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. 2025 Aug 5;10(32):36421–36431. doi: 10.1021/acsomega.5c04792

Processing and Characterization of Aerospace Composites for Advanced Radome Applications

Sadam Ali Khichi 1,2, Abdul Waheed Bhutto 1, Inamullah Maitlo 2, Ali Dad Chandio 3,*
PMCID: PMC12368624  PMID: 40852280

Abstract

This research presents the development and characterization of innovative composite materials for aerospace radome applications. Radomes, which protect radar and antenna systems in hostile environmental conditions, must maintain structural integrity while minimizing signal attenuation, especially in the high-frequency Ku-band (10–14.5 GHz). The study explores the use of commercially available reinforcements such as Kevlar and quartz combined with cyanate ester matrices. Three manufacturing techniqueshand layup, infusion molding, and autoclave processingwere employed to fabricate composite samples. Mechanical characterization included tensile, impact, and flexural strength tests, as per American Society for Testing and Materials (ASTM) standards, while electromagnetic testing was conducted using a state-of-the-art anechoic chamber. Among both combinations, the cyanate ester/quartz composite manufactured through autoclave processing demonstrated superior electromagnetic performance with an insertion loss of less than 1 dB across the Ku-band while maintaining adequate mechanical properties. Kevlar-based composites showed higher mechanical strength but slightly compromised signal transparency. This work contributes to the field by providing a data-driven framework for optimizing material selection and processing techniques in aerospace composite manufacturing, aligning with national goals for indigenously developed high-performance materials.

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Introduction

A radome, derived from the words “radar” and “dome”, is a structural enclosure that protects radar equipment, antennas, and sensitive electronic systems from environmental hazards such as rain, wind, ice, and temperature fluctuations while allowing electromagnetic signals to pass through with minimal distortion or attenuation. These enclosures play a crucial role in ensuring the reliable performance and longevity of communication systems deployed across a wide range of platforms, including ships, satellites, aircraft, submarines, ground stations, and military vehicles. The design and development of radomes present a unique engineering challenge as the materials used must simultaneously exhibit high mechanical strength and excellent radio frequency (RF) transparency. Ideally, a radome should be “invisible” to electromagnetic waves within the operational frequency range to avoid any signal degradation, boresight error, or attenuation that could impact the system’s performance. Radomes operating in the aerospace industry should combine properties that are unique to this part of the industry: high mechanical strength, high dielectric characteristics, and the ability to resist extreme environmental conditions. Conventional composite materials are usually compromised in achieving a trade-off between mechanical strength and optimum electromagnetic transparency. It is urgently needed to have innovative composite systems that will accommodate the multifunctional needs of next-generation aerospace radomes.

As communication technologies evolve and shift toward higher frequency bands, especially the C and Ku bands, the need for radomes capable of operating efficiently at these frequencies has become critical. , However, environmental factors and the dielectric nature of the radome materials often introduce complex issues, such as signal delay, deflection, and insertion loss. These factors can impair system accuracy and reduce the efficiency of satellite ground systems. For large radomes, typically constructed in the shape of a spherical frustum mounted on a cylindrical tower, the effects of wind loads become significant, necessitating rigorous structural analysis and design optimization −

Recent studies have provided insight into the materials and design strategies required for effective radome construction. Notably, Leslie explored radomes’ historical and geopolitical significance. While others focused on the performance of ceramics and high-performance fibers such as UHMWPE and Kevlar. Composite structures using these reinforcements in epoxy or cyanate ester matrices offer a balance between mechanical resilience and low dielectric loss. Meanwhile, Kausar et al. and Sahu et al. provided advanced methodologies for tailoring composite configurations using hybrid sandwich structures and frequency-selective surfaces to enhance electromagnetic transmission. The increasing complexity of these materials and configurations necessitates a multidisciplinary approach, as emphasized in Tiede et al., who reviewed frameworks for optimizing composite performance under varying operational conditions.

This study proposes the research and optimization of aerospace-grade composites that can meet the requirements of mechanical strength and radio frequency (RF) transparency as a way of dealing with the material performance trade-off of radome design in aerospace. The aerospace setting of radomes requires that the trade-off between structural or mechanical properties and electromagnetic properties created be such that both structural and electromagnetic performances are balanced, especially in the Ku-band (10–14.5 GHz) operational frequency range. Conventional materials can hardly satisfy both of these requirements.

In this paper, the preparation and testing of epoxy-based composite systems inside the reinforced quartz and aramid (Kevlar) fibers have been concentrated on due to their superior mechanical properties as well as small levels of dielectric constants. The study tackles three different and commonly applied manufacturing methods, hand layup, vacuum infusion molding, and autoclave processing, to draw some conclusions about their impact on the structural and electromagnetic properties of the composite.

The mechanical performance is tested according to ASTM standards, and RF transparency is tested with the help of an anechoic chamber. Comparison of processing routes gives invaluable information on optimizing composite fabrication routes toward the generation of high-tech radome applications out of commercially available materials and processed at scalable methods.

The core objective of this research is to optimize the radome manufacturing process by evaluating the structural and RF performance of different material combinations, refining processing parameters, and analyzing the cost-effectiveness of each approach. This work aims to support the advancement of indigenous aerospace materials for satellite communication, particularly in critical national projects. Through its findings, the study intends to provide a scientific foundation for developing radomes that are not only structurally robust and durable but also offer minimal signal loss, ensuring reliable operation in advanced satellite communication systems.

Materials

The composite materials used in this study were carefully selected based on their mechanical strength, thermal stability, and electromagnetic properties suitable for radome applications. All materials were procured from reputable commercial suppliers to ensure their quality and traceability.

Reinforcement fibers included aramid (Kevlar 49, 99% purity), obtained from DuPont, Wilmington, Delaware, USA, known for its high tensile strength and low dielectric constant. Quartz fiber (HP-12 grade, 99.9% purity) was sourced from Saint-Gobain Quartz, Solon, Ohio, USA, chosen for its superior electromagnetic transparency and thermal resistance. Cyanate ester resin Primaset PT-30 (99% purity) was obtained from Lonza Group AG, Visp, and Valais, Switzerland, which is valued for its low moisture absorption, high glass transition temperature, and stable dielectric properties. All materials were stored under standard conditions as per the manufacturer’s recommendations to preserve chemical stability and mechanical integrity before processing.

Manufacturing Techniques and Sample Preparation

The initial step of composite fabrication required the selection of materials (as shown in Figure ) according to the mechanical and electromagnetic properties. The reinforcement fabrics contained fibers aligned at both 0 and 90° directions to achieve balanced mechanical properties throughout their planar dimension.

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Process of the synthesis of a composite of cyanate ester with Kevlar and cyanate ester with quartz.

Three different methods were used to manufacture composite laminates through hand layup, vacuum-assisted resin infusion (VARI), and autoclave processing, in which 60% is fiber consumption and 40% is matrix consumption. The hand-laid method required manual resin impregnation of woven fabric layers before they were placed inside a mold. Air bubbles were removed, and layers were pressed together through the servicing of the vacuum bagging procedure. In the case of the hand layup process, the curing of the composite takes place at room temperature, and it takes 24 h. To enhance thermal and mechanical properties, the postcure cycle is used at 60 °C for 2 h and 80–100 °C for 2–4 h. Vacuum bagging may be used to reduce air entrapment, but the process operates under atmospheric pressure. Hydraulic resin infusion proceeded by positioning dry reinforcement fabrics inside molds, which contained peel ply and flow media, and then subjecting the combined components to vacuum pressure reaching near −0.8 bar. During the infusion process, the vacuum pressure continued to operate for one more hour to ensure full fiber-matrix saturation. During autoclave processing, the cyanate ester resin prepress combined with fibers was merged inside vacuum bags before entering a high-pressure autoclave at 3–5 bar, which enhanced the reinforcement alignment and minimized material imperfections.

The production of composite panels included standardized curing protocols for every fabrication approach to maintain uniformity. The curing process occurred through either programmable autoclave treatments. All composite samples were cured to a standard thermal profile, which consisted of a temperature ramp of 2 °C/min to 180 °C and a dwell of 2 h at 180 °C. In the case of oven curing (applied in hand layup and infusion molding), the external pressure was not applied; the samples were put into a regular air circulation oven. Conversely, during autoclave processing, the same ramp and hold were used, yet it was at a constant pressure of 0.7 MPa at all times during heating and dwell. Also, a vacuum was used before curing in infusion and autoclave techniques to eliminate trapped air and the least amount of voids. The major difference is that external pressure is used in the autoclave processing, which increases the level of compaction, wetting of the fiber, and removal of the void. The heating process occurred at 2 °C/min until the temperature stopped at 100 °C and was held for 0.5 h. The second temperature increase led to an increase to 160 °C at the same rate while keeping the resin in the cyanate ester state at this temperature for more than 2 h to complete polymerization. The controlled cooling process at 1.5–2 °C per minute was used to decrease the temperature from autoclave conditions to room temperature, thus avoiding thermal shock and internal stress formation.

