Microhardness Level Research Articles

Effect of ultrasonic surface rolling process by disparate static pressure on a carburized layer of 17CrNiMo6 steel

Considering the elevated application requirements for transmission gears in terms of wear and corrosion resistance, this study investigates the impact of different static pressures (0.4 MPa, 0.6 MPa, 0.8 MPa) on the structure and protective properties of vacuum-carburized USRP layers, leveraging the technical advantages offered by vacuum carburizing and USRP. The findings demonstrate that using USRP facilitates fine-grained strengthening, and the thickness of the grain refinement layer increases proportionally with static pressure. The 0.6 MPa static pressure exhibits the most pronounced positive impact on grain refinement. The application of varying static pressures results in different surface integrity and microhardness levels within the carburized layer, thereby impacting the layer's wear and corrosion resistance. The coating exhibits the highest hardness and lowest roughness at a static pressure of 0.6 MPa, thereby demonstrating superior wear resistance. In addition, the 0.6 MPa layer with the lowest roughness has the best corrosion resistance.

Структурные особенности и трибологические свойства многослойных высокотемпературных плазменных покрытий

Introduction. Multilayer high-temperature coatings obtained using plasma spraying, are studied. The combination of layers of different chemical and phase compositions made it possible to increase wear resistance by 1.5–2.0 times. The purpose of this work is to study the influence of the chemical composition of sprayed coatings on the phase composition, structure, micromechanical and tribological characteristics under conditions of dry sliding friction of surface layers. Materials and methods of research. Coatings A and B consist of sequentially sprayed layers. The first and second layers were sprayed in a reducing atmosphere: the first layer was a heat-resistant self-fluxing powder of two systems: 1 – Fe-Cr-Si-Mn-B-C for coating A and 2 – Fe-Ni-Si-Mn-B-C for coating B; the second layer was a mixture of self-fluxing powder with iron powder in a 1:1 ratio. The third layer was obtained by spraying iron powder in an oxidizing atmosphere to form a metal oxide coating. To create a layer of scale on the surface, coated specimens were subjected to high-temperature annealing at a temperature of 1,000 ℃. The chemical composition and nature of the distribution of elements over the thickness of the coatings were determined by micro-X-ray spectral analysis using a TWSCAN scanning electron microscope with an Oxford energy-dispersive attachment. Microhardness and micromechanical properties were studied using an instrumental microhardness tester of the Fischerscope HM2000 XYm system at a load of 0.980 N. Determination of tribological properties was carried out on a laboratory installation using the “finger-disc” scheme at loads of 30, 75, 100 and 130 N. To measure roughness parameters and obtain 3-D profilometry of surfaces after testing, a non-contact profilometer-profiler Optical profiling system Veeco WYKO NT 1100 was used. Results and discussion. Metallographic studies have shown that the formed multilayer coatings consist of an internal metal layer and an external oxide layer with a total thickness of the entire coating up to 800–850 μm. It is established that the first sprayed layer has the highest level of microhardness, which is due to the high-volume fraction of the strengthening phases contained in it (~ 95 %). It is shown that the coating A has increased wear resistance, which is expressed by minimal weight loss (~ 1.5 times less than that of the coating of the coatimg B), the friction coefficient was f = 0.3 for coating A and f = 0.4 for coating B. The study of wear surfaces has shown that for all selected test loads under sliding friction conditions, the coating of both compositions was preserved, even at a maximum load of 130 N.

Boriding of Low-Carbon Steel by Plasma Method: Microstructure and Coating Properties

In materials science, steel boration is a promising type of thermochemical diffusion process, the purpose of which is the introduction of hard and wear-resistant boride particles into the surface layers of the metal. The main disadvantage of boration is the fragility of borated layers, especially boride. Currently, there are promising methods for the formation of a composite structure, which are based on the treatment of surface layers of steels with concentrated energy flows (laser, electron beam and plasma). The work includes studies of microstructure, hardness measurements, analysis of phase composition, and determination of chemical composition in local locations, wear tests under various conditions, and adhesion testing of the coatings obtained after plasma boration. As the degree of alloying of the molten layer decreases in the direction from the surface to the base metal, zones of overeutectic, eutectic and pre-eutectic types with a different combination of structural components are formed. The material obtained after borating a mixture of 40% B + 10% Fe is characterized by the highest level of microhardness, which is 1000...1300 HV. The highest results in friction tests are provided by boration of a powder mixture of 40% B + 10% Fe.

