A. Tontisakis, W. Simpson, J. Lincoln, R. Dhawan A
M. Opliger B
A Axiom Materials, Inc., Santa Ana, CA 92705
B National Institute for Aviation Research, Wichita, KS 67260
Oxide-Oxide ceramic matrix composites (Ox-Ox CMCs) enable improved performance properties relative to titanium, inconel and other high-temperature alloys, as high-temperature components in the aerospace and advanced energy sectors due to their low density, oxidation and corrosion resistance, and high heat resistance. However, the high surface porosity and roughness of Ox-Ox CMCs can be problematic in some applications, especially where airflow, drag and friction play a factor in overall performance. The present study explores the application of an Ox-Ox CMC surfacing film designed to co-cure and sinter with composite parts manufactured using Ox-Ox pre-impregnated fabrics with the intention of providing improved surface finish. This surfacing film aims to reduce surface roughness, improve smoothness and airflow, reduce surface porosity and improve resistance to thermal cycling by minimizing microcracking. The film is based on Ox-Ox chemistry compatible with current systems in the market and can withstand temperatures up to 1400°C. Surface properties and mechanical properties are evaluated and reported on Ox-Ox CMC laminates prepared with and without surfacing film. Results indicate that the Ox-Ox CMC surfacing film provides meaningful improvement in surface quality and in damage tolerance. Marginal reduction in fiber volume percentage was observed due to the increased matrix contribution from the surfacing film.
Ox-Ox CMCs have significant application momentum as high-temperature oxidation-sensitive components, particularly in the aerospace industry.1-2 Ox-Ox CMCs have lower densities than standard high-temperature superalloys and allow jet engines to run at higher temperatures, leading to fewer emissions of carbon dioxide and nitrogen oxides, both of which contribute to global warming.3-4 Ox-Ox CMCs also resist acoustic fatigue better than the standard high-temperature metallic superalloys.5 Although Ox-Ox CMCs are more orientable than metallic superalloys, and able to form more complex geometries, they aren’t as smooth. Surface roughness plays a role in applications where airflow and drag are critical performance characteristics. For example, in exhaust mixer applications, the efficiency of the hot core-engine exhaust to mix with the cooler bypass air directly affects the engine efficiency and noise.6 Ox-Ox CMCs also have high surface porosity, which leads to erosion and wear.7-9 The gas and liquid permeability of Ox-Ox CMCs lead to water vapor corrosion at temperatures above 1200°C.10 The present work seeks to provide engineering solutions for the reduction of Ox-Ox CMC component surface roughness and surface porosity through (a) the development of a matrix rich surfacing film and (b) the characterization of the physical and mechanical properties of laminates with and without a surfacing film.
Surface roughness in Ox-Ox CMCs closely relates to the exposed surface fabric geometry. Tightly wound, smaller diameter fiber tows create smoother surfaces, and in turn, higher diameter fiber tows create coarser surfaces. As industry moves to higher denier fabrics to reduce cost, surface roughness becomes a more serious engineering concern. A ceramic matrix rich film with a thin ceramic carrier helps to alleviate this issue. There is widespread precedent in the use of polymer matrix rich surfacing films to decrease surface roughness in Polymer Matrix Composite (PMC) components. Polymer matrix composite component manufacturers use resin-rich, adhesive-based surfacing films to create a more cosmetic surface finish. Other variations of PMC surfacing films are used to protect the composite surface from sanding, bead blasting and/or thermal cycling. These surfacing films are ideal for composite components that need protection from the harsh rigors of flight.11 In addition to protecting the exposed fiber, surfacing films reduce manufacturing costs by eliminating post-sintering steps such as grinding and polishing. Grinding and polishing take time, produces waste and may damage the fiber, causing it to fray or pull out of the matrix. The manufacturing profile for the automated coating of Ox-Ox CMC components is shown in Fig. 1.
Table 1 presents a summary of the properties of the coated ceramic fabrics used in the present study.12-13 Photos of select fabrics, presented in Fig. 2, show the visual differences observed when changing deniers and pick counts (tows/in). NextelTM 610 8 harness satin 1500 denier fabric is the most common architecture used for Ox-Ox CMC prepreg in industry and literature, and therefore will be considered the baseline for this study.14 Moving to 3000 and 4500 denier fabrics, the expectation is that the fiber tows will spread after the sizing is burned away to make a more uniform fabric without open spaces between the fiber tows. Fabrics have balanced 4 to 8 harness satin (HS) weaves.
For the present study, an alumina-silica based resin system (AX-7810, Axiom Materials, Inc.) was used to coat the various fabric architectures. The fibers used were all alumina (NextelTM 610, 3M Corporation). With consideration toward the surface roughness of higher denier fabrics, this study explores the compatibility of an Ox-Ox CMC surfacing film with prepregs based on various higher denier fibers and fabric architectures, and the resultant composite properties.
