Influence of pre-existing martensite on the wear resistance of metastable austenitic stainless steels

The effect of pre-existing martensite on the sliding wear behavior of a commercial metastable austenitic stainless steel was investigated. Two different steel conditions were considered: annealed (with a fully austenitic microstructure) and cold rolled, consisting of mixtures of austenite and martensite. Wear tests were carried out using ball on disc technique at constant velocity and different sliding distances. Correlation between microstructure and wear mechanisms was performed by X-ray diffraction, electron back-scattered diffraction and focus ion beam. Results show that wear resistance decreases at increasing the amount of pre-existing martensite. In this sense, more strain-induced martensite developed for cold rolled samples, hardening the surface and consequently reducing wedge formation, which induced material removal from the surface. The detailed analysis of the wear track demonstrated the formation of an ultrafine-grain layer just below the surface, not only for annealed but also for cold rolled steel.Peer ReviewedPreprin

Small-scale mechanical response at intermediate/high temperature of 3D printed WC-Co

The present work is focused on WC-Co dry-electropolished. A systematic nanomechanical study of a 3D printed WC-Co grade is investigated. In doing so, nanoindentation technique is implemented and the main deformation/damage mechanisms induced at the submicrometric lenght scale are investigated at different temperatures, from room up to 600ºC. In general, three different approaches are followed to accomplish this research: (1) assessment of intrinsic hardness values as a function of crystallographic orientations from a room temperature up to 600ºC and (2) the determination of effective hardness and flow stress through the Tabor's equation of the metal cobalt binder. Finally, the elastic strain to break was also determined for the main crystallographic orientations for the WC particles as a function of the temperature. The preliminary results highlight that the strenght reduction with increasing temperature is attributed to metallic binder softening. On the other hand, the WC particles presents an isotropic behavior when the testing temperature is over 500ºC because, to inside these particles, the dislocations and the stacking faults are the main deformation mechanisms induced at intermediate/high testing temperature.Postprint (published version

ElectroCatalytic Activity of Nickel Foam with Co, Mo, and Ni Phosphide Nanostructures

In this study, the electrocatalytic activity of nickel foam, which is activated by cobalt, molybdenum, and nickel phosphide nanostructures, is prepared by the plasma hydrothermal method for use in the release of hydrogen and oxygen. The morphology and crystallographic structure of the synthesized phosphide specimens were examined by means of scanning electron microscopy, Fourier-transform infrared spectroscopy, and X-ray diffraction. Moreover, the electrolysis activity for these sets of specimens was investigated using the Tafel polarization curve or linear sweep voltammetry, cyclic voltammetry, as well as by means of the electrochemical impedance spectroscopy technique. Preliminary results show that nickel phosphide presents the highest electrocatalytic activity than the other phosphides developed in this research. In this regard, it presents an electrocatalytic activity to release hydrogen and oxygen of around -1.7 and 0.82 mV, which is measured at a current density of 100 mA·cm-2, respectively.Peer ReviewedPostprint (published version

Exploring the effects of laser surface modification on AISI 301LN steel: a micro-mechanical study

This article investigates the surface hardening capability of a metastable austenitic TRansformation Induced Plasticity (TRIP) stainless steel, particularly on AISI 301LN, by laser texturing. This technology produces microstructural surface changes in terms of both phase transformation and grain size modification and, as a direct consequence, the laser influences the surface characteristics, mainly hardness and roughness. In this sense, the key parameters (laser power, scanning speed and position of the focal length) were investigated by using a Design of Experiments (DoE) in detail to better understand the correlation between texturing parameters, microstructural and mechanical changes, always at the superficial level. From all the aforementioned information, the results show that the maximum surface hardening is obtained by increasing the laser power and decreasing the scanning speed. Furthermore, by reducing the focal distance, the depth of the microstructural evolution layer is more significant, while the width is less affected. Finally, a suitable model was developed to correlate the processing parameters here investigated with the resulting surface integrity, in terms of mechanical properties, by means of a regression equation.This research was funded by AGAUR, Agency for Administration of University and Research (Agència de Gestió d’Ajuts Universitaris i de Recerca), grant number FI-SDUR 2020.Peer ReviewedPostprint (published version

Effect of laser surface texturing on Schmid factor and plastic deformation mechanisms on AISI 301LN steel

