High-temperature Composites Research Articles

Preform-based toughening technology for RTMable high-temperature aerospace composites

This article describes the efforts that led to the development of surface-loaded preforms that may be used to significantly improve the compression-after-impact strength of high-temperature composites and correspondingly to dramatically reduce the area of damage because of impact. Moreover, by matching the toughening polymer surface-loaded and design of the surface pattern, in-plane mechanical properties are unaffected or even improved over laminates made from the identical materials. The proprietary preforms, so-called ES™-Fabrics, may be handled and infused with the high-temperature RTMable resins such as bismaleimide and polyimide in exactly the same manner as traditional fabrics without surface modification. The RTM conditions for the preform-based toughening is fully compatible with the traditional process procedure, making the technology cost-effective in production. This technology represents a key enabler for the use of low-cost RTM processes for high-temperature resins to supplant prepreg as the building-block material of choice for aeronautical composite structures.

Preparation and Properties of MgAl<sub>2</sub>O<sub>4</sub>-CaAl<sub>12</sub>O<sub>19</sub> High Temperature Composite

The utilization of lightweight refractories plays an important role in reducing the energy consumption of industrial furnaces. In this paper, MgAl2O4-CaA112O19 high temperature composite was synthesized via solid state reaction using magnesite, dolomite and industrial alumina as raw materials. The influences of raw materials and reaction temperature on phase compositions and microstructure of the composite were investigated by XRD and SEM，respectively. The parameters to prepare MgAl2O4-CaA112O19 high temperature composite were optimized. The results show that the optimum reaction conditions for synthesizing MgAl2O4-CaA112O19 composite is the CA6/MA weight ratio of 2:3, and the reaction temperature of 1500°C for 4h. The CaA112O19 crystals showed laminated or plate-like structure, and the MgAl2O4 showed spherical morphology. The reaction temperature had little effect on the phase compositions of MA-CA6 composite in this experiment. The content of Al2O3 in the raw material affected the phase composition of MA-CA6 composite.With the increase of the CaA112O19 amount, the bending strength of the composite decreased.

The degradation behavior of C/SiC composites in complex environment

Abstract The advanced high temperature composites have been considered as structural or functional material in complex and harsh environments, which usually comprise of various oxidizing particles such as molecular oxygen (MO), atomic oxygen (AO) and molecular water. This study aimed to investigate C/SiC behaviors in single factor or combination environments. In the MO oxidation, an inert surface is formed on SiC. At the AO oxidation, the Si of SiC is preferentially etched off leaving a certain depth of C layer on the sample. Two types of coupled oxidations defined as mode A (first MO and then AO oxidation), and mode B oxidation (first AO and then MO oxidation) were investigated systematically in this study, and found that the prior oxidative effect majorly determined the corresponding coupled oxidation behavior. From microscopic observation and fractural test, the mode B oxidation induced a more serious degradation upon the material than that of the mode A, which is illustrated by the proposed erosion mechanisms for the modes A and B.

Phase behavior of serpentine mineral by carbothermal reduction nitridation

In this paper, the effects of reductant addition and reaction temperature on phase behavior of serpentine by in situ carbothermal reduction nitridation (CRN) treatment were studied. A brief discussion on the synthesis mechanism of CRN process was also included. The results show that temperature and carbon content were two essential factors that determined the nitridation of serpentine and the formation of β-Si3N4 grains. Serpentine could be transformed into high-temperature composites containing Mg2SiO4, β-Si3N4, Si2N2O and β-SiC at temperatures higher than 1400°C. At 1550°C, more Si2N2O was nitrided to β-Si3N4 as the increase of carbon content, in the final products of samples with carbon addition excess 100wt.% of theoretical quantity, Mg2SiO4 coexisted with β-Si3N4 as the main crystal phases. This transformation provided a feasible way of dealing with serpentine waste for high-temperature utilization.

Global Patent Scenario of Materials: An IP Analysis

There is an ever increasing demand and innovation worldwide in the area of materials research in aerospace industry. Research on materials is the focused area in aerospace sector because reducing aircraft/spacecraft weight and/or increasing the temperature capabilities is the key factor in increasing the performance. Hence there is always an urge on the development of advanced materials especially in the area of light weight, High temperature resistant materials. There are different ways of keeping abreast on current state of art on the development of advanced materials namely reading Journal publications, attending conferences/seminars or performing intellectual property (IP) analysis specifically looking at patents. Patent portfolio indicates the technology and research direction of a company. Intellectual property plays a key role in the present global competitive world. It is not only an asset for the company but also helps in increasing the bottom line of an organization. Hence, an attempt was made to look at the global patent scenario of materials used for aerospace industry. A high level scan of aerospace materials patent portfolio in the past 20 years was done to understand the current technology focus and trend of materials research. The study was restricted to Aerospace/Aviation Players. Four different types of Materials are considered for the study namely Light weight metals (includes Al and Ti alloys), Superalloys, High temperature coatings and Composites. A further deep dive analysis on the above materials has been done to understand the Industry focus on various technology areas such as Manufacturing, Inspection, Application etc. This analysis helps technologists in understanding the current state of art and research focus in Materials area specific to Aerospace/Aviation Industry

