Mechanical Behavior Investigation of 4H-SiC Single Crystal at the Micro-Nano Scale

Impact of metastable graphene-diamond coatings on the fracture toughness of silicon carbide

Silicon carbide has excellent mechanical properties such as high hardness and strength, but its applications for body armor and protective coating solutions are limited by its poor toughness. It has been demonstrated that epitaxial graphene-coated SiC can enhance SiC mechanical properties due to the pressure-activated phase transition into a sp3 diamond structure. Here, we show that atomically thin graphene coatings increase the hardness of SiC even for indentation depths of ∼10 μm. Very importantly, the graphene coating also causes an increase of the fracture toughness by 11% compared to bare SiC, which is in contradiction with the general indirect variation of hardness and fracture toughness. This is explained in terms of the presence of a diamond phase under the indenter while the rest of the coating remains in the ultra-tough graphene phase. This study opens new venues for understanding hardness and toughness in metastable systems and for the applications of graphene-coatings.

Effect of Strain Rate on Nano-Scale Mechanical Behavior of A-Plane (112¯0) ZnO Single Crystal by Nanoindentation

In this study, nanoindentation tests at three different strain rates within 100 nm indentation depth were conducted on an a-plane (112¯0) ZnO single crystal to investigate the effect of strain rate on its nano-scale mechanical behavior. The load-indentation-depth curves, pop-in events, hardness and Young's moduli of an a-plane (112¯0) ZnO single crystal at different strain rates were investigated at the nano-scale level. The results indicated that, with the indentation depth increasing, the load increased gradually at each maximum indentation depth, h_ma, during the loading process. A distinct pop-in event occurred on each loading curve except that corresponding to the h_max of 10 nm. The applied load at the same indentation depth increased with the increasing strain rate during the nanoindentation of the a-plane (112¯0) ZnO single crystal. The higher strain rate deferred the pop-in event to a higher load and deeper indentation depth, and made the pop-in extension width larger. The hardness showed reverse indentation size effect (ISE) before the pop-in, and exhibited normal ISE after the pop-in. Both the hardness and the Young's modulus of the a-plane (112¯0) ZnO single crystal increased with the increasing strain rate, exhibiting the positive strain-rate sensitivity.

Giant Increase of Hardness in Silicon Carbide by Metastable Single Layer Diamond-Like Coating

Silicon carbide (SiC) is one of the hardest known materials. Its exceptional mechanical properties combined with its high thermal conductivity make it a very attractive material for a variety of technological applications. Recently, it is discovered that two-layer epitaxial graphene films on SiC can undergo a pressure activated phase transition into a sp³ diamene structure at room temperature. Here, it is shown that epitaxial graphene films grown on SiC can increase the hardness of SiC up to 100% at low loads (up to 900 µN), and up to 30% at high loads (10 mN). By using a Berkovich diamond indenter and nanoindentation experiments, it is demonstrated that the 30% increase in hardness is present even for indentations depths of 175 nm, almost three hundred times larger than the graphene film thickness. The experiments also show that the yield point of SiC increases up to 77% when the SiC surface is coated with epitaxial graphene. These improved mechanical properties are explained with the formation of diamene under the indenter's pressure.

Atomic Insights into Fracture Characteristics of Twisted Tri-Layer Graphene

Graphene twistronics have recently gained significant attention due their superconductive behavior as a consequence of their tunable electronic properties. Although the electronic properties of twisted graphene have been extensively studied, the mechanical properties and integrity of twisted trilayer graphene (tTLG) under loading is still elusive. We investigated the fracture mechanics of tTLG with a twist angle of ±1.53° utilizing molecular dynamics simulation. This twist angle was chosen because it is known to exhibit highly superconductive behavior. The results indicate that tTLG does not preserve the excellent mechanical properties typically associated with graphene, with toughness and fracture strain values much lower in comparison. The Young’s modulus was an exception with values relatively close to pristine graphene, whereas the tensile strength was found to be roughly half of the intrinsic strength of graphene. The fracture toughness, fracture strain and strength converge as the crack length increases, reaching 0.26 J/m3, 0.0217 and 39.9 GPa at a crack length of 8 nm, respectively. The Griffth critical strain energy is 19.98 J/m2 and the critical stress intensity factor Kc is 4.47 MPa M1/2, in good agreement with that of monolayer graphene in the experiment. Our atomic insights might be helpful in the material design of twisted trilayer graphene-based electronics.

Study on Damage of 4H-SiC Single Crystal through Indentation and Scratch Testing in Micro–Nano Scales

In this paper, a series of indentation tests in which the maximum normal force ranged from 0.4 to 3.3 N were carried out to determine the fracture toughness of 4H-SiC single crystals. The results indicated that an appropriate ratio of the distance from the indentation center to the radial crack tip to the distance from the indentation center to the indentation corner is significant to calculate fracture toughness of 4H-SiC single crystals. The critical condition with no cracks on the edge of the indentation was obtained through a fitting method. The surface morphologies of the groove were analyzed by scanning electron microscopy (SEM). Plastic deformation was observed and characterized by the smooth groove without cracks and ductile chips on the edge of the groove in the initial stages of scratch. With increased normal force, median cracks, radial cracks, and microcracks appeared in turn, followed by the crack system no longer being able to stably extend, causing the brittle fracture to dominate the material removal. The size of the edge damages were measured through SEM and the experimental data highly agreed with the predicted curve. A modified calculation model considering elastic recovery of the sample by the indenter during the scratching process was suggested. These results prove that elastic recovery of 4H-SiC single crystals cannot be ignored during ultra-precision machining.