METHODS OF INCREASING THE DEFORMABILITY OF CERAMIC MATERIALS AND CERAMIC MATERIALS MADE THEREBY

Abstract

Methods of increasing the deformability of ceramic materials, as a nonlimiting example, titanium dioxide, particularly through the introduction of defects, and to ceramic materials produced by such methods. Such a method increases the deformability of a ceramic material by introducing high-density pre-existing defects and oxygen vacancies in the ceramic material during a flash sintering process and then forming nano scale stacking faults and nanotwins in the ceramic material. Such a ceramic material contains high-density pre-existing defects and oxygen vacancies and nano scale stacking faults and nanotwins, and may exhibit deformability in a temperature range of room temperature to 600° C.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Nos. 62/815,738 filed Mar. 8, 2019, and 62/843,406 filed May 4, 2019. The contents of these prior patent applications are incorporated herein by reference.

STATEMENT REGARDING GOVERNMENT FUNDING

This invention was made with government support under Contract No. N00014-17-1-2087 and N0014-16-2778 awarded by Office of Naval Research. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

The present disclosure generally relates to methods of increasing the deformability of ceramic materials, as a nonlimiting example, titanium dioxide, particularly through the introduction of defects, and to ceramic materials produced by such methods.

This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.

Ceramic materials often offer numerous advantages over metals, such as corrosion resistance, high hardness, wear resistance, high melting temperature and low density, and thus have been widely used as structural materials for various environments. However, the applications of ceramics have frequently been prohibited by their brittle nature at low temperatures. In general, the plastic deformation of crystalline metallic materials proceeds through the activity of dislocations at room temperature (RT), and/or diffusion creep at elevated temperatures. In comparison to metallic materials, ceramics with ionic or covalent bonding are strong but mostly brittle at low temperature, such as room temperatures, due to a lack of dislocations to accommodate plasticity. Fracture toughness of ceramics is much lower than that of deformable metallic materials. The plastic deformation of ceramic materials has been intensively investigated at high temperatures, whereby diffusional creep or grain boundary sliding become significant.

Room-temperature plasticity remains largely absent in most ceramic materials. It has been shown that fracture can be suppressed to some extent in a few ceramics at low temperatures, such as single crystal SrTiO₃ and MgO, and polycrystalline ZrO₂ (zirconia) and YSZ (yttria-stabilized zirconia). For example, YSZ, which experiences martensitic phase transformations, may exhibit superelasticity at room temperature depending on the doping concentration and its grain size. Another approach to improve plasticity of ceramics is significant grain refinement. It has been demonstrated that plastic deformation of ceramics at low temperature is occasionally possible if their grain sizes are in the range of a few nanometers. However, the sintering of ceramics while maintaining such small grain sizes is challenging.

In order to produce advanced ceramic materials with low porosity, various sintering techniques have been extensively investigated. In general, the conventional sintering typically requires high temperature and long sintering time and thus often leads to significant grain coarsening. With the assistance of an electrical field and pressure, spark plasma sintering has been widely explored to sinter ceramics materials to full density over a short period of time, generally on the order of several minutes. A recently discovered technique, flash sintering, however, can be used to sinter fully densified ceramics within even shorter time, generally within a few seconds, without pressure, and at temperatures much lower than conventional sintering temperature. During flash sintering, a ramp heating process and a moderate electrical field are applied. Once reaching the onset of the “flash temperature,” a densification process occurs instantaneously, followed by a sudden increase in electrical conductivity. In spite of significant interest in flash sintering of ceramics, investigations on the deformability of flash-sintered ceramics remain scarce.

Titanium dioxide (TiO₂; titania) has diverse and broad applications, such as solar cells, semiconductors, and water purification, and has been intensively investigated in the past decades The mechanical properties of TiO₂ prepared by different techniques have been widely investigated. For instance, rutile TiO₂ single crystals of various stoichiometry have been tested at temperatures varying from room temperature to 1300° C. by compression. Prior studies showed that TiO₂ was brittle when tested below 600° C., and most specimens fractured before yielding. Plastic deformation becomes feasible in TiO₂ only when test temperatures exceed about 600° C.

Thus, there is an ongoing desire to achieve room temperature plasticity in ceramic materials, including but not limited to titanium dioxide.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides methods of increasing the deformability of ceramic materials, as a nonlimiting example, titanium dioxide, particularly through the introduction of defects, and to ceramic materials produced by such methods.

According to one aspect of the invention, a method of increasing the deformability of a ceramic material comprises introducing high-density pre-existing defects and oxygen vacancies in the ceramic material during a flash sintering process and then forming nano scale stacking faults and nanotwins in the ceramic material.

