High-Temperature Water Vapor Effects JAm. Cera Soc,86|8l128-9102003) ournal Consequence of Intermittent Exposure to Moisture and Salt Fog on the High-Temperature Fatigue Durability of Several Ceramic-Matrix Composites Larry P, Zawada, James Staehler, *and Steve Steel Materials and Manufacturing Directorate, Air Force Research Laboratory, AFRL/MLLN Wright-Patterson AFB. Ohio 45433-7817 Fatigue behavior of four ceramic-matrix composites(CMCs) degradation is accelerated whenever moisture is was documented at 1000 C, and a fifth composite was docu- a humid day contains from 0.003 to 0.025 atm( mented at 1200 C. Additional fatigue specimens were cycle 1×10°Pa)ofH2O. For co ustors. the com for set blocks of cycles, removed from the fatigue machine, and from burning hydrocarbon fuel contain -10%H,O exposed in a cyclie corrosion tester for 24 h with a fog of I atm). For combustors operating at 10 atm, the hot gas deionized water and a fog of deionized water containing 0.05 contains "10-15 vol% of H,O. The degradation in SiC/SiC wt% NaCL. BN-fiber-coated NicalonMisinc and Nicalon/ CMCs under turbine engine combustor conditions has been thor- AlO3 experienced a pronounced decrease in fatigue life oughly documented. A thorough discussion about CMC degrada- (86%)with salt fog exposure. Nicalon/C experienced rapid tion mechanisms in combustors is given by More and co- loss of the SiC exterior seal coat and a 30% decrease in life workers 2 and by Ferber et al. 3 with salt fog exposure Nextel610/AS and Nextel720/AL,O3 The components at the back of the turbine engine(divergent demonstrated no loss in fatigue performance or retain nozzle or exhaust-washed structures) are exposed to high-velocity strength with water or salt fog exposure. Changes to the hot exhaust gases containing a high moisture content and are periodically exposed to moisture whenever there rain or con- they influenced moisture sensitivity. Bn fiber coatings, BN densation of moisture. Fighter aircraft, such as the F-18 Hornet, the or BN/SiC, alternate matrix prepreg, and matrix filler type F-16 Falcon, and the F-15 Eagle, are considered all-weather had no influence on improving moisture resistance. Direct fighters, which means they fly in all types of weather, including exposure to moisture fog produced accelerated rates of rain. While parked on the ground, the exhaust nozzles and degradation in the BN fiber coating and greatly decreased structures are normally covered by a waterproof tarp, but the tarps atigue durability need to be removed before engine start Direct impingement of rain on these structures periodically occurs as a normal part of L Introduction operations during the service life of military aerospace platforms Periodic washing of the aircraft also results in exposure to water. lE class of materials known as ceramic-matrix composites Although CMC parts occasionally are exposed to rain, they also CMCs)continues to be an attractive choice for many aero- are exposed to salt fog from ocean mists and exhaust de space turbine engi plications, CMCs retain strength to much temperature alloys used in jet engines. Salt fog (h,o t nacl) than conventional nickel-based superal However. it is the exposure also has a strong effect on Sic-fiber strength."In GKOspect of higher material operating temperatures and decreased addition, salts are associated with the dissolution(fluxing) of the ooling air that is most attractive to the aerospace design commu- normally protective oxide scales, such as SiO2. as well as other nity. Higher gas temperatures and less cooling air translate directly oxides, such as B,O3. Richardson and Kowalik have shown that hot corrosion of Nicalon M/C with Na SO, occurs above 900%C. with less cooling air for the combustor has the important effect of extensive degradation of the protective Sic surface coating and decreasing the production of nitrous oxides (NO,), which are loss of the carbon-matrix material. Thus. water and salt fog are an detrimental to earth's protective ozone layer. Currently, CMCs are important factor for long-term mechanical performance at high eing demonstrated in turbine components, including combustor temperature. Environmental testing early in the evaluation process liners, turbine nozzles, shrouds, transition ducts, diffusers, and can help determine if these environmental effects are a problem. exhaust structures, such as divergent flaps and seals, as well as There is extensive literature on the subject of moisture and how structures that have hot exhaust gases flowing over them(exhaust- it affects the oxidation of SiC. 7-2Opila and Hann and Pila. 10 washed structures have shown that the presence of water vapor increases the rate of dence of accelerated degradation after only a few hours of hot SiO, growth on SiC at high temperature and that it leads to accelerated rates of SiC recession. There is also extensive literature dation of fibers and fiber on the subject of moisture and how it affects the degradation of BN fiber coatings. -20 Sensitivity of Bn to moisture has been shown to be strongly dependent on the quality of the BN, impurities present in the BN and moisture, levels of moisture, temperature, E.J. Oplia-contributing editor and amount of matrix cracking. most of the work to date has involved exposures and oxidative studies of coupons without the application of stress. The goal of the work reported in this article is to assess the overall extent that moisture affects the high- Manuscript No 186663. Received September 13, 2002; approved April 1, 2003 mperature fatigue durability of CMCs. Initial results on two Materials Society(TMS), Seattle, WA, February tn-o, z2 A c- idation of High- CMCs from two earlier investigations by Lee et al. and Steel et Temperature Materials at the 131st Annual Meet al.2 are reviewed and discussed along with the results for three 1282
