Oxide glasses are well known for their brittle nature, but new experimental research out of an international research consortium has revealed surprisingly high ductility in thin films of amorphous aluminum oxide (a-Al2O3), or alumina, at room temperature. As reported in Science (doi:10.1126/science.aav1254), the plasticity is activated when an a-Al2O3 sample free of flaws is subjected to a high stress at varying strain rates.
Inorganic oxide glasses are appealing materials for electronic applications because they can be tailored for many different functional properties. They are chemically and thermally stable and transparent to visible light. However, at room temperature the glasses are prone to sudden, catastrophic failure. This has limited their usefulness.
According to conventional theory, inorganic oxide glasses become ductile only when a high temperature activates a relaxation mechanism such as viscous flow or creep. Viscous flow occurs only above a critical temperature; viscous creep typically requires an external load and a critical temperature. Both viscous flow and viscous creep require a critical temperature well above room temperature so the material is effectively a solid at 300 K.
As a graduate student at Tampere University in Finland, project leader Erkka Frankberg was studying the boundaries of plastic deformation in ceramics and glasses when preliminary results hinted that it might be possible to create ductile alumina at room temperature. Working with Erkki Levänen at Tampere University, Karine Masenelli-Varlot at the Université de Lyon in France, and Fabio Di Fonzo at the Istituto Italiano de Tecnologia, Frankberg initiated a collaborative effort to experimentally measure the viscosity of flawless a-Al2O3 thin films at room temperature. “We didn’t have exact knowledge that this would be possible, but there was a chance, so we took it,” he explains.
The researchers prepared thin films of defect-free a-Al2O3 by pulsed laser deposition, with thicknesses of 40 nm and 60 nm. The samples were probed by a custom micromechanical testing device located inside a transmission electron microscope (TEM) at room temperature. The testing device could apply up to 1 mN of force and had two modes, applying either shear-compressive stress or tensile stress to samples. For each mechanical test, the applied force and sample displacement were measured in situ over time and verified by TEM images.
Transmission electron microscope (TEM) images of a freestanding sample under zero strain (a) and after increasing strain until the point of fracture (b). The change in length is due to tensile stress. The sample is highlighted with a white border; a film can be seen partially overlapping the sample on the bottom right but does not impact the test. Scale bars are 500 nm. Reprinted with permission from Frankberg et al., Science 366 (6467), 864 (2019).
While observing a test through the TEM, Frankberg realized the team could be on to something important. “[The sample] stretched and it stretched,” he recalls. The researchers observed samples elongate by up to 100% under mixed shear/compression loading. Furthermore, the deformation depended on the rate at which the load was applied, implying that a viscous relaxation mechanism had been activated.
Measurements revealed a log-log linear relationship between viscosity and strain rate. As strain rate went to zero, viscosity approached infinity and the material became solid-like. When finite strain was applied, viscosity decreased and the material became plastic. TEM images implied that the Al2O3 samples remained amorphous during deformation and could be considered a supercooled liquid.
The researchers ran atomistic simulations to explore the cause of this unusual behavior. The simulations showed initial plastic deformation caused by density changes in a-Al2O3, but this accounted for less than 2% of the permanent elongation of the axis under tension. The density saturated at around 25% of the total tensile strain and at higher strains the plastic deformation was caused only by viscous creep. In contrast to conventional theory, creep was activated almost solely by the applied load.
According to the simulations, room-temperature plasticity was facilitated by a bond-switching phenomenon well known in amorphous materials. The load prompted individual aluminum atoms to move into open spaces and replace existing oxygen bonds with bonds to neighboring oxygen atoms. As the number of individual bond-switching events accumulated, weaker local atomic groups yielded and enabled flow across the material.
The striking ductility of room-temperature a-Al2O3 challenges the conventional view of what is possible with amorphous oxide materials according to Morten Smedskjær, an expert on oxide glass materials at Aalborg University in Denmark, who was not associated with this research. He notes that it will be interesting to explore whether mechanical activation can induce bond switching, and therefore plasticity, in other disordered oxides.
The atomistic simulations spotlight likely requirements for plasticity in oxide glasses at room temperature. According to the research team, bond switching between neighboring atoms is facilitated by low effective activation energy and high atomic density—both of which hold in a-Al2O3. Furthermore, fractures are often initiated at the locations of defects and cavities, so a flawless material is key.
Frankberg, now a Marie Curie Research Fellow at the Instituto Italiano de Tecnologia, and his collaborators are currently studying this process in detail, working toward a comprehensive theory that would enable researchers to predict which glasses might exhibit plasticity at room temperature. “At this point we still don’t have enough knowledge to make direct predictions,” Frankberg says. “And that means we don’t have a full theory yet.”
Room-temperature ductile oxide glasses have wide-ranging potential in areas ranging from flexible electronics to batteries and displays, but that requires significant scaling. There are no theoretical restrictions to scaling, according to the researchers. “Instead, the challenge appears fully technological, given that we lack processing technology that could produce such flaw-free amorphous materials at a macroscopic scale,” the authors wrote. According to Frankberg, tackling this challenge will likely require a community effort that involves academic and industry research teams.
