S. Mall,J.L Ryba Composites Science and Technology 68(2008)274-282 275 and the matrix, thus increasing the access continually for reduction in the intermediate temperature range (i.e, from more exposure to the oxidizing environment. On the other 450 to 900C) under harsh environment. Therefore, the hand, BN also experiences oxidation, but not readily as the test temperatures of this study were selected in such man- ner that they were below this intermediate range, within In general CMCs experience embrittlement when this range, and above this range, respectively. Tests were exposed to oxidizing environment, and it has been referred conducted under 100% steam and laboratory air environ to as the oxidation embrittlement. This affects primarily ments to highlight oxidizing environments'effects In addi fiber/matrix interphase causing degradation in strength tion, monotonic tests were also conducted to establish the and toughness of the CMC. Oxidation embrittlement in baseline data. Finally, the detailed microscopic analyses several SiC/SiC CMC systems has been investigated in pre- were conducted to document the failure and damage vious studies [4-6] as well as in other CMC systems [7-12]. mechanisms It has been noted that the SiC/SiC CMC systems with BN on in the inte 2. Experi mediate temperature range(from 450 to 900C)under harsh environment since cracks in matrix material allow 2 material the outside environment to penetrate inside and attack the fiber/matrix interphase [7-12). This is due to oxidiza- As mentioned earlier, Syl- iBN/BN/SiC CMC was the tion of fiber/matrix interphase region forming the boria test material of this study. Honeywell Advanced Compos- (B2O3), which reacts with SiC. This generates a borosilicate ites, Inc. manufactured composite panels with Syl fibers melt, which is then solidified as a glass. This bonds fibers provided by NASa Glenn Research Center. The composite gether causing an embrittlement. In other words, oxida- material consisted of 8 plies of woven(5 Harness Satin) Syl tion in SiC/SiC CMC systems with BN interphase results tows containing 800 fibers. These preforms(i.e,woven in a solid brittle glass replacing the functional interphase plies) were treated in several steps. First, in situ BN pre- material, and thereby removes the inherent toughness forms had the interphase bn layer applied by chemical nechanisms. Thus, oxidation embrittlement is a concern vapor infiltration(CVI) process, resulting in a 10.64+ during the application of SiC/SiC CMC systems in humid 0.34% weight gain. Then, a thin layer of SiC was applied environments. One such example is the combustor liner by Cvi to the bn coated woven preform, resulting in a gas turbine engines [13] 53.40+.99% weight gain. SiC particle slurry was then One way to alleviate the oxidation embrittlement con- infiltrated into the porous network, resulting in a 30.10+ cern in the SiC/SiC CMC systems is to modify the fiber/ 1. 50%weight gain. Finally, molten Si was melt-infiltrated matrix interphase. One such modification involves the (Mi) to nearly fill the porous network, resulting in a pre-application of in situ grown BN layer on SiC fiber 13.92+0.80% weight gain. The resulting matrix was pre- before reinforcing in SiC matrix along with BN interphase dominantly SiC with some silicon (Si). The final volume [14]. This CMC system consisted of Syl fibers with in situ fractions were: for fibers, 37%, for BN, 6.6%, for CVI layer of boron nitride(iBN), Bn interphase and Sic SiC, 19.3%, for MI SiC and Si (including the porosity), matrix, and it is referred to as"Syl-iBN/BN/SiC". This 37.05%. The test specimens were cut in a dog-bone config- CMC system has been characterized for its stress rupture uration with the following nominal gage dimensions: and fatigue behavior under humid environment by the first 2. 1 mm thickness, 10.2 mm width and 100 mm length author and his colleagues [15-17], however these character-(Fig. 1) izations have been conducted in the intermediate tempera ture range (from 550 to 750C)only. In order to 2. 2. Test setup understand fully, it is necessary to investigate its behavior at other elevated temperatures also. This is focus of the All tests were conducted on a servo-hydraulics mechan present study. The specific objective of the present study ical testing machine equipped with hydraulic water-cooled haracterize the tensile stress rupture behavior of grips, a compact two-zone resistance-heated furnace, and Syl-iBN/BN/SiC at three elevated temperatures, 400 two temperature controllers. a digital controller was systems with bn intes hase have iet, sin ticant strength as po ire the rest da contests were conducted es ther in Fig. I. Dog-bone shaped specimenand the matrix, thus increasing the access continually for more exposure to the oxidizing environment. On