Defective Surface Layer Research Articles

Investigation of tool traverse speed on surface integrity of nano composites processed by friction stir process

This work examines the effects of different tool traverse speeds on the surface integrity of AA7075 reinforced with graphene oxide (GO), silicon carbide (SiC) and graphene nanoplatelets (GNPs) nanoparticles during Friction Stir Processing (FSP). Three different traverse speeds of 20, 25 and 30 mm/min, respectively, were used for FSP operations. The resulting nanocomposites were thoroughly analysed using methods including optical microscopy, tensile testing, fracture morphology analysis, microhardness testing, X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (FESEM-EDAX) and non-contact surface interferometry. The results show that the creation of a refined microstructure with equiaxed grains was aided by tool traverse speeds between 20 and 25 mm/min. Notably, an Ultimate Tensile Strength (UTS) of 358 MPa, microhardness of 139 HV, refined crystalline size of 47.162 nm and average roughness (Ra) of 1.53 μm were obtained with the AA7075+SiC nanocomposite under tool traverse speed of 25 mm/min. On the contrary, surface layer defects developed at a traversal speed of 30 mm/min. Through the use of mechanical stirring and severe plastic deformation (SPD), this study aids in changing the microstructure of the airplane frames.

Self-Modification of Defective TiO2 under Controlled H2/Ar Gas Environment and Dynamics of Photoinduced Surface Oxygen Vacancies.

In recent years, defective TiO2 has caught considerable research attention because of its potential to overcome the limits of low visible light absorption and fast charge recombination present in pristine TiO2 photocatalysts. Among the different synthesis conditions for defective TiO2, ambient pressure hydrogenation with the addition of Ar as inert gas for safety purposes has been established as an easy method to realize the process. Whether the Ar gas might still influence the resulting photocatalytic properties and defective surface layer remains an open question. Here, we reveal that the gas flow ratio between H2 and Ar has a crucial impact on the defective structure as well as the photocatalyic activity of TiO2. In particular, transmission electron microscopy (TEM) in combination with electron energy loss spectroscopy (EELS) revealed a larger width of the defective surface layer when using a H2/Ar (50 %-50 %) gas mixture over pure H2. A possible reason could be the increase in dynamic viscosity of the gas mixture when Ar is added. Additionally, photoinduced enhanced Raman spectroscopy (PIERS) is implemented as a complementary approach to investigate the dynamics of the defective structures under ambient conditions which cannot be effortlessly realized by vacuum techniques like TEM.

Improving the Quality of Ceramic Products by Removing the Defective Surface Layer

The surface of ceramic products manufactured using diamond grinding is replete with shallow scratches, deep grooves and other defects. The thickness of the defective layer amounts to 3–4 µm and it must be removed to increase wear resistance of the products when exposed to intense thermomechanical loads. In this study, removal of the defective layers from samples made of ZrO2, Al2O3 and Si3N4 with a beam of fast argon atoms was carried out with a stripping rate of up to 5 µm/h. To prevent contamination of the source of fast argon atoms by the sputtered dielectric material, the beam was compressed and passed to the sample through a small hole in a wide screen. Due to the removal of the defective layer, abrasive wear decreased by an order of magnitude and the adhesion of coatings deposited on the cleaned ceramic surfaces improved significantly.

Technological principles for removal of defective surface layer on grinded ceramic products by etching with accelerated argon

The article describes the technological principles and equipment for “dry” ion-plasma etching of the surface layer of SiAlON dielectric ceramics and subsequent deposition of (TiAl)N coatings. The proposed approach made it possible to ensure the high-performance removal of a defective layer formed during diamond grinding from the ceramic samples’ surface. Accelerated argon atoms with an energy of 5 keV are used as a source of energy impact on ceramic samples, which bombard the surface layer and ensure its sputtering at a rate of 9 μm/h. It is shown that the (TiAl)N coating deposition on ceramic samples after the preliminary removal of the defective layer reduces the volumetric wear under abrasive action by a factor of 2.6 compared to the initial samples subjected to diamond grinding.

Features of ablation and modification of Al<sub>2</sub>O<sub>3</sub>‒TiC ceramic surface structure during two-coordinate pulsed laser treatment

The main regularities of influence of two-coordinate pulsed laser treatment parameters on processes of ablation and structural modification of Al2O3‒TiC ceramics were established. The ambiguous influence of laser treatment parameters mode on the intensity of the ablation process was revealed. It was shown that structural modification nature is determined by parallel processes of fragmentation, melting and fusion of original ceramics grains and defective surface layer with the formation of new elements was established. The influence of laser treatment parameters mode on nature of structural modification is manifested through the size and number of newly formed elements of surface structure.

