Cube Corner Indenter Research Articles

Mechanically Biased Self-Assembly of Quantum Dots by Nanoindentation

AbstractNanoindentations were created in the GaAs(100) surface to act as strain centers to bias the nucleation of self-assembled InAs quantum dots providing for patterned growth. Indents were generated using loads below 450 μN with a sharp cube corner indenter. Growth of InAs quantum dots on indent patterns is performed using molecular beam epitaxy (MBE). The effect of indent spacing and size on the patterned growth is investigated. The structural analysis of the quantum dots including spatial ordering, size, and shape are characterized by ex-situ atomic force microscopy (AFM). Results reveal that the indent patterns clearly bias nucleation with dot structures selectively growing on top of each indent. It is speculated that the biased nucleation is due to a combination of favorable surface strain and multi-atomic step formation at the indent sites, which leads to increased adatom diffusion on the patterned area.

Deformation and failure mechanism of nano-composite coatings under nano-indentation

Two nano-composite coatings based on nc-TiC particles in an a-C:H matrix are deposited via closed-field unbalanced reactive magnetron sputtering. The compositions of the coatings are varied by changing the acetylene gas flow during the depositions. A Cr/Cr–Ti/Ti–TiC graded interlayer is introduced between substrate and coating. Electron probe micro-analyses (EPMA) show that the Ti content of the coatings varies between 31.7 and 11.5 at.%. The coatings exhibit a hardness ( H) of 20.0 and 15.7 GPa, and a Young's modulus ( E) of 229.4 and 136.6 GPa, respectively, as measured through nano-indentations. Cube corner indentations are performed to probe the fracture toughness of the coatings through the determination of critical indentation loads ( L r) at which radial cracks start to propagate. Transmission electron microscopy (TEM) observations and energy-filtered TEM are employed to characterize the coatings nanostructures. The variation in Ti content is accompanied by a variation in TiC particle size and volumetric fraction, as well as a change in the columnar structure of the coatings. A focus ion beam (FIB) slicing technique is employed to prepare samples from nano-indented locations of coated Silicon and stainless steel (SS) substrates. TEM inspection of the FIB sliced samples determines that the most brittle phase in the coating is the C-enriched columnar boundary, and identifies the location of failure within the interlayer. As a consequence of the different nanostructure, the coatings exhibit different elastic recovery properties and toughness.

Radial Fracture During Indentation by Acute Probes: II, Experimental Observations of Cube-corner and Vickers Indentation

The companion article proposed a model for radial crack development at sharp contacts. The major extension of this model from previous works is the inclusion of a ‘wedging’ mechanism, to form a three-stress-field description of indentation crack evolution. Here, the amplitude terms of the three stress-intensity factors comprising the model are calibrated from experimental in situ and post situ inert-environment radial crack measurements on soda-lime glass. These values are scaled to predict radial crack evolution during cube corner and Vickers indentation of fused silica and soda lime glass in inert and ambient air environments. Both the conventional two-field and the proposed three-field model predictions are compared with radial crack lengths measured during indentation load-unload cycles (through the transparent materials with an in-situ apparatus). The three-field model is shown to be a great improvement over the two-field model in the description of crack evolution at cube-corner indentations, particularly with respect to the significant crack extension during loading and the attainment of a maximum crack length during unloading. The three-field model is consistent with observations of Vickers fracture in soda-lime glass and is able to reproduce the features of radial fracture evolution on the ‘anomalous’ glass, fused silica.

Fracture toughness, hardness and elastic modulus of hydrogenated amorphous carbon films deposited by chemical vapor deposition

Fracture toughness, hardness and elastic modulus of hydrogenated amorphous carbon thin films produced from butene gas deposited by chemical vapor deposition on silicon were measured by depth sensing indentation. Voltage bias varying from − 60 to − 400 V and pressures of 2 and 8 Pa were used for deposition. Cube corner indentation produces film chipping at loads lower than 400 mN. A new approach to measure toughness was proposed to determine the energy released during film chipping. Fracture toughness results from this new approach are in between of the ones obtained from methods proposed in the literature.

