Excellent Mechanical Performance Research Articles

Laser Directed Energy Deposition Additive Manufacturing of Al7075 alloy: Process Development, Microstructure, and Porosity

7xxx series aluminum alloys are priority materials adopted in aerospace because of their lightweight property and excellent mechanical performance. The combination of lightweight material and additive manufacturing techniques can significantly promote lightweight design in many industries. This study developed the optimal process for manufacturing Al7075 alloy by laser-aided directed energy deposition and revealed the material properties of the as-deposited Al7075 specimens. Firstly, single-track experiments were conducted by varying laser power, printing speed, and powder delivery rate in broad ranges. Results indicated that the printing speed and powder delivery rate impact the surface finish and shape of single-track beads considerably. Block specimens printed with the optimal parameters selected from the single-track experiments present crack-free quality and have a high relative density of up to 99.89%. Second phases with rich Cu, Mg, and Zn elements along the inter-dendritic regions and grain boundaries were observed from the microstructure inspection. The composition of zinc in the deposited samples is lower than usual, which leads to a relatively low microhardness.

Solving Ambiguity in EBSD Indexing of Long-Period Stacking Ordered (LPSO) Phase in Mg with Template Matching Approach

Magnesium (Mg) alloys with long-period stacking ordered (LPSO) phases are receiving increasing interest because of their excellent mechanical performance. The close similarity in atomic stacking sequences between different LPSO polytypes and Mg lattice often leads to ambiguous indexing in electron backscatter diffraction (EBSD), a commonly used material characterization technique. Instead of the Hough transformation approach used in commercial software, an alternative indexing approach, which can catch subtle differences by matching experimental patterns with simulated ones, is explored in this study. Our results, showing ~94% of mapping data being correctly indexed as the target phase, 14H LPSO, demonstrate the capability of not only resolving the LPSO phases but also distinguishing different LPSO polytypes. This approach offers a valuable, if not unique, solution for the microscale characterization of LPSO phases, enabling precise microstructure tuning to further promote the mechanical properties of Mg alloys.

Chemically Recyclable, Reprocessable, and Mechanically Robust Reversible Cross-Linked Polyurea Plastics for Fully Recyclable Aramid Fiber Reinforced Composites.

Aramid fiber reinforced composites (AFRCs) have received increasing attention because of their excellent comprehensive performance including high mechanical strength, high modulus, and light weight. However, full recycling of AFs from ARCFs is difficult to achieve. Herein, fully recyclable ARCFs are fabricated using reversible cross-linked polyurea plastics (PUHA) as the matrix. PUHA plastics are fabricated by cross-linking linear polyurea using hemiaminal groups. By changing the main chain structures, two types of PUHA plastics are prepared with excellent mechanical performance, which is comparable to that of traditional engineering plastics. PUHA plastics can be reprocessed at least five times without losing their original mechanical properties because of the dynamic exchangeability of the hemiaminal groups. Meanwhile, PUHA plastics can be rapidly depolymerized into linear polyurea under acidic conditions. When PUHA plastics are used as a matrix to fabricate AFRCs, the AFRCs exhibit excellent mechanical strength. Moreover, due to the simple chemical recycling ability of PUHA plastics, AFRCs can be fully decomposed into intact AFs and linear polyurea with high purity. This work presents the use of reversible cross-linked polyurea plastics in the fabrication of fully recyclable AFRCs and provides the future direction of developing fully recyclable and high-performance fiber-reinforced composites.

Effect of Diatomite on the Mechanical and Electrical Properties of Polyurethane/Epoxy Resin Composites

ABSTRACT Epoxy resin is widely utilized for the encapsulation of power semiconductor devices, necessitating excellent insulation properties, mechanical performance, and moisture resistance. In this study, hexadecyltrimethoxysilane was employed to modify diatomite, which was then combined with polyurethane-modified epoxy resin to fabricate a series of filled composites with varying diatomite content. The influence of diatomite content on the mechanical properties, thermal properties, and insulation performance of the polyurethane/epoxy resin composites was investigated. Experimental results showed that, under the synergistic effect of polyurethane and diatomite, the composite containing 4 wt.% diatomite exhibited improvements of 13.95% and 61.11% in tensile strength and flexural strength, respectively, compared to the epoxy resin. The elongation at break and flexural displacement also increased by 22.46% and 68.23%, respectively. Furthermore, the incorporation of diatomite effectively compensated for the thermal losses caused by polyurethane, enhancing thermal stability. Electrical property tests indicated that the addition of diatomite reduced the dielectric constant and conductivity of the epoxy resin while simultaneously improving its breakdown strength.

