Future insights on high temperature ceramics and composites for extreme environments

Abstract

This document summarizes key research directions as they emerged during the proceedings of the Inaugural Orton Workshop that aimed to define scientific areas of current interest to the ceramics community around the theme of: “High Temperature Ceramics and Composites for Extreme Environments.” The topic was selected due to the timely interest in such materials to meet the needs of hypersonic aviation and space exploration and habitation. Experts from funding agencies supporting ceramics research, thought‐leaders from academia with expertise spanning materials processing, characterization, and modeling, as well as research and development leaders from key (aviation‐related) industries, gathered to evaluate the state‐of‐the‐art in this field and to address key questions with the intent of accelerating research and development efforts on all fronts. Highlights of the work presented and of the discussion and brainstorming sessions are provided here. It was the purpose of the organizers (The Orton Ceramic Foundation and the Orton Chair in Ceramic Engineering at OSU) to establish this event as a service to the Ceramics community in the spirit the founder of the field of Ceramic Engineering, Dr. Edward Orton Jr.

HIGH TEMPERATURE CERAMICS TECHNOLOGIES: PAST-PRESENT-FUTURE

It was the need for materials for nuclear reactors that provided the impetus for the research into high temperature ceramics in the second half of the 20th century. “The ceramist's creative and inventive instinct is essential to the resolution of the problems associated with nuclear energy research and development. Many more ceramists should be at the front of this challenge” urged the late Stephen D. Stoddard in 1974. B₄C, HfO₂, Gd₂O₃ headlined the long list of carbides and oxides that garnered much attention and were subsequently implemented as ceramic control materials in hot pressed pellets in nuclear reactor rods. At the same time, interest was expressed in using advanced ceramics as “uncooled” components in gas turbine engines due to their higher melting points, lower densities, and higher specific strengths relative to superalloys. ZrB₂, SiC, Si₃N₄ have since emerged as the most common materials for such more modern high temperature applications. Since then, the major developments in high temperature ceramic technology have focused more on the large-scale manufacturing and process optimization rather than on the thorough microstructural examination of the known materials or the development of novel material systems and unconventional synthesis routes.

With the demonstrated success by GE aviation of its all-ceramic LEAP gas turbine engine that incorporates Ceramic Matrix Composite (CMC) components, a new era has dawned for high temperature materials. The SiC–SiC composite technology employed by GE is considered a “mature” material, rigorously tested over a period of 20 years at the cost of several billion dollars. This qualification also applies to the historically emphasized families of Hf-, Zr-, Ta-, and Ti- borides, carbides, and nitrides in monolith form, as support matrix, or as a coating. The prevailing practice has become to choose among materials that have been extensively studied and validated by others to avoid the historically expensive and trying task of validating materials yourself. However, there is an emerging need to push the boundaries of the ceramics field forward at a faster pace. What are the factors that drive innovation, and what makes for a successful new technology with fast maturation and a rapid adoption by the relevant industries? How might ceramics scientists and engineers go about realizing these breakthroughs? For ceramists to utilize our creative and inventive instinct praised above, we need be aware of the challenges ahead. Table 1 displays the list of participants at the Inaugural Orton Workshop.

TABLE 1 Participants list

Participant	Affiliation
Thomas Nixon	Rolls-Royce
Todd Steyer	The Boeing Company
Tania Kashyap	Pratt & Whitney
Jim Steibel	GE Aviation
Kara Bridges	Pratt & Whitney
Randall Hay	Wright-Patterson Air Force Base
Lynnette Madsen	National Science Foundation
Frances Hurwitz	National Aeronautics and Space Administration
Jessica Krogstad	University of Illinois Urbana-Champaign
Olivia Graeve	University of California, San Diego
Alexander Mukasyan	University of Notre Dame
David Poerschke	University of Minnesota, Twin Cities
Kathleen Sevener	University of Michigan
Gregory Thompson	University of Alabama
Samantha Daly	University of California, Santa Barbara
Yury Gogotsi	Drexel University
Yunfeng Shi	Rensselaer Polytechnic Institute
Allan Katz	Wright-Paterson Air Force Base
Joseph Homeny	Edward Orton Jr. Ceramic Foundation
Mark Lawson	Edward Orton Jr. Ceramic Foundation
Perena Gouma	The Ohio State University
Garrick Ackart	Edward Orton Jr. Ceramic Foundation
Sheikh Akbar	The Ohio State University
Michael Mills	The Ohio State University
Gregory Morscher	University of Akron

EXPLORING NOVEL METHODS AND EXPANDING TRADITIONAL METHODS FOR HIGH TEMPERATURE CERAMICS SYNTHESIS Combustion synthesis

Self-propagating high temperature synthesis (SHS) of high temperature ceramics belongs to the combustion category of ceramic processing methods and is depicted through a series of images in Figure 1.^1–7 The unique characteristic of the SHS process is that brief exposure of raw materials to moderately high temperatures or some similar stimulus activates an exothermic reaction that releases enough heat to sustain the reaction without prolonged heating or other driving force input by the user. The most notable advantages of SHS over conventional synthesis processes are the relative ease of reaction initiation, the fact that the reaction sustains itself without further energy input, the exceptional purity of products that survive the high temperatures evolved by the reaction, and the high reaction and yield rates since all starting material reacts and the reaction front can move as fast as 0.25 m/s. Three major parameters control SHS processes. First, there is the energy generation; whether such a reaction can occur is dictated by the energy evolved by the reaction and its value relative to the activation energy of the reaction. Then, there is also the spatial distribution of this energy evolved by the reaction and the mass transport, which must both be considered since they will decide if the thermodynamics of the energy generation will mechanistically suffice in a real sample.

