Search Results (236)

Worn roller mill roll shafts made of Steel 45 require cost-effective surface restoration; plasma transferred arc (PTA) deposition of Ni–Cr–B–Si + WC composite coatings is a promising approach, yet the effect of arc current on coating quality remains insufficiently characterised for this substrate. Six coatings were deposited from PS-12NVK-01 powder (65 wt.% PG-10N-01 + 35 wt.% WC) at arc currents of 50–100 A on Steel 45 substrates using a ZTW3501DC PTA system; coatings were characterised by SEM, EDS mapping, XRD (HighScore Plus, PDF-2), Vickers microhardness profiling, and ball-on-flat tribological testing. EDS analysis revealed that compositional dilution increases from 18.1% at 60 A to 46.6% at 100 A; XRD identified WC + Cr₃C₂ + Ni₃B + Ni₂B + (Fe,Ni)γ at 50 A, transitioning through Cr₇C₃ + W₂C dominance at 80 A to an Fe₀.₆₄Ni₀.₃₆ matrix at 100 A; and coating thickness peaked at 2.70 mm at 80 A. The 60 A coating yielded the highest surface hardness (887 ± 76 HV, >4× the substrate), the lowest specific wear rate (4.00 × 10⁻⁶ mm³/(N·m), ~22× lower than uncoated Steel 45), and minimum dilution (18.1%), identifying 60 A as the most favourable deposition current for the restoration of roller mill roll shafts under the process parameters employed. Full article

In this work, to address the limitation of low strength and hardness of single-phase CoCrFeNi high-entropy alloy, SiC particles were introduced as a reinforcing phase to prepare CoCrFeNi matrix composites with SiC contents of 0 wt%, 1 wt%, 2.5 wt% and 5 wt% via spark plasma sintering (SPS). It was preliminarily predicted that SiC particles would be uniformly distributed along grain boundaries of the CoCrFeNi matrix. During sintering, partial SiC decomposes at high-temperature, high-activity interfaces, regulating carbide precipitation and phase structural evolution, while residual undecomposed SiC remains at grain boundaries to pin boundaries and refine grains, thereby synergistically enhancing mechanical properties and wear resistance. Microstructural characterization reveals that all samples maintain a face-centered cubic (FCC) solid-solution matrix, and samples with non-zero SiC addition contain Cr₇C₃ carbides, which are mostly distributed at grain boundaries. With the increase in SiC content, mechanical performance is remarkably improved compared with the unreinforced CoCrFeNi matrix: the hardness rises from 198.8 HV to 321.7 HV, the yield strength is greatly enhanced from 242.5 MPa to 673.4 MPa, and the tensile strength increases from 557.9 MPa to 755.7 MPa. The improved yield strength originates synergistically from grain refinement, solid-solution strengthening, grain-boundary strengthening and dislocation strengthening. By clarifying the influence of microstructural defects on critical shear stress (τ₀) and normal fracture stress (σ₀), the intrinsic mechanism governing tensile mechanical performance and ductile–brittle fracture transition was revealed. This optimized CoCrFeNi/SiC composite exhibits excellent strength–hardness comprehensive performance, showing promising application potential for high-load, wear-resistant and structural service components under severe tribological and pressure conditions. Full article