Cryogenic mechanical performance and gas-barrier property of epoxy resins modified with multi-walled carbon nanotubes

Type V linerless hydrogen storage vessels made of all-composite face the challenges of cryogenic mechanical failure and hydrogen leakage of composites. The cryogenic mechanical performance and gas-barrier property of the carbon fiber reinforced polymer for Type V vessels are largely determined by the epoxy matrix. Carbon nanotubes are widely used as polymer reinforcement material to enhance the cryogenic mechanical performance and gas-barrier property of epoxy resins. In this work, multi-walled carbon nanotubes (MWCNTs) were dispersed into epoxy resins to prepare MWCNTs-modified epoxy resins. The cryogenic mechanical performance, gas-barrier and thermal properties of the modified resin were then investigated. The experimental findings revealed a 34.34% reduction in the gas permeability coefficient for the nanocomposites containing 0.9 wt% MWCNTs compared with the epoxy matrix. The tensile strength and failure strain at cryogenic temperature (77 K) experienced enhancements of 26.99% and 41.53%, respectively. In addition, the thermal stability and glass transition temperature of the modified resin were improved. As a result, the MWCNTs-modified epoxy resin exhibits improved gas-barrier property in addition to excellent cryogenic mechanical performance, making it ideal for manufacturing Type V linerless hydrogen storage vessels.

High-temperature mechanical performance and damage behavior of molybdenum tailings modified fly ash-based geopolymers

Molybdenum tailings modified fly ash based geopolymers were an eco-friendly building material with excellent mechanical performance and fracture toughness. In this work, cubic compressive test and acoustic emission technique were used to evaluate the effect patterns of molybdenum tailings contents and thermal temperature conditions on the high-temperature mechanical performance and damage behaviors of molybdenum tailings modified fly ash-based geopolymers. The molybdenum tailings contents were 0 %, 10 %, 20 %, 30 % and 40 %, and the target temperatures were 300 °C, 600 °C and 900 °C, respectively. The results of compressive tests showed that the low contents of molybdenum tailings could limit the growth of macroscopic cracks during the loading process and improve the compressive strength of fly ash-based geopolymers when the heat treatment temperature did not exceed 600 °C. The acoustic emission results showed that 20 % molybdenum tailings contents could improve the structural stability and high-temperature mechanical performance of fly ash-based geopolymers, reduced the growth of microcracks in the specimen during the loading process, and delayed the formation and cracking of macrocracks, but did not change the specimen's compressive damage mode. The 40 % contents of molybdenum tailings changed the compressive damage mode of the fly ash-based geopolymers, delayed the cracking of the matrix structure under the peak loading, and significantly improved its resistance to plastic deformation.

Rapid on-site nondestructive surface corrosion characterization of sintered nanocopper paste in power electronics packaging using hyperspectral imaging

Sintered nanocopper (nanoCu) paste, exhibiting excellent electrical, thermal, and mechanical performances, offers promise for interconnections in wide bandgap (WBG) semiconductors operating at higher temperatures. However, sintered nanoCu is prone to severe corrosion in environments containing H2S, with on-site characterization methods for the composition of corrosion products currently lacking. In this study, a novel method was proposed for the rapid characterization of corrosion products during the corrosion process based on hyperspectral imaging (HSI) technology. Sintered nanoCu samples were subjected to 336 h H2S gas corrosion tests with bulk Cu as the reference, followed by correlating the corrosion element content with hyperspectral characteristic parameters. Then, the morphology and composition of corrosion products were researched using focused ion beam scanning electron microscope (FIB-SEM) and transmission electron microscope (TEM) analysis. The results showed that (1) during the corrosion process, a linear relationship was established between the Cu, O elemental atomic contents on the sample surfaces and their hyperspectral characteristic parameters. (2) The elemental atomic content of S exhibited an exponential relationship with the characteristic parameter. (3) The change rate in the spectral characteristic parameters during the corrosion process reflected the severity of corrosion, which was confirmed by comparing the thickness of the corrosion products of the sintered nanoCu and bulk Cu. This study offers a foundation for the further investigation of rapid on-site characterization of sintered nanoCu corrosion involving H2S.