Thermally Sprayed Coatings for the Protection of Industrial Fan Blades.

This paper presents a study on thermally sprayed coatings. Coatings produced by high-velocity oxygen-fuel spraying HVOF and plasma spraying deposited on the A03590 aluminum casting alloy are tested. The subject of this research concerns coatings based on tungsten carbide WC, chromium carbide Cr3C2, composite coatings NiCrSiB + 2.5%Fe + 2.5%Cr, mixtures of tungsten and chromium powders WC-CrC-Ni, mixtures of carbide powders with the Cr3C2-NiCr + the composite 5% NiCrBSi and WC-Co + 5% NiCrBSi. The aim of this research is to find a coating most resistant to the erosive impact of particles contained in the medium centrifuged by industrial rotors. The suitability of the coating is determined by its high level of microhardness. The hardest coatings are selected from the coatings tested and subjected to abrasion tests against a sand particle impact jet and the centrifugation of a medium with corundum particles. It is found that the most favorable anti-erosion properties are demonstrated by a coating composed of a mixture of tungsten carbide and chromium carbide WC-CrC-Ni powders. It is concluded that the greatest resistance of this coating to the erosive impact of the particle jet results from the synergistic enhancement of the most favorable features of both cermets.

Increasing the Strength and Impact Toughness of Carbon Steel Using a Nanosized Eutectoid Resulting from Time-Controlled Quenching.

High-carbon steels are normally used as tool materials. The use of such steels for construction is limited due to their increased brittleness and poor weldability. However, it appears that high-carbon steels possess certain hidden reserves for enhanced plasticity and strength if properly heat-treated. An unconventional heat treatment was applied to carbon eutectoid steel (0.8 wt.% C) in order to increase its strength and impact toughness simultaneously. Samples for tensile and impact testing were held at 800 °C for different time ranges from 3 min to 9 min with subsequent cooling in oil. It was established that for each type of sample, an optimal holding time exists that is responsible for increased strength and high impact toughness. The hardness and microhardness levels of the surface and under-surface regions of the samples reached 390 HV after optimal heat treatment. An X-ray revealed a shift of the (211)α-peak to the lower 2-theta angles after heat treatment with the optimal holding time; this indicates an increase in carbon content in alpha solid solutions of approximately 0.12 wt.%. Thus, a nanostructured mixture of low-carbon martensite and thin cementite plates is formed in the under-surface region of carbon eutectoid steel after heat treatment, with a controlled holding time at the austenitizing temperature.

Producing multilayer laminated composite of aluminium/copper by friction stir additive manufacturing process

ABSTRACT The objective of this study was to assess the feasibility of producing Al-Cu multi-layered composite through the friction stir additive manufacturing (FSAM) process and to analyse the microstructure and mechanical properties of the fabricated composite. The composite was fabricated at a rotational speed of 1100 rpm, traverse speed of 60 mm/min and a 2° tilt angle. The macrostructure, microstructure, and intermetallic formation were observed and analysed. The macro and microstructure results indicated that achieving defect-free build is feasible in the selected parameter settings. In-depth examinations using energy-dispersive x-ray spectroscopy (EDS) and x-ray diffraction (XRD) unveiled the presence of intermetallic compounds (IMCs) such as AlCu, Al2Cu, and Al4Cu9 within the stir zone in various regions of the build. Notably, inhomogeneous microhardness levels ranging from 56.7 HV0.1 and 324.6 HV0.1 were noted, corresponding to distinct microstructural features and the various IMCs within the build. The Al-Cu composite demonstrated an excellent balance of strength and ductility, with an ultimate tensile strength of 187.7 MPa and an elongation of 19%. These findings bring out the exceptional opportunity presented by FSAM process for fabricating innovative Al-Cu multilayered composites with unique mechanical properties.

Effects of experimental in-office bleaching gels incorporated with co-doped titanium dioxide nanoparticles on dental enamel physical properties.