Ni/Siral catalysts with different Si/Al ratios were prepared by incipient wetness impregnation (IWI) to assess the impact of support composition on Ni²⁺ speciation and ethylene oligomerization (EO) performance. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS), H₂ temperature-programmed reduction (TPR), X-ray diffraction (XRD), NH₃ temperature-programmed desorption (TPD), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) with energy-dispersive X-ray (EDX) analysis, and diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS). The EO catalysts were tested in a fixed-bed reactor at 225 °C under 11 bar ethylene and at 120 °C under 26 bar ethylene. Ni/Siral-70 was the most active catalyst investigated, but Ni/Siral-30 also exhibited good performance. The active sites were inferred to be isolated Ni²⁺ ions on amorphous SiO₂-Al₂O₃ containing interstitial Al³⁺ ions that enhance Brønsted acidity; Ni/Siral-70 displayed the highest concentration of these sites based on CO DRIFTS. Formation of NiAl₂O₄ surface species limited the activity of Ni/Siral-30 and especially Ni/Siral-5. The catalysts were also tested using a simulated ethane oxidative dehydrogenation (ODH) product stream containing 44% ethylene, 44% ethane, 4.5% methane, 2% H₂, 4.5% CO₂, 0.9% propylene, and 0.1% CO. The simulated ODH mixture gave lower EO conversion than 50/50 ethylene/N₂ at 225 °C and 11 bar over Ni/Siral-30, consistent with catalyst poisoning. In contrast, EO conversion over the Ni/Siral-70 catalyst was unaffected under these conditions. Catalyst testing at 120 °C and 26 bar revealed catalyst poisoning by feed impurities for both catalysts. Low-temperature/high-pressure EO activity was not recovered by simple thermal regeneration of Ni/Siral-30 at 300 °C. Full article

Clay minerals in coal play a key role in controlling mineralogical composition, geochemical signatures, and the industrial behavior of coal and its combustion residues. This study investigates the occurrence, provenance, and potential applications of clay minerals in Carboniferous ash-rich coals from the Bogatyr, Lenin, and Saradyr coal mines in northeastern Kazakhstan. A total of 60 coal samples were analyzed using XRD, SEM–EDS/BSE, XRF, and ICP-OES following acid leaching. Based on ash yield, 52 samples were classified as coal (<50% ash), while 8 samples were classified as carbonaceous shale or mudstone (>50% ash). Mineralogical assemblages show clear variability among the studied mines. Saradyr samples are strongly quartz-dominated with lower clay proportions, Bogatyr samples exhibit highly heterogeneous quartz–clay–mica assemblages, whereas Lenin samples are relatively more clay-rich and dominated by kaolinite and illite-group minerals. Across all samples, kaolinite is the dominant clay mineral (16.6–46 wt.%), occurring mainly as authigenic pore- and cell-filling aggregates. Minor phases include illite–muscovite (7.1–29.9 wt.%), illite–smectite (up to 7.6 wt.% in Bogatyr), and smectite–montmorillonite (0.4–0.7 wt.%). Clay minerals occur as discrete particles, coatings, and pore fillings, contributing to ash formation; however, their correlation with ash yield is weak (R = 0.03–0.05), reflecting heterogeneous mineral inputs and diagenetic overprinting. All geochemical data are reported on a high-temperature coal ash (HTA) basis (815 °C). Geochemical indices (CIA, CIW, CIX) and Al₂O₃/TiO₂ ratios (1.8–17.4) indicate variable provenance and moderate to high weathering intensity, reflecting mixed mafic to intermediate source rocks. A total of 23 trace elements were identified. Au occurs at trace levels (up to 0.02 ppm), while selected rare earth elements (REE: Ce, Dy, Eu, La, Nd, Sm, Y, Yb) average 0.2–0.3 ppm, indicating negligible economic recovery potential. REEs show a strong positive correlation with clay minerals (r = 0.93), indicating adsorption and minor structural incorporation. In contrast, Au correlates with As, V, Zn, Cu, Ni, and Nb, suggesting sulfide association. HTA is enriched in SiO₂–Al₂O₃ phases dominated by kaolinite and quartz, indicating strong potential for cement, geopolymer, ceramic, and zeolite applications. Full article

