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While the use of low-melting-point metals as sintering aids for high-entropy carbide (HEC) ceramics has been well established, their existence can compromise hardness due to residual metallic inclusions. This study demonstrates an innovative strategy to meet this challenge, where (Ti,Zr,Nb,Ta,Mo)C high-entropy carbide ceramics with ultrafine grains and enhanced hardness are obtained through chromium (Cr)-metal-assisted spark plasma sintering (SPS) at a temperature as low as 1600 °C. The results show that the addition of 5 vol% Cr promotes the formation of highly densified single HEC phase ceramics with a high relative density (98.4%) and an ultrafine-grained microstructure (0.17 μm). This low-temperature densification mechanism can be attributed to Cr’s solid-solution effect within the matrix and the increased carbon vacancies generated during sintering. The grain size of the (Ti,Zr,Nb,Ta,Mo)C ceramics with 5 vol% Cr metal addition is significantly smaller than that of Cr-free (Ti,Zr,Nb,Ta,Mo)C ceramics sintered at 2000 °C (3.03 μm) or via traditional low-temperature liquid-phase sintering (1.3–1.5 μm). Importantly, the addition of 5 vol% Cr substantially increased the hardness of the ceramics, with a remarkable increase from 23.57 to 28.16 GPa compared to that of the pure (Ti,Zr,Nb,Ta,Mo)C ceramics, owing to the fine-grain strengthening and solid-solution strengthening mechanisms. This work highlights the uniqueness of Cr metal as a sintering aid in achieving densification and hardness improvements in (Ti,Zr,Nb,Ta,Mo)C ceramics, offering a promising strategy for improving the properties of HEC materials for further development in the near future.

Carbonate rock strengthening techniques are widely used in cultural heritage preservations. Recently, there has been a growing interest in their application in the oil and gas sector to improve the hardness and stability of fracture surfaces compared to the predominant host mineral. In this study, two rock lithologies were treated (chalk and limestone) at ambient temperature using 0.9 M NaF and 0.1 M ZnSO4 solutions with the aim of improving the hardness of the rock surfaces by transforming their mineralogy. The carbonate rocks were characterized before and after treatment using various techniques to establish changes in their mineralogical compositions (using x-ray diffraction (XRD), scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM–EDS), and X-ray photoelectron spectroscopy (XPS) techniques) and petrophysical/mechanical properties (using Brinell test, impulse hammer, scratch test, steady-state gas injection). The reaction rates were monitored using inductively coupled plasma optical emission spectrometry and ion chromatography (ICP-OES) over a period of 2–5 days at 4–6 h intervals. It was found that the NaF treatment yielded a 4% and 6% improvement in indentation resistance of limestone and chalk, respectively. The ZnSO4 treatment produced the most notable improvement in rock hardness (presented by Young's modulus), with a 21% improvement for chalk and a 17% improvement for limestone, respectively. The novel findings in this study suggest that NaF and ZnSO4 are potential carbonate rock strengthening agents that can be utilized in various industrial applications, such as, for example, solving conductivity reduction problems arising in hydraulic fractures due to rock softening. [ABSTRACT FROM AUTHOR]

• Cr introduction enhances continuous Ag precipitation in Cu-Ag alloys. • Cr addition refines Ag precipitates and generates minuscule Cr precipitates. • 0.1 % to 0.5 % Cr doping substantially increases Cu-3 %Ag alloy hardness. Incorporating Cr into Cu-Ag alloys offers potential for enhancing mechanical properties, yet the intricate influence of microstructural characteristics at varying Cr concentrations on performance remains unclear. In this study, Cu-3 %Ag-(0.1–4 %Cr) alloys were investigated, optimizing performance through sequential solution processing and aging. After aging at 450 °C/2h, Cu-3 %Ag binary alloys featured solely discontinuous Ag precipitates, while Cr-containing alloys exhibited prominent continuous Ag precipitation. Modest Cr content significantly increased alloy hardness, with a 37.4 % improvement in the 0.5 % Cr alloy. Concurrently, Cr introduction caused a modest decline in electrical conductivity. These findings illuminate the strengthening mechanisms in Cu-Ag-Cr alloys, emphasizing the substantial presence of continuous Ag precipitates and fine Cr precipitates during aging, collectively enhancing the alloy's microstructure. [ABSTRACT FROM AUTHOR]