Consequence of Intermittent Exposure to Moisture and Salt Fog on the High-Temperature Fatigue Durability of Several Ceramic-Matrix Composites Larry P. Zawada, James Staehler,* and Steve Steel Materials and Manufacturing Directorate, Air Force Research Laboratory, AFRL/MLLN, Wright-Patterson AFB, Ohio 45433-7817 Fatigue behavior of four ceramic-matrix composites (CMCs) was documented at 1000°C, and a fifth composite was documented at 1200°C. Additional fatigue specimens were cycled for set blocks of cycles, removed from the fatigue machine, and exposed in a cyclic corrosion tester for 24 h with a fog of deionized water and a fog of deionized water containing 0.05 wt% NaCl. BN-fiber-coated NicalonTM/SiNC and Nicalon/ Al2O3 experienced a pronounced decrease in fatigue life (86%) with salt fog exposure. Nicalon/C experienced rapid loss of the SiC exterior seal coat and a 30% decrease in life with salt fog exposure. Nextel610/AS and Nextel720/Al2O3 demonstrated no loss in fatigue performance or retained strength with water or salt fog exposure. Changes to the constituents of Nicalon/SiNC were evaluated to determine if they influenced moisture sensitivity. BN fiber coatings, BN or BN/SiC, alternate matrix prepreg, and matrix filler type had no influence on improving moisture resistance. Direct exposure to moisture fog produced accelerated rates of degradation in the BN fiber coating and greatly decreased fatigue durability. I. Introduction THE class of materials known as ceramic-matrix composites (CMCs) continues to be an attractive choice for many aerospace turbine engine applications. CMCs retain strength to much higher temperatures than do metals, and they offer lower density than conventional nickel-based superalloys. However, it is the prospect of higher material operating temperatures and decreased cooling air that is most attractive to the aerospace design community. Higher gas temperatures and less cooling air translate directly to increased thrust and decreased fuel consumption. In addition, less cooling air for the combustor has the important effect of decreasing the production of nitrous oxides (NOx), which are detrimental to earth’s protective ozone layer. Currently, CMCs are being demonstrated in turbine components, including combustor liners, turbine nozzles, shrouds, transition ducts, diffusers, and exhaust structures, such as divergent flaps and seals, as well as structures that have hot exhaust gases flowing over them (exhaustwashed structures). Many of these demonstration components have exhibited evidence of accelerated degradation after only a few hours of hot time. The degradation involves oxidation of fibers and fiber coatings, and the degradation is accelerated whenever moisture is present. Air on a humid day contains from 0.003 to 0.025 atm (1 atm 1 105 Pa) of H2O. For combustors, the combustion products from burning hydrocarbon fuel contain 10% H2O (PH2O 0.1 atm). For combustors operating at 10 atm, the hot gas stream contains 10–15 vol% of H2O. The degradation in SiC/SiC CMCs under turbine engine combustor conditions has been thoroughly documented. A thorough discussion about CMC degradation mechanisms in combustors is given by More and coworkers1,2 and by Ferber et al.3 The components at the back of the turbine engine (divergent nozzle or exhaust-washed structures) are exposed to high-velocity hot exhaust gases containing a high moisture content and are periodically exposed to moisture whenever there is rain or condensation of moisture. Fighter aircraft, such as the F-18 Hornet, the F-16 Falcon, and the F-15 Eagle, are considered all-weather fighters, which means they fly in all types of weather, including rain. While parked on the ground, the exhaust nozzles and structures are normally covered by a waterproof tarp, but the tarps need to be removed before engine start. Direct impingement of rain on these structures periodically occurs as a normal part of operations during the service life of military aerospace platforms. Periodic washing of the aircraft also results in exposure to water. Although CMC parts occasionally are exposed to rain, they also are exposed to salt fog from ocean mists and exhaust deposits. Fused salt deposits accelerate the hot corrosion of most hightemperature alloys used in jet engines. Salt fog (H2O NaCl) exposure also has a strong effect on SiC-fiber strength.4,5 In addition, salts are associated with the dissolution (fluxing) of the normally protective oxide scales, such as SiO2, as well as other oxides, such as B2O3. Richardson and Kowalik6 have shown that hot corrosion of NicalonTM/C with Na2SO4 occurs above 900°C, with extensive degradation of the protective SiC surface coating and loss of the carbon-matrix material. Thus, water and salt fog are an important factor for long-term mechanical performance at high temperature. Environmental testing early in the evaluation process can help determine if these environmental effects are a problem. There is extensive literature on the subject of moisture and how it affects the oxidation of SiC.7–12 Opila and Hann8 and Opila9,10 have shown that the presence of water vapor increases the rate of SiO2 growth on SiC at high temperature and that it leads to accelerated rates of SiC recession. There is also extensive literature on the subject of moisture and how it affects the degradation of BN fiber coatings.13–20 Sensitivity of BN to moisture has been shown to be strongly dependent on the quality of the BN, impurities present in the BN and moisture, levels of moisture, temperature, and amount of matrix cracking. Most of the work to date has involved exposures and oxidative studies of coupons without the application of stress. The goal of the work reported in this article is to assess the overall extent that moisture affects the hightemperature fatigue durability of CMCs. Initial results on two CMCs from two earlier investigations by Lee et al.21 and Steel et al.22 are reviewed and discussed along with the results for three other CMCs. In addition, results from two specially designed E. J. Oplia—contributing editor Manuscript No. 186663. Received September 13, 2002; approved April 1, 2003. Presented at the Symposium on Water Vapor Effects on Oxidation of HighTemperature Materials at the 131st Annual Meeting of the Minerals, Metals, & Materials Society (TMS), Seattle, WA, February 18–20, 2002. *Member, American Ceramic Society. High-Temperature Water Vapor Effects J. Am. Ceram. Soc., 86 [8] 1282–91 (2003) 1282 journal
Consequence of Moisture and Salt Fog on High-Temperature Fatigue of Ceramic-Matrix Composites 283 moisture investigations are used to qualify to what extent the at 1200%C to investigate the potential of this CMC. At 1200C, the high-temperature fatigue behavior of certain types of CMC is Al,O3 matrix should show no sign of softening or weakening, and affected by the direct exposure to moisture the n720 fiber should be relatively stable for the 27.7 h anticipated for reaching run out Fatigue testing was conducted in laboratory air using a servo- I. Materials hydraulic fatigue machine. The fatigue cycle used a load rat of 0.05 and a frequency of I Hz. Run out was defined as 100 000 To thoroughly explore the role of moisture on the fatigue cycles. Results of the standard fatigue experiments were used to durability of CMCs, five different CMCs were purchased from enerate a fatigue diagram of stress versus cycles to failure for their manufacturer and evaluated. all the cmcs have been used as each CMC. These results were also used as the base line for the an eight-harness-satin-weave(8HSW) fiber architecture, and each fatigue plus moisture experime of these material systems has been described in detail else- where -14 Therefore, only a brief description is given here for An environmental chamber with temperature control, humidity control(not used), and fog- generating capability was used to each CMC. Three of the CMCs contain ceramIc-grade Nicalon investigate the influence of water fog and salt fog exposu fibers. The first is Nicalon/Al,O3 from Lanxide Corp. The CMC contains an Al,O, matrix, and the fibers are coated with a dual high-temperature fatigue performance. Chamber temperature was layer of BN and then SiC. The bn is there for debonding, whereas controlled at 35.C and atmospheric pressure. During operati the SiC is applied over the BN to protect it during matrix synthesis. compressed air(gauge pressure of 0. 