According to another aspect of the invention, a ceramic material is provided with deformability in a temperature range of room temperature to 600° C. The ceramic material contains high-density pre-existing defects and oxygen vacancies and nano scale stacking faults and nanotwins.

Technical effects of methods and ceramic materials as described above preferably include the ability to significantly increase deformation plasticity in a range of ceramic materials, including but not limited to TiO₂.

Other aspects and advantages of this invention will be appreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Some of the drawings shown herein may include dimensions. Further, some of the drawings may have been created from scaled drawings or from photographs that are scalable. It is understood that such dimensions or the relative scaling within the drawings are by way of example, and are not to be construed as limiting. It should be recognized that all the figures shown are not to scale.

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

FIG. 1 contains images that includes a schematic and micrographs pertaining to microstructures of TiO₂ prepared by flash sintering. Image (a) is a schematic of a flash sintering process performed on TiO₂. Image (b) is an SEM image of an unpolished flash-sintered TiO₂. Image (c) is a bright-field (BF) TEM micrograph showing an area of a flash-sintered TiO₂ containing stacking faults. Images (d) and (e) are TEM micrographs showing high-density dislocations within TiO₂ grains of a flash-sintered TiO₂. Image (f) is a graph plotting ultimate/fracture strain (%) as a function of testing temperatures for flash-sintered TiO₂, conventional TiO₂, and other conventional ceramic systems, including SiCN, YSZ, and TiC, and provides a comparison of temperature-dependent deformability of flash-sintered TiO₂ with the conventional ceramic materials. Fracture strain is very low at low temperatures and increases with temperatures for most of the conventional ceramics. However, the flash-sintered TiO₂ exhibits significantly enhanced deformability even at room temperature (solid magenta circle data points).

FIG. 2 shows results of uniaxial in situ microcompression tests on the conventional and flash-sintered TiO₂ at different temperatures, room temperature vs. 400° C., at a constant strain rate of 5×10⁻³ s′. Images (a1)-(a6) are representative stress-strain curves of conventional sintered TiO₂ tested at room temperature and 400° C. All pillars experienced brittle catastrophic fractures at an average true strain of 2% for room temperature tests and 3% for 400° C. tests. Images (b1)-(b6) are stress-strain curves of flash-sintered TiO₂ tested at room temperature showing work hardening to a maximum flow stress of 2-2.5 GPa and relatively continuous flow stress-strain curves with small serrations. Stress-strain curves of different colors were obtained from different individual pillars. The in situ movie SEM snapshots of a pillar (red data in FIG. 2, Image (b1)) compressed to different strain levels show the formation of successive high-density slip bands. No fracture was observed up to a strain of 8%. (Images (c1)-(c6)). At 400° C., large isolated serrations manifested by sharp load-drops were observed. Each load-drop usually corresponds to the formation of a major shear band as indicated by arrows. The inset of FIG. 2, Image (c6), shows a magnified view of multiple shear bands generated during deformation.

FIG. 3 contains TEM micrographs of several flash-sintered TiO₂ pillars after compression tests at different temperatures to a similar strain, of 8 to 10%. Loading direction is indicated by yellow arrows in each figure. All images were taken from [010] zone axis. Image (a1) is a low-magnification TEM image of a pillar compressed at room temperature. A majority of the wide straight shear bands are stacking faults. Images (a2)-(a3) are examples of high-density stacking faults formed on two sets of {101} planes with an intersection angle of 66°. Image (a4) is an HRTEM micrograph showing a twin boundary decorated with stacking faults. Image (b1) is a low-magnification TEM image of a pillar compressed at 400° C. A majority of the wide straight shear bands were twin boundaries. Image (b2) shows examples of deformation twins as confirmed by the insert SAD pattern. Image (b3) shows HRTEM images of typical twin boundaries that are either sharp or decorated with stacking faults. Image (b4) is a dark-field TEM image of stacking fault segments formed after compression. Image (el) is a low-magnification TEM image of the top part of a pillar (about 2 μm from the surface) compressed at 600° C. No shear bands were observed. Image (c2) shows high-density dislocations of a pillar and Image (c3) shows stacking fault segments formed near the pillar top surface. Image (c4) shows an example of a stacking fault segment. Partials cross-slipped on two different {101} planes with an intersection angle of 66°.

