Mismatch Stress Research Articles

Phase stability, thermophysical properties, thermal shock behavior and CMAS resistance of Sc2O3-CeO2 co-stabilized ZrO2 TBCs

In this study, 16 mol% CeO2–2 mol% Sc2O3 co-stabilized ZrO2 (4S16CZ) coating was prepared by atmospheric plasma spraying (APS) technology. Parameters related to the application of 4S16CZ as a thermal barrier coatings (TBCs) were studied, including high-temperature stability, thermal insulation performance, coefficient of thermal expansion (CTE), thermal shock behavior and CaO-MgO-Al2O3-SiO2 (CMAS) resistance. After heat treatment at 1400 °C for 100 h, the 4S16CZ coating maintained a single non-transformable tetragonal phase (t'), exhibiting excellent high-temperature stability. Additionally, at the test temperatures (25 °C–1000 °C), the thermal conductivity of the 4S16CZ coating ranged from 0.79 W/(m·K) to 1.02 W/(m·K), lower than that of the ZrO2 coating stabilized by 7.6 wt% Y2O3 (YSZ, 0.88–1.13 W/(m·K)), The CTE of the 4S16CZ coating was from 8.3 × 10−6 K−1 to 11.2 × 10−6 K−1, which was comparable to the YSZ. Moreover, the thermal shock behaviors of 4S16CZ TBCs and 4S16CZ/YSZ TBCs were evaluated at 1050 °C. The thermal durability of the 4S16CZ/YSZ TBCs was higher than that of 4S16CZ TBCs due to the buffering effect of the YSZ layer. The main reasons for the failure of both TBCs were the oxidation of the bond coat, thermal mismatch stress, low fracture toughness and sintering effects. In CMAS isothermal corrosion tests, CMAS exhibited low wettability on the 4S16CZ coating. Compared with the YSZ coating, the 4S16CZ coating showed better resistance to CMAS attacks.

Thermal shock behaviors and failure mechanisms of EB-PVD thermal barrier coatings using radiant heating

In this work, the thermal shock behaviors and failure mechanisms of the thermal barrier coatings (TBCs) were investigated with the self-built thermal shock platform. The results showed that the thermal shock cycles significantly could affect the microstructure and element distribution of TBCs. Firstly, the size of columnar gaps in the top coating (TC) layer increased with the increase of the thermal shock cycles. And, the transverse micro-cracks generated near the tip of the columnar gaps under the action of gradient stress. Moreover, Al elements in the bond coating (BC) significantly affected the growth mechanism of thermally grown oxide (TGO). The growth rate of the mixed oxide layer would be accelerated when the concentration of Al element in the BC layer dropped to a certain value. Finally, the accumulation of the thermal mismatch stress induced transverse cracks initiation and propagation in the porous mixed oxides layer, and the adhesion strength of TBCs decrease. When the transverse crack was connected with the columnar gaps, the TBCs would peeling off and failure at the interface of TC and TGO.

Interface stress calculation and failure behavior of TBCs on the surface of aluminum alloy with anodic oxidation

Aluminum protective coatings have poor performance due to the significant difference in coefficient of thermal expansion (CTE) between the substrate and the coating. In this study, porous anodic aluminum layer, which have high bonding, well matching and grew in situ on the substrate of aluminum alloy (4032) using anodic oxidation, and then thermal barrier coatings were produced on the surface by laser surface modification and atmospheric plasma spraying (APS). The aim of this way is to improve the interface thermal shock resistance (TSR) of the coatings. The results showed that the coatings with anodic aluminum layer (highest group average of 444 cycles) have a much higher number of thermal shock cycles than metallic bond coatings. Furthermore, the failure analysis suggested that the bonding stability of the coatings is influenced by the surface state, surface mosaic structure, as well as such as CTE and surface roughness. Eventually, a double-layer mismatch stress model was developed to characterize the residual stresses caused by deformation mismatch in the coatings. After performing calculations，from a force perspective τa≈10τb, and from a work perspective Wfa≈3.34Wfb.

The fabrication of flexible and large‐size inverted metamorphic five‐junction solar cells

AbstractThe inverted metamorphic AlGaInP/AlGaAs/GaAs/InGaAs/InGaAs five‐junction solar cells with the bandgap energies of 2.0/1.67/1.42/1.18/0.83 eV were grown by metal–organic chemical vapor deposition on GaAs substrates. The mismatch stress between the two bottom (In0.17Ga0.83As and In0.46Ga0.54As) cells and GaAs substrate was nearly fully relaxed owing to the growth of AlInGaAs compositionally step‐graded buffers. By employing the low‐temperature epilayer transfer technology with only one‐time temporary bonding, the flexible five‐junction solar cells with the size of 12 cm2 were successfully fabricated. The conversion efficiency was 35.1% with the open‐circuit voltage of 4.73 V under AM1.5G spectrum. The excellent performance of the large‐size five‐junction solar cells is attributed to the low amount of mismatch dislocations in the material as well as the simple fabrication process of the flexible device.

Effect of carbon nanotube substrate temperature on the evolution mechanism of microstructure in FeCoNiCrCu coatings

The microstructure and substrate temperature dependence of FeCoNiCrCu high-entropy alloys (HEAs) coatings on carbon nanotube (CNT) surfaces were investigated using experimental and molecular dynamics simulations. It was found that the crystal structure of FeCoNiCrCu underwent the evolution of amorphous, stacking faults (SFs), multiple-fold twins, and columnar grains with an increase in the CNT substrate temperature. When the CNT substrate temperature was less than 800 K, the SFs and microtwin formation was induced by the mismatch stress at the interface of HEAs and CNT. Meanwhile, the refinement of SFs was caused by continuous crossing and cutting of dislocations. When the CNT substrate temperature was greater than 800 K, multiple-fold twin were induced by thermal stress at the surface of HEAs coating. During the evolution of the fivefold twin, the first microtwin was formed from the surface of the HEAs, and the remaining four microtwin growth from the center successively. These results provide a new method to design and control the interface structure between HEAs and CNTs.