1 MPa) was humidified by A unique process involving metal oxidation is used to create the passing through a bubble tower, after which it was mixed with the Al,O, matrix. The second CMC is Nicalon/SINC(Nicalon/silicon desired solution in a nozzle. The nozzle atomized the solution and nitrocarbide)from Dow Corning Corporation, and the fibers are air to a corrosive fog. The chamber used -0.76 L of solution/h coated with BN and then Si, N, Multiple polymer infiltration and Operation of the chamber in the fog mode resulted in air that was pyrolysis(PIP) cycles are used to synthesize the amorphous fully saturated with water(PHo s 0.055 atm) and a thick fog of matrix. The third CMC is Nicalon/C from Hitco Technologies water particles. Test specimens were suspended near the center of This Cmc has no fiber coating. but does have b, c filler added to the chamber by racks. During the exposures, the specime the matrix to serve as a glass former and a Sic exterior coating to became fully saturated as solution condensed on them, but were protect the inhibited carbon matrix from oxidation. The matrix is never immersed in water. Specimens were arranged with sufficient thesized using pip followed by chemical infiltration space between test pieces for adequate fog circulation, and in such (CVI) of carbon. The fourth and fifth CMCs are oxide/oxide a way that did not allow condensation from one specimen to fall on CMCs that use a porous matrix to impart toug into the CmC another specimen. During the exposure, the test specimens were in One CMc is a Nextel 610/aluminosilicate(N610/AS)from Gen- continual contact with water and air eral Electric, and the other is a Nextel 720/Al,O3(N720/A)from ionized water was used for the water fog exposures, while COI Ceramics, Neither of these all-oxide CMCs has a coating deionized water mixed with salt(Nacl)was used for the salt fog applied to the fibers. There is no evidence of an interphase between exposures. A Nacl concentration of 0.05 wt% was used. This the fibers and matrix after processing. The matrix of both CMCs is concentration was selected to be 2 orders of magnitude above nthesized using a sol-gel method. the concentrations at typical flying altitudes for most aircraft An additional aspect of this investigation was to determine what and 2 orders of magnitude below what is normally found in the onstituents in the Cmc contributed to moisture-induced degra- saltwater oceans. Salt concentration in the water of the earth's dation. The Nicalon/SiNC system( 8HS W) was selected for further oceans is 3. 4 wt% near the shore and 3. 6% at the center of the study, because it demonstrated accelerated degradation when oceans. The salt concentration of ocean saltwater was not used exposed to moisture. Factors that were changed included the this investigation, because it already had been shown to have ber-matrix interphase, fiber orientation of polymer cross- a very pronounced effect on SiC fibers" and produced failure link chemistry, and filler. The variations of the CMC were during the first few thousand cycles of testing for the Nicalon/ BN-Si3Na interphase, 0/90 lay-up, basic prevtesdvarepPerepreg, tal salt fog exposure were applied alternately to the specimens.The BN-Si3N4 interphase, 0/90 lay-up, optional prepreg, Si3Na filler In performing each experiment, cyclic loading and environmen- 11BN-Si3Na interphase, quasi-isotropic lay-up. Sis Na filler; (iv)BN interphase, 0/90 lay-up, basic prepreg, SiC run-out condition for the fatigue plus moisture experiments filler; and(v)BN interphase, 0/90 lay-up, optional prepreg, SiC filler cluded a total of eight blocks of fatigue cycling at 1000C and The fiber coating changes and prepreg changes are a direct seven 24 h salt fog exposures. Stress levels were selected to be the attempt at identifying those compositions that are more resistant to same as for the standard isothermal fatigue experiments. The moisture-induced degradation. Cloth lay-up, prepreg, and filler measured life at each stress level was divided into separate blocks hanges should have an impact on the structure and porosity of the The increments were 5%. 10%.15%. 20%. 25%.50%.75%. and matrix. The goal is to alter geometry and distribution of the 00%. Each test specimen was first fatigued at 1000C at the elected stress level for 5% of the expected fatigue life. At the en internal oxidation. This has been well documented for carbon/ of the block of fatigue cycles, the specimen was held at zero carbon composites, and, more recently, it has been demonstrated in applied load and rapidly cooled to room temperature. The test specimen was then removed from the fatigue test frame and placed hanges should also influence matrix crack density and crac in the environmental chamber for a 24 h exposure. After 24 h, the distributions within the CMC. It is also speculated that these test specimen was placed in a drying oven under slight vacuum at hanges might affect the residual stress state, thus influencing the 37C for-12 h. This step was required to remove pockets of water stress level at which matrix cracks open up under tensile load that would remain in the large pores of the specimen. Initial trial runs of the test procedure revealed that, if the water remained in the large pores, it produced small localized delaminations during Il. Experiments heating. After the specimen was dried, it was placed back into the fatigue test frame, ramped to temperature in -5-10 min, held for (PL) stress and for selection of appropriate stress levels for the procedure was repeated five times. After the first five blocks, the atigue tests. Tensile and fatigue specimens were 150 mm long cycle increment was then increased to 25% for the remaining three with a dogbone shape and a ection width of 10 mm blocks of fatigue testing. Testing was terminated when either the Tension tests were performed in ory air at room temperature specimen failed or 100% estimated fatigue life was reached d at 1000oc for four CMcs wi N720/A system was tested Several specimens that reached the run-out condition were tension
