The effect of thermal stressing on perfluoropolyalkylethers at elevated temperatures

Fultz et al. have reported that the thermo‐oxidative properties of linear PFPAEs can be improved by stressing the fluid at elevated temperature (371°C) in the presence of air. A study of M‐50 steel coupons exposed to unstressed and stressed linear PFPAE fluids at 260 °C and 330 °C each reveal complex surface layers. For the coupon exposed to the unstressed fluid at 260 °C, a subsurface layer is observed between the iron oxide and iron substrate that has been characterized as being composed of FeF₂. In contrast, the coupon exposed to the stressed fluid has a marked increase in the iron oxide thickness ∼2–3 times) when compared to the unstressed sample and shows no evidence of a buried fluorine‐containing layer. An increase in temperature (330 °C) in the stressed fluid O–C test was required to form a subsurface FeF₂ layer. It is proposed that the elimination of the fluorine layer found on the M‐50 substrate increases the upper temperature limit found from the oxidation–corrosion studies. The increase in the oxide layer thickness implies that the FeF₂ layer found in the unstressed sample acts like a diffusion barrier which inhibits the outward movement of Fe⁰ and the decreased rate of iron oxide growth. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of Ag and Ti3SiC2 on Tribological Properties of TiAl Matrix Self-lubricating Composites at Room and Increased Temperatures

Dry sliding tribological properties of TiAl matrix self-lubricating composites (TMSC) containing Ag, Ti₃SiC₂, Ag and Ti₃SiC₂ were investigated from 25 to 800 °C under ball-on-disk test conditions against Si₃N₄ counterface under the same conditions of 10 N-0.234 m/s. The results indicated that the tribological properties were strongly dependent on the lubricant additives. TMSC with the addition of Ag and Ti₃SiC₂ (TAT) exhibited the lower friction coefficients (0.32–0.43) and less wear rates (1.23–4.13 × 10^?4 mm³ N^?1m^?1) in the wide temperature range of 25–800 °C. The excellent tribological properties of TAT over the wide temperature range were attributed to the synergetic effect of Ag and Ti₃SiC₂ lubricants, silver diffusion forming a rich-silver smooth tribo-film on the frictional surface of TAT at low and moderate temperatures from 25 to 400 °C, while Ti₃SiC₂ oxidation reaction forming rich-oxide tribo-film on the worn surface of TAT at higher temperatures of 600 and 800 °C.     

Tribological Behavior of Ti3AlC2 Against SiC at Ambient and Elevated Temperatures

In this paper, we reported the tribological behavior of Ti₃AlC₂ disk sliding against SiC ball from room temperature (RT) to 1,000 °C. The tribological properties are highly dependent of testing temperature. At RT, the coefficient of friction (CoF) is as low as 0.34 in the steady state, but the wear rate is relative high (4.26 × 10^?4 mm³/Nm). At 200 and 400 °C, the CoF is as high as 1.21, and the wear rates are very high, about on the order of 10^?3 mm³/Nm. From 600 to 1,000 °C, however, Ti₃AlC₂ exhibits quite low wear rate on the order of 10^?6 mm³/Nm and relative moderate CoF, 0.60–0.80. The compacted continuous oxide layer at 600 °C and above might be responsible for the outstanding wear resistance.     

Biobased Polyalphaolefin Base Oil: Chemical,Physical, and Tribological Properties

