Digital image correlation after focused ion beam micro-slit drilling: A new technique for measuring residual stresses in hardmetal components at local scale

A new method has been developed for measuring residual stresses at the surface of hardmetal components with higher spatial resolution than standard X-ray diffraction methods. It is based on measuring the surface displacements produced when stresses are partially released by machining a thin slit perpendicularly to the tested surface. Slit machining is carried out by focused ion beam (FIB). Measurement of the displacement fields around the FIB slit are performed by applying an advanced digital image correlation algorithm based on Fourier analysis with sub-pixel resolution. This method compares SEM images of the same area of the hardmetal surface before and after slitting. The method has been successfully applied to as-ground and femto-laser textured surfaces showing good correlation with the standard sin2 ψ XRD technique. It is concluded that texturing induced by laser pulses in the femtoseconds regime is not perfectly adiabatic, since residual stresses are reduced by 15

An investigation on the mechanics of nanometric cutting for hard-brittle materials at elevated temperatures

Due to their exceptional physical and chemical properties such as high strength, high thermal conductivity, high stability at high temperature, high resistance to shocks, low thermal expansion and low density, silicon and silicon carbide (SiC) have become consummate candidates for optoelectronics, semiconductor and tribological applications. In particular, 3C-SiC, as a zinc blende structured SiC, possesses the highest fracture toughness, hardness, electron mobility and electron saturation velocity amongst the SiC polytypes. Thus, it has drawn substantial attention as a candidate substrate material for nano-devices which require high performance in extreme environments.Nanometric cutting is a promising ultra precision manufacturing process for manufacturing of 3D silicon and SiC based components which require submicron form accuracy and nanometric smooth finish. However, silicon and 3C-SiC have poor machinability at room temperature due to their relatively low fracture toughness and high hardness. A common understanding is that the yield strength and hardness of silicon and 3C-SiC will reduce under high temperature. As such, their fracture toughness increase which will ease plastic deformation and improve their machinabilities primarily as a result of thermally-generated intrinsic defects and thermal softening processes. However, the extent has never been reported although this knowhow could be vital in implementing the hot machining of silicon and SiC with the assistance of laser processing.This dissertation aims to gain an in-depth understanding of nanoscale mechanisms involved in nanometric cutting of hard-brittle materials such as silicon and 3C-SiC at elevated temperatures through molecular dynamics (MD) simulation and experimental trials. To this end, three-dimensional MD models of nanometric cutting were developed and different types of interatomic potential functions i.e. Tersoff, modified Tersoff, ABOP and SW were adopted to describe the interactions between atoms. In order to obtain reliable results, the equilibrium lattice constants were calculated at different temperatures for the employed potential functions. To perform the MD simulations, LAMMPS software was employed on a HPC service which was coupled with OVITO to visualise and post-process the atomistic data. Material flow behaviour, cutting chip characteristics, cutting forces and specific cutting energy, yielding stresses, stress and temperature on the cutting edge of the diamond tool, tool wear, defect-mediated plasticity and amorphization processes were calculated and analysed to quantify the differences in the cutting behaviour at different temperatures. Furthermore, In-situ high temperature nanoscratching (~500°C) of silicon wafer under reduced oxygen condition through an overpressure of pure Argon was carried out using a Berkovich tip with a ramp load at low and high scratching speeds. Ex-situ Raman spectroscopy and AFM analysis were performed to characterize high pressure phase transformation, nanoscratch topography, nanoscratch hardness and condition of the nanoindenter tip in nanoscratching at room and elevated temperatures.MD simulation results showed that the workpiece atoms underneath the cutting tool experienced a rotational flow akin to fluids. Moreover, the degree of flow in terms of vorticity was found higher on the (111) crystal plane, signifying better machinability on this orientation. Furthermore, it was observed that the degree of turbulence in the machining zone increases linearly with machining operation temperature. The cutting temperature showed significant dependence on the location and position of the stagnation region in the cutting zone of the substrate. In general, when cutting was performed on the (111) plane, the stagnation