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(pronounced “care-x”) is a spin-off from Italy’s National Research Council – Institute of Science Technology and Sustainability for Ceramics () in Faenza. The company was co-founded by CEO Giorgio Montanari, along with CNR-ISSMC research director Diletta Sciti (chief product officer for K3RX) and senior researcher Luca Zoli (chief manufacturing officer for K3RX).

“We have worked with ultra-high temperature [UHT] ceramics for years,” says Sciti. “These materials are capable of very high temperatures, even above 3000°C, but they are very brittle. My dream was to make UHT ceramics unbreakable. That's why we developed composites, adding fiber to increase toughness.”

“These ultra-high temperature ceramic matrix composites [UHTCMC] are a unique product, enabling near-zero erosion components operating above 2000°C with increased durability compared to other materials,” says Montanari. “Thus, they offer reduced cost and pollution because the parts can be used repeatedly.”

K3RX has demonstrated high performance in flat parts up to 1 centimeter thick and up to 40 centimeters in diameter for more complex geometries, including testing for thermal shock and repeated arcjet and plasma wind tunnel exposure. These parts include rocket nozzles, nose cones, leading edges, spacers and thermal protection system (TPS) tiles, reaching technology readiness levels (TRL) 5-6, with successful tests of integrated assemblies including threaded connectors also made of the same UHTCMC. K3RX is working to commercialize its UHTCMC with 10-20 customers in space, defense, energy and braking applications.

Patents and process options

The founding of K3RX began after Sciti had completed work as part of the 4-year EU-funded project Next Generation Ceramic Composites for Combustion Harsh Environments and Space (), which started in 2016. “After that project ended, we paid for exclusive use of patents awarded to Diletta and Luca working with other CNR researchers,” explains Montanari. “But we have further advanced these technologies, making and testing many prototypes to achieve certification and optimize the production for specific applications. Diletta and Luca have more than 200 publications on this work.”

K3RX currently uses two main processes, both with three steps. The first process comprises impregnation of a preform with a preceramic slurry, sintering that into a UHTCMC and then machining to produce the final part. “This is very similar to UHTC processing,” says Sciti, “and it can be very fast with just a single densification cycle.” This cycle time can range from a few hours to a day, depending on part size, and she notes it can be further accelerated by using spark plasma sintering.

The second process involves impregnation, shaping and pyrolysis. “This is more akin to a typical PIP process,” says Zoli. Here, he refers to polymer infiltration and pyrolysis (PIP), which is used widely to produce CMC using carbon fiber and phenolic resin, for example. “For this process, we impregnate a reinforcement with a special polymer system and then use layup, press molding or winding to create a near-net shape, followed by pyrolysis to create the UHTCMC. Some trimming or drilling may be needed, but not machining, to create the part shape as required in the first process. We can make larger-dimensioned components compared to the first process, but the final performance characteristics are a little bit lower.”

“This PIP process also requires the 4-10 cycles that you read in CMC literature,” says Sciti. “It depends on what kind of porosity you want and the final application temperature. The latter determines how you tailor the temperature of the pyrolysis. Higher service temperature requires a more refractory matrix after pyrolysis, so you need to crystallize the matrix properly.”

Fiber and matrix possibilities

The figure at right shows typical matrix materials that have been used to produce UHTCMC in various research publications. But let’s start first with fibers. “For now, we mainly use carbon fiber,” says Sciti, “but we are also looking at silicon carbide [SiC] and others. It depends on what is required. If we need to light weight, it’s better to go with carbon fiber. If we want increased oxidation resistance at a lower temperature, then SiC fiber performs well. But the cost of SiC fiber is at least 10 times that of carbon fiber, most of it comes from Japan and the rate of production is not very high. So, we prefer to use carbon fiber.”

Zoli notes another issue. “In my opinion, for a UHTCMC to withstand more than 2000°C, it doesn’t make sense to reinforce your matrix with SiC fiber that withstand only 1800°C. If 40-50% of the volume is SiC fiber, then this is not a UHTCMC.”

Regarding matrix materials, Sciti explains that because the K3RX processes are versatile, it can use a wide range of matrix formulations. “We can use any UHTC we want, but we have a reference composition, which was developed during the C3HARME project and is based on a formulation with zirconium diboride [ZrB₂] and SiC in the matrix. We have a lot of testing on this using carbon fiber reinforcement.”

