Thermal Aging Process Research Articles

Comparative study of thermoplastic liner materials with regard to mechanical and permeation barrier properties before and after cyclic thermal aging

Abstract Lightweight pressure vessels of type IV for hydrogen storage consist of a thermoplastic inner liner, commonly from polyethylene or polyamide. The liner is the permeation barrier against the compressed gas and must prevent the formation of cracks, also after temperature changes, for example after refueling processes. In the present work high-density polyethylene, cross-linked polyethylene, polyamide 6 and polyamide 12 were characterized by tensile tests, single notch impact tests and permeations measurements before and after a cyclic thermal aging process. The aging only lead to slight changes of mechanical properties due to post-crystallization, but to a significant decrease of permeation properties. This decrease was contributed to weakened, amorphous regions where chain splitting occurred. Considerable differences in properties resulted from different peroxide cross-linking times of polyethylene at the same temperature. A longer holding time at 200 °C led to an improvement in impact strength by a factor of more than three. However, the permeation properties decreased by about 50 %, indicating that peroxide cross-linking in the melt inhibited the formation of crystalline regions.

The effect of thermal aging on the composition of pyrolysis oil fuel derived from typical waste plastics

This study reveals the influence of thermal aging on the pyrolysis oil fuel components of typical waste plastics, including polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC). Plastic samples were continuously heated in an aging oven at 100 °C to simulate thermal aging. The pyrolysis volatile compositions of fresh and aged plastic samples were comparatively studied by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) at 500 °C. The volatile composition analysis results showed that thermal aging changes the pyrolysis oil composition, and the influence strongly depends on the type of plastic. For PE and PP, which underwent random scission after thermal aging treatment, the influence of thermal aging on the heavy oil fraction was more obvious than that on the light oil and diesel oil fractions. For PVC samples, the thermal aging process caused structural defects and turned the color of the PVC light yellow. After 128-h thermal aging, the maximum weight loss rate decreased by 35.8%, and the fraction of C5-C9 components and aromatics first increased and then decreased. The maximum values were 80.21 ± 1.62% and 73.69 ± 0.73%, and the minimum values were 65.58 ± 2.63% and 63.98 ± 0.39%, respectively.

Improved contact resistance and solderability of electrodeposited Au-Sn alloy layer with high thermal stability for electronic contacts

We investigated the effect of thermal aging on the contact resistance and solderability of Au-Sn and Au-Co alloy films. The Au-Sn alloy exhibited lower contact resistance and better solderability compared to the conventional Au-Co alloy. In the Au-Sn alloy, an Au-Sn–Ni intermetallic compound layer was formed at the interface between the electroplated Ni and Au-Sn layers during the thermal aging process. The resulting Au-Sn–Ni layer effectively blocked the diffusion of underlying nickel to the surface, leading to improved solderability and enhanced thermal stability of the contact resistance. Meanwhile, the results revealed that a significant amount of nickel diffused to the surface of the Au-Co alloy during the thermal treatment, and the nickel atoms on the surface were oxidized to NiO or Ni2O3, which increased the contact resistance and deteriorated solderability by acting as a barrier layer.

Thermal aging of a polyamide 66—experimental analysis and modeling

The present paper presents an analysis of the thermal aging process of a polyamide 66 at two different temperatures: −6 °C and 60 °C, and throughout a time span of 9000 h. The paper proposes a unified expression to estimate the mechanical behavior of the polyamide in tensile tests at the different aging stages. The goal is to present an algebraic equation able to describe the stress–strain curves that would be obtained using the ASTM D638-14 standard for a wide range of aging times, without the actual necessity of performing all the tests. It is shown that with a minimum of three tests performed at different aging stages, it is possible to obtain a reasonable approximation of the stress–strain curve for this range of time.