Evaluating In-Plane Thermal Expansion of Two-Dimensional Layered Materials via Effective Descriptors

Two-dimensional layered materials show promising applications in miniaturized devices, such as transistors, spintronics, and field emitters. However, substantial thermal management issues, including thermal mismatch and thermal stress, may degrade device performance. To address such challenges, the thermal expansion (TE) anisotropy determined by the structure feature of layered material needs to be well understood. Here, we propose two new descriptors to evaluate the TE behavior of layered materials, namely the axial elastic deviation factor σi and the axial net thermal stress fi along the ith direction. The former, defined as the normalized elastic element difference of material elastic tensor C and compliance tensor S, can distinguish whether the thermal expansion of a material is driven by phonons (with small σi) or elastic property (with large σi) with few computational costs. The latter, axial stress (in GPa/K) induced by temperature, shows an accurate determination of the positive or negative thermal expansion along different in-plane directions of layered materials. Based on the analysis of descriptors, we found that PtS2 and PtSe2 are featured with a larger axial elastic deviation factor (>23%). Considering the elastic property, we for the first time report the in-plane negative thermal expansion in PtS2 (−1.2 ppm/K) and PtSe2 (−0.8 ppm/K). Our work provides a unified understanding of TE causes of layered materials via effective descriptors, which can serve as a guideline for high-throughput screening of thermal expansion materials and subsequent device design.

The effect of size, orientation and temperature on the deformation of microcast silver crystals

Monocrystalline microwires of silver are produced by means of a novel casting process that combines precise mold production by silicon microfabrication with molten metal pressure infiltration. The wires have a shape amenable to tensile testing and a diameter from a few to several tens of micrometer. Displacement-controlled in-situ tensile tests are conducted at room-temperature, 200 °C and 400 °C. Data are compared with bulk counterparts of the same silver cast to have a diameter near 1 mm. Results show clear evidence of small-scale plasticity. The influence of size, crystal orientation, and temperature on the deformation, yield, strain burst statistics and strain hardening of these micrometric dense metal samples is analyzed. Yield stress values agree with the single arm source model provided that the density of forest dislocations comprises a scale-dependent component likely resulting from thermal mismatch stresses. Statistics of strain burst amplitudes agree with an exponentially truncated power-law complementary cumulative distribution. The power-law exponent agrees with predictions of mean-field theory, while the cutoff intensity exhibits no visible dependence on sample diameter or test temperature and agrees with predictions of current theory.

Impact of cooling rate on mechanical properties and failure mechanism of sandstone under thermal–mechanical coupling effect

High geo-temperature is one of the inevitable geological disasters in deep engineering such as resource extraction, space development, and energy utilization. One of the key issues is to understand the mechanical properties and failure mechanism of high-temperature rock disturbed by low-temperature airflow after excavation. Therefore, the experimental and numerical investigation were carried out to study the impact of cooling rate on mechanical properties and failure mechanism of high temperature sandstone. First, uniaxial compression experiments of high temperature sandstone at different real-time cooling rates were carried out to study the mechanical properties and failure modes. The experimental results indicate that the cooling rate has a significant effect on the mechanical properties and failure modes of sandstone. The peak strain, peak stress, and elastic modulus decrease with an increase in cooling rate, and the fragmentation degree after failure increases gradually. Moreover, the equivalent numerical model of heterogeneous sandstone was established using particle flow code (PFC) to reveal the failure mechanism. The results indicate that the sandstone is dominated by intragrain failure in the cooling stage, the number of microcracks is exponentially related to the cooling rate, and the higher the cooling rate, the more cracks are concentrated in the exterior region. Under axial loading, the tensile stress is mostly distributed along the radial direction, and the damage in the cooling stage is mostly due to the fracture of the radial bond. In addition, axial loading, temperature gradient and thermal stress mismatch between adjacent minerals are the main reasons for the damage of sandstone in the cooling stage. Moreover, the excessive temperature gradient in the exterior region of the sandstone is the main reason for the damage concentration in this region.

Minor titanium addition markedly improves the co-deformability of copper-tantalum composites

We investigated the microstructures of equal channel angular extrusion (ECAE) processed sintered composites of copper (Cu) and tantalum (Ta). These composites were produced with nominal Ta volume percentages ranging from 25% to 75%. Additional composites of the same nominal Ta content were prepared with a minor amount of titanium (Ti) added to facilitate interphase bonding. Ti addition was found to markedly improve the strength and co-deformability of the composites, allowing reliable extrusion of Cu–Ta composites with novel microstructures. A major influence of the Ti appears to be to soften the Ta, which contributes to improved co-deformability by reducing the flow stress mismatch between the composite constituents. A change in the fracture surface morphology in the Ti-modified composite compared to the unmodified composite suggests that the Ti may also improve cohesion between Cu and Ta by reducing Ta surface oxides.

In-situ EBSD study on the microstructure evolution of an Al–Mg–Si–Fe alloy with different precipitation free zones during tension at cryogenic and ambient temperatures

AA6101 (Al-0.44Mg-0.39Si-0.12Fe) extrudate was subjected to solid solution treatment followed by either water quenching (WQ) or air quenching (AQ) before a peak artificial aging (T6) to produce different precipitation free zone widths. These samples are then tested using uniