Surface features of HfMoNbTiZr refractory high entropy alloy after thermal oxidation in air, encouraging growth of an external oxide layer, are investigated. Thermal oxidation at 400 and 500 degrees C for durations >6 h formed an external complex oxide (consisting of HfO2, MoO2/MoO3, Nb2O5, TiO2, ZrO2) layer, whose growth can be modelled by the Larson-Miller approach with respect to temperature and duration. This layer had got a hardness of similar to 20 GPa and provided excellent protection against dry sliding wear at room temperature under Hertzian contact pressure of similar to 0.6 GPa, regardless from the thickness, which varied in between 0.29 and 4.65 mu m. However, scratch tests revealed the role of the oxide layer thickness on its mechanical stability. During scratch testing, similar to 0.55 mu m thick oxide layer possessed better integrity with the underlying substrate without any evidence of spallation. Nevertheless, similar to 0.55 mu m thick oxide layer was obtained after thermal oxidation at 400 degrees C for 12 h in this study, Larson-Miller approach can be used to estimate different temperatures and/or durations providing this critical thickness. In this regard, the equation of T(20+logt)=14186 has been proposed.

This study investigates the influence of plastic deformation of commercially pure titanium (cp-Ti) substrates on the tribological performance of micro-arc oxidation (MAO) coatings. Hydrostatic extrusion (HE) was employed to refine the microstructure of cp-Ti, producing ultrafine-grained (UFG) titanium, which was compared with coarse-grained (CG) cp-Ti. Both substrates were subjected to the MAO process, and the fabricated MAO coatings were analysed through X-ray diffraction (XRD), electron microscopy techniques (SEM and TEM), energy dispersive spectrometry, atomic force microscopy (AFM), and tribological testing (hardness, scratch and wear tests). The results show that HE-treated cp-Ti substrates promoted faster and thicker MAO coating formation, with superior adhesion and tribological properties compared to CG cp-Ti. The UFG substrate led to increased hardness and wear resistance, largely due to the higher density of grain boundaries and electrical resistance of the substrate, which accelerated oxide layer growth. It has been demonstrated that plastic deformation of the substrate via HE significantly enhances the performance of MAO coatings, providing improved surface protection for biomedical applications.

This study investigates the structural features and mechanical durability of the oxide coatings fabricated on HfMoNbTiZr refractory high entropy alloy via micro-arc oxidation in two different electrolytes (namely, sodium silicate- and sodium aluminate-based). Structural characterizations revealed that fabricated coatings consisted of oxides of the refractory elements (Hf, Zr, Nb, Ti) except Mo and the oxides of the main constituents of the electrolyte (Si and Al). According to the results of hardness and scratch tests, the Al2O3-containing coating (fabricated in sodium aluminate-based electrolyte) has got higher hardness and scratch resistance compared to SiO2-containing coating (fabricated in sodium silicate-based electrolyte). Moreover, the SiO2-containing coating detached from the substrate at 100 degrees C under sliding contact, while the Al2O3-containing coating remained intact with the substrate up to temperature of 300 degrees C. Therefore, it is concluded that Al2O3-containing coating has a potential to provide better protection against sliding contact for the HfMoNbTiZr alloy both at room and high temperatures.

Failure analysis was conducted on two diesel engine exhaust valves manufactured by joining martensitic and austenitic steels (stem and head parts, respectively). While one of these valves failed by cracking, the other one failed by fracture from the austenitic steel head. Microscopic and spectroscopic examinations conducted on the cracked valve revealed evidences of overheating-induced oxidation (i.e., burning). In the case of the fractured valve, observation of beach marks on the fracture surface suggested the progress of failure via fatigue following burning. Based on these findings, it is recommended that the valve head material should be replaced with a nickel-based superalloy to prolong the service life

This study investigates the behavior of an in-house cast directionally solidified nickel-based superalloy (i.e. CM247LC grade alloy) at low and high temperatures (760 and 982 °C, respectively). For this purpose, creep and tensile tests were conducted on the alloy after the heat treatment consisted of solutionizing and double aging. Following creep and tensile tests, microstructural examinations were made on the fracture surface and cross-sections of samples by using optical, digital, and electron microscopes. The results showed that the dominant creep mechanism of the CM247LC alloy was dislocation creep at both temperatures. However, the higher temperature accelerated creep failure via de-binding at the matrix/carbide interface and caused severe structural degradation.