FIG. 4 contains high-resolution scanning TEM (STEM) micrographs of twin boundaries and stacking faults in flash-sintered TiO₂ after compression at 400° C. and DFT calculations of generalized stacking fault (GSF) energy in TiO₂. Image (a) is an integrated differential phase contrast (iDPC) image of a twin boundary showing the atomic arrangement of both Ti and O columns. For clarity, Ti columns above and below the twin boundary are shown in yellow and green, respectively. Image (b) is a magnified area as shown by red dash-line in Image (a). Selected Ti and O columns close to the twin boundary are shown by empty circles. Missing 0 columns surrounding Ti on the twin boundary plane are clearly observed, indicating the 0 deficiency close to the twin boundary. Image (c) is a graph plotting the variation of GSF energies with displacement. The unstable stacking fault energy is high, about 1.9 J/m², while the stable stacking fault energy is much lower, 30-40 mJ/m². Image (d) is a graph showing stacking fault and twin fault formation energy vs. 0 vacancy position. The embedded figures show (left) a basic stacking fault (unit cell vector shift) and (right) a basic twin fault (supercell). The solid red line indicates the fault energy without a vacancy.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of this disclosure is thereby intended.

Ceramic materials have been widely used for structural applications. However, most ceramics have rather limited plasticity at low temperatures. A majority of ceramics fracture well before the onset of plastic yielding. The brittle nature of ceramics arises from the lack of dislocation activity and the need for high stress to nucleate dislocations. This disclosure describes deformability of TiO₂ prepared by a flash sintering technique. The in situ studies leading to this disclosure show that flash-sintered TiO₂ can be compressed to about 10% strain under room temperature without noticeable crack formation. The room temperature plasticity exhibited by the flash-sintered TiO₂ reported below was attributed to the formation of nanoscale stacking faults and nanotwins, which may be assisted by the high-density pre-existing defects and oxygen vacancies introduced by the flash sintering process. Distinct deformation behaviors were observed in flash-sintered TiO₂ deformed at different testing temperatures, ranging from room temperature to 600° C. Potential mechanisms that may render ductile ceramic materials are discussed.

In investigations reported below, it was observed that by using in situ micropillar compression tests performed inside a scanning electron microscope, prominent room temperature plasticity was observed in flash-sintered TiO₂ with an average grain size of about 10 μm. In contrast to conventionally sintered TiO₂, flash-sintered TiO₂ inherits a high density of stacking faults and dislocations introduced during the non-equilibrium sintering process. Compared to the previously reported low-temperature plastic deformation in flash-sintered 3YSZ (zirconia stabilized by 3% yttria (Y₂O₃)), where phase-transformation-induced plastic deformation and dislocation-assisted plastic deformation were observed in the testing temperatures below and above 400° C., respectively, flash-sintered TiO₂ in the investigations presented no phase transformation and its obvious plastic deformation phenomena in the testing temperatures ranging from room temperature to 600° C. were correlated with abundant preexisting defects, such as stacking faults, nanotwins, and dislocations. The investigations suggested that flash sintering may be an effective approach to significantly improve the low-temperature plasticity of ceramic materials in general.

Methods and materials employed in the investigations are described below. Experimental results and a discussion of several aspects of the invention are then described.

Flash Sintering: Rutile TiO₂ powder (Inframat Advanced Materials, Product #22N-0814R, 50±20 nm particle size) was pressed uniaxially to make cylindrical green bodies. The dimensions of the green bodies were 6 mm diameter by 6±0.5 mm height with 40 to 45% density. A pressure of 10 kPa was used to maintain consistent electrical contact between the green body and electrodes. Specimens were heated at a rate of 10° C./min to a pre-flash temperature of 900° C. An electric field of 60 V/cm was applied across the sample faces, which resulted in a small current passing through the specimen which rose gradually with time. Once the pre-flash temperature was reached, the electric field was applied across the samples faces. The rise in conductivity of the sample leads to a non-linear rise in the current until it reached the limit set of 1.5 A/cm² in the feedback loop. The power supply was switched from voltage control to current control and held constant for 1 minute. The samples were then cooled down to room temperature at a rate of 25° C./min without any applied electric field.

Microstructure characterization by TEM: Plan-view TEM samples of flash-sintered TiO₂ were prepared through a conventional approach, which included manual grinding, polishing, dimpling and final polishing in an Ar ion milling system (PIPS II, Gatan). Low energy Ar ion polishing (2 kV) was used to minimize ion milling-induced damage. An FEI Talos 200X TEM/STEM microscope with ChemiSTEM technology (X-FEG and SuperX EDS with four silicon drift detectors) operated at 200 kV was used in this study for microstructure characterization and energy-dispersive X-ray spectroscopy (EDS) chemical mapping. In addition, high-resolution scanning transmission electron microscopy (HRSTEM) images were obtained using a modified FEI Titan STEM TEAM 0.5 with a convergence semi-angle of 17 mrad operated at 300 kV at the National Center for Electron Microscopy at the Lawrence Berkeley National Laboratory. The integrated differential phase contrast (iDPC) images were obtained using an FEI Themis Z with a Schottky electron emitter, an electron energy monochromator, and a 5th order probe spherical aberration corrector operated at 300 kV at the Materials Research Lab (MRL) at the University of Illinois, Urbana-Champaign.

