Abstract
The effect of thermal shock and oxidation-induced damage on the thermal diffusivity of unidirectional Nicalon-LAS glass-ceramic composites is presented in this study. The data presented show that thermal diffusivity measurements provide a sensitive nondestructive method whereby damage progression may be assessed. Samples were exposed to isothermal oxidation and thermal shock environments. In addition, combined cycles of oxidation and thermal shock were also evaluated. The thermal diffusivity transverse to the fibers was measured to detect changes in material integrity. Significant decreases up to 23% were observed in the thermal diffusivity of the material.
| Original language | English |
|---|---|
| Pages (from-to) | 73-87 |
| Number of pages | 15 |
| Journal | Journal of Composite Materials |
| Volume | 37 |
| Issue number | 1 |
| DOIs | |
| State | Published - 2003 |
Funding
Graham Samuel Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0405, USA; Sandia National Laboratories McDowell David L. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0405, USA Lara-Curzio Edgar Dinwiddie Ralph B. Wang Hsin Porter Wallace Oak Ridge National Laboratory, Oak Ridge, TN 37831-6064, USA 01 2003 37 1 73 87 The effect of thermal shock and oxidation-induced damage on the thermal diffusivity of unidirectional Nicalon-LAS glass–ceramic composites is presented in this study. The data presented show that thermal diffusivity measurements provide a sensitive nondestructive method whereby damage progression may be assessed. Samples were exposed to isothermal oxidation and thermal shock environments. In addition, combined cycles of oxidation and thermal shock were also evaluated. The thermal diffusivity transverse to the fibers was measured to detect changes in material integrity. Significant decreases up to 23% were observed in the thermal diffusivity of the material. sagemeta-type Journal Article search-text Nondestructive Characterization of Thermal Shock and Oxidation-Induced Damage by Flash Diffusivity SAMUEL GRAHAM* AND DAVID L. MCDOWELL Woodruff School of Mechanical Engineering Georgia Institute of Technology Atlanta, Georgia 30332-0405, USA EDGAR LARA-CURZIO, RALPH B. DINWIDDIE, HSIN WANG AND WALLACE PORTER Oak Ridge National Laboratory Oak Ridge, TN 37831-6064, USA ABSTRACT: The effect of thermal shock and oxidation-induced damage on the thermal diffusivity of unidirectional Nicalon-LAS glassceramic composites is presented in this study. The data presented show that thermal diffusivity measurements provide a sensitive nondestructive method whereby damage progres- sion may be assessed. Samples were exposed to isothermal oxidation and thermal shock environments. In addition, combined cycles of oxidation and thermal shock were also evaluated. The thermal diffusivity transverse to the fibers was measured to detect changes in material integrity. Significant decreases up to 23% were observed in the thermal diffusivity of the material. INTRODUCTION THE DEVELOPMENT OF toughness in fiber reinforced ceramicmatrix composites (CMCs) has primarily relied upon the existence of a weak fibermatrix interface. Weak interfaces have a propensity to debond in the presence of crack propagation which allows crack bridging and a reduction in crack tip driving forces. This behavior reduces flaw sensitivity and allows for stable, multiple site damage evolution prior to failure. With the use of silicon carbide fibers, this weak interface is often facilitated by a compliant layer of carbon or boron nitride as a result of CVI processing or an in situ reaction [1,2]. These interface materials are the catalyst in producing toughening and energy dissipative *Author to whom correspondence should be addressed. Currently with Sandia National Laboratories. Journal of COMPOSITE MATERIALS, Vol. 37, No. 1/200373 0021-9983/03/01 007315 $10.00/0DOI: 10.1106/002199803028994 2003 Sage Publications deformation mechanisms in relatively low temperature environments. However, under prolonged high temperature exposure, these interfacial materials are susceptible to volatilization through oxidation reactions. The changes in the integrity of the interface adversely affects the performance, properties, and expected service life of these materials [3,4]. Reactions which may degrade the interfacial structure are enhanced by long term high temperature exposure combined with matrix microcracking and fibermatrix debonding. The intended service environments for both structural and nonstructural ceramic composites will undoubtedly involve high temperature oxidizing environments with rapidly varying thermal transients (thermal shock) [5]. Therefore, the avoidance of oxidation and thermal shock environments may be difficult and at times, unrealistic in applications involving silicon carbide reinforced ceramics. Oxidation and thermal shock processes will induce damage in ceramic composites by destroying the compliant fiber matrix interfacial layer, and generating matrix microcracks and thermally induced debond cracks. These physical changes in