region (irrespective of the cutting temperature) was observed to locate at an upper position than that for the (010) and (110) planes. Also, at high temperatures, the stagnation region was observed to shift downwards than what was observed at room temperature. Another point of interest was the increase of subsurface deformation depth of the workpiece while cutting the (111) crystal plane at elevated temperatures.;Dislocation nucleation and formation of stacking faults were identified in conjunction with amorphization of silicon as the meditators of crystal plasticity in single crystal silicon during nanometric cutting process on different crystallographic planes at various temperatures. MD simulations revealed strong anisotropic dependence behaviour of dislocation activation and stacking fault formation. Likewise, while cutting 3C-SiC on the (110), formation and subsequent annihilation of stacking fault-couple at high temperatures, i.e. 2000 K and 3000 K, and generation of the cross-junctions between pairs of counter stacking faults meditated by the gliding of Shockley partials at 3000 K were observed. An observation of particular interest, while cutting 3C-SiC, was the shift to the (110) cleavage at cutting temperatures higher than 2000 K. The initial response of both the silicon and 3C-SiC substrates was found to be solid-state amorphization for all the studied cases. Further analysis through virtual X-ray diffraction (XRD) and radial distribution function (RDF) showed the crystal quality and structural changes of the substrate during nanometric cutting. No symptom of any atom-by-atom attrition wear and plastic deformation of the diamond cutting tool was observed during nanometric cutting of silicon irrespective of the cutting plane or the cutting temperature under vacuum condition. However, while cutting 3C-SiC, cutting tool showed severe wear and plastic deformation. It was found that the atom-by-atom attrition wear and plastic deformation of the diamond cutting tool could be alleviated while cutting 3C-SiC at high temperatures. Nevertheless, chemical wear i.e. dissolution-diffusion and adhesion wear is plausible to be accelerated at high temperatures.Raman spectroscopy was successfully used to identify the formation of metastable silicon phases during nanoscratching experiments at room and high temperatures. The probability of forming high pressure phases of Si-III and Si-XII was found to increase above the threshold load of 5 mN during room temperature nanoscratching experiment at low scratching speed. At high scratching speed, small remnants of Si-XII and Si-III phases were detected when the scratching load was greater than a threshold value i.e. ~9.5 mN. When high temperature nanoscratching was carried out at low and high speeds, no remnants of polymorph phases were observed along the nanoscratch residual track, suggesting the transition of metastable silicon phases (Si-III and Si-XII) into thermodynamic stable Si-I. Further analysis using AFM showed that the residual scratch morphologies and nanoscratch hardness were profoundly influenced by the temperature and scratching speed.Due to their exceptional physical and chemical properties such as high strength, high thermal conductivity, high stability at high temperature, high resistance to shocks, low thermal expansion and low density, silicon and silicon carbide (SiC) have become consummate candidates for optoelectronics, semiconductor and tribological applications. In particular, 3C-SiC, as a zinc blende structured SiC, possesses the highest fracture toughness, hardness, electron mobility and electron saturation velocity amongst the SiC polytypes. Thus, it has drawn substantial attention as a candidate substrate material for nano-devices which require high performance in extreme environments.Nanometric cutting is a promising ultra precision manufacturing process for manufacturing of 3D silicon and SiC based components which require submicron form accuracy and nanometric smooth finish. However, silicon and 3C-SiC have poor machinability at room temperature due to their relatively low fracture toughness and high hardness. A common understanding is that the yield strength and hardness of silicon and 3C-SiC will reduce under high temperature. As such, their fracture toughness increase which will ease plastic deformation and improve their machinabilities primarily as a result of thermally-generated intrinsic defects and thermal softening processes. However, the extent has never been reported although this knowhow could be vital in implementing the hot machining of silicon and SiC with the assistance of laser processing.This dissertation aims to gain an in-depth understanding of nanoscale mechanisms involved in nanometric cutting of hard-brittle materials such as silicon and 3C-SiC at elevated temperatures through molecular dynamics (MD) simulation and experimental trials. To this end, three-dimensional MD models of nanometric cutting were developed and different types of interatomic potential