moisture investigations are used to qualify to what extent the high-temperature fatigue behavior of certain types of CMC is affected by the direct exposure to moisture. II. Materials To thoroughly explore the role of moisture on the fatigue durability of CMCs, five different CMCs were purchased from their manufacturer and evaluated. All the CMCs have been used as an eight-harness-satin-weave (8HSW) fiber architecture, and each of these material systems has been described in detail elsewhere.21,22 Therefore, only a brief description is given here for each CMC. Three of the CMCs contain ceramic-grade Nicalon fibers. The first is Nicalon/Al2O3 from Lanxide Corp. The CMC contains an Al2O3 matrix, and the fibers are coated with a dual layer of BN and then SiC. The BN is there for debonding, whereas the SiC is applied over the BN to protect it during matrix synthesis. A unique process involving metal oxidation is used to create the Al2O3 matrix. The second CMC is Nicalon/SiNC (Nicalon/silicon nitrocarbide) from Dow Corning Corporation, and the fibers are coated with BN and then Si3N4. Multiple polymer infiltration and pyrolysis (PIP) cycles are used to synthesize the amorphous matrix. The third CMC is Nicalon/C from Hitco Technologies. This CMC has no fiber coating, but does have B4C filler added to the matrix to serve as a glass former and a SiC exterior coating to protect the inhibited carbon matrix from oxidation. The matrix is synthesized using PIP followed by chemical vapor infiltration (CVI) of carbon. The fourth and fifth CMCs are oxide/oxide CMCs that use a porous matrix to impart toughness into the CMC. One CMC is a Nextel 610/aluminosilicate (N610/AS) from General Electric, and the other is a Nextel 720/Al2O3 (N720/A) from COI Ceramics. Neither of these all-oxide CMCs has a coating applied to the fibers. There is no evidence of an interphase between the fibers and matrix after processing. The matrix of both CMCs is synthesized using a sol–gel method. An additional aspect of this investigation was to determine what constituents in the CMC contributed to moisture-induced degradation. The Nicalon/SiNC system (8HSW) was selected for further study, because it demonstrated accelerated degradation when exposed to moisture. Factors that were changed included the fiber–matrix interphase, fiber orientation, type of polymer crosslink chemistry, and filler. The variations of the CMC were (i) BN–Si3N4 interphase, 0/90 lay-up, basic prepreg, Si3N4 filler; (ii) BN–Si3N4 interphase, 0/90 lay-up, optional prepreg, Si3N4 filler; (iii) BN–Si3N4 interphase, quasi-isotropic lay-up, basic prepreg, Si3N4 filler; (iv) BN interphase, 0/90 lay-up, basic prepreg, SiC filler; and (v) BN interphase, 0/90 lay-up, optional prepreg, SiC filler. The fiber coating changes and prepreg changes are a direct attempt at identifying those compositions that are more resistant to moisture-induced degradation. Cloth lay-up, prepreg, and filler changes should have an impact on the structure and porosity of the matrix. The goal is to alter geometry and distribution of the porosity. It is known that extensive porosity promotes rapid internal oxidation. This has been well documented for carbon/ carbon composites, and, more recently, it has been demonstrated in combustor tests of SiC/SiC composites.1–3 In addition, such changes should also influence matrix crack density and crack distributions within the CMC. It is also speculated that these changes might affect the residual stress state, thus influencing the stress level at which matrix cracks open up under tensile load. III. Experiments Tension tests were conducted to identify the proportional limit (PL) stress and for selection of appropriate stress levels for the fatigue tests. Tensile and fatigue specimens were 150 mm long with a dogbone shape and a gauge section width of 10 mm. Tension tests were performed in laboratory air at room temperature and at 1000°C for four CMCs, while the N720/A system was tested at 1200°C to investigate the potential of this CMC. At 1200°C, the Al2O3 matrix should show no sign of softening or weakening, and the N720 fiber should be relatively stable for the 27.7 h anticipated for reaching run out. Fatigue testing was conducted in laboratory air using a servohydraulic fatigue machine.21,22 The fatigue cycle used a load ratio of 0.05 and a frequency of 1 Hz. Run out was defined as 100 000 cycles. Results of the standard fatigue experiments were used to generate a fatigue diagram of stress versus cycles to failure for each CMC. These results were also used as the base line for the fatigue plus moisture experiments. An environmental chamber with temperature control, humidity control (not used), and fog-generating capability was used to investigate the influence of water fog and salt fog exposure on high-temperature fatigue performance. Chamber temperature was controlled at 35°C and atmospheric pressure. During operation, compressed air (gauge pressure of 0.1 MPa) was humidified by passing through a bubble tower, after which it was mixed with the desired solution in a nozzle. The nozzle atomized the solution and air to a corrosive fog. The chamber used 0.76 L of solution/h. Operation of the chamber in the fog mode resulted in air that was fully saturated with water (PH2O 0.055 atm) and a thick fog of water particles. Test specimens were suspended near the center of the chamber by racks. During the exposures, the specimens became fully saturated as solution condensed on them, but were never immersed in water. Specimens were arranged with sufficient space between test pieces for adequate fog circulation, and in such a way that did not allow condensation from one specimen to fall on another specimen. During the exposure, the test specimens were in continual contact with water and air. Deionized water was used for the water fog exposures, while deionized water mixed with salt (NaCl) was used for the salt fog exposures. A NaCl concentration of 0.05 wt% was used. This concentration was selected to be 2 orders of magnitude above the concentrations at typical flying altitudes for most aircraft and 2 orders of magnitude below what is normally found in the saltwater oceans. Salt concentration in the water of the earth’s oceans is 3.4 wt% near the shore and 3.6% at the center of the oceans. The salt concentration of ocean saltwater was not used in this investigation, because it already had been shown to have a very pronounced effect on SiC fibers4 and produced failure during the first few thousand cycles of testing for the Nicalon/ SiNC composite. In performing each experiment, cyclic loading and environmental salt fog exposure were applied alternately to the specimens. The run-out condition for the fatigue plus moisture experiments