| 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 to 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 |