The test specimens required finishing processes after their curing stage was complete. The conventional milling process is used to produce composite sheets with a uniform thickness of 2.5 mm. The tests required sizes of the panels that were produced from these panels through the utilization of precision cutting tools that adhered to ASTM standards, as shown in Figure . All specimens for mechanical testing received three specific dimensions: tensile required 250 × 25 × 2.5 mm sizes, and impact needed 63.5 × 12.7 × 3 mm, while the flexural test utilized 127 × 12.7 × 3.2 mm dimensions. The evaluation of physical properties involving density, along with porosity and matrix-to-reinforcement ratios, used test specimens cut to 300 × 300 × 2.5 mm dimensions.

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Specimen geometry for mechanical and physical testing. All samples were fabricated with a fiber volume fraction of 60% and a matrix volume fraction of 40%.

Characterization and Testing

The crystalline nature of the silica composite was characterized by XRD. X-ray diffraction analysis was performed using a Bruker Model D8 Advance (manufactured by Bruker Corporation in the USA) for pattern measurement with a Cu Kα radiation source (λ = 1.5406 Å). The scanning range was 10–80° (2θ), and the data were employed in the identification of the crystalline phase in the composite. To evaluate the functional groups of the composite, FTIR spectra of the samples were obtained. The sample spectra were recorded between 4000 and 400 cm–1 using a model number PerkinElmer Spectrum 100 FTIR spectrometer. All samples had uniform dimensions (10 × 10 × 1 mm) to ensure consistency in optical path length and transmittance. The surface morphology and particle distribution of the composite were analyzed using SEM and EDX. High-resolution images of the coatings were taken using a model number JEOL 6390 SEM (manufactured by JEOL Ltd., Tokyo, Japan). The EDX analyses involved an acceleration voltage of 15 kV and a high vacuum value. Tests were conducted according to ASTM standards to ensure reproducibility and reliability. Tensile properties were measured following ASTM D3039M-17 using a universal testing machine (model ID: 24700IE016) at a cross-head speed of 2 mm/min. Specimens were tested under ambient conditions (28 °C and 73% humidity) with a gauge length of 100 mm and a cross section of 3 mm × 3 mm.

Impact resistance was evaluated using an ASTM D256-10 via a Charpy impact test setup (model ID: 24700IE008). The specimens were notched (U-type, 2 mm depth) to induce a stress concentration, and dimensions were maintained at 63.5 × 12.7 × 3 mm. Flexural performance was assessed under ASTM D790-17 using a three-point bending configuration with span lengths between 32 and 56 mm and a span-to-depth ratio of 16:1. The universal testing machine (model ID: 24700IE017) was operated at a cross-head speed of 5 mm/min.

Additionally, density measurements were performed using Archimedes’ principle to calculate mass-to-volume ratios, ensuring consistency in structural assessments. Electromagnetic performance was tested in an anechoic chamber across the 10–14.5 GHz range using standard gain horn antennas equipped with dielectric lenses to simulate plane wave conditions. Insertion loss was measured using a vector network analyzer (VNA), and dielectric properties were calculated using the Nicholson–Ross–Weir (NRW) method based on S-parameter data. Calibration was conducted using a reference material to compensate for system losses. Measurements were repeated 3 times per sample to ensure repeatability, with time-gated signal analysis applied to minimize reflections and alignment errors. The overall measurement uncertainty was maintained within ±0.05 dB. Additionally, density was measured using Archimedes’ principle to support structural assessment and correlate material composition with electromagnetic behavior.

Results and Discussion

XRD was employed to determine the type of crystalline structure and the distribution of the different phases present in the cyanate ester composites reinforced with Kevlar and quartz fibers. Figure reveals that both composites display separate diffraction peaks; however, the more intense and pointed peaks in the cyanate ester/Kevlar composite (red) indicate greater crystallinity than that seen in the cyanate ester/quartz composite (black), where the peaks are broader and not as strong. The prominent diffraction peaks for cyanate ester/Kevlar composites are found at approximately 2θ = 18.5, 20.5, and 22.7°, corresponding to the ordered molecular packing of the Kevlar polymer chains and intense intermolecular hydrogen bonding. , These prominent peaks indicate that Kevlar fibers, when combined with the cyanate ester matrix, enhance crystalline domain formation because of their semicrystalline nature and well-aligned molecular chains. On the other hand, the composite based on quartz shows broader peaks, reflecting a higher degree of amorphous structure. , Quartz fibers themselves are naturally amorphous or weakly crystalline in polymeric matrices and tend to cause disruption of the ordered packing and, hence, result in a majority amorphous phase in the composite. The sharper and stronger the peaks in the XRD, the more crystalline the compound is, the bigger and better-polarized the crystalline domains are, and the fewer defects in the composite structure. This is normally due to the result of a better processing environment (e.g., autoclave curing) that results in better axe alignment and fewer voids, resulting in steeper and higher intensity diffraction patterns.