Traps inhomogeneity induced conversion of Shockley–Read–Hall recombination in NiO/β-Ga2O3 p+–n heterojunction diodes

In this Letter, the trap inhomogeneity within β-Ga2O3 is correlated with the conversion of Shockley–Read–Hall (SRH) recombination in NiO/β-Ga2O3 p+–n heterojunction diodes. For the virgin epi-wafer, both near-surface traps E2 (EC-0.82 eV) and E3 (EC-1.11 eV) and bulk E2* traps (EC-0.76 eV) are identified by a transient capacitance analysis, and the corresponding forward current–voltage characteristics of diodes are well fitted in the framework of field-dependent SRH recombination. The SRH recombination rates for E2, E3, and E2* traps are determined to be 1.3 × 107, 8.6 × 108, and 2.4 × 108 s−1, respectively. In this circumstance, carrier transport under forward bias is governed by trap-assisted tunneling through E3 traps with high recombination rates, and the hysteresis is pronounced. With the removal of the defective surface layer, E2 and E3 traps are almost completely eliminated, together with the reduced density of E2* traps to 5.6 × 1014 cm−3. The resultant diode performs an improved rectification ratio of >1011 at ±3 V and an enhanced reverse breakdown voltage of 1692 V. The elimination of near-surface traps leads to the conversion of carrier transport into the conventional SRH recombination, accompanied by the negligible forward hysteresis characteristics. The established fundamental correlation of carrier transport and traps within Ga2O3 is beneficial to develop a high-performance power rectifier toward practical applications.

Influence of Defects in Surface Layer of Al2O3/TiC and SiAlON Ceramics on Physical and Mechanical Characteristics

The paper studies the influence of diamond grinding, lapping, and polishing on the surface layer and defectiveness of the Al2O3/TiC and SiAlON ceramic samples. The index of defectiveness ID, which is the product of the defect density and the defective layer’s thickness (Rt), and a method for its evaluation are proposed to quantify the defectiveness of the ceramic surfaces. Lapping reduces the Rt parameter by 2.6–2.7 times when the density of defects was decreased by 2 times. After polishing, the Rt parameter decreases to 0.42 μm for Al2O3/TiC and 0.37 μm for SiAlON samples. The density of defects decreases many times after polishing: up to 0.005 and 0.004, respectively. The crack resistance of the polished samples increased by 5–7%. The volumetric wear of polished samples decreased by 1.5–1.9 times compared to the ground ones after 20 min of abrasion wear. The polished samples show a decrease in the coefficient of friction at 800 °C and a decrease in the volumetric wear by 1.5 and 1.3 times, respectively, compared to the ground ones after 200 m of friction distance. The volumetric wear at high-temperature friction of sliding for polished specimens was 55% and 42% less than for the ground ones, respectively.

Evaluating the Lifetime of Quarto Stand Rolls on the Basis of the Contact Fatigue Criterion

One of the principal failures of the supporting and working rolls on the Quarto rolling system is the spalling of their surfaces due to the gradual accumulation of defects in contact volumes of a material. Nowadays to prevent such kind of degradation failures it is necessary to check out the stationary condition of the contact strength, which doesn’t answer to the question of the duration of the roll staying in the working state, i.e., its resource (lifetime). In the article a new physical-analytical model of the roll’s failure is suggested by the criterion of the surface layer fracture and the corresponding methodology of the expected resource calculation. The approach is based on the kinetic concept of damage of solid bodies, the energy (thermodynamic) condition of fracture of solid bodies and the fundamentals of reliability forecasting for technical objects. The feature of the suggested methodology is that the definition of the failure moment for the supporting or working rolls of the Quarto system derived from the condition of the current density of defects in surface layer of a material reaching the critical value, which is the function of the enthalpy of a material melting..