functions i.e. Tersoff, modified Tersoff, ABOP and SW were adopted to describe the interactions between atoms. In order to obtain reliable results, the equilibrium lattice constants were calculated at different temperatures for the employed potential functions. To perform the MD simulations, LAMMPS software was employed on a HPC service which was coupled with OVITO to visualise and post-process the atomistic data. Material flow behaviour, cutting chip characteristics, cutting forces and specific cutting energy, yielding stresses, stress and temperature on the cutting edge of the diamond tool, tool wear, defect-mediated plasticity and amorphization processes were calculated and analysed to quantify the differences in the cutting behaviour at different temperatures. Furthermore, In-situ high temperature nanoscratching (~500°C) of silicon wafer under reduced oxygen condition through an overpressure of pure Argon was carried out using a Berkovich tip with a ramp load at low and high scratching speeds. Ex-situ Raman spectroscopy and AFM analysis were performed to characterize high pressure phase transformation, nanoscratch topography, nanoscratch hardness and condition of the nanoindenter tip in nanoscratching at room and elevated temperatures.MD simulation results showed that the workpiece atoms underneath the cutting tool experienced a rotational flow akin to fluids. Moreover, the degree of flow in terms of vorticity was found higher on the (111) crystal plane, signifying better machinability on this orientation. Furthermore, it was observed that the degree of turbulence in the machining zone increases linearly with machining operation temperature. The cutting temperature showed significant dependence on the location and position of the stagnation region in the cutting zone of the substrate. In general, when cutting was performed on the (111) plane, the stagnation region (irrespective of the cutting temperature) was observed to locate at an upper position than that for the (010) and (110) planes. Also, at high temperatures, the stagnation region was observed to shift downwards than what was observed at room temperature. Another point of interest was the increase of subsurface deformation depth of the workpiece while cutting the (111) crystal plane at elevated temperatures.;Dislocation nucleation and formation of stacking faults were identified in conjunction with amorphization of silicon as the meditators of crystal plasticity in single crystal silicon during nanometric cutting process on different crystallographic planes at various temperatures. MD simulations revealed strong anisotropic dependence behaviour of dislocation activation and stacking fault formation. Likewise, while cutting 3C-SiC on the (110), formation and subsequent annihilation of stacking fault-couple at high temperatures, i.e. 2000 K and 3000 K, and generation of the cross-junctions between pairs of counter stacking faults meditated by the gliding of Shockley partials at 3000 K were observed. An observation of particular interest, while cutting 3C-SiC, was the shift to the (110) cleavage at cutting temperatures higher than 2000 K. The initial response of both the silicon and 3C-SiC substrates was found to be solid-state amorphization for all the studied cases. Further analysis through virtual X-ray diffraction (XRD) and radial distribution function (RDF) showed the crystal quality and structural changes of the substrate during nanometric cutting. No symptom of any atom-by-atom attrition wear and plastic deformation of the diamond cutting tool was observed during nanometric cutting of silicon irrespective of the cutting plane or the cutting temperature under vacuum condition. However, while cutting 3C-SiC, cutting tool showed severe wear and plastic deformation. It was found that the atom-by-atom attrition wear and plastic deformation of the diamond cutting tool could be alleviated while cutting 3C-SiC at high temperatures. Nevertheless, chemical wear i.e. dissolution-diffusion and adhesion wear is plausible to be accelerated at high temperatures.Raman spectroscopy was successfully used to identify the formation of metastable silicon phases during nanoscratching experiments at room and high temperatures. The probability of forming high pressure phases of Si-III and Si-XII was found to increase above the threshold load of 5 mN during room temperature nanoscratching experiment at low scratching speed. At high scratching speed, small remnants of Si-XII and Si-III phases were detected when the scratching load was greater than a threshold value i.e. ~9.5 mN. When high temperature nanoscratching was carried out at low and high speeds, no remnants of polymorph phases were observed along the nanoscratch residual track, suggesting the transition of metastable silicon phases (Si-III and Si-XII) into thermodynamic stable Si-I. Further analysis using AFM showed that the residual scratch morphologies and nanoscratch hardness were profoundly influenced by the temperature and scratching speed