Microcompression test: Micropillars of flash-sintered TiO₂ of about 3 μm in diameter, and a diameter-to-height aspect ratio of about 1:2 were prepared using focused ion beam (FEI quanta 3D FEG) and a series of concentric annular milling and polishing with progressively de-escalated currents were adopted to reduce the tapering angle. Micropillar compression experiments were performed using a Hysitron PI 87 R PicoIndenter equipped with a piezoelectric actuator on the capacitive transducer that enabled the collection of force-displacement data inside a scanning electron microscope (FEI quanta 3D FEG). Moreover, a 20 nm diamond flat punch tip designed for high-temperature compression experiments was used to conduct in situ compression experiments, and the geometric variation of micropillars was synchronized to evolving force-displacement curve. For high-temperature in situ compression setups, the flat punch tip was fastened to a probe heater, and the specimens were clamped by a V-shaped molybdenum clamp to a ceramic heating stage. The temperature on two heating terminals was simultaneously ramped up at a rate of 10° C./min and isothermally preserved for 30 minutes before implementing every single compression experiment to eliminate the thermal-driven drifts on both probe and stage sides. An average drift rate of less than 0.5 nm/s was estimated in the preloading process for 45 seconds, and the estimated force noise level was less than 8 μN prior to compression. The specimen displacement during the compression test was systematically measured and corrected during in situ SEM studies.

Atomistic Simulations: Atomistic simulations used density functional theory with the Perdew-Burke-Enzerhoff (PBE) exchange correlation functional, as implemented in the VASP software version 5.4.4 with a projector-augmented-wave (PAW) basis. The plane wave cutoff was set to 450 eV, and PAWs with 6 and 4 electrons in valence for 0 and Ti, respectively, were used. Simulations of stacking faults without point defects used a 1×1×8 supercell of a [100]×[011]×[011] interface cell, and a 3×3×1 k-point mesh. The stacking fault was introduced by adding slip to the out-of-plane lattice vector, creating one stacking fault per supercell. Simulations with point defects used a 3×3×6 supercell of the primitive interface cell, one vacancy per supercell (i.e. 1 vacancy per 9 interface plane sites), and only the Γ-point. Unstable stacking fault calculations determined a low energy path between stable configurations using the variable-cell nudged elastic band method as implemented in the TSASE python software. Twin boundary calculations used a 3×3×6 supercell, with half of the layers reflected on the (100) plane, creating two twin boundaries per supercell.

FIG. 1 shows schematics of flash sintering and the microstructure of flash-sintered TiO₂ are shown in FIG. 1. The density of flash-sintered TiO₂ (900° C. for 1 min) was measured to be about 98%. Conventional sintered (1350° C./5 h) TiO₂ (about 95% density) and zero-field sintered TiO₂ (1100° C. without an applied electric field, about 95% density) were also prepared for comparison studies. Micropillars were fabricated using focused ion beam (FIB) lithography on the polished surface of the sintered TiO₂ cylinder for in situ SEM compression studies. Bright-field (BF) TEM micrographs in FIG. 1, Images (c)-(e), show the existence of stacking faults and a high-density of dislocations in the flash-sintered TiO₂. For both conventional and zero-field sintered TiO₂, the grain interiors are relatively clean, with few pre-existing defects. FIG. 1, Image (f), compares ultimate/fracture strain (%) as a function of testing temperatures for flash-sintered TiO₂, conventional TiO₂ and several other conventional ceramic systems. All data were obtained from compression studies. Clearly, among all the data points, the flash-sintered TiO₂ exhibits significantly enhanced deformability even at room temperature.

Stress-strain behaviors of conventional and flash-sintered TiO₂ obtained from in situ microcompression studies from room temperature to 600° C. at a constant strain rate of 5×10-3 s-1 are shown in FIG. 2. For conventional sintered TiO₂ tested at both room temperature and 400° C. (FIG. 2, Images (a1)-(a6)), all pillars experienced brittle catastrophic fractures at an average true strain of 2% for room temperature tests and 3% for 400° C. tests. Similarly, micropillar compression tests on the zero-field sintered TiO₂ performed at room temperature and 400° C. at a constant strain rate of 5×10⁻³ s⁻¹ showed brittle catastrophic fractures at an average strain of about 2% at room temperature, and about 3.5% at 400° C.