2. MATERIALS AND METHODS
Prepregs of AX-7810 (Axiom Materials, Inc.), described in Table 1, were prepared by application of a solvent-based alumina-silica slurry (AX-810, Axiom Materials, Inc.) on to various Nextel fabrics, with automated coating equipment. The prepregs were coated to a known matrix content and volatile content and wound with release liners on a supporting roll. CerFaceTM (AX-8810, Axiom Materials, Inc.), a ceramic-paper supported surfacing film based on alumina chemistry, was coated onto mylar film using automated film coating equipment.15 The film was coated to a known areal weight and volatile content. Laminates were prepared from the AX-7810 prepregs both with and without the surfacing film using each fabric variation. The surfacing film laminates were laid up with a single ply of surfacing film on each surface. Samples without surfacing film were manufactured with a target thickness of 0.100-0.130 inches (2.5-3.3 mm). Laminates were cured in an autoclave and sintered in a high-temperature kiln. Laminate physical properties were evaluated, including fiber volume, matrix volume, porosity, density, per-ply thickness, and surface roughness. Laminates were cut into specimens for testing of flexural properties per ASTM C1341, interlaminar shear properties per ASTM D2344, and tension properties per ASTM C1275.16-18 In order to evaluate the thermal effects, tension properties were tested at 900°C per ASTM C1359.19 Mechanical testing was conducted using an MTS Servo-Hydraulic testing machine at Axiom Materials Inc, Fig. 3.
Tensile strength after impact testing was performed at the National Institute for Aviation Research (NIAR). Unnotched tensile specimens were machined and were tested before and after impact in accordance with ASTM D5766-11.20 Impacted specimens were impacted using ASTM D7136-15 for guidance, but modifications were made to accommodate the tension specimens, which were narrower than the standard compression specimen referenced in ASTM D7136-15 (1.5” x 12” versus 4” x 6”).21-22 These modifications included a smaller striker tip (3/8” versus 5/8” diameter – hemispherical) and smaller support fixture cut-out (1” x 1.5” versus 3” x 5”). Additionally, a lower impact energy was chosen for these CMC specimens (160 in-lb/in versus 1500 in-lb/in) since ASTM D7136-15 is intended for PMC specimens, which have better impact resistance than CMCs and because a smaller striker tip was used. Only AX-7810-DF11-5HS3000D prepreg was chosen to evaluate as it is the standard industry prepreg.
Surface roughness analysis was performed at KRÜSS in Germany on laminates manufactured with and without the surfacing film using the confocal microscope technique. The measurements were performed with the KRÜSS surface roughness analyzer (SRA) controlled by the Itom software 3.1. Both linewise and areal roughness were measured. For linewise roughness, roughness along 5 lines of 5mm length were averaged to determine the arithmetic mean deviation of the assessed profile. For areal roughness, an area of about 7mm x 7mm was scanned to calculate roughness with the Itom software. The parameters in Table 2 were used to measure the variables in Table 3.
Additional surface roughness analysis was performed at the National Institute for Aviation Research in Wichita, Kansas on laminates manufactured with and without surfacing film. Laminates were evaluated using the Keyence VK-X1100 Laser Confocal Microscope in a dark laboratory to ensure peripheral lighting was not impacting the quality of the scans or measurements. All surface measurements were taken and evaluated per ISO 4287 with the Keyence Multi-File Analyzer software. The parameters in Table 4 were used to measure the variables in Table 3.
Cross sectional images by SEM were taken using an FEI Magellan 400 XHR SEM at UCI. Micro-CT images were collected at 3MTM at a resolution of 3 microns using a Bruker Skyscan 1172 scanner operated at X-ray source settings of 80kV and 120µA with application of a 0.5mm Aluminum filter.
3. RESULTS AND DISCUSSION
Micro-CT scan images were taken of laminates manufactured with and without surfacing film in Fig.4.
Laminates manufactured with surfacing film were evidently thicker, but initial concerns emerged over internal consolidation of the AX-7810 prepreg. The matrix rich layers of surfacing film were thought to increase resin flow through the laminate during consolidation, leading to matrix rich areas and lower fiber volumes. The micro-CT show consistent ply consolidation within each laminate and give insight into the surfacing film integration with NextelTM prepreg. This film-prepreg interface shows some cracking and delamination near the edges of the cut specimen, but the same damage is observed within standard laminates, indicating damage during machining. The center of the laminate, however, shows consistent integration between the surfacing film and the prepreg.
Though the consolidation appears consistent, the fiber volume within the laminates remains undefined. Fiber volumes for surfacing film laminates were thought to decrease due to addition of matrix rich layers on the surfaces of the laminate. To determine the thickness of these surfacing film layers, an average film-thickness was calculated using SEM cross-sectional images. Nine images from each laminate were taken at the film-prepreg interface, Fig. 5. The averaged thicknesses yielded different results for each fabric architecture, defined in Table 5.
SEM images show adequate surfacing film integration with the NextelTM prepreg, indicating a successful co-cure. The surfacing film film-thickness averages are consistent for the DF11 fabrics, but lower for the 4500D laminate. This is most likely due the increase of matrix, from the surfacing film that fills in the larger gaps/ridges of the fabric, caused by thicker fiber bundles. Surfacing film thicknesses were subtracted from the overall thickness of each laminate to calculate the internal per-ply thickness values found in Table 6.
The reduction in the fiber volume and density of the surfacing film laminates is a result of the increased matrix volume on the surfaces of each laminate. A slight increase in the per-ply thickness of the surfacing film laminates indicates reduced consolidation during cure, leading to matrix rich areas within the laminate. This is thought to be a result of the resin rich surfacing film and the flow of this resin through the medium and fiber preform.
The surface of these resin rich surfacing film laminates was analyzed and compared to that of laminates manufactured without a surfacing film. Linewise surface roughness comparison data are presented in Fig. 6-12 and Tables 7-8.