included a total of eight blocks of fatigue cycling at 1000°C and seven 24 h salt fog exposures. Stress levels were selected to be the same as for the standard isothermal fatigue experiments. The measured life at each stress level was divided into separate blocks. The increments were 5%, 10%, 15%, 20%, 25%, 50%, 75%, and 100%. Each test specimen was first fatigued at 1000°C at the selected stress level for 5% of the expected fatigue life. At the end of the block of fatigue cycles, the specimen was held at zero applied load and rapidly cooled to room temperature. The test specimen was then removed from the fatigue test frame and placed in the environmental chamber for a 24 h exposure. After 24 h, the test specimen was placed in a drying oven under slight vacuum at 37°C for 12 h. This step was required to remove pockets of water that would remain in the large pores of the specimen. Initial trial runs of the test procedure revealed that, if the water remained in the large pores, it produced small localized delaminations during heating. After the specimen was dried, it was placed back into the fatigue test frame, ramped to temperature in 5–10 min, held for 5 min at temperature, and fatigue cycling was initiated. This procedure was repeated five times. After the first five blocks, the cycle increment was then increased to 25% for the remaining three blocks of fatigue testing. Testing was terminated when either the specimen failed or 100% estimated fatigue life was reached. Several specimens that reached the run-out condition were tension August 2003 Consequence of Moisture and Salt Fog on High-Temperature Fatigue of Ceramic-Matrix Composites 1283
1284 Journal of the American Ceramic Sociery-Zawada et al. Vol 86. No. 8 tested to measure residual tensile strength. The specimen fracture urfaces were observed using Sem to determine damage and failure modes The effect of interrupting the fatigue test, but without environ- Fiber Form: 8HSW mental fog exposure, was also investigated to determine what effect interrupting the fatigue tests had on fatigue study, first-generation Nicalon/SINC samples with a PL of 75 MPa were fatigued at 100 MPa and 1000oC for blocks of 5000 cycles and then thermally cycled using the same cooling and heating rates as for the fog experiments. After 25 000 cycles, the increment w changed to 25 000 cycle intervals. Results from these interrupted t and thermally cycled tests were compared with identical speci mens that were isothermally fatigued. This gave a one-to-on comparison and documented the effect of the thermal cycles Results (1 Tensile Strain(%) Multiple tension tests(typically three)were performed at room emperature and elevated temperature for the five CMCs. Average Fig. 1. Tensile stress versus strain behavior for Nicalon/Al,o mechanical behavior values from the tension tests are shown in Table I The stress-strain traces for Nicalon/Al,O, are shown in Fig toom-temperature behavior exhibits a well-defined PL. Increasing to failure. It is very difficult to identify a traditional Pl for this the temperature to 1000.C has a distinct effect on the stress versus CMC. Temperature has no effect on the stress-strain response, but rain behavior. At temperature, the PL increases slightly, the there is a slight decrease in the uts strain to failure decreases, and the ultimate tensile strength ( UTs) Behavior of the N720/A system(Fig. 5)is also essentially linear is essentially unchanged. Significant changes in the stress-strain to failure Tests at 1200%C result in a lower modulus value and behavior as a function of temperature make designing components significantly more strain to failure. However, there is little de crease in the uts very difficult and is an early indicator that this CMC may not be a good candidate material system for operation at 1000C Tensile behavior of the Nicalon/SINC system is shown in Fig (2) Fatigue, Fatigue plus Salt Fog, and Fatigue plus 2. This CMC also demonstrates a well-defined PL. Above the Pl, Water Fog the response remains essentially linear up to the UTS. The tensile The results of the fatigue plus salt fog experiment for Nicalon/ AL,O, are shown in Fig. 6. Run out is not achieved during the tress-strain traces decreasing within a tight range. There is no fatigue tests until the stress level decreases to 75 MPa. This stress bservable effect of temperature on fast-fracture behavior The tensile behavior of the Nicalon/C is noticeably different controlling feature for long service life. Fatigue plus salt fog tests om the first two CMCs(Fig. 3). There is a small initial linear at 75 MPa ran out, but at 100 MPa, the life of the test specimen is region up to 55 MPa, followed by a subtle transition to slightl significantly decreased. In addition, the retained strength after run out at 75 MPa decreases to 1 14 MPa for the isothermal fatigue linear behavior out to failure. Picking a PL stress level for this specimen and 69 MPa for the salt fog specimen. These results material is very subjective. Such behavior is likely the result of the during fatigue testing pronounced matrix cracking that occurs during synthesis of the and that severe degradation occurs during the salt fog experiment. matrix. At very low loads, these matrix cracks start to open and to This CMC demonstrates poor durability during fatigue, and th extend, but, with the low-fracture-energy carbon matrix, this is not retained strength values are low. Therefore, no further fatigue plus a sudden or rapid event, as shown for the first two CMCs. The salt fog experiments have been conducted ress-strain traces at both temperatures are essentially identical The UTS of this CMC is 250 MPa, and the uts at 1000.C is slightly higher than at 23C. Overall, temperature has little effect on fast-fracture behavior The behavior of the N610/As system is shown in Fig 4. This Load Rate =0.05 m CMC exhibits stress versus strain behavior that is essentially linear Fiber Form: 8HSW Table 1. Average mechanical Behavior Values from Room- and Elevated-Temperature Tension Tests on Five Different cmcs CMC system UTS (MPa) (GPa) (o) PL (MPa) Nicalon/Al 031000 Nicalon/SiNC Nicalon/SiNC Nicalon/ 1000 670.60 Nextel610/AS 700.36100 0.00.1020.3040.50.6 Nextel6IO/AS Strain(%) Nextel720/A 690.2480 Nextel720/A 0.30 Fig. 2. Tensile stress versus strain behavior for Nicalon/SiNC