In contrast, the flash-sintered TiO₂ tested at room temperature (FIG. 2, Images (b1)-(b6)) showed work hardening to a maximum flow stress of 2-2.5 GPa, and the stress-strain curves had small serrations. The in situ SEM movie snapshots of a pillar (FIG. 2, Image (b1)) compressed to different strain levels showed the formation of successive slip bands (indicated by arrows). No fracture was observed up to a strain of about 12%. For pillars tested at 400° C. (FIG. 2, Images (c1)-(c6)), prominent load-drops dominated the stress-strain curves. Each load-drop corresponded to the formation of one major shear band as indicated by arrows. The inset of FIG. 2, Image (c6), showed a magnified view of multiple shear bands generated during deformation. No crack was observed in the deformed pillar.

When the test temperature was 200° C., a mixture of small serrations and large load-drops were observed in the stress-strain curves. The maximum flow stress was about 1.5 GPa. At a strain of about 4%, a shear band emerged. Interestingly, the shear band broadened or propagated as indicated by yellow arrows. For the pillars tested at 600° C., the maximum flow stress was further decreased to about 1 GPa, after a few percents of strain. In addition, as compared to room temperature and 400° C., no obvious stress serrations were observed. SEM snapshots showed a relatively smooth surface without noticeable slip bands in the deformed pillars. These pillars deformed in a ductile manner without indication of fracture.

To investigate the influence of deformation on the evolution of microstructure, post-deformation TEM analyses were performed on the flash-sintered TiO₂ pillars tested to a strain of 8 to 10%. As shown in FIG. 3, Image (a1), for the pillar compressed at room temperature, the deformed portion of the pillar contained a high density of parallel slip bands. The slip bands intersected with the left surface of the pillar and led to prominent shear offsets as shown in FIG. 2, Image (b6). The pillar also contained several nanopores, but no cracks were observed to initiate from any of these nanopores. Several locations (indicated by red boxes) were selected to examine the microstructure in detail. FIG. 3, Image (a2) shows a majority of these parallel slip bands are composed of a high density of stacking faults along planes, as confirmed by the streaking lines in the inserted selected area diffraction (SAD) pattern. The average spacing between stacking faults was about 10 nm. Although a majority of stacking faults are parallel to one direction, FIG. 3, Image (a3), captures another set of high-density stacking faults on {101} planes forming an intersection angle of 66° relative to the stacking faults in the other direction. Twin boundaries also formed, but their density was low. High-resolution TEM (HRTEM) micrograph in FIG. 3, Image (a4) shows an example of a twin boundary decorated with stacking faults.

The pillars compressed at 400° C. had microstructures drastically different from those tested at room temperature. Although FIG. 3, Image (b 1) shows that the deformed pillar also contained high-density slip bands, a careful examination shows that these slip bands were primarily twin boundaries as confirmed by the inserted SAD pattern in FIG. 3, Image (b2). The average twin spacing was about 150 nm. The complex contrast in grain interiors may arise from the intersection of dislocations with twin boundaries. HRTEM micrograph in FIG. 3, Image (b3) shows two typical types of twin boundaries, relatively sharp coherent twin boundaries and stacking fault decorated twin boundaries. Although twin boundaries were the predominant defects in the deformed pillar, stacking faults were also observed as shown in FIG. 3, Image (b4). These stacking faults, parallel to the direction of predominant twin boundaries, were mostly fragmented, appearing as discontinuous segments.

For pillars compressed at 600° C., no shear bands were observed (FIG. 3, Image (c1)), in agreement with SEM observations. A high-density of dislocations and stacking fault segments formed near the pillar top surface as shown in FIG. 3, Images (c2) and (c3). FIG. 3, Image (c4), shows an example of stacking fault segments intersect on two sets of {101} planes with an intersection angle of 66°.

Preexisting defects in flash-sintered TiO₂: Ceramic materials often have a low dislocation density due to the nature of their ionic and covalent bonds. Dislocations appear in ceramics during high-temperature deformations. However, a high density of dislocations and stacking faults were observed in flash-sintered TiO₂ before any deformation (FIG. 1). The as-received nanocrystalline TiO₂ powders had no preexisting dislocations. These dislocations may be generated during flash sintering wherein the rapid sintering introduces significant plastic deformation at high temperatures within a short time, and the consequent rapid cooling process may be able to “freeze” a large number of dislocations in the flash-sintered TiO₂. Furthermore, it has been reported that defect catastrophes frequently occur during flash sintering. The colossal generation of electrons, holes, and point defects, can enhance electrical conductivity, promote phase transformations, and accelerate sintering simultaneously. Both Joule heating and the applied electric field are important for defect catastrophes. An in situ biasing study of rutile TiO₂ inside TEM showed that under the presence of electric field (±1.5V), oxygen (O) vacancies were created at low temperatures and then migrated under the electric field. Eventually, O vacancies coalesced into stacking faults. These stacking faults had the form of vacancy discs on {110} and {121} planes and were bounded by partial dislocation loops. It is reasonable to speculate that a similar but more aggressive process takes place during flash sintering of the TiO₂ under the influence of the electric field. Massive formation and migration of O vacancies occur simultaneously at the onset of flash sintering and leading to the creation of a high-density of defects after flash sintering.

To examine point defects in flash-sintered specimens, atomic-resolution TEM experiments were conducted to investigate the flash-sintered TiO₂. FIG. 4, Images (a) and (b), show the high-resolution scanning TEM (STEM) micrographs of a flash-sintered TiO₂ pillar deformed at 400° C. An integrated differential phase contrast (iDPC) STEM image of a twin boundary projected from [101] zone axis is shown in FIG. 4, Image (a). Both Ti and O columns can be resolved. The Ti columns above and below the twin boundary are intentionally shown in yellow and green colors, and the O columns are shown in red color. FIG. 4, Image (b), shows the magnified view of the red box in FIG. 4, Image (a). A few Ti and O columns adjacent to the twin boundary are shown by empty circles for clarity. Evidently, O columns were missing at the twin boundary, forming defective twin boundaries. It is worth mentioning that there were twice as many Ti columns along the twin boundary and stacking faults as compared to the perfect crystal lattices in flash-sintered TiO₂. Such a phenomenon could be an indication that the lack of O may lead to severe lattice distortion after twinning or faulting. Strain and lattice distortion near the stacking faults, which may arise from O vacancies, have been observed. Future investigation of the O partial pressure on the formation of defects during flash sintering as well as the influence of O stoichiometry on the mechanical behavior of flash-sintered TiO₂ are necessary to further understand the mechanisms of defect formation and mechanical response of flash-sintered TiO₂.

Defect-assisted room-temperature plasticity in flash-sintered TiO₂: Most ceramics, including TiO₂, are brittle especially when tested at low temperatures. Few ceramics have shown limited deformability at ambient temperatures, such as single crystal SrTiO₃ and MgO, and polycrystalline ZrO₂ and YSZ. MgO can be plastically deformed at room temperature as dislocations in MgO are mobile at relatively low stresses. ZrO₂ exhibits superelasticity because of the martensitic phase transformation. Prior studies show that bulk TiO₂ has no plasticity unless deformed above 600° C.

The micropillar compression studies of this disclosure showed that the zero-field and the conventional sintered TiO₂ fractured at an average strain of about 2% at room temperature, and at about 3% strain when tested at 400° C. (FIG. 2). The poor deformability of these TiO₂ was in line with prior studies on the brittle nature of conventional sintered TiO₂. In contrast, it was surprising to observe significant plasticity in the flash-sintered TiO₂ deformed at room temperature. No catastrophic failure was observed in the flash-sintered TiO₂ after a strain level of greater than 8% as shown in FIG. 2, Image (b6). Numerous stress-strain plots showed good reproducibility. FIG. 1, Image (f), compares ultimate/fracture strain (%) as a function of testing temperatures for flash-sintered TiO₂, conventional TiO₂ and several other conventional ceramic systems. All data were obtained from compression studies. As shown in FIG. 1, Image (f), fracture strain, in general, was very limited at low temperatures (about 2% or less) and increases with testing temperatures for most conventional ceramics, such as TiO₂, SiCN, YSZ, and TiC. In contrast, the flash-sintered TiO₂ in the current study exhibits significantly improved deformability well below 600° C., and the large plastic strain at room temperature (exceeding 10%) has not been reported in any TiO₂ tested previously.

In ductile metallic materials, plasticity is accommodated by the nucleation and migration of dislocations. As the lattice friction stress is low in metallic materials, dislocations can be highly mobile and improve the plasticity of metallic materials. However, the corresponding friction stress in ceramic materials is typically high due to the strong directional interatomic bonding. What makes the situation worse is the resolved shear stress required for dislocation nucleation in ceramics is exceptionally high, on the order of the theoretical shear strength for perfect crystals under athermal conditions. Hence the high friction stress and resistance to dislocation nucleation lead to the brittle behavior in most ceramics at low to intermediate temperatures. Nanotwinned metals have shown high strength and plasticity as twin boundaries can significantly increase the work hardening ability of metals. However twin boundaries are generally absent in ceramic materials, and twin boundary or stacking faults dominated plasticity is scarce in ceramics. In comparison, flash-sintered TiO₂ already contains abundant pre-existing defects, such as dislocations and short segments of stacking faults, which may foster plastic deformation at room temperature. Moreover, O vacancies and the pre-existing defects could also facilitate the nucleation and propagation of defects, such as the high-density of stacking faults and nanotwins formed during deformation at room temperature.