Plasma electrolytic oxidation (PEO) treatment typically enhances the wear resistance of lightweight alloys but also significantly increases the coefficient of friction (COF). To address this limitation, particularly for PEO treated magnesium alloys, this study explores the use of MoS2-coated steel counterfaces under lubricated sliding conditions up to 100 degrees C. The results showed that PEO-coated AZ31 sliding against MoS2-coated steel exhibited consistently low COF and wear rates across the temperature range of 25 degrees C to 100 degrees C. At temperatures above 50 degrees C, a stable MoS2-rich tribolayer formed on the PEO surface, effectively reducing friction and wear, particularly as the oil film thinned with increasing temperature. A slight increase in COF was observed between 75 degrees C and 100 degrees C, attributed to the formation of MoOS, which reduced the lubricating effectiveness of the MoS2 tribolayer. In contrast, uncoated AZ31, lacking a tribolayer, showed significant increases in friction and wear due to surface oxidation and thermally activated plastic deformation. This study demonstrates that pairing PEO-coated AZ31 with MoS2-coated steel counterfaces is an effective strategy for lowering COF and wear rates at temperatures up to 100 degrees C, indicating potential for improving the tribological performance of magnesium alloys in high-temperature applications such as the automotive and aerospace industries.

This study focuses on fabricating AlAl matrix compositeComposites layers on commercially pure AlAl and 70757075 alloy by hot pressing to improve wear resistance at elevated temperaturesTemperature. As a reinforcement particle, Al2Ce intermetallicAl2Ce intermetallic particles synthesized by vacuum arc melting was chosen for its better chemical compatibility with AlAl matrix. Structural characterizationsCharacterization revealed the good binding between Al2Ce-p and AlAl matrix and the success of Al2Ce-p in increasing hardnessHardness of AlAl and 70757075 alloy. Results of the wear tests conducted at 200 °C against alumina balls under the load of 2 N showed 2.6- and 6.0-times lower wear rate for the AlAl matrix compositeComposites layer covered and 70757075 matrix compositeComposites layer covered samples as compared to their monolithic states, respectively.

This study investigates the wear micromechanisms of D2 steels under impact-sliding conditions, offering insights into their performance when used in applications such as trimming dies for high-strength steel sheets where they undergo plastic deformation and chipping. Two D2 steel samples, both with a bulk hardness of 59.7 HRC but different matrix hardnesses and carbide distributions, are tested by using an impact-sliding wear test rig at Hertzian contact pressures exceeding 2 GPa. The sample with a softer matrix exhibits wear primarily through delamination caused by plastic deformation. This initiates cracks at the matrix/primary carbide interface, leading to material loss in the form of large chips. In contrast, the steel with a harder matrix shows reduced wear due to its resistance to plastic deformation. Initially, wear occurs through the fracture of primary carbides. However, with prolonged loading, the matrix begins to soften, adopting a wear mechanism similar to the D2 steel with softer matrix. Notably, smaller primary carbides are associated with improved wear resistance by limiting the initiation sites for cracks, especially at the matrix/primary carbide interface. This understanding enables the selection and design of heat treatments to optimize D2 steel microstructure, thus improving resistance to impact-sliding wear damages observed in processes like trimming.

In this study, failure analysis was conducted on a 51CrV4 steel leaf spring cracked during hot forming by using a light optical and energy dispersive spectroscopy equipped scanning electron microscopes. Microstructural surveys revealed presence of several surface and internal cracks at the prior austenite grain boundaries, where trace elements (Cu, Sn, Sb and Pb) accumulated. Finite element analysis revealed that the location of the crack was the region of the leaf spring where the maximum principle stress developed. It was finally concluded that, the leaf spring material lost its hot ductility under tensile loading at the employed processing temperature upon intergranular liquation.

