Effects of moisture on fatigue behavior of SiC/SiC composite at elevated temperature☆
Introduction
The technological advancement for many structural components is contingent upon the continued development of high-performance materials, which can survive in demanding operating environments. As a result, the demand for advanced materials in aerospace and other high-temperature applications has steadily increased over the past few decades. Unfortunately, the supply has not maintained the pace. A good example is the limited success engineers have had in designing and manufacturing advanced supersonic and hypersonic aerospace vehicles. Much of the progress in this area has been hampered by the non-availability of structural materials, which can withstand the necessary operating conditions, since in many cases materials are required to operate in environments where severe thermal-mechanical and fatigue loads can be expected.
Ceramic materials are a natural fit for these environments as a result of their innate resistance to heat, chemicals and wear. What makes ceramic materials so appealing is that they possess high strength and modulus even at elevated temperatures. Ironically however, the phenomenon, which provides the ceramic material with its exceptional strength and stiffness, also causes the ceramic to be very brittle. As a result, when monolithic ceramics fail, they typically fail catastrophically and without warning. Hence, these materials are not amenable to many structural applications. With the addition of high-strength reinforcing fibers/fabrics; however, the overall strength of the ceramic, as well as its fracture toughness, can be increased significantly. For this reason, ceramic matrix composites (CMCs) are currently receiving a great deal of attention. CMCs are the leading candidates for a number of applications in turbine engines for aircraft and future space vehicles (reusable launch vehicles, space operation vehicles etc), land-based power generation systems, and automobiles.
CMCs are materials that have chemically and/or physically distinct constituent of fiber with interphase distributed within a continuous constituent of matrix. CMCs have oxide and/or non-oxide constituents. Nextel 720/alumina is an example of oxide fibers reinforced in oxide matrices CMCs [1], while silicon carbide (SiC) fibers reinforced in silicon carbide matrix is an example of non-oxide CMCs [2]. There are several commercially available SiC fibers, for example, Nicalon (Nic), Hi-Nicalon (HN) and Sylramic (Syl) [2], [3], [4]. Although these all are made of SiC, they have different mechanical properties at elevated temperature and/or in oxidizing environments. Carbon (C) and boron nitride (BN) have been choices as the interphase materials in the SiC/SiC CMCs [2]. However, there has been considerable drop in interest in C as interphase due to its poor performance in oxidizing environments since C oxidizes and volatizes which causes degradation in the CMCs performance [5]. BN is thus currently the most desirable interphase material for CMCs during their applications in air-breathing gas turbine engines, since it possesses similar debonding and sliding properties as C with higher durability in the oxidizing environment [6].
Fiber/matrix interphase is an important constituent of CMCs to improve their performance under stressed-oxidative environment in intermediate temperature regime, so efforts are always underway to modify the interphase to achieve better properties of CMCs. One such effort involves the pre-application of in situ BN layer to SiC fiber before reinforcing in SiC matrix along with BN interphase [7]. This method has been developed by NASA where CMC system consisted of Syl fibers, BN interphase, and SiC matrix, and is referred to as “Syl-iBN/BN/SiC”. This CMC system is one of strongest and the most creep resistant systems [7]. This will be material of this study. This CMC system has a potential use in conditions involving the elevated temperature and humidity. One such application is the combustor liner in gas turbine engines [6]. The current SiC/SiC CMC system have shown a significant strength reduction in the intermediate temperature range (from 450 to 900 °C) under harsh environment where cracks in matrix material allow the outside environment to penetrate inside and attack the fiber/matrix interphase [8], [9], [10], [11], [12], [13]. This is of great concern to engine designers due to presence of harsh environments, such as moisture. Therefore, the effects of the moisture on CMC performance at elevated temperature are, in general, a very important issue. The focus of this study was in this direction where the combined effects of humidity and temperature on the fatigue performance of the aforementioned newly developed SiC/SiC CMC system, Syl-iBN/BN/SiC, was investigated at an intermediate elevated temperature under humid environment. Also test were performed under the dry condition with no moisture in order to study the humidity effects.
Section snippets
Material
As mentioned earlier, Syl-iBN/BN/SiC CMC was used in this study. Honeywell Advanced Composites, Inc. manufactured composite panels with Syl fibers provided by NASA Glenn Research Center. The composite material consisted of eight plies of woven (5 Harness Satin) Syl tows containing 800 fibers each (20 epi). These performs were treated in several steps. First, in situ BN preforms had the interphase BN layer applied by CVI, resulting in a 10.64 ± 0.34% weight gain. Then, a thin layer of SiC was
Monotonic test
Fig. 2 shows the monotonic tensile stress–strain curves at three different test environments of the Syl-iBN/BN/SiC CMC system used in this study. From this, several observations can be made. The ultimate tensile strength is dependent on the temperature, but no obvious dependence on moisture content could be observed. At 750 °C, there were 14 and 22% reductions in ultimate tensile strength for the 0% (dry) and 60% moisture content conditions in comparison to its counterpart at room temperature
Conclusions
Fatigue performance of a ceramic matrix composite (CMC) at an elevated temperature in presence of moisture was investigated. The tested CMC consisted of silicon carbide fiber (Syl) reinforced in silicon carbide (SiC) matrix with boron nitride (BN) interphase, Syl-iBN/BN/SiC. Tension–tension fatigue tests were conducted at 10 Hz with a stress ratio of 0.1 at 750 °C: (1) under a dry environment test condition (with 0% moisture content) and (2) under a humid environment test condition (with 60%
Acknowledgement
The support of Ruth Sikorski and Ted Fecke, Propulsion Directorate, Air Force Research Laboratory and Jim DiCarlo, Ceramics Branch, NASA-Glenn Research Center is highly appreciated.
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The views expressed in this article are those of the authors and do not reflect the official policy or position of the United States Air Force, Department of Defense, or the U.S. Government.