Review of recent progress in nanoscratch testing

Nanoscratch testing, as an important technique for the assessment of the mechanical failure behaviour and adhesion strength of ceramic coatings and a simulation tool of single asperity contact in tribological experiments, is increasingly becoming an established nanomechanical characterisation method. This paper reviews recent work in nanoscratch testing in different engineering applications including thin ceramic films, automotive organic coatings, chemical- mechanical polishing and biomaterials. In the main part of the paper, nanoscratch results from experiments performed using NanoTest systems fitted with tangential force sensors and spherical indenters as scratch probes are presented and discussed. The types of nanoscratch tests described include constant load nanoscratches, ramped load nanoscratch tests and multipass repetitive unidirectional constant load nanoscratch tests (nanowear). The results are discussed in terms of critical load sensitivity to intrinsic and extrinsic factors, impact of scan speed and loading rate, influence of probe radius and geometry, estimation of tip contact pressure, influence of surface roughness and film stress and thickness, and finally role of ploughing on friction evolution

Bioresorbable Composite Stents for Enhanced Response of Vascular Smooth Muscle Cells

Formation of arterial plaque and stenosis is one of the main cardiovascular disease risk factors. Stenting is a popular approach to increase the inner diameter of the artery and provide an acceptable lumen gain. This is achieved by applying internal pressure to the arterial wall. Despite the desirable outcomes of this procedure, there are complexities and challenges that are being discussed among scholars in this area. Restenosis is one of these complications, in which smooth muscles cell start proliferation and remodeling in response of induced mechanical stresses. Another important issue is the placement of the stent and possible migration due to the continuous deformation and special contact state between tissue and stent struts. Finally, the mechanical properties of the stent and application of novel materials in order to improve its performance are the critical topics that also have been elaborated in the current research work. First of all, we developed a multi-scale model which is able to calculate load distribution in RVE scale and can be useful to assess the mechanical stresses experienced by smooth muscle cells. Moreover, stent migration has been simulated by using finite element modeling, and the effect of stent structure on this complication has been explained. Finally, the application of novel nano composite materials in stent design has been discussed. Developing 3D printed steel-PLLA and MgPLLA particle composites and the effect of added phases in micromechanical properties of composites has been evaluated. Advisor: Linxia G

Mechanical Characterizations of 3D-printed PLLA/Steel Particle Composites

The objective of this study is to characterize the micromechanical properties of poly-L-lactic acid (PLLA) composites reinforced by grade 420 stainless steel (SS) particles with a specific focus on the interphase properties. The specimens were manufactured using 3D printing techniques due to its many benefits, including high accuracy, cost effectiveness and customized geometry. The adopted fused filament fabrication resulted in a thin interphase layer with an average thickness of 3 μm. The mechanical properties of each phase, as well as the interphase, were characterized by nanoindentation tests. The effect of matrix degradation, i.e., imperfect bonding, on the elastic modulus of the composite was further examined by a representative volume element (RVE) model. The results showed that the interphase layer provided a smooth transition of elastic modulus from steel particles to the polymeric matrix. A 10% volume fraction of steel particles could enhance the elastic modulus of PLLA polymer by 31%. In addition, steel particles took 37% to 59% of the applied load with respect to the particle volume fraction. We found that degradation of the interphase reduced the elastic modulus of the composite by 70% and 7% under tensile and compressive loads, respectively. The shear modulus of the composite with 10% particles decreased by 36%, i.e., lower than pure PLLA, when debonding occurred

Effect of thermal annealing and carbon implantation on the functional properties of nanocomposite TiSiN coatings on steel

