Structural
State-of-the-art structural and spectroscopic characterization of the grown samples will be an important activity throughout the consortium. It will be linked closely to the synthesis activity and will provide feedback to it, in order to optimize the structural and optical properties of the grown materials. A number of complementary techniques will be used.
For the investigation of the crystalline structure of the grown material systems, we shall be using X-Ray Diffraction. This activity will be taking place at the HO. Due to the rather complicated topologies involved, such as the expected very small grain size, and the low diffraction volume of the specimens, the XRD experiments will be performed at grazing incidence asymmetric geometry using parallel-beam optics. It refers to most of the nanocomposite materials under study, both with a-C and SiC matrices.
Density and sp3 fraction are crucial parameters characterizing the nanostructured materials. The mass density of diamond is 50% larger than graphite, because the inter-layer bonding of graphite takes considerable space. This difference carries over into the amorphous phase, and indeed a linear correlation of density and sp3 fraction is expected for a mixed sp2/sp3 network. Thus, the density is a useful indicator of sp3 fraction, particularly in hydrogen-free carbons. The presence of hydrogen complicates matters, as CHx groups also occupy a lot of space, so CHx groups reduce the density. The density can be measured by weight gain during the deposition, by floatation or by X-ray reflectivity. The latter technique is the one to be used extensively in the consortium to measure density. The HO and PA2 will be involved in this activity. X-ray reflectivity is a technique widely used to analyze thin films and multilayers in the optoelectronics industry. It can be used to give a parameter-free measurement of the density. The refractive index of a solid for X-rays is slightly less than 1, so that X-rays have a critical angle for total external reflection. The critical angle is a glancing angle, 0.20° for an X-ray wavelength of 1.39 Å. The refractive index depends on the density of all electrons, valence and core. This allows the mass density of a carbon film to be derived from the critical angle. In practice, the C film is on a substrate such as silicon. For a high density film like ta-C, the X-rays are reflected at the outer surface and only a single critical angle is seen. On the other hand, if the film has a rather low density like evaporated a-C or a-C:H, then the critical angles of both film and Si are observed, and the overall reflection edge must be simulated to derive the film’s density. Above the film critical angle, the reflected waves from the top surface and the film-substrate interface produce interference fringes, whose periodicity gives the film thickness.
It is often attempted to derive the sp3 fraction from the optical spectra in the energy range 1–6 eV. This is based on the idea that the excitations of π and σ electrons are separate, and that the π excitations occur at a lower energy than the σ excitations. A variant of this is the Spectroscopic Ellipsometry (SE) method, which could even give a real-time or in-situ measurement of the sp3 fraction and thickness of the growing film and which is going to be used extensively by PA2 for this purpose. It is possible to obtain reasonable estimates of the sp3 fraction when compared to EELS values. In addition, SE is very powerful technique for the study of nanocomposite materials through the application of Effective Medium Theory analyses, which are capable of identifying the size, distribution and volume fractions of the inclusions into the matrix.
For the extraction of the optical response and related parameters (dielectric function, optical gaps, absorption coefficient, etc.), use will also be made of the SE method, at PA2. This analysis is important for the optimization of absorption/emission properties of the carbon coatings for solar collectors, as well as for an indirect verification of the sp3 fractions in our samples.
Mechanical
This task is dedicated to the nanocale characterization of all nanostructured materials and systems grown within the Unit. Specimens synthesized through PE-CVD, LP-CVD, PLD and MS will be characterized at the nanoscale from a mechanical, chemical, and morphological point of view. A state-of the art instrumented nanoindenter, already installed and operational at our premises, and an atomic force microcope (to be purchased) will contribue to this end. These activities will complement structural and spectroscopic characterization and will provide input parameters and control for the atomistic simulatios and computer modeling.