the material microstructure will degrade the load and heat carrying capability of the material and may ultimately lead to failure. Thus, it is imperative to study the behavior of CMCs under oxidation and thermal shock loading, and to develop methods which may be used to assess material integrity. The effect of oxidation and thermal shock environments on CMCs have been characterized through several studies [3,4,6,7]. Oxidation of silicon carbide reinforced composites often involves a rapid decrease in mass from pipeline oxidation of the fiber matrix interface. Continued oxidation exposure results in a slow, steady weight gain from the formation of SiO2, which may eventually seal the interfaces and induce embrittlement. In thermal shock studies, composite specimens are held at a predetermined temperature for a fixed period of time and then quenched in a low temperature medium to induce temperature gradients in the sample. The temperature gradients have the potential to induce matrix microcracking, fibermatrix debonding, and debond cracks. In addition to degrading the elastic and thermophysical properties of the material, these damage entities provide additional paths for oxidation to reach the interior of the composite. In both oxidation and thermal shock studies, the performance of CMCs has often been characterized through destructive methods. These characterization methods have included fiber push-out tests, residual strength measurements, stress-rupture test, thermogravi- metric analysis (TGA), and various microscopy techniques [2,8,9]. Residual strength and creep-rupture tests provide necessary data concerning changes in material performance while under stress. However, they are relatively insensitive to early stages of damage development. Fiber push-out tests and electron microscopy are destructive methods which cannot be used to directly assess material performance. The data obtained from these tests have been key in the analysis of CMCs by revealing the microstructural mechanisms which influence macroscale material responses. However, destructive analysis methods may prove to be costly or less desirable when expensive or unique engineering components are involved. Very few in situ or periodic nondestructive monitoring methods for assessing oxidation and thermal shock damage have been explored. Studies have shown the sensitivity of ultrasonic characterization of elastic moduli changes in the analysis of thermally shocked and oxidized ceramic composites [1012]. In addition, thermal diffusivity measurements have been performed on silicon nitride and alumina composites which have been exposed to single cases of thermal shock and isothermal oxidation [13,14]. Thermal diffusivity measurements offer a relatively simple test method which may be easily implemented into many periodic inspection routines. This method may be used to quickly measure the 74S. GRAHAMET AL. thermal diffusivity both through the thickness of composite samples as well as in the plane of components. Such tests may complement other inspection methods as part of a qualitative assessment of the integrity of a composite component. By measuring changes of thermal diffusivity of components, it is possible to obtain sensitive variations in properties due to oxidation and thermal shock induced microstructural damage. However, to increase the viability of using thermal diffusivity monitoring, studies must be performed which establish links between changes in thermal diffusivity to the microstructural damage entities. The intent of this research is to investigate the use of thermal diffusivity measurements to assess damage development in CMCs subjected to complex oxidationthermal shock histories. In this program, the effect of oxidation and thermal shock damage on unidirectional Nicalon-reinforced Lithium Alumino Silicate (LAS-II) glassceramic composites were studied. This material was used as a model glassceramic composite system which exhibits a complex behavior based on thermal exposure. Material samples were subjected to varying degrees of oxidation prior to thermal shock exposure to study the coupling between oxidation and thermal shock damage. In addition, multiple cycles of oxidation and thermal shock were performed in order to study the progression of damage subsequent to an initial thermal shock exposure. Thermal diffusivity measurements were taken after each step of the process to assess both the early stages and progressive nature of the damage development. Subsequent destructive methods and TGA were also employed to verify conclusions drawn from the thermal diffusivity data. EXPERIMENTAL PROCEDURE The Nicalon-LAS II glassceramic composites used in this study were fabricated at the United Technologies Research Center by standard hot pressing methods. The material contained low oxygen content ceramic grade Nicalon fibers on the order of 15 mm in diameter at a volume fraction of 36%. The matrix composition was very similar to Corning 9608 glassceramic with TiO2 being replaced by ZrO2, and the addition of Nb2O5 for reaction