the other hand, BN also experiences oxidation, but not readily as the carbon. In general CMCs experience embrittlement when exposed to oxidizing environment, and it has been referred to as the oxidation embrittlement. This affects primarily fiber/matrix interphase causing degradation in strength and toughness of the CMC. Oxidation embrittlement in several SiC/SiC CMC systems has been investigated in pre￾vious studies [4–6] as well as in other CMC systems [7–12]. It has been noted that the SiC/SiC CMC systems with BN interphase have a significant strength reduction in the inter￾mediate temperature range (from 450 to 900 C) under harsh environment since cracks in matrix material allow the outside environment to penetrate inside and attack the fiber/matrix interphase [7–12]. This is due to oxidiza￾tion of fiber/matrix interphase region forming the boria (B2O3), which reacts with SiC. This generates a borosilicate melt, which is then solidified as a glass. This bonds fibers together causing an embrittlement. In other words, oxida￾tion in SiC/SiC CMC systems with BN interphase results in a solid brittle glass replacing the functional interphase material, and thereby removes the inherent toughness mechanisms. Thus, oxidation embrittlement is a concern during the application of SiC/SiC CMC systems in humid environments. One such example is the combustor liner in gas turbine engines [13]. One way to alleviate the oxidation embrittlement con￾cern in the SiC/SiC CMC systems is to modify the fiber/ matrix interphase. One such modification involves the pre-application of in situ grown BN layer on SiC fiber before reinforcing in SiC matrix along with BN interphase [14]. This CMC system consisted of Syl fibers with in situ layer of boron nitride (iBN), BN interphase and SiC matrix, and it is referred to as ‘‘Syl-iBN/BN/SiC’’. This CMC system has been characterized for its stress rupture and fatigue behavior under humid environment by the first author and his colleagues [15–17], however these character￾izations have been conducted in the intermediate tempera￾ture range (from 550 to 750 C) only. In order to understand fully, it is necessary to investigate its behavior at other elevated temperatures also. This is focus of the present study. The specific objective of the present study was to characterize the tensile stress rupture behavior of Syl-iBN/BN/SiC at three elevated temperatures, 400 C, 750 C, and 950 C. As stated earlier, the SiC/SiC CMC systems with BN interphase have a significant strength reduction in the intermediate temperature range (i.e., from 450 to 900 C) under harsh environment. Therefore, the test temperatures of this study were selected in such man￾ner that they were below this intermediate range, within this range, and above this range, respectively. Tests were conducted under 100% steam and laboratory air environ￾ments to highlight oxidizing environments’ effects. In addi￾tion, monotonic tests were also conducted to establish the baseline data. Finally, the detailed microscopic analyses were conducted to document the failure and damage mechanisms. 2. Experiments 2.1. Material As mentioned earlier, Syl-iBN/BN/SiC CMC was the test material of this study. Honeywell Advanced Compos￾ites, Inc. manufactured composite panels with Syl fibers provided by NASA Glenn Research Center. The composite material consisted of 8 plies of woven (5 Harness Satin) Syl tows containing 800 fibers. These preforms (i.e., woven plies) were treated in several steps. First, in situ BN pre￾forms had the interphase BN layer applied by chemical vapor infiltration (CVI) process, resulting in a 10.64 ± 0.34% weight gain. Then, a thin layer of SiC was applied by CVI to the BN coated woven preform, resulting in a 53.40 ± 5.99% weight gain. SiC particle slurry was then infiltrated into the porous network, resulting in a 30.10 ± 1.50% weight gain. Finally, molten Si was melt-infiltrated (MI) to nearly fill the porous network, resulting in a 13.92 ± 0.80% weight gain. The resulting matrix was pre￾dominantly SiC with some silicon (Si). The final volume fractions were: for fibers, 37%, for BN, 6.6%, for CVI SiC, 19.3%, for MI SiC and Si (including the porosity), 37.05%. The test specimens were cut in a dog-bone config￾uration with the following nominal gage dimensions: 2.1 mm thickness, 10.2 mm width and 100 mm length (Fig. 1). 2.2. Test setup All tests were conducted on a servo-hydraulics mechan￾ical testing machine equipped with hydraulic water-cooled grips, a compact two-zone resistance-heated furnace, and two temperature controllers. A digital controller was employed to generate and control all test commands as well as to acquire the test data. Tests were conducted either in Fig. 1. Dog-bone shaped specimen. S. Mall, J.L. Ryba / Composites Science and Technology 68 (2008) 274–282 275