To evaluate the physical properties of enamel submitted to hydrogen peroxide (HP) incorporated with titanium dioxide nanoparticles (NP) co-doped with nitrogen and fluorine and irradiated with violet LED light (LT). Enamel-dentin disks were randomly allocated (T0) into groups, according to HP (HP6, HP15, or HP35) and NP (no NP, 5NP, or 10NP) concentrations, and irradiated or not with LT. A negative control (NC) group was set. After three bleaching sessions (T1, T2, and T3), specimens were stored in saliva for 14days (T4). Enamel surface microhardness number (KHN), surface roughness (Ra), cross-sectional microhardness (ΔS), energy-dispersive spectroscopy (EDS), scanning electron (SEM), and polarized light (PLM) microscopies were performed. Surface KHN was significantly influenced by NP over time, independently of LT irradiation. At T3 and T4, gels with 5NP and 10NP exhibited no KHN differences compared to NC and baseline values, which were not observed under the absence of NP. NP incorporation did not statistically interfere with the ΔS and Ra. PLM images exhibited surface/subsurface darkening areas suggestive of demineralizing regions. SEM demonstrated some intraprismatic affection in the groups without NP. EDS reported a higher enamel calcium to phosphorus ratio following 10NP gels applications. Gels with NP maintained the enamel surface microhardness levels and seemed to control surface morphology, upholding the mineral content. None of the proposed experimental protocols have negatively influenced the enamel surface roughness and the cross-sectional microhardness.

Investigation on the scale-dependent behavior of microstructure characteristics of laser powder bed fused TiB2/AlSi10Mg composite

The pre-alloyed TiB2/AlSi10Mg composite, a new high-strength aluminum alloy developed for laser powder bed fusion (LPBF) technology, offers promising applications in lightweight and multi-scaled structures. However, thermal behavior during LPBF is markedly scale-dependent, leading to microstructural variations that significantly affect the load-bearing capacity of multi-scaled structures. Therefore, this study systematically investigates the scale-dependent behavior of microstructure characteristics of this composite. Utilizing a hatching scanning strategy, it was found that the marginal zones of samples are predominantly composed of coarse Al cell and Al grain structures, contrasting with the fine microstructures in the central zones. With increasing structure scale, cell and grain structures in both the marginal and central zones become more refined, with cell sizes reducing by 49 %–72 % (∼3.02 μm → 0.86–1.55 μm). Particularly, the minimum-scaled structures also feature broken eutectic Si particles and nanopores. The essence is primarily due to the low heat dissipation with higher peak temperature and longer duration time at high temperatures in both the small-scale structures and marginal zones. Additionally, smaller structures correlate with reduced microhardness and tensile strength, accompanied by the “softening” of the marginal zones. The strength of the minimum-scaled structure is only half that of the standard sample. Our findings suggest a scale threshold of 2.0 mm for researching scale effect. Encouragingly, incorporating additional contour scanning significantly counteracts the adverse influence of the scale effect. Owing to the combined influence of extended inter-layer time and laser remelting, all samples demonstrate a distinctly refined microstructure. This results in consistently high levels of microhardness and strength, with the “hardening” of the marginal zones. Eventually, the relationship between mechanical properties and microstructure sizes is established. This study provides valuable insights into the innovative designs and engineering applications of multi-scaled structures in LPBF using various materials.

Comparative Evaluation of Microhardness and Solubility of Different Combinations of Antibiotic Powders Added to Glass Ionomer Cement: An In Vitro Study.

The imbalance between remineralization and demineralization leads to the formation of secondary caries. Fluoride-releasing ability has been the characteristic property of glass ionomer cement (GIC), but it is uncertain if this property alone will be sufficient for the cessation of the growth of the organisms. Therefore, a restorative material with additional bacteriostatic properties needs to be introduced. To evaluate the microhardness and solubility of the conventional GIC after adding different combinations of antibiotic powders. In this study, the three groups were conventional GIC (group I), GIC + metronidazole + ciprofloxacin (group II), and GIC + metronidazole + amoxicillin/clavulanic acid (group III). The concentration of the double antibiotic combination was maintained at 1.5% w/w. The antibiotic powders were added to the GIC and evaluated for microhardness and solubility of the specimens. The mean microhardness level of group II was the highest, which was statistically significant using analysis of variance (ANOVA) with a p-value of 0.022. The comparison of each group's solubility in different solutions was not statistically significant. Along with the conventional properties of GIC, an additional therapeutic gain can also be obtained by incorporating various combinations of antibiotics, thereby arresting the progression of caries at the site of infection itself. This new approach shall help in community health programs, where treatment of a large population needs to be done within a short span of time and arrest the progression of caries activity in deep caries. Benson TL, Sogi S, Jain M, et al. Comparative Evaluation of Microhardness and Solubility of Different Combinations of Antibiotic Powders Added to Glass Ionomer Cement: An In Vitro Study. Int J Clin Pediatr Dent 2024;17(6):619-624.

