Author: Mario Kotlár

Microstructure, Mechanical and Tribological Properties of Advanced Layered WN/MeN (Me = Zr, Cr, Mo, Nb) Nanocomposite Coatings

Smyrnova K., Sahul M., Haršáni M., Pogrebnjak A., Ivashchenko V., Beresnev V., Stolbovoy V., Čaplovič L., Čaplovičová M., Vančo L., Kusý M., Kassymbaev A., Satrapinskyy L., Flock D.

Nanomaterials, 12 (3), art. no. 395

https://doi.org/10.3390/nano12030395

Abstract

Due to the increased demands for drilling and cutting tools working at extreme machining conditions, protective coatings are extensively utilized to prolong the tool life and eliminate the need for lubricants. The present work reports on the effect of a second MeN (Me = Zr, Cr, Mo, Nb) layer in WN-based nanocomposite multilayers on microstructure, phase composition, and mechanical and tribological properties. The WN/MoN multilayers have not been studied yet, and cathodic-arc physical vapor deposition (CA-PVD) has been used to fabricate studied coating systems for the first time. Moreover, first-principles calculations were performed to gain more insight into the properties of deposited multilayers. Two types of coating microstructure with different kinds of lattices were observed: (i) face-centered cubic (fcc) on fcc-W2N (WN/CrN and WN/ZrN) and (ii) a combination of hexagonal and fcc on fcc-W2N (WN/MoN and WN/NbN). Among the four studied systems, the WN/NbN had superior properties: the lowest specific wear rate (1.7 × 10−6 mm3/Nm) and high hardness (36 GPa) and plasticity index H/E (0.93). Low surface roughness, high elastic strain to failure, Nb2O5 and WO3 tribofilms forming during sliding, ductile behavior of NbN, and nanocomposite structure contributed to high tribological performance. The results indicated the suitability of WN/NbN as a protective coating operating in challenging conditions. 

Combined in Situ Photoluminescence and X-ray Scattering Reveals Defect Formation in Lead-Halide Perovskite Films

Nada Mrkyvkova*, Vladimír Held, Peter Nádaždy, Riyas Subair, Eva Majkova, Matej Jergel, Aleš Vlk, Martin Ledinsky, Mário Kotlár, Jianjun Tian, Peter Siffalovic

Phys. Chem. Lett. 2021, 12, 41, 10156–10162

https://doi.org/10.1021/acs.jpclett.1c02869

Abstract

Lead-halide perovskites have established a firm foothold in photovoltaics and optoelectronics due to their steadily increasing power conversion efficiencies approaching conventional inorganic single-crystal semiconductors. However, further performance improvement requires reducing defect-assisted, nonradiative recombination of charge carriers in the perovskite layers. A deeper understanding of perovskite formation and associated process control is a prerequisite for effective defect reduction. In this study, we analyze the crystallization kinetics of the lead-halide perovskite MAPbI3–xClx during thermal annealing, employing in situ photoluminescence (PL) spectroscopy complemented by lab-based grazing-incidence wide-angle X-ray scattering (GIWAXS). In situ GIWAXS measurements are used to quantify the transition from a crystalline precursor to the perovskite structure. We show that the nonmonotonous character of PL intensity development reflects the perovskite phase volume, as well as the occurrence of the defects states at the perovskite layer surface and grain boundaries. The combined characterization approach enables easy determination of defect kinetics during perovskite formation in real-time.

Formation of CuCrCoFeNiO high entropy alloy thin films by rapid thermal processing of Cu/CrNiO/FeCo multilayers.

Formation of CuCrCoFeNiO high entropy alloy thin films by rapid thermal processing of Cu/CrNiO/FeCo multilayers.

Wang, Anni; Oliva Ramírez, Manuel; Caplovicova, Maria; Vretenar, Viliam; Böttcher, Julius; Hopfeld, Marcus; Kups, Thomas; Flock, Dominik; Schaaf, Peter

Surface and coatings technology, ISSN 0257-8972, 405 (2021), 126563

doi.org/10.1016/j.surfcoat.2020.126563

Abstract

This study presents the synthesis of High Entropy Alloy (HEA) films starting from elemental Cu and binary alloy CoFe and CrNiO multilayers, followed by rapid thermal processing (RTP). By that, the HEA films (HEAFs) were formed by phase formation via short-range and fast diffusion processes. Multilayers with a total thickness of 760 nm consisting of 16 repetitions of a Cu (11 nm)/CrNiO (16.5 nm)/CoFe (20 nm) sequence were annealed at temperatures from 600 °C to 1000 °C for 5 min. The reaction products were then analyzed by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM) combined with electron energy loss spectroscopy (EELS), in order to identify the phase transformations and elemental distributions. A duplex FCC structure containing CrCoFeNiO HEA and pure Cu phase was successfully synthesized at 600 °C and 800 °C by the solid-state reaction. CuCrCoFeNiO HEA formed within in a Cu nanocrystalline matrix. As the annealing temperature increased, the oxygen content in the films decreased, Both HEA and Cu possess significant 〈111〉 preferred orientation. The HEA phase demonstrated a typical microstructure of alloys with intensive nano-twins. Moreover, the grain growth kinetics of the HEA phase was evaluated, and the activation energy was found to be 185(10) kJ/mol. This is comparable to that of conventional stainless steel (~150 kJ/mol) and less than half of the value for CrCoFeNi bulk (434 kJ/mol). A surface energy-driven grain growth mechanism of the HEAFs via multilayer alloy formation is proposed in this study. The mechanical properties, hardness and Young’s modulus, were measured via nanoindentation, and the strengthening mechanism was proposed and compared with current literature.

Covalent Diamond–Graphite Bonding: Mechanism of Catalytic Transformation

Covalent Diamond–Graphite Bonding: Mechanism of Catalytic Transformation

Semir Tulić, Thomas Waitz, Mária Čaplovičová, Gerlinde Habler, Marián Varga, Mário Kotlár, Viliam Vretenár, Oleksandr Romanyuk, Alexander Kromka, Bohuslav Rezek, Viera Skákalová

ACS Nano, 2019, 1344621-4630

doi.org/10.1021/acsnano.9b00692

Abstract

Aberration-corrected transmission electron microscopy of the atomic structure of diamond–graphite interface after Ni-induced catalytic transformation reveals graphitic planes bound covalently to the diamond in the upright orientation. The covalent attachment, together with a significant volume expansion of graphite transformed from diamond, gives rise to uniaxial stress that is released through plastic deformation. We propose a comprehensive model explaining the Ni-mediated transformation of diamond to graphite and covalent bonding at the interface as well as the mechanism of relaxation of uniaxial stress. We also explain the mechanism of electrical transport through the graphitized surface of diamond. The result may thus provide a foundation for the catalytically driven formation of graphene–diamond nanodevices.