The proposed synthesis activities of the Unit will yield materials with sub-micrometric characteristics. The nanomechanical properties of the material systems will therefore be probed using instrumented indentation and an atomic force microscope. Instrumented indentation is probably the only commercially available equipment that allows quantitative measurements of mechanical properties at length-scales on the order of 10-9 m and forces in the 10-8 N regime. The advent of instrumented indentation has therefore enabled fundamental studies in the nanomechanical response of metals, ceramics, polymers, and composites. Current technology allows for contact-based deformation of nanoscale load and displacement resolution and had been leveraged for both general mechanical characterization of small material volumes (e.g. thin films adhered to substrates40 and free standing nanowires) and unprecedented access to the physics of deformations processes such as dislocation nucleation in crystals. The technique consists of establishing contact between the indenter material (typically diamond) and the indented material whose mechanical properties are of interest. Elastic contact mechanics provides a convenient framework for linking the measured indentation modulus with the elastic properties of the material; in turn yield design methods can be used to link the measured indentation hardness to the strength properties of the material. The energy dissipated during indentation, which can be measured by integrating the experimentally obtained load-displacement response, can serve as an index of material ductility. While instrumented indentation was originally developed (in the late 1980’s) mainly for metals, the application of this technique to more complicated metal/ceramic nanocomposites, which are characterized by a multi-phase nature, poses several challenges, as the underlying analysis assumes a material of homogeneous nature. The technique has been recently extended to nanocomposites and an analysis similar to the one developed for homogeneous solids can be employed, provided that the length scale of indentation is carefully controlled and the number of indentation tests is significantly increased so that the results can be analyzed statistically. For the purposes of the three projects described herein the elastic modulus, hardness, and a ductility coefficient will be measured for all specimens. While elastic modulus measurements and hardness measurements are common practice in indentation testing the extraction of ductility coefficients is a developing area of significant scientific and technological interest. It is therefore the intention of this research to extend the indentation analysis in this regime by defining appropriate measures that quantify the ductility of materials; among the candidates that will be explored are:
(a) the critical load to fracturing and b) the energy dissipated during indentation. Furthermore, it is the intention to link these measured quantities with microstructural features of the constructed materials (i.e., volume fractions of inclusions, material chemistry, morphology, etc.) so that we will develop the scientific basis that will allow material performance optimization. Volumetric proportions of the constituent phases composing the nanocomposite systems will be provided through the structural/chemical characterization process. Atomic force microscopy and scan electron microscopy (available at HO) will provide the morphological arrangement of the components in space. Nanoindentation and AFM will give the intrinsic mechanical properties of the individual materials and their composite response. Nanoindentation experiments will be performed at the HO. AFM tests will be performed both at HO and at PA1 (University of Cyprus). These three crucial parameters (material chemistry, morphology, nanoscale mechanical response) are essential inputs to the micromechanical models that will be developed and provide benchmarking values for the atomistic simulations.
We plan to offer a complete range of nanomechanical and tribological tests, including nanoindentation, nanoscratch and wear, nanoimpact and fatigue, elevated temperature indentation, and indentation in fluids. These advanced techniques coupled with recent analysis methods will be exploited in our studies. The main objective will be to approximate as closely as possible the service state conditions of all envisioned products (solid lubricants and protective coatings, solar collector coatings and carbon sensors) and test their nanomechanical behavior in that environment.
The ultimate goal is to create through a complete cycle of simulations-synthesis-characterization the scientific basis for material and product optimization. Such an approach will simultaneously contribute to the socioeconomic development of Cyprus through cutting end technology-oriented research and the generation and dissemination of new knowledge.
Nanoscale Characterization
Structural
State-of-the-art structural and spectroscopic characterization of the grown samples will be an important activity throughout the consortium. It will be linked closely to the synthesis activity and will provide feedback to it, in order to optimize the structural and optical properties of the grown materials. A number of complementary techniques will be used.
For the investigation of the crystalline structure of the grown material systems, we shall be using X-Ray Diffraction. This activity will be taking place at the HO. Due to the rather complicated topologies involved, such as the expected very small grain size, and the low diffraction volume of the specimens, the XRD experiments will be performed at grazing incidence asymmetric geometry using parallel-beam optics. It refers to most of the nanocomposite materials under study, both with a-C and SiC matrices.
Density and sp3 fraction are crucial parameters characterizing the nanostructured materials. The mass density of diamond is 50% larger than graphite, because the inter-layer bonding of graphite takes considerable space. This difference carries over into the amorphous phase, and indeed a linear correlation of density and sp3 fraction is expected for a mixed sp2/sp3 network. Thus, the density is a useful indicator of sp3 fraction, particularly in hydrogen-free carbons. The presence of hydrogen complicates matters, as CHx groups also occupy a lot of space, so CHx groups reduce the density. The density can be measured by weight gain during the deposition, by floatation or by X-ray reflectivity. The latter technique is the one to be used extensively in the consortium to measure density. The HO and PA2 will be involved in this activity. X-ray reflectivity is a technique widely used to analyze thin films and multilayers in the optoelectronics industry. It can be used to give a parameter-free measurement of the density. The refractive index of a solid for X-rays is slightly less than 1, so that X-rays have a critical angle for total external reflection. The critical angle is a glancing angle, 0.20° for an X-ray wavelength of 1.39 Å. The refractive index depends on the density of all electrons, valence and core. This allows the mass density of a carbon film to be derived from the critical angle. In practice, the C film is on a substrate such as silicon. For a high density film like ta-C, the X-rays are reflected at the outer surface and only a single critical angle is seen. On the other hand, if the film has a rather low density like evaporated a-C or a-C:H, then the critical angles of both film and Si are observed, and the overall reflection edge must be simulated to derive the film’s density. Above the film critical angle, the reflected waves from the top surface and the film-substrate interface produce interference fringes, whose periodicity gives the film thickness.