The properties of a biobased polyalphaolefin with a viscosity of 40 cSt at 100 °C (BPAO-40) were investigated relative to a commercial petroleum-based PAO of similar viscosity at 100 °C (PAO-40). BPAO-40 was synthesized by oligomerization of a mixture of alpha olefins, with and without terminal methyl esters. These olefins were obtained from vegetable oils via a biorefinery process. In contrast to BPAO-40, commercial PAO-40 is synthesized only from non-functionalized alpha olefins. Thus, BPAO-40 is not only biobased, but also has a unique chemical structure, which makes it a functionalized PAO. The effect of chemical structure (presence or lack of methyl ester functionalization) on chemical, physical, and tribological properties of these two base oils was investigated. The investigation showed that, relative to the commercial non-functionalized PAO-40, the functionalized BPAO-40 displayed the following properties: higher density at 40–100 °C, lower number average molecular weight, higher polydispersity index, higher viscosity index, lower oxidation stability (pressurized differential scanning calorimetry), higher total acid number, higher free fatty acid, lower four-ball anti-wear coefficient of friction (COF) and lower wear scar diameter (WSD), higher elastohydrodynamic (EHD) lubricant film thickness under boundary conditions (low speeds and high temperature), lower EHD traction coefficient at 40 and 100 °C, similar pressure–viscosity coefficient, lower COF, lower WSD, and higher relative film thickness on a high-frequency reciprocating rig tribometer under boundary conditions (low speeds).     

Design and construction of a new temperature-controlled chamber for light and confocal microscopy under monitored conditions: biological application for plant samples

A new light microscope–temperature‐controlled chamber (LM–TCC) has been constructed. The special feature of the light microscope–temperature‐controlled chamber is the Peltier‐element temperature control of a specimen holder for biological samples, with a volume capacity of 1 mL. This system has marked advantages when compared to other approaches for temperature‐controlled microscopy. It works in a temperature range of −10°C to +95°C with an accuracy of ±0.1°C in the stationary phase. The light microscope–temperature‐controlled chamber allows rapid temperature shift rates. A maximum heating rate of 12.9°C min⁻¹ and a maximum cooling rate of 6.0°C min⁻¹ are achieved with minimized overshoots (≤1.9°C). This machinery operates at low cost and external coolants are not required. Especially with samples absorbing irradiation strongly, temperature control during microscopy is necessary to avoid overheating of samples. For example, leaf segments of Ficaria verna exposed to 4500 μmol photons m⁻² s⁻¹ in a standard microscopic preparation show a temperature increase (δT) of 18.0°C, whereas in the light microscope–temperature‐controlled chamber this is reduced to 4°C. The kinetics of microscope‐light induced δT are described and infrared thermography demonstrates the dissipation of the temperature. Chloroplasts of the cold adapted plant Ranunculus glacialis show the tendency to form stroma‐filled protrusions in relation to the exposure temperature. The relative number of chloroplasts with protrusions is reduced at 5°C when compared to 25°C. This effect is reversible. The new light microscope–temperature‐controlled chamber will be useful in a wide range of biological applications where a rapid change of temperature during microscopic observations is necessary or has to be avoided allowing a simulation of ecologically relevant temperature scenarios.     

Friction factor evaluation by FEM and experiment for TA15 titanium alloy in isothermal forming process

The friction at die–workpiece interface is an important parameter in metal forming processes, which affects the metal flow, cavity fill, surface quality, etc. The friction in the forming process is influenced by material properties and forming conditions. The friction in forming process of TA15 (Ti–6Al–2Zr–1Mo–1V) titanium alloy under high temperatures (isothermal forming) and low strain rates is studied here by ring compression test. The friction calibration curves are elaborated by means of finite element method. The research on the calibration curves and friction factor at the loading speed v?=?0.1–1.0?mm/s and the conventional forging temperature (950°C) and near-beta forging temperature (970°C) is carried out. The influence of loading speed on friction calibration curves is similar to the influence on friction factor m: at the low (m <about 0.1) or high (m >about 0.7) friction condition, the influence of loading speed can be ignored, however the influence is notable in the midst magnitude (about 0.2–0.5). The temperature variation from 950°C to 970°C has little influence on friction calibration curves, but has notable influence on m under lubricated condition.     