Characterization of ultra-low-load(µN) nanoindents in GaAs(100) using a cube corner tip

In this study, nanoindentations are produced and characterized for the future patterning ofnanostructures. Nanoindentation is performed on Si-doped (n-type) vertical gradient freeze(VGF) GaAs(100) and epitaxial GaAs(100) using a diamond cube corner indenter. Unlikeprevious research, the uniqueness of this work is in nanoindentation of GaAs at ultra-low loads(<400 µN). Indentations of less than 200 nm in width are produced, and the mechanical properties ofthe two materials including hardness and elastic modulus are determined. The smallestindentations achieved are less than 60 nm in width and less than 2 nm deep. The width,depth, shape, and volume of the indents are determined as a function of appliedload using atomic force microscopy (AFM). Also, the ratio of pile-up volume toindent volume is determined. The experimental findings are discussed in relation toexisting theories of indentation and for the directed self-assembly of nanostructures.

Evidence for nanoindentation-induced phase transformations in germanium

Nanoindentation experiments were performed using Berkovich and cube-corner indenters to investigate whether nanoindentation-induced phase transformations, such as those observed in silicon, also occur in germanium. Although the indentation load-displacement curves for germanium do not show the unloading pop-out or elbow phenomena observed in silicon, clear evidence for phase transformations was obtained by scanning electron microscopy (SEM) and micro-Raman spectroscopy. SEM showed that there is extruded material around the contact periphery of cube-corner hardness impressions that is metalliclike in its flow characteristics, just as in silicon. Micro-Raman spectroscopy revealed more direct evidence by identifying amorphous and what may be the crystalline BC8 (Ge-IV) phase. The fact that these phenomena are observed primarily and reproducibly only for the cube-corner indenter suggests that the contact geometry significantly affects the transformation behavior. Results are discussed in terms of possible deformation mechanisms and how they may be influenced by the indenter geometry.

Nanohardness and fracture toughness of combustion chemical vapor deposition deposited yittria stabilized zirconia–alumina films

Composite films with compositions of 100% yttria stabilized zirconia (YSZ) and YSZ with 15, 30, 45, 62.8 (eutectic composition), 80 mol% alumina and 100% alumina films were deposited onto sapphire substrates using combustion chemical vapor deposition. Precursors of yttrium 2-ethylhexanoate, zirconium 2-ethylhexanoate and Al acetylacetonate dissolved in toluene at a total metal ion concentration of 0.002 M were used to produce films up to 1 μm thick. Flame temperatures at the substrate surfaces were 1550±50 °C and deposition rates fell between of 0.76–1.7 μm/h, depending on composition. Nanohardness, determined with a Berkovich indenter, was constant at about 15 GPa for compositions less than 100% alumina. The 100% alumina films were about twice as hard as other films. The films' fracture toughness, determined with a cube corner indenter, increased with alumina content from 1.76±0.46 MPa m 0.5 with no alumina to 2.49±0.32 MPa m 0.5 at 30 mol% alumina. Further alumina increases had little effect on fracture toughness, with about 2.2 MPa m 0.5 being the fracture toughness at 100% alumina. Eutectic composition films, that were annealed for 2.5–10 h at 1500 °C, displayed coarsening of the second phase YSZ particles. Film hardness decreased by about half (∼22 to ∼11 GPa) after five or more hours of annealing, while fracture toughness reached a maximum of 3.33 MPa m 0.5 after a 5 h anneal.

Analysis of Nanoscale Deformation in GaAs(100): Towards Patterned Growth of Quantum Dots

AbstractNanoindentations were created in the GaAs(100) surface to act as strain centers for the controlled nucleation and patterning of InAs quantum dots. A systematic approach is taken to understand the indent deformation processes that may lead to precision patterning capabilities for a broad range of nanostructures. Indents were generated using loads below 450 μN with a sharp cube corner indenter. Site-specific cross-sectional thinning of nanoindents (down to 100 nm in size) has been achieved using the in-situ ‘lift-out’ technique. This allowed for observation of subsurface deformation by transmission electron microscopy (TEM). The crystal was observed to deform solely by dislocation activity with no evidence of amorphization, twinning, fracture, or phase transformation. It is shown that the single phase deformation of GaAs can be well characterized and controlled, which should allow for exploitation of the dislocation strain field to bias nucleation of self-assembled quantum dots.