Krüss results show a 58% reduction in linewise roughness for 8HS1500D and 5HS3000D laminates with surfacing film and a 41% reduction in linewise roughness for TW4500D laminates with surfacing film (Fig. 6). The surfacing film roughness values are consistent across all fabric weaves with an average of 2.7µm (Table 7). For the laminates without surfacing film, the reduced surface roughness for the laminate with TW4500D prepreg as compared to the other denier prepregs was thought to be a factor of fiber gapping. As seen in the SEM cross sectional images of the surfacing film, resin in the TW4500D prepreg filled deeper into the gaps and ridges of the fabric leading to a thinner surfacing film layer. The tighter gaps of the thinner fiber bundle fabric, presented in Fig. 7, may have impeded resin flow leading to a more inconsistent and rougher surface.
NIAR surface roughness analysis show a reduction in linewise roughness of laminates with surfacing film, compared to those without, increasing from 51% to 78% with increasing denier. The surfacing film roughness values are consistent for 8HS1500D and 5HS3000D laminates, with an average of 5.3µm (Table 8). The TW4500D surfacing film laminate has a lower roughness of 3.0µm, thought to be a factor of gap filling, as described previously and showcased in Fig. 5. The difference in results from Krüss and NIAR are thought to be caused by a difference in equipment and analysis location. Further investigation into the differences between surface analytical techniques is warranted.
In addition to linewise roughness, areal surface roughness data were measured and are presented in Fig. 13-16 and Tables 9-10.
Krüss results show a 52% reduction in areal roughness for 8HS1500D laminates with surfacing film, a 44% reduction in areal roughness for 5HS3000D laminates with surfacing film and a 28% reduction in areal roughness for TW4500D laminates with surfacing film. The surfacing film roughness values are consistent across all fabric weaves with an average of 3.3µm (Table 9). For the laminates without surfacing film, the reduced surface roughness for laminates with increasing denier was thought to be a factor of fiber gapping, as stated previously.
NIAR surface roughness analysis show a reduction in areal roughness of laminates with surfacing film, compared to those without, increasing from 52% to 72% with increasing denier. The surfacing film roughness values are consistent for 8HS1500D and 5HS3000D laminates, with an average of 5.9µm (Table 10). The TW4500D surfacing film laminate has a lower roughness of 4.6µm, thought to be a factor of gap filling, as described previously and showcased in Fig. 5. The difference in results from Krüss and NIAR are thought to be caused by a difference in equipment and analysis location. Further investigation into the differences between surface analytical techniques is warranted.
The three-dimensional surface roughness topography for all samples was generated. Krüss surface topography was generated from 7mm x 7mm areal scans and filtered with a 2.5mm cut-off. Maps were created with the Mountainsmaps software. NIAR three-dimensional images were created in the Keyence multi-file analyzer software from 8mm x 10.5mm areal scans. Three-dimensional surface images are plotted below in Fig. 17-18.
The maps show large gaps/ridges in standard prepreg laminates and more consistent surface features in laminates with surfacing film.
Mechanical properties were tested to determine the impact a layer of surfacing film has on the mechanical strength of the bulk material. The CMC surfacing film was projected to add no mechanical strength, therefore, to more accurately compare properties, the surfacing film ply-thickness was subtracted from the overall laminate thickness to calculate ultimate stress and modulus. This standardization is common in the mechanical characterization of surfacing films or fiberglass plies used for galvanic corrosion protection of carbon reinforced PMCs. Standard prepreg, without surfacing film, mechanical properties were averaged from historical in-house results recorded according to ASTM standards. Ambient-temperature mechanical property data recorded at Axiom Materials Inc are presented in Fig. 19-21 and Tables 11-13.
Mechanical data shows a slight reduction in surfacing film laminate strength attributed to a decrease in consolidation between layers of prepreg. Matrix tensile strength is the “key parameter that controls the strength and failure strain of the CMC lamina.”23 So an increase in matrix content between plies leads to an increase in matrix crack propagation pathways, weakening the laminate. CMC tensile coupons were then tested at 900°C according to ASTM C1359, the data is presented in Fig. 22 and Table 14.
Tensile strength data at temperature indicates a slight reduction in tensile strength of laminates with surfacing film attributed to a decrease in consolidation between layers of prepreg. This, again, is attributed to matrix cracking, the critical damage mechanism for CMCs.
Tensile strength after impact testing was performed according to ASTM D7136-15 and ASTM D5766-11 with the modifications detailed in the “Evaluation” section. The unnotched and after impact tensile strength data are presented in Fig. 23 and Table 15.
Mechanical data show a slight reduction in after-impact tension strength for laminates with surfacing film, attributed to a decrease in consolidation between layers of prepreg. After-impact testing results show large variation between specimen, so it is recommended that the data presented for tension after impact are used for comparative purposes rather than engineering or design purposes.
Pulsed thermography was used to image the specimen after impact. Thermal images show impact damage through the specimens (Fig. 24).
Thermal images of the laminates with surfacing film show more localized impact damage as compared to laminates without surfacing film which indicates a reduction in damage propagation during impact. The resin rich surfacing film protects the interior fiber preform by taking majority of the damage and keeping the impact to a smaller area.
An Ox-Ox CMC surfacing film that enables a significant reduction in surface roughness, has been presented and characterized. The Ox-Ox CMC surfacing film offers a smooth surface with a cosmetic finish and localized impact protection without significant reduction in mechanical properties. Surfacing film integrated well with Ox-Ox CMC prepregs, but resulted in lower fiber volumes. This impedance to consolidation resulted in slight reductions in the mechanical strength. Future research should be directed toward consolidation improvement, product standardization and design-quality data development of Ox-Ox CMC surfacing film technology to enable more flexibility in engineering design and optimized application results.