HVOF-Sprayed Adaptive Low Friction NiMoAl–Ag Coating for Tribological Application from 20 to 800 °C

An adaptive NiMoAl–Ag composite coating was deposited by high-velocity oxy fuel spraying, and its tribological properties from 20 to 800 °C under unlubricated conditions were evaluated using a CSM high-temperature tribometer. Scanning electron microscopy, X-ray diffraction and Raman spectroscopy were used to characterize the coating and corresponding wear tracks to determine the lubrication mechanisms. The results showed that the friction coefficient of the NiMoAl–Ag composite coating was around 0.3 from 20 to 600 °C and reached the lowest value of 0.09 at 800 °C. Meanwhile, wear rates of the coating were maintained on the order of 10^?5 mm³/N m at the test temperatures except for 400 and 600 °C. Characterization of the NiMoAl–Ag coating revealed that silver provided lubrication below 400 °C. Ag₂Mo₂O₇ and Ag₂MoO₄, which were formed through tribochemical reactions, acted as high-temperature lubricants above 400 °C. It was especially proposed that silver in a nearly molten state was effective in reducing the friction of the NiMoAl–Ag coating at 800 °C. Moreover, a comprehensive lubrication mechanism model of an NiMoAl–Ag composite coating at 800 °C was established to explain the extremely low friction coefficient and wear rate of the coating.     

Influence of temperature on a low-cycle fatigue behavior of a ferritic stainless steel

The main objective of this study is to reveal the effect of dynamic strain ageing (DSA) on a ferritic stainless steel with detail relation to monotonic and cyclic responses over a wide range of temperatures. For assessing the effect of strain rate on mechanical properties, tensile test results are studied at two different strain rates of 2×10^?3/s and 2×10^?4/s. Typical responses of this material are compared with other alloy in literatures that exhibits DSA. Serrations in monotonic stress-strain curves and anomalous dependence of tensile properties with temperatures are attributed to the DSA effect. The low cycle fatigue curves exhibit prominent hardening and negative temperature dependence of half-life plastic strain amplitude in temperatures between 300°C–500°C which can be explained by DSA phenomenon. The regime for dependence of marked cyclic hardening lies within the DSA regime of anomalous dependence of flow stress and dynamic strain hardening stress with temperature and negative strain rate sensitivity regime of monotonic response. It is believed that shortened fatigue life observed in the intermediate temperature is mainly due to the adverse effect of DSA. An empirical life prediction model is addressed for as-received material to consider the effect of temperature on fatigue life. The numbers of load reversals obtained from experiment and predicted from fatigue parameter are compared and found to be in good agreement.     

Experimental investigation on ejector cooling system performance at low generator temperatures and a preliminary study on solar energy

In this paper, the performance of an ejector cooling system with constant-area mixing chamber was investigated at low generator temperatures. The cooling system was tested by using hot water as driving fluid and R123 as working fluid. The effects of operating temperatures on the cooling capacity and on the coefficient of performance (COP) of the system were experimentally investigated at an ejector area ratio of 7.17, which is suitable for solar cooling. As a result, COP of 0.42 was obtained at a vapor generator temperature of 74°C, evaporator temperature of 10°C, and at a critical condenser temperature of 29°C. A solar application of the system with single-glazed selective-type collectors in a horizontal position was conducted as a preliminary study. In the solar application, a cooling capacity of 1080 W for a 9.2 m² collection area at an evaporator temperature of 10°C was obtained.     

Tribological Performance of Boronized,Nitrided, and Normalized AISI 4140 Steel against Hydrogenated Diamond-Like Carbon-Coated AISI D2 Steel

In the current work, AISI 4140 steel was pack-boronized at 950°C for 3 h and gas-nitrided at 550°C for 72 h. All specimens used in this work were prepared from the same steel bar. A 3-µm-thick diamond-like carbon (DLC) coating (a-C:H) was deposited on the AISI D2 high-carbon, high-chromium, cold-worked tool steel by a plasma-assisted chemical vapor deposition technique. Normalized, boronized, and nitrided steel pins were tested against DLC-coated AISI D2 steel at various normal loads (15, 30, 60, and 80 N) for 1,000 and 3,000 m sliding distance in ambient air. Specific wear rate of all pins decreased with increasing load, and a similar trend was observed for the coefficient of friction (COF). Microscopic and energy-dispersive spectroscopic (EDS) analysis confirmed the role of the transfer layer for a low COF with increasing load. At all loads, the specific wear rate of boronized pins was lower than that of the nitrided and normalized pin specimens. Boronized pins showed a specific wear rate in the range of 0.27 × 10^?8 to 0.44 × 10^?8 mm³/Nm and the COF was about 0.1.     

