Pavel Krakhmalev

Experimental study of the relationship between temperature and adhesive forces for low-alloyed steel, stainless steel, and titanium using atomic force microscopy in ultrahigh vacuum

Experimental study of the relationship between temperature and adhesive forces for low-alloyed steel, stainless steel and titanium using atomic force microscopy in ultra-high vacuum

Measurements of the critical strain for rippling in carbon nanotubes

We report measurements of the bending stiffness in free standing carbon nanotubes, using atomic force microscopy inside a scanning electron microscope. Two regimes with different bending stiffness were observed, indicative of a rippling deformation at high curvatures. The observed critical strains for rippling were in the order of a few percent and comparable to previous modeling predictions. We have also found indications that the presence of defects can give a higher critical strain value and a concomitant reduction in Young’s modulus.

Mechanical behavior of carbon nanotubes in the rippled and buckled phase

We have studied the mechanical behavior of multi-walled carbon nanotubes for bending strains beyond the onset for rippling and buckling. We found a characteristic drop in the bending stiffness at the rippling and buckling onset and the relative retained stiffness was dependent on the nanotube dimensions and crystallinity. Thin tubes are more prone to buckle, where some lose all of their bending stiffness, while thicker tubes are more prone to ripple and on average retain about 20% of their bending stiffness. In defect rich tubes, the bending stiffness is very low prior to rippling, but these tubes retain up to 70% of their initial bending stiffness.

MICROSTRUCTURAL AND THERMAL STABILITY OF SELECTIVE LASER MELTED 316L STAINLESS STEEL SINGLE TRACKS

To remove residual stresses, an as-built SLM object is usually post- treated. This treatment can affect the microstructure, changing the final mechanical characteristics. This investigation is focused on the microstructural characterisation of 316L austenitic stainless steel in as-built and annealed conditions. The SLM microstructure was relatively stable up to 900°C, when cell boundaries start to disappear. At higher temperatures, an insignificant grain coarsening was detected. These microstructural changes caused a gradual drop in the hardness. The obtained result is background for the future development of post-treatment regimens to achieve a high level in the final mechanical properties of SLM objects.

Tensile properties and microstructure of direct metal laser sintered Ti6Al4V (ELI) alloy

Direct metal laser sintering (DMLS) is an additive manufacturing technology used to melt metal powder by high laser power to produce customised parts, light-weight structures, or other complex objects. During DMLS, powder is melted and solidified track-by-track and layer-by-layer; thus, building direction can influence the mechanical properties of DMLS parts. The mechanical properties and microstructure of material produced by DMLS can depend on the powder properties, process parameters, scanning strategy, and building geometry. In this study, the microstructure, tensile properties, and porosity of DMLS Ti6Al4V (ELI) horizontal samples were analysed. Defect analysis by CT scans in pre-strained samples was used to detect the crack formation mechanism during tensile testing of as-built and heat-treated samples. The mechanical properties of the samples before and after stress relieving are discussed.

Abrasion of ultrafine WC-Co by fine abrasive particles

Abstract Abrasive wear of a series of WC-(5%-14%, mass fraction) Co hardmetals was investigated employing coarse and fine SiC abrasive under two-body dry abrasion conditions with pin-on-disc and edge-on-disc test arrangements. Unexpectedly, it is found that submicron grades demonstrate substantially higher wear rates comparing with the coarse grades if fine abrasive is utilized in pin-on-disc tests. Such a behavior is attributed to changes in a ratio of abrasive size to size of hard phase as finer abrasive is used. The edge-on-disc test demonstrates that edge wear may be described in two stages with the highest wear rates at the beginning stage. This behavior is associated with a transition of wear mechanisms as edge is wider due to wear. Compared with the ultrafine grades of the same Co content, the coarse grades demonstrate higher wear rates at the beginning, but lower wear rates at the final stage. Wear rates and mechanisms observed at final stage correlate well to the results observed for pin-on-disc tests employing fine abrasive.