Enhanced Strengthening and Toughening of T6-Treated 7046 Aluminum Alloy through Severe Plastic Deformation

The 7046 aluminum alloy possesses a favorable fatigue property, corrosion resistance and weldability, but its moderate strength and plasticity limit its wider application and development. In the present study, severe plastic deformation (SPD) was applied prior to T6 treatment to significantly enhance the strength and toughness of the 7046 aluminum alloy. The results show that the alloy processed by four passes of equal channel angular pressing (ECAP) at 300 °C prior to T6 treatment exhibits an excellent mechanical performance, achieving an ultimate tensile strength (UTS) and elongation (EL) of 485 MPa and 19%, respectively, which are 18.6% and 375% higher than that of the T6 alloy. The mechanical properties of the alloy are further improved by an additional room temperature (RT) rolling process, resulting in a UTS of 508 MPa and EL of 23.4%, respectively. The increased presence of η′ and Al6Mn phases in the 300°C4P-R80%-T6 and 300°C4P-T6 alloys contributes to a strengthening and toughening enhancement in the SPD-processed T6 alloy. The findings from this work may shed new insights into enhancing the 7046 aluminum alloy.

Scalable wet-spinning multilevel anisotropic structured PVDF fibers enhanced with cellulose nanocrystals-exfoliated MoS2 for high-performance piezoelectric textiles

The high-performance fabric-based piezoelectric nanogenerators (PENGs) is the ideal choice for the next generation of wearable electronics. However, large-scale fabrication of high-performance fabric-based PENGs remains a great challenge. In this study, cellulose nanocrystals (CNCs) exfoliated MoS2 (C@M) are integrated into the poly (vinylidene fluoride) (PVDF) matrix, which are successfully fabricated PVDF/C@M fibers (PCF) by wet-spinning and post-drawing processes. Accompanied by the stress-induced anisotropic alignment of C@M and PVDF molecular chains during post-drawing, the interfacial interactions between C@M and PVDF dipoles greatly promote the formation of the electroactive β-phase of PVDF as well as the orientation of the dipoles. The PCF demonstrate excellent mechanical performance with a tensile strength of 301 MPa and toughness of 68.5 MJ·m−3. Subsequently, the PCF are woven into fabrics and subsequently assembled into a flexible PENG (PCF-PENG), exhibiting excellent piezoelectric coefficient (d33 = 40.3 pC·N−1), power density of 56.25 μW·cm−2. Furthermore, scenarios such as energy harvesting, motion detection, and posture recognition of the PCF-PENG are demonstrated, providing a feasible proposal for the large-scale fabrication of electronic textiles.

Preparation and characterization of biochar/polypropylene composites from recycled waste plastics and agricultural waste-reed straw

To alleviate the increasingly serious plastic pollution problem, developing biomass-plastic composite materials has become a research hotspot. In this study, waste polypropylene (PP) and reed biochar (RC) are used as raw materials to prepare biochar/polypropylene composites (BPCs), and γ-aminopropyl triethoxysilane (KH550) is used as an interfacial modifier to improve the interficical compatibility between RC and PP. Benefiting from the coupling effect of KH550, the obtained KH550-modified reed charcoal/polypropylene (K-RC/PP) composites exhibit enhanced tensile and bending strengths compared to the RC/PP. When 2.4 wt% of KH550 is applied, the tensile and flexural strengths of K-RC/PP reach 22.77 MPa and 49.6 MPa, respectively. Moreover, the interfacial modification simultaneously improves the thermal stability and hydrophobicity of the composites, which not only significantly reduces the fire risks but also improves the durability of the material. This work provides a feasible solution for developing biochar/plastic composites with excellent mechanical performances and high fire safety.