Residual stress determination on lithium disilicate glass-ceramic by nanoindentation

A recently developed method for measuring residual stresses around crystals embedded in the glass matrix, using nanoindentation with cube corner indenter, was used in a lithium disilicate glass-ceramic. We observed that there is a tangential tensile stress at the edge of the crystals that opens and propagates the crack in a direction normal to crystal surfaces. The residual stresses around the crystals are concentrated in the region at distances smaller than 100μm from the crystal surfaces. At greater distances the stresses are smaller. The limitations and improvements to the method are discussed.

Growth of Ti3SiC2 thin films by elemental target magnetron sputtering

Epitaxial Ti3SiC2(0001) thin films have been deposited by dc magnetron sputtering from three elemental targets of Ti, C, and Si onto MgO(111) and Al2O3(0001) substrates at temperatures of 800–900°C. This process allows composition control to synthesize Mn+1AXn (MAX) phases (M: early transition metal; A: A-group element; X: C and∕or N; n=1–3) including Ti4SiC3. Depositions on MgO(100) substrates yielding the Ti–Si–C MAX phases with (101¯5), as the preferred orientation. Samples grown at different substrate temperatures, studied by means of transmission electron microscopy and x-ray diffraction investigations, revealed the constraints of Ti3SiC2 nucleation due to kinetic limitations at substrate temperatures below 700°C. Instead, there is a competitive TiCx growth with Si segregation to form twin boundaries or Si substitutional incorporation in TiCx. Physical properties of the as-deposited single-crystal Ti3SiC2 films were determined. A low resistivity of 25μΩcm was measured. The Young’s modulus, ascertained by nanoindentation, yielded a value of 343–370GPa. For the mechanical deformation response of the material, probing with cube corner and Berkovich indenters showed an initial high hardness of almost 30GPa. With increased maximum indentation loads, the hardness was observed to decrease toward bulk values as the characteristic kink formation sets in with dislocation ordering and delamination at basal planes.

Nanoscratching on surfaces: the relationships between lateral force, normal force and normal displacement

The relationship between lateral force, normal force and normal displacement has been investigated. Surfaces of diverse materials were nanoscratched at constant rate. Nanoindents with cube corner, Berkovich or cono-spherical indenter indicate proportionality of (normal force) ∼ (normal displacement) 3/2 up to considerable forces, sometimes approaching the 10 mN range, but phase transformations under pressure may change the slope. This differs from the varying Meyer exponents reported at forces higher by two to three magnitudes when applied to Vickers indenters. Nanoscratching on diverse materials consistently reveals proportionality of (lateral force) ∼ (normal force) 3/2 . This relationship is valid for amorphous (fused quartz) and crystalline materials such as silicon, silica, strontium titanate and the polar molecular crystals of thiohydantoin and ninhydrin with hydrogen bonds, or pure van-der-Waals crystals of tetraphenylethylene. Anisotropies of the crystals and the type of destruction (breaking covalent bonds, or hydrogen bonds, or van-der-Waals interactions; abrasion, or molecular movements) do not affect the obviously universal relationship but only the proportionality factor with the dimension μN -0.5 which describes materials response to the scratching action for the particular crystal face and anisotropic direction. The new quantitative relationship is usable for the calculation of scratch work and of other quantities related to the lateral force on the basis of the new empirical constants. Both new relationships from nanoindents and nanoscratches are unified in the (lateral force) ∼ (normal displacement) 9/4 relationship and experimentally verified.

