High-pressure Phase Transformation Research Articles

Influence of temperature and crystal orientation on tool wear during single point diamond turning of silicon

Owing to the capricious wear of cutting tools, ultra precision manufacturing of silicon through single point diamond turning (SPDT) operation becomes a challenging task. It thus becomes non-trivial to understand the contribution of temperature and crystal orientation during the SPDT process in order to suppress tool wear. Molecular dynamics (MD) simulation is an appropriate tool to study nanoscale processes occurring at the femtosecond/picosecond timescale which cannot otherwise be studied experimentally or by the finite element method (FEM). Accordingly, MD simulation has been deployed with a realistic analytical bond order potential (ABOP) formalism based potential energy function to simulate the single point diamond turning operation of single crystal silicon in order to understand the influence of temperature and crystal orientation on the tool wear mechanism. Results showed the strong influence of crystal orientation on the wear resistance of a diamond tool; cubic orientation performed better than dodecahedral orientation. It was also observed that high pressure phase transformation (HPPT) in the cutting zone was accompanied by the formation of dangling bonds of silicon. Under the influence of cutting temperature, the newly formed dangling bonds of silicon chemically combine with the pre-existing dangling bonds on the surface of the diamond tool resulting in the formation of silicon carbide (SiC), the main appearance of which was evident at the tool flank face. Continuous abrasion of the diamond cutting tool with SiC causes sp3–sp2 disorder of the diamond tool. Hence, both these processes proceed in tandem with each other. The mechanism proposed here is in good agreement with a recent experimental study, where silicon carbide and carbon like particles were observed using X-ray photoelectron spectroscope (XPS) technology after machining a silicon wafer with a diamond tool.

Ductile mode material removal and high-pressure phase transformation in silicon during micro-laser assisted machining

Micro-laser assisted machining is a novel micro/nano machining technique developed for ductile mode machining of ceramics and semiconductors. Ductile mode material removal is possible in a nominally brittle material due to the high-pressure phase transformation (HPPT) phenomenon during the machining process. This paper investigates the mechanism of machining by analyzing the HPPTs under different scratch conditions, with and without laser heating. Micro-Raman (μ-Raman) spectroscopy studies of the nano-scratched regions provided evidence for HPPT in single crystal Silicon (Si). Annealing of the high-pressure phases into a recrystallized diamond structure (Si-I) at higher laser powers and its effect on machining characteristics is also discussed. This has been the first time that HPPT is reported in the material removed region where the material's phase transformation and laser heating occur simultaneously and instantaneously (the annealing process occurs instantaneously and is not time dependent in this case).

The influence of peak shock stress on the high pressure phase transformation in zirconium

At high pressures zirconium is known to undergo a phase transformation from the hexagonal close packed (HCP) alpha phase to the simple hexagonal omega phase. Under conditions of shock loading, the high-pressure omega phase is retained upon release. However, the hysteresis in this transformation is not well represented by equilibrium phase diagrams and currently models that accurately represent such a solid-solid phase transformation coupled with the multi-phase plasticity likely under shock conditions do not exist. For this reason, the influence of peak shock stress on the retention of omega phase in Zr is explored in this study. In-situ VISAR measurements along with post-mortem metallographic and neutron diffraction characterization of soft recovered specimens have been utilized to quantify the volume fraction of retained omega phase, morphology of the shocked alpha and omega phases, and qualitatively understand the kinetics of this transformation. This understanding of the role of peak shock stress will be utilized to address physics to be encoded in our present macro-scale models.

Replacing diamond cutting tools with CBN for efficient nanometric cutting of silicon

Diamond is the hardest known naturally-occuring material and, for this reason, it is widely believed to be a superior cutting tool material for nanometric cutting of silicon for various ultra precision engineering applications. In this paper, a molecular dynamics (MD) simulation has been done to assess the performance of second ultra-hard material, cubic boron nitride (CBN), during the nanometric cutting of single crystal silicon. While cutting silicon with a CBN tool, MD results indicate a high magnitude of high pressure phase transformation (HPPT) in the cutting zone compared to what is achievable with a diamond tool. It was found that CBN caused the transformation of silicon from its stable diamond cubic lattice structure to FCC lattice structure in the cutting zone which produced better metallic response during cutting. Thus, the CBN tool generated a better ductile regime by the virtue of HPPT. Based on further analysis, it was concluded that a CBN tool can be more conducive for nanometric cutting of silicon and an effective substitute for a costly diamond tool.