tested to measure residual tensile strength. The specimen fracture surfaces were observed using SEM to determine damage and failure modes. The effect of interrupting the fatigue test, but without environmental fog exposure, was also investigated to determine what effect interrupting the fatigue tests had on fatigue life. In this study, first-generation Nicalon/SiNC samples with a PL of 75 MPa were fatigued at 100 MPa and 1000°C for blocks of 5000 cycles and then thermally cycled using the same cooling and heating rates as for the fog experiments. After 25 000 cycles, the increment was changed to 25 000 cycle intervals. Results from these interrupted and thermally cycled tests were compared with identical specimens that were isothermally fatigued. This gave a one-to-one comparison and documented the effect of the thermal cycles on fatigue live. IV. Results (1) Tensile Multiple tension tests (typically three) were performed at room temperature and elevated temperature for the five CMCs. Average mechanical behavior values from the tension tests are shown in Table I. The stress–strain traces for Nicalon/Al2O3 are shown in Fig. 1. Room-temperature behavior exhibits a well-defined PL. Increasing the temperature to 1000°C has a distinct effect on the stress versus strain behavior. At temperature, the PL increases slightly, the strain to failure decreases, and the ultimate tensile strength (UTS) is essentially unchanged. Significant changes in the stress–strain behavior as a function of temperature make designing components very difficult and is an early indicator that this CMC may not be a good candidate material system for operation at 1000°C. Tensile behavior of the Nicalon/SiNC system is shown in Fig. 2. This CMC also demonstrates a well-defined PL. Above the PL, the response remains essentially linear up to the UTS. The tensile behavior at 23° and 1000°C is essentially identical, with all the stress–strain traces decreasing within a tight range. There is no observable effect of temperature on fast-fracture behavior. The tensile behavior of the Nicalon/C is noticeably different from the first two CMCs (Fig. 3). There is a small initial linear region up to 55 MPa, followed by a subtle transition to slightly nonlinear increase in strain with increasing stress, followed by linear behavior out to failure. Picking a PL stress level for this material is very subjective. Such behavior is likely the result of the pronounced matrix cracking that occurs during synthesis of the matrix. At very low loads, these matrix cracks start to open and to extend, but, with the low-fracture-energy carbon matrix, this is not a sudden or rapid event, as shown for the first two CMCS. The stress–strain traces at both temperatures are essentially identical. The UTS of this CMC is 250 MPa, and the UTS at 1000°C is slightly higher than at 23°C. Overall, temperature has little effect on fast-fracture behavior. The behavior of the N610/AS system is shown in Fig. 4. This CMC exhibits stress versus strain behavior that is essentially linear to failure. It is very difficult to identify a traditional PL for this CMC. Temperature has no effect on the stress–strain response, but there is a slight decrease in the UTS. Behavior of the N720/A system (Fig. 5) is also essentially linear to failure. Tests at 1200°C result in a lower modulus value and significantly more strain to failure. However, there is little decrease in the UTS. (2) Fatigue, Fatigue plus Salt Fog, and Fatigue plus Water Fog The results of the fatigue plus salt fog experiment for Nicalon/ Al2O3 are shown in Fig. 6. Run out is not achieved during the fatigue tests until the stress level decreases to 75 MPa. This stress level is below the PL and clearly identifies the PL as the controlling feature for long service life. Fatigue plus salt fog tests at 75 MPa ran out, but at 100 MPa, the life of the test specimen is significantly decreased. In addition, the retained strength after run out at 75 MPa decreases to 114 MPa for the isothermal fatigue specimen and 69 MPa for the salt fog specimen. These results identify that substantial degradation occurs during fatigue testing and that severe degradation occurs during the salt fog experiment. This CMC demonstrates poor durability during fatigue, and the retained strength values are low. Therefore, no further fatigue plus salt fog experiments have been conducted. Fig. 1. Tensile stress versus strain behavior for Nicalon/Al2O3. Fig. 2. Tensile stress versus strain behavior for Nicalon/SiNC. Table I. Average Mechanical Behavior Values from Roomand Elevated-Temperature Tension Tests on Five Different CMCs CMC system Temperature (°C) UTS (MPa) Modulus (GPa) Strain (%) PL (MPa) Nicalon/Al2O3 23 196 187 0.56 60 Nicalon/Al2O3 1000 193 153 0.39 72 Nicalon/SiNC 23 197 107 0.32 85 Nicalon/SiNC 1000 214 101 0.42 75 Nicalon/C 23 221 73 0.47 54 Nicalon/C 1000 254 67 0.60 56 Nextel610/AS 23 205 70 0.36 100 Nextel610/AS 1000 173 77 0.26 83 Nextel720/A 23 144 69 0.24 80 Nextel720/A 1200 140 55 0.30 35 1284 Journal of the American Ceramic Society—Zawada et al. Vol. 86, No. 8
August 2003 Consequence of Moisture and Salt Fog on High-Temperature Fatigue of Ceramic-Matrix Composites 285 250F Load Rate=0.05 mr Loading Rate: 0.05 mm/s Fiber Form: 8HSW Fiber Form: 8HS 200 150 23°c 5 1000c 020 0.00.1 050.6 0.0 010 Strain (%) Fig. 5. Tensile stress versus strain behavior for N720/A Fig. 3. Tensile stress versus strain behavior for Nicalon/C The second material investigated was the Nicalon/SINC, results BN-coated fibers experienced greatly accelerated degradation out during standard fatigue testing is achieved at 100 MPa. The selected for further study using only deionized water. The ol are shown in Fig. 7 and are discussed in detail by Lee et al.- Run Therefore, the Nicalon/SiNC and Nicalon/Al,O, systems w stress-strain traces in Fig 3 show that 100 MPa is above the Pl of tive was to determine if the salt was causing the rapid degradation 75 MPa. Thus, during fatigue testing, this CMC reaches run out or if it was primarily the moisture. Results for the fatigue plus sa even with extensive matrix cracking. However, once fatigue fog are compared with the results of fatigue plus water fog in Table ombined with salt fog, there is a significant decrease in life at 100 Il. Presented in Table ll are the maximum fatigue stress levels and MPa. Fatigue life is decreased by I order of magnitude for each of corresponding decrease in life as a result of the the three stress levels studied and clearly identifies that the salt fog For Nicalon/SINC. the decrease in life as a result of water fo exposures result in an additional aggressive degradation mecha- exposure is the same as for the salt fog exposures. Three specimens nism that dominates the failure process. tested at 125 MPa are all within -1000 cycles of each other, and The fatigue results for Nicalon/C are shown in Run out their lives are extremely short. This clearly identifies that th is achieved at 75 MPa for the isothermal fatigue te 的能 primary culprit is moisture and that any contribution to degrada- g tests, and the 75 MPa stress level is actually tion from the small concentration of salt is overwhelmed by the ress of 55 MPa. The