This PhD research contributes to the part of advanced materials technology. The machining industry currently faces tremendous pressures with the need for durable cutting tools suitable for eco-friendly high speed machining operations becoming acute. In this thesis innovative design and synthesis strategies are explored to tailor the properties of nanocomposite coatings. Advanced characterisation techniques are applied to identify the mechanisms that control the mechanical, tribological, and corrosion behaviours of these coatings. Cutting tools protected by these coatings are anticipated to exhibit a unique combination of superior toughness and greater resistance to wear and corrosion, providing significant economic and environmental benefits. The thin ceramic coatings are commonly applied to various kinds of steel cutting and machining tools to enhance their mechanical and tribological properties. The most common ceramic coating is TiN. But the major issues that hamper the application of TiN are high friction co-efficient (typically~0.5), lower hardness, lower thermal stability (~5000 C) and lower corrosion resistance. To address some of these problems, TiSiN nanocomposite coatings are developed, which have super-hardness, better thermal stability (~10000C) and better corrosion resistance. But the as-deposited TiSiN coating still has high co-efficient of friction (~0.4) and high residual stress (~7-9 GPa) which consequently affect the adhesion and toughness of the coating. This project aims to address these problems by (a) the application of carbon implantation to modify the structure and chemistry of the surface layer of the nanocomposite coatings with reduced friction and residual stress; and (b) thermal annealing of the nanocomposite coating to reduce the residual stress with enhanced fracture toughness, better corrosion resistance and more thermal stability. In addition, the role of microstructure, residual stress and defects of these hard coating in corrosive environment will be studied. For this research, a combination of microstructural and mechanical properties characterization, corrosion analysis, tribological test and finite element modelling facilities will be used. The study includes focused ion beam (FIB) milling and transmission electron microscopy (TEM), Synchrotron X-Ray Diffraction (XRD), X-ray Photo Spectroscopy (XPS), Energy dispersive X-Ray (EDX), nanoindentation, nano-scratching, potentio-dynamic polarization cell and Atomic force microscopy(AFM)

Solar selective performance, Opto-dielectric and mechanical characteristics of vacuum fabricated metal nitride thin film coatings

In the present study, metal nitride based sputtered thin film coatings such as Mo/Si, CrN/Si, and Mo:CrN/Si were investigated for their solar selective surface and mechanical applications. Despite a large number of literature is available in the area of solar selective applications of metal nitride based thin film coatings, these materials are still to be commercialized for their practical device applications. In view of this, we chose metal nitride based thin film coatings e.g., Mo, CrN and Mo:CrN to realize their structural, morphological, elemental compositions, optical, dielectric and mechanical properties in as-deposited, and annealed conditions. Detail analyses of these features were carried out using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-Ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), UV-Vis spectroscopy and FTIR, nanoindentation and finite element modeling (FEM). In addition to these, the first principle based density functional theory (DFT) integrated with the B3LYP hybrid functional plus LANL2DZ basis sets based infrared (IR), Raman and ultraviolet-visible (UV-Vis) analysis were also carried out to probe the electronic structural and optical properties of pristine and Mo-doped CrN clusters in the non-crystalline phase. Optical analysis showed that in the visible range of the solar spectrum, the CrN coatings exhibit the highest solar absorptance of 66% while the lowest thermal emittance value of 5.67 was recorded for the CrN coating doped with Mo. As a result, the highest solar selectivity of 9.6, and the energy band-gap of 2.88 eV were achieved with the Mo-doped CrN coatings. On the other hand, optical studies of the annealed coatings showed that with the rise in annealing temperature up to 700 °C, the solar absorptance of CrN coatings increased from 61% to 89% and slightly decreased at 800 °C, while the optical band-gap energy dropped from 2.62 to 1.38 eV but slightly increased to 1.48 eV at 800 °C. Nanoindentation results indicated that as the annealing progresses, the hardness and elastic modulus values of CrN coatings are lowered. Further optical studies of Mo-doped CrN coatings showed that as the annealing temperature increased up to 700 oC, the solar absorptance is increased from 55% to its maximum value of 86%, and the optical band-gaps were dropped from 2.48 to 1.14 eV. Nanoindentation and finite element modeling studies of Mo-doped CrN coatings indicated that as the annealing progresses, both the hardness and elastic modulus values of these coatings reduced. A first principle based DFT simulation on Cr4N4, Cr4MoN3, Cr4Mo2N2, Cr3MoN4, Cr3Mo2N3, and Cr2Mo2N4 clusters indicated that the Cr4Mo2N2 structure was chemically and energetically the most stable species among the six clusters considered. The DFT based electronic analysis revealed that Cr4MoN3 and Cr3Mo2N3 clusters possess magnetic susceptibility while the infrared (IR), Raman and ultraviolet-visible (UV-Vis) studies indicated that the clusters formed by Cr4N4 and Cr4Mo2N2 are naturally stable and able to function as light harnessing materials to be used in solar selective surfaces

