Pencil sketch graphene films as solid lubricant on steel surface: Observation of transition to grapehene/amorphous carbon

During recent years, graphene as a solid lubrication material have been thoroughly studied under nano or micro scales, but rarely reported at industrial conditions. In present work, graphene films as solid lubricant were prepared on the surface of 201 stainless steel substrates by pencil sketch. And then the friction tests from 5 to 65 N were carried out via a homemade tribo-tester and used GCr15 balls (ø = 5 mm) as friction pairs. Not surprisingly, graphene films cannot bear the loads beyond 5 N, but interestingly, via gradually increasing the loads, graphene films show prominent load performance and steady state of friction coefficients at about 0.12 while the loads varied from 5 to 65 N. Compared with bare steel, the coefficient of graphene films reduced by about 80%, and the wear volume reduced to 1/28 when variable load (from 5 N to 30 N) were applied. Raman spectra shown that the structure of graphene had been changing into diamond-like carbon films with graphene distributed inside, which was confirmed by HRTEM that graphenes were coming with amorphous carbon. Considering the roughness of steel wafers (170 nm), one can speculate that, with graphene films' protection, the steel has no abrasion but plastic deformation instead. It is concluded that the shearing force induced the film densification via sp² to sp³ changing that enforced cross-linking. This cross-linking carbon matrix was responsible for high load bearing and the graphene exfoliated into graphene under shearing force contribute to low steady-state friction. Benefiting from sketch, one can get a lubrication film on any substrates with complex topography, our results shed light on the growth of graphene films for industrial use.     

多元醇对对苯二异氰酸酯基聚氨酯微孔弹性体的形态与性能影响

以对苯二异氰酸酯(PPDI)、1,4-丁二醇、水、聚四氢呋喃醚多元醇(PTMEG)和氢化端羟基丁二烯多元醇(HLBH)为原料,采用两步法制备出聚氨酯微孔弹性体样品。通过傅里叶变换衰减全反射红外光谱(FTIR-ATR)、动态机械分析(DMA)、差示扫描量热仪(DSC)、万能材料试验机等技术手段对样品的微相分离、耐低温性能、动态生热进行了系统表征。结果表明,两种多元醇结构对泡孔尺寸影响不大,微孔尺寸在100~300μm之间,其中以150μm尺寸左右的泡孔居多;HLBH制备的聚氨酯微孔弹性体硬段形成的氢键数量多于PTMEG制备的微孔弹性体,具有更好的微相分离;由于较好的微相分离结构,HLBH样品在-30~150℃具有很宽的模量平台区,而PTMEG样品受软段的低温结晶影响,在0℃以下模量急剧上升,HLBH样品低温下的刚度变化优于PTMEG样品;同时HLBH样品的滞后生热亦小于PTMEG样品,具有更好的动态疲劳性能。     

Tensile testing of freestanding polymer thin films with non-standard geometries

While an increasing requirement from both fundamental and application, a direct tensile testing of freestanding polymer thin films in the air remains a challenge, mainly due to their extreme fragility. This study aims to establish a facile and reliable testing system, and investigate the effect of specimen geometry on the testing. First, a thin film transfer and adhesive guide frame gripping technique is introduced. With low length-to-width ratio specimens, tensile testing of rectangle non-standard polystyrene films with thickness down to 45 nm is achieved on a commercial universal testing machine. By changing specimen width and gauge length individually, a scale law between system compliance and specimen width is then verified. After corrected by compliance, a constant modulus of ~3.15 GPa for 45–4319 nm thick polystyrene films can be obtained. This study provides a potential strategy to overcome operational difficulties in performing tensile testing of freestanding polymer thin films.     

On the strain measurement and stiffness calculation of carbon fibre reinforced composites under quasi-static tensile and tension-tension fatigue loads

The applicability of different strain measurement techniques for carbon/epoxy laminates under quasi-static tensile and tension-tension fatigue loads was studied. Strain gauges, mechanical extensometers, digital image correlation and 2 D camera systems were applied on laminates tested at angles of 0°, 45°, 60°, 90° and ±45°. In addition, displacements recorded by the servo-hydraulic piston were monitored and compared to local strain measurement techniques. Representative examples that illustrate characteristics and limits of each technique in quasi-static and fatigue tests are discussed. Influences of the respective method of strain measurement, the specimen surface, fibre direction and processes in the specimens during tests on the recorded stress-strain behaviour and on the calculated stiffness are presented. Recommendations for accurate strain measurement of anisotropic laminates based on the results are made.     