Studying the residual stress homogenization and relief in aerospace rolling ring of GH4169 alloy using ageing treatment

In the manufacturing process of aerospace large ring parts, a large number of exterior materials are removed, which makes the ring parts become a thin-walled structure with low stiffness. In the actual production process of the aerospace industry, the processing deformation caused by a series of internal and external factors is ubiquitous. The studies show that the main factor causing the machining deformation is the initial residual stress (RS) inside the blank ring. The ring rolling process is accompanied by local severe plastic deformation, resulting in large and uneven RS. The homogenization and reduction of initial RS are of great significance to reduce the adverse effect of machining deformation. The research object in this study is a sizeable GH4169 alloy aerospace rolling ring. First, a long-term natural ageing treatment was designed for homogenizing residual stresses in sizeable GH4169 alloy ring. Second, a finite element model was established to simulate the rolling process and the thermal ageing process. The corresponding rolling and thermal ageing experiments were carried out to verify the simulation results. The purpose of this process is to explore and verify the effect of natural ageing and artificial ageing on homogenizing and reducing the RS in the rolling ring. The results show that the homogenization of RS distribution was enhanced significantly by natural ageing treatment, while the RS magnitudes descended by 15~35%. Combined with the subsequent thermal stress relief, the RS is significantly reduced or even eliminated. This study is of great significance to the homogenization and reduction of RS in large GH4169 alloy ring.

Long Term Stability Evaluation of Stress Improvement Effect on Ni-Based Alloy Welds by Various Residual Stress Improvement Methods

Stress corrosion cracking (SCC) is one of serious aging degradation problems for the Alloy 600 components of pressurized water reactors (PWRs). In order to prevent SCC, various methods such as water jet peening (WJP), laser peening (LP), surface polishing have been used to introduce compressive stresses at the surfaces of the PWR components. However, it has been reported that such compressive residual stress introduced by these methods might be relaxed during the practical operation, because of high temperature environment. In this study, the hardness reduction behavior of the Alloy 600 processed by LP, Buff and WJP in the thermal aging process has been investigated to estimate the stability of the residual stress improving effect by each method, based on the fact that there is a correlation between the compressive residual stress relaxation and the decrease of hardness. The behavior of the residual stress relaxation in the processed materials in the high temperature environment has been discussed with kinetic analysis.

Storage Life Evaluation and Aging Mechanism Analysis of Epoxy Resin Potting Materials

The tensile shear samples of potting epoxy resin materials were accelerated under 90°C, 100°C and 110°C temperature conditions. The tensile shear strength was used as the aging characteristic index during the aging test. After normalizing the data in the descending section, the storage life evaluation and analysis is performed based on the Arrhenius model. The retention rate of tensile shear strength is 50% as the critical failure condition. The storage life at 25°C is predicted to be 21.7 years. During the thermal oxygen aging process, the epoxy bonding material’s adhesive interface first undergoes post-curing reaction under the action of heat, and then undergoes oxidative degradation under the action of thermal oxygen. The sample interface is more susceptible to oxidative degradation than the interior. In the oxidative degradation stage, the part of the epoxy resin connected to the hydroxyl group is oxidized to the aldehyde group, and the methylene group connected to the N in the curing agent is oxidized to C=O, forming C=O in the amide, and the molecular backbone is broken And produce low molecular weight polymers.

Clarifying the effect of moisture absorption and high-temperature thermal aging on structure and properties of polyimide film at molecular dynamic level