Investigation of effect of post weld heat treatment on microhardness and microstructure of auto TIG welded stabilized SA213 TP347H weldments

Investigation explores the weldability of Niobium stabilized TP347H austenitic stainless steel tubes through the application of optimized welding parameters using Auto TIG welding. This research investigates the influence of post weld heat treatment conditions on microhardness, microstructures, sensitization due to Cr23C6 precipitation, presence of Niobium carbides and formation of twin boundaries in the weldment. Extended post weld heat treatment duration resulted in notable decline in microhardness from 280 to 180 VHN within initial 5-h of soaking. Nevertheless, further prolongation of soaking time (10–15 h) exhibited subsequent rise in microhardness levels to 200 VHN in base, weld and heat affected zone. Increase in hardness is attributed to the emergence of annealing twins in base metal and the precipitation of Niobium carbides within weld and heat affected zone, confirmed by advanced analytical scanning electron microscope. Energy dispersive spectrum line mapping reveals presence of Cr23C6 precipitation following 1-h soak at 720 °C. Subsequent prolongation of soaking time (5-h) demonstrates desensitization in the base metal. Additionally, cooling rate of 300 °C/hr during post weld heat treatment reduces required time for microstructure homogenization, resulting in increased hardness compared to 100 °C/hr rate in the weldment. However, beyond a 5-h soaking time, the cooling rate has a negligible impact on microhardness. Ultimate Tensile Stress of 674.82 and 665.28 N/mm2 for TP347H was achieved against the minimum requirement of 515 N/mm2 as per American Society of Mechanical Engineers Section IX standard.

Tungsten heavy alloy: Nano-crystallinity and alloying induced low temperature sintering, microstructure and mechanical properties

In this study, a tungsten heavy alloy with a composition of 90 W-4.9Ni-2.1Fe-3.0Cu was processed using powder metallurgy. Pure elements were ball-milled to develop a mechanically alloyed nano-grained solid solution to reduce the required sintering temperature. The alloy was then liquid-phase sintered in a hydrogenated argon atmosphere at 1250 °C. Microstructural characterization revealed that the high-energy ball milling process successfully developed a tungsten solid solution with a reduced tungsten grain size to a level of 15 nm, facilitating effective low-temperature (i.e., 1250 °C) liquid-phase sintering with 88% relative density. The tungsten grains grew mostly in a polygonal shape, coalesced to a large size, and had strong interfacial integrity with the nickel‑iron‑copper matrix. Mechanical properties characterization revealed a significantly high microhardness level of 380 HV, compressive yield strength of 910 MPa, and ultimate compressive strength of 1326 MPa. The applied compressive stress induced intergranular crack propagation leading to the fracture of the sample with a failure strain of 25.6%.

Development of technology for obtaining nanostructured composite coating of Kh20N80–Cr–TiC

The paper presents results of a comprehensive study on the development of technology for the production of composite nanostructured functional coatings based on the Kh20N80 – chromium – TiC system using the supersonic cold gas dynamic spraying method. Composite coating powders are obtained by chromium plating of a matrix powder made of alloy Kh20N80 followed by the application of a reinforcing coating of TiC nanoparticles. The resulting coating has a high level of microhardness, modulus of elasticity and resistance to wear. A distinctive feature of the resulting coating is its reliable operation without destruction for a long time (more than 4800 hours).