Combined synchrotron X-rays and image correlation analyses of biaxially deformed W/Cu nanocomposite thin films on Kapton

International audienceAbstract In-situ biaxial tensile tests within the elastic domain were conducted with W/Cu nanocomposite thin films deposited on a polyimide cruciform substrate thanks to a biaxial testing machine developed on the DiffAbs beamline at SOLEIL synchrotron. The mechanical behavior of the nanocomposite was characterized at the micro-scale and the macro-scale using simultaneously synchrotron X-ray diffraction and digital image correlation techniques. Strain analyses for equi-biaxial and non equi-biaxial loading paths have been performed. The results show that the two strain measurements match to within 1 × 10-4 in the elastic domain for strain levels less than 0.3% and for both loading paths

SUPERFACT: A Model Fuel for Studying the Evolution of the Microstructure of Spent Nuclear Fuel during Storage/Disposal

The transmutation of minor actinides (in particular, Np and Am), which are among the main contributors to spent fuel α-radiotoxicity, was studied in the SUPERFACT irradiation. Several types of transmutation UO$_{2}$-based fuels were produced, differing by their minor actinide content ($^{241}$Am, $^{237}$Np, Pu), and irradiated in the Phénix fast reactor. Due to the high content in rather short-lived alpha-decaying actinides, both the archive, but also the irradiated fuels, cumulated an alpha dose during a laboratory time scale, which is comparable to that of standard LWR fuels during centuries/millenaries of storage. Transmission Electron Microscopy was performed to assess the evolution of the microstructure of the SUPERFACT archive and irradiated fuel. This was compared to conventional irradiated spent fuel (i.e., after years of storage) and to other $^{238}$Pu-doped UO$_{2}$ for which the equivalent storage time would span over centuries. It could be shown that the microstructure of these fluorites does not degrade significantly from low to very high alpha-damage doses, and that helium bubbles precipitate

An atomistic investigation on the nanometric cutting mechanism of hard, brittle materials

