AH.Cea.Sox86161981-90(2003) urna Characterization and High-Temperature Mechanical Behavior of an Larry P. Zawada, Randall S. Hay, Shin S. Lee, and James Staehler Materials an Air Force Research Laboratory, Wright-Patterson Air For Ohio 45433 An oxide/oxide ceramic fiber-matrix composite(CMC) has been extensively characterized for high-temperature aerospac porous, cracked matrices for high-temperature aerospace applica- structural applications. This CMC is called GEN-IV, and it and an aluminosilicate matrix is called GEN-IV. 4, 5, 2I and it is has a porous and cracked aluminosilicate matrix reinforced by referenced in the following text as N6lO/AS. A review of the 3M Nextel 610 M alumina fibers woven in a balanced eight mechanisms and mechanical properties of porous-matrix CMCs harness weave(SHSW). This CMC has been specifically has been given elsewhere. 22 designed without an interphase between the fiber and matrix The following characteristics of N610/AS porous-matrix com- and it relies on the porous matrix for flaw tolerance. Stress- posites are evaluated: (i) long-time phase and microstructural strain response is nearly linear to failure and without a stability: (ii) high-temperature, short-time stress-strain well-defined proportional limit in tension and compression (iii)fatigue; and (iv) creep and creep rupture. Compari In-plane shear and interlaminar strength increases with in other CMCs are made and possible explanations for the creasing temperature. The 1000 C fatigue limit in air at 105 behavior are discussed cycles is 160 MPa, and the residual tensile strength of run-out pecimens is not affected by the fatigue loading. The creep- rupture resistance above 1000'C is relatively poor, but it can lL. Materials and Experiments be improved with a more-creep-resistant fiber. (1) Composite Fabrication 3M Nextel 610 fibers are used to manufacture tile of NolO/AS The Nextel 610 fiber is 99 wt% polycrystalline a-Al,O,, with a L. Introduction density of 3.88 g/cm, an average grain size of 0. I um, and an average filament diameter of 12 um. Eight harness satin weave H fracture toughness and damage tolerance is engineered (8HSW) cloth of Nextel 610 was prepregged with a mixture of fine o most fiber-reinforced ceramic-matrix composites (CMCs)by tailoring properties of the fiber-matrix interface. The A Os powder and a SiO,forming polymer. Twelve individ- fiber-matrix interface must deflect matrix cracks and allow fiber ual prepregged cloths were stacked on top of each other as a aminate. The laminate was warm molded in an autoclave to pullout afterward. Mechanical properties of CMCs break down if the coating is not stable in the application environment. 3.s ered in air at -1000C. This pr roduce a dense green-state ceramic tile. The tile was pressureless Carbon- and BN- are the usual fiber-matrix interphases in SiC-fiber CMCs. Unfortunately, carbon coatings begin to oxidize converted the polymer to porous So cess removed organics and at-450C, and the gap left by oxidation may fill with the Sio oxidation product of SiC and form a strong fiber-matrix bond that (2) Experiments seriously degrades CMC mechanical properties. .m Amorphous (A) Microstructure Characterization: The composi and imperfectly crystalline BN are moisture sensitive and easily oxidize. o, 2 Similar to SiO2, B, O, forms a strong fiber-matrix sity (seven specimens) was measured using the Archi pycnometer. bond that degrades CMC properties. Oxidation is a serious adsorbs moisture from the air. the specimens were carefully obstacle to long-term use of CMCs with carbon or BN fiber- outgassed before measurement. The total pore surface area was matrix interfaces at intermediate and high temperatures measured using the Brunauer-Emmett-Teller (BET) method An approach to flaw-tolerant CMCs that are also oxidation (eight different measurements on seven specimens). Fiber volume resistant is oxide/oxide CMCs with fibers that are " strongly fractions were measured in three specimens polished at 45 to the bonded to a matrix deliberately made weak by incorporation of fiber axes. A microscope( Model Metallovert, Leitz) mounted with high porosity and microcracks. Instead of crack deflection and a video camera, video monitor, and computer running image diffuse microcracking in the s matrix. Early modeling of mage analysis of the fiber volume fractions pus)was used for hese materials suggests that the fiber bundles must be heterog Fracture surfaces of failed specimens were characterized using enously distributed in the matrix, have higher coefficient of thermal expansion (CTE) than the matrix, and have a Mode I As-fabricated specimens and specimens heat-treated for 3000 h at fracture energy twice the Mode I fracture energy of the ma- 982 C(1800 F) were characterized using optical microscopy trix.9.2 General Electric has developed oxide/oxide CMCs with SEM(Model 360FE, Leica), and TEM operating at 200 kV(Model 2000 FX, JEOL Tokyo, Japan ). SEM specimens and TEM thin sections were prepared using a method described elsewhere. TEM specimens were mounted on copper grids and ion milled at F. Zok--contnbuting editor 7kV, with the last 15 min at 4 kv. Most specimens were carbon coated. Some of the TEM specimens were observed without carbon coating so that the fine structure of the matrix porosity could be better resolved. For these specimens, care was taken to uscript No, 188004. Received January 17, 2002; approved January 28, 2003 align and stigmate the microscope for the particular condenser lens ber, American Ceramic societ