However, K3RX can tailor both materials and process depending on individual application requirements. “Especially with the first process,” says Sciti, “we can use more of a certain UHTC or more SiC, but we’ve also tried other types of matrix materials, all with good results.” She notes there are some adjustments needed in terms of the fiber matrix interface, but a wide variety of formulations are possible. Still, each matrix has its pros and cons. “It is possible to increase the temperature capability by selecting a higher melting point UHTC, like hafnium diboride [HfB₂], but this costs 10 times more than ZrB₂. That may be justified in some cases, because HfB₂ has a much higher resistance to oxidation. But it also increases weight. This is why we developed our technology to tailor the properties as needed.”

Time and temperature

For CMC and UHTCMC, the selection of fibers and matrix materials is not just a matter of maximum temperature, but also how long the part must withstand this. Short exposures or excursions don’t require the same formulations as long-duration or frequently repeated service periods.

“Currently, our UHTCMC have been tested at 2200°C for up to 30 minutes,” says Sciti, “and up to 2500°C for less than 5 minutes. To do this type of testing for hours, for example, requires facilities that are usually in the hands of just a few companies globally. But then all the results are kept secret and not disseminated. So, this is a challenge, but we want to do this type of testing as well.”

She notes UHTCMC can also behave differently at lower temperatures. “The carbon fibers in our baseline UHTCMC are very sensitive to oxidation, even at temperatures like 1000°C. The matrix must create a protection layer on the fibers to protect them, but this layer is only active at temperatures higher than 1200°C. Thus, the matrix may not be as good in protecting the fibers at say 1000°C versus at higher temperatures. There are different components in the matrix, and each has its own behavior with oxidation. Thus, there are different combinations of reactions at different temperatures which create different results. To understand UHTCMC properties at lower temperatures, more testing is needed.”

The K3RX baseline UHTCMC is currently undergoing such testing, selected as part of the ESA’s European Materials Ageing (EMA) program, installed earlier this year on the Bartolomeo platform attached to the International Space Station (ISS), where it will be exposed to space for 12-18 months.

Testing, proven performance

During the work with the ESA to qualify its materials for the EMA program, K3RX tested its baseline UHTCMC above 1800°C in an arcjet to simulate the material withstanding reentry from space. “The material performed very well, with almost zero ablation,” says Montanari.

Sciti notes that once the materials are tested and come back to Earth, K3RX will be able to analyze the effect of the deep vacuum of space on the materials. “This is important because it’s not possible to perfectly reproduce those conditions on Earth. However, these composites were already tested in much worse conditions, in terms of temperature, oxidation and corrosion, because they were tested in the plasma wind tunnels at CIRA and at DLR. The response of the material was very good. They can survive long-time exposure, but the conditions for Bartolomeo are different. For example, there is a jump between -150°C and 150°C, and you also have atomic oxygen that can deteriorate the surface of the sample.”

In addition to its baseline UHTCMC, K3RX also submitted a sample that was arcjet tested. “This will simulate the material after reentry, where it is slightly oxidized, and now it is being exposed to deep space,” says Sciti. “We will then be able to compare the reference material and the oxidized material to help assess the performance of this UHTCMC in reusable rocket and vehicle applications.”

Montanari notes that K3RX materials as well as finished parts have been tested by many universities, research centers and aerospace organizations across Europe with successful results. These include Airbus, the German Aerospace Center (DLR, Stuttgart and Cologne), the ESA European Space Research and Technology Centre (ESTEC, Noordwijk, Netherlands) and the European Synchrotron Radiation Facility (ESRF, Grenoble, France) as well as Safran and ArianeGroup, both headquartered near Paris. In Italy, the list includes Tier 1 space launcher/equipment supplier Avio (Colleferro), aerospace and defense Tier 1 Leonardo (Rome), braking systems supplier Brembo (Curno), the Centro Italiano Ricerche Aerospaziali (CIRA, Capua) and Federico II University (Naples).

The figure at right above compares the K3RX baseline UHTCMC (yellow) with competitive materials including carbon fiber-reinforced carbon (C/C) and SiC (CMC) as well as Tungsten metal. “Our UHTCMC is very strong compared to these other materials,” says Montanari, “and excels where you need components with high resistance to oxidation and wear.” For the latter, Sciti explains that C/C are porous materials, while K3RX composites are dense, ceramic-based composites with very little porosity.

Additional results from testing include:

• Excellent thermal shock resistance with dimensional stability and non-brittle behavior, qualified as a “non-fragile” material by the ESA.
• High resistance to most corrosive and abrasive media.
• Self-healing capability to self-repair defects created during exposure to thermomechanical stress.