An attempt has been made to enhance the wear resistance of Al-12Si alloy by forming a dual-layer coating consisting of an Al-12Si matrix composite layer (Al-12Si +-40 vol % Al2Ce) and an external composite oxide layer. While the hardness of monolithic Al-12Si alloy was-110 HV, the Al-12Si matrix composite layer has got hardness of-140 HV and exhibited-6 times higher sliding wear resistance due to the suppression of the plasticity-induced adhesive wear by Al2Ce particles. Applying micro-arc oxidation (MAO) as a post-treatment favoured an oxide layer (Al2O3) on the Al-12Si alloy and a composite oxide layer (CeO2 + Al2O3) on the Al-12Si matrix composite layer. The MAO coatings fabricated on the Al-12Si alloy and Al-12Si matrix composite layer, with hardness of-11 GPa and-13 GPa, respectively, provided more than two orders of magnitude in-crease in the wear resistance by shifting the wear mode to elasticity-dominated wear. However, the critical load imposing the complete detachment of the MAO coating by intense cracking and spallation during testing was determined as 3 N for the MAO'ed Al-12Si alloy and 5 N for the MAO'ed Al-12Si matrix composite layer. The wear resistance of the MAO coating of the Al-12Si matrix composite layer was-2 times higher than that of the MAO coating of the Al-12Si alloy below the critical test loads, where the coatings remain intact with the substrate.

This study was initiated to improve surface hardness and wear resistance of a HfNbTaTiZr refractory high entropy alloy (RHEA) by gas nitriding at a medium temperature (600 degrees C) for 3 h. Structural characterizations conducted by X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) equipped scanning electron microscope (SEM) revealed that nitriding led to formation of a 1.5 mu m thick surface layer containing precipitates of oxides and nitrides of the alloying elements. Detection of oxides within the surface layer was attributed to the presence residual oxygen in the nitriding atmosphere. Nevertheless, the employed gas nitriding provided remarkably higher scratch resistance compared to the untreated state, as the results of increment in the surface hardness and development of larger compressive residual stress.

This study investigated the friction and wear characteristics of a plasma electrolytic oxidation (PEO)-coated Mg–Al alloy (AZ31) in sliding contact against steel using graphene nanoplatelets (GNPs) containing ethanol as a lubricant. The results revealed that the typically high coefficient of friction (COF) of PEO-coated surfaces under dry sliding (0.74) was notably reduced to 0.18 during the sliding tests conducted in GNP-free ethanol. When the ethanol contained 5 × 10−4 wt.% GNPs, the COF of the uncoated AZ31 alloy further dropped to 0.17. The PEO-coated surfaces achieved a significantly lower COF of 0.07 and demonstrated a marked reduction in wear rate, attributed to the formation of a tribolayer incorporating graphene. These findings highlight the significant potential of GNP-incorporated ethanol to improve the tribological performance of PEO-coated AZ31, presenting a promising avenue for advancing lightweight, sustainable, and efficient automotive technologies.

Tungsten (W) is the most promising material for future plasma-facing materials. Its alloys or composites reinforced with various compounds have been studied to improve the irradiation resistance of W. This study aimed to investigate the effect of boron-containing reinforcement in the W matrix against He + ion irradiation. Cerium hexaboride (CeB 6 ) particulates, which have high neutron shielding properties, were incorporated into the W matrix pre-alloyed by 1 wt% Ni (W1Ni). CeB 6 powders were home-made and prepared from CeO 2 /Mg/B 2 O 3 powder blends via mechanochemical synthesis and purification steps. 1, 5, and 10 wt% CeB 6 powders were added to pre-alloyed W1Ni by mechanical alloying, and then they were consolidated by using pressureless sintering (PS, 1400 degrees C, 1 h) and spark plasma sintering (SPS, 1410 degrees C, 1 min) techniques. CeB 6 particlereinforced W1Ni composites contained different amounts of reinforcements were prepared by two different sintering methods and were compared with respect to their compositional, microstructural, and microhardness properties and wear and irradiation behaviors. Based on the results, increasing the CeB 6 reinforcement amount in the composite triggered the formation of the W 2 B phase, especially in the W1Ni - 10CeB 6 composite after both sintering methods. The mechanical and irradiation properties were enhanced more by increasing the CeB 6 amount in the case of using the SPS method. When compared to other sintered samples, the SPS ' ed W1Ni - 10CeB 6 composite has the lowest specific wear rate of -4 x 10 - 7 mm 3 /Nm and the maximum hardness value of -21 GPa. According to surface deformation, the W1Ni - 5CeB 6 composite exhibited comparatively higher resistance to He + ion irradiation.