Thermal characteristics of an ammonia-charged closed-loop pulsating heat pipe

At this age, engineering applications are demanding effective ways of heat recovery and energy savings for their optimum performance. Among other cooling techniques, pulsating heat pipes have emerged as a convenient and cost effective thermal design solution due to its excellent heat transfer capability, high thermal efficiency and structural simplicity. The paper presents an experimental study on the operational limit of an aluminum closed-loop pulsating heat pipe (CLPHP) charged with ammonia. It consists of total 14 turns of aluminum pipe with 3 mm inner and 4 mm outer diameter. Ammonia was used as working fluid with 3 different filling ratios such as 0.4, 0.6 and 0.8. Operation orientations were vertical, 30°, 45°, 60°, 90° and 180° inclinations. Constant electric heat input of 36 W was applied to the heating block and temperature rise in various sections was monitored till steady state was reached. Temperature was measured at different locations of the CLPHP by using thermocouples. The effects of operational orientations and filling ratios were investigated on heat transfer by working fluid Q̇_php (Watt), overall heat transfer coefficient U (W/m² °C) and thermal resistance R (°C/W) considering the measured temperature. The result shows that, 0.4 and 0.6 fill ratios and inclination angle of 30º give better result than any other arrangements for CLPHP.

     

Tribological properties of Ni-17.5Si-29.3Cr alloy at room and elevated temperatures

The tribological properties of Ni-17.5Si-29.3Cr alloy against Si₃N₄ were studied on a ball-on-disc tribotester between room temperature and 1000 °C. The effects of temperature on the tribological properties of the alloy were investigated. The worn surfaces of the alloy were examined using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results indicated that the tribological behavior of the alloy expressed some differences with increase in testing temperature. At low and moderate temperatures (below 800 °C), the alloy showed excellent wear and oxidation resistances, and the wear rate of the alloy remained in the magnitude of 10^?5 mm³/Nm; but at elevated temperature (800–1000 °C), the wear and oxidation resistances decreased, and the wear rate of the alloy increased up to 10^?4 mm³/Nm. The friction coefficient decreased from 0.58 to 0.46 with the rising of testing temperature from 20 to 600 °C, and then remained nearly constant. The wear mechanism of the alloy was mainly fracture and delamination at low and moderate temperatures, and transformed to adhesive and oxidation at elevated temperatures.     

Tribological behavior of graphite-containing nickel-based composite as function of temperature,load and counterface

Nickel-based graphite-containing composites were prepared by powder metallurgy method. Their mechanical properties at room temperature and friction and wear properties from room temperature to 600 °C were investigated by a pin-on-disk tribometer with alumina, silicon nitride and nickel-based alloy as counterfaces. The effects of graphite addition amount, temperature, load, sliding speed and counterface materials on the tribological properties were discussed. The micro-structure and worn surface morphologies were analyzed by scanning electron microscope (SEM) attached with energy dispersive spectroscopy (EDS). The results show that the composites are mainly consisted of nickel-based solid solution, free graphite and carbide formed during hot pressing. The friction and wear properties of composites are all improved by adding 6–12 wt.% graphite while the anti-bending and tensile strength as well as hardness decrease after adding graphite. The friction coefficients from room temperature to 600 °C decrease with the increase of load, sliding speed while the wear rates increase with the increasing temperature, sliding speed. The lower friction coefficients and wear rates are obtained when the composite rubs against nickel-based alloy containing molybdenum disulfide. Friction coefficients of graphite-containing composites from room temperature to 600 °C are about 0.4 while wear rates are in the magnitude of 10^?5 mm³/(N m). At high temperature, the graphite is not effective in lubrication due to the oxidation and the shield of ‘glaze’ layer formed by compacting back-transferred wear particles. EDS analysis of worn surface shows that the oxides of nickel and molybdenum play the main role of lubrication instead of graphite at the temperature above 400 °C.     