Tip-radius effect in finite element modeling of sub-50 nm shallow nanoindentation

An accurate representation of the indenter geometry is essential for correct finite element simulations of shallow indentations of less than 50 nm using a 90° cube-corner indenter. A nonlinear regression method for estimating the tip radius of an indenter is presented, which takes into account that initially the contact is only with the spherical surface of the tip and subsequently also includes contact with the equivalent conical surface. The tip radius of a Berkovich indenter is estimated by a finite element modeling (FEM) best-fitting method. Using the estimates of tip radii, the yield strength of gold in the film of a gold/silicon system was estimated from the best fit between FEM simulations and nanoindentation experiments using the 90° cube corner indenter, which compared favorably with an FEM simulation and nanoindentation data using a Berkovich indenter.

Acoustic Emission Analysis of Nanoindentation-Induced Fracture Events

ABSTRACTMonitoring of acoustic emission (AE) activity was employed to characterize the initiation and progression of local failure processes during nanoindentation-induced fracture. Specimens of various brittle materials were loaded with a cube-corner indenter and AE activity was monitored during the entire loading and unloading event using an AE transducer mounted inside the specimen holder. As observed from the nanoindentation and AE response, there were fundamental differences in the fracture behavior of the various materials. Post-failure observations were used to identify particular features in the AE signal (amplitude, frequency, rise-time) that correspond to specific types of fracture events. Furthermore, analysis of the parametric and transient AE data was used to establish the crack-initiation threshold, crack-arrest threshold, and energy dissipation during failure. It was demonstrated that the monitoring of AE signals yields both qualitative and quantitative information regarding highly local failure events in brittle materials.

Micro-Raman Mapping and Analysis of Indentation-Induced Phase Transformations in Germanium

ABSTRACTAlthough it has been confirmed by diamond anvil cell experiments that germanium transforms under hydrostatic pressure from the normal diamond cubic phase (Ge-I) to the metallic β-tin phase (Ge-II) and re-transforms to Ge-III (ST12 structure) or Ge-IV (BC8 structure) during release of the pressure, there are still controversies about whether the same transformations occur during nanoindentation. Here, we present new evidence of indentation-induced phase transformations in germanium. Nanoindentation experiments were performed on a (100) Ge single crystal using two triangular pyramidal indenters with different tip angles - the common Berkovich and the sharper cube-corner. Although the indentation load-displacement curves do not show any of the characteristics of phase transformation that are well-known for silicon, micro-Raman spectroscopy in conjunction with scanning electron microscopy reveals that phase transformations to amorphous and metastable crystalline phases do indeed occur. However, the transformations are observed reproducibly only for the cube-corner indenter.

Cross-Sectional TEM Studies of Indentation-Induced Phase Transformations in Si: Indenter Angle Effects

ABSTRACTNanoindentations were made on a (100) single crystal Si wafer at room temperature with a series of triangular pyramidal indenters having centerline-to-face angles ranging from 35° to 85°. Indentations produced at high (80 mN) and low (10 mN) loads were examined in plan-view by scanning electron microscopy and in cross-section by transmission electron microscopy. Microstructural observations were correlated with the indentation load-displacement behavior. Cracking and extrusion are more prevalent for sharp indenters with small centerline-to-face angles, regardless of the load. At low loads, the transformed material is amorphous silicon for all indenter angles. For Berkovich indentations made at high-load, the transformed material is a nanocrystalline mix of Si-I and Si-III/Si-XII, as confirmed by selected area diffraction. Extrusion of material at high loads for the cube-corner indenter reduces the volume of transformed material remaining underneath the indenter, thereby eliminating the pop-out in the unloading curve.

Micro-Raman Mapping and Analysis of Indentation-Induced Phase Transformations in Germanium

ABSTRACTAlthough it has been confirmed by diamond anvil cell experiments that germanium transforms under hydrostatic pressure from the normal diamond cubic phase (Ge-I) to the metallic β-tin phase (Ge-II) and re-transforms to Ge-III (ST12 structure) or Ge-IV (BC8 structure) during release of the pressure, there are still controversies about whether the same transformations occur during nanoindentation. Here, we present new evidence of indentation-induced phase transformations in germanium. Nanoindentation experiments were performed on a (100) Ge single crystal using two triangular pyramidal indenters with different tip angles - the common Berkovich and the sharper cube-corner. Although the indentation load-displacement curves do not show any of the characteristics of phase transformation that are well-known for silicon, micro-Raman spectroscopy in conjunction with scanning electron microscopy reveals that phase transformations to amorphous and metastable crystalline phases do indeed occur. However, the transformations are observed reproducibly only for the cube-corner indenter.