Solid lubricant coatings deposited by plasma-assisted deposition techniques represent modern way to reduce the coefficient of friction in lubricant-free sliding contacts. Molybdenum disulphide (MoS 2) is perhaps the most known and applied solid lubricant coating; however, its use is strictly limited by low hardness and environmental sensitivity. To improve mechanical and tribological properties, we doped MoS 2 coating with nitrogen. Mo-S-N self-lubricant films were deposited by pulsed d. c. High Target Utilisation Sputtering (HiTUS) in reactive atmosphere. The effect of deposition conditions on chemical composition, structure and mechanical properties of MoS x and Mo-S-N coatings was studied; films with the most promising properties have been selected for tribological testing. MoS x film with the elemental composition sulphur to molybdenum (S/Mo ratio) 1.6 exhibited the coefficients of friction (COFs) in humid air 0.17 and 0.07 for loads 2 and 15 N, respectively. Mo-S-N films were prepared with nitrogen content in a range of 11 to 50 at., whereas S/Mo ratio varied from 1.35 to 0.4. Mo-S-N films were amorphous or nanostructured with nanograins of molybdenum disulphide. Hardness increased with N content up to 14 GPa for film with the highest content of nitrogen. Friction behaviour in humid air was evaluated using a ball-on-disk tribometer. Globally, the doping with N resulted in hardness in Mo–S–N films one order of magnitude higher and wear rate two orders of magnitude lower than in an undoped one, keeping the friction coefficient at the same level or even lower. These coatings showed remarkable friction coefficients in humid air from 0.28 to 0.05 with loads from 2 to 15 N, respectively. The excellent friction properties were attributed to the formation of a thin molybdenum disulphide tribofilm at the sliding interface. HiTUS represents a very promising way of producing thin films on the thermally sensitive substrates (e.g. bearing steel) with desired properties. Display Omitted • Even small N addition enhances the density and hardness of MoS 2 coatings. • Hardness increased with nitrogen content from 0.8 to 14 GPa. • Moderate N content led to amorphization of the coating. • Low friction is due to formation of an easy-shear layer at the sliding interface. • Friction coefficient decreases with increasing applied load. [ABSTRACT FROM AUTHOR]

Abstract: NbN/AlN nano-multilayer films with AlN layer thickness (l AlN) ranging from 2.2 to 12.2nm have been deposited on Si(100) substrate by reactive magnetron sputtering in Ar/N2 mixtures. The l AlN dependent structural and mechanical properties for resulting NbN/AlN multilayers have been evaluated by means of low-angle X-ray reflectivity, X-ray diffraction, transmission electron microscope, pole figure measurements and nanoindentation tests. The finding is that at small l AlN both hexagonal wurtzite-AlN(0002) and face-centered cubic (fcc) AlN(111) are coherent with the fcc NbN(111), and the increase of l AlN can promote coherent growth of fcc-NbN(111)/w-AlN(0002) due to minimization of total energy and formation of a strong NbN(111)/AlN(0002) fiber texture. The hardness of all NbN/AlN multilayers lies in between 32.7 and 37.5GPa with increasing l AlN from 2.2 to 12.2nm, which is obviously higher than that calculated by using a simple rule of mixture, showing that the remarkable hardness enhancement implements at a wide range of l AlN from 2.2 to 12.2nm for NbN/AlN multilayer system. The high-hardness value in a wide range of l AlN can be mainly attributed to Koehler mechanism and structural barriers (fcc/hexagonal) to dislocation motion between NbN and AlN layers. [Copyright &y& Elsevier]

Highlights: [•] The coherent relationship of forms in multilayer. [•] The remarkable hardness enhancement implements at a wide range of modulation periodicity. [•] Good wear property appears in multilayers with small modulation periodicity. [ABSTRACT FROM AUTHOR]