Temperature-dependent ratcheting of PTFE gaskets under cyclic compressive loads with small stress amplitude

Uniaxial stress-controlled ratcheting experiments on PTFE gaskets under cyclic compressive loads with small stress amplitude were performed. The effect of temperature on the deformation behavior was considered. Results showed that the compressive modulus decreases rapidly when the temperature increases from 100 °C to 200 °C. Compressive ratcheting deformation with cycles increase significantly with the increases of temperature. The ratcheting deformations at 100 °C, 150 °C and 200 °C are nearly two, three and five times that at room temperature, respectively. Most of ratcheting deformation mainly occurs during the first 20 cycles because the subsequent ratcheting rate and strain range are small and much less than those in the previous cycles. The accumulated deformation under cyclic loads with small stress amplitude is relatively approach to the static compressive creep with the same peak stress. Therefore, the accumulated deformation with time of PTFE gaskets obtained by cyclic compression with small stress amplitude can be estimated by the corresponding static creep deformation with good accuracy under the approximate stress rate and the same temperature, especially at room temperature.     

Elastic coils: deformation micromechanics of coir and celery fibres

Natural fibres, such as flax and hemp, are typically chosen as reinforcing elements in composites to replace traditional glass fibres due to their high stiffness, strength and low strain to failure. Some plant fibres such as coir and celery however possess high strains to failure, which could be utilised in a composite to enhance toughness. This paper reports on the use of Raman spectroscopy to follow the molecular deformation of single fibres of coir and celery. The technique is also used to characterise the orientation of the cellulose structure within the fibres. It is shown by mechanical testing of fibres that both celery and coir possess a non-linear stress–strain curve. Coir fibres however exhibit high strain to failure, whereas celery fibres are shown to have a much lower value of this parameter, despite having a similar coiled fibrillar structure. It is shown by using polarised Raman spectroscopy, and rotating the specimens with respect to the polarisation axis of the laser and measuring the intensity of the 1095 cm⁻¹ Raman band, that both celery and coir fibres combine both axial and transverse orientation, due to their coiled structures. This is also confirmed by birefringence measurements. By following the shift in the central position of this Raman band as a function of cyclic deformation of the fibres, it is shown that the coir fibres recover their molecular deformation, whereas the celery does not show the same level of recovery. This difference between the fibres is postulated to be due to the fact that coir possesses an interlaced fibrillar structure, which remains intact, whereas the celery sub-fibrils unravel and orient towards the fibre axis during deformation.     

Modelling of micro-electrochemical machining parameters used for machining of holes on copper plate

The electrochemical machining is a non-conventional machining process based on the electrolysis principle. It is used to machine various features on conducting engineering materials with the required accuracy and precision. So an attempt has been made to machine micro-holes on a 300 μm thick copper plate as an anode and a hollow stainless steel tool electrode of 250 μm diameter as the cathode. The machining operation is performed on the in-house developed micro-ECM experimental setup with controlled machining parameters voltage, concentration, and duty factor varied in three levels. A full factorial experimental plan is used to study the output responses material removal rate (MRR), radial overcut (ROC), circularity, and taper angle (TA). Later an adaptive neuro-fuzzy inference system (ANFIS) model has been developed and shown the effectiveness of the developed model in the paper. The Sugeno fuzzy model has been used in ANFIS to generate the fuzzy rules required for the model. Out of 27 experiments, 22 machining data are used for training the model, and the remaining 5 machining data are used for testing the developed model. The average error observed between the ANFIS predicted values and experimental values of MRR is 12.56%, circularity is 43.09%, ROC is 13%, and TA is 27.53%, respectively.     