Accurate prediction of the stacking fault energy (SFE) is critical for controlling deformation mechanisms, specifically transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP), in high-manganese (high-Mn) austenitic steels, which are of growing importance in automotive and structural applications that demand exceptional strength–ductility combinations. This study presents a systematic comparative evaluation of six supervised machine learning (ML) models—Multiple Linear Regression (MLR), Random Forest (RF), Extra Trees (ETs), Gradient Boosting (GB), Support Vector Regression (SVR), and a stacking ensemble—trained on a curated, outlier-cleaned experimental database of Fe-Mn-C-Si-Al-Cr-Ni-N spanning SFE values from 5.0 to 63.0 mJ/m² (mean 23.7 ± 11.2 mJ/m²). After Z-score outlier removal (|Z| > 3) and 80/20 train–test splitting with nested 5-fold cross-validation hyperparameter optimization using GridSearchCV, ET and GB achieved training R² values of 0.988 and 0.990, respectively, confirming that SFE is highly predictable from alloy composition alone. The stacking ensemble delivered the best generalization on the independent held-out test set (test R² = 0.603, RMSE = 5.60 mJ/m², MAE = 4.86 mJ/m²), outperforming all the individual learners. Random Forest feature importance analysis identified Al (22.3%), Fe (20.5%), and Mn (17.7%) as the three most influential compositional variables, collectively explaining 60.6% of the predicted variance. Pearson correlation analysis confirmed that Al was the strongest individual linear predictor (r = +0.421, p < 0.001), whereas Fe showed a significant negative correlation (r = −0.327, p < 0.001). Mn, C, and the remaining elements showed no statistically significant linear correlations with SFE, underscoring the dominance of nonlinear compositional interactions. Composition–SFE design maps derived from the GB model delineate the TRIP/TWIP regime boundaries in the Mn–C and Mn–Al composition spaces, providing a validated computational tool for targeted high-Mn steel alloy design. Full article

Silicomanganese slag (SiMnS), a Mn-bearing by-product from silicomanganese alloy production, is often stockpiled in large quantities and may pose environmental concerns due to potential metal leaching. This study develops an OPC-rich hybrid SiMnS–steel slag–fly ash–OPC-activated composite binder, referred to as SMSAB, in which OPC accounts for 55% of the solid precursor mass. Different alkali contents and sodium silicate moduli were investigated, and the optimised paste was characterised in terms of mechanical strength, reaction products, pore structure, carbon-footprint and heavy-metal leaching. The best performance was obtained at an alkali content of 4% and a sodium silicate modulus of 1.0, giving 28-day compressive and flexural strengths of 65.13 MPa and 3.37 MPa, respectively. XRD, SEM-EDS, FTIR and MIP results showed that the main reaction products were C-(A)-S-H, N-A-S-H and C-N-A-S-H gels, which refined the pore structure and produced a dense matrix. The reduction in Mn leaching may be associated with physical encapsulation, possible charge-balancing interactions within gel structures, changes in Mn-related bonding environments and the presence of Mn-bearing phases. Leaching concentrations of Zn, Mn, Cr, Cu and Ni satisfied the Grade III groundwater limits used in China. The calculated carbon intensity of SMSAB was 3.97 kg·(m³·MPa)⁻¹, indicating a favourable strength-to-emission balance compared with the reference systems considered. It should be noted that the present work examines paste specimens only; aggregate skeleton, traffic loading, freeze–thaw cycling and wet–dry/moisture cycling were not included. Therefore, the results demonstrate binder-level potential rather than direct qualification of SMSAB as a pavement base or subbase material. Full article

The poor tribological and mechanical performance of Al alloys hinders their use in practical applications where low COF and high durability are required. This study examined and evaluated a novel laser-sintered Ni-Cr coating to improve the load-carrying capacity and tribological performance of an Al alloy (Al 6061) substrate. The authors demonstrate that laser sintering cycle count is a decisive process variable governing coating dispersion, microstructural consolidation, and tribological performance in Ni-Cr coatings fabricated via Selective Laser Sintering (SLS). Increasing the laser cycle count progressively refined the surface morphology, improved coating dispersion, and strengthened interparticle bonding. As a result, the average durability after three cycles was seven times that after one laser cycle, accompanied by markedly improved COF. To further improve durability and load-carrying capacity, Ti₃SiC₂ was introduced into the Ni-Cr coating. The coating containing 10 wt% Ti₃SiC₂ exhibited a 20-fold increase in durability, extending the time to failure to approximately 70,000 s (700 m) while maintaining a low coefficient of friction (~0.48) compared with the coating containing no Ti₃SiC₂. The greater durability of the Ni-Cr-10wt%Ti₃SiC₂ coating in this novel study was attributed to improved adhesion to the substrate, better particle distribution during sintering, and greater load-carrying capacity. While further process changes do not yield feasible samples, this study showed that surface properties can be improved within the available small-process regime. Overall, laser sintering of a Ni-Cr-10wt%Ti₃SiC₂ coating shows promise as a means to improve the tribological and mechanical performance of Al 6061. This study should aid researchers and other stakeholders in fabricating well-adhering, durable, and tribotactic composite coatings on Al6061 and similar material systems. Full article