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XRD patterns of cyanate ester composites reinforced with Kevlar and quartz fibers.

XRD results confirm that the reinforcing material chosen for the composite impacts its crystal structure. Kevlar forms crystals more easily, which may strengthen the material and give it greater thermal stability, but quartz offers a less hard and flexible structure.

We use FTIR spectra in Figure to see the chemical interactions, functional groups, and types of cross-linking that occur in Kevlar and quartz-reinforced cyanate ester composites as they are processed by hand layup, vacuum infusion, and autoclave. Each fiber shows characteristic absorption bands, but their intensity and sharpness change from one type and process to another. The amide groups of Kevlar can be located with the help of FTIR spectroscopy, and their characteristic peaks are under observation. The sharp peaks in the range of 1600–1700 cm–1 are characteristic of the C=C stretching of aromatic rings. Aromatic ring C=C stretching vibrations are found in the FTIR spectrum at 1600–1475 cm–1. Strong, sharp peaks usually appear in the region of 1600 and 1500 cm–1, and this is evidence of aromatic structures having delocalized 4-electrons. Such bands are typical of materials such as cyanate ester and Kevlar, while the peak at ∼1350 cm–1 is characteristic of the C–N stretching, and the peak at about 1200–1250 cm–1 can be related to the C–O–C symmetric stretching, which is indicative of the ether linkages commonly present in cyanate ester polymers. ,

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FTIR of Kevlar- and quartz-reinforced composites of cyanate ester with hand layup, infusion molding, and autoclave processes. The main vibrations are C=C (1600–1700 cm–1), C–N (∼1350 cm–1), C–O–C (1200–1250 cm–1), and N–H stretching (3300–3400 cm–1) in Kevlar composites. Quartz composites show Si–O–Si peaks (1000–1100 cm–1). Sample dimensions were 10 × 10 × 1 mm, tested by using PerkinElmer Spectrum 100 FTIR.

For Kevlar-reinforced composites, there is an extra broad peak in the region of 3300–3400 cm– 1 that arises due to N–H stretching by the amide groups of Kevlar. This peak is more intense in hand layup and infusion, indicative of the partial hydrogen bonding between the Kevlar amide groups and the cyanate matrix. These peaks confirm the amide bond in Kevlar, and the pristine peak can refer to the interaction with the matrix or the processing effect. Autoclave processing tends to weaken this peak marginally, probably due to improved curing and cross-linking, which decreases the free −NH groups. Si–O–Si stretching vibrations at 1000–1100 cm– 1 characterize quartz-reinforced composites. , These are more prominent for autoclave-cured samples, indicating greater interfacial adhesion and improved matrix wetting under conditions of an elevated temperature and pressure. Lower intensity and broader appearance of the same peaks for hand layup and infusion samples indicate lower cross-linking efficiency and interfacial compatibility. The spectra show that there are definite chemical differences in the processing methods. To illustrate, based on the conclusion of the quartz-based composites (purple and yellow curves) in the 1750–1350 cm–1 region, the autoclave-treated sample (yellow) has sharper, more pronounced peaks, especially at 1650 and 1400 cm–1, which denote a more thoroughly cured sample and the formation of stronger C=N or C=C bonds. In aid of this, a magnified representation of this area (1750–1350 cm–1) would be useful in showing chemical differences between autoclave and hand layup. Overall, samples processed in an autoclave show sharper and stronger bands of absorption, reflecting better curing and greater cross-linking density. This attests to the enhanced quality of composites manufactured under controlled temperature and pressure, resulting in better polymerization of the cyanate ester resin. FTIR results established the existence of signature functional groups in the epoxy matrix as well as in the fibers. Curing of the epoxy resin and chemical compatibilities were also attained, as there were crops in the region of −OH, C–O–C, and aromatic C=C stretching peaks. There were no noticeable changes in the peaks, indicating weak chemical degradation or modification of the composites.

