[Image above] Bearing balls made of silicon nitride. Researchers in China discovered a way to plastically deform nanopillars made from this ceramic by harnessing a dual-phase structural configuration. Credit: Lucasbosch, Wikimedia (CC BY-SA 3.0)
By Laurel Sheppard
Ceramics are well known for their brittleness. However, it wasn’t until the 1950s that scientists confirmed the reason for why ceramics so strongly resist deformations—because of their dislocation structure.
Dislocations, or line defects, are areas where atoms are arranged anomalously compared to the rest of the perfect crystal structure. When stress is applied, movement of these dislocations allows atoms to slide over each other, leading to plastic deformation of the material rather than brittle fracture.
Ceramics have far fewer and less mobile dislocations than metals due to both innate and process-related reasons. For one, ceramic materials contain mostly covalent and ionic bonds, which are stronger than metallic bonds and so restrict movement of dislocations. Additionally, the conventional sintering process for ceramics causes grain coarsening, which lowers the number of dislocations in the structure.
Researchers have explored different ways to improve a ceramic’s ability to plastically deform, for example, by using alternative processing routes to retain dislocations or applying mechanical pressure to generate dislocations. Some of these recent advances were covered in the November 2022 episode of Ceramic Tech Chat, which featured Xufei Fang sharing his knowledge and research on this topic.
Fang is junior group leader in the nonmetallic inorganic materials research group at the Technical University of Darmstadt in Germany. In the episode, he explained that most research to date on ceramic dislocations has involved oxide ceramics. However, a recent study led by researchers at Tsinghua University in China demonstrated the possibility of plastic deformation in nonoxide ceramics as well, specifically silicon nitride.
The researchers of the recent study explain that the deformation of nonoxide ceramics is complicated by their bonding structure. Compared to oxide ceramics, which feature mainly nondirectional ionic bonds, nonoxide ceramics contain mainly covalent bonds, which exhibit strong and directional characteristics.
“As such, realizing the plastic deformability of covalently bonded crystalline ceramics has been a longstanding and critical challenge,” they write.
Theoretically, plastic deformation could be achieved by breaking covalent bonds in a very small volume and then immediately healing the area through new bond formations, a phenomenon the researchers term “bond switching.” However, to achieve bond switching, the energy barrier for local atomic rearrangements must be lowered to enable a relatively easy transition of one bonding configuration to another. Simultaneously, the lattices on both sides of the slip plane would need to have roughly the same unit atomic distance to ensure successive atomic translations.
In their paper, the researchers propose that a dual-phase structure with a coherent (well-matching) lattice interface may meet these requirements. Under normal conditions, silicon nitride has two polymorphs, an alpha phase and beta phase. These phases have similar hexagonal lattice structures but different lattice constants along the c direction (the alpha constant is twice that of the beta one).
Silicon nitride is usually synthesized via liquid-phase sintering, which results in a sample that only contains the beta phase. To produce silicon nitride containing both phases, the Tsinghua-led researchers used a spark plasma sintering process that involved sintering a powder mixture of Si3N4: Al2O3: Y2O3 (weight ratio of 90:6:4) at 1,550℃ for 5 minutes with 30 MPa pressure and a heating rate of 150℃/min.
Five samples with different percentages of alpha-beta coherent interfaces were prepared by adjusting the alpha-phase content in the initial powder mixture. Analysis with high-angle annular dark-field scanning transmission electron microscopy showed the angle mismatch between the alpha and beta subgrains was less than 1.8°, indicating a high degree of lattice matching.
Mechanical properties were evaluated through nanopillar compression tests. This approach was selected to minimize the ambiguities caused by structural defects resulting from the lower sintering temperature and shorter holding time.
The nanopillar-shaped samples showed higher strength and plastic deformation values than conventional single-beta-phase silicon nitride. The sample with a coherent interface proportion of 32% exhibited the best performance, with a fracture strength of about 11 GPa and plastic strain of about 20%. This strength is more than twice that of conventional single-beta-phase silicon nitride (4.7 GPA), which demonstrates no plastic deformation.
Closer inspection during the compression tests confirmed that a stress-induced beta-to-alpha phase transformation was behind the plastic deformation success. This observation was surprising because, until now, researchers have only witnessed alpha-to-beta phase transformations in silicon nitride due to the higher stability of the beta phase.
The researchers attributed the beta-to-alpha phase transformation to movement of the coherent interfaces during compression. Specifically, the transformation involved
- Sliding at the beta/alpha interface
- Intralayer transformation from beta to alpha phase
- A second intralayer transformation from beta to alpha phase
- Sliding at the beta/alpha interface, resulting in a stacking layer of the alpha phase
Ultimately, this process results in all three neighboring layers in the beta phase transforming into the alpha phase. Eventually, the remaining beta phase layer will also transform into the alpha phase as the four-step process continues.
Interestingly, this beta-to-alpha transformation is a diffusionless solid-state process that takes place with no liquid participation. In contrast, the well-known alpha-to-beta transformation is a thermally induced dissolution–reprecipitation process with the presence of a liquid phase.
The newly observed beta-to-alpha phase transformation is superficially similar to the martensitic phase transformation in the oxide ceramic ZrO2, with both being a diffusionless process that leads to plastic deformation. But the researchers emphasize there are key differences between these transformations. Specifically, while the ionically bonded ZrO2 transformation is a displacive process realized by lattice shearing, the covalently bonded silicon nitride transformation is instead a reconstructive process realized through an extra rotation of the [NSi3] units.
In the conclusion, the researchers state that this dual-phase solution to plastic deformation could likely be extended to other nonoxide ceramics, for example, by harnessing the cubic and hexagonal phases of silicon carbide.
In a perspective piece on the research published in Science, Tampere University academy postdoctoral researcher Erkka Frankberg applauds the study and notes how it provides an opportunity to extend the understanding of plastic deformation in ceramics at low temperatures.
“Future studies should investigate whether bulk Si3N4 gains similar benefits from the stress-induced microstructural transformations as ZrO2 and whether the required coherent interfaces remain stable at temperatures and process conditions that are relevant for practical applications,” he concludes.
The paper, published in Science, is “Plastic deformation in silicon nitride ceramics via bond switching at coherent interfaces” (DOI: 10.1126/science.abq7490).