5. DECLARATION OF COMPETING INTEREST
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Axiom Materials Inc has been issued a patent for an Ox-Ox CMC surfacing film.
The authors acknowledge technical contributions from Myles Brostrom, Juan Contreras, Stan Fast, Frederick Fiddler, Matt Jones, Julian Lamas, Raymund Paguirigan, Marc Simpson, Carlos Tornell, Hanna Wright. This research was funded by Axiom Materials Inc.
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JOHN LINCOLN, BARRETT JACKSON, AMY BARNES, AARON BEABER, LARRY VISSER
Axiom Materials, Inc., Santa Ana, CA 92705; [email protected]
Composites Horizons, Covina, CA 92697-7075
3M Company, Advanced Materials Division, St. Paul, MN ...
JOHN LINCOLN, BARRETT JACKSON, AMY BARNES, AARON BEABER, LARRY VISSER
Axiom Materials, Inc., Santa Ana, CA 92705; [email protected] Composites Horizons, Covina, CA 92697-7075 3M Company, Advanced Materials Division, St. Paul, MN 55144
ABSTRACTOxide-oxide ceramic matrix composites are gaining increasing attention as a mainstream material option for high temperature components in the aerospace and advanced energy sectors. As the material moves from bench to production, cost reductions are required to ensure that the solutions are market-competitive with titanium and other high temperature alloys. In parallel, a more comprehensive portfolio of fabric geometries and data are desirable to enable flexibility in both engineering and design. 3M, maker of Nextel™ ceramic fibers, joins CMC prepreg developer and manufacturer, Axiom Materials, Inc., and CMC parts designer and fabricator, Composites Horizons, in developing data for the present work. The team jointly compares the properties of Oxide-Oxide CMCs fabricated from conventional Nextel fabric architectures with those of new, lower cost fabric designs.
INTRODUCTIONOxide-oxide ceramic matrix composites (OxOx CMCs) now have significant application momentum in oxidation-sensitive component applications. Yet both the transition to OxOx CMC, as well as the engineering of new OxOx CMC hardware, has been gradual as a result of its high cost relative to conventional materials or as an element of any new component design. The global market for OxOx CMC components is positioned for near-term growth and on the cusp of mainstream adoption provided that reasonable cost profiles can be achieved. Particularly in the case of the aerospace sector, cost reduction initiatives have become especially high-priority as the OxOx CMC cost / value proposition crosses that of machined titanium components for turbine engine and other high temperature hardware. The present work seeks to provide engineering solutions for cost-conscious OxOx CMC design through (a) the development of lower cost fiber and fabric architectures, and (b) characterization of their physical and mechanical properties. Indeed, there is precedent in using higher denier fibers and fabrics to achieve reduced costs. Similar trends have been observed for carbon fiber composites, which has progressed from 1k and 3k fibers initially, to upward beyond 50k fibers to reduce the cost of finished components.1 Like carbon components, costs associated with the manufacturing process for OxOx CMC components are weighted heavily toward the cost of the fiber and/or fabric. Insofar as these costs can be reduced, component level costs can likewise be reduced. To date, most OxOx CMC technical property data and research has been based on the use of NextelTM 610 and 720 fibers,2-7 and most notably styles DF-11 and EF-11 (1500 denier). The 1500 denier yarns are the finest produced commercially and are the most expensive fibers from a manufacturing standpoint due to small fiber bundle size and high demand placed on fiber manufacturing lines. 3M has demonstrated that higher denier yarns may be made by increasing the number of filaments in the tow bundle and keeping the individual filament diameters similar, 8 which results in a significant cost advantage in fiber production. As the cost savings available from higher deniers of the Nextel fibers are of primary interest, relative costs of various fiber deniers are presented in Figure 2. The fiber types used in the present study for both grades Nextel 610 and 720 are 1500, 3000, 4500 and 10,000 denier. [caption id="attachment_1969" align="aligncenter" width="841"] Figure 1: Current (6/2016) relative price per pound for structural ceramic fibers at various deniers (3M)[/caption] Figure 1 expresses the 1500 denier fibers as 100%, and each higher denier is reported as a lower percentage of the 1500 denier price. The comparative pricing shows that fiber costs can be significantly reduced in transitioning to higher denier fiber tows. Further, it is worth noting that changes at the fiber level also have the potential to impact cost other points in the supply chain. Figure 2 expresses the typical flow of large-volume OxOx CMC supply chain from fiber manufacturing to finished component. [caption id="attachment_1968" align="aligncenter" width="1024"] Figure 2: Typical OxOx CMC supply chain from fiber to component[/caption] Weaving costs, for example, can be reduced through the use of higher denier yarn configurations, simpler weaving patterning & setup, or reduced total yardages in the case of heavier fabric architectures. Prepregging costs can likewise be reduced with heavier fabrics (lower overall yardage), as would part fabrication costs (reduced layup & yardage). Therefore, from a holistic supply chain standpoint, additional cost savings are expected through the use of modified fibers and fabrics. In preparation for the present study, a variety of woven fabrics were designed from various deniers