Strength and microstructure of geopolymer recycled brick aggregate concrete after high temperatures

Geopolymer recycled brick aggregate concrete (GRBC), as a new environmentally friendly material, exhibits excellent mechanical performance at ambient temperature, but its properties after exposure to high temperatures are not clear. In this paper, the strength and microstructure of GRBC after high temperatures were investigated and compared with ordinary recycled brick aggregate Portland cement concrete (ORBC) through compressive strength, splitting tensile strength, and microscopic tests. The main parameters varied in the strength tests were temperature (20 °C, 200 °C, 400 °C, 600 °C, and 800 °C) and recycled brick aggregate (RBA) replacement ratio (0 %, 25 %, 50 %, 75 %, and 100 %). The results indicated that the strength of GRBC decreased less than that of ORBC after high temperatures. The compressive strength retention rate of GRBC after exposure to 800 °C was 0.359−0.456, and the tensile strength retention rate was 0.415−0.442. The strength of GRBC with 50 % RBA replacement ratio was about 0.70 that of the case without RBAs. The microscopic test results showed that the structure of GRBC was denser than that of ORBC, and the interfacial transition zone (ITZ) between matrix and RBAs was less affected by the temperature. The first weak zone after high temperatures was RBA itself for GRBC, but ITZ between matrix and RBAs for ORBC. The X-ray diffraction analysis indicated that the phase composition of GRBC and ORBC after high temperatures was basically similar to that at ambient temperature. The strength of GRBC was mainly provided by the N-A-S-H and C-A-S-H gels, while the strength of ORBC was mainly relied on the C-S-H gel.

Fracture properties of bamboo fibrous composites: A systematic review

As a fast-growing grass, bamboo has received a great deal of attention due to the excellent mechanical performance. The unique properties of bamboo come from its natural composite structure - a kind of natural fiber reinforced composites, which is composed of bamboo fibers (reinforcement) and parenchyma tissue (matrix). Based on raw bamboo, man-made bamboo fibrous composites such as engineered bamboo and bamboo fiber reinforced polymeric composites have emerged. These materials eliminate some inherent shortcomings of bamboo, which can provide more stable material performance and achieve dimension alteration according to requirements. All of these bamboo fibrous composites have orthotropic structure, the mechanical properties of which in the longitudinal direction far exceed that in the transverse direction, and the tensile strength and compressive strength can reach 111–114.8 and 104.7–115.7 MPa respectively. Besides, the interfacial zone between fiber and matrix is weak, in which some defects are usually existed, such as micro-cracks and adhesive voids. As a result, cracks in the composites are inevitable, which are easy to extend in the longitudinal direction and result to fracture failure. Consequently, fracture studies become necessary for evaluating the crack-resistance ability of bamboo fibrous composites. This paper systematically introduces the previous research works on the fracture properties of bamboo fibrous composites. Firstly, this paper summarized the test methods for different fracture modes including Double cantilever beam (DCB), Single-edged notched beam (SENB), Compact tension (CT), etc., meanwhile the corresponding calculation theories such as Compliance calibration (CC), Modifed compliance calibration (MCC) and Modified beam theory (MBT) are also elucidated. Subsequently, the fracture behaviors and mechanisms of various bamboo fibrous composites are analyzed, in which the fracture parameters and crack-resistance capabilities are compared. In addition, considering the limitations of current research, the future research in need on the fracture properties of bamboo fibrous composites are discussed, the conclusion can be used in providing important reference for the safety evaluation of bamboo fibrous composites.