Efficient joining of hard metals to steels is crucial for supporting sustainable manufacturing under emissions strategies to minimize CO₂. CoNiCrFeCu high-entropy alloy containing scandium (Sc) was designed as a filler for laser braze-welding of WC-Co and steel. The designed compositions with different Sc levels were melted and cast in a high-vacuum non-consumable arc furnace. The results showed that the as-cast microstructure was a complex mixture of a networked Ni₂Si, elongated Cr-Fe-Co solid-solution phase, and Fe-Ni-Co-Cu solid-solution phase. Scandium was shown to have formed compounds with nickel/cobalt and copper. The TG-DSC analysis confirmed that the melting points of the designed compositions were between 973.7 °C and 981.5 °C. The maximum spreading area of the CoNiCrFeCu-0.9Sc composition on AISI 1045 steel was 64.83 mm², and on the WC-Co cermet it was 78.63 mm². The interface between the fusion zone and AISI 1045 steel exhibited an epitaxial growth of dendrites from the steel base metal. The interface between WC-Co and the fusion zone exhibited a partial penetration of brazing filler into the Co matrix, forming a metallurgical bonding between the dissimilar materials. Sc, as an alloying element in the filler metal, enhanced the bond formation because it decreased the solidus temperature and increased wetting. Full article

Lightweight high-entropy alloys are primarily designed to overcome the strength-to-density ratio limitations of conventional counterparts and often consist of elements with drastically different melting temperature and vapor pressure. Their chemistry, therefore, imposes challenges on alloy synthesis, particularly through liquid metal engineering routes, since elements with high vapor pressure (e.g., Mg, Zn, Li) vaporize before the higher-melting-point ingredients (e.g., Cu, V, Ni) are fully molten, resulting in volatile element loss. To overcome this challenge, a novel pressure-assisted induction melting (PAIM) process was developed and the proprietary furnace for its implementation was designed and built. The system allows precision melting of up to 10 cm³ of an alloy at temperatures up to 1700 °C while addressing the partial pressure requirements during the melting progress. The chamber is prepared using rough vacuum and re-filled with inert gas such as argon with the operating pressure range from about 10⁻⁴ MPa up to maximum of 1.6 MPa (233 psi). The alloy chemical composition can be modified in situ by feeding solid additives at specific melting stages through the isolated airlock without disrupting the pressure conditions within the chamber. The viability of the concept was verified by synthesis of two lightweight non-equimolar high-entropy alloys: Mg-rich Mg₅₀(MnAlZnCu)₅₀ and Al-rich Al₃₅Mg₃₀Si₁₃Zn₁₀Y₇Ca₅. The experiments showed that sequential multi-step melting procedures, designed based on inputs from FactSage computational analysis, when combined with PAIM synthesis, allowed manufacturing fully dense and chemically homogenous complex alloy compositions with optimal volumes for materials discovery research. Full article