of both 3M™ Nextel™ 610 and 720 fiber. Nextel 610 is >99% alpha alumina, which is the higher strength fiber, and 720 is an 85% alpha alumina and 15% silica, which as fired is a mix of alpha alumina and mullite, making it the more refractory fiber.8 A presentation of mechanical properties of base fiber tows at various deniers is of value in order to interpret the greater implication of their use in the fabrics. Table 1 expresses the fibers used in the present study, alongside their reported breaking strengths. Typical single filament and strand strengths are listed from historical process data. [caption id="attachment_1978" align="aligncenter" width="715"] Table 1: – Single Strand and Tow Break Loads[/caption] By keeping the nominal diameter of each filament the same, the breaking load of the filament is independent of denier from 1500-10,000 denier. However, with the number of filaments increasing in each tow, measuring the strength of each tow becomes more problematic. Test method ASTM 2256 is followed where pneumatic grips are used to clamp the tow. With 1500 and 3000 denier a typical brittle ceramic break can be observed. As the filament count increases beyond 3000 denier, the test curves indicate that not all the filaments are breaking at the same time. Because of the strand break load testing characteristics as shown in the table above, fiber properties is based on the single filament break loads, crystal size, and crystal phase. A comparison of the single filament test results shows that the fundamental fiber quality is comparable for low and high denier. As single filament breaking strength is relatively unaffected by higher denier yarn production, higher denier tows are in turn expected to be usable in the woven fabrics of OxOx CMCs to achieve similar performance to lower denier tows. Since much of the early research activity started from 1500 denier fabric (DF-11 and EF-11), new fabric architectures were largely based around the approximate fiber volume and thickness of DF-11 and EF-11, with some exceptions to explore boundaries. Nascent near-property offsets to DF-11 and EF-11 include DF-11-10,14,17 & EF-11-13,14,16,17, which were prepared from 3,000 and 4,500 denier yarns. In addition to offsets for the 1500 denier fabric, other fabrics in the test matrix explore an increased amount of fiber in each direction of the weave, in turn helping to determine the effect on composite properties using thicker and/or fewer plies. Lastly, for 3M™ Nextel™ 720 a unidirectional-type fabric (EF-20) was prepared, where the bulk of the fiber is 10,000 denier in the warp direction. For EF-20, 1500 denier is used at a much lower pick count in the fill direction to give the fabric enough stability to be handled and prepregged. Table 2 presents a summary of the properties of the uncoated ceramic fabrics used in the present study. Photos of select fabrics, presented in Figure 3, show the visual differences observed when changing deniers and pick counts (tows/in). Fabric EF-11 (identical to DF-11 in construction) is the fabric to which others should be compared. In the 3000 denier and DF-24-8-10,000 fabrics, the expectation is the tows will spread after the sizing is burned away to make a more uniform fabric without open spaces between tows. Fabrics have balanced 4 to 8 harness satin (HS) weaves except for EF-20, which is the semi-unidirectional (UD) fabric. [caption id="attachment_1977" align="aligncenter" width="1187"] Table 2: 3M™ Nextel™ 610 and 720 fabrics evaluated[/caption] [caption id="attachment_1967" align="aligncenter" width="1024"] Figure 3: Images of select woven Nextel sized fabrics[/caption] With consideration toward the cost benefits of increased fiber denier, and on supply chain savings through modified fabric architectures, this study explores the effects of higher denier fibers and fabric architectures on the resultant composite properties.
EXPERIMENTALFabrics of Nextel 610 and Nextel 720 described in Table 2 were woven and heat cleaned. Fabrics were coated to a known matrix content with proprietary ceramic slurries formulated and blended at Axiom Materials, Inc. and Composites Horizons using laboratory-based coating techniques. For Nextel 610 fabrics, an aluminum-silicate slurry was used. For Nextel 720 fabrics, an alumina slurry was used. Matrix absorption and fabric handling characteristics were observed and recorded. Prepregged fabrics were laid up at Composites Horizons into multi-ply 0°/90° laminates with a target thickness of 0.100-0.130 inches (2.5-3.3 mm). EF-20 was laid up into a 0° laminate because of its unidirectional weave pattern. Automated coating behavior for DF-11, DF-19, EF-11, EF-19, and EF-20 were independently explored using a single pass solution prepreg treater at Axiom Materials, Inc. Laboratory-coated laminates were processed via autoclave cure and sintered at Composites Horizons. Physical properties were evaluated including fiber volume, matrix volume, porosity, density, and per-ply thickness. Laminates were cut into specimens for testing of flexural properties per ASTM C1341, interlaminar shear properties per ASTM D2344 and tension properties per ASTM C1275. In order to evaluate the thermal effects, tension properties of Nextel 610 composites were tested after aging at 900°C, 1050°C, and 1150°C (each at 10hr, 100hr, and 500hr exposures), and tension properties of Nextel 720 composites were tested after aging at 1100°C, 1200°C, and 1275°C (each at 10hr, 100hr, and 200hr). Mechanical tests were conducted at 3M, Axiom Materials Inc., and Exova laboratories. Round robin testing was also conducted to assess laboratory biases for interlaminar shear and tension.