Low‐temperature‐SEM study of dihedral angles in the ice‐I/sulfuric acid partially molten system

The transport and mechanical properties of partially molten materials are influenced by the wetting behaviour of the melt with respect to the crystalline solid. The equilibrium microstructure of an ice + melt system was examined using low‐temperature scanning electron microscopy. The samples were prepared by spraying a liquid solution of H₂O‐H₂SO₄ into liquid nitrogen and packing the frozen particulates into aluminium capsules. Samples were then sintered at –35 °C or –55 °C (above the eutectic temperature, T_E=–62 °C) for various durations and were quenched in liquid nitrogen to capture the equilibrium microstructure. This paper reports the first quantitative measurements of dihedral angle in this system. The measured median dihedral angle between the solid and vitrified melt is approximately 26 ± 2° at –35 °C and increases slightly as temperature decreases and approaches the solidus (32 ± 3° at –55 °C).     

Dry sliding wear behaviour of Ti–6Al–4V pin against SS316L disc in vacuum condition at high temperature

ABSTRACT

Ti–6Al–4V alloy exhibit a unique combination of mechanical, physical, and chemical properties; that pronounced its desirability for implementation in the fields of aerospace, automobile, and chemical industries. The mechanisms, namely, strain rate response/adiabatic shear band (ASB) – effect of plastic deformation, tribo-chemical reaction and formation of the mechanically mixed layer (MML), can control wear behaviour of the alloy. Hence, the present work investigates the influence of these mechanisms in governing the tribological characteristics of the Ti6Al4V alloy aganist SS316L steel. The experiments were executed on a pin-on-disc tribometer under vacuum (2?×?10^?5?Torr), by varying the temperature (25, 100, 200, 300 and 400°C) at constant sliding speed (0.01?ms^?1) and load (137.3?N) conditions. Compression test was carried out at distinct strain rate (0.001 and 10 s^?1) and temperature (25, 100 and 400°C) values, to investigate the occurrence of ASB. The scanning electron microscopy and energy dispersive X-ray spectroscopy analyses were used to evaluate the formation of appendage layers (oxides and MML) and the composition of wear debris, respectively. The wear rate of the Ti–6Al–4V alloy decreased with increment in temperature (room condition to 400°C) inside vacuum environ, governed by ASB and the presence of oxide layers.     

Mechanical and tribological properties of NiCr–Al2O3 composites at elevated temperatures

The effect of Al₂O₃ content on the mechanical and tribological properties of Ni–Cr alloy was investigated from room temperature to 1000 °C. The results indicated that NiCr–40 wt% Al₂O₃ composite exhibited good wear resistance and its compressive strength remained 540 MPa even at 1000 °C. The values obtained for flexural strength and fracture toughness at room temperature were 771 MPa, 15.2 MPa m^1/2, respectively. Between 800 °C and 1000 °C, the adhesive and plastic oxide layer on the worn surface of the composite was claimed to be responsible for low friction coefficient and wear rate.     