Prototypes, customers, reducing cost

K3RX produced flying parts during the C3HARME project, including leading edges, flaps and nozzles, says Montanari. “Now, we are further developing and producing nose cones, winglets, thermal shield tiles and complete TPS assemblies, including spacers, screws and nuts. We are also making brake discs where there is a need for performance in extreme conditions, and we see an opportunity for aerospace brake rotors and stators to increase performance as new types of aircraft are developed. Our materials and parts capabilities are also a good fit for hypersonics.”

K3RX is also working with companies in the energy sector, including discussions with the global energy company Eni (Rome, Italy). “There is a growing demand for high-temperature materials in novel nuclear technologies and in other decarbonization initiatives,” says Montanari. “We also see special applications in transport and industry.”

“We are working with 10-20 companies,” he continues, “mainly supplying prototypes and finished components, but in some cases also semi-finished components to customers who want to control some steps of the production process. We are a Tier 1 and Tier 2 supplier, but also an R&D partner, and our supply chain is 100% European or American except for some fiber from Japan. Our layup and curing partners are in Germany, Spain and Italy, and our tooling partners are in Italy and Germany.”

K3RX is in the process of closing its first round of investment, which includes several European investors, mainly large companies in the aerospace and defense, energy and braking systems markets. “We will need a second round of financing in the future, so we are having discussions also with international investors,” says Montanari.

TRL, assemblies, cost, future developments

K3RX has reached TRL 5-6 across most of these applications, notes Sciti. “We have already assembled components to a substructure and tested these in representative service environments. For example, we have produced a large nozzle up to 12 centimeters external diameter and ~12 centimeters in length that has been tested in a rocket bench, as well as a 194 × 240-millimeter tile that was tested for TPS performance. It was integrated into a structure that was made of other types of CMC. We have demonstrated that we can integrate our parts into systems that perform well.

“We have also tested a TPS tile where both it and the fixation elements were made of our material,” continues Sciti. “And we’ve made spacers with the same materials. So, the whole system can have the same oxidation resistance as the main component. I think this is important and was already a goal in in the C3HARME project.”

Although K3RX has not yet investigated in situ joining of parts — that is, uniting preforms during the composite step to become an integrated CMC during sintering or PIP — it is one of the planned next R&D topics. “We believe that most of the joining methods that work for CMC should also be possible for us to use with our UHTCMC,” says Sciti. “But the joining method needed really depends on the application and time and temperature requirements. If the UHTCMC is thick enough to allow for the temperature to decrease at the joining point, then even a joint with a lower temperature resistance may be sufficient. On the other hand, if you just want a small section of UHTCMC, then you must design the joining method accordingly. There are various approaches.”

Another development topic is cost reduction. Traditionally, CMC have been expensive due to the lengthy manufacturing processes. This has been even more of a concern for UHTCMC, especially when using more exotic matrix combinations. “We are working to evaluate our cost versus more typical C/C and CMC materials,” says Montanari. “Even though we have a premium product, cost is always a consideration. We know that with sufficient scale, our cost could be reduced by 75% — so decreasing to 25% of what we have today as a result of industrialization.”

Zoli explains how this is possible. “If you look at graphite that is used in furnace applications, it is made using PIP that requires at least 10 cycles, reaching 3000°C to obtain a graphite and not amorphous carbon. So, it’s a very long and expensive process, but the cost of graphite is low compared to traditional C/C. And this is due to companies like SGL Carbon [Meitingen, Germany], which has produced this material on an industrial scale for decades. So, the cost may be only 40 euros/kilogram but if someone asked me to produce graphite, it could be 1,000 euros/kilogram due to the small series. Of course, for K3RX, our UHTCMC raw materials have a cost. But even when buying a larger amount of powder or fibers, we have seen a significant drop. So, for every step of the process, we have possibilities to decrease our final cost. And this is something we are exploring.”

In the meantime, the size and TRL of the UHTCMC parts that K3RX is producing have not been matched in published information. “We’ve mostly seen R&D scale only,” says Sciti. “But we are also one of the few publishing so many results about prototypes and test results. This capability is not frequently shown by other researchers in this field. When you do UHTCMC for such extreme temperatures, the work tends to go down to much lower TRL. But we think it is important to continue to show our capabilities in terms of performance, size of parts and assemblies.”

“Of course, we continue to make developments and advance our technology,” says Montanari. “But we also need to spread the information regarding our products and help develop the market for UHTCMC in aerospace, defense, energy, braking and industrial applications. Our mission is to be a game-changer in these markets.”