Effect of flame-sprayed Al–12Si coatings on the failure behaviour of pressurized fibre-reinforced composite tubes

Internal pressurization testing at room temperature was performed to evaluate the effect of flame-spraying aluminum-12silicon on the leakage behavior of fibre-reinforced composite tubes fabricated by filament winding, and covered with garnet sand. A group of specimens were completely coated with flame-sprayed metal particles and another set remained uncoated. t-tests were conducted at a significance level of α = 0.05 to analyze the effect of the coating on the leakage behavior of the specimens. Ultraviolet imaging was used to illustrate a characteristic leakage pattern of the composite tubes. Coated specimens exhibited slightly higher mean leakage and burst pressures than the uncoated specimens. The t-test results provided strong statistical evidence that the flame spraying process employed did not damage the filament wound tubes covered with garnet sand.     

Temperature dependence of the formation of the passivation layer on carbon steel in high alkaline environment of concrete pore solution

In this study, the formation and the strength of the passive layer on carbon steel in concrete pore solution were investigated for temperatures ranging from room temperature to 850 °C. Two sets of carbon steel specimens were first exposed to different temperatures for an hour. One group cooled to room temperature in the air and the other one in the water. The steel specimens were then immersed in concrete pore solution and the passive layer formed on the steel was investigated. Results showed that in the extreme conditions, i.e. very high temperature, and water-cooled condition, the passive layer rarely formed on the steel. Increasing the exposure temperature led to a decrease in the thickness of the passive layer. This decrease was more apparent when steel was cooled in water compared to air cooled specimens. Results from this study contribute to the knowledge base that may be used to advance the state of the art in predictive models for steel reinforcement in concrete exposed to high temperatures (e.g., fire, power plants).     

Doping of three-dimensional porous carbon nanotube-graphene-ionic liquid composite into polyaniline for the headspace solid-phase microextraction and gas chromatography determination of alcohols

In this work, ionic liquid (IL, i.e. 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate), carboxyl multiwall carbon nanotubes (MWCNTs) and reduced graphene oxide (rGO) were used to prepare three-dimensional porous material (MWCNTs-rGO-IL) by one-step self-assembly, then it was co-electrodeposited with polyaniline (PANI) on stainless steel wires by cyclic voltammetry. The resulting coating (PANI-MWCNTs-rGO-IL) was characterized by using FT-IR and scanning electron microscopy etc, and it showed porous structure and had high thermal stability. Furthermore, it was found to be very suitable for the headspace solid-phase microextraction of alcohols (i.e. octanol, nonanol, geraniol, decanol, undecanol and dodecanol). By coupling with gas chromatography, wide linear ranges and low limits of detection (i.e. 2.2–28.3 ng L⁻¹) were obtained for the alcohols. The coating also presented good repeatability and reproducibility; the relative standard deviations for intra-fiber and fiber-to-fiber were less than 5.6% (n = 5) and 7.0% (n = 5) respectively. In addition, the proposed method was successfully applied to the determination of alcohols in tea drinks, and the recoveries for standards added were 85.6–114%.     

Synthesis of NIPU by the carbonation of canola oil using highly efficient 5,10,15‐tris(pentafluorophenyl)corrolato‐manganese(III) complex as novel catalyst

For the green synthesis of polyurethane (PU), non‐isocyanate routes are worthy alternatives. In the present work, we have explored 5,10,15‐tris(pentafluorophenyl)corrolato‐manganese(III) complex as novel catalyst for coupling reaction between epoxidized canola oil and CO₂ (gaseous) to introduce cyclic carbonate moieties in the oil and further used it to obtain non‐isocyanate PU, generally abbreviated as NIPU, by curing with different diamines. The results obtained indicated a 1/4th of the reduction in reaction time with the use of 5,10,15‐tris(pentafluorophenyl)corrolato‐manganese(III) complex as catalyst as compared to the previously reported literature data. As per the reported studies, the corrole metal complex has not been used for this reaction earlier. The structure of products and intermediates were confirmed by using different characterization techniques like ¹H NMR and FTIR spectroscopies. The thermal and mechanical behavior of final product was analyzed by TGA and universal testing machine, respectively. The non‐isocyanate PU obtained showed a good thermal stability up to 200°C and a tensile strength of up to 8 MPa. The effect of structure of diamines on the properties of non‐isocyanate PU was also extensively studied.     