barrier formation. After fabrication, the plates were subsequently devitrified by a post heat treatment at 920 C for 24 h. Samples were then cut using a low speed diamond saw into rectangular platelets which were 6.35 mm long (fiber direction), 12.7 mm wide, and 2.4 mm thick. The platelets were subsequently polished on one face parallel and transverse to the fiber axis to observe changes in microstructure morphology by microscopy. These polished samples were then used in all oxidation and thermal shock experiments presented in following sections. The thermal diffusivity of the composites were measured by the transient heating technique referred to as flash diffusivity [15]. Room temperature thermal diffusivity measurements were performed with heat loss correction by the method of Clark and Taylor [16] using the Kioski parameter estimation method [17]. Samples were mounted and irradiated on the plane parallel to the fiber direction by a 4800 W/s Xenon flash lamp with a flash pulse duration of 6 ms. The rear surface temperature response was monitored by a liquid Nitrogen cooled infrared detector. Typical scan times were of the order of 4 s. These measurements provided the thermal diffusivity through the thickness of the specimen, transverse to the fiber direction. Changes of the thermal diffusivity in this transverse direction are the most sensitive to microstructural changes associated with the damage induced by oxidation and thermal shock processes. The thermal diffusivity Nondestructive Characterization by Flash Diffusivity75 data points presented in this paper are an average of at least 10 measurements with a standard deviation on the order of 1%. Oxidation Experiments In order to characterize damage induced by oxidation and thermal shock interactions using flash diffusivity, samples of Nicalon-LAS II were first analyzed under oxidizing environments. Based on the application temperature of this material system [18], samples were oxidized in air at temperatures of 500 and 900 C for periods up to 48 h in high temperature ovens. Oxidation at 500 C involved only the removal of the carbon interfacial coating whereas the oxidation at 900 C included oxidation of the Nicalon fiber. Samples were periodically removed from the ovens, allowed to thermally equilibrate to room temperature, and their thermal diffusivity measured. The thermal diffusivity transverse to the fiber direction was monitored since oxidation is expected to only degrade the thermal conductance of the fibermatrix interface. The increase in interfacial thermal resistance will be manifested by a pronounced degradation of thermal diffusivity transverse to the fiber axis. The polished surfaces of the samples were also observed for any apparent changes using optical microscopy. Interior sections of the samples were cut, polished, and examined using an Hitachi S4100 scanning electron microscope. To provide further insight into the oxidation damage and to supplement the thermal diffusivity data, TGA was performed. Samples were ultrasonically cleaned in acetone and dried at 100 C in an oven prior to testing. The TGA samples were oxidized in flowing dry air at a rate of 50 mL/min and temperatures of 500 and 900 C up to 48 h. This was done in order to simulate the oxidation experiments and gather information on weight loss due to interfacial pipeline oxidation. Thermal Shock Experiments In order to investigate the complex interaction between oxidation and thermal shock damage, cyclic thermal shock tests were employed. By varying the thermal history prior to a thermal shock event, these tests provide a means to combine the effects of oxidation and thermal stress-induced damage to analyze material performance. By performing these tests in a cyclic manner, the progression of damage throughout a thermal history containing a repetition of events may be observed. The data gained from these tests allows for the evaluation of material performance when reexposed to oxidizing environments after an initial thermal shock event. The effects of damage induced in the composite microstructure was again evaluated by changes observed in the thermal diffusivity transverse to the fiber axis. For the thermal shock tests, samples were mounted in a porous alumina holder and heated on one side with an impinging flame held at a fixed distance. The sample temperature was measured using two thermocouples mounted on the top and bottom. Samples were heated to a predetermined temperature based on the top thermocouple reading and held for one minute. Typical temperature fluctuations were of the order of 20 C. Prior to thermal shock, the top thermocouple was removed and the sample was quenched using high velocity laboratory air. The thermal transient time was observed to be less than one second, although the exact temperaturetime history was not recorded. 