Cross-Sectional TEM Studies of Indentation-Induced Phase Transformations in Si: Indenter Angle Effects

ABSTRACTNanoindentations were made on a (100) single crystal Si wafer at room temperature with a series of triangular pyramidal indenters having centerline-to-face angles ranging from 35° to 85°. Indentations produced at high (80 mN) and low (10 mN) loads were examined in plan-view by scanning electron microscopy and in cross-section by transmission electron microscopy. Microstructural observations were correlated with the indentation load-displacement behavior. Cracking and extrusion are more prevalent for sharp indenters with small centerline-to-face angles, regardless of the load. At low loads, the transformed material is amorphous silicon for all indenter angles. For Berkovich indentations made at high-load, the transformed material is a nanocrystalline mix of Si-I and Si-III/Si-XII, as confirmed by selected area diffraction. Extrusion of material at high loads for the cube-corner indenter reduces the volume of transformed material remaining underneath the indenter, thereby eliminating the pop-out in the unloading curve.

Nanoindentation stress–strain curves as a method for thin-film complete mechanical characterization: application to nanometric CrN/Cr multilayer coatings

The mechanical behavior of CrN/Cr multilayer coatings deposited by rf magnetron sputtering has been investigated by nanoindentation measurements performed with indenters of different geometries. Nanoindentation stress–strain curves generated from these measurements allow us to characterize the complete mechanical behavior of these coatings in the elastic, elastoplastic, plastic and fracture deformation regimes. In particular, indentation measurements carried out with a 100-μm-radius spherical indenter allowed us to study the elastic deformation regime and estimate the yield stress parameter through the initial indentation yielding point. The elastoplastic deformation regime has been studied using a 5-μm-radius spherical indenter and the stationary yielding regime (fully plastic regime) has been investigated with a pyramidal indenter of Berkovich geometry. The use of a pyramidal cube-corner indenter allowed us to study coating fracture characteristics. Nanometric CrN/Cr multilayer structures as well as single CrN and Cr coatings have been characterized. The study has shown that multilayered coatings with period thicknesses less than 46 nm present values of yield stress, Young’s modulus, hardness and toughness higher than those for single-layer CrN and Cr coatings.

Nanoindentation Method for Measuring Residual Stress in Brittle Materials

The lowered threshold load for cracking with the cube‐corner indenter has been used in developing a method that can be used to measure residual stresses in small volume brittle materials. By studying a series of orthogonal cracks generated at loads not exceeding 10 mN with the cube‐corner indenter, a variation of crack length with position around a large Vickers impression in soda–lime glass was observed. Using an indentation fracture mechanics approach residual stresses were evaluated at the positions where the cube‐corner indents had been made. The stress values thus evaluated were generally higher than those reported in the literature where micro‐Vickers indents had been used to measure the stresses. Possible reasons for the disparity are discussed.

Micromechanical properties of a laser-induced iron oxide–aluminum matrix composite coating

A laser-based technique was used to deposit Fe3O4 on A319Al, producing an Fe3O4/Al reaction composite coating. Scanning Auger microscopy indicated a reaction between oxide particles and aluminum-forming Fe–Al intermetallic compounds, Al2O3, and various intermediate reaction products. Analysis of the coating region, fractured in vacuo, indicated substantial toughness of the material due to extremely refined microstructure with finely distributed oxide and intermetallic particles and strong interfacial bonding between particles and the matrix. Mechanical properties of the coating were evaluated by nanoindentation techniques employing both Berkovich and cube-corner indenters. Hardness and elastic modulus values were found to be uniform at 1.24 and 76 GPa, respectively. No radial cracking was observed for either the Berkovich or cube-corner indenters. These results indicate that the laser-induced rapidly solidified composite material is tough and fracture resistant.