Влияние добавок карбида и борида вольфрама на структуру и микротвердость эквиатомного CrFeNi-покрытия, сформированного короткоимпульсной лазерной наплавкой

A coating based on a single-phase medium-entropy CrFeNi alloy with a face centered cubic structure has good ductility, relatively high anti-corrosion properties, low cost, but insufficient strength for its widespread use. It is assumed that adding strengthening particles in the form of tungsten carbides and borides to the CrFeNi equiatomic coating will lead to an increase in its mechanical properties. This work studies the influence of tungsten carbide and boride additives on the structure and microhardness of a CrFeNi equiatomic coating. The coatings were formed by layer-by-layer short-pulse laser cladding with preplaced powder on a multifunctional laser installation equipped with a solid-state laser with a lamp pump based on an Nd:YAG crystal. The change in phase composition when adding strengthening particles was detected using X-ray diffraction analysis and transmission electron microscopy (TEM). Both methods confirmed the precipitation of Cr23C6 chromium carbide in the deposited coatings. TEM photographs indicate that the precipitated phase is distributed along the grain boundaries of the -solid solution. The study found that the addition of 6 wt. % WC and 3 wt. % WB increases the level of microhardness of the CrFeNi coating by 26 % (from 340±6 to 430±12 HV 0.025). This occurs due to the presence of Cr23C6, WC particles in the structure and possible microdistortions of the crystal lattice of the -phase as a result of doping with tungsten atoms released during the dissolution of tungsten borides and carbides in the process of high-temperature short-pulse laser heating.

Дослідження відновлення корпусних деталей автомобільних двигунів методом електродугової металізації

Road transport ensures a rational organization of production and transportation of products. The cylinder block is a basic part, mechanisms and engine parts are attached to it. Engine blocks are operated with high loads. Defects arise and develop, which can be eliminated during major repairs. Improving the methods of restoring worn cylinder blocks of automobile engines is an urgent task. The possibility of restoring parts with such defects by gas-thermal methods was considered. The total level of defects that can be eliminated by applying a restorative coating by the method of electric arc metallization is 37%. The analysis of literary sources shows that surfacing and welding methods do not ensure the quality of the restored part. Gas-thermal methods are often used in the process of repairing parts. The most technological and effective is electric arc metallization. Productivity of electric arc metallization can be very high. Operating costs of using electrometallization are small. The equipment is relatively simple. Restoration of parts by electric arc metallization increases wear resistance and durability of the engine. To assess the repeatability of block defects, we analyzed a sample of 20 truck cylinder blocks from different manufacturers. It was established that the resource of the block depends on: wear of the main bearing beds, holes, cracks, damage to the thread and breakage of pins and holes. Laboratory experiments on the application of electroplating coating were carried out on samples of high-strength cast iron. Preparatory processing of samples - shot blasting. Experiments on sputtering of samples were carried out with known powder wires PG-SR4 and PG-SR4+3% Al with a diameter of 2.0 mm. Spraying was carried out with an EM-17 electric arc metallizer. Tests on the adhesion strength of the coating to the base metal were carried out using the adhesive method. From the research results, we can see that the coating applied with PG-SR4+3%A1 wire is destroyed at the joint under a higher load than the corrosion formed by PG-SR4 wire. At the same time, the adhesion strength also exceeds, and is 22.9 MPa. The microhardness of the coating built up with powder wire PG-SR4+3%A1 exceeds the level of microhardness of the surface built up with wire PG-SR4. Microhardness in both cases. decreases with distance from the surface of the deposited layer. Conclusions. From the conducted research, it follows that the electric arc restoration does not have a thermal effect on the part, provides high adhesion strength of the applied layer, which prevails over the majority of traditional methods of restoration of parts. In addition, the method of electric arc metallization is characterized by low energy consumption, small-sized and mobile technological equipment. Therefore, there is a need to continue the research of electric arc metallization in the restoration of worn surfaces of car parts.

What role did really tin bronze play in the Argaric society?

The transition from arsenic copper to tin-bronze in ancient metallurgy has long been attributed to the superior physical and mechanical properties of tin-bronze. However, recent archaeometallurgical studies have cast doubt on this theory, suggesting that the functional and productive advantages of tin-bronze over arsenical copper may not be as clear-cut as traditionally thought. In this paper we present the results of metallographic and microhardness tests conducted on the metallic assemblages from several Bronze Age Argaric sites (Southeast Iberia). Compositional analyses of more than 700 copper-based objects revealed a distinct correlation between the use of tin-bronze for ornaments and arsenical copper for functional objects. This fact suggests that the choice of tin-bronze was influenced by factors beyond mere productivity. The results presented in this paper show that both arsenic copper and tin-bronze could exhibit similar mechanical properties. According to them, their microhardness levels depend on the final processes of their manufacture and the intensity of these processes, rather than on the alloy's composition. This challenges the notion that bronze was adopted solely for its functional efficiency. Therefore, alternative interpretations must be considered to explain the adoption of this new alloy.