76S. GRAHAMET AL. Heating the sample on one surface induced temperature gradients through the thickness direction and is more typical of conditions that nonstructural CMCs may experience in turbine engine environments [5]. Oxidation effects were included in these tests by oxidizing several samples for a period of 1 and 24 h prior to thermal shock. The isothermal oxidation was performed in laboratory ovens in air at 500 and 900 C. Samples were removed, allowed to thermally equilibrate to room temperature, then thermally shocked as previously described. Cyclic thermal shock tests were performed by reintroducing some of the samples to an oxidizing environment for 1 h and repeating the thermal shock. This procedure was performed for a total of five cycles. Thermal diffusivity changes were measured after the first and fifth cycles and then compared in order to discern any progressive changes in the microstructure. RESULTS Oxidation Prior to oxidation and thermal shock experiments, measurements of the thermal diffusivity transverse to the fiber axis resulted in a value of 0.0087 cm2/s. This value is consistent with other reported values for transverse thermal diffusivity for Nicalon-LAS II [19]. The effect of oxidation on the thermal diffusivity of Nicalon-LAS II is shown in Figure 1. In Figure 1, each data point shown is an average of ten thermal diffusivity measurements with a standard deviation typically on the order of 1%. The data show that Figure 1. Change in thermal diffusivity of Nicalon-LAS II transverse to the fibers during oxidation at 500 and 900 C. Nondestructive Characterization by Flash Diffusivity77 the thermal diffusivity decreases much more severely after exposure to oxidation at 500 C when compared to oxidation at 900 C. The decrease in thermal diffusivity after exposure to 900 C was observed to range between 23% and is within the normal statistical scatter of the measurement method. This small decrease is seen to stabilize within the first hour of exposure. When oxidized at 500 C, the thermal diffusivity decreases rapidly and finally stabilizes after approximately a 23% decrease in value. The response of Nicalon-LAS II to oxidation as shown in Figure 1, is interesting and unique to glassceramic matrix composites reinforced with Nicalon fibers. The degradation of the thermal diffusivity at 500 C is based upon the fact that the carbon interface is slowly oxidized away which increases the thermal resistance of the fibermatrix interface. The ineffectiveness of the 900 C oxidation exposure in degrading the thermal diffusivity is based on the oxidation kinetics of this glassceramic composite. It has been shown in several studies that glassceramic composites, reinforced with Nicalon fibers, have the ability to rapidly form a silica glass layer. This protective layer seals off the interior of the composite to further degradation by oxidation. This silica layer forms as a result of the following reactions [20]: C O2 ! CO2 SiC 2O2 ! SiO2 CO2 SiC 3 2O2 ! SiO2 2CO The oxidation of Nicalon fibers processed in glassceramic matrices occurs at a much higher rate than that observed for as-received Nicalon fibers [21]. The increased kinetics maybe facilitated by the migration of the metallic cations (Li, Al, Mg) into the fiber [1,21]. These elements are found in the carbon rich region near the edge of the fiber. This layer may act to form an oxide bridge during the oxidation of carbon and help to quickly seal the interfacial gap left behind by the volatilized carbon (Figure 2). The absence of the formation of the silica plugs has been observed in composites where little or no metal cations have migrated into the Nicalon fiber as in Nicalon-LAS III composites [4]. The presence of the oxide layer is displayed in the optical micrographs of the polished sample surfaces in Figure 3. After 24 h of oxidation, the fiber and matrix regions are clearly distinguishable in the sample exposed to 500 C. The surface of the sample exposed to 900 C has developed a thick SiO2 glaze which makes the fiber and matrix regions much harder to distinguish. The development of this oxide layer has been reported for LAS composites subjected to high temperature oxidizing environments for as little as 15 min [7]. To further understand the data shown in Figure 1, a TGA was performed on the samples. The results of these tests are shown in Figure 4. The comparison of data in Figures 1 and 4 is strikingly similar. It is clear that the sample exposed to 900 C environment quickly loses weight which stabilizes within the first hour of exposure much like the thermal diffusivity data. During this period, the rate of carbon loss at the fiber matrix interface is quickly overcome by the formation of SiO2 on the fiber surfaces. The growth of the SiO2 prevents further oxidation of the carbon interface which is then manifested as a slow, long term weight gain. In the sample oxidized at 500 C, the weight loss continues in a rapid manner over approximately a 10 h period. The weight loss finally stabilizes and remains constant after approximately 15 h of exposure. The fact that no weight gain is seen in this sample suggests that at 500 C, the temperature is too low for any significant formation of SiO2. This allows a complete oxidation of the