Figures and display SEM and EDX analyses of the composites cyanate ester/Kevlar and cyanate ester/quartz made by hand layup, resin infusion, and autoclave.

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SEM micrographs of the cross-sectional area of composite samples, captured on a 50 μm scale. The images show the fiber-matrix interface, voids, and resin dispersion. These samples were not fractured but were cross-sectioned for detailed microstructural analysis.

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EDX spectra of cyanate ester composites: (A) cyanate ester/Kevlar composite; (B) cyanate ester/quartz composite.

The SEM images show that both the reinforcement and fabrication techniques affect the way fibers are placed, resin is absorbed, and fusion takes place at the interface. The structure of cyanate ester/Kevlar composites made using the hand layup method appears random and disoriented, with lots of voids and resin-surrounded spots. From the fiber pullout and gaps between the matrix and the reinforcement, it is clear that the Kevlar fibers do not get properly wet by the resin, leading to limited bonding. Alternatively, resin infusion results in better fiber alignment, but there are still some voids and fiber-matrix separation. The structure formed in the autoclave process is the densest, since it has the fewest voids and the most successful encapsulation of the fibers by the matrix material. −

Using the hand-laid-up method in cyanate ester/quartz composites leads to unscaled fibers and imperfect contact with the resin. Since the surface is uneven and rough, the resin has not penetrated as expected. With the help of the infusion method, the fiber is stuffed more densely, and the resin covers more, even though some small voids remain uncovered. Using the autoclave method results in carbon fiber material with a compact and even structure, tightly bonded fibers, a polished surface upon fracturing, and almost no voids. High pressure and temperature in both systems in the autoclave give the best shape of the fibers with the matrix, making it clear that these factors help produce the best fiber-matrix interaction. ,,

EDX analysis was carried out to verify the main elements present in the two composite materials. The cyanate ester/Kevlar composite is displayed in spectrum 6A, while the cyanate ester/quartz composite is in spectrum 6B. For spectrum 6A (cyanate ester/Kevlar), the high peaks belong to carbon (C), oxygen (O), and nitrogen (N), which indicate both the Kevlar’s aromatic polyamide structure and the organic properties of the cyanate ester resin. Aluminum (Al), silicon (Si), and potassium (K) can enter a sample during processing or sample handling. Since the composite is rich in organic matter, high carbon is present, and nitrogen indicates the presence of Kevlar’s amide groups. Kevlar is high in nitrogen since it has amide groups (H–C–N–H), which have atoms of nitrogen in them. This proves the existence of Kevlar and the effective inclusion of fibers in the matrix.

Silicon and oxygen are the main elements detected in spectrum 6B (cyanate ester/quartz), signaling that this mix is reinforced by quartz (SiO2). Although the organic matrix contains carbon, its level relative to spectrum 6A is much lower. The characteristically strong Si peak proves that the quartz fiber was used and that the main element in quartz is silica.

The enhanced distribution and bonding of the autoclave and infusion specimens can be associated with more uniform elemental distribution, indicating greater resin infiltration and compatibility between the matrix and reinforcement. − The two types of composites in Figure differed when analyzed by using EDX spectra. Carbon, nitrogen, and oxygen are predominant in the cyanate ester/Kevlar composite, with strong peaks indicating its organic composition of the Kevlar and the resin. On the contrary, the cyanate ester/quartz composite shows the prevalence of the silicon peak along with oxygen, which means that there is quartz (SiO2). In the quartz sample, the nitrogen level is quite low, whereas the silicon content in the Kevlar composite is low. Such a dispersion proves the unique chemical composition of every reinforcement cementation.

Cyanate ester composites with Kevlar and quartz fibers have specific density trends linked to both fiber reinforcement and the manufacturing process. As depicted in Figure b, quartz composite parts display higher densities in hand layup, infusion molding, and autoclave than Kevlar-reinforced composite (as shown in Figure a) parts in all cases. This development is consistent with the natural structure, as the density of quartz fibers is higher (2.65 g/cm3) than that of Kevlar fibers (1.44 g/cm3). Autoclave-curing samples gave the highest density in both cases, coming closely behind infusion molding, and hand layup gave the lowest density value of the three. The high density enhances the mechanical strength through reduction of porosity, but with little decrease in the electromagnetic transmittance because of an increased dielectric loss. The reason for this is the dispersion of fibers, consolidation of the matrix, and a reduction in air bubbles in autoclave processing, which takes advantage of the temperature and pressure. Improved compaction closes many interstitial voids and helps the fibers stick tightly to the matrix, which increases the overall material density. As a consequence, combinations of reinforcement type and processing method dictate the final density, and this density can impact mechanical and thermal features. ,−

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(a) Density of cyanate ester/Kevlar composites; (b) density of cyanate ester/quartz composites fabricated using different processing techniques: hand layup, infusion molding, and autoclave. Each processing method involved the fabrication of 10 samples per composite system. The error bars represent the standard deviation, indicating the variation in density values among the 10 independently prepared samples for each method.