In this comprehensive study, the multifaceted impact of laser surface texturing (LST) on AISI 301LN stainless steel is explored. Changes in the microstructure, mechanical properties, and grain characteristics are examined. The dynamic relationship between Schmid factor evolution and plastic deformation in this stainless steel alloy is unveiled through the analysis of grain statistics and individual grain scrutiny. It is revealed that LST initiates the formation of strain-induced a’-martensite, grain refinement, and substantial hardness enhancements. Notably, an a’-martensite crystalline size of 2.05 Å is induced by LST. Furthermore, a 12% increase in tensile strength is observed after LST along with an 11% boost in yield strength. However, reductions of 19% in elongation to fracture and 12% in the area reduction are experienced.This research was funded by AGAUR, Agency for Administration of University and Research (Agència de Gestió d’Ajuts Universitaris i de Recerca), grant number FI-SDUR 2020.Peer ReviewedPostprint (published version

Experimental analysis of manufacturing parameters’ effect on the flexural properties of wood-PLA composite parts built through FFF

This paper aims to determine the flexural stiffness and strength of a composite made of a polylactic acid reinforced with wood particles, named commercially as Timberfill, manufactured through fused filament fabrication (FFF). The influence of four factors (layer height, nozzle diameter, fill density, and printing velocity) is studied through an L27Taguchi orthogonal array. The response variables used as output results for an analysis of variance are obtained from a set of four-point bending tests. Results show that the layer height is the most influential parameter on flexural strength, followed by nozzle diameter and infill density, whereas the printing velocity has no significant influence. Ultimately, an optimal parameter set that maximizes the material’s flexural strength is found by combining a 0.2-mm layer height, 0.7-mm nozzle diameter, 75% fill density, and 35-mm/s velocity. The highest flexural resistance achieved experimentally is 47.26 MPa. The statistical results are supported with microscopic photographs of fracture sections, and validated by comparing them with previous studies performed on non-reinforced PLA material, proving that the introduction of wood fibers in PLA matrix reduces the resistance of raw PLA by hindering the cohesion between filaments and generating voids inside it. Lastly, five solid Timberfill specimens manufactured by injection molding were also tested to compare their strength with the additive manufactured samples. Results prove that treating the wood-PLA through additive manufacturing results in an improvement of its resistance and elastic properties, being the Young’s module almost 25% lower than the injected material.Preprin

Small scale fracture behaviour of multilayer TiN/CrN systems: Assessment of bilayer thickness effects by means of ex-situ tests on FIB-milled micro-cantilevers

TiN/CrN multilayered PVD coatings are known to exhibit outstanding micromechanical properties and wear resistance. On the other hand, information on their small scale fracture behaviour is rather scarce. The present work aims to address it by testing to failure FIB-milled microbeams of multilayer TiN/CrN systems with different bilayer periods (8, 19 and 25 nm). In doing so, these micrometric specimens are first FIB notched, and thus deflected by means of a nanoindentation system. It is found that multilayer architecture translates into a beneficial synergic effect regarding critical load for reaching unstable failure; and thus, on energy absorption at fracture. Such behaviour is associated with small scale crack deflection as main toughening mechanism.Peer ReviewedPostprint (author's final draft

Geometrically Necessary Dislocations on Plastic Deformation of Polycrystalline TRIP Steel

In this study, the main deformation behavior in terms of geometrically necessary dislocations (GND) was investigated on a transformation induced plasticity (TRIP) stainless steel by using sharp indentation at nanometric length scale. Results evidence that austenitic grains display an isotropic behavior on terms of GND, the main deformation mechanism being the Frank–Read source activated at local level

Small-scale assessment of corrosion-induced damage in hardmetals

In this work, the effect of corrosion-induced damage on the mechanical response of hardmetals was evaluated at small-scale level by means of nanoindentation and nanoscratch. Damage was introduced in a controlled way through immersion in acidic solution. It is found that surface degradation associated with corrosion leads to a strong reduction of hardness and elastic modulus, as compared to non-corroded samples. Similarly, significant differences are observed in nanoscratch response, regarding not only width and depth of tracks but also deformation mechanisms developed as contact load is progressively increased. Damage was already evidenced in corroded surfaces at scratching loads one order of magnitude lower than for virgin specimens. Cracking and fragmentation of individual WC grains, together with chipping of at the track edges were the main deformation and fracture micromechanisms identified. Changes in nanoindentation and nanoscratch response and damage scenario are discussed on the basis of the corrosion-induced changes within the intrinsic microstructural assemblage of hardmetals.Postprint (published version