The demand for ultra precision machined devices and components is growing at a rapid pace in various areas such as the aerospace, energy, optical, electronics and bio-medical industries. Because of their outstanding engineering properties such as high refractive index, wide energy bandgap and low mass density, there is a continuing requirement for developments in manufacturing methods for hard, brittle materials. Accordingly, an assessment of the nanometric cutting of the optical materials silicon and silicon carbide (SiC), which are ostensibly hard and brittle, has been undertaken. Using an approach of parallel molecular dynamics simulations with a three-body potential energy function combined with experimental characterization, this thesis provides a quantitative understanding of the ductile-regime machining of silicon and SiC (polytypes: 3C, 4H and 6H SiC), and the mechanism by which a diamond tool wears during the process. The distinctive MD algorithm developed in this work provides a comprehensive analysis of thermal effects, high pressure phase transformation, tool wear (both chemical and abrasive), influence of crystal anisotropy, cutting forces and machining stresses (hydrostatic and von Mises), hitherto not done so far. The calculated stress state in the cutting zone during nanometric cutting of single crystal silicon indicated Herzfeld–Mott transition (metallization) due to high pressure phase transformation (HPPT) of silicon under the influence of deviatoric stress conditions. Consequently, the transformation of pristine silicon to β-silicon (Si-II) was found to be the likely reason for the observed ductility of bulk silicon during its nanoscale cutting. Tribochemical formation of silicon carbide through a solid state single phase reaction between the diamond tool and silicon workpiece in tandem with sp3-sp2 disorder of carbon atoms from the diamond tool up to a cutting temperature of 959 K has been suggested as the most likely mechanism through which a diamond cutting tool wears while cutting silicon. The recently developed dislocation extraction algorithm (DXA) was employed to detect the nucleation of dislocations in the MD simulations of varying cutting orientation and cutting direction. Interestingly, despite of being a compound of silicon and carbon, silicon carbide (SiC) exhibited characteristics more like diamond, e.g. both SiC iii workpiece and diamond cutting tool were found to undergo sp3-sp2 transition during the nanometric cutting of single crystal SiC. Also, cleavage was found to be the dominant mechanism of material removal on the (111) crystal orientation. Based on the overall analysis, it was found that 3C-SiC offers ease of deformation on either (111) , (110) or (100) setups. The simulated orthogonal components of thrust force in 3C-SiC showed a variation of up to 45% while the resultant cutting forces showed a variation of 37% suggesting that 3C-SiC is anisotropic in its ease of deformation. The simulation results for three major polytypes of SiC and for silicon indicated that 4H-SiC would produce the best sub-surface integrity followed by 3C-SiC, silicon and 6H-SiC. While, silicon and SiC were found to undergo HPPT which governs the ductility in these hard, brittle materials, corresponding evidence of HPPT during the SPDT of polycrystalline reaction bonded SiC (RB-SiC) was not observed. It was found that, since the grain orientation changes from one crystal to another in polycrystalline SiC, the cutting tool experiences work material with different crystallographic orientations and directions of cutting. Thus, some of the grain boundaries cause the individual grains to slide along the easy cleavage direction. Consequently, the cutting chips in RB-SiC are not deformed by plastic mechanisms alone, but rather a combination of phase transformation at the grain boundaries and cleavage of the grains both proceed in tandem. Also, the specific-cutting energy required to machine polycrystalline SiC was found to be lower than that required to machine single crystal SiC. Correspondingly, a relatively inferior machined surface finish is expected with a polycrystalline SiC. Based on the simulation model developed, a novel method has been proposed for the quantitative assessment of tool wear from the MD simulations. This model can be utilized for the comparison of tool wear for various simulation studies concerning graphitization of diamond tools. Finally, based on the theoretical simulation results, a novel method of machining is proposed to suppress tool wear and to obtain a better quality of the machined surface during machining of difficult-to-machine materials

Digital image correlation after focused ion beam micro-slit drilling: A new technique for measuring residual stresses in hardmetal components at local scale

A new method has been developed for measuring residual stresses at the surface of hardmetal components with higher spatial resolution than standard X-ray diffraction methods. It is based on measuring the surface dis-placements produced when stresses are partially released by machining a thin slit perpendicularly to the tested surface. Slit machining is carried out by focused ion beam (FIB). Measurement of the displacement fields around the FIB slit are performed by applying an advanced digital image correlation algorithm based on Fourier analysis with sub-pixel resolution. This method compares SEM images of the same area of the hardmetal surface before and after slitting. The method has been successfully applied to as-ground and femto-laser textured surfaces showing good correlation with the standard sin2 psi XRD technique. It is concluded that texturing induced by laser pulses in the femtoseconds regime is not perfectly adiabatic, since residual stresses are reduced by 15%

Correlation between surface damage and mechanical properties at micro- and nanometric length scale for WC-Co hardmetals

