Nanoscale Elastic-plastic Transitions, Mechanical Properties and Deformation Mechanisms of SiC Materials

Abstract

This PhD thesis reports the stress distributions, nanoscale elastic-plastic properties and indentation-induced deformation mechanism of four SiC-based materials by using nanoindentation with a Berkovich indenter. These materials include amorphous silicon carbide (a-SiC) and cubic silicon carbide (3C-SiC) thin films, 4H-SiC and 6H-SiC single crystals.
Firstly, nanoindentation analysis of a-SiC reveals that localized deformation occurs due to local readjustment of small clusters of atoms. The deformation mechanism is also explained by shear transformation zone (STZ) based amorphous plasticity theory. In addition, the average values of elastic modulus and hardness of the sample in elastic and elastic plastic regime were found to be 170 ± 10 and 11.0 ± 0.8 GPa, respectively. By integrating Johnson cavity model and Hertzian contact theory, yield strength (14.4 GPa), maximum shear stress (7.7 GPa), plastic zone size (30-300 nm) and plastic work ratio (0.18-0.40) were also determined. For 4H-SiC single crystal, the nanoindentation tests were carried out over a range of indentation loads starting from 50 to 9000 µN. Load-displacement curves were utilized to identify useful information about the elastic-plastic response of the 4H-SiC. No piling-up of specimen was found. Indentation size effect (ISE) was observed in the samples which were validated by Nix-Gao and proportional specimen models. Hardness and elastic modulus were measured to be 36 ± 2 and 413 ± 8 GPa, respectively, in load independent regions. Energy-based theory was also used to determine various mechanical properties including the so-called true hardness (119 GPa), maximum shear stress (23.5 GPa) and plastic energy (125-425 nm). The calculated values of maximum tensile strength (13.5 GPa) and cleavage strength (33 GPa) confirm that deformation is caused by pop-in nucleation. Transmission electron microscope showed that basal plane is more susceptible to slip in comparison to other planes.
Secondly, the response of the materials under various loading rates was investigated on 6H-SiC single crystal. Increase in critical pop-in load, pop-in displacement and shear stress with increasing loading rate further indicates that no phase transformation occurred in 6H-SiC specimen. In addition, elastic modulus and hardness were found to be ~383 and ~ 37-50 GPa, respectively. The maximum shear stress at the first pop-in event was also determined as 24 GPa. The outcome of the maximum tensile strength (13.5 GPa) and cleavage strength (31 GPa) revealed that deformation in 6H-SiC was also caused due to dislocation nucleation. Moreover, homogeneous nucleation of dislocation occurred in 6H-SiC because theoretical shear strength (25 GPa) and maximum shear stress (24 GPa) were in close agreement.
Thirdly, the nanoindentation tests were carried out on 3C-SiC thin film over a range of indentation loads starting from 50 to 5000 µN. Increasing trend of critical pop-in load and shear stress due to increase in loading rate further negates the possibility of phase transformation in 3C-SiC thin film. The ISE was observed in the 3C-SiC sample which was validated by Nix-Gao model. Depth independent hardness was also calculated on the basis of proportional resistance model. In addition, other mechanical properties including the Sakai hardness (104 GPa), indentation volume (from 2104 to 340104 nm3) and work of indentation (20 nJ/µm3) were also determined. The rise of piles-up was observed by the use of scanning probe microscopy. Analysis of experimental results reveals that pop-in was caused by dislocation nucleation rather than tensile stresses. The deformation in the slip planes was confirmed by using maximum shear stress, Schimdt factor analysis and inter-planner spacing. Moreover, it was also found that glide plane, i.e., {111} ⟨112⟩, is more susceptible to slip rather than other planes.
Finally, the main contributions of this work are summarized and the further work is suggested.

Date of Award	25 Aug 2017
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yaogen SHEN (Supervisor)