Assembling Zn2+ on surface of kevlar NanoFibers as functionalized separator for dendrite-free Zn anode and high-performance Zn metal batteries

The proliferation of wearable electronic products has made the development of flexible energy storage devices increasingly important. Among various options, aqueous Zn metal batteries are promising due to their low cost, safety, non-toxicity, environmental friendliness, and abundant element storage. However, the growth of dendrites on the anode surface of Zn metal batteries is a significant challenge to efficient transport of Zn2+, which leads to reduced battery capacity or even failure. This limitation hinders practical applications for these batteries. This study develops a functionalized separator with high ion transport performance and excellent mechanical performances by electrostatic assembling deprotonated PPTA− molecular chains with Zn2+ using Kevlar as a raw material. The successful assembly of Zn2+ is confirmed through chemical composition and functional group analysis. The prepared separator significantly shows the increased the transfer number of Zn2+ and decreased activation energy of Zn2+ deposition. The separator is used in Zn||Zn symmetric batteries, achieving a reversible deposition/stripping process of more than 180 h at a current density of 1 mA cm−2 and a deposition capacity of 1 mAh·cm−2. When applied to Zn||MnO2 batteries, an initial discharging specific capacity of up to 220 mAh·g−1 is obtained at a high current density of 2 A g−1, and the battery could be cycled stably for more than 1, 200 cycles, with a smooth and flat Zn anode surface and no obvious dendrite growth. The separator has good potential for flexible energy storage applications due to its excellent mechanical strength, which enables it to maintain high electrochemical activity even after bending 500 times at 180°. Overall, the functionalized separator effectively inhibits dendrite growth, improves the electrochemical stability of Zn metal batteries, and demonstrates significant application potential for other flexible energy storage applications.

Increasing softening point of isotropic spinnable asphalt by two-step preparation from ethylene tar residue modified with bio tar residue

Carbon fiber prepared from asphalt may be a critical type of low-weight chemical material of excellent mechanical performances. Here, an isotropic spinnable asphalt (EBA) was prepared from ethylene tar residue (ETR) by a two-step method of bio tar residue (BTR) through co-polymerization-air blowing, and isotropic asphalt carbon fibers were prepared from the product asphalt. The main types of molecules contained in the two feedstocks were determined by 1H NMR and LDI-TOF MS analyses. The co-polymerization behavior and thermal stability of the asphalt blends were analyzed using polarized light microscopy and thermogravimetric analyzer, respectively. Finally, the morphology and mechanical properties of carbon fibers made from EBA were evaluated by scanning electron microscope and universal testing machine. The results show that BTR contains much more oxygen-containing functional groups and unique long-chain sawtooth molecular structure; the former can promote the reactivity of the co-polymerization reaction, and the latter can limit the formation of large-plane cyclic aromatic hydrocarbon polymers due to the spatial resistance effect to greatly reduce the content of the mesophase. The softening point of the product asphalt shows a tendency of increasing and then decreasing with the increase of the proportion of BTR in the raw material mixture, and a reaction mechanism based on linear and bulky molecules is proposed to explain it by analogy with the structure of macromolecules. Consequently, the EBA made from a mixture of ETR and BTR at ideal moderate proportions has excellent performance in terms of softening point and mesophase content than ETRO made from ETR alone, and the carbon fiber made has a high tensile strength of 1.82 GPa with an elastic modulus of 38.9 GPa, which has reached the level of high tensile strength carbon fiber. Therefore, the use of inexpensive renewable energy bio-asphalt in the modification of heavy oil to obtain high softening point isotropic asphalt for preparing isotropic carbon fiber with excellent properties is achievable.

Investigation on the effects of deposition strategy on the mechanical characteristics and microstructure of austenitic stainless steel 308LSi manufactured via wire arc additive manufacturing