Pressure effects on energy gaps and phase transitions in ZnAl2Se4

The full potential linear augmented wave method has been used for an ab-initio study of structural, electronic and high pressure phase transformation features of ZnAl2Se4. The exchange and correlation potential (XC) was calculated using the Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) and the modified GGA due to Engel–Vosko (EV-GGA). The geometry optimization was performed by calculating the total energy as a function of the three variables u, c/a and V. Using the sets of data obtained we determined the equilibrium volume, V0, the bulk modulus, B0, and the pressure derivative of the bulk modulus, B0′. The results show that ZnAl2Se4 is a direct wide band gap semiconductor at ambient conditions (P = 0, T = 0). With increasing hydrostatic pressure of the unit cell (i.e., with decreasing size) both the direct (Γ—Γ) and the indirect (Γ—H) energy gaps first increase and at a pressure of 9.2 GPa both gaps become equal and start decreasing as the pressure increases further but with a change in the nature of the energy band gap from direct to indirect. With increasing pressure a transition from tetragonal to spinel phase has been observed at a pressure of 3 GPa.

High pressure phase transformation in iron under fast compression

We observe kinetic features—velocity loops—at the α to ϵ phase transformation of iron, similar with the ones reported when water is frozen into its ice VII phase under comparable experimental conditions. By using a phase nucleation and growth kinetic model with pressure dependent phase interface velocity we find that the thermodynamic path followed by the sample is strongly dependent on the drive conditions and sample characteristics. The velocity loops become broader and shallower at slower compressions, while on faster time–scales, e.g., for laser drivers, the loops form at higher velocities and may eventually disappear.

High pressure structural phase transition and elastic properties of Ga1-xInxAs semiconducting compounds

The present paper addresses the high-pressure phase transformation and mechanical properties of Ga1-xInxAs (x = 0.25, 0.5 and 0.75) by formulating an effective interionic interaction potential. This potential consists of the long-range Coulomb and charge transfer caused by the deformation of the electron shells of the overlapping ions and the Hafemeister and Flygare type short-range overlap repulsion extended upto the second neighbor ions and the van der Waals (vdW) interaction. The estimated values of phase transition pressure and the vast volume discontinuity in pressure-volume (PV) phase diagram indicate the structural phase transition from zinc blende (B3) to rock salt (B1). The equation of state curves plotted between V (P)/ V (0) and pressure are for both the zincblende (B3) and rocksalt (B1) structures. Further, the variations of the second and third order elastic constants with pressure have followed a systematic trend, which are almost identical to those exhibited by the observed data measured for other compounds of this family.

High-pressure phase transformation of nanometric ZnSb prepared by mechanical alloying

Nanometric orthorhombic ZnSb was prepared by mechanical alloying and its structural stability was studied as a function of pressure. The changes were followed by Raman and x-ray absorption spectroscopy. Between 11.0 and 14.6 GPa, we observed an irreversible phase transformation from the orthorhombic phase to a hexagonal hP1 phase. A similar transformation had been previously observed at 7 GPa for bulk ZnSb. The difference in the phase transformation pressures is attributed to an interfacial component of the nanometric structure.

Synchrotron X-ray diffraction studies of LiMn 2O 4 and Li 4Mn 5O 12 structures at high pressure

Experimental study of the phase transition in the lithium-excess lithium–manganese spinel over the pressure range 0–13 GPa has shown its suppression. This observation implies that the Li/Mn ratio, associated with the manganese oxidation state, plays an important role in the high-pressure phase transformation of the lithium–manganese spinels. As indicated by the results, the occupation of the octahedral sites with Li + and Mn 4+ ions stabilizes the crystallographic structure of this cubic spinel even at a high pressure.

Ab initio molecular dynamics simulation of a pressure induced zinc blende to rocksaltphase transition in SiC

The high-pressure induced phase transformation from the zinc blende to rocksalt structurein SiC has been studied by the ab initio molecular dynamics method. The simulationsshowed that SiC passes through a tetragonal intermediate state before transforming to amonoclinic phase at 160 GPa. The mechanism for this phase transformation agrees wellwith recent ab initio MD simulations, in which the applied pressure was as high as∼600 GPa, but in the present study the transformation occurs at much lower pressure.

Finite element modeling of high-pressure deformation and phase transformation of silicon beneath a sharp indenter

Finite element modeling of high-pressure deformation and phase transformation of silicon beneath a sharp indenter

Finite element modeling of high-pressure deformation and phase transformation of silicon beneath a sharp indenter

Modeling of the mechanical response of single crystalline silicon to a sharp indenter is an essential step for the optimization of wafer manufacturing processes. In this paper, deformation of silicon during indenter loading and unloading was analyzed by the finite element method, and the changes of stress field and high-pressure phase distribution were dynamically simulated. We found that the deformation of silicon in nanoindentation can be simply characterized by two factors: one is the elastic strain of each high-pressure phase and the other is the equivalent elastic strain of each phase transformation. In loading, indentation energy is absorbed mostly by phase transformation, and accumulated as the elastic strain of the high-pressure phases. The distribution pattern of the high-pressure phases beneath the indenter is independent of the indentation load, and the depth of the phase-transformed region is approximately twice the indentation depth. In unloading, high-pressure phases except the β-Sn phase undergo reverse phase transformations. The β-Sn phase does not transform back to the diamond phase but changes to other non-equilibrium phases, which becomes the dominant reason for residual strain. During unloading, the non-equilibrium phase expands from the diamond phase region toward the indenter tip, while the boundary between the non-equilibrium phase and the diamond phase remains unchanged. The unloading mechanism is independent of the change in the maximum indentation load and the presence/absence of pop-out events.