carbon interface of 78S. GRAHAMET AL. Figure 3. Surface of Nicalon-LAS II samples after a 24h exposure to oxidation at: (A) 500 C; (B) 900 C. Figure 2. Progression of interfacial oxidation in Nicalon reinforced glassceramics: (A) as received; (B) pipeline carbon oxidation; (C) SiO2, seal coat forming on Nicalon; (D) SiO2 seal covering Nicalon fiber and interfacial gap preventing/slowing interfacial oxidation. Nondestructive Characterization by Flash Diffusivity79 the sample which is manifested by a 23% decrease in the thermal diffusivity and 0.5% weight loss. The insignificant loss of mass and thermal diffusivity of the 900 C sample suggests that the integrity of the carbon interface has remained intact within the interior of the composite. Therefore, little or no change in sample properties and performance is expected. Such behavior has been seen in several glassceramic composite systems reported in the literature [3,20,22]. Thermal Shock The effect of thermal shock on the thermal diffusivity of Nicalon-LAS II is shown in Figure 5. In the data shown, oxidation of the samples is considered negligible since the samples were only held at the high temperature for 1 min. In Figure 5, the thermal diffusivity prior to and after thermal shock is displayed. From these data, it is evident that thermal shock of the samples at temperatures greater than 900 C causes a significant decrease in transverse thermal diffusivity. Upon inspection of the cross section of the samples, thermal debond cracks were seen which propagated across the specimen near the heated and quenched surface (Figure 6). The lower portion of the specimen, as shown in Figure 6, contains no visible cracks. The micrograph shown in Figure 6 is from a sample quenched from 1000 C. The cracking near quenched surface is facilitated by through thickness temperature gradients and was seen on several cross sections which were sectioned from the sample. Similar results have been predicted for homogenous anisotropic materials with steady-state through thickness temperature gradients [23]. Figure 4. TGA analysis of weight change of Nicalon-LAS II samples oxidized at 500 and 900 C. 80S. GRAHAMET AL. In this case, the highest level of energy release rate for crack propagation is predicted at a depth of 20% of the thickness below the hot surface. The wide crack opening displacement in Figure 6 is attributed to the large tensile radial stresses which are induced due to thermal expansion mismatch in the constituents. In Nicalon-LAS II, the fiber has a much larger coefficient of thermal expansion than the matrix which; the values are 4.0106C1and 1.0106C1, respectively. During the quenching process of thermal shock, this mismatch induces tensile radial stresses across the fibermatrix interface which facilitates debonding and the wide crack opening displacements. These cracks tend to run through fiber rich regions where the tensile residual stresses are the highest [24]. In these regions, the interfacial debonds may easily link together to form a macroscopic crack. Crack opening displacements on the order of a fiber diameter have also been observed in Nicalon-LAS composites which have compressive radial stresses for this same quench temperature [6]. To ensure this cracking was not induced by the low speed diamond saw, approximately 500 mm was removed from the sample by diamond grinding. Analysis of the polished surfaces still revealed the presence of the debond cracks. It is evident from these data that the thermal expansion mismatch and the strength of the interface will govern the development of thermal debond cracks during thermal shock. The presence of such cracks were not observed at temperatures of 500 C as the thermal diffusivity was unaffected by thermal shock at this temperature. It is also interesting to note that previous studies on this material system have shown that Nicalon-LAS is very resistant to thermal shock [13]. However, in these studies, residual strength from flexure testing and the observation of transverse matrix cracks were used to evaluate degradation in material performance. Due to compressive residual stresses in the matrix in the longitudinal (fiber) direction, the presence of transverse matrix cracks are difficult to observe without placing the sample under tensile loading. Also, thermal shock-induced damage is only observed near the surface of the sample and does Figure 5. The change in thermal diffusivity of Nicalon-LAS II after a single thermal shock exposure with no prior oxidation. Nondestructive Characterization by Flash Diffusivity81 not propagate far into the bulk of the composite [6]. This is expected since the low thermal diffusivity of the material promotes severe temperature gradients near the surface during thermal shock. A significant accumulation of distributed damage is necessary to cause a substantial change in the macroscopic stressstrain behavior and strength of the composite. Therefore, these test methods are not as sensitive to the type of damage development seen in glassceramic composites as methods