salt fog experiments primary degradation caused by moisture. However, once the stress life of 30% over the standard fatigue experiments level is decreased to below the PL, the two specimens tested at 75 The last system salt fog tested was the N610/AS ite,and MPa run out, thus highlighting the contribution of matrix cracking the fatigue results are shown in Fig. 9. This CMC exhibits to accelerated degradation excellent fatigue resistance at 1000C, with run out very near the Decreased life with water fog wa UTS of the CMC. The introduction of salt fog has no effect on the alonAl O3 system. Two specimens stress of fatigue life or retained strength values. In fact, the tensile strength 75 MPa failed before reaching run out. failed at after fatigue testing is typically 5%10% higher than the as- a stress level where specimens exposed to salt fog ran out. Such processed value findings indicate that there is some variability in the Pl value and The three CMCs with Nicalon fibers experienced decreased lives with the introduction of salt fog, and the two Cmcs with Tension Test T=1000°c 10o0c 0.05 mm/s Fiber 8HSW 0.00.1 Cycles(N Strain(%) Fig. 6. Fatigue diagram of stress versus cycles to failure for Nicalon/ Al,, Specimens were subjected to fatigue and interrupted fatigue plus Fig 4. Tensile stress versus strain behavior for N610/AS
The second material investigated was the Nicalon/SiNC; results are shown in Fig. 7 and are discussed in detail by Lee et al.21 Run out during standard fatigue testing is achieved at 100 MPa. The stress–strain traces in Fig. 3 show that 100 MPa is above the PL of 75 MPa. Thus, during fatigue testing, this CMC reaches run out even with extensive matrix cracking. However, once fatigue is combined with salt fog, there is a significant decrease in life at 100 MPa. Fatigue life is decreased by 1 order of magnitude for each of the three stress levels studied and clearly identifies that the salt fog exposures result in an additional aggressive degradation mechanism that dominates the failure process. The fatigue results for Nicalon/C are shown in Fig. 8. Run out is achieved at 75 MPa for the isothermal fatigue tests and the salt fog tests, and the 75 MPa stress level is actually above the PL stress of 55 MPa. The salt fog experiments result in a decrease in life of 30% over the standard fatigue experiments. The last system salt fog tested was the N610/AS composite, and the fatigue results are shown in Fig. 9. This CMC exhibits excellent fatigue resistance at 1000°C, with run out very near the UTS of the CMC. The introduction of salt fog has no effect on the fatigue life or retained strength values. In fact, the tensile strength after fatigue testing is typically 5%–10% higher than the asprocessed value. The three CMCs with Nicalon fibers experienced decreased lives with the introduction of salt fog, and the two CMCs with BN-coated fibers experienced greatly accelerated degradation. Therefore, the Nicalon/SiNC and Nicalon/Al2O3 systems were selected for further study using only deionized water. The objective was to determine if the salt was causing the rapid degradation, or if it was primarily the moisture. Results for the fatigue plus salt fog are compared with the results of fatigue plus water fog in Table II. Presented in Table II are the maximum fatigue stress levels and corresponding decrease in life as a result of the exposures. For Nicalon/SiNC, the decrease in life as a result of water fog exposure is the same as for the salt fog exposures. Three specimens tested at 125 MPa are all within 1000 cycles of each other, and their lives are extremely short. This clearly identifies that the primary culprit is moisture and that any contribution to degradation from the small concentration of salt is overwhelmed by the primary degradation caused by moisture. However, once the stress level is decreased to below the PL, the two specimens tested at 75 MPa run out, thus highlighting the contribution of matrix cracking to accelerated degradation. Decreased life with water fog was also observed for the Nicalon/Al2O3 system. Two specimens tested at a fatigue stress of 75 MPa failed before reaching run out. These specimens failed at a stress level where specimens exposed to salt fog ran out. Such findings indicate that there is some variability in the PL value and, Fig. 3. Tensile stress versus strain behavior for Nicalon/C. Fig. 4. Tensile stress versus strain behavior for N610/AS. Fig. 5. Tensile stress versus strain behavior for N720/A. Fig. 6. Fatigue diagram of stress versus cycles to failure for Nicalon/ Al2O3. Specimens were subjected to fatigue and interrupted fatigue plus salt fog exposure. August 2003 Consequence of Moisture and Salt Fog on High-Temperature Fatigue of Ceramic-Matrix Composites 1285
1286 Journal of the American Ceramic Sociery-Zawada et al. Vol 86. No. 8 T=1000°c T=1000°c 0o0°c atique 150 口 Fatigue ▲ Fatigue+ Salt Fog ue t Salt Fo 10010110210310410510° Cycles(N) ycles(N) 7. Fatigue diagram of stress versus cycles to failure for Nicalon/ diagram of stress cles to failure for n610/A SINC. Specimens were subjected to fatigue and interrupted fatigue plus salt subjected to fatigue and interrupted fatigue plus salt fog ubsequently, in fatigue resistance of this CMC. However, it does Al,O3 CMC has excellent strength retention after air exposure at document that H,O is responsible for the accelerated de 1200° for 1000h The n720/A nts were performed only using deionized It was speculated that, if changes were made to the constituents water, because the N61O/As system showed no influence of the in the Nicalon/SiNC CMC, there might be a change in the moisture alt. This CMC was tested at 1200C, and the results are shown in sensitivity. Results from these experiments are shown in Table Ill Fig. 10. These results have been discussed in more detail else- None of these chang pacted moisture resistance. Neither the where by Steel et al. 2 but are also included here for completeness fiber coating nor the alternate prepreg material improved resis In summary, the behavior mirrored that observed for the N61oAs tance to moisture. It was hoped that the changes would produce a composite, in that the fatigue limit was near the UTS and that the more uniform microstructure, with a change in the morphology of specimens either failed at low cycle counts or ran out. The run-out the porosity. It has been observed that porosity contributes to the stress levels were repeated with the fatigue plus water fo experiments, and all these ran out as well. Thus, this CMC system mlos adation of the material by providing paths for oxygen and ure to penetrate into the interior of the test specimen experienced no degradation from the fatigue process or the fatigue However, optical observations of