Methodology for improving the mechanical and operational properties of aircraft structural elements during its maintenance using gas pulse treatment

A methodology has been developed to ensure airworthiness by improving the reliability of structural elements of aviation and airfield equipment, which differs from the currently available ones by using new effective non-deformational technologies based on pulsating subsonic gas flows during all types of maintenance and repair. New technologies have been developed to improve the mechanical and operational properties of aircraft structural elements, applicable in the production of aviation equip-ment, in the process of its maintenance and repair, characterized by the ability to increase them in a complex, without deterioration of any parameters. The nature of the impact of unsteady subsonic gas flows on the structure and properties of aircraft structural elements made of metal and polymer materials, the stress states of products, and the corresponding physical model has been created. It is proposed to implement measures for the use of new non-deformational technologies for restoring operational properties, as well as improving the reliability and extending the life cycle of elements, parts and assemblies of aircraft, airfield equipment by transport companies that exceed the currently used efficiency, efficiency and envi-ronmental friendliness, making appropriate changes to the maintenance and repair regulations. The direction of improving the methodology for forecasting the technical condition of aircraft structural elements is indicated, which differs from the existing ones by taking into account data on the density of crystal structure defects in surface layers.

Reconstructing the amorphous and defective surface for efficient and stable perovskite solar cells

The surfaces of perovskite solar cells (PSCs) are significant in determining the devices’ efficiencies and stabilities. Here, we first uncover that the 4-tert-butylpyridine (tBP), as an essential additive in hole transport layers (HTLs), could recrystallize the amorphous and defective perovskite surface layers and passivate the defective sites on grain surfaces. The reconstruction induces a larger surface work function and mitigates the interface energy level misalignment between perovskite and HTLs, enlarging the photovoltage of the device. Then, we engineer the chemical bonding strength and develop a more effective HTL additive 4-tert-butylpiperidine (tBPp), which possesses a stronger interaction with perovskite surface defective sites than tBP. With the enhanced adsorption, the tBPp-reconstructed perovskite surface exhibits lower densities of defects and better stability under the stimuli of heat, light and humidity. As a result, the optimized tBPp PSC reaches a champion efficiency of 24.2% with much better operation stability. Tracked at the maximum power point under a continuous bias, the unsealed devices in a N2 atmosphere can nearly maintain their initial efficiency after continuous light exposure for over 1200 h. Our findings provide an underlying understanding of the HTL additives, which markedly affect the efficiency and stability of n-i-p PSCs.

Methodological approach to ensuring the operational stability of ceramic tools

The developed methodological approach to ensuring the operational stability of ceramic tools, based on creation of functional surface layer after removing defective surface layer formed after grinding is presented. The advantages of functional surface layer on ceramic tools are confirmed by tests. Ill. 3. Ref. 28.

Plasma-Beam Processing of Tools Made of SiAlON Dielectric Ceramics to Increase Wear Resistance When Cutting Nickel–Chromium Alloys

This research aimed at an increase in wear resistance of round cutting plates manufactured with SiAlON dielectric ceramics through deposition of wear-resistant coatings. To increase effectiveness of the coatings, their adhesion was improved by the removal of defective surface layers from the cutting plates before the deposition. As the depth of caverns and grooves appearing on the cutting plates due to manufacturing by diamond grinding reached 5 µm, a concentrated beam of fast argon atoms was used for the removal of defective layers with a thickness exceeding the depth of caverns and grooves. At the equal angles of incidence to the front and back surfaces of the cutting wedge amounting to 45 degrees, two-hour-long etching of rotating cutting plates provided removal of defective layers with thickness of ~10 µm from the surfaces. After the removal, the cutting edge radius of the plates diminished from 20 to 10 µm, which indicates the cutting plates’ sharpening. Wear-resistant TiAlN coatings deposited after the etching significantly improve the processing stability and increase wear resistance of the cutting plates by not less than 1.7 times.

Grain refinement effect on the Ti-45Nb alloy electrochemical behavior in simulated physiological solution

In this study, the influence of microstructural refinement induced by the high-pressure torsion (HPT) on the corrosion resistance of the Ti-45Nb (mass%) alloy was investigated. The alloy characteristics before and after the HPT deformation were analyzed by electron backscatter diffraction (EBSD), scanning transmission electron microscopy (STEM), x-ray diffraction (XRD), and Vickers microhardness measurements, while the alloy corrosion behavior in simulated physiological conditions was examined by potentiodynamic polarization measurements and electrochemical impedance spectroscopy (EIS) analysis. Detailed microstructural analyses revealed that the HPT deformation led to significant grain refinement of the Ti-45Nb alloy exhibiting an ultra-fine grained (UFG) microstructure along with a substantial increase of hardness. Results also indicated that the grain refinement did not affect the alloy phase composition since β-Ti and Ti4Nb phases were present in the microstructure before and after the HPT deformation. Even though the Ti-45Nb alloy in both, coarse-grained (CG) and UFG, conditions shows high corrosion resistance in Ringer's solution at 37 °C, it was observed that the HPT treatment additionally improved the alloy corrosion properties. Namely, more rapid formation of the passivating layer with better barrier properties on the UFG alloy surface was recorded and resulted in better corrosion resistance of the alloy after HPT deformation. An increase of the grain contact area in the refined microstructure caused an increase of the diffusive transfer along the grain boundaries, accelerated the formation of a less defective protective barrier surface layer, and promoted the alloy surface passivation in the simulated physiological conditions.