A comparative study of sequentially layer-deposited and co-deposited Co–Mn oxides as potential redox capacitors

Layers of cobalt and manganese oxides were co-deposited or deposited on top of each other or next to each other by potentiostatic method onto stainless steel substrate. Deposition potentials of 1 and −1 V for the anodic and cathodic depositions were employed. Specific capacitance values in the range of 38.5–78 F g⁻¹ were found with cobalt oxide on top of manganese oxide having the lowest and manganese oxide on top of cobalt oxide having the highest capacitances. The usefulness of the electrodes was characterized by cyclic voltammetry, charge–discharge cycling, and electrochemical impedance spectroscopy in 2 M NaOH electrolyte for redox supercapacitor applications. The latter presented the best charge/discharge behavior with no voltage drop due to lower ohmic resistance in prepared substrate; although the steadiest current observed in the course of voltammetry was due to the former. The evaluated double layer and specific capacitances for co-deposited sample according to the impedance studies were 1.75 and 47.5 F g⁻¹, respectively, being in good agreement with voltammetric measurements.     

Protective coatings for stainless steel for SOFC applications

In this paper, a coating procedure based on spin coating of metal oxide polymer precursors on stainless steel, which decreases the oxide scale growth rate, is evaluated. The yttrium and cobalt solutions were used as polymer precursors, while a ferritic stainless steel Crofer 22 APU was used for the deposition of protective coatings. The thickness of deposited protective film was about ~500 nm. The effectiveness of protective layer was evaluated by cyclic thermogravimetry, oxide scale electrical conductivity, and X-ray diffractometry. The results show that steel coated with yttrium polymer precursor has better properties than uncoated or cobalt-coated sample.     

Influence of strain rate on the compressive yield stress of CMDB propellant at low,intermediate and high strain rates

The mechanical properties of composite modified double base (CMDB) propellant significantly depend on the strain rate. In particular, the yield stress increases dramatically at higher strain rates. To study this behaviour, low, intermediate and high strain rate compression testing (1.7 × 10⁻⁴ to 4 × 10³ s⁻¹) of CMDB propellant at room temperature was conducted by using a universal testing machine, a hydraulic testing machine and a split Hopkinson pressure bar (SHPB) system, respectively. The yield stress was observed to increase bilinearly with the logarithm of strain rate, with a sharp increase in slope at a strain rate of 5 × 10¹ s⁻¹, which was supported by dynamic mechanical analysis (DMA) testing. The Ree-Eyring model, involving two rate-activated processes, was employed to predict the yield behaviour of CMDB propellant over a wide range of strain rates. The predictions are in excellent agreement with the experimental data.     

Fatigue crack propagation mechanisms in poly(methyl methacrylate) by in situ observation with a scanning laser microscope

Crack propagation tests were performed on an amorphous polymer, poly(methyl methacrylate), to investigate fatigue crack propagation mechanisms. A scanning laser microscope with a newly developed tensile testing machine was used to observe in situ crack propagation in compact‐type specimens. A crack usually propagated within the craze located at the crack tip under both static and cyclic loading conditions. When a crack stably propagated into the craze under static loading conditions, bright bands composed of the broken craze were observed at the edges along the crack wakes. However, there were successive ridges and valleys in place of bright bands along the crack wakes under cyclic loading conditions. When stable fatigue cracks were propagated at the loading half‐cycle in each cycle, new craze fragments appeared that were similar to the bright bands under static loading. However, the thickness of these fragments decreased in the following loading cycle, and a new valley was formed. This suggested that the valleys were formed by the contact between the fracture surfaces near the crack tip during unloading. Fatigue crack propagation is thought to be due to fibrils weakened by crack closure between fracture surfaces. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 3103–3113, 2001     

Ageing mechanism and mechanical degradation behaviour of polychloroprene rubber in a marine environment: Comparison of accelerated ageing and long term exposure