In the present work, the effect of the electric potential provided by the bipolar pulsed power supply during micro-arc oxidation of titanium in disodium hydrogen phosphate (Na2HPO4) electrolyte on the microstructure and chemical composition of the formed oxide coating was studied. The surface topography and microstructure of obtained coatings were investigated using scanning and transmission electron microscopy. In addition, the chemical analysis was performed with two complementary methods: energy-dispersive X-ray spectroscopy and more accurate but surface-sensitive X-ray photoelectron spectroscopy. These investigations revealed the formation mechanism of the titanium oxide coating, depending on whether an anodic or cathodic cycle is forced by the power supply. During the anodic cycle, the simultaneous interaction of negatively charged O2− ions and HPO42− anions with Ti4+ ions released from the substrate occurs. Introducing the cathodic cycle allows a small amount of Na to be incorporated into the coating material, mainly in the areas close to the porosity, which was not possible with the use of the direct current (DC) or unipolar pulsed power supply. Moreover, an increase in applied voltage increased rutile content and improved the crystallinity of the TiO2-based MAO coating.

This study investigates the high temperature wear behaviour of a WE43 Mg alloy after covering it with single and dual layer coatings. For this purpose, cold spray and micro-arc oxidation processes were employed individually and sequentially. Single-layer coatings fabricated by cold spray and micro-arc oxidation processes were Al/Al2O3 composite and MgO-based ceramic, respectively. Sequential application of cold spray and micro arc oxidation processes induced dual layer coating upon synthesizing an external Al2O3-based layer over the Al/Al2O3 composite layer. Results of the wear tests conducted under the load of 2 N revealed the superior resistance of the dual layer coated sample against the rubbing action of the counterface compared to single layer coatings. Thus, the presence of a relatively hard and tough external Al2O3-based layer over the Al/Al2O3 composite layer sustained protection up to the temperature of 320 degrees C, where the dominant wear mechanism was fatigue wear. However, the increase in the test temperature to 350 degrees C caused detachment of the external Al2O3-based layer. Reduction of the wear test load from 2 to 1 N resulted in the remaining of external Al2O3-based layer intact with the underlying Al/Al2O3 composite layer even at a test temperature of 350 degrees C. It is therefore concluded that the combination of cold spray and micro-arc oxidation processes is promising to broaden the reliable use of WE43 and other Mg alloys in wear related applications at high service temperatures.

Thermal barrier coatings (TBC) have been developed to reduce the surface temperature of hot components in gas turbine engines. To get superior oxidation and mechanical properties, “The Yttria Stabilized Zirconia (YSZ)” top coat and β-NiAl bond coat are deposited by Electron Beam Physical Vapor Deposition (EB-PVD) and Chemical Vapor Deposition (CVD) processes, respectively. In this study, after structural characterization of the TBC formed on the directionally solidified (DS) CM247LC superalloy, the nanomechanical properties of the top and bond coats were determined using the nanoindentation technique. The results showed no significant differences in their elastic modulus despite the more than two times higher hardness of the top coat than the bond coat (18.4 GPa and 7.2 GPa, respectively). Energy Dispersive Spectrometry (EDS) equipped with Scanning Electron Microscope examinations revealed that Al and N- and N-rich zones within the bond coat have an underlying diffusion zone. In addition, thermal-grown oxide (TGO) film was detected at the interface of the top and bond coat.

This study is initiated with the aim of regulating the release of silver (Ag) as an antibacterial agent from the micro‐arc oxidation (MAO) coating. Herein, an external 5 wt% Tin(II) chloride (SnCl2) containing biodegradable polycaprolactone (PCL) layer is formed on the 0.8 wt% Ag‐incorporated MAO coating by the dip coating method. 5 wt% SnCl2 addition into PCL provides a steady release of Ag into concentrated simulated body fluid (1.5X SBF) from the underlying MAO coating at 37 °C. When the Ag release rate is taken into consideration, it is quantified as 0.0089 and 0.0586 ppm day−1 for PCL‐covered MAO and PCL‐free MAO coatings, respectively. It is finally concluded that the preliminary result of this study can be promising for minimizing the in vivo adverse effects of Ag+ ions arising from rapid release as well as maintaining antibacterial efficacy for prolonged periods, which is ideal for preventing the risk of postimplantation infections.