Abstract: Polytetrafluoroethylene (PTFE)/Al, PTFE/Cu, and PTFE/Ti multilayer thin films have been deposited in order to investigate effects of interface energy on mechanical properties. PTFE, which has a low surface energy of 19.2mJ/m2, was used to introduce a large interface energy into multilayer thin films. PTFE thin film was deposited by rf magnetron sputtering using a PTFE sheet target. Al, Cu, and Ti were deposited by dc magnetron sputtering. The multilayer thin films were fabricated sequentially without breaking vacuum. Substrate used was aluminosilicate glass. The modulation period was changed from 6.7 to 200nm. The total thickness was about 200nm for all samples. The internal stress of metal layers changed from tensile to compressive and increased with decreasing modulation period for all of PTFE/Al, PTFE/Cu, and PTFE/Ti. Both hardness enhancement and superelasticity were observed in the results of nanoindentation measurements. The energy dissipated during nanoindentation process (one load and unload cycle) decreased with decreasing modulation period. The minimum value of the ratio of dissipated/loaded energy was <40%, which is smaller than the values obtained for monolithic PTFE or metal films (about 73% for PTFE and 87% for Al, 72% for Cu, and 71% for Ti, respectively). This meant that the PTFE/metal nano-multilayer thin films became more elastic with decreasing modulation period. The tendency of change in the mechanical properties strongly correlated to internal stress. Mechanisms involved in anomalous behaviors in film hardness and elasticity were discussed based on the relationship to interface energy, interface stress, and internal stress, induced by multilayering of the films. It is concluded that a large compressive stress introduced in the thin films increased the energy needed to deform elastically or plastically the thin film during indentation, resulting in the increase in hardness and elasticity. The nanoindentation analysis of the multilayer thin films emphasized that in PTFE/metal multilayer thin films mechanical properties of the films depend on interface stress induced by the accumulated interface energy, being independent of bulk materials properties composing thin films, resulting in increase in hardness and elasticity. [Copyright &y& Elsevier]

[Display omitted] • Solid lubricant Mo-Se-Ti coatings were deposited by High Target Utilisation Sputtering (HiTUS). • Addition of Ti improves the density and mechanical properties of MoSe 2 coatings. • Ti content above 18 at.% lead to amorphization of the coatings. • Extremely low coefficients of friction (0.02) and wear rate (8 × 10 - 8 mm3N-1m−1) achieved in humid air. • Mo-Se-Ti coating with ~ 25 at.% Ti showed the best tribological performance of the all tested coatings. In this work, we doped sputtered MoSe 2 coatings with Ti to improve their mechanical and tribological properties. Mo-Se-Ti coatings were prepared onto steel discs by High Target Utilisation Sputtering (HiTUS). The Ti was in the range 7–31 at.%, which led to Se/Mo ratios of 1.7–2.7. The Mo-Se-Ti coatings possessed a polycrystalline structure for the lower Ti contents and became amorphous when Ti content was above approx. 18 at.%. The hardness of the initially soft (0.3 GPa) pure MoSe 2 coatings was dramatically improved with Ti additions (up to 7.5 GPa). Sliding experiments confirmed excellent tribological properties: coefficient of friction values from 0.02 to 0.08 and very low wear rates (8 × 10 - 8 mm3N-1m−1) of the Ti-containing coatings tested in the humid air at the loads in the range of 2–45 N. The outstanding self-lubricant properties of Mo-Se-Ti coatings were attributed to the transformation of the topmost amorphous part of the coating into crystalline MoSe 2 tribofilm while preserving inner parts of the coating in as-deposited state ensuring good load-bearing capacity. There is strong experimental evidence that Ti has a gettering effect in the tribological contact, which protects MoSe 2 tribofilm against oxidation. [ABSTRACT FROM AUTHOR]

Abstract: Multilayer oxide coatings consisting of amorphous Al2O3 and crystalline TiO2 nanolayers have been deposited using reactive pulsed d.c. magnetron sputtering at different partial pressures of oxygen. Hardness enhancement has been observed in oxide multilayer coatings with amorphous Al2O3 as the majority component. These coatings had greater hardness-to-modulus ratios and showed greater resistance to wear over monolithic Al2O3 and TiO2 majority phase multilayers. Multilayer films retain their high hardness up to ~800 °C in air; some hardness enhancement in the Al2O3 majority phase multilayer coating remains even after 1 h of air annealing at 1000 °C. The hardness decrease at elevated temperatures is due to the roughening of interfaces between nanolayers, which can be attributed to the annealing-driven change of crystallographic texture of TiO2 layers. [Copyright &y& Elsevier]