Under the conditions of laser power of 1500 W, scanning speed of 5 mm/s, spot diameter of 3.5 mm, and powder feeding rate of 10 r/min, this study systematically investigated the influence of different tempering temperatures (200 °C and 600 °C) on the microstructure, friction and wear properties, and corrosion resistance of laser cladding Ni25 coatings, as well as the underlying mechanisms. The phase composition, microstructure, chemical composition, wear resistance, and corrosion resistance of the coatings were characterized and analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), pin-on-disk friction and wear tests, and electrochemical workstations. The results showed that the as-clad coating was composed of γ-Ni supersaturated solid solution and various metastable borides/carbides (such as Cr₃B₄), presenting fine-grained and non-equilibrium features. Tempering at 200 °C mainly achieved stress relaxation, enhancing and shifting the diffraction peaks to the left without changing the phase composition, while tempering at 600 °C drove significant diffusion-type phase transformation, leading to the decomposition of metastable Cr₃B₄ and the precipitation of stable phases such as Ni₂Si, accompanied by grain growth and microstructure coarsening. Friction tests indicated that the coating tempered at 600 °C exhibited the lowest average friction coefficient (0.679) and wear volume (0.0582 mm³) due to stable microstructure and hard phase strengthening, demonstrating the best wear resistance. However, electrochemical tests revealed a “trade-off” effect: the fine-grained microstructure of the as-clad coating, with its uniform composition, had the lowest corrosion current density (8.10 × 10⁻⁵ A/cm²) in 3.5% NaCl solution, showing the best resistance to uniform corrosion, while tempering, especially at 600 °C, caused grain growth, coarsening of the second phase, and micro-galvanic effects, slightly reducing the anodic dissolution resistance and increasing the corrosion current. This study clarified that heat treatment can significantly enhance the mechanical and tribological properties of Ni25 coatings by regulating their transformation from metastable to stable states, but at the potential cost of some corrosion resistance, providing a theoretical basis for optimizing post-treatment processes for different service conditions (wear resistance or corrosion resistance). Full article

The growing importance of renewable energy sources and the implementation of circular economy principles highlight the need for the rational management of biomass combustion by-products. The aim of this study was to assess the chemical composition of ash produced through the combustion of various biomass fractions from four varieties of common hazel (Corylus avellana L.) in the context of its potential for secondary use. The analysis covered the shells, husks, leaves, and shoots of the following varieties: Kataloński, Olbrzymi z Halle, Olga, and Webba Cenny. Combustion was carried out under laboratory conditions at a temperature of 550 °C, and the content of macro- and micro-element oxides (P₂O₅, K₂O, CaO, SO₃, Cl, SiO₂, MnO, Fe₂O₃, NiO, CuO) and potentially toxic elements (ZnO, TiO₂, Cr₂O₃) was determined using the EDXRF method. The results showed significant variation in the chemical composition of the ash depending on the biomass fraction and variety. The highest P₂O₅ content was found in the leaves of the Kataloński variety (5.02), whilst the highest K₂O concentration was found in the husk of the Olga variety (47.33%). The maximum CaO content was found in the leaves of the Webba Cenny variety (32.60). The leaf and husk fractions were characterised by the highest content of nutrients of fertilising importance, whilst the shells exhibited the lowest values for most macronutrients. The content of potentially toxic elements was low. The results obtained indicate that the selective utilisation of specific fractions of hazel biomass can increase the efficiency of mineral recovery whilst maintaining environmental safety, in line with the principles of the circular economy. Full article

Atmospheric plasma spraying (APS) represents a critical solution for enhancing the durability of agricultural components, such as harrow discs, which are subjected to synergistic wear and corrosion during soil cultivation. This study presents experimental results evaluating the electrochemical corrosion behavior and thermal shock resistance of discs coated via atmospheric plasma thermal spraying. Both metallic and ceramic materials, in powder form, from established manufacturers were used to produce the coatings, and the three types of coatings (two metallic and one ceramic) have the following chemical compositions and trade names: W₂C/WC12Co (Metco71NS), Cr₂O₃-4SiO₂-3TiO (Metco136F) and Co25.5Cr10.5Ni7.5W0.5C (Metco45C-NS). The coatings were analyzed using electron microscopy to evaluate the surfaces following corrosion testing. The ceramic coating based on the Cr₂O₃-4SiO₂-3TiO demonstrated the highest protective efficiency by increasing the charge transfer resistance from 307 Ω/cm² to 2213 Ω/cm² for the ceramic coating. It provided a superior physical barrier, reducing the corrosion current density from 0.140 mA/cm² for unprotected substrate to 0.004 mA/cm², representing an improvement of nearly two orders of magnitude. These findings demonstrate that implementing Cr₂O₃-4SiO₂-3TiO ceramic systems can significantly extend the operational lifespan of soil-engaging components, providing a cost-effective strategy for reducing maintenance intervals and material loss in aggressive agricultural environments. Full article