A series of pyromellitic dianhydride (PMDA)/4,4′-oxydianiline (ODA) polyimide (PI) films with different moisture content were experimentally prepared and corresponding PI/H2O composite models were constructed and analyzed based on molecular dynamic (MD) simulation. The elongation at break increased from 47.2% to 59.7% when the moisture content of the PI film reached 2%, which was attributed to the presence of water molecules that increased the mobility of molecular chains. The fractional free volume and XRD patterns of the composite models illustrated that the average packing distance of PI chains increased with moisture absorption and weakened intermolecular charge-transfer complex formation, therefore endowing the wet PI films better transmittance. The thermal decomposition mechanism of PMDA/ODA, 3,3′,4,4′-biphenyldianhydride (BPDA)/ODA and BPDA/p-phenylene diamine (PDA) based PI films were also investigated by thermogravimetric-infrared radiation (TG-IR) and MD simulation. The IR spectra of gas products indicated that the thermal decomposition of PI mainly generated CO2, CO, NH3 and HCN, and the PI with different chemical structures had the same thermal decomposition products. The MD results showed that the C–O bond in the ODA structural unit was broken first during the thermal decomposition, which could well explain that the BPDA/PDA exhibited better thermal stability compared with PMDA/ODA and BPDA/ODA. This work paved the way for the investigation of the macromolecular structural evolution of PI materials during moisture absorption and thermal aging process.

Mechanical properties and reaction mechanism of microwave-activated crumb rubber-modified asphalt before and after thermal aging

The anti-aging properties and related mechanism of microwave-activated crumb rubber-modified asphalt (ACRMA) were evaluated in this study. Physical and rheological tests were conducted on ACRMA before and after simulated thermal aging, and several microcosmic tests were performed to analyze the aging mechanism. The stiffness, plasticity, and viscoelasticity of ACRMA were less affected by aging than crumb rubber-modified asphalt (CRMA). After aging, ACRMA exhibited better high-temperature performance and permanent deformation resistance than CRMA. The thermal aging process of ACRMA was analyzed, and the anti-aging mechanism model was established. The anti-aging performance of ACRMA was better than that of CRMA.

Detailed Study on the Mechanical Properties and Activation Energy of Natural Rubber/Chloroprene Rubber Blends during Aging Processes

Selection of a suitable thermal aging process could render desirable mechanical properties of the rubbers or blended rubbers. In this work, the effect of the aging processes on the mechanical properties and activation energies of natural rubbers (NR) and NR/chloroprene rubbers (CR) blends with low CR contents (5–10%) was investigated. Three aging processes including heat aging (at 110°C for 22 hours), mechanical aging (under dynamic loading to 140% strain for 16000 cycles), and complex aging (heat and mechanical aging) were studied. The results revealed that the compatibility of CR in natural rubber matrix had a significant effect on the dynamic properties of the blended rubber and negligible effect on the static properties. The changes in activation energies of the blended rubber during aging processes were calculated using Arrhenius relation. The calculated changes (ΔUc, ΔUd, and ΔUT) in activation energies were consistent with the results of mechanical properties of the blended rubber. Interestingly, the change in activation energies using complex aging conditions (ΔUc) was mostly equal to the total changes in activation energies calculated separately from heat aging (ΔUT) and mechanical aging (ΔUd) conditions. This indicates that, in complex aging conditions, the heat and dynamic loading factors act independently on the properties of the blended rubber.

Effect of crystal structure of nickel substrates on interfacial behaviors in Sn/Ni soldered joints

The effects of crystal structure of Nickel on the interfacial phase and growth mechanism were clarified in Sn/Ni soldered joints by using polycrystal nickel and single crystal nickel substrate. It was found that the intermetallic compounds (IMCs) phase, Ni3Sn4, was formed at the interface of Sn/polycrystal nickel joints; while, it changed into NiSn4 when using single crystal nickel substrate. At the following thermal aging process, Ni3Sn4 and NiSn4 exhibited different behaviors: Ni3Sn4 grew faster than NiSn4 showing a much thicker layer. Kinetics analysis indicated that different diffusion mechanisms governed the IMCs layer growth, namely Ni3Sn4 was controlled by diffusion of the reaction elements, whereas NiSn4 was controlled by reaction rate.