Increasing the titanium alloys durability due to modifications by vibro-abrasive processing of the strengthened surface layer

It is shown that vibroabrasive treatment of hardened titanium alloy specimens contributes to an increase in their durability. This effect is explained by the removal of a defective surface layer containing microcracks and subject to the influence of residual tensile stresses. It is shown that the proposed stage of vibro-abrasive processing with ceramic granules makes it possible to almost completely remove iron introduced after vibro-impact surface hardening, which makes it possible to exclude the operation of etching in nitric acid from the technological process. The current state of research on durability in world science is briefly presented.

Comparative analysis of methods for improving the quality of hardened pipe and bar stock for agricultural machinery parts

The problem of obtaining a high-quality surface is acute in thermal, chemical-thermal and thermomechanical processing of products of rolling production. But, due to the fact that many metallurgical plants are insufficiently equipped with cleaning tools for these types of processing, the required surface quality of metal products is not ensured. Operations related to the removal of defects in the surface of rolled products employ from 30 to 60% of the workers in rolling shops. The need for cleaning leads to a rupture of the production flow, since the metal must be pre-cooled for inspection and cleaning of defects. As a result, the total cost of cleaning is 2 - 3 times higher than the cost of performing the main technological operations - heating and deformation of the metal. In this regard, the combined methods of hot rolling with simultaneous hardening and elimination of surface defects are of particular interest, which gives significant energy savings and excludes etching operations in solutions of sulfuric and hydrochloric acids. The paper considers new methods for stripping rolled stock before deformation in the high-temperature thermomechanical treatment (HTMT) mode of cylindrical hot-rolled solid and hollow billets with screw compression (SC) deformation, combining, along with a high-performance method of shaping and strengthening of rolled products, environmentally friendly (acid-free) methods of surface cleaning, as well as methods of removing surface layer defects combined with mechanical processing. At the same time, simultaneously with cleaning the surface and increasing the accuracy of rolled products, the structure of the surface layer is formed by the mechanism of phase transformations during thermomechanical processing. The type of fracture changes at low-temperature destruction from brittle to ductile. It should also be noted, that there is the need for further development of such well-proven cleaning methods as blade and waterjet processing, which allow, when cleaning rolled products from any steel grades from scale, to implement the most reliable cleaning system, ensure effective removal of surface and deeper defects, increase yield, reduce energy consumption and fit well into the continuous rolling mill line. According to recent studies, waterjet treatment increases the fatigue life of hollow cylindrical parts by up to 15%.

Identifying the Soft Nature of Defective Perovskite Surface Layer and Its Removal Using a Facile Mechanical Approach

<h2>Summary</h2> The presence of a defective layer composed of nanocrystals and amorphous regions at the surface of perovskite films has been shown to initialize the degradation of perovskites and cause nonradiative recombination. Here, we report the discovery that these defective surface layers are mechanically softer than the crystalline regions. The defective surface layer has a weaker bonding with the crystalline layer underneath it, which enables a facile approach to mechanically peel-off these defective layers using adhesive tapes. The chosen low-cost tape has an appropriate bonding force with perovskites, so the peeling does not damage the crystalline region and embedded interfaces underneath. The tape-treated devices retained 97.1% of the initial efficiency after operation at the near maximum power point under one sun illumination for 1,440 h at 65°C. This method is universally effective in enhancing the stability of various commonly used perovskite compositions and is compatible with the scaling up of perovskite solar modules.