Structure and mechanical properties anisotropy of a steel product manufactured by layer-by-layer electric arc wire 3D printing

The work presents the study of structure and mechanical properties anisotropy of a metal wall obtained using electric arc wire 3D printing (WAAM) with ER70S-6 wire. The layers were deposited in the protective gases of carbon dioxide and argon. As a result of structural studies, it was found that the internal structure of the model product in form of a wall can be divided into three zones. Repeated heating, cooling cycles and degree of accumulated heat influence the formation of different wall zones. As a result of rapid heat removal to the substrate during deposition of the first layers, the wall base (zone 1) contains large elongated grains with acicular ferrite structure. The wall middle part (zone 2) consists of ferrite-pearlite structure, which was formed as a result of recrystallization under conditions of repeated heating and cooling during 3D printing. The size of ferrite grains in zone 2 varies from 11 to 16.3 µm with increasing the number of layers. The gradual accumulation of heat during 3D printing led to the formation of structures in zone 3 under conditions of overheating and a reduced cooling rate. As a result, the wall upper part (zone 3) consists of large ferrite grains (up to 29.8 μm), sorbite, and a small proportion of Widemanstatten ferrite and acicular ferrite. It is shown that the most uniform level of mechanical characteristics (σ0.2 = 340 MPa, σu = 470 MPa, ε = 28 %) correspond to the samples cut from zone 2 in a direction parallel to 3D prin­ting direction. The samples cut in the vertical direction relative to 3D printing and from zone 3 show the lowest level of microhardness and mechanical characteristics (σ0.2 = 260 MPa, σu = 425 MPa, ε = 20 %).

Methods of structural surface engineering in solving problems of multifactorial improvement of materials performance

Problem. Structural surface engineering encompasses a complex of scientific disciplines and technological methods of directed change of physical and chemical properties of materials surface layers by modification, deformation, application of films, coatings, protective layers, with the help of various combined methods. In the course of the study several different methods of structural engineering of the surface are considered for solving problems of multifactor increase of the level of exploitation characteristics of materials. The methods described in the article are characterized by different physics of the process on the way to obtaining the result, but are aimed at modifying the structure and properties of the surfaces to which they are applied. Goal. Consideration of different methods of influence on the surface of ferrous and non-ferrous alloy objects, analysis of their influence on the modification of the structural state and properties of the surface layers that were investigated, discussion of the peculiarities of each of the considered technologies. Method. The first direction includes technologies that include a friction component, namely thermofriction treatment (TFT) for thermofriction hardening (TFH), supplementary thermofriction hardening (STH) or thermofriction welding (TFW). The second direction is the technology involving the use of anodic-cathodic electrolysis mode in alkali-silicate electrolyte – micro-arc oxidation (MAO). The third direction is the technology of vacuum-arc PVD-method, which involves obtaining chromium coatings. The article describes the features and results of application of such technologies, as well as the expediency of using this or that method for materials of different classes, schemes of the corresponding installations are presented. Results. The result of additional hardening of U8A steel surface from the microhardness level of 7.2 GPa to 14.7 GPa by DTFZ method after its thermal hardening practically to the maximum possible level is shown. The microstructure of the cross section of the pre-hardened U8A steel sample after DTFZ is presented, where the degree and character of surface hardening can be reliably seen. It is emphasized that in previous studies a consistently effective hardening of steels of various classes was achieved, up to a level of 22 GPa in 65G steel. The structure and properties of coatings on low-alloy aluminum alloys AB and AD1 formed in alkali-silicate electrolyte in the anodic-cathodic mode of MDO in the process of application of the microarc oxidation method have been studied. It is shown that the method of MDO in alkali-silicate electrolyte allows to obtain coatings with thickness up to 300 μm, coating growth rate ~ 2 μm/min and coating hardness 10-20 GPa. The coatings have high adhesion to the substrate, have a layered structure, the properties of the coatings are determined by the properties of the base layer. The coatings have a crystalline structure and consist of the following phases: γ-Al2O3, α-Al2O3, mullite (3Al2O3⋅2SiO2), the ratio between the phases depends on the electrolysis conditions. It is established that phase formation begins with the γ-Al2O3 phase, which in the process of further growth of the coating turns into the α-Al2O3 phase or interacts with silicon oxide to form the mullite phase. The possibility of obtaining high-quality chromium nitride-based hard coating by vacuum-arc sputtering is shown.