Abstract: In the present work SiC and B4C monolayers as well as SiC/B4C multilayer coatings have been investigated with respect to their composition and mechanical properties. The coatings have been deposited on silicon substrates and polished cemented carbide inserts by non-reactive dual radio frequency (r.f.) magnetron sputtering from stoichiometric, high-purity silicon carbide (99.5%) and boron carbide (99.9%) targets. Amorphous stoichiometric SiC and B4C have been achieved with high hardness of 2950 HV0.01 and 4160 HV0.01 with a residual stress of −3.4 GPa and −2.9 GPa, respectively. The number of monolayers in the multilayer system with a constant total layer thickness has been varied in order to investigate the influence of the number of interfaces on film composition and properties. Additionally, the monolayer thickness ratio for a constant modulation period (double layer thickness) was varied. In this multilayer system no notable hardness enhancement could be observed. Despite the absence of the hardness enhancement, the toughness is expected to be enhanced due to stress relaxation and a reduction of crack propagation by crack dissipation along the interfaces. An abrupt interface is considered to be an indispensable requirement. However, the strain fields and the difference of the mechanical properties in the interfacial region are also of importance. [Copyright &y& Elsevier]

Abstract: AlN/TiN multilayer thin films with the layer thickness ranging from 1 nm to 50 nm were synthesized at 400 °C using a dual-cathode magnetron sputtering. Two series of samples were prepared under conditions of epitaxially matching orientations and far from the epitaxy to study the possible influence of the relative orientation of the constituents on the formation of coherent structures and hardness enhancement of superlattices. Both XRD and TEM observations showed that in large composition modulation period films each constituent grows under its own growth kinetic, leading to the formation of randomly oriented nanocrystalline layers. Decreasing progressively the layer thickness to beneath ≈ 10 nm favours the alignment of (002) basal plane of h-AlN on (111) plane of c-TiN, and results in development of strong (111) texture, prerequisite for stabilisation of c-AlN and the formation of epitaxially coherent structures. The increase of hardness coincides with the contraction of TiN lattice and transition of structures from a randomly oriented nanocrystalline layers to highly (111) textured superlattices. When the relative orientation of the constituents is far from the epitaxy relation, a partial crystallographic coherence of nanocrystalline domains can be obtained with (200) texture, leading to moderate enhancement of hardness. [Copyright &y& Elsevier]

By controlling the thickness of individual layers (20–70 nm) and substrate temperature (∼50–800 °C), a number of AlN/TiN multilayer films were grown in a reactive magnetron sputtering on the Si(100) and WC–Co substrates. The films were characterized using grazing incidence X-ray diffraction (XRD), nanoindentation, and cross-sectional electron microscopy [transmission electron microscopy (TEM) and HRTEM]. The measured hardnesses of multilayer films were found to be always higher than the rule of mixture, but the magnitude of hardness enhancement was found to be dependant on the bilayer thickness and deposition temperature. The obtained results are discussed in terms of interface-related phenomena, as well as structural modifications, that occur within the individual layers of TiN and AlN, i.e., change of grain size, transition of preferred orientation from (111) to (200), localized epitaxial growth between TiN and AlN crystallites, and development of biaxial residual stresses. [Copyright &y& Elsevier]

The first objective of this paper is to deposit new hard coatings based on compositionally modulated CN/TiCN/TiN multilayer films by dual ion beam sputtering using a combination of TiN, TiC and C targets and an Ar–N2 mixture discharge gas. The second objective is to evaluate the physical and chemical characteristics of the multilayer with respect to their practical applications. In this work, we have studied the composition, morphology, hardness and friction coefficient of the films. The deposition parameters were controlled to obtain films that combine ultrahard behaviour with good tribological and wear properties. [Copyright &y& Elsevier]

The effects of bilayer thickness and chromium content on the microstructures and mechanical properties of nanolayered TiAlCrN thin films were studied. The films were deposited onto the WC–Co substrates using cathodic arc method where 4 TiAl alloyed targets and 2 Cr targets were mounted alternately. Complex multilayers were produced by rotation of the sample holder and by controlling deposition conditions. Addition of chromium favors the formation of columnar structures in TiAlCrN layers. In thick bilayer films (Λ=400 nm) the TiAlN layer periodically interrupts the formation of columns in TiAlCrN, while for thinner bilayer films (Λ=10 nm) the columns are not interrupted leading to the formation of perfectly columnar films. Both Cr content and bilayer thickness contribute to hardness enhancement. The addition of Cr favored the formation of the harder cubic (Cr,Al)N phase rather than the softer wurtzite, hcp-(Al,Ti)N phase. The hardness enhancement due to multilayering can be explained by two factors; a grain refinement that leads to an increase in hardness and the formation of columnar structures with a preferred orientation. Indentation induced crack morphology appeared very sensitive to film microstructures, where well organized straight through thickness cracks occurred while indentation of TiAlCrN caused a network of irregular cracks due to the greater contribution of column grain boundary sliding. [Copyright &y& Elsevier]