Mechanical and Microstructure Study of Nickel-Based ODS Alloys Processed by Mechano-Chemical Bonding and Ball Milling

Due to the need to increase the efficiency of modern power plants, land-based gas turbines are designed to operate at high temperature creating harsh environments for structural materials. The elevated turbine inlet temperature directly affects the materials at the hottest sections, which includes combustion chamber, blades, and vanes. Therefore, the hottest sections should satisfy a number of material requirements such as high creep strength, ductility at low temperature, high temperature oxidation and corrosion resistance. Such requirements are nowadays satisfied by implementing superalloys coated by high temperature thermal barrier coating (TBC) systems to protect from high operating temperature required to obtain an increased efficiency. Oxide dispersive strengthened (ODS) alloys are being considered due to their high temperature creep strength, good oxidation and corrosion resistance for high temperature applications in advanced power plants. These alloys operating at high temperature are subjected to different loading systems such as thermal, mechanical, and thermo-mechanical combined loads at operation. Thus, it is critical to study the high temperature mechanical and microstructure properties of such alloys for their structural integrity.;The primary objective of this research work is to investigate the mechanical and microstructure properties of nickel-based ODS alloys produced by combined mechano-chemical bonding (MCB) and ball milling subjected to high temperature oxidation, which are expected to be applied for high temperature turbine coating with micro-channel cooling system. Stiffness response and microstructure evaluation of such alloy systems was studied along with their oxidation mechanism and structural integrity through thermal cyclic exposure. Another objective is to analyze the heat transfer of ODS alloy coatings with micro-channel cooling system using finite element analysis (FEA) to determine their feasibility as a stand-alone structural coating.;During this project it was found that stiffness response to increase and remain stable to a certain level and reduce at latter stages of thermal cyclic exposure. The predominant growth and adherent Ni-rich outer oxide scale was found on top of the alumina scale throughout the oxidation cycles. The FEA analysis revealed that ODS alloys could be potential high temperature turbine coating materials if micro-channel cooling system is implemented

Investigation into laser re-melting of inconel 625 HVOF coating blended with WC

High velocity oxy-fuel (HVOF) spraying of Diamalloy 1005 powders mixed with WC particles onto steel (304) is considered and laser re-melting of the resulting coatings is examined. Laser re-melting process is modeled to determine the melt layer thickness while temperature increase is formulated using the Fourier heating law. The morphological and metallurgical analyses prior and post laser re-melting process are carried out using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) technique is used to determine the residual stress developed in the coating while the analytical formulation is adopted to predict the residual stress levels at the coating base material interface. The indentation tests are carried out to determine the Young’s modulus and fracture toughness of the coating prior to laser re-melting. Corrosion resistance of coating is measured using potentiodynamic polarization technique prior and post laser treatment process. The predictions of the melt layer thickness are in good agreement with experimental results. The presence of WC particles modifies temperature rise and its gradient in the coating while affecting the Young’s modulus, residual stress levels, and fracture toughness of the coating. The differences in the thermal properties of Inconel 625 powders and WC particles result in formation of small size cellular structure through polyphase solidification. WC dissolution in the central region of the large polycrystalline cells is observed due to the loss of carbon through carbonic gas formation. The results of corrosion tests prevail that significant improvement of corrosion resistance can be achieved after laser treatment process

Scratching the surface: Elastic rotations beneath nanoscratch and nanoindentation tests

In this paper, we investigate the residual deformation field in the vicinity of nanoscratch tests using two orientations of a Berkovich tip on an (001) Cu single crystal. We compare the deformation with that from indentation, in an attempt to understand the mechanisms of deformation in tangential sliding. The lattice rotation fields are mapped experimentally using high-resolution electron backscatter diffraction (HR-EBSD) on cross-sections prepared using focused ion beam (FIB). A physically-based crystal plasticity finite element model (CPFEM) is used to simulate the lattice rotation fields, and provide insight into the 3D rotation field surrounding a nano-scratch experiment, as it transitions from an initial static indentation to a steady-state scratch. The CPFEM simulations capture the experimental rotation fields with good fidelity, and show how the rotations about the scratch direction are reversed as the indenter moves away from the initial indentation