Synthesis and sintering of high-temperature composites based on mechanically activated fly ash

Amount of fly ash which is and yet to be generated in the coming years highlights the necessity of developing new methods of the recycling where this waste can be reused in significant quantity. A new possibility for fly ash utilization is in high-temperature application (thermal insulators or/and refractory material products). As such, fly ash has to adequately answer the mechanical and thermal stability criteria. One of the ways of achieving it is by applying mechanical activation procedure on fly ash. In present study, fly ashes from two different power plants were mechanically activated in a planetary ball mill. Mechanically treated fly ashes were cemented with two different binders: standard Portland cement and high-aluminates cement. Physico-chemical analysis and investigation of mineralogical components of composites are emphasized, due to the changes occurred in fly ash during mechanical activation and sintering of composites. Macro-performance of the composites was correlated to the microstructure of fly ash studied by means of XRD and SEM analysis. Thermal stability of crystalline phases was investigated with DTA. Highlight was placed on determination of relationship between mechanically activated fly ash and obtained composites microstructure on one side and behavior of sintered composites on the other side.

Potential Composite Structures for NASA Future Launch Vehicles and Crew Spacecraft

Because of their well known specific strength, stiffness and excellent durability properties, advanced composites are being considered for primary structures in launch vehicles, crew modules and various components for increased performance and cost reduction. Therefore, it is prudent and beneficial to review the engineering practices and lessons learned in connection with their use in related aeronautic and energy applications, where they are already replacing formerly used metallic materials. Examples of composite components will be shown for the reusable space shuttle orbiter where a number of different composite systems performed very satisfactorily. In addition, very large potential polymer composite designs for future launch vehicles will be discussed. Among them are payload shrouds, interstage structures and the typical intertank shell, wherein thrust booster rockets are often attached between the core stage propellant and oxidizer tanks. In addition, cryogenic propellant composite vessels of different sizes and shapes were fabricated with mostly excellent results, although some spectacular failures were also observed. High pressure composite overwrapped vessels, with and without metallic liners, will be described. Compared to widely used metallic materials, some special features of composites are listed. Relevant design allowables, depending on mission requirements, will be summarized and currently used design practice for aircraft and spacecraft in the US will be reviewed. The well-known “building - block” approach, which is often used to design military as well as civilian aircraft, will be summarized. Some of the most popular micromechanics and macromechanics computer programs used to analyze composite structures, especially with finite elements, will be listed. Although very high temperature composites like carbon/carbon, carbon/ceramic and ceramic/ceramic fiber/matrix systems are also selectively used, the focus in our discussion will be on advanced polymer matrix, carbon/glass fiber systems.

Characterisation of thermal barrier coatings and ultra high temperature composites deposited in a low pressure plasma reactor

A low pressure plasma process working at 600–800 Pa was used to deposit from aqueous solution ZrO 2–4 mol% Y 2O 3 (Yttria partially stabilized Zirconia–YpSZ) layers and stacks of Ta 2O 5/YpSZ layers for use as thermal barrier coatings (TBC). The observation of the cross section revealed a high porosity. The thermal diffusivity of the layers (1 × 10 −7 m 2 s −1) was measured by a laser flash technique and compared with values obtained on air plasma sprayed material (3 × 10 −7 m 2 s −1). The plasma reactor were also used to deposit ZrB 2–ZrO 2–SiC layers used as Ultra High Temperature Composite (UHTC) from aqueous solutions of zirconyl and Boron nitrates containing suspensions of SiC. Layers up to 100 μm thick were obtained on SiC substrates. XRD was used to study the crystallinity of the layer. The presence of ZrB 2 and SiC phases was confirmed after the deposition. XRD analysis showed that heat treatment at 1073 K under oxidizing conditions led to the loss of ZrB 2 and the appearance of ZrO 2 phases. To understand the behaviour of the layers to interaction with atomic oxygen (combustion for TBC and spacecraft re-entry phase for UHTC), we have measured the atomic oxygen recombination coefficient to determine the number of adsorption sites on the surface of the coatings. This was accomplished by using a low pressure plasma reactor coupled with optical spectroscopic measurements as a diagnostic technique.