The grain size and porosity also affected the deformability of TiO₂. The grain size of flash-sintered and conventional sintered TiO₂ was about 10 and about 50 respectively, and about 1 μm for zero-field sintered TiO₂. It was likely that the micropillars (diameters of about 3 μm) fabricated from the flash-sintered and conventional sintered TiO₂ specimens contained one or few grains, while the pillars from zero-field sintered TiO₂ contained multiple grains. Although smaller grain size often improves the plasticity of ceramics, the flash-sintered TiO₂ with large grains has significant plasticity at room temperature compared to the brittle fracture of zero-field sintered TiO₂. It is also worth mentioning that the existence of nano/micropores inside the materials may also have affected the mechanical behavior under compression. The pores inside conventional sintered and zero-field sintered TiO₂ were dominated by intergranular pores, and the shapes of these pores were usually triangular or irregular. In comparison, the pores inside flash-sintered TiO₂ were usually intragranular pores, and their shapes were either circular or faceted as shown in FIG. 1. The investigations suggested that the intergranular pores can act as the sources for crack formation. In comparison, the intragranular pores in flash-sintered TiO₂ had little or no deleterious effect on the plasticity or fracture behavior as shown in FIG. 3, Images (a1), (b1), and (c1).

DFT calculations showed that the stacking fault energy of TiO₂ was low (comparable to some metals like Ag and Cu), 30-40 mJ/m², but the unstable stacking fault energy, which gives an approximation of the barrier to dislocation nucleation and motion, was very high, about 1.9 J/m², as shown in FIG. 4, Image (c). Furthermore, the existence of an O vacancy in planes next to stacking faults did not significantly increase the stacking fault energy of TiO₂ as shown in FIG. 4, Image (d). The density of deformation induced stacking faults was low in conventional TiO₂ deformed at room temperature due to the high unstable stacking fault energy. However, in the flash-sintered TiO₂, there were abundant O vacancies and preexisting short stacking fault segments, which may foster the formation of high-density long stacking faults during deformation. These stacking faults once formed after deformation were stable due to the low stacking fault energy.

Temperature-dependent deformation behavior in flash-sintered TiO₂: In general, conventional sintered TiO₂ will fracture before yielding at room temperature, and hence no yield strength values were reported for TiO₂ at room temperature. The yield stress of TiO₂ tested at 500° C. may be about 800 MPa based on a prior high-temperature test of TiO₂ single crystal specimen. It has also been reported that the flow stress of TiO₂ was sensitive to test temperature. At high temperatures (greater than 600° C.), the primary slip systems in TiO₂ were {101}/<101> and/or {110}/<001> slip systems, and the former slip system was often more active. Low-temperature (less than 600° C.) plasticity in TiO₂, however, has not been reported to date. Surprisingly, significant plasticity was observed in flash-sintered TiO₂ tested at room temperature in the present investigations.

The difference between the continuously serrated stress flows at room temperature (FIG. 2, Image (b 1)) versus the consecutive sudden load-drops at 400° C. (FIG. 2, Image (c1)) was remarkable. Nanoscale stacking fault formation was the predominant deformation mechanism during compression at room temperature (FIG. 3, Images (a1)-(a4)), while deformation-induced twin bands became more pronounced at 400° C. (FIG. 3, Images (b1)-(b3)). The habit planes of the stacking faults and twin bands were both {101}. It has been shown that twinning occurs more readily at higher temperatures in Al₂O₃ and CaCO₃. It has been shown previously the propagation of stacking faults (dislocations) in certain systems is sluggish compared with twin boundaries, and the velocity of twin propagation normally increases with increasing test temperature. Also, the twinning stress is inversely proportional to temperature, i.e., lower temperatures lead to higher twinning stress. Consequently, the deformation behavior of flash-sintered TiO₂ at room temperature was dominated by the stress-induced nanoscale stacking faults that propagate slowly, and thus the corresponding stress-strain curves were continuous with few and small serrations, while the sharp load-drops at 400° C. were caused by the formation and rapid propagation of deformation twins. When the test temperature was 200° C., the deformability was controlled by a mixture of stacking faults and deformation twins, and thus the stress-strain curve has a mixture of large stress-drops and small serrations.

