Vacuum and controlled atmosphere furnaces for hypersonics and other next-generation aerospace materials

Hypersonic missiles and vehicles are an emerging class of aerospace technology that is developing rapidly toward active use in military and potentially commercial applications.

These machines can achieve sustained speeds of Mach 5 or greater within the Earth’s atmosphere, i.e., at altitudes below about 90 km. While conventional intercontinental ballistic missiles can also achieve hypersonic speeds during atmospheric reentry, they follow a high-arching ballistic trajectory with limited maneuverability, in contrast to the real-time in-flight maneuverability offered by hypersonic systems. As such, hypersonic missiles are preferrable for precision strikes,¹ while in the commercial realm, airliners are excited by the possibility of drastically shortened journey durations with hypersonic vehicles.²

Because hypersonic missiles and vehicles move at extreme speeds within the Earth’s atmosphere, they are subject to significant atmospheric compression and friction effects.³ These effects result in considerable aerodynamic heating of the leading edges, nose tips, and exhaust-washed structures, from 1,800°C to more than 3,000°C (3,200°F to 5,400°F).

Traditional aerospace materials such as aluminum, stainless steel, and titanium cannot be used at these elevated temperatures without thermal protection engineering. In contrast, an emerging portfolio of materials including refractory metals, carbon–carbon composites, ultrahigh-temperature ceramics (UHTCs), and ceramic matrix composites (CMCs) can more easily deal with this extreme heat.

UHTCs and CMC materials typically are composed of metal carbides, borides, and nitrides, which means they are traditionally processed at very high temperatures. Currently, the leading candidate materials are silicon carbide (melting/decomposition point: 2,730°C)⁴ and zirconium diboride (melting point: ~3,246°C)⁵ due in part to their reasonable raw material costs.

Processing of UHTCs, CMCs, and other advanced materials for aerospace applications includes one or more of the following high-temperature processing steps, often using vacuum and controlled atmosphere furnace technology:

• Chemical vapor infiltration
• Chemical vapor deposition
• High-temperature sintering
• Graphitization
• Silicon melt infiltration of carbon–carbon composites

Each stage of the product development cycle—from laboratory-scale research and development to prototype development to production-scale manufacturing—requires a portfolio of specialized furnaces to achieve the goals of each stage.

This article takes a closer look at the types of furnace solutions available to develop, process, and commercialize these high-performance materials, with examples pulled from the product offerings of furnace manufacturer Centorr Vacuum Industries (Nashua, N.H.). Laboratory-scale research and development Laboratory-scale R&D activities focus mostly around the development, fabrication, and testing of small-scale parts, which requires a small, adaptable furnace.

Laboratory-scale research and development

Laboratory-scale R&D activities focus mostly around the development, fabrication, and testing of small-scale parts, which requires a small, adaptable furnace.

A furnace design to answer this need is the LF graphite vacuum furnace (Figure 1). First designed in 2012, the LF is a robust, low-cost development furnace with temperature capability up to 3,000°C in vacuum or inert gas. This temperature range covers most hypersonic, UHTC, and other applications. For example, some current customers fit the small 3” (75 mm) diameter × 4” (100 mm) height hot zone with small graphite crucibles to fire graphite-based powders for applications in battery and electric vehicle technology.

Research-based universities such as Dalhousie University in Nova Scotia, Canada, modified the base LF system by adding a small binder/off-gassing trap and positive pressure exhaust tower for processing of nonoxide ceramics produced by additive manufacturing. These samples include silicon-based ceramics (silicon carbide and silicon nitride), high-entropy ceramics, and cermet systems.

Subsequent laboratory applications require a larger hot zone furnace for processing bigger samples, and the Series 10 graphite tube furnace is an economical choice (Figure 2). This tube furnace, which has a hot zone of 4” (100 mm) diameter × 16” (400 mm) height and 3,000°C maximum temperature, is based on a 50+ year old furnace design, although the traditional alumina or quartz tube has since been replaced with a solid graphite tube. Operating in vacuum or partial/positive pressures of argon, R&D centers use this furnace to process carbon powder formulations to maximize the percent conversion to graphite, as not all carbon-based starting materials will convert to crystalline graphite.