Effects of acrylonitrile content on thermal characteristics and thermal aging properties of carbon black-filled NBR composite

In seal applications, thermal characteristics and thermal aging properties of rubber are essential factors in determining the applicable temperature range and lifetime of the seal. In this study, thermal characteristics and thermal aging properties of the acrylonitrile butadiene rubber composite in response to the acrylonitrile (ACN) content were investigated. Thermal stability, glass transition temperature ( T g), and dynamic property were determined by thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis, respectively. The results showed that T g, thermal decomposition temperature, and tan δ were increased as the ACN content increased. To investigate the thermal aging properties of the composite, the composite was subjected to an accelerated thermal-oxidative and thermal aging process at 100°C for 168 h. The aged composites were evaluated by investigating the change in cross-link density, mechanical properties, and elastic recovery. The change in the chemical functional group was also studied using attenuated total reflectance Fourier transform infrared spectroscopy. The results indicate that subjecting the composite to thermal-oxidative and thermal aging process decreased the elongation at break and elastic recovery, while the cross-link density, Shore A hardness, and 100% modulus were increased. All of the properties were also dependent on the ACN content as well as the aging conditions.

Terahertz Dielectric Spectroscopy Based Thermal Aging Analysis of Polypropylene

Polypropylene (PP) is a widely used common type of polymer with excellent chemical and mechanical properties. Thermal aging is a main culprit in degrading its lifetime and effectiveness. In this article, terahertz (THz) dielectric frequency response of PP during the process of thermal aging is monitored and analyzed to understand the underlying mechanism and to qualify THz dielectric spectroscopy as a useful tool for time-stamping and benchmarking the thermal aging of polymer materials. Using an improved THz material parameter extraction algorithm for thin films, the real and imaginary parts of the dielectric permittivity of standard PP test coupons were obtained at various stages of the thermal aging process. In agreement with theoretical analysis based on a molecular-level understanding of the thermal aging dynamics, the experimental results indicated THz wave transmission loss of PP correlated quantitatively with the degree of its thermal aging.

Trimesic acid assisted synthesis of cerium-manganese oxide for catalytic diesel soot elimination: Enhancement of thermal aging resistibility

In this work, trimesic acid was introduced in the preparation of cerium-manganese oxide, and its effect on thermal aging resistibility for catalytic diesel soot elimination was investigated systematically. The prepared catalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption–desorption, X-ray diffraction (XRD), Raman spectra, X-ray photoelectron spectroscopy (XPS) and H2-temperature programmed reduction (H2-TPR). The experimental results show that trimesic acid addition has no significant effect on the morphologies of cerium-manganese oxide catalyst, and also contribute litter to the suppression of catalyst particles sintering and textual features destroying. In addition, the crystal structure and high-temperature (800 °C) phase separation of CeO2-MnOx solid solution are not improved by the trimesic acid modification. But, noteworthily, trimesic acid assisted synthesis can obviously promote Mn4+ generation and keep up oxygen vacancy concentration and hence improve the product of highly active oxygen species (especially O2-) of CeO2-MnOx catalyst during the thermal aging process. Furthermore, trimesic acid assistance can significantly alleviate the loss of CeO2-MnOx metal-support interaction caused by thermal aging, and maintain the ability to supply active oxygen at low-temperature (approximately 200–400 °C). Finally, after thermal aging at 800 °C for 12 h, the trimesic acid assisted synthesized CeO2-MnOx catalyst shows significantly better catalytic soot oxidation activity. Thus this work reveals that besides stabilizing textual and structural properties, stabilizing and optimizing electronic structure is another feasible pathway to enhance the thermal aging resistibility of CeO2-based soot oxidation catalysts.