Mechanical properties of high-temperature composites made from mixtures of tungsten-coated and uncoated NiAl intermetallide granules

387 An interesting possibility is the production of high� temperature composites with a honeycomb structure by sintering powder based on NiAl (or simple alloys based on NiAl) when each grain is coated with a thin layer of tungsten or successive layers of tungsten and molybdenum [1]. We may note the following research findings: ⎯nonuniform thickness of the tungsten coating on an individual grain (in the limiting case, no coating on a section of the grain’s surface); ⎯rupture of the coating in technological processes associated with deformation; ⎯breakdown of the continuous tungsten film to individual droplets, in the form of a tungsten alloy with Ni and Al, formed at temperatures near the melt� ing point of NiAl [2]. These processes may convert the composite from a material with a honeycomb structure to a dispersely hardened material. In the present work, we attempt to determine what proportion of grains with intact coat� ings is sufficient for the composite with an initial hon� eycomb structure to retain its high strength (within the spread of mechanical test data). The loss of honeycomb structure, with degradation to a dispersely hardened material, is simulated by mix� ing NiAl powders with and without tungsten coatings. The grain size of stoichiometric NiAl (obtained by crushing and sifting cast material) is 15–17 μm; the deposited (with carbonyls) tungsten layer on the grains is around 0.5 mm in thickness. In pressing the powders (pressure up to 70000 atm, without a protective atmo� sphere, for 5–10 s at 1350–1400°C), cracks may appear in the tungsten layer on the grains (according to observations on metallographic samples after etching with Marble’s reagent): over the perimeter of a single grain, 0.5–1.5 cracks may be observed. After pressing, the samples are cylindrical (height 5–6 mm; diameter 5–6 mm). The porosity of all the materials is no more than 8 vol %, with sufficiently uniform distribution over the volume. The whole range of compositions is investigated, from 100% pure NiAl to 100% composite with honeycomb structure. Powder mixtures are prepared in a UZDN�2T ultrasound system, by mixing weighed portions (10 g) in ethyl alcohol for 5 min. After complete drying, the material undergoes dry mechanical mixing for 10 min. The monolithic samples obtained are characterized by uniform distribution of the components over the pressed volume. Table 1 summarizes the alloy compo� sition. The mechanical behavior of the materials is studied by measuring the microhardness at 20°C and deter� mining the fracture stress of thin samples (disks) on

Spark Plasma Sintering of ZrB 2–SiC–ZrC ultra-high temperature ceramics at 1800 °C

The compositional effects in ZrB 2–SiC–ZrC ultra high temperature composites with four different compositions were investigated via Spark Plasma Sintering (SPS) at a maximum temperature of 1800 °C. Density, Rockwell hardness, and thermal conductivity were measured, along with structural X-ray diffraction (XRD) and microstructural characterization. The relative amounts of SiC and ZrC had an influence on the composites’ density, mechanical and thermal properties.

Measurement and Distribution Pattern of Temperature Field within the Preform during CLVI

CLVI has the fastest efficient of densification in the preparation of high temperature composites, compared with traditional isothermal CVI technique. In this paper, the internal temperature field of the preform during the process of CLVI was tested synchronously through multiple thermocouples. The change rules of the temperature filed inside the preform was got by analyzing the data obtained from the above test through Origin software. The liquid hydrocarbon keeps the temperature of the outer preform constant at its own boiling point, whereas the inner part of the preform is hot enough to pyrolyze the vaporized carbon precursor, causing a steep thermal gradient through the preform thickness. The thermal gradient along the radial direction in the preform can get about 1000°C during the deposition. In this case, densification occurs preferentially inside the preform, and then outward.

High-temperature (to 1670°C) air oxidation of AlN–Si3N4 and AlN–Si3N4–(Ni–Cr–Al) ceramic materials

The kinetics and mechanism of high-temperature air oxidation of powders and compact materials with different compositions in the AlN–Si3N4 and AlN–Si3N4–(Ni–Cr–Al) systems are studied using nonisothermal (up to 1500°C) and isothermal (1350 and 1670°C) thermogravimetry, DTA, XRD, petrography, and EMPA. During heating, Al4O4–x N x and Si2N2O oxynitrides, α-Al2O3 and α-SiO2 (crystobalite) oxides, and mullite-based Al2O3 ∙ SiO2 solid solution of the boundary composition form during the first oxidation stage, and 3Al2O3 ∙ 2SiO2 mullite forms at higher temperatures (>1350°C) in the upper scale layer and aluminum-nickel chromite spinel in the intermediate (barrier) scale layer. The AlN–Si3N4 and AlN–Si3N4–(Ni–Cr–Al) ceramics may be regarded as hightemperature composites (HTCs) owing to their high corrosion resistance.