Image formation mechanisms in scanning electron microscopy of carbon nanotubes, and retrieval of their intrinsic dimensions.

We present a detailed analysis of the image formation mechanisms that are involved in the imaging of carbon nanotubes with scanning electron microscopy (SEM). We show how SEM images can be modelled by accounting for surface enhancement effects together with the absorption coefficient for secondary electrons, and the electron-probe shape. Images can then be deconvoluted, enabling retrieval of the intrinsic nanotube dimensions. Accurate estimates of their dimensions can thereby be obtained even for structures that are comparable to the electron-probe size (on the order of 2 nm). We also present a simple and robust model for obtaining the outer diameter of nanotubes without any detailed knowledge about the electron-probe shape.

Functionalization of Biomedical Ti6Al4V via In Situ Alloying by Cu during Laser Powder Bed Fusion Manufacturing

The modern medical industry successfully utilizes Laser Powder Bed Fusion (LPBF) to manufacture complex custom implants. Ti6Al4V is one of the most commonly used biocompatible alloys. In surgery practice, infection at the bone–implant interface is one of the key reasons for implant failure. Therefore, advanced implants with biocompatibility and antibacterial properties are required. Modification of Ti alloy with Cu, which in small concentrations is a proven non-toxic antibacterial agent, is an attractive way to manufacture implants with embedded antibacterial functionality. The possibility of achieving alloying in situ, during manufacturing, is a unique option of the LPBF technology. It provides unique opportunities to manufacture customized implant shapes and design new alloys. Nevertheless, optimal process parameters need to be established for the in situ alloyed materials to form dense parts with required mechanical properties. This research is dedicated to an investigation of Ti6Al4V (ELI)-1 at % Cu material, manufactured by LPBF from a mixture of Ti6Al4V (ELI) and pure Cu powders. The effect of process parameters on surface roughness, chemical composition and distribution of Cu was investigated. Chemical homogeneity was discussed in relation to differences in the viscosity and density of molten Cu and Ti6Al4V. Microstructure, mechanical properties, and fracture behavior of as-built 3D samples were analyzed and discussed. Pilot antibacterial functionalization testing of Ti6Al4V (ELI) in situ alloyed with 1 at % Cu showed promising results and notable reduction in the growth of pure cultures of Escherichia coli and Staphylococcus aureus.

VALIDATION OF MINIATURISED TENSILE TESTING ON DMLS TI6AL4V (ELI) SPECIMENS

Direct metal laser sintering (DMLS) is a relatively new technology that is developing rapidly. Since DMLS material is created by melting/solidifying tracks and layers from powder, even building geometry can influence the mechanical properties. To certify a material, the testing specimens must be designed and manufactured according to the appropriate standards. Miniaturised tensile DMLS samples could be a good alternative for express quality control, and could reduce the cost of DMLS-specific testing. In this study, as-built and stress-relieved miniaturised tensile DMLS Ti6Al4V (ELI) specimens with different surface qualities were investigated. The fracture surfaces and mechanical properties of the mini-tensile specimens were analysed and compared with standard full-sized specimens also manufactured by DMLS. The obtained data showed the applicability of mini-tensile tests for the express analysis of DMLS objects if a correction factor is applied for the calculation of the load-bearing cross-section of the specimen.

Large variations in the onset of rippling in concentric nanotubes

We present a detailed experimental study of the onset of rippling in highly crystalline carbon nanotubes. Modeling has shown that there should be a material constant, called the critical length, describing the dependence of the critical strain on the nanotube outer radius. Surprisingly, we have found very large variations, by a factor of three, in the critical length. We attribute this to a supporting effect from the inner walls in multiwalled concentric nanotubes. We provide an analytical expression for the maximum deflection prior to rippling, which is an important design consideration in nanoelectromechanical systems utilizing nanotubes.