At 600° C., no slip bands were observed in flash-sintered TiO₂, and the stress-strain curves appeared smooth with minor serrations. Post-compression TEM studies showed that a high density of dislocations and stacking fault segments formed after compression at 600° C. (FIG. 3, Images (c1)-(c3)). No shear bands were detected. Diffusion creep may not be significant as the test temperature was still well below the melting point of TiO₂ (about 1800° C.), about 0.33 Tm.

It has been shown that conventional TiO₂ can sustain certain plastic strain when deformed above 600° C. Post compression TEM studies of conventional TiO₂ in the current study showed that there were few scattered stacking faults and twin boundaries accompanied by numerous intragranular cracks.

It should be noted that although flash-sintered TiO₂ contains a small number of preexisting stacking faults, deformation at room temperature induces a significant increase of stacking fault density separated with a spacing of a few nanometers. A majority of these stacking faults may arise from the migration of partials from preexisting dislocations. As the density of deformation induced stacking faults is high, the stress-strain curve appears relatively smooth with small serrations. In addition, the effect of crystal orientation on the mechanical behavior of flash-sintered TiO₂ in this study shall be briefly discussed. In general, the mechanical behavior of ceramics was largely determined by the crystal orientation. In this study, as the average grain size of the flash-sintered TiO₂ was about 5 μm, most flash-sintered pillars have single-crystal or bicrystal-like nature, and these pillars may have different crystal orientation depending on the position at which the pillars were made from the flash-sintered specimens. One may expect that the reproducibility of stress-strain curves should be poor in this study if the crystal orientation plays a significant role in the plasticity of pillars. However, the reproducibility was in general good at all test temperatures. Such a phenomenon suggested that crystal orientation plays a relatively insignificant role in determining the mechanical behavior of flash-sintered TiO₂. The high-density of pre-existing defects may have a major impact on the mechanical behavior of the flash-sintered TiO₂. In addition, residual stress due to orientation dependent coefficient of thermal expansion (CTE) mismatch or defect annihilation if occurred during the pillar fabrication may add some uncertainty on the reproducibility of micropillar compression studies. However, given the comparable large grain size in both conventional and flash-sintered TiO₂, the difference of residual stress between the two types of specimens may be insignificant. Also, the reasonable reproducibility of stress-strain curves for TiO₂ pillars tested at various temperatures suggested that the microstructures developed in flash-sintered TiO₂ may be similar in these pillars, containing high-density defects. For instance, preexisting defects can be readily identified in the lower (largely undeformed) portion of the pillar in FIG. 3, Images (a1) and (b1). Therefore, pre-existing defects that were introduced during flash-sintering process survived after the pillar fabrication.

It should also be noted that non-uniform grain and pore size distributions exist in the current flash-sintered samples, preventing reliable macroscopic mechanical testing. Continuous efforts have been devoted to explore the flash-sintering mechanisms and to achieve better control on the sample uniformity and quality. Promising results have been demonstrated by AC field sintering, controlled current ramping, SPS-flash combined process, and others. Given the potentials in microstructure and defect density control offered by the flash sintering process, flash-sintered ceramics hold great potentials in demonstrating improved ductility in macroscopic specimens under moderate temperatures or room temperature, compared to conventional sintered samples.

Based on the investigations reported above, in situ micropillar compression tests showed that flash-sintered TiO₂ exhibits unexpected significant plasticity at room temperature, sustaining 10% strain without noticeable cracks. The high density of pre-existing defects and O vacancies introduced during the non-equilibrium flash sintering process facilitated the nucleation of dislocations. Room temperature deformation was dominated by the formation of nanoscale stacking faults, followed by the creation of nanotwins when tested at 200-400° C. Dislocation glide takes over the deformation mechanism at about 600° C. The investigations strongly suggested that the flash sintering method of this disclosure presents great potential in promoting deformation plasticity in a range of ceramic materials.

Those skilled in the art will recognize that numerous modifications can be made to the specific implementations described above. The implementations should not be limited to the particular limitations described, as other implementations may be possible. Therefore, the scope of the invention is to be limited only by the following claims.

Claims

1. A method of increasing the deformability of a ceramic material, the method comprising introducing high-density pre-existing defects and oxygen vacancies in the ceramic material during a flash sintering process and then forming nano scale stacking faults and nanotwins in the ceramic material.

2. The method of claim 1, wherein the ceramic material is titanium dioxide (TiO2).

3. A ceramic material with deformability in a temperature range of room temperature to 600° C., the ceramic material containing high-density pre-existing defects and oxygen vacancies and nano scale stacking faults and nanotwins.

4. The ceramic material of claim 3, wherein the ceramic material is titanium dioxide (TiO2).