Transverse relaxation characteristic and stress relaxation model considering molecular chains of HTPB coating based on pre-strained thermal aging

In order to accurately describe the transverse relaxation characteristic and stress relaxation modulus of HTPB coating during pre-strain thermal aging process, a one month thermal aging test was carried out at 70 °C with pre-strain of 0%, 3%, 6% and 9%, respectively. The low-field 1H NMR and stress relaxation modulus tests were carried out for HTPB coating at different aging stages. The stress relaxation model considering the molecular chains was proposed according to the changes of crosslinking chain and dangling chain of HTPB coating during pre-strain aging. The results showed that with the increase of aging time, the decay rate of transverse relaxation curve became faster, the transverse relaxation time decreased, the value of combined parameter qMrl increased, the proportion of crosslinking chain decreased, while the proportion of dangling chain increased. Moreover, the stress relaxation modulus increased, the crosslinking network structure of HTPB coating became denser and the degree of crosslinking increased. At the initial aging stage, the pre-strain will destroy the crosslinking network structure of HTPB coating to a certain extent. With the increase of aging time, the effect of pre-strain will gradually weaken and the influence of aging on materials will gradually increase. The correlations between the stress relaxation model considering the molecular chains and the test results were more than 0.9950, which can accurately describe the stress relaxation modulus of HTPB coating during the pre-strain thermal aging process.

Evaluation of the Bond Strengths between Dental Porcelain and Cobalt-Chromium Metal Frameworks Manufactured with Different Techniques after the Thermal Aging Process.

Purpose The present study is aimed at examining the bond strength of cobalt-chromium (Co-Cr) metal frameworks, prepared through different techniques, to a single type of low-temperature porcelain system after the thermal aging process. Methods A hundred and twenty Co-Cr alloy framework specimens were prepared using conventional casting, CAD/CAM, and two commercially different laser sintering devices, and dental porcelain was applied to the specimens. A single type of dental porcelain (Kuraray Noritake Dental Inc., Tokyo, Japan) was applied to the specimens. After the subgroups were determined, half of the specimens were subjected to a thermal aging process. Bond strength of specimens was evaluated using a 3-point bending test. The surfaces of the fractured specimens were evaluated using a stereomicroscope. The metal-porcelain bonding area of samples randomly selected from 8 groups has been examined with SEM under ×1000 magnifications. Normality distribution of obtained data was examined using by a Kolmogorov-Smirnov test. The obtained data of the present study was statistically analyzed with a statistical package program (SPSS for Windows 22.0, Chicago, IL, USA). Results There was a statistically significant difference between CAD/CAM and the other three methods, and the bonding value of the CAD/CAM group was the highest among the groups. Besides, the bond strength between dental porcelain and 4 differently produced metal frameworks was high enough to surpass the acceptable threshold (>25 MPa) according to the ISO 9693. There was no statistically significant difference between thermal aging applied and nonapplied groups. Conclusions Based on this study, it could be shown that the metal-ceramic bond strength is dependent on the manufacturing method used, but it is independent of the thermal aging application. It was found that the bond strength values of all samples with and without thermal aging application exceeded the minimum acceptable value of 25 MPa recommended by the ISO 9693.

Microstructure evolution and tensile properties of an Al added high-Cr ODS steel during thermal aging at 650 °C

Abstract Effects of thermal aging at 650 oC up to 5000 h on the microstructure evolution and tensile properties of 16Cr-3Al ODS steel were investigated. Laves phase and σ phase were identified after thermal aging for 250 h. The Laves phase tended to form at the grain boundaries and exhibited different morphologies at fine grains’ boundaries and coarse grains’ boundaries, while the σ phase mainly located at fine grain zones. The grain structure and misorientation evolution during the aging process were observed by the electron back-scattered diffraction (EBSD) method. It was found that the area fraction of coarse grains increased while the geometrically necessary dislocation (GND) density decreased during thermal aging process, which induced the misorientation evolution. Grain growth mode was discussed by combination with transmission electron microscopy (TEM) analysis. In the zones with uniform grain size distribution, the growth direction of grain boundaries was from the grains with a high number density of precipitates towards those with a low number density of precipitates. Room temperature tensile tests were conducted, and the results of tensile strength, yield strength, and elongation during different stages of thermal aging indicated that 16Cr-3Al ODS steel had a relatively good thermal aging stability at 650 oC.