Effect of Heat Treatment Temperature on Microstructure and Mechanical and Tribological Properties of Cold Sprayed Ti-6Al-4V Coatings

In this study, Ti-6Al-4V (Ti-64) coatings were prepared on commercial Ti-64 substrates via a high-pressure cold spray process. The coatings were heat treated at different temperatures of 400–1000°C to investigate the effect of heat treatment temperature on their microstructure and mechanical and tribological properties. The increased heat treatment temperature from 400 to 600°C promoted diffusion between sprayed Ti-64 particles. Recrystallization of the sprayed particles was found at the heat treatment temperature of 800°C and grain growth was found in the microstructure of the coating heat treated at 1000°C. The highest and lowest hardnesses of the heat-treated coatings were found at heat treatment temperatures of 400 and 800°C, respectively. Therefore, the lowest and highest specific wear rates of the coatings were consistently found at 400 and 800°C due to their highest and lowest abrasive wear resistances associated with their highest and lowest surface hardnesses, respectively. The coating heat treated at 400°C showed the highest surface hardness of 470.1 Hv and lowest specific wear rate of 69.6 × 10^?14 m³/Nm. It could be concluded that the microstructure and mechanical and tribological properties of the Ti-64 coatings were significantly influenced by heat treatment temperature.     

Determination of magnetic field using a Fabry–Perot cavity containing novel nanoparticles

Mn_0.75Zn_0.25Fe₂O₄ nanoparticles were used to characterize magnetic fields using an all-fiber Fabry–Perot interferometer. The 20-nm nanoparticles were fabricated with citrate and displayed a coercive field of approximately 10?mT. The nanoparticles were dispersed in oleic acid to prepare a magnetic fluid to fill a Fabry–Perot structure fabricated by arc splicing with conventional single-mode and hollow core photonic crystal fibers. This device provided sensitivity and resolution of 0.11?dB/mT and 0.09?mT, respectively. Thermal analysis indicated that the magnetic measurements are weakly depending on temperature (0.7?pm/°C and 7?×?10^?3?dB/°C). This device offers low-cost fabrication, simple implementation and may be used in several industrial applications.     

Microstructural,Mechanical, and Tribological Properties of Rice Husk–Based Carbon: Effect of Carbonizing Temperature

In this study, we investigated the microstructural, mechanical, and tribological properties of rice husk (RH)-based carbon carbonized at various carbonizing temperatures under dry conditions. All samples exhibited amorphous carbon structures and the X-ray diffraction spectra of the samples carbonized at 1300 and 1400?°C indicated the presence of a polymorphic crystals of silica. The hardness increased with temperature due to the densification of the structure and the presence of the hard crystalline silica. At low normal loads, the mean friction coefficient of the material decreased as the carbonizing temperature was increased from 600 to 800?°C and slightly decreased as the carbonizing temperature was further increased from 800 to 1400?°C. At the highest load, all samples, except for that carbonized at 600?°C, exhibited extremely low friction coefficients (around 0.05). The wear rates of the all samples were smaller than 10^?5 mm³/N·m, indicating that RH carbon exhibits sufficient wear resistance. A Raman spectroscopic analysis of the worn surface of a steel ball revealed that the transfer layer at 600?°C had a less graphitic structure compared to the other carbonizing temperature. Based on these findings, we recommend an optimal carbonizing temperature for applications of sliding materials exposed to dry sliding contact.     

Magnetic and eddy-current testing of hardened constructional steel subjected to combined strain-thermal treatment

The potentialities of magnetic and eddy-current methods for testing a structural state, wear resistance, and mechanical properties of steel 50 (0.51 wt. % C) subjected to quenching from 850°C, nanostructurizing frictional treatment with a hard alloy indenter, and tempering at the temperatures of 100–600°C are studied. It is shown that the structural state of constructional steel 50 that is subjected to combined strain-thermal treatment can be tested using the eddy-current method upon low temperature (to 250°C) tempering and in the tempering temperature range of 250–450°C, the coercitimetric method and based on the maximum permeability in the temperature range of 100–350°C, based on the residual magnetic induction for major and minor magnetic hysteresis loops in the temperature ranges of 100–400°C and 250–600°C, respectively, and based on the specific resistivity in the temperature range of 100–300°C. Correlations between electromagnetic parameters and hardness, mechanical properties, wear intensity under the conditions of dry frictional that point to the possibility of using eddy-current and magnetic testing of the quality of optimized strain-thermal treatment, which provides a good complex of wear resistance and mechanical characteristics of constructional steel, were established.