Cemented carbides (WC-Co) are ceramic-metal composite materials made by hard tungsten carbide particles bonded through a metallic binder matrix, mainly of cobalt. It is a hard material characterized by an exceptional combination of strength, toughness and wear resistance. As result, cemented carbides are first choice materials for cutting tools and wear parts. However, final shaping of these components usually require diamond grinding. During this hard machining, surface integrity may became altered, particulary in terms of compression stresses and/or microcracking. Such defects can locally affect the mechanical properties. The aim of this investigation is to analyze the influence of the surface finish quality on the mechanical properties at the surface level for WC-Co materials as well as the influence over the properties of a TiN coating deposited on hardmetal substrates. The study has been done at micrometric (to analyze the general properties) and nanometric scale (local properties aiming to capture residual stress state effects) by using nanoindentation and nanoscratch testing. Tests done in plain view prove that roughness plays an important role in the assessment of mechanical properties at the surface, as it induces significant scatter, as compared to results determined on cross-sections. Finally, a sequential polishing process has been done in order to extract the polishing rate for cemented materials as well as study how roughness affects the mechanical properties measured. This process points out that roughness can mask surface damage, like cracks or chipping, among others. As a final conclusion, an optimized protocol is proposed to study the mechanical properties of the samples with high roughness and exhibiting a compressive residual stress state

Surfaces functionality of precision machined components: Modelling, simulation, optimization and control

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University, 23/07/2008.This research develops an analytical scientific approach for investigating the high precision surface generation and the quantitative analysis of the effects of direct factors in precision machining. The research focuses on 3D surface characterization with particular reference to the turning process and associated surface generation. The most important issue for this research is surface functionality which is becoming important in the current engineering industry. The surface functionality should match with the characterization parameters of the machined surface, which can be expressed in formula form as proposed in chapter 4. Modelling and simulation are extensively used in the research. The modelling approach integrates the cutting forces model, thermal mode% vibration model, tool wear model, machining system response model and surface topography model. All of those models are integrated as a whole model. The physical model with such as direct inputs is formed. The major inputs to the model are tooling geometry and the process variables. The outputs from the modelling approach are cutting force, surface texture parameters, dimensional errors, residual stress and material removal rate. MATLAB and Simulink are used as tools to implement the modelling and simulation. According to the simulation results, it is found that the feed rate has the most profound effect on in surface generation. The influence of the vibrations between the cutting tool and the workpiece on the surface roughness may be minimised by the small feed rate and large tool nose radius. Surface functionality simulation has been developed to model and simulate the surface generation in precision turning. The surface functionality simulation model covers the material and tool wear as well. It shows that chip formation is resulted from cutting forces. Cutting trials are conducted to validate the modelling and simulation developed. There are positive results that show the agreement between the simulation and experimental results. The analysis of the results of turning trials and simulations are conducted in order to find out the effects of process variables and tooling characteristics on surface texture and topography and machining instability. From the research, it can be concluded that the investigation on modelling and simulation of precision surfaces generation in precision turning is performed well against the research objectives as proposed. Recommendations for future work are to improve the model parameters identification, including comprehensive tool wear, chip formation and using Neural Networks modelling in the engineering surface construction system.Universiti Teknologi Tun Hussien Orin (UTHM

A study on the effects of laser shock peening on the microstructure and substructure of Ti–6Al–4V manufactured by Selective Laser Melting

Ti‐6Al‐4V was fabricated by powder-bed fusion using different laser scanning strategies. The microstructure and deformation properties were investigated in the as-built condition, and also after the material had been subjected to a laser-shock-peening (LSP) treatment. The microstructure in each condition was surveyed using 3D optical microscopy, EBSD, and TEM. The post-manufacture residual stresses were determined. The results indicate a correlation between the residual stresses and the substructures observed in TEM: tensile residual stresses from the surface down to 1 mm depth were observed in the as-built material, corresponding to extensive deformation through twinning of the 101̅2 type and wavy slip structures; while after LSP the alloy showed a variety of dislocation arrangements, especially planar and in significantly higher density, along with 112̅2 twins and with the presence of compressive residual stresses. The findings indicate that the deformation capability is mechanistically aided by the peening process, which effectively promotes the replacement of tensile residual stresses by compressive ones, offering routes for potentially improving the mechanical properties of the additively manufactured Ti‐6Al‐4V, as well as its usability.</p