Prediction of incommensurate crystal structure in Ca at high pressure.

Ca shows an interesting high-pressure phase transformation sequence, but, despite similar physical properties at high pressure and affinity in the electronic structure with its neighbors in the periodic table, no complex phase has been identified for Ca so far. We predict an incommensurate high-pressure phase of Ca from first principle calculations and describe a procedure of estimating incommensurate structure parameters by means of electronic structure calculations for periodic crystals. Thus, by using the ab initio technique for periodic structures, one can get not only reliable information about the electronic structure and structural parameters of an incommensurate phase, but also identify and predict such phases in new elements.

Phase transformation and residual stress probed by Raman spectroscopy in diamond-turned single crystal silicon

Single-point diamond turning of monocrystalline semiconductors is an important field of research within brittle materials machining. Monocrystalline silicon samples with a (100) orientation have been diamond turned under different cutting conditions (feed rate and depth of cut). Micro-Raman spectroscopy and atomic force microscopy have been used to assess structural alterations and surface finish of the samples diamond turned under ductile and brittle modes. It was found that silicon undergoes a phase transformation when machined in the ductile mode. This phase transformation is evidenced by the creation of an amorphous surface layer after machining which has been probed by Raman scattering. Compressive residual stresses are estimated for the machined surface and it is observed that they decrease with an increase in the feed rate and depth of cut. This behaviour has been attributed to the formation of subsurface cracks when the feed rate is higher than or equal to 2.5 μm/rev. The surface roughness was observed to vary with the feed rate and the depth of cut. An increase in the surface roughness was influenced by microcrack formation when the feed rate reached 5.0 μm/rev. Furthermore, a high-pressure phase transformation induced by the tool/material interaction and responsible for the ductile response of this typical brittle material is discussed based upon the presented Raman spectra. The application of this machining technology finds use for a wide range of high quality components, for example the creation of a micrometre-range channel for microfluidic devices as well as microlenses used in the infrared spectrum range.

Structural phase transitions in high-pressure wurtzite to rocksalt phase in GaN and SiC

Ab initio molecular dynamics simulations are employed to study the atomistic mechanisms and pathways of high-pressure phase transformation in GaN and SiC. Our simulations bring a fundamental level of understanding of the wurtzite to rocksalt phase transformation that undergoes inhomogeneous displacements via a tetragonal atomic configuration, and suggest that the transition path may be independent of the presence of d electrons on the cation in GaN. The discrepancies between experimental and theoretical studies of transition paths are discussed.

Comparison between numerical simulations and experiments for single-point diamond turning of single-crystal silicon carbide

Single-point diamond turning (SPDT) experiments conducted on single-crystal 6-H silicon carbide (SiC) have shown chip formation similar to that seen in the machining of metals. The ductile nature of SiC is believed to be the result of a high-pressure phase transformation (HPPT), which generates a plastic zone of material that behaves in a metallic manner. This metallic behavior is the basis for using AdvantEdge, a metal machining simulation software, for comparison to experimental results. Simulations (2D) were carried out by matching the SPDT experimental conditions, which were conducted at nanometer (nm) depths of cut and varying tool rake angles. The experiments were performed by machining the circumference of the single-crystal wafer, thereby conforming to a 2D orthogonal cut (plunge cuts, or an infeed, achieved the depth of cut, and no cross feed was incorporated). The cutting and thrust forces generated from the experiments under ductile cutting conditions compared favorably with the simulation. As the depth of cut is decreased (250 nm, 100 nm, and 50 nm), the experimental conditions transition from a brittle to ductile behavior, with the 50 nm cuts being dominated by the ductile regime. Thus, the forces from the experiment and the simulations are in much better agreement for the smaller depths of cut, that is, below the critical depth of cut that establishes the ductile-to-brittle transition, as ductile conditions exist in both the simulation and experiments. The differences in the results that do arise are assumed to be primarily due to a springback of the material leading to increased rubbing on the flank face.