This study examines the impact of deposition strategy on ASS 308LSi thin-walled structure manufactured via the Wire Arc Additive Manufacturing (WAAM) technique on the microstructural characteristics, tensile strength, microhardness, Charpy impact testing, and fracture morphology of the WAAM 308LSi. The analysis of the microstructure reveals that the deposition strategy promotes transitioning from columnar to equiaxed fine grain structure. The tensile strength results show that specimens with a 45° deposition strategy exhibit lower anisotropy and higher tensile properties compared to those with a 0° deposition strategy, with improvements of 33.1% in the transverse direction and 26.7% in the longitudinal deposition directions, respectively. The microhardness in WAAM SS308LSi demonstrates variations in the bottom, middle, and top regions, with the highest average value observed at a 45° deposition strategy (213.3 ± 6.5 HV, 201.1 ± 10.7 HV, and 191.5 ± 5.2 HV) as well. The impact testing results indicate that the highest absorbed energy occurs at a 45° deposition strategy, with 75 ± 4.2 J and 74 ± 4.0 J for the transverse and longitudinal directions, respectively. The fractures observed during testing exhibit ductile characteristics, with the presence of dimples and particles. This study demonstrates the significant potential of the 45° deposition strategy with the implementation of double-sided substrate deposition, resulting in a refined microstructure, nearly isotropic behavior, and excellent mechanical performance.

Effect of deep cryogenic treatment on microstructure and mechanical properties of Fe50Mn30Co10Cr10 high entropy alloy fabricated by laser metal deposition

As a typical additive manufacturing technology, laser metal deposition (LMD) has been widely used to process high entropy alloy (HEA). However, deposited products usually have poor mechanical properties. In this study, deep cryogenic treatment (DCT) is proposed to enhance the mechanical performance of the laser melting deposited Fe50Mn30Co10Cr10 high entropy alloy. The tensile strength, hardness and strength-plasticity product of the HEA after 48 h of immersion by DCT processing were 901 MPa, 286HV and 33.3 GPa%, respectively. Compared with the as-deposited state, the HCP ε phase content and tensile strength of the deep cryogenic treated (DCTed) sample increased by 2.2 times and 30.8 %, respectively, while the average grain size decreased by 58.0 %. Furthermore, the friction coefficient and wear rate of the DCTed samples reduced by 25.6 % and 30.3 %, respectively. The high temperature gradient soaking in liquid nitrogen induced dynamic recrystallization, refining the FCC γ grain structures in the HEA. Meanwhile, the samples contained more HCP ε phase due to the transformation-induced plasticity (TRIP) effect triggered by thermal stress. The results indicate that the effects of fine-grain strengthening and second-phase particle dispersion strengthening contribute to the excellent mechanical performance. DCT processing provides a simple and effective way for overcoming the strength-ductility trade-off in the additive manufactured Fe50Mn30Co10Cr10 HEA.

Convection Heat Transfer and Performance Analysis of a Triply Periodic Minimal Surface (TPMS) for a Novel Heat Exchanger

Heat exchangers are necessary in most engineering systems that move thermal energy from a hot source to a colder location. The development of additive manufacturing technology facilitates the design and optimization of heat exchangers by introducing triply periodic minimal surface (TPMS) structures. TPMSs have shown excellent mechanical and thermal performance, which can improve heat energy transfer efficiency in heat exchangers. This current study intends to design and develop efficient, lightweight heat exchangers for aerospace and space applications. Using the TPMS structure, a porous construction encloses a horizontal tube that circulates heated fluid. Low-temperature water circulates inside a rectangular box that houses the complete system to remove heat from the horizontal pipe. Three porous structures, the gyroid, diamond, and FKS structures, were employed and examined. Porous models with various porosities and surface areas (15 cm2 and 24 cm2) were investigated. The results revealed that the gyroid structure exhibits the highest Nusselt number for heat removal (Nu max = 2250), confirming the highest heat transfer and lowest pressure drop among the three structures under investigation. The maximum Nusselt number obtained for the FKS structure is less than 1000, whereas, for the diamond structure, it is near 1250. A linear variation in the average Nusselt number as a function of the structure surface area was found for the FKS and diamond structures. In contrast, nonlinearity was observed in the gyroid structures.