It is often attempted to derive the sp3 fraction from the optical spectra in the energy range 1–6 eV. This is based on the idea that the excitations of π and σ electrons are separate, and that the π excitations occur at a lower energy than the σ excitations. A variant of this is the Spectroscopic Ellipsometry (SE) method, which could even give a real-time or in-situ measurement of the sp3 fraction and thickness of the growing film and which is going to be used extensively by PA2 for this purpose. It is possible to obtain reasonable estimates of the sp3 fraction when compared to EELS values. In addition, SE is very powerful technique for the study of nanocomposite materials through the application of Effective Medium Theory analyses, which are capable of identifying the size, distribution and volume fractions of the inclusions into the matrix.
For the extraction of the optical response and related parameters (dielectric function, optical gaps, absorption coefficient, etc.), use will also be made of the SE method, at PA2. This analysis is important for the optimization of absorption/emission properties of the carbon coatings for solar collectors, as well as for an indirect verification of the sp3 fractions in our samples.
Mechanical
This task is dedicated to the nanocale characterization of all nanostructured materials and systems grown within the Unit. Specimens synthesized through PE-CVD, LP-CVD, PLD and MS will be characterized at the nanoscale from a mechanical, chemical, and morphological point of view. A state-of the art instrumented nanoindenter, already installed and operational at our premises, and an atomic force microcope (to be purchased) will contribue to this end. These activities will complement structural and spectroscopic characterization and will provide input parameters and control for the atomistic simulatios and computer modeling.
The proposed synthesis activities of the Unit will yield materials with sub-micrometric characteristics. The nanomechanical properties of the material systems will therefore be probed using instrumented indentation and an atomic force microscope. Instrumented indentation is probably the only commercially available equipment that allows quantitative measurements of mechanical properties at length-scales on the order of 10-9 m and forces in the 10-8 N regime. The advent of instrumented indentation has therefore enabled fundamental studies in the nanomechanical response of metals, ceramics, polymers, and composites. Current technology allows for contact-based deformation of nanoscale load and displacement resolution and had been leveraged for both general mechanical characterization of small material volumes (e.g. thin films adhered to substrates40 and free standing nanowires) and unprecedented access to the physics of deformations processes such as dislocation nucleation in crystals. The technique consists of establishing contact between the indenter material (typically diamond) and the indented material whose mechanical properties are of interest. Elastic contact mechanics provides a convenient framework for linking the measured indentation modulus with the elastic properties of the material; in turn yield design methods can be used to link the measured indentation hardness to the strength properties of the material. The energy dissipated during indentation, which can be measured by integrating the experimentally obtained load-displacement response, can serve as an index of material ductility. While instrumented indentation was originally developed (in the late 1980’s) mainly for metals, the application of this technique to more complicated metal/ceramic nanocomposites, which are characterized by a multi-phase nature, poses several challenges, as the underlying analysis assumes a material of homogeneous nature. The technique has been recently extended to nanocomposites and an analysis similar to the one developed for homogeneous solids can be employed, provided that the length scale of indentation is carefully controlled and the number of indentation tests is significantly increased so that the results can be analyzed statistically. For the purposes of the three projects described herein the elastic modulus, hardness, and a ductility coefficient will be measured for all specimens. While elastic modulus measurements and hardness measurements are common practice in indentation testing the extraction of ductility coefficients is a developing area of significant scientific and technological interest. It is therefore the intention of this research to extend the indentation analysis in this regime by defining appropriate measures that quantify the ductility of materials; among the candidates that will be explored are:
(a) the critical load to fracturing and b) the energy dissipated during indentation. Furthermore, it is the intention to link these measured quantities with microstructural features of the constructed materials (i.e., volume fractions of inclusions, material chemistry, morphology, etc.) so that we will develop the scientific basis that will allow material performance optimization. Volumetric proportions of the constituent phases composing the nanocomposite systems will be provided through the structural/chemical characterization process. Atomic force microscopy and scan electron microscopy (available at HO) will provide the morphological arrangement of the components in space. Nanoindentation and AFM will give the intrinsic mechanical properties of the individual materials and their composite response. Nanoindentation experiments will be performed at the HO. AFM tests will be performed both at HO and at PA1 (University of Cyprus). These three crucial parameters (material chemistry, morphology, nanoscale mechanical response) are essential inputs to the micromechanical models that will be developed and provide benchmarking values for the atomistic simulations.
We plan to offer a complete range of nanomechanical and tribological tests, including nanoindentation, nanoscratch and wear, nanoimpact and fatigue, elevated temperature indentation, and indentation in fluids. These advanced techniques coupled with recent analysis methods will be exploited in our studies. The main objective will be to approximate as closely as possible the service state conditions of all envisioned products (solid lubricants and protective coatings, solar collector coatings and carbon sensors) and test their nanomechanical behavior in that environment.
The ultimate goal is to create through a complete cycle of simulations-synthesis-characterization the scientific basis for material and product optimization. Such an approach will simultaneously contribute to the socioeconomic development of Cyprus through cutting end technology-oriented research and the generation and dissemination of new knowledge.