FIGURE 1. A Series of thermal images captured during an SHS reaction (the dotted line represents a pellet inside of vacuum encapsulation)14 (courtesy of M. Pawar)

SHS processing of high temperature ceramics offers the scalable synthesis, ease of manufacturing, and high purity product at a low energy cost. The spontaneous nature of SHS reactions is also the reason this processing method has not yet been fully realized at the industrial scale. Once initiated, SHS reactions can evolve heating rates on the order of 10⁴ or 10⁵ K/s, driving reaction temperatures to between 2000 and 4000 K, so SHS reactions are not considered fully controllable. Consequently, it is not yet possible to obtain tailored microstructures from SHS processes. Given the fact that all types of carbides nitrides, sulfides, borides, intermetallic compounds have been successfully produced by SHS and that there is evidence of one-step near-net shape processing using this technique, it is worth the larger Ceramics community looking deeper into the inner workings of SHS processes.^8,9 Currently, SHS processing is used when there is no other method available to produce a given material, such as super refractory carbides.¹⁰

Electric field activated combustion also falls under this combustion classification as a subset of SHS where a self-sustained reaction is initiated by application of an external electric field.^11,12 The required value of the electric field varies with the reacting material system and its features, but a threshold value exists. For Si and C to react and produce SiC via an electric field activated SHS process, a threshold voltage of 8.9 V/m was found to be required. Furthermore, novel nanostructures have been obtained by mixing nitrate salts and an organic fuel in the form of a slurry then inducing an SHS reaction. Upon formation of a melt, homogeneous compounds are formed, such as hexaboride nanocubes.¹³

Novel processing of high temperature ceramics of lower dimensionalities—2D and 1D configurations 2D-MXenes

Another unconventional processing method for the synthesis of high-temperature ceramics, such as hexagonal boron nitride and early transition metal carbides, among many others, involves the etching, most commonly of M_n+1AX_n (MAX) phases.¹⁵ In this case, M is an early transition metal, A is most often a group IIIA or IVA element, X is C or N, and n can vary between 1 and 3 depending on the oxidation state of M. Such etching leaves behind 2D layers of M_n+1X_n ceramic compounds. These are called the MX(ene) family of ceramics. While MAX phase etching may be the most common MXene preparation by etching, other more complicated final products may be prepared by etching such as those appearing in Figure 2.¹⁶ Through the relatively short history of these materials, they have been primarily explored in functional applications, but they may be of interest to consider as reinforcements in structural composites.

FIGURE 2. (A) Scanning electron micrograph of hydrofluoric acid-etched Ti3AlC2. (B) X-ray diffraction patterns of Nb2AlC before and after hydrofluoric acid etching16

Electrospun fibers and preforms of high temperature ceramics

The uncommon process of electrospinning involves the electrostatic drawing of continuous fibers that can be collected in an aligned state or as nonwoven mats.^17,18 While this technique was originally developed for polymer systems, significant and relatively simply adaptations to well-established sol–gel processes have made electrospinning available to ceramics processing. Both single crystal and polycrystalline ceramic fiber configurations, as well as 3D self-supported mats, have been reported. The ability to prepare borides, carbides, and nitrides, combined with the availability of high-throughput techniques for scalable processing, may prove electrospinning highly competitive for processing preforms and fibers for composites.

ADVANCED, IN-SITU, AND OPERANDO STRUCTURAL, CHEMICAL AND MECHANICAL PROPERTY CHARACTERIZATION Characterization in simulated operating conditions

There is an inherent difficulty in studying high temperature ceramic materials in the environments they will operate in, which explains the scarcity of characterization data for high temperature ceramics in simulated operating conditions. One solution to this problem is the use of contactless instrumentation for thermo-mechanical testing. Several researchers have developed variations of an electro-magnetic mechanical apparatus for mechanical testing at high temperature where resistive heating controls sample temperatures while Lorenz forces are used to bend the sample and a laser displacement instrument measures deformation.¹⁹ Recent advancements in such instrumentation now account for sample prebending during thermal exposure to enable measurement of activation energies for deformation processes and dislocation movement.