As R&D activities begin to focus on particular material compositions, larger furnaces are needed to synthesize meaningful sizes and quantities of candidate materials prior to scaling up for manufacture. The next larger lab furnace in the Centorr portfolio used for carbon/graphite work is the Series 45 graphite top-loading furnace (Figure 3). With a hot zone measuring approximately 6” (150 mm) diameter × 6” (150 mm) height and rated for 3,200°C maximum temperature, it offers a larger useable firing footprint at higher temperatures than the Series 10 furnace.

Characterization and prototyping stage

Once the final candidate materials are processed, aerospace design engineers need to test meaningfully sized samples of the materials at high temperature under mechanical loading. Centorr’s Testorr® line of furnaces can be combined with mechanical test stands for measurement of mechanical properties at high temperature.

For example, Wichita State University’s National Institute for Aviation Research uses multiple Testorr units to measure tension, compression, and shear properties of ceramic matrix composites, refractory metals, and other materials at high temperature. This particular design is rated for temperatures up to 2,700°C in vacuum or inert gas, which allows it to better simulate some aspects of hypersonic service environments (Figure 4).

An important task of the R&D and prototyping stages is to work out processing parameters that will be translated to production-scale manufacturing processes. For example, simple carbon structures will react with air during reentry and suffer damaging effects at temperatures as low as 500°C. Therefore, any carbon–carbon materials or solid carbon shapes used in hypersonic applications must be protected with advanced ceramic coatings for durability and oxidation resistance.

Chemical vapor deposition is one such coating deposition process, and one of the most popular protective coatings is silicon carbide. The coating is deposited on substrate parts by flowing hydrogen gas through a bubbler of liquid methyltrichlorosilane (MTS; CH₃SiCl₃) gas. Newer systems use a heated evaporator to vaporize the MTS liquid in a hydrogen carrier gas stream. The combination of hydrogen and MTS is introduced at partial pressures into the furnace hot zone inside a graphite retort, where the gases “crack” or decompose, depositing microns-thick coatings of silicon carbide onto the part’s surface.

Production stage

Once the advanced materials are properly characterized and prototyped, it is time to look at equipment for full-scale production manufacturing. The furnace configurations for these processes can be either conventional front-loading designs, or for reasons of floor space savings and gas flow dynamics, may be oriented in vertical top- or bottom-loading designs.

The most popular of Centorr’s front-loading systems is the Sintervac® front-loading graphite furnace (Figure 5). This unit has integral graphite retort and dual gas flow to the main chamber and retort. Sintervac furnaces are rated for temperatures between 1,600°C–2,600°C in sizes from 2 ft³ to 135 ft³ (56 to 3,500 liters). These furnace systems include durable rotary piston pumping systems with inline binder traps and particulate filters to protect the pumping systems from damage from abrasive ceramic particulates. The internal graphite retort compartmentalizes the off-gassing that takes place and prevents it from escaping into the hot zone, where the oxide byproducts can attack and degrade the graphite heating elements and rigid graphite board insulation.

One common application for this type of furnace is melt infiltration of carbon–carbon composites to improve the physical properties and oxidation resistance of the composite. When processed in partial pressures (or even at positive pressures) of argon, silicon will melt at approximately 1,450°C. The silicon liquid and vapor infiltrate into the void spaces of the porous carbon–carbon composite via capillary action. The infiltrated silicon reacts with the free carbon in the carbon–carbon fiber structure, forming a silicon carbide matrix around the carbon–carbon fiber structure.

Firms such as Exothermics (Amherst, N.H.) use this process for missile and aerospace applications. The silicon carbide matrix structure provides an environmental barrier to oxidation during reentry into Earth’s atmosphere and improves the matrix’s temperature performance to approximately 1,600°C in air.

Smaller production units were also developed for carbon–carbon work at temperatures from 2,450°C and 2,600°C. The addition of dedicated water-cooled filtration traps and 10-µ particulate filters helps deal with the heavy off-gassing expected from processing of carbon–carbon materials.

