| dc.description.abstract | Nanomechanical responses of brittle materials, such as glass and ceramics, are important for various industrial applications. Understanding their deformation and failure mechanisms would offer new knowledge and help design materials with better performance. Among brittle materials, boron carbide is of interest due to its low density, high hardness, and chemical inertness. However, the nanomechanical responses of boron carbide are less known compared to other brittle materials. In this work, we performed nanoindentation at various loads with a Berkovich indenter on the undoped, B-doped, and B/Si co-doped boron carbide samples to investigate their nanomechanical responses. Pop-in events were observed in the load-displacement curves for all samples. The loads to trigger the first pop-in in the B-doped boron carbide are rather scattered. In contrast, consistent distributions of the first pop-ins, which corresponds Microbial communities have a widespread distribution on earth, ranging from soil to human gut. Each microbial community often contains various bacterial species. Interactions between species and their activities within microbial communities shape the environment where they live. To survive in such an environment, bacteria develop diverse strategies to interact with each other. To study these strategies, we developed a genetically tractable two-species model system composed of Bacillus subtilis and Streptomyces venezuelae. The interaction between the two species activates a mobile response in B. subtilis.
To uncover the molecular basis underlying this interspecies interaction, I first characterized the mobile response as sliding motility and identified the motility inducer as a ribosome-targeting antibiotics, chloramphenicol (Cm) and its brominated derivative at subinhibitory concentrations. Moreover, I determined that the sliding response is tied to the protein synthesis stress. To understand the connection between the subsequent responses after protein synthesis stress and the underlying genetic determinants, I used a combination of transcriptomics and metabolomics approaches to identify the key factors that contribute to the initiation of sliding motility. Results from these experiments suggest that a regulatory network leading to metabolic reprogramming governs the sliding response in response to antibiotic insults. Finally, I examined how B. subtilis survives in the presence of antibiotic-producing streptomycetes and found that the intrinsic antibiotic resistance coupled with sliding motility enables B. subtilis to fit into their shared habitat. Overall, these findings indicate that bacteria can sense the invasion from competitors via detecting low-dose antibiotics and respond by adjusting their metabolic activities governed by complex gene regulation systems to control their physiological adaptations.the onset of plasticity, were noted in the undoped and B/Si co-doped boron carbide. With that, the power-law fitting and the standard Hertzian contact fitting were applied for these materials to derive the maximum shear stress induced by nanoindentation, with 25.94±0.57 GPa for the undoped boron carbide and 24.47±0.52 GPa for the B/Si co-doped boron carbide. Besides, cracking (both the surface and subsurface ones) was also induced by nanoindentation. The surface cracks were studied through scanning electron microscopy. Surface crack lengths were measured and then applied to estimate the indentation fracture toughness through the model assuming half-penny cracks. The estimated indentation fracture toughness values for undoped, B-doped, and B/Si co-doped boron carbide are 3.15±0.65 MPa m^(1/2),4.91±0.66MPa m^(1/2), and 2.79±0.39 MPa m^(1/2), respectively. To verify the assumption, serial focused ion beam cross-sectioning was employed to reveal the subsurface cracks beneath the 500mN-load indentation impressions. The results suggested the model overlooked the contribution from edge cracks and lateral cracks. To sum up, from the nanomechanical responses, we revealed that 1) the maximum shear stress required to trigger the first pop-in event is higher in the undoped boron carbide, and 2) the B-doped boron carbide displays the highest indentation fracture toughness although the true value is underestimated. This work also illuminated how the dopants influence the deformation behavior behind the physical phenomena. | en |
