracking properties relative to composites consisting of the con￾ventional all-CVI matrix.7 There are two reasons for the im￾proved matrix cracking properties. First, the Si-infiltrant fills in most of the macro-porosity of the CVI–SiC matrix, thereby re￾moving the stress concentrators in the CVI–SiC matrix at cross￾over points in the woven structure, which are observed to be the site of matrix crack initiation at lower stresses.7,8 Second, the volume expansion of Si upon cooling below its melting point and the lower thermal expansion coefficient of Si compared with SiC places the matrix under residual compression in the ma￾trix5,9 relative to the fiber. However, the MI matrix itself is ex￾pected to be less creepresistant than an all-CVI–SiC matrix because of its reduced CVI–SiC content and the low creep re￾sistance of the Si infiltrant at temperatures near its melting point (14101C). This study was aimed at understanding the tensile creep be￾havior of the MI composite system at 13151C in air, including the effect of creep on the resulting stress–strain response and ultimate properties of the system. There have been only limited studies reported on creep behavior in SiC–SiC MI CMCs, and no studies on after-creep behavior of these materials.10 There have been, however, some studies on creep and after-creep be￾havior on other ceramic composite, namely glass ceramic and Si3N4 matrix systems.11–13 Specifically, composite systems in which the fiber is more creep resistant than the matrix, it has been shown that at creep stresses lower than the matrix cracking stresses, the matrix relaxes during creep and transfers the load to the fiber.11 It has been shown in these materials that the stress to cause matrix cracking in the after-creep specimens is actually higher11,13 relative to that in the as-produced material. As de￾scribed in this paper, a similar phenomenon appears to be op￾erative in the MI system studied here. II. Experimental Procedure Details about composite processing, constituent content, room and some elevated temperature properties can be found in Mo￾rscher and Pujar5 for the three different panels that tensile spec￾imens for this study came from. The melt-infiltrated composites all consisted of eight plies of stacked five harness satin (7.1 tow ends/cm) 2D woven 0/90 HNS fabric, a CVI BN interphase, a CVI–SiC matrix layer, followed by SiC slurry infiltration and molten Si infiltration. The three panels varied in their relative constituent contents, as shown in Table I.5 Note that panels labeled A1 and A2 consisted of similar fractions of CVI SiC and Si whereas panel B had a significantly lower fraction of CVI SiC content and consequently a significantly greater fraction of Si content. Journal J. Am. Ceram. Soc., 89 [5] 1652–1658 (2006) DOI: 10.1111/j.1551-2916.2006.00939.x r 2006 The American Ceramic Society 1652 E. Lara-Curzio—contributing editor w Author to whom correspondence should be addressed. e-mail: vijay.pujar@goodrich. comz Senior Research Scientist residing at NASA Glenn Research Center. Manuscript No. 21068. Received October 11, 2005; approved December 19, 2005