Finely dispersed refractory compounds for high-temperature ceramic matrix composite applications

Application of finely dispersed refractory compounds in the creation of high-temperature ceramic matrix composites (HTCMC) is briefly reviewed. Examples of successful combination of nanomaterials and HTCMC technologies are given.

High Temperature VARTM of Phenylethynyl Terminated Imides

Depending on the part type and quantity, fabrication of composite structures using vacuum-assisted resin transfer molding (VARTM) can be more affordable than conventional autoclave techniques. Recent efforts have focused on adapting VARTM for the fabrication of high temperature composites. Due to their low melt viscosity and long melt stability, certain phenylethynyl terminated imides (PETI) can be processed into composites using high temperature VARTM (HT-VARTM). However, one of the disadvantages of the current HT-VARTM resin systems has been the high porosity of the resultant composites. For aerospace applications, the desired void fraction of less than 2% has not yet been achieved. In the current study, two PETI resins, LaRC PETI-330 and LaRC PETI-8 have been used to make test specimens using HT-VARTM. The resins were infused into ten layers of IM7-6K carbon fiber 5-harness satin fabric at 260 or 280 °C and cured at temperature up to 371 °C. Initial runs yielded composites with high void content, typically greater than 7% by weight. A thermogravimetric-mass spectroscopic study was conducted to determine the source of volatiles leading to high porosity. It was determined that under the thermal cycle used for laminate fabrication, the phenylethynyl endcap was undergoing degradation leading to volatile evolution. This finding was unexpected as high quality composite laminates have been fabricated under higher pressures using these resin systems. The amount of weight loss experienced during the thermal cycle was only about 1% by weight, but this led to a significant amount of volatiles in a closed system. By modifying the thermal cycle used in laminate fabrication, the void content was significantly reduced (typically ∼ 3% or less). The results of this work are presented herein.

Effect of yttria on crystallization and microstructure of an alumina–YAG fiber prepared by aqueous sol–gel process

Continuous fiber development is needed for high performance and high temperature composites. Various methods have been used to make ceramic fibers. In this research, composite fibers (yttrium aluminum garnet (YAG)/Al 2O 3) were prepared by a sol–gel method using aqueous solution. They were synthesized from aluminum salt, aluminum metal, yttrium oxide and water used as solvent. Transparent gel fibers were obtained by immersing a thin wire into the viscous sol, then pulling it out by hand. The obtained fibers contained very fine grains with diameter ranging from 10 to 80 μm after heat treatment. When yttria content was increased, the crystallization of YAG shifted to a lower temperature, whereas the transformation temperature to α-Al 2O 3 shifted to a higher temperature. Nevertheless, the fibers with different amounts of yttria contained alumina and YAG after heat treatment at 1400 °C. The composite fibers had vermicular structure and were denser than alumina fibers. The yttria percent concerning the limits of this study (≤10 wt%) effected on fiber diameter. As the yttria content was increased, the fiber diameter increased, whereas grain size and densification of the composite fibers decreased.

Titanium matrix composite lattice structures

Cellular materials made from high temperature composites with efficient load supporting lattice topologies and small cell sizes would create new options for light weight, high temperature structures. A method has been developed for fabricating millimeter cell size cellular lattice structures with a square truss topology from 240 μm diameter Ti–6Al–4V coated SiC monofilaments. The compressive stiffness of the lattice structures was well approximated by that of the monofilament fraction in the loading direction. The strength of the lattices was controlled by elastic buckling and the results were accurately represented by an elastic buckling model. The post peak compressive deformation was accommodated by elastic buckling of the SiC fibers and plastic deformation of the titanium coating until the onset of monofilament fracture. The specific stiffness and strength of the lattices were found to be between 2 and 10 times that of other cellular structures and thus appear to be promising candidates for high temperature, ultralight weight load supporting applications.