Effect of thermal aging on high viscosity modified asphalt binder: Rheological property, chemical composition and phase morphology

Currently, the high viscosity modified asphalt (HVMA) is widely used in the porous asphalt pavement (PA) in China. HVMA provides good raveling resistance and rutting resistance for PA due to the excellent rheological property of it. However, the high construction temperature and the well-developed void structure of PA lead to a severe aging of HVMA. In order to understand the thermal aging behavior and mechanism of HVMA, the changes of rheological property, chemical composition, and phase morphology of HVMA, SBS modified asphalt (SBSMA) and PEN70 asphalt (70#A) during thermal aging process were investigated in this study. Rotational viscosity (RV) test, frequency sweep test, and multiple stress creep recovery (MSCR) test were used to evaluate the rheological property of HVMA. Then, Fourier transform infrared spectroscopy (FTIR) test and Gel permeation chromatography (GPC) test were used to determine the effect of thermal aging on the chemical composition of HVMA. Moreover, the phase morphology of HVMA, and SBSMA were observed by Fluorescence microscopic (FM) test. Test results indicate that the rotational viscosity, softening point, and strain recovery of HVMA show a decreasing trend initially and then increase with the process of thermal aging. The polymer phase structure of HVMA degrades obviously in short-term aging process, which leads to a deteriorated rheological property of HVMA. After long-term aging, the oxidation of base asphalt is the main reason for the changes in rheological property of HVMA. The aging resistance of HVMA is better than 70#A and SBSMA. As the amount of polymers increases, the aging resistance of HVMA is further improved. During thermal aging process, the polymer phase structure of HVMA is more abundant than SBSMA, which contributes to the superior anti-aging property of HVMA.

The effect of niobium addition on the microstructure and properties of cast iron used in cylinder head

Cylinder head is one of the most important parts of heated components in the engine, while cracking due to low strength and large residual stress at high temperature is the main failure mode of cylinder head. With the growing demand of good performance and high reliability, gray cast iron with trace alloying element and optimized annealing process are probably effective ways in obtaining suitable materials. The effect of niobium addition on the microstructure and properties of cast iron used in cylinder head is studied in this paper. The results show that, the tensile strength, toughness, fatigue properties and thermal fatigue properties of gray cast iron are improved by adding trace niobium elements. The specimen with 0.20% niobium shows better comprehensive mechanical properties than others. The addition of trace niobium elements can not only refine the graphite, eutectic cell, carbide and phosphorus eutectic, but also reduce the pearlite lamellar space, thus strengthening the matrix. The cylinder head is made by cast iron with 0.20% niobium addition, and the annealing process of thermal aging is optimized. The residual stress of cylinder head can be maintained at a low level by adopting annealing process with higher holding temperature, longer holding time, lower cooling rate and lower outlet temperature. The analysis of microstructure, mechanical properties and residual stress for niobium alloyed cast iron is helpful for solving the cracking problem of cylinder head, and providing scientific basis for improving engine reliability.

Investigation of long-term thermal aging-induced damage in oxide/oxide ceramic matrix composites

Long-term thermal aging is a typical factor affecting the thermo-mechanical fatigue life for hot-end components in the gas turbine. The present work focuses on the development of thermal aging-induced damage in 2-D woven oxide/oxide ceramic matrix composites from micro-mechanism and macroscopic mechanical performance. The porosity evolution and mechanical performance after long-term thermal aging were characterized through mercury intrusion measurements and uniaxial compressive tests, respectively. The results show that the decrease of micro-porosity directly reflects the irreversible evolution of material microstructure in the thermal aging process, and the decrease of compressive strength after aging is the macroscopic reflection of the microstructure variation. The porosity increment of matrix was thus used to characterize the thermal aging-induced damage, establishing a unique analysis model between the increment of micro-porosity under thermal aging and the corresponding degradation of material compressive strength. The experimental results are in good agreement with the established model.