Influence of overlap rate and ultrasonic compaction on mechanical performance for additive manufacturing of continuous carbon fiber reinforced polylactic acid composites

The additive manufacturing (AM) technology of continuous fiber reinforced thermoplastic composites (CFRTPCs) is gaining attention in the composites industry due to its excellent mechanical performance, potential designability and light-weight structures. Process parameters of AM have a significant impact on mechanical performance and forming accuracy. There is still a Lack of clear design basis for hatch spacing as structural parameter. This study focused on the effect of overlap rate on AM performance of continuous carbon fiber (CCF)/polylactic acid (PLA) composites through the mechanical test, surface morphology, defects characterization and failure analysis, and investigated the adaptability of ultrasonic compaction post-treatment to overlap rate settings. The results indicated that proper overlap state between filaments was beneficial for mechanical properties. The optimal interlaminar shear strength (ILSS) increased to 22.72 MPa by 56 % and tensile strength improved by 17.5 % to 721.65 MPa at various overlap rates. Surface roughness remained stable at approximately 33 μm. Ultrasonic compaction achieved a maximum ILSS up to 24.84 MPa with improvement of 10.26 %, and tensile strength reached to 736.76 MPa with 2.09 % improvement, showing excellent adaptability to overlap rate design. Research findings will improve understanding of digital adjustment mechanisms for printing parameters and provide design guidance for high-performance AM technology of CFRTPCs.

Investigation on mechanical and tribological properties of PTFE nanocomposites reinforced by surface-modified graphene using molecular dynamics simulations

Surface-modified nanoparticles are commonly used to improve the mechanical properties and wear resistance of polytetrafluoroethylene (PTFE). However, fewer studies have been devoted to quantitatively revealing the action mechanism of graphene (Gr) modified with different functional groups on the mechanical and tribological properties of PTFE. Herein, the effects of four functional groups (−OH, −NH2, −COOH, and −COOCH3 functional groups) on the surface of Gr nanosheets on the mechanical and tribological properties of PTFE nanocomposites are studied using molecular dynamics simulations. The results indicate that the incorporation of functional groups to the Gr surface is able to significantly improve the mechanical properties and wear resistance of the nanocomposites, and the COOH-functionalized Gr nanosheet shows the best reinforcing effect due to the synergistic effect of its own high surface roughness and strong interfacial interaction between itself and the matrix. It is also found that the friction coefficient of the nanocomposites is obviously increased by the inclusion of functionalized Gr nanosheets, and the greater the surface roughness of the functionalized Gr nanosheet, the more significant the growth of the friction coefficient of the nanocomposites. The pull-out test and confined shear simulation reveal that due to the increased interfacial shear strength and the isolation of functional groups, an inhomogeneous transfer film is formed at the friction interface, leading to a decreased anti-friction property. This study provides some guidance for the future design and development of polymer nanocomposites with excellent mechanical and tribological performance for use in extreme service conditions.

Precision depth-controlled isolated silver nanoparticle-doped diamond-like carbon coatings with enhanced ion release, biocompatibility, and mechanical performance

Silver doped diamond-like carbon (Ag/DLC) coatings are in high demand for biomedical applications such as artificial implants, surgical instruments, and medical devices. However, recent reports indicate that the excess Ag concentration required in typically made Ag/DLC coatings significantly reduces their mechanical performance and biocompatibility. Here, we propose a novel single-step approach to precisely dope small quantities of Ag in the form of isolated nanoparticles embedded at defined depths in a DLC matrix. This new Ag/DLC coating architecture is designed to release controlled Ag ion levels to fight infection in the early post-surgery stages, while a confined Ag amount maintains the excellent mechanical and biocompatibility performance of the underlying DLC coating when compared to typically made Ag/DLC coating designs. Coatings of pure DLC, typically made Ag/DLC with Ag doped throughout the carbon matrix and the new Ag/DLC design with precise Ag doping, are made using a modified magnetron sputtering system. The coatings are characterised for structural, mechanical, ion leaching, and biocompatibility profiles against L929 fibroblast cells. Results indicate that the new Ag/DLC coating requires only 2 at.% Ag to release a similar level of Ag ions (∼0.6 ppm) to a typical Ag/DLC coating with a much higher Ag content of 17 at.%. The new Ag/DLC coating design also outperforms the typical design with a 63 % increase in hardness, 100 % higher Young's modulus, and 21 % higher biocompatibility. The enhanced biomechanical performance of the proposed new Ag/DLC architecture could have significant potential for coating of future medical devices.