Structure and properties of rails after extremely long-term operation

The paper reveals regularities and mechanisms of structure-phase states and properties formation of of differentially hardened 100-m rails of DT 350 category after the passed tonnage of 1411 mln. tons brutto. The formation of highly defective surface layer with nanosize (40–50 nm) grain-subgrain structure of pearlite colonies and submicrocrystal (150–250 nm) structure grains with structure free ferrite is detected. The change of hardness, microhardness, crystal lattice parameter, microdistorsion level, scalar and excess dislocation density on the rails head section are analyzed. The possible mechanisms of cementite plates’ transformation at extremely long-term operation are discussed.

Reviving reversible anion redox in 3d-transition-metal Li rich oxides by introducing surface defects

The reversible anion redox of O2−/(O2)n− in 3d-transition-metal based Li layered oxides (LLO) has received renewed attention due to its capability of hosting additional redox centers, which can further improve the energy density of Li-ion batteries. However, over-oxidized (O2)n− was found to be unstable upon cycling, resulting in an irreversible crystal structure transformation that lowers long-term cycling stability. Herein, we demonstrated that the anion redox can be tuned through surface defect engineering, which consequently improves the cycling instability. By reconstructing the atomic configuration of the surface, a highly defective surface layer with oxygen vacancies is achieved, substantially enhancing the reversibility of anion redox as well as the stability of bulk crystal structure. The modified LLO expresses a high performances of discharge capacity (94.5%), redox potential (>3.0 V during discharge) and energy density (90.2%) after 100 cycles. Ex-situ XPS measurements confirm a high reversibility of the O2−/(O2)n− redox couple, which is supported by DFT calculations showing that the oxygen vacancies formed at the fully lithiated state of LLO mitigate the over-oxidization of oxygen and the formation of unstable superoxides ((O2)-) through a reductive coupling mechanism.

Efficient waste polyvinyl(butyral) and cellulose composite enabled carbon nanofibers for oxygen reduction reaction and water remediation

Waste polyvinyl(butyral) (W-PVB) collected from the windshields of end-of-life vehicles has drawn considerable interest as a complementary and abundant resource. However, large amounts of W-PVB are still being buried in landfills every year owing to a lack of recycling techniques. As an alternative, we report the fabrication of carbon nanofibers from natural cellulose and W-PVB composites using a facile electrospinning, carbonization, and KOH activation approach. Interestingly, volatiles and residual carbon from a W-PVB matrix through carbonization produce highly porous carbon nanofibers and a defective graphitic surface layer, respectively. As a result of the large surface area (698.1 m2 g−1) and pore volume (0.2919 cm3 g−1) from abundant micropores, as well as the high density of active sites from defects, resulting carbon nanofiber shows a superior performance in environmental applications. It serves as a metal-free and un-doped carbon catalyst with a half-wave potential of 0.76 V vs RHE for the oxygen reduction reaction and a 99.6% removal of rhodamine B from water as an adsorbent for water remediation. This simple strategy can open a new approach to the design and synthesis of various classes of W-PVB-based composites, which will broaden the reuse of W-PVB in renewable and sustainable applications.

Tribological properties of swollen nitrile rubber under dry and wet sliding conditions

When nitrile rubber contacting with water under the external force during the service life, swelling and sliding wear occur. Since nitrile has similar polarity to water, the interaction between water and nitrile rubber and its influence on mechanical and tribological behaviors of the nitrile rubber play an important role in rubber performance, which is extremely necessary to be studied systematically. In this study, immersion experiments were conducted on three nitrile rubber samples which contained different acrylonitrile weight percentages of 18%, 26% and 41% (i.e. N18, N26, and N41) respectively. The hardness, tensile, dry and wet unidirectional sliding wear tests were further conducted. To reveal the mechanisms, the nuclear magnetic resonance (NMR) analysis and scanning electron microscopy (SEM) analysis were performed and interpreted accordingly. The results showed that cross-linking break based structural damage, substance dissolution and surface layer defects occurred after swelling, resulting in decreases in hardness and tensile strength and increases in permanent deformation fracture rates of the nitrile rubbers. The friction coefficient value of the un-swollen NBRs was positively related to the wear loss and decreased with the increase of the acrylonitrile content. During the wear process under dry conditions, the un-swollen rubbers presented hardening in aging while the swollen samples softened along with the deterioration of adhesive wear with higher steady-state friction coefficient value and wear loss. Under wet friction conditions, the susceptible wear behavior of the swollen rubbers induced reductions in friction coefficient and increases in wear loss. The swelling degree and its effects on mechanical and tribological behavior decreased with the increase of the acrylonitrile content. The swelling influences on the tribological behavior of nitrile rubber affected and acted on the entire wear process.