Mo-Se-N films were prepared by pulsed direct current (DC) High Target Utilisation Sputtering (HiTUS) in reactive Ar + N 2 atmosphere. Here, the effect of nitrogen doping was studied. MoSe x film with Se/Mo atomic ratio ~ 2 exhibited polycrystalline structure and the lowest coefficients of friction (COFs) in humid air from 0.025 to 0.1 for loads in the range 2–45 N. Mo-Se-N coatings were deposited with N concentrations ranging from 1 to 40 at.%, whereas Se/Mo ratio varied from 0.6 to 2. Mo-Se-N coatings formed amorphous structures for the N contents above 7 at.% and increased hardness proportional with the N content up to 9 GPa. The addition of nitrogen also resulted in a general decrease in wear rate of two orders of magnitude when compared to pristine films while retaining a reasonably low coefficient of friction. Mo-Se-N films showed notable COF values in humid environment ranging from 0.22 to 0.015 when tested using loads from 2 to 45 N. The excellent friction properties of Mo-Se-N films were associated to the crystallisation of a MoSe 2 tribofilm in the wear scar. Moreover, we showed that modern pulsed DC HiTUS technology represents a suitable way of producing thin films with a variety of elemental compositions and desired mechanical and tribological properties, even from sensitive, semi-conducting and extremely low thermally conductive MoSe 2 targets. Unlabelled Image • Solid lubricant Mo-Se-N coatings were deposited by High Target Utilisation Sputtering (HiTUS). • Addition of nitrogen improves the density and mechanical properties of MoSe 2 coatings. • N content above 7 at.% led to amorphization of the coating. • Hardness of Mo-Se-N coatings increased from 0.3 to 9 GPa. • Coefficient of friction in the humid air < 0.02 [ABSTRACT FROM AUTHOR]

• The hardness of boiled potato was significantly increased through the combined use of LA and CC. • Pre-soaking of the potato slice in LA promoted gelation of the pectin. • Pre-soaking of the potato slice in LA and CC inhibited PG activity. Potatoes usually suffer from greatly decrease of hardness after boiling, which limits their processing potential in food industry. Moreover, methods for enhancing the hardness of potatoes after boiling are underexplored. In this study, the hardness of potato slices after boiling were increased from 288 g to 2342 g by the combined treatment of lactic acid (LA) and calcium chloride (CC). Through the analysis of the microstructure of the potato cells, the molecular weight distribution and natural sugar ratio of different soluble pectin fractions, and the enzymatic activities (polygalacturonase, PG and pectin methylesterase, PME), the possible mechanism behind the hardness enhancement by LA and CC pretreatment, namely the direct link between pectin and potato structure was revealed. The obtained results confirmed the target spot for enhancing the hardness of potatoes after boiling lay in PG activity and gelation of the pectin, which also could be used to help other plants resist the heat process if pectin existed in their cell wall. [ABSTRACT FROM AUTHOR]

An efficient strategy that can guide the synthesis of materials with superior mechanical properties is important for advanced material/device design. Here, we report a feasible way to enhance hardness in transition-metal monocarbides (TMCs) by optimally filling the bonding orbitals of valence electrons. We demonstrate that the intrinsic hardness of the NaCl- and WC-type TMCs maximizes at valence electron concentrations of about 9 and 10.25 electrons per cell, respectively; any deviation from such optimal values will reduce the hardness. Using the spark plasma sintering technique, a number of W 1- x Re x C ( x = 0-0.5) have been successfully synthesized, and powder X-ray diffractions show that they adopt the hexagonal WC-type structure. Subsequent nanoindentation and Vickers hardness measurements corroborate that the newly developed W 1- x Re x C samples ( x = 0.1-0.3) are much harder than their parent phase (i.e., WC), marking them as the hardest TMCs for practical applications. Furthermore, the hardness enhancement can be well rationalized by the balanced occupancy of bonding and antibonding states. Our findings not only elucidate the unique hardening mechanism in a large class of TMCs but also offer a guide for the design of other hard and superhard compounds such as borides and nitrides.