In this study, Ti-xNb (x = 0-40 wt%) alloys produced by the powder metallurgy were borided with the aim of clarifying the effect of Nb on the structural and mechanical properties of the boride layer. After smearing the paste prepared from nano boron powder on the surfaces of the alloys, boriding was conducted at three different temperatures (900, 1000 and 1100 degrees C) for 8 h in a vacuum atmosphere. Unlike those formed at 900 degrees C, boriding temperatures of 1000 and 1100 degrees C provided thicker and homogenous boride layers. However, the boriding temperature of 1100 degrees C induced cracking within the boride layer of the Ti-40Nb - 40Nb alloy. For these reasons, the optimum boriding temperature was determined as 1000 degrees C. Increase in the Nb content not only increased the fraction of beta-Ti phase in the microstructure of the sintered alloy at the expense of alpha-Ti, but also induced NbB2 2 in the structure of the boride layer along with TiB2. 2 . While Nb-poor alpha-Ti grains favoured the growth of TiB2, 2 , TiB2 & sdot;NbB2 2 & sdot;NbB 2 mixture preferentially developed over the Nb-rich beta-Ti grains. As the result of this, the hardness of the boride layer tended to decrease with increasing Nb content of the substrate. For example, the average hardness of the boride layers formed on Nb-free Ti and Ti-40Nb - 40Nb alloy were measured as 2674 HV 0.025 and 2460 HV 0.025 , respectively. But regardless from the hardness, the boride layers provided a good protection for the underlying substrates against dry sliding contact and triggered abrasive wear on the contact surface of the counterface (WC-Co ball). The presence of NbB2 2 in the boride layer led to a reduction in abrasive wear of the counterface. This finding revealed that in any wear-related application, where borided Ti alloys were intended to be used, it is better to choose high Nb-containing Ti alloys instead of alpha-Ti to minimize the wear of the tribocouple via reducing the abrasion at the counter body.

In this study impact-sliding wear performance of a 316L austenitic stainless steel (ASS), having widespread usage in various engineering applications under complex loading conditions, has been examined at room temperature (RT) and 180 °C under contact pressures as high as 2.5 GPa. Impact-sliding wear tests were carried out against 10 mm diameter 52100 grade bearing steel balls for 1000 loading cycles. The wear tracks formed on the samples were then examined by a 2-D contact profilometer, an energy dispersive X-ray spectroscopy (EDX) equipped scanning electron microscope (SEM) and a Raman spectroscopy. Despite the detection of tribo-oxides on the impact and sliding zones, mainly plasticity dominated wear was operative at RT. Tests conducted at RT and 180 °C favoured hematite + magnetite and hematite type tribo-oxides on the worn surface, respectively. The increase of test temperature to 180 °C caused an increase in the wear rate at the impact zone but a decrease at the sliding zone of the wear track. While the former was the result of softening of the ASS, the latter was due to the formation of thicker tribo-oxides on the contact surface. Although the reduction of sliding wear rate above a critical temperature (300–400 °C) was previously associated with the formation of protective tribo-oxides on the worn surfaces, this study revealed that higher contact pressure reduces this critical temperature via increasing the frictional heating under sliding contact. Thus, contact pressure of 2.5 GPa favoured the formation of protective tribo-oxides at a lower temperature (i.e. 180 °C) for the examined ASS/bearing steel tribo-pair.

Cu micrometalic particles with the supersonic velocity (3 Mac) were applied to determine the optimum microparticle bombardment effect on the ZnO:Al thin-film surface derived by sol–gel method (which allows mixing at the atomic level to form colloidal particles). The mechanical damage deriving by the Cu particles with high kinetic energy has indicated the practical coating parameters (presenting surface related aspects) for its fabrication steps to use in the diot applications. The key parameters of the practical ohmic contact deposition on the film surface (describing the functional behavior of the ZnO:Al/p-Si heterojunction) were examined to develop low ohmic contact resistance (derived by using Cu layer) for use in optoelectronic devices. The annealing of ZnO:Al/p-Si heterojunction (at 700 °C in vacuum) has supported to obtain a suitable metal contact with optimum low resistance by using the cold gas dynamic spraying technique. The conductive and rectifier behaviors of ZnO:Al/p-Si heterojunction have indicated the utilization of the Cu stack layer without the need for the extra thermal annealing treatment of Cu ohmic contact (after the annealing of ZnO:Al/p-Si heterojunction according to the specific analyses for applications in optoelectronics). The generated damage depended on the acceleration of Cu particles through the trapezium structure of the ZnO:Al surface (annealed at 800 °C). The Cu particles with high purity have been provided to avoid the cracked surface (annealed at 700 °C in vacuum). The developed in-depth surface performance has emphasized the relation to the control of the surface properties at the atomic level (by using the sol–gel dip-coating technique). The annealing process (affecting the thickness of the film) has indicated the control of the temperature as the key parameter for avoiding the mechanical damage (depending on the bombardment of the dense micrometallic particles at ultra-high speed) on the film surface.