Methods of Structural Engineering of Surface in Solving the Problems of Multifactorial Increase of the Level of Operational Characteristics of Materials

In the course of the study, several different methods of surface structural engineering are reviewed. The methods described in this paper are characterized by different process physics on the way to obtaining the result, but they are aimed at modifying the structure and properties of the surfaces to which they are applied. Among them, two different technological directions are considered. The first area involves technologies that include a friction component, namely thermofriction treatment (TFT) for thermofriction strengthening (TFS), additional thermofriction strengthening (ATFS) or thermofriction welding (TFW). The second direction is a technology that involves the use of an anode-cathode electrolysis mode in an alkaline-silicate electrolyte – micro-arc oxidation (MAO). The paper describes the features and results of the application of such technologies and the feasibility of using this or that method for materials of different classes, and presents schemes of the corresponding installations. The result of additional hardening of the surface of U8A steel from a microhardness level of 7.2 GPa to 14.7 GPa using the ATFS method after its thermal hardening to almost the maximum possible level is shown. The microstructure of the cross-section of a prehardened specimen of U8A steel after ATFS is presented, where the degree and nature of surface hardening are reliably visible. It is emphasized that in previous studies, consistently effective hardening of steels of various classes has been achieved, even up to the level of 22 GPa in 65G steel. Regarding the method of microarc oxidation, the structure and properties of coatings on low-alloy aluminum alloys AB and AD1 formed in an alkaline-silicate electrolyte in the anode-cathode MAO mode were investigated. It is shown that the method of MAO in alkaline-silicate electrolyte allows to obtain a coating thickness of up to 300 μm, a coating growth rate of ~ 2 μm/min, and a coating hardness of 10-20 GPa. The coatings have high adhesion to the substrate; they have a layered structure. The properties of the coatings are determined by the properties of the base layer. The coatings have a crystalline structure and consist of the following phases: γ-Al2O3, α-Al2O3, mullite (3Al2O3·2SіO2), the ratio between the phases depends on the electrolysis conditions. It has been established that phase formation begins with the γ-Al2O3 phase, which in the process of further coating growth turns into the α-Al2O3 phase or interacts with silicon oxide to form the mullite phase.

Sol-Gel Synthesis of Nanosized Powders and Obtaining Ceramic Composites Based on Zircon and Zirconium Oxide

Nanosized precursor powders of (1 – x)ZrSiO4‒xZrO(OH)2 are synthesized by the sol-gel method with the separate precipitation of components to obtain (1 – x)ZrSiO4‒xZrO2 ceramic composites. The thermal behavior of precursor powders is studied by differential scanning calorimetry and thermogravimetry (DSC/TG). Ceramic composites with a high level of microhardness are obtained by sintering powders, preliminarily calcined at 850°C, in air in the temperature range 1000‒1300°C. In the future, such ceramic composites can be used as matrices for solidification and isolating high-level waste (HLW).

The effects of gastric acid on pediatric restorative materials: SEM analysis.

In this study, we aimed to demonstrate changes in the surface roughness and microhardness of three different restorative materials routinely used in pediatric dentistry (composite, compomer and resin-modified glass ionomer cement (RMCIS)) in response to continuous daily exposure to gastric acid. Twelve samples of each of type of restorative material were prepared. Eleven of the specimens were included in the gastric acid cycle. The microhardness and surface roughness of ten samples were measured before and after the cycle. Another sample included in the cycle was compared with the sample not included in the cycle by scanning electron microscopy (SEM). There was a significant difference between the groups in terms of roughness scores following gastric acid cycle (p = 0.039). RMCIS material possessed the highest roughness value. A significant difference was identified in terms of microhardness levels before and after the gastric acid cycle (p = 0.001). The most significant change was observed in the compomer material. SEM analysis, performed after the gastric acid cycle, revealed that most cracks were identified in RMCIS material; this was followed by compomer and composite materials, respectively. Our analysis indicates that the restorative materials used frequently in pediatric dental procedures, show increased surface roughness and reduced microhardness when exposed to gastric acid.