The mechanical properties of cyanate ester composites, shown in Figure and Table , are largely affected by both the type of reinforcement and the method used for processing. They were tested mechanically under room temperature to guarantee an application in radome situations that are conventional in service circumstances, since they are mainly subjected to ambiences. During all of the manufacturing processes, the tensile strengths of Kevlar-reinforced composites were always higher than those of quartz-reinforced ones. With autoclave treatment, cyanate ester/Kevlar composites reached a maximum tensile strength of 1005 MPa and had an elongation at break of 9.7%, compared to quartz, whose strongest point was 895 MPa with a stretch of 8.9%. The trends are similar in hand layup and infusion molding, with Kevlar composites being about 60–100 MPa stronger than quartz-based composites. However, compared to Kevlar, quartz fibers are stiffer but brittle, which results in lower tensile performance and leads to earlier material breakdown. Furthermore, different processing methods produce different results: Autoclave processing gave the strongest mechanical features, while infusion and hand laying were next. Higher temperatures and pressures in the autoclave process result in improved fiber-matrix bonding, lower void contents, and improved resin flow. This culminates in composites that are stronger and more ductile, as well as better from the standpoint of overall mechanical performance when compared to the alternative techniques. It demonstrates the degree of fiber wetting, resin access, and removal of voids with every technique. Because the autoclave uses heat and pressure, composites made with it have better tensile strength and elongation. According to these findings, both what materials are used and how they are made contribute to a composite’s mechanical performance, which is vital in certain structural and high-performance applications. ,−

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Tensile stress–strain curves of cyanate ester composites processed using three different fabrication techniques: hand layup, infusion molding, and autoclave. (a) Cyanate ester/Kevlar composites. (b) Cyanate ester/quartz composites. Each processing method involved the fabrication and testing of three individual samples per composite system. The curves represent the average tensile response, and the error bars show the standard deviation across the three replicates for each method.

1. Tensile Strength and Elongation at Break of Cyanate Ester Composites Reinforced with Kevlar and Quartz Fibers Processed by Hand Layup, Infusion Molding, and Autoclave Techniques.

  cyanate ester/Kevlar
 cyanate ester/quartz
processing tensile strength (MPa) elongation at break (%) tensile strength (MPa) elongation at break (%)
hand layup 975 9.3 875 8.5
infusion molding 915 8.8 845 8.3
autoclave 1005 9.7 895 8.9
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a

SD = ±12.45.

b

SD = ±0.42.

c

SD = ±12.12.

d

SD = ±0.5.

Figure and Table demonstrate that composites made with cyanate ester/Kevlar absorb much more energy during impact than those made with cyanate ester/quartz, regardless of the processing method. Out of the three Kevlar-based samples, the impact energy measured was highest for the autoclave-processed composite, slightly lower for the hand layup, and lowest for the infusion-molded sample. Compared to the other groups, cyanate ester/quartz composites achieved much lower impact energies: between 3.00 and 3.95 J, with their highest level produced by the autoclave method. Such differences are due to the characteristics of the fibers. Kevlar is an aramid fiber that is tough and strong and not easily broken because of its very ordered polymers and many hydrogen bonds. As a result of its structure, Kevlar handles an impact by releasing energy more rapidly than other materials. Unlike E-fibers, quartz fibers are sturdy but do not bend enough to reduce the energy involved in causing damage. − Moreover, using an autoclave improves the bond between the fiber and the matrix, reduces empty spaces, and boosts the ability of composites to bear and resist loads, following reports in the literature. , Consequently, the selection of materials and their preparation play major roles in determining the impact of the composite.

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Impact energy (J) of cyanate ester/Kevlar and cyanate ester/quartz composites processed by hand layup, infusion molding, and autoclave.

