Evaluation Of Surface Finish Technology in the Manufacture of Oxide-Oxide Ceramic Matrix Composites White Paper
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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Dec 15, 2020