RESULTS AND DISCUSSIONUpon initial weaving, sized fabrics of heavier deniers appeared to have more fiber gapping than weave patterns of 1500 denier. However, it was observed that the heat cleaning (desizing) operation had a significant impact on relaxing the fabric tows and spreading them out into a more even plane with greater coverage. This was an important discovery because the presence of rounder, unspread tow bundles of higher denier have the potential to impact the properties of the resultant composite due to inconsistent microstructure and/or increased thicknesses. This is similarly observed in comparing heat-cleaned thicknesses with sized thicknesses in Table 2. The extent to which the heavier denier and weave pattern fabrics accept ceramic matrix impregnation was also evaluated. It was observed that all fabrics identified in Table 2 were readily impregnable in the laboratory with relative ease. Further, automated prepregging operations using ceramic slurries were conducted on DF-19 (3,000 denier 3M™ Nextel™ 610), and EF-19 (3,000 denier Nextel 720), and EF-20 (10,000 denier Nextel 720), all of which demonstrated good impregnation performance and handling through the treating equipment, and little difficulty in roll winding or packaging. The expectation is that fabrics having as high as 20,000 denier could be prepregged using automated methods with relative ease. Evaluating prepreg drape characteristics is likewise central in determining if modified fabric architectures are suitable for largescale production. Base assessment of prepreg drape characteristics indicated that all fabrics of 3,000 denier or lower were not of concern during layup for radii of 1" (25mm) or greater, including fabric styles DF-19 and EF-19. DF-19-16-4500 exhibited fiber resistance and breaking and is suggested for components with gentler radii. Lastly, it was determined that 10,000 denier semi-unidirectional fabric EF-20 was able to be contoured in the 90° direction (perpendicular to dominant fibers) for tubular structures of appx. 1" (25mm) or greater. Composite laminate properties are presented in Tables 3 and 4. The characterization of laminate properties provides insight into finished composite laminate quality and microstructure, as well as standard properties such as density, ply count, and dimension. [caption id="attachment_1976" align="aligncenter" width="854"] Table 3: Physical properties for Nextel 610 laminates in the present study[/caption] [caption id="attachment_1975" align="aligncenter" width="829"] Table 4: Physical properties for 3M™ Nextel™ 720 laminates in the present study[/caption] To the degree that properties are outside of general expectations, physical properties can also indicate the presence of any improperly prepared or processed laminates. No unusual characteristics were observed for any of the fiber deniers or fabric geometries evaluated. Even at relatively thicker fabrics and higher fiber deniers, similarity was observed in porosity content, matrix content, and density. Volumetric properties were within normal ranges of experimental and measurement variance error. [caption id="attachment_1966" align="aligncenter" width="1024"] Figure 4: Per ply thicknesses of prepreg laminates from various Nextel 610 and 720 fabrics[/caption] Per ply thicknesses (PPT), of value from a design perspective, are plotted in Figure 4. Comparisons may been be drawn between heat-cleaned fabric thickness (refer to Table 2), and postprocessed, composite PPTs. The data indicates that a reduction in PPT between heat-cleaned fabrics and finished part component readings can be expected, although variation in the change is significant and deemed to be largely geometry-dependent, and affected by the behavior of both (a) the fibers laying down during composite processing, and (b) the flow and behavior of the matrix around the fiber. The composite thicknesses are useful to the part designer in determining the number of plies of a particular fabric style to use in costing and/or engineering OxOx CMC components. Composite thicknesses also provide framework for establishing cost per unit thickness of components using various prepregs. In order to provide some understanding of composite microstructure, SEM images were taken of various composite laminates in the present study, and are presented in Figures 5-8. [caption id="attachment_1965" align="aligncenter" width="1024"] Figure 5: SEM Images of EF-11 OxOx CMC (1,500 Denier)[/caption] [caption id="attachment_1964" align="aligncenter" width="1024"] Figure 6: SEM Images of DF-11 OxOx CMC (1,500 Denier)[/caption] [caption id="attachment_1963" align="aligncenter" width="1024"] Figure 7: SEM Images of DF-11-10-4500 OxOx CMC (4,500 Denier)[/caption] [caption id="attachment_1962" align="aligncenter" width="1024"] Figure 8: SEM Images of DF-24-8-10k OxOx CMC (10,000 Denier)[/caption] The images are polished cross sections that show both the matrix and the fibers. What is obvious is the shape of the fiber changing from round to more of an oval to dog bone shape as the denier increases. This is the result of a drying phenomenon that occurs when the fibers are spun during initial manufacturing. As is shown in Table 1, the single filament strength is not affected by the shape change of the fibers at higher deniers. Nextel 610 OxOx CMC Results: Ambient-temperature mechanical property data collected on OxOx CMC produced from various Nextel 610 fabrics are presented in Figures 9 and 10. [caption id="attachment_1961" align="aligncenter" width="1024"] Figure 9: Tensile and flexural properties for Nextel 610™ composite laminates[/caption] [caption id="attachment_1960" align="aligncenter" width="1024"] Figure 10: Interlaminar shear properties for 3M™ Nextel™ 610 composite laminates[/caption] Both tensile data and flexural data indicate a mild downward trend with increasing fabric thickness and/or denier. What is significant about both tensile and flexural properties is that they suffer less degradation for higher denier and/or thicker weaves than was the expectation at the onset of this research. Properties remained relatively constant, despite drastic changes in filament geometry and fabric architecture. Interlaminar shear properties