Multiscale damage characterization in CMCs

The nature of mechanical damage in CMCs exemplifies the need for understanding mechanical damage at multiple length scales because that is required to separately understand how and where cracks initiate, analyze crack propagation through the matrix, and observe the effects of accumulated damage.^20,21 In-situ scanning electron microscopy-coupled-digital image correlation has been used to comprehensively investigate the initiation and propagation of mechanical damage in tensile-loaded SiC–SiC composites across these critical length scales. A related investigation that also included manual crack opening displacement measurements highlighted the incongruity of matrix cracks in SiC–SiC composites. As depicted in the representative micrograph reproduced in Figure 3, no cracks were observed passing fully through the gage of the tensile specimens.²² This observation casts doubt on the common assumption made in computer modeling of such materials that all cracks are full through-cracks.

FIGURE 3. Several partial cracks in a SiC-SiC tensile specimen22

PROTECTIVE COATINGS FOR HIGH TEMPERATURE CERAMICS

A first step in preventing failure is always understanding the mechanism behind it, and this is equally applicable to thermal barrier coatings (TBCs).^23–27 One such study of TBC failure examined a phase transformation in yttria-stabilized zirconia (YSZ) from the t’ phase to the monoclinic phase. This study used transmission electron microscopy and X-ray diffractometry to identify the formation of a lamellar structure composed of Y-rich and Y-lean domains that ultimately transform into the monoclinic phase. This concluded that the coarsening rate of these lamellae, depicted in Figure 4, ultimately determines the phase stability of conventional YSZ TBCs.²⁸

FIGURE 4. (A) Lamellar packets in the sample aged for 7.8 h at 14821C showing different crystallographic orientations along all possible {111} planes but with a common [001] direction. (B) A detail of lamellae converging in a tetrahedral-like pattern, also schematically depicted in (C). (C) The mismatch between neighboring lamellae is similar along the [−101] and [0–11] directions but different along [−110].28

Not all coatings relevant to CMCs are meant to protect from thermal or environmental damage; boron nitride (BN) is applied to SiC fibers primarily to facilitate pullout. It was once also thought to sacrificially delay oxidative embrittlement of the SiC fibers, but a recent investigation concluded that BN coatings do not sacrificially protect SiC fibers from oxidation as implied by calculated values of requisite oxygen activity to initiate oxidation of these materials.²⁹ Instead, it was found that rates of oxidant ingress, ultimately controlled by the diffusivity of the oxidant through the matrix rather than the porosity of the matrix, control the magnitude of oxidation damage.

FUTURE TRENDS

Discussions focused on the immediate needs of contemporary extreme-environment applications for high temperature ceramics. This conversation continuously returned to the tremendous task of overcoming industrial aversion to adopting new materials and resulted in a three-part strategy. The first part of this strategy is thoughtfully focusing research efforts, the second is development of evaluation techniques, and the third is ongoing, targeted communication between academic researchers and industry.

Intentionally focusing research on materials with multiple possible applications will maximize the impact for two reasons. This is beneficial for researchers since it increases the chance that their material will be a good fit for at least one application. Moreover, it is beneficial for industry since using a single material for multiple applications simplifies manufacturing concerns. Beyond carefully selecting research topics, ideologically focusing on designing a solution in collaboration with others, rather than simply optimizing your one part, is critical.

Materials evaluation in simulated extreme operating environments is critical to accelerating material maturation. Participants foresee much upcoming work on designing testing instrumentation for such characterization with an especially great focus on nondestructive characterization. Moreover, general consensus was that there is enormous opportunity for anyone that can enable widespread access to such testing equipment. Beyond this, expectations are that advanced modeling tools will manifest alongside the development of specialized instrumentation for this purpose. Such upcoming modeling tools are expected to apply machine learning in combination with well-established physical models to predict part lifetimes under the influence of synergistic degradation mechanisms.

The final part of this strategy is to model future academic-industrial collaboration after successful previous government-industry collaborations. At the beginning of a collaborative project, there must be a clearly defined roadmap to industrial adoption with evaluation landmarks along the way that both parties agree on. Having clearly communicated expectations from the start and open communication along the way will seriously accelerate this material maturation process.

Another recurring theme in the discussions was the issue of encouraging work in this subject area, at every level. Such intrinsically interdisciplinary research is self-selective enough that establishment of a public, dynamic funding source dedicated to related projects seems necessary to enable more collaborative work. Moreover, greater public funding in this typically privately funded research area would accelerate the field more than an equal dollar amount of additional private funding by yielding results that are not proprietary or confidential like so much work in this field has historically been. Building on the importance of access to the preceding body of work, there was also agreement that the establishment of a public knowledgebase would be an enormous step toward accelerating the field. An entity like ACerS or NIST would be best suited for this, and it would be for the purpose of allowing transparent access to the cumulative body of work thus far in this field so researchers would know exactly what has already been done. Additionally, participants agreed that industry must drive efforts to develop educated workers for all levels of involvement with these projects from initial research and development mass production. At every level, this is work that few are equipped to perform, and most of those people found themselves in this field by happenstance; that must change. Finally, there was also agreement on the need for stronger educational influence on policy makers. Widespread work in this field will only be feasible with regulatory assistance ensuring consistent availability and controlled cost of the raw materials needed for such work.