Ústav materiálových věd a inženýrství

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Effect of microstructure on machinability of extruded and conventional H13 tool steel
(ELSEVIER SCI LTD, 2025-06-09) Kolomý, Štěpán; Malý, Martin; Doubrava, Marek; Sedlák, Josef; Zouhar, Jan; Čupera, Jan
H13 tool steel samples were fabricated using material extrusion to explore their machinability, offering a promising alternative to laser powder bed fusion for producing complex parts like moulds and cores. Three material states were studied: as-built (AB), heat-treated additively manufactured (HTAM), and heat-treated wrought (HTW). Machining tests focused on cutting speed, feed per tooth, and cooling conditions (dry/flood), while tracking their effect on cutting forces, surface roughness, hardness, microstructure, and residual stresses. Heat treatment significantly reduced porosity (similar to 45 % decrease between AB and HTAM) and transformed the microstructure to full martensite, increasing hardness and cutting forces. Interestingly, the HTAM sample showed lower cutting forces than HTW-by 23.7 % in dry and 24.5 % under flood cooling. HTW generally produced smoother surfaces at lower cutting parameters, but its roughness increased at higher conditions compared to HTAM. The softest AB sample experienced the highest surface hardening (similar to 12 %) when machined at low cutting speeds, while the HTW sample showed most uniform plastic deformation, extending up to similar to 50 mu m below the surface. Dominantly tensile residual stresses were measured in HTW, while AB and HTAM showed mainly compressive residual stresses under dry conditions. This study highlights viability of extruded H13 for industrial use, particularly in mould applications.
Cryogenic Treatment of Martensitic Steels: Microstructural Fundamentals and Implications for Mechanical Properties and Wear and Corrosion Performance
(MDPI, 2024-01-23) Jurči, Peter; Dlouhý, Ivo
Conventional heat treatment is not capable of converting a sufficient amount of retained austenite into martensite in high-carbon or high-carbon and high-alloyed iron alloys. Cryogenic treatment induces the following alterations in the microstructures: (i) a considerable reduction in the retained austenite amount, (ii) formation of refined martensite coupled with an increased number of lattice defects, such as dislocations and twins, (iii) changes in the precipitation kinetics of nano-sized transient carbides during tempering, and (iv) an increase in the number of small globular carbides. These microstructural alterations are reflected in mechanical property improvements and better dimensional stability. A common consequence of cryogenic treatment is a significant increase in the wear resistance of steels. The current review deals with all of the mentioned microstructural changes as well as the variations in strength, toughness, wear performance, and corrosion resistance for a variety of iron alloys, such as carburising steels, hot work tool steels, bearing and eutectoid steels, and high-carbon and high-alloyed ledeburitic cold work tool steels.
Erosion development in AISI 316L stainless steel under pulsating water jet treatment
(Elsevier, 2024-01-26) Hloch, Sergej; Poloprudský, Jakub; Šiška, Filip; Babinský, Tomáš; NAGH, Akash; Chlupová, Alice; Kruml, Tomáš
Erosion of solids by liquid droplets is a phenomenon which is a compromise between mechanical properties of the material and droplet hydrodynamic parameters. While a number of studies deal with the deformation of drops, the deformation evolution inside the material has not yet been revealed, mainly from the point of view of the time action of the impinging drops The mechanical response of AISI 316L was investigated under gradually increasing numbers of impingements of liquid droplets, with a droplet volume of Vd approximately equal to 0.9 mm3, generated by an ultrasonic pulsating water jet with the frequency f = 40 kHz from 1 to 20 s. The surface roughness and the wear rates were determined using a laser profilometer. The cross-section of the selected samples was subjected to microhardness measurement with a load of 0.150 N in a 2D grid, which included the entire perimeter of the deformed area. The minimal microhardness measurement grid under the groove had dimensions of 15 x 15 indents, equal to an area of approximately 450 x 600 mu m. A maximum hardness increase was observed at the lowest measured depth of 30 mu m. An increase in hardness was observed at 300 mu m below the surface. The hardening in the deeper subsurface area was most likely caused by shear stress. This shows the high degree of similitude between the solid and liquid droplet impingements. The results indicate that the currently accepted theory on the development of erosion over time has shortcomings, as demonstrated in this work by the ratio between the utilised droplet diameter and the grain size of the material.