Polymers are widely used in marine environments due to their excellent properties and good weathering resistance. Despite this extensive use, their long term behaviour in such an aggressive environment is still not well known. To assess the polymer durability within reasonable durations, it is essential to perform accelerated ageing tests to accelerate the degradation kinetics but without any modification of the degradation process. This study therefore proposes and validates accelerated ageing tests to study marine ageing of a silica-filled chloroprene rubber (CR) used for offshore applications. Several accelerated ageing protocols are investigated for temperatures ranging from 20 to 80 °C in renewed natural seawater. The ageing consequences are characterized using physical measurements (FTIR, solid state NMR) and mechanical testing based on monotonic tension tests. Instrumented micro-indentation tests are also employed, in order to describe accurately the ageing gradients through sample thickness. The measurements obtained on the samples cut from accelerated specimens are compared to those obtained from the topcoat of an offshore flowline aged under service conditions for 23 years. For both kinds of specimens, polychloroprene develops rapid material changes most clearly represented by a considerable increase in stiffness, which allows the accelerated ageing protocols to be validated.     

Breakdown potential modelling of austenitic stainless steel

The breakdown potential is a crucial factor in the study of pitting corrosion resistance of stainless steel. This work aims to demonstrate the advantage of different chemometric techniques to estimate the breakdown potential of austenitic stainless steel. In order to predict pitting corrosion behaviour of stainless steel, a total of 60 samples of this alloy were subjected to electrochemical tests varying chloride ion concentration, pH and temperature. The experimental values of the breakdown potential, in addition to the tested environmental factors, were used to construct the predictive models based on support vector machines and artificial neural networks. A multiple‐comparison study based on statistic tests was applied to determine the optimal configuration for each technique. According to the results, support vector machines became a suitable and reliable technique to be applied in the modelling of the breakdown potential of austenitic stainless steels. This technique outperformed the models based on artificial neural networks and provided a useful tool to compare the pitting corrosion resistance of stainless steel in different environmental conditions without recourse to polarization tests. Therefore, this model presented a relevant meaning in science and engineering applications. Copyright © 2014 John Wiley & Sons, Ltd.     

Investigation of infill-patterns on mechanical response of 3D printed poly-lactic-acid

This paper presents the effect of infill patterns (IPs) on the mechanical response of 3D printed specimens by conducting the low-velocity impact test (LVI) and compression test. The poly-lactic acid (PLA, purity 98 wt% >) material has selected and printed using fused deposition modeling (FDM, speed 20 mm/s, layer height 0.2 mm, no of layers 30, extruded at 200 °C) with four different IPs: triangle, grid, quarter cubic, and tri-hexagon. The LVI test on velocity-time, energy-time and force-displacement, and the compression responses have examined and presented in this study. The LVI test was carried out to determine the penetration energy level, energy absorption capacity (toughness), stiffness, and strength of PLA porous parts (60% infill density) for implant/tissue/recyclable product applications. The results have shown that the triangular pattern has produced the highest absorbed energy in LVI test (penetration energy 7.5 J, and stiffness 668.82 N/mm) due to more sheared/contact layers’ perpendicular to impactor (hemispherical insert); while the grid pattern exhibited the highest compressive strength (72 MPa) due to more layers aligned along the compressive loading direction The SEM fracture surface image of Triangular IP has produced effective raster and layer bonding, less number of voids, more amount of circular beach markings, and absence of ratchet lines leading to possess improved mechanical properties.     

Analysis of Molecular and Morphological Effects on Slow Crack Growth in Modern PE Pipe Grades by Cyclic Fracture Mechanics Tests

Summary: Three different polyethylene (PE) pipe grades as well as three different lots of one of the grades were investigated by cyclic tests with cracked round bar (CRB) specimens, concerning resistance to slow crack growth. To enhance the test sensibility and proof its applicability for a quick quality assurance method various molecular and morphological characterizations on compression molded plates were carried out, with special attention on the influence of molecular and morphological differences, as well as lot to lot variations on the resistance to slow crack growth. The cyclic CRB tests allowed a ranking of the different pipe grades and lots with short testing times per material and testing machine, as a function of failure time as well as of crack initiation time with further reduction of testing time of about 50%. Moreover the ranking corresponded to the expectations based on the molecular and morphological properties of the materials, where only minor changes in the molecular mass distribution and the co-monomer concentration in case of lot to lot variations were proofed reliably.