appears generally unaffected in Nextel 610 OxOx CMC laminates of heavier architecture, although there is some scatter in the data. The assumption for interlaminar shear properties was that an increase in the variation in composite microstructure would track with an increase in fiber denier, and in turn a decline in properties, but this was not observed. Thermal aging data and high-temperature test data are presented for Nextel 610 OxOx CMC in Figures 11 and 12, respectively. The impact of thermal aging on tension strength after various aging times and temperatures is in alignment with historical data for Nextel 610 fiber performance, which suggest a decline in fiber mechanical performance whose occurrence begins at approximately 900-1000°C.9 Tensile data at various temperatures is also in alignment with fiber performance expectation, where a decline in strength can be expected to correspond directly to temperature.9 Notably, data trends are tight and relatively independent of fiber denier or weave pattern. [caption id="attachment_1959" align="aligncenter" width="1024"] Figure 11: Tensile strengths of 3M™ Nextel™ 610 OxOx CMC laminates as processed (AP), 10/100/500 hrs at 900°C, 10/100/500 hrs at 1050°C, and 10/100/500 hrs at 1150°C[/caption] [caption id="attachment_1958" align="aligncenter" width="1024"] Figure 12: Tensile properties of Nextel 610 OxOx CMC laminates as processed (21°C), at 1000°C, 1050°C, and 1150°C[/caption] Nextel 720 OxOx CMC Results: Ambient-temperature mechanical property data collected on OxOx CMC produced from various Nextel 720 fabrics are presented in Figures 13 and 14. Tension and flexural data indicate a stable trend with increasing fabric thickness and/or denier for balanced-weave fabric composites. EF-20, a unidirectional fabric of unbalanced weave, should not be considered in assessing trends. Similar to Nextel 610 composites, Nextel 720 OxOx CMCs do not appear to have significant mechanical property reduction for increased thicknesses or deniers. [caption id="attachment_1957" align="aligncenter" width="1024"] Figure 13: Tensile and flexural properties for 3M™ Nextel™ 720 composite laminates[/caption] [caption id="attachment_1956" align="aligncenter" width="1024"] Figure 14: Interlaminar shear properties for Nextel 720 TM composite laminates[/caption] Nextel 720 OxOx CMC interlaminar shear properties followed a similar flat trend to Nextel 610 in properties, although slightly upward with weight / fiber denier in this case. In drawing a relative comparison to the industry-standard EF-11 fabric, results presented for tension, flexural, and interlaminar shear properties are similar to those of heavier denier and courser weave fabrics. In the case of EF-20 semi-unidirectional fabric, the results for tensile & flexural modulus and interlaminar shear properties are in alignment with the general expectation of increased values in the 0° direction relative to same-thickness laminates of balanced weave. Tensile and flexural strength properties did not meet the expectation of an increase in strength for the additional fiber aligned with test direction. The impact of thermal aging on tension and interlaminar shear properties is presented for Nextel 720 OxOx CMC laminates after various aging times and temperatures in Figures 15 and 16. Results for tension are in alignment with expectations for fiber strength performance, which is reported to decline at or around 1100-1200°C.9 Interlaminar shear data at various temperature aging suggests long-term property reduction onset at approximately 1250°C. Data trends are for both properties are relatively independent of fiber denier or weave pattern. [caption id="attachment_1955" align="aligncenter" width="1024"] Figure 15: Tensile properties of 3M™ Nextel™ 720 OxOx CMC laminates as processed (AP), 10/100/500 hrs at 1100°C, 10/100/500 hrs at 1200°C, and 10/100/200 hrs at 1275°C[/caption] [caption id="attachment_1954" align="aligncenter" width="1024"] Figure 16: Interlaminar shear properties of Nextel 720 OxOx CMC laminates as processed (AP), 10/100/500 hrs at 1100°C, 10/100/500 hrs at 1200°C, and 10/100/200 hrs at 1275°C[/caption] Interlaboratory Testing: Round robin testing was conducted between various laboratories to identify any expectations of bias for tensile and interlaminar shear data. Data are presented in Figure 17. [caption id="attachment_1953" align="aligncenter" width="1024"] Figure 17: Interlaboratory test data for tensile strength and interlaminar shear strength[/caption] Site-to-site comparability for tensile strength was reasonable, while there was significant scatter for interlaminar shear data. Based on the variation in shear data recorded for this study, it is recommended that the data presented for shear are used for comparative purposes rather than engineering or design purposes.
CONCLUSIONFiber and fabric architectures enabling significant component-level cost reductions for OxOx CMC have been presented. While primary cost savings are achieved at the fiber level, secondary benefits may also be realized at other points in the OxOx CMC supply chain in transitioning to the heavier fabric designs. OxOx CMC prepregs of higher-denier fabrics have been produced, evaluated, and converted into composites for mechanical characterization. Prepregs layups may be completed on tight contours using up to 3,000 denier. Gentler geometries are advised for parts intended to be produced with 4,500 denier or above. Data and observations indicate that the transition to heavier denier yarns and architectures have relatively minimal effect on mechanical properties or on thermal stability of OxOx CMCs. Future research should be directed toward product standardization and design-quality data development for higher-denier fabrics to enable more flexibility in engineering design. It is recommended that part contours and layup characteristics are carefully considered in selecting the lowest cost fabric suitable for the application.
ACKNOWLEDGEMENTSThe authors acknowledge technical contributions from Jonathan Kemling, Moses Omafuaire, Adam Schendel, Mike Davidson, Giovanny Guanche, and Michelle Smith. This research was funded jointly by 3M, Axiom Materials, Inc., and Composites Horizons.