2. Mechanical Properties of Cyanate Ester/Kevlar and Cyanate Ester/Quartz Composites Processed Using Hand-Laid, Infusion Molding, and Autoclave Methods. Data Include Impact Energy (J), Maximum Deflection (mm), and Flexural Strength (MPa) with Corresponding Standard Deviations.

  cyanate ester/Kevlar
 cyanate ester/quartz
processing impact energy (J) max deflection (mm) flexural strength (MPa) impact energy (J) max deflection (mm) flexural strength (MPa)
hand layup 8 4.20 229.44 3 2.89 105.98
infusion molding 7.8 4.32 240.75 3.2 2.09 165.35
autoclave 8.65 4.95 295.52 3.95 4.98 188.12
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a

SD = ±0.12

b

SD = ±0.05.

c

SD = ±15.67.

d

SD = ±0.042.

e

SD = ±0.012.

f

SD = ±10.98.

The flexural strength results shown in Figure indicate that cyanate ester/Kevlar composites also exhibit better flexural properties than their quartz counterparts for all types of processing. The autoclave-processed Kevlar composite, however, had the highest flexural strength at 295.52 MPa, followed by infusion molding (240.75 MPa) and hand-laid (229.44 MPa). Conversely, the quartz-reinforced samples recorded lower results, with the autoclave sample recording 188.12 MPa, infusion molding at 165.35 MPa, and hand layup at 105.98 MPa. The enhanced flexural strength in the Kevlar-based systems can be explained in terms of Kevlar’s high modulus and fiber continuity, which contribute to higher resistance against bending and crack growth under stress. Moreover, autoclave processing provides the best resin curing, void reduction, and fiber-matrix interface improvement, all of which play roles in enhanced stress distribution and bending capability. Quartz fibers, however, being more brittle and microcracking prone under flexure stress, have decreased capability in the bending condition. ,, Gibson and Callister demonstrated through literature that the structure and bonding of fillers, the material matrix, and flaws produced during manufacture are important to the strength of polymer matrix composites. For this reason, cyanate ester/Kevlar compositesmainly made using an autoclaveare preferred when a high bending strength is needed.

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Ternary plot representing the normalized mechanical properties (tensile strength, flexural strength, and load) of the cyanate ester/Kevlar and cyanate ester/quartz composites. All values are unitless and normalized concerning the maximum value in each category.

The electromagnetic analysis of the six composite specimenscyanate ester-Kevlar and quartz fiber-reinforced, processed by autoclave, infusion molding, and hand layupillustrates a distinct effect of the fiber type and processing procedure on their dielectric response and insertion loss over the Ku-band (10–14.5 GHz) in Figure . For all of the samples, the Kevlar-autoclave composite had the best results, with an expected insertion loss between 0.4 and 0.6 dB and a loss tangent (tan δ) between 0.004 and 0.006. The values are well below the usual 1 dB expected for aerospace radome use, confirming the radome’s excellent performance. The low dielectric constant of Kevlar (∼3.5) and built-in stability, together with the high integrity of autoclave processingaveraging uniform resin distribution, maximum wetting of the fibers, and zero voidssubstantially lower dielectric discontinuities, scattering, and energy absorptionresulting in very little signal attenuation. Kevlar composites processed by infusion molding and hand layup exhibited moderately elevated insertion losses, approximated at 0.6–0.8 and 0.8–1.1 dB, respectively. This trend is also related to a gradual rise in the effective dielectric constant and loss tangent (up to ∼0.010) with rising void content, resin-rich areas, and nonuniform fiber distribution. These microstructural defects, more common in hand layup processes, introduce localized dielectric mismatches and scattering centers that enhance energy loss in the transmission of high-frequency signals.

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Comparison of the dielectric loss tangent (tan δ) and estimated Ku-band insertion loss (dB) for cyanate ester composites reinforced with Kevlar and quartz fibers, fabricated using autoclave, infusion molding, and hand layup techniques. Each data box represents the results obtained from 25 independent samples (n = 25).

Quartz-reinforced composites had higher dielectric constants (∼3.7–4.0) and similar or greater loss tangents based on the processing method. Since quartz is lower in dielectric loss and autoclaving is more effective, the quartz-autoclave sample had a tan δ of 0.003–0.005 and an insertion loss of no more than 0.7 dB. Yet, insertion losses measured 0.7–0.9 dB in quartz-infusion samples and 1.0–1.3 dB in quartz-hand layup samples. Although quartz fiber dielectric properties are good, since they are denser and more brittle, they are more likely to have micro cracks and an uneven surface in manual layups or rapid infusion, which can lead to more electromagnetic energy scattering and loss. − Dielectric breakdown testing was performed to assess electrical safety, revealing the maximum electric field that the composites can withstand before failure, thereby supporting their suitability for high-voltage and radome applications.