based on measuring thermal or elastic properties. It has been shown in several studies that changes in elastic and thermal properties were observed well before strength degradation following thermal shock [12,13]. Residual strength tests were not performed in this study due to the sample geometry, but would be valuable in comparing the sensitivity of the methods to microstructural damage development. Based on those previous studies, relatively small changes in residual strength are also expected in this material system. The interaction between oxidation and thermal shock damage is shown in Figures 7 and 8. These samples were held at 500 and 900 C for 1 and 24 h prior to thermal shock, respectively. Figure 6. Optical micrograph sample cross section after thermal shock from 1000 C. Debond cracks (A) were observed near the quenched surface and ran across a majority of the width of the specimen. This extensive damage was not seen further away from the quenched surface as shown (B). Figure (C) displays the typical type of debond cracking which was observed in specimens thermally shocked at 900 C and above. 82S. GRAHAMET AL. The thermal diffusivity of the samples was measured at room temperature before oxidation, after oxidation, and after thermal shock. This was done in order to determine the contribution of each process to the overall degradation of thermal diffusivity. The effect of oxidation prior to the thermal shock was similar to the data shown in Figure 1. Figure 7. Change in thermal diffusivity of Nicalon-LAS II after 1 h of oxidation followed by thermal shock. Data show changes in thermal diffusivity for both single and cyclic oxidation/thermal shock loading. Figure 8. Change in thermal diffusivity of Nicalon-LAS II after 24h of oxidation followed by thermal shock. Data show changes in thermal diffusivity for both single and cyclic oxidation/thermal shock loading. Nondestructive Characterization by Flash Diffusivity83 This includes 5 and 20% decrease in thermal diffusivity due to oxidation of the sample exposed to the 500 C environment for 1 and 24 h, respectively. Subsequent thermal shock induced no further changes in thermal diffusivity even in the case of the completely oxidized sample. The sample exposed to oxidation at 900 C showed a negligible change in thermal diffusivity due to the formation of a protective silica layer. Subsequent thermal shock, however, induced a significant change in thermal diffusivity, on the order of 710%. Similar to the data in Figure 5, subsurface thermal cracking is the main feature which causes the decrease in thermal diffusivity. The oxidation at 900 C prior to thermal shock does very little to change the dynamics of crack formation during thermal shock, because the interior of the composite is protected from any oxidation effects. Figures 7 and 8 also depict the results from the cyclic thermal shock experiments. After the initial oxidation and thermal shock exposure, these samples were reexposed to 4 additional cycles of oxidation for 1 h followed by thermal shock. After the initial 1 h oxidation, the sample cyclically shocked at 500 C shows a larger decrease in thermal diffusivity than the sample exposed to the single thermal shock cycle. The decrease in thermal diffusivity here is comparable to the oxidation of Nicalon-LAS II exposed to 500 C for 5 h shown in Figure 1. Thus, damage which is occurring under cyclic thermal shock at 500 C is most likely associated with continued oxidation of the sample. In Figure 8, the sample initially exposed to oxidation at 500 C for 24 h shows no further decrease in thermal diffusivity after cyclic thermal shock. Therefore, no progressive changes in the microstructure of Nicalon-LAS II is expected even with multiple thermal shock exposures. The samples initially exposed to oxidation of 1 and 24 h at 900 C produced similar results under cyclic thermal shock. The similarity in the behavior is due to the fact that the 1 and 24 h oxidation exposure only affects a small region near the surface of the material and leaves the bulk of the composite intact. A further decrease in thermal diffusivity of the samples cyclically shocked at 900 C is clearly seen in both Figures 7 and 8. Unlike the sample cyclically shocked at 500 C, this decrease is not simply explained by continued oxidation of the sample. Thermal shock acts to compromise the protective SiO2 coating and allows limited oxidation upon reexposure to a 900 C oxidation environment. This barrier eventually reseals itself by forming on any newly exposed material surfaces. In addition, it is highly probable that the thermal shock- induced damage propagates during additional thermal shock exposures. This process continues during each thermal shock cycle and slowly degrades the thermal diffusivity as long as the thermal shock-induced damage continues to propagate. From the results presented in Figures 58, thermal shock causes a further degradation in thermal diffusivity above and beyond oxidation damage in samples exposed to 900 C. For samples exposed to a 500 C