Atomic tuning of 3D printed carbon surface chemistry for electrocatalytic nitrite oxidation and reduction to ammonia
(ROYAL SOC CHEMISTRY, 2024-11-26) Gao, Wanli; Michalička, Jan; Pumera, Martin
Nitrite contamination in agricultural and industrial wastewater presents a critical impact on environmental sustainability, demanding efficient strategies for monitoring and remediation. This study addresses this challenge by developing cost-effective electrocatalysts for both nitrite detection and conversion to value-added ammonia. 3D printed carbon materials are explored as bifunctional platforms for the electrochemical nitrite oxidation reaction (NO2OR) and nitrite reduction reaction (NO2RR). Benefiting from the inherent Ti-dominated metallic impurities and intrinsic surface features of carbon nanotubes, 3D printed carbon electrodes exhibit electrocatalytic activity for both reactions. To enhance this activity, we further introduce an effective fabrication methodology that combines 3D printing of carbon substrates with precise surface modification using atomic layer deposition (ALD) of TiO2. The resulting TiO2-coated carbon electrode demonstrates significantly improved electrocatalytic properties. For NO2OR, it exhibits a peak current density of 0.75 mA cm-2 at 1.53 V vs. RHE, while for NO2RR, it achieves a yield rate of 630.5 mu g h-1 cm-2 with a faradaic efficiency of 81.9% at -1.06 V vs. RHE. This enhancement in electrocatalytic activity is primarily attributed to the formation of abundant interfaces between the conductive carbon and ALD-coated TiO2. The developed methodology not only enables precise modification of 3D printed carbon surface chemistry but also presents a scalable method for electrocatalyst production.
Enhancing Alkaline Hydrogen Evolution Reaction on Ru-Decorated TiO2 Nanotube Layers: Synergistic Role of Ti3+, Ru Single Atoms, and Ru Nanoparticles
(Wiley, 2025-05-01) Thalluri, Sitaramanjaneya Mouli; Rodriguez Pereira, Jhonatan; Michalička, Jan; Kolíbalová, Eva; Hromádko, Luděk; Šlang, Stanislav; Pouzar, Miloslav; Sopha, Hanna Ingrid; Zazpe Mendioroz, Raúl; Macák, Jan
Synergistic interplays involving multiple active centers originating from TiO2 nanotube layers (TNT) and ruthenium (Ru) species comprising of both single atoms (SAs) and nanoparticles (NPs) augment the alkaline hydrogen evolution reaction (HER) by enhancing Volmer kinetics from rapid water dissociation and improving Tafel kinetics from efficient H* desorption. Atomic layer deposition of Ru with 50 process cycles results in a mixture of Ru SAs and 2.8 +/- 0.4 nm NPs present on TNT layers, and it emerges with the highest HER activity among all the electrodes synthesized. A detailed study of the Ti and Ru species using different high-resolution techniques confirmed the presence of Ti3+ states and the coexistence of Ru SAs and NPs. With insights from literature, the role of Ti3+, appropriate work functions of TNT layers and Ru, and the synergistic effect of Ru SAs and Ru NPs in improving the performance of alkaline HER were elaborated and justified. The aforementioned characteristics led to a remarkable performance by having 9 mV onset potentials and 33 mV dec(-1) of Tafel slopes and a higher turnover frequency of 1.72 H-2 s(-1) at 30 mV. Besides, a notable stability from 28 h staircase chronopotentiometric measurements for TNT@Ru surpasses TNT@Pt in comparison.

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