AFRL scientists characterized and evaluated the high-temperature mechanical behavior of fiber-reinforced ceramic matrix composite (CMC) materials used in aerospace structural applications. Researchers examined four principal characteristics of a porous matrix composite that General Electric developed for the aerospace industry. Their evaluations resulted in an increased understanding of the materials and their potential for applications in military and commercial aerospace products.

Engineers can design high fracture toughness and damage tolerance into most fiber-reinforced CMCs; to do so, they tailor the properties of the fiber matrix interface so that matrix cracks will deflect around the fiber and allow it to move. The friction inherent to this interaction leads to toughness and flaw tolerance. Engineers most commonly select carbon (C) and boron nitride (BN) as the interfaces to tailor for silicon carbide (SiC)-fiber CMCs. However, C coatings begin oxidizing at about 450°C, leaving a gap that can fill with silica (SiO2), the oxidation byproduct of SiC. The result is a strong fiber matrix bond that seriously degrades CMC mechanical properties. Both amorphous and crystalline forms of BN are sensitive to moisture and oxidize easily, forming B2O2. Similar to SiO2, B2O2 forms a strong fiber matrix bond that degrades CMC properties. Consequently, engineers consider oxidation a serious obstacle to long-term use of CMCs that contain C or BN fiber matrix interfaces and which are subject to intermediate and high temperatures.

Figure 1. Transmission electron microscope (TEM) micrograph of as-processed N610/AS fiber showing very fine matrix porosity
Conversely, porous matrix, oxideoxide CMCs are flaw-tolerant and inherently oxidation-resistant. Fabricating the CMC with a porous matrix and without a fiber coating dissipates matrix crack energy via pervasive microcracking. By preventing crack tip stress that causes fibers to fracture, the weak porous matrix leads to tough, damage-tolerant behavior. For the CMC to exhibit this behavior, the fibers must have both a thermal expansion coefficient higher than that of the matrix and a tensile fracture energy twice as great as the shear fracture energy of the matrix.

AFRL scientists examined N610/AS, one of several porous matrix, oxideoxide CMCs that General Electric developed for high-temperature aerospace applications. The N610/AS composite consists of 3M Nextel™ 610 fiber and an aluminosilicate matrix. The research team investigated four key characteristics of the composite: (1) long-time phase and microstructural stability; (2) high-temperature, shorttime, stress-strain response; (3) fatigue; and (4) creep and creep rupture.

Figure 2. TEM micrograph of N610/AS fiber showing matrix porosity coarsening after heat treatment at 982° C for 3,000 hours
The researchers also compared N610/AS-based CMCs to other CMCs in order to formulate possible explanations for the N610/AS composite's behavior. They found that the N610/AS CMC exhibited reasonable tensile strength and fatigue performance at both room temperature and high temperatures, and they determined that its tensile and compressive strength are moderate at room temperature compared to other CMCs. However, unlike most other CMCs, the N610/AS CMC's fatigue performance does not change significantly with temperatures up to 1000°C. This result is consistent with the limited observable porosity coarsening in the matrix (see Figures 1 and 2). The 3M Nextel 610 fiber had low creep resistance and limited use time above 1000°C. In addition, the low-modulus, porous, and precracked aluminosilicate matrix had low in-plane and interlaminar strength. Both the fiber and the matrix exhibited significant mechanical property degradation above 1000°C.

Mr. Larry P. Zawada, Dr. Randall S. Hay, and Dr. Peter S. Meltzer (Anteon Corporation), of the Air Force Research Laboratory's Materials and Manufacturing Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn/index.htm. Reference document ML-H-05-32.