The autoclave-processed samples had superior electromagnetic behavior than their hand layup and infusion equivalents, but one sample, the Kevlar-autoclave composite, stood out from the rest with the lowest insertion loss and loss tangent as well as structural integrity. The results render it the best choice for high-end radome applications, where low signal attenuation, high strength, and dielectric stability are paramount.

Conclusions

The manufacture of Kevlar- or quartz-reinforced cyanate ester composites was carried out using hand-latch-up, infusion molding, and autoclave techniques. The Kevlar composites demonstrated acute crystalline peaks at 2 theta = 18.5, 20.5, and 22.7, whereas the quartz composites showed broad amorphous patterns. All samples displayed C=C stretching (1600–1700 cm–1), C–N stretching (1350 cm–1), and C–O–C stretching (1200–1250 cm–1), which were confirmed by FTIR; Kevlar composites showed an additional N–H band at 3300–3400 cm–1 that became slightly less intense after autoclave curing, suggesting further cross-linking. Mechanical tests ranked Kevlar-autoclave the highest, with tensile strength = 1005 MPa, elongation = 9.7%, impact energy = 8.65 J, max deflection = 4.95 mm, and flexural strength = 295.5 MPa. Kevlar-hand layup and Kevlar-infusion followed at 975 MPa/9.3% and 915 MPa/8.8%, respectively. Quartz composites trailed, with the best being quartz-autoclave (895 MPa/8.9%, impact = 3.95 J, flexural = 188.1 MPa). Quartz composites came second, with the highest being quartz-autoclave (895 MPa/8.9%). These trends were echoed in strain capacity, which showed greater fiber chain bonding and reduced pore volume in autoclave laminates. At 1014.5 GHz, electromagnetic measurements revealed that Kevlar-Autoclave had the least insertion loss (0.40.6 dB) and loss tangent (0.004–0.006); Kevlar-infusion and Kevlar-hand layup had 0.6–0.8 and 0.8–1.1 dB, respectively. Quartz/autoclave reached 0.7 dB (tan/delta = 0.003–0.005), but quartz/infusion and quartz/hand layup increased to 0.7–0.9 and 1.0–1.3 dB, respectively. These findings were also confirmed by SEM cross sections, i.e., the Kevlar-autoclave exhibited such characteristics of a dense, well-wet laminate with few microcracks and voids; the hand layup and infusion options had more porous laminates and microdefectives, particularly in the quartz samples. The Kevlar/cyanate ester composite treated with the autoclave was found to provide optimal crystallinity, microstructural integrity, mechanical strength, ductility, and Ku-band electromagnetic transparency, and, hence, it served as the optimal choice as compared to other composites in the advanced radome applications.

These results suggest that an autoclave treatment of Kevlar-strengthened composites of cyanate ester may provide the perfect properties of mechanical strength and electromagnetic transparency. As such, they are very well suited to high-performance aerospace radome applications, in which strength, durability, and signal fidelity are the primary considerations.

Limitations

The test of this study was constrained to an ambient test without environmental or thermal cycling. The sizes of samples used in mechanical and RF testing were comparatively small, and the fact that the results were obtained using different batches may limit the scope of the applicability. Future research would involve the utilization of a greater number of data, environmental testing in controlled environments, and dielectric breakdown testing of the environment to better understand the operational environment.

Acknowledgments

The authors would also like to acknowledge the Department of Metallurgical Engineering, NED University of Engineering and Technology, Karachi, Pakistan, for their overall support. In addition, we must thank the Department of Chemical Engineering and the Department of Metallurgy and Materials Engineering at Dawood University for their support, too. Also, thanks to Dr. Sohaib Akbar for his valuable guidance and assistance, particularly in composite synthesis, etc., throughout this project.

The data underlying this study are available throughout the manuscript.

Conceptualization, A.W.B.; methodology, S.A.K.; data curation, S.A.K. and I.M.; writingoriginal draft preparation, S.A.K.; supervision, A.W.B. and I.M.; project administration, A.D.C. All authors have read and agreed to the published version of the manuscript.

The authors received no funding for this work. It is a self-funded project.

The authors declare no competing financial interest.

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Data Availability Statement

The data underlying this study are available throughout the manuscript.

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