A shock-induced phase transformation in a LiTaO3 crystal

The high-pressure phase transformation behavior of LiTaO3 crystal has been studied by both Hugoniot measurements and first-principle calculations. We observe a discontinuity in shock velocity (D) versus particle velocity (UP) relation, a two-wave structure below 37.9 GPa, and a three-wave structure above 37.9 GPa. These data confirm that a shock-induced phase transformation of LiTaO3 occurs. The onset pressure of the phase transformation (37.9 GPa) defined by our new shock compression data is higher than the early shock wave value (19 GPa) reported by Stantonand Graham [P. L. Stanton and R. A. Graham, J. Appl. Phys. 50, 6892 (1979)]. A first-principle calculation of the zero degree isotherm for rhombohedral phase (R3c space group) is in good agreement with our low-pressure experimental data. The calculated zero degree isotherm for orthorhombic phase (Pbnm space group) is in concord with our high-pressure shock compression data.

High-pressure phase transformation as the mechanism of ductile chip formation in nanoscale cutting of silicon wafer

In nanoscale cutting of silicon wafer, it has been found that under certain conditions ductile mode chip formation can be achieved. In order to understand the mechanism of the ductile chip formation, experiments and molecular dynamics (MD) simulations have been conducted in this study. The results of MD simulations of nanoscale cutting of silicon showed that because of the high hydrostatic pressure in the chip formation zone, there is a phase transformation of the monocrytslline silicon from diamond cubic structure to both β silicon and amorphous phase in the chip formation zone, which results in plastic deformation of the workpiece material in the chip formation zone, as observed in experiments. The results further showed that although from experimental observation the plastic deformation in the ductile mode cutting of silicon is similar to that in cutting of ductile materials, such as aluminium, in ductile mode cutting of silicon it is the phase transformation of silicon rather than atomic dislocation that results in the plastic deformation.

Shock-induced phase transition of z-cut lithium tantalate single crystal

High-pressure phase-transition behaviors of z-cut lithium tantalate single-crystal have been studied by Hugoniot measurements at our two-stage light-gas gun and DFT-PWP calculations. A distinct discontinuity was discovered on the D-u (shock-wave velocity versus particle velocity) relation. An elastic-plastic two-wave structure was observed from the VISAR measured particle velocity profiles at low pressures (25.9 GPa and 32.6 GPa), while three-wave structure appeared in the measured particle velocity profiles at the final pressure of 42.9 GPa and 53.0 GPa. Both facts indicate a shock-induced phase transition of LiTaO3 samples occurred with an onset pressure of 37.9GPa. The theoretically calculated 0K pressure versus compression ratio (P-V/V0) curve for the rhombohedral phase (R3c space group) is in good agreement with the low-pressure experimental data, while that for orthorhombic phase (Pbnm space group) is in accord with the results by deducing thermal pressure contribution from the measured shock-compression data at high pressures. This suggests that the high-pressure phase has orthorhombic symmetry. High-pressure phase transformation behaviors including the transition pressure and structures, which are unclear in current literature, have been clarified in this paper by our new shock-wave data and ab-initio calculations. These behaviors were demonstrated to be in close similarity with that of its isomorphous crystal LiNbO3. The present work is significant for the investigations of shock-induced phase-transition of similar single-crystal materials.

High pressure elasticity and phase transformation in brucite, Mg(OH)2

The first pressure derivatives of the second-order elastic constants \({\rm d}C^{\prime}_{IJ}/{\rm d}p\) have been calculated for brucite, Mg(OH)2 from the second- and third-order elastic constants. The deformation theory and finite strain elasticity theory have been used to obtain the second- and third-order elastic constants of Mg(OH)2 from the strain energy of the lattice. The strain energy ϕ is calculated by taking into account the interactions up to third nearest neighbors in the Mg(OH)2 lattice. ϕ is then compared with the strain dependent lattice energy from continuum model approximation to obtain the expressions of elastic constants. The complete set of six second-order elastic constants CIJ of brucite exhibits large anisotropy. Since C33 (= 21.6 GPa), which corresponds to the strength of the material along the c-axis direction, is less than the longitudinal mode C11 (= 156.7 GPa), the interlayer binding forces are weaker than the binding forces along the basal plane of Mg(OH)2. The 14 nonvanishing components of the third-order elastic constants, CIJK, of brucite have been obtained. All the CIJK of brucite are negative except the values of C114 (= 230.36 GPa), C124 (= 75.45 GPa) and C134 (= 36.98 GPa). The absolute values of the CIJK are, in general, one order of magnitude greater than the CIJ’s in the Mg(OH)2 system as usually expected for a crystalline material. To our knowledge, no previous data are available to compare the pressure derivatives of brucite. The pressure derivatives of the two components viz., C14 and C33 become negative \(({\rm d}C^{\prime}_{14}/dp = -1.51\;{\rm and}\;{\rm d}C^{\prime}_{33}/{\rm d}p= -1.41)\) indicating an elastic instability in brucite while under pressure. This may be related to the phase transition of brucite largely involving rearrangements of H atoms revealed in the Raman spectroscopic, powder neutron diffraction and synchrotron X-ray diffraction studies.