the cut edges of test specimens plus water fog exposure. Residual strength tests showed the fatigue revealed that the geometry and shape of the porosity was not specimens to be stronger than the as-received specimens. Clearly, altered significantly. These observations suggested that significant this was exceptional fatigue behavior, especially for a test tempe mprovements in processing of this CMC may some day result in ature of 1200C. Excellent retained strength was not unexpecte decreased sensitivity to moisture because Campbell et al. have shown that this CMC looses only Results of the fatigue and interrupted fatigue experiments are for 1000 h. Carelli et al. 2 have also shown that a N720/mullite- number of times the interrupted experiments were cooled 0o and -7% of its load-carrying capability after air exposure at 1200C shown in Table IV. Table Iv shows the cycles to failure temperature and then heated back to test temperature. The rupted fatigue specimens demonstrated a decrease in life of 72406 ue exper 250 °c T=1000°c V. Discussion re fatigue life of CMCs is generally associated with the onset of no gation, run out of 100 000 cycles is observed to be at stress levels equal to or slightly above the Pl. However, exposure to water fog lowers the run-out stress level to below the PL. Even though the Nicalon/SiNc and Nicalon/c systems have extensive matrix cracks in 5% ufficiently sealed, at least at the surfaces, so that oxidation species do not have a path to penetrate into the Cmc. For Nicalon/SINC the matrix is in residual compression, and this should also aid in keeping cracks closed. All three Nicalon-fiber CMCs are appa ently dense enough to limit any substantial oxygen diffusion into the CMC as long as the existing matrix cracks remain closed. As the existing cracks open, they provide an easy path for oxygen Cycles(n) transport throughout the matrix, promoting rapid oxidation, Kallur et al. studied the high-temperature fatigue behavior of Nicalon/ Fig. 8. Fatigue diagram of stress versus cycles to failure for Nicalon/C. SiNC using humidity exposures as well as salt fog, similar to this pecimens were subjected to fatigue and interrupted fatigue plus salt fo investigation. The test specimens were unique in that they con- tained many machined holes with a nominal diameter of 1. 8 mm
subsequently, in fatigue resistance of this CMC. However, it does document that H2O is responsible for the accelerated degradation. The N720/A experiments were performed only using deionized water, because the N610/AS system showed no influence of the salt. This CMC was tested at 1200°C, and the results are shown in Fig. 10. These results have been discussed in more detail elsewhere by Steel et al.22 but are also included here for completeness. In summary, the behavior mirrored that observed for the N610/AS composite, in that the fatigue limit was near the UTS and that the specimens either failed at low cycle counts or ran out. The run-out stress levels were repeated with the fatigue plus water fog experiments, and all these ran out as well. Thus, this CMC system experienced no degradation from the fatigue process or the fatigue plus water fog exposure. Residual strength tests showed the fatigue specimens to be stronger than the as-received specimens. Clearly, this was exceptional fatigue behavior, especially for a test temperature of 1200°C. Excellent retained strength was not unexpected, because Campbell et al.23 have shown that this CMC looses only 7% of its load-carrying capability after air exposure at 1200°C for 1000 h. Carelli et al.24 have also shown that a N720/mullite– Al2O3 CMC has excellent strength retention after air exposure at 1200°C for 1000 h. It was speculated that, if changes were made to the constituents in the Nicalon/SiNC CMC, there might be a change in the moisture sensitivity. Results from these experiments are shown in Table III. None of these changes impacted moisture resistance. Neither the fiber coating nor the alternate prepreg material improved resistance to moisture. It was hoped that the changes would produce a more uniform microstructure, with a change in the morphology of the porosity. It has been observed that porosity contributes to the degradation of the material by providing paths for oxygen and moisture to penetrate into the interior of the test specimen. However, optical observations of the cut edges of test specimens revealed that the geometry and shape of the porosity was not altered significantly. These observations suggested that significant improvements in processing of this CMC may some day result in decreased sensitivity to moisture. Results of the fatigue and interrupted fatigue experiments are shown in Table IV. Table IV shows the cycles to failure and number of times the interrupted experiments were cooled to room temperature and then heated back to test temperature. The interrupted fatigue specimens demonstrated a decrease in life of 24% as compared with the isothermal fatigue experiments. V. Discussion High-temperature fatigue life of CMCs is generally associated with the onset of nonlinear stress–strain behavior. In this investigation, run out of 100 000 cycles is observed to be at stress levels equal to or slightly above the PL. However, exposure to water fog lowers the run-out stress level to below the PL. Even though the Nicalon/SiNC and Nicalon/C systems have extensive matrix cracks in them from processing and 5% porosity, they are sufficiently sealed, at least at the surfaces, so that oxidation species do not have a path to penetrate into the CMC. For Nicalon/SiNC, the matrix is in residual compression, and this should also aid in keeping cracks closed. All three Nicalon-fiber CMCs are apparently dense enough to limit any substantial oxygen diffusion into the CMC as long as the existing matrix cracks remain closed. As the existing cracks open, they provide an easy path for oxygen transport throughout the matrix, promoting rapid oxidation. Kalluri et al.25 studied the high-temperature fatigue behavior of Nicalon/ SiNC using humidity exposures as well as salt fog, similar to this investigation. The test specimens were unique in that they contained many machined holes with a nominal diameter of 1.8 mm. Fig. 7. Fatigue diagram of stress versus cycles to failure for Nicalon/ SiNC. Specimens were subjected to fatigue and interrupted fatigue plus salt fog exposure. Fig. 8. Fatigue diagram of stress versus cycles to failure for Nicalon/C. Specimens were subjected to fatigue and interrupted fatigue plus salt fog exposure. Fig. 9. Fatigue diagram of stress versus cycles to failure for N610/AS. Specimens were subjected to fatigue and interrupted fatigue plus salt fog exposure. 1286 Journal of the American Ceramic Society—Zawada et al. Vol. 86, No. 8