In contrast to melt infiltration, chemical vapor infiltration drives gaseous reactants into the porous matrix where the gas reacts with the porous structure to form a dense matrix. The chemical vapor infiltration process is used to fabricate larger parts for hypersonics, such as rocket motors and missile components, and carbon–carbon aircraft brakes. For these applications, Centorr designed a line of vertical top- and bottom-loading chemical vapor infiltration units (Figure 6), with sizes ranging from 52” to 80” (1,320 mm to 2,000 mm) in diameter with heights from 80” up to 120” (2.0 to 3.0 meters).

In the chemical vapor infiltration process, gases including hydrogen, methane, and propane are fed into the furnace chamber at high flow rates and at temperatures approaching 1,000°C–1,100°C. The methane and propane gases break down and deposit carbon deep into the matrix of the carbon–carbon fibrous parts. The cycles can be very long, approaching seven to 10 days, for the material to fully densify, and multiple cycles are usually necessary.

Low operating pressures require extremely large mechanical pumping systems with large vacuum blowers or boosters. These furnaces include water-cooled “tar” traps (with a heated stripping system) and large Dollinger particulate filters for handling the resin off-gas byproducts.

These furnaces are almost always induction heated, using multizone induction coils and large, thick-wall graphite susceptors for optimal temperature uniformity. The insulation design uses carbon black powder, which is economical as well as highly efficient for temperature reduction.

While more conventional rigid or flexible graphite board or felt materials can be used, experience shows that the degree of infiltration of carbon resins over time will affect the density and porosity of the insulation pack (as it does the load material), causing degradation and densification of the insulation. The denser insulation results in high coil water temperatures, which compromises hot zone life. Specialized carbon black installation and removal equipment is required by end users to maintain the insulation efficiency of the furnace hot zone. Because gas flow in the furnace is critically important, special diffusor plates or plenums are used to uniformly direct gas flow across the entire geometry of the parts.

Once the advanced materials undergo chemical vapor infiltration, they are still composed of a carbon base material, which needs to be converted to a more orderly crystalline graphite structure to impart the durability and strength required in aerospace applications. To accomplish this conversion, the material needs to be heated at temperatures greater than 2,300°C to convert the carbon to a highly ordered graphite structure, a process called graphitization.

The graphitization process employs similar furnace designs to the chemical vapor infiltration process, but the induction heating power supply is changed to the more conventional single zone coil and the vacuum pumping systems are smaller with no tar traps needed. Load sizes of 3,000–5,000 lbs (1,360–2,268 kgs) are possible. Both the smaller and larger chemical vapor infiltration and graphitization units have large, water-cooled heat exchangers inline with large cooling fans, which reduce cooling times from 10+ days to less than 175 hours.

A smaller graphitization unit was also developed in a 30” (762 mm) diameter × 40” (1,000 mm) height size rated to 2,900°C maximum temperature in a vertical bottom-loading configuration for processing smaller parts in lower volumes for aerospace brakes.

The silicon carbide chemical vapor deposition units for laboratory applications discussed previously are also needed for production-size volumes. One of the larger systems available is a 53” (1,350 mm) diameter × 83” (2,108 mm) height graphite hot zone furnace rated for 1,600°C operation for chemical vapor deposition or infiltration of silicon carbide (Figure 7). Due to tight temperature uniformity requirements, these units are multizone control with graphite hot zones constructed of rigid graphite board for process durability. The pumping systems can be either “dry” or “liquid ring” designs for processing the acidic off-gas materials. A post-exhaust chemical scrubber system is required to safely neutralize the hydrogen chloride off-gases.

Enabling the next generation of aerospace materials

The difficult design requirements of next-generation aerospace technologies will continue to push the existing limits of material performance. Characterization and development of new materials will be critical to the success of these aerospace programs, and vacuum and controlled atmosphere furnaces will play a role in the production of these materials. Centorr Vacuum Industries will continue to support this industry through its diverse product offerings, ranging from laboratory-scale testing up to production scale.