This study investigated the properties of a composite NiCrFeSiB coating with fine-dispersed WC additives, deposited by the HVOF method. The NiCrFeSiB powder alloy with WC additives was applied to a steel substrate. The WC content in the coating was 10, 15, and 20% by weight. The particle size distribution of the mixture ranged from 3 to 10 µm. The WC used was the WC8 alloy (92% WC, 8% Co). The levels of stress, phase composition, hardness, wear resistance, and coating structure were investigated. The studies revealed that the structure was primarily composed of the γ-Ni-Fe solid solution phase, with secondary phases including Ni₃B, Fe₃B, (Cr,)₂B, and carbides of the W₂C, WC, M₇C₃ type. A small amount of the initial WC particles was also present. The use of a fine-dispersed NiCrFeSiB powder mixture with WC particles resulted in a nearly twofold increase in hardness and wear resistance compared to the same parameters of the coating without WC. The coating with 20% WC exhibited the highest hardness. However, its wear resistance was lower than that of the coating with 15% WC. This fact could be explained by a slight difference in the phase composition and an increase in the proportion of the unsolidified WC phase in the structure. This led to the spalling of fine particles and a reduction in wear resistance. The study demonstrated the feasibility of using a fine-dispersed NiCrFeSiB coating with WC additives without additional remelting. Similar hardness and wear resistance results were achieved immediately after HVOF spraying when using a fine-dispersed NiCrFeSiB + 15% WC/Co mixture with a 92/8 composition. This simplification of the technology reduced the coating application process time. It also lowered production costs by eliminating the remelting stage. Full article

This study systematically investigates the role of Ni in Co₂SiO₄ in a bimetallic (Co²⁺,Ni²⁺)₂SiO₄ olivine-type system and the materials’ catalytic efficiency in a model Heck–Mizoroki coupling reaction. Thus, a series of olivines with varying (Co²⁺,Ni²⁺)₂SiO₄ compositions (0–100% Ni) was synthesised and characterised by ICP-OES, FTIR/Raman, P-XRD and XPS analysis. Ideal mixing of metals was achieved with (49:51) Co:Ni. Catalytic testing revealed distinct conversion vs. time profiles, with the (69:31) Co:Ni olivine exhibiting the best overall performance, combining good reactivity with near-perfect selectivity (>99%) and improved stability. Mechanistic pathways were probed through product scope analysis, reactant–product temporal profiling, leaching and radical scavenging experiments. Results suggest a radical-assisted Heck–Mizoroki mechanism. Spectroscopic data correlated Co²⁺ and Ni²⁺ incorporation with M1 and M2 site occupancy, where Ni²⁺ M2 sites enhanced reactant activation and intermediate stability and Co²⁺ in the M1 site enhanced product release, though also homocoupling in Co₂SiO₄. Minimal leaching was observed for all bimetallic catalysts. These findings highlight the tunability of bimetallic olivines for C–C coupling reactions via controlled cation distribution. Full article

Coal combustion residues are often useful components for the cement industry. This study represents a material characterization and screening analysis by focusing on the mineralogical, physicochemical, and petrographic compositions of fly and bottom ash samples from four Greek power plants in order to evaluate their suitability and potential in industrial applications, especially as fillers in cement manufacturing. Proximate analysis revealed LOI values exceeding ASTM C618-22 limits. The sum of SiO₂, CaO, and Al₂O₃ classifies the studied samples as Class C except one. Iron and magnesium oxides are among the major components, while S, Ni, and Sr are also contained in significant amounts. Calcite, quartz, and plagioclases dominate, corresponding to their geochemical profile, while secondary mineral phases (i.e., neo-formed minerals during coal combustion) such as natrolite and gehlenite, were also identified. Relatively high amounts of carbonized organic matter and unburnt organic particles point to the incomplete combustion process, revealing the risk of slagging into the combustion chamber; this is confirmed through the high slagging and fouling indices. The amount of the magnetic fraction is low; magnetic spherules with complex surface structures and a wide range of spherule sizes were observed. While the pozzolanic character of the samples is strong, high values of LOI, S content, and carbonized organic material make them suitable for the cement industry after further treatment only. Full article