Synthesis and High Thermal Stability of Double-Walled Carbon Nanotubes Using Nickel Formate Dihydrate as Catalyst Precursor

Nickel formate dihydrate is demonstrated to be an effective catalyst precursor for selectively synthesizing double-walled carbon nanotubes (DWNTs) with high thermal stability by using a hydrogen arc discharge technique. High-resolution transmission electron microscopy (HRTEM) observations show that the assynthesized DWNTs are of high quality with good structural integrity and narrow diameter distribution (1.983.47 nm and 1.32-2.81 nm, respectively, for the outer and inner diameter). Thermogravimetric analysis (TGA) in air indicates that the DWNTs have excellent oxidation resistance up to 800 °C, approaching to that of graphitized multiwalled carbon nanotubes and higher than that of DWNTs prepared by other routes or methods. Moreover, Raman analysis and HRTEM observations reveal that these DWNTs can keep good structural stability and other carbon species are removed after oxidation at 650 °C in air flow. The formation of DWNTs with high thermal stability is attributed to the cleaning effect of active radicals such as hydrogen and oxygen originated from the decomposition of nickel formate dihydrate and the in situ defect-healing effect induced by high arc current in the arc growth process. This approach opens various application possibilities for DWNTs with high thermal stability to be used in high-temperature composites, nanoelectronic devices, and electron filed emitters operating at high currents, etc.

An alumina–YAG nanostructured fiber prepared from an aqueous sol–gel precursor: Preparation, rheological behavior and spinnability

Continuous-fiber development is needed for high-performance and high-temperature composites. Various methods have been used to make ceramic fibers. In this research, composite fibers (YAG/Al 2O 3) were synthesized from an aqueous solution of aluminum powder, aluminum chloride hexahydrate and yttrium oxide by the sol–gel method. Transparent fibrous gels were obtained by immersing a thin wire into the viscous sol, then pulling it out by hand. Rheological measurements suggested that the sols exhibited spinning when they were viscous and Newtonian or near Newtonian to the time of gelation. The sols became non-spinnable when thixotropic flow behavior was observed. It was observed that the time of hydrolysis–condensation, drawing, gelation and length of the gel fibers decreased with increasing the amounts of either acid or yttrium oxide. The time of hydrolysis–condensation and gelation decreased with increase in the amount of aluminum powder, whereas the time of drawing and spinnability increased. The desired viscosity of the sols for spinning was between 2000 and 5000 P. The dried gel fibers had a smooth surface with diameter ranging from 20 to 200 μm. The composite fiber showed fine-grained microstructure (100–200 nm) after heat treatment at 1400 °C.

Synthesis and High Thermal Stability of Double-Walled Carbon Nanotubes Using Nickel Formate Dihydrate as Catalyst Precursor

Nickel formate dihydrate is demonstrated to be an effective catalyst precursor for selectively synthesizing double-walled carbon nanotubes (DWNTs) with high thermal stability by using a hydrogen arc discharge technique. High-resolution transmission electron microscopy (HRTEM) observations show that the as-synthesized DWNTs are of high quality with good structural integrity and narrow diameter distribution (1.98−3.47 nm and 1.32−2.81 nm, respectively, for the outer and inner diameter). Thermogravimetric analysis (TGA) in air indicates that the DWNTs have excellent oxidation resistance up to ∼800 °C, approaching to that of graphitized multiwalled carbon nanotubes and higher than that of DWNTs prepared by other routes or methods. Moreover, Raman analysis and HRTEM observations reveal that these DWNTs can keep good structural stability and other carbon species are removed after oxidation at 650 °C in air flow. The formation of DWNTs with high thermal stability is attributed to the cleaning effect of active radicals such as hydrogen and oxygen originated from the decomposition of nickel formate dihydrate and the in situ defect-healing effect induced by high arc current in the arc growth process. This approach opens various application possibilities for DWNTs with high thermal stability to be used in high-temperature composites, nanoelectronic devices, and electron filed emitters operating at high currents, etc.

Co-continuous composites for high-temperature applications

The present work demonstrates a new method for creating a high-temperature composite. First, silica is reacted in liquid aluminum. This creates a highly aligned, near single crystal alumina structure that has about 25% open volume that is filled with aluminum. This open space is subsequently re-infiltrated with a refractory metal (NiAl or a nickel alloy), creating an interpenetrating phase or co-continuous composite. Experiments with this material were carried out to assess the degree to which this co-continuous structure (with phases of very different coefficients of thermal expansion) is susceptible to thermal cycling damage. Both the Al–Al 2O 3 and NiAl–Al 2O 3 materials were examined and the former case is compared to an axially reinforced Al–Al 2O 3 composite with a similar alumina volume fraction. The results showed that unlike traditional composites co-continuous composites were quite resistant to thermal cycling damage, and the NiAl–Al 2O 3 showed no thermal cycling damage as noted by no strength loss or cracking, but the elastic modulus of the composite was related to its environmental history (the composite elastic modulus was observed to change by 25%, and in a reversible way). This is postulated as being due to the role hydrogen may have in controlling interface sliding behavior at the NiAl and Al 2O 3 interfaces. Overall this work points to a potentially useful processing route and architecture for high-temperature composite materials.