REFERENCES1Das, S., Warren, J., West, D., “Global Carbon Fiber Composites Supply Chain Competitive Analysis,” (2016) Oak Ridge National Laboratory Technical Report ORNL/SR-2016/100. 2Zok, F.W. "Developments in Oxide Fiber Composites," (2006). J. of Am. Cer. Soc., 89 3309-3324. 3Volkmann, E., Tushtev, K., Koch, D., Wilhelmi, C., Goring, J., Rezwan, K., (2015) "Assessment of three oxide/oxide ceramic matrix composites: Mechanical performance and effects of heat treatments," Composites Part A: Applied Science and Manufacturing, 68, 19-28. 4Wilson, D.M., Visser, L.R. “High performance oxide fibers for metal and ceramic composites,” (2001) Composites - Part A: Applied Science and Manufacturing, 32 (8) 1143-1153. 5Wilson, D.M. “Statistical tensile strength of Nextel 610 and Nextel™ 720 fibres,” (1997) Journal of Materials Science, 32 (10), 2535-2542. 6Axiom Materials, Inc. AX-CMC-610 Technical Data Sheet. Revision Date 3/2/16 7Askarinejad, S., Rahbar, N., Sabelkin, V., Mall, S., "Mechanical behavior of notched oxide/oxide ceramic matrix composite in combustion environment: experiments and simulations," (2015) Composite Structures, 127, 77-86. 8Taylor, M.D. “Chemistry and Manufacture of Alumina and Aluminosilicate Fibers,” (1999) Fine ceramic fibers, ed. M. Berger and A. Bunsell. 93M NextelTM Ceramic Textiles Technical Notebook, 11/04
Dec 15, 2020
White Paper: An Evaluation of Low-Cost High-Denier Fabrics for High Temperature Oxide-Oxide Ceramic Matrix Composites
Johnny Lincoln, Wylie Simpson, Antonios Tontisakis
Axiom Materials, Santa Ana, CA
With major development programs underway and a growing interest in more complex oxide-oxide ceramic matrix composite (CMC) fabric weaves ...
Johnny Lincoln, Wylie Simpson, Antonios Tontisakis
Axiom Materials, Santa Ana, CA
ABSTRACTWith major development programs underway and a growing interest in more complex oxide-oxide ceramic matrix composite (CMC) fabric weaves and resin systems, an investment in production-scale manufacturing is required to satisfy the demand. CMC prepreg has traditionally been manufactured through a “hand-prepregging” process making scale-up difficult. Due to limitations in the resin system and the process itself the result is CMC prepreg with poor uniformity and infiltration into the fiber. Complex fabric weaves, such as lower cost high denier fabrics, require a resin system and coating process that can infiltrate the fabric’s thick fiber bundles. Axiom Materials, Inc has invested in production-scale coating equipment that allows continuous ceramic fabric impregnation. Batch to batch variation is eliminated through controlled alignment features and tensioning devices and infiltration of the fiber bundle is uniform and consistent. CMC prepreg manufactured with production scale coating equipment allows for resin system and fabric variability resulting in a more traditional and commercial product with a wide range of uses. Axiom Materials, Inc evaluates the properties of high denier oxide-oxide ceramic prepreg manufactured with production-scale coating equipment [caption id="attachment_1994" align="aligncenter" width="1024"] Figure 1: CMC Production Flowchart.[/caption]
OX-OX CMC PROPERTIESOx-Ox CMC vs Metals (Ti & Ni) » High Temperature (Operating temperatures up to 1200°C) » Higher Strength » Lower Density (2.5 vs ~ 4.5g/cc) » Low dielectric » Orientable properties » Corrosion resistance » Low creep / high modulus Ox-Ox CMC vs Monolithic » Higher Strength » Lower Density (2.5 vs 3.0-3.7g/cc) » Orientable properties » Damage tolerance / durability / stiffness [caption id="attachment_1999" align="aligncenter" width="618"] Figure 2: Material property charts comparing CMCs to other materials.[/caption]
OX-OX CMC APPLICATIONSAerospace / Hypersonics / Missiles: turbine engine components, reentry surfaces, nozzles, exhaust ducts/tubes, heat shields, propulsion, high-temperature radomes, high-temperature sensing equipment Energy / Industrial: refractory hardware, incineration, heat shielding, gas turbines and microturbines, oxide fuel cells, tubing Automotive / Motorsports: exhaust ducts, mufflers, high wear components, brake ducting [caption id="attachment_2001" align="aligncenter" width="1024"] Figure 3: Examples of CMC applications.[/caption]
CMC MATERIALS CAPABILITY[caption id="attachment_2006" align="aligncenter" width="1024"] Figure 4: CMC Materials Chart.[/caption] SB: Solvent-Based WB: Water-Based AX-810 solvent-based Alumina-Silica slurry selected as fabric coating resin for evaluation
NEXTELTM CERAMIC FABRIC[caption id="attachment_2011" align="aligncenter" width="1021"] Table 1: 3MTM NextelTM Ceramic Fiber Properties[/caption] [caption id="attachment_2010" align="aligncenter" width="1024"] Table 2: 3M NextelTM Ceramic Fabric Properties[/caption] [caption id="attachment_2009" align="aligncenter" width="1024"] Figure 5: NextelTM 610 Fabric Architectures. (Left) 8HS1500D; (Middle Left) 5HS3000D; (Middle Right) 2x2TW4500D; (Right) Spread Tow PW100000D[/caption] High-Denier Woven Fabrics Build on Industry Standard » Large tow bundles at lower pic count to mimic Nextel 1500 Denier Tow, 8 Harness Satin Weave » Direct substitution for components already in production Weaves that Increase Drapability » Ability to form more complex structures Larger Tow Bundles » Greater per-ply thickness leads to less material required to target thickness
- Reduced Yardage - Lower Fabrication Labor - Reduced WasteLower Cost AUTOMATED COATING PROCESS
Dec 15, 2020