environment and thermally shocked, oxidation of the fiber matrix interface is the dominate degradation mechanism. Thermal shock dominates at higher temperatures due to the ability of glassceramics to protect themselves from oxidation. It has been suggested by several researchers to use short, high temperature exposures in oxidizing environments to protect Nicalon reinforced glassceramic composites from oxidation damage [20,22]. However, this type of protection is very limited since mechanical overloads or thermal shock will erradicate this protection. Evidence of this is shown in the data in Figure 9. In these data, several samples were oxidized in air at 900 C for 24 h to form the protective SiO2 seal. One sample was then subsequently exposed to a 500 C oxidation environment for 4 h. No change in thermal diffusivity was seen in this sample which suggests that the silicate glass barrier does provide protection as noted by other researchers. A second sample was subsequently 84S. GRAHAMET AL. thermally shocked at 900 C and then exposed to a 900 C environment for 4 h. In this case, a 7% decrease in thermal diffusivity is seen after thermal shock. No further decrease in thermal diffusivity was seen after the subsequent 4 h 900 C oxidation exposure. This indicates the thermal shock-induced damage does not propagate during the oxidation exposure and the sample reseals itself to prevent significant oxidation. A third sample was thermally shocked from 900 C and then subsequently exposed to an oxidizing environment at 500 C. In this case, a substantial decrease in thermal diffusivity was seen after thermal shock, on the order of 10%. Subsequent exposure to 500 C oxidation for 4 h resulted in an additional 5% decrease in thermal diffusivity. This indicates that the protective SiO2 layer was damaged and permitted oxidation of the carbon interface to proceed in the material. These results show that glassceramics which rely on an SiO2 barrier as protection are highly susceptible to degradation which is dependent on thermal exposure. CONCLUSIONS The use of flash thermal diffusivity to monitor oxidation and thermal shock damage has been demonstrated in this paper. The results have shown that the use of flash thermal diffusivity measurements provides an effective method whereby the progression of damage may be qualitatively determined in CMCs. The determination of the integrity of Nicalon reinforced CMCs becomes very important due their propensity to lose toughness when oxidized. The monitoring of thermal diffusivity in Nicalon-LAS II shows substantial changes in properties when either oxidation or thermal shock damage is induced. The method provides the advantage of being a quick, nondestructive tool which is sensitive to oxidation and thermal shock-induced damage in early stages of development. Although Figure 9. Effect of subsequent thermal exposure on Nicalon-LAS II after thermal shock damage. Samples were first oxidized at 900 C for 24h to form protective coating. Data show that thermal shock and subsequent low temperature exposure renders the silicate barrier as a fragile form of oxidation protection. The legend is given as oxidation temperaturethermal shock T/subsequent oxidation temperature. Nondestructive Characterization by Flash Diffusivity85 rear surface temperature measurements are often made in flash diffusivity tests, the technique is amenable to front surface measurements using lasers and infrared thermography. Such tests are invaluable when access to only a single surface of a component is feasible. This allows increased versatility of the method for inservice inspections in gas turbine engine components, for example. The results presented in this paper show that the effect of oxidation and thermal shock on the thermal diffusivity of Nicalon-LAS II has a complex behavior depending on service environment. At high temperatures where a SiO2, protective layer forms, thermal shock damage tends to control the degradation of the thermal diffusivity. At temperatures below which this layer forms, oxidation controls the degradation of thermal diffusivity. The use of thermal diffusivity measurements under cyclic thermal shock conditions clearly show a progressive degradation in the material property. This degradation is due in part to continued oxidation and/or the propagation of thermal shock-induced damage, depending on the temperature environment. Although residual strength tests have shown little or no degradation of strength after thermal shock of Nicalon-LAS II, the thermal diffusivity measurements show significant changes in thermal properties. These data also suggest that the subsequent environmental exposure after thermal shock significantly affects the integrity of the material. 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| Funders | Funder number |
|---|---|
| Lockheed Martin Energy Research Corp. | |
| Office of Transportation Technologies | |
| U.S. Department of Energy | DE-AC05-96OR22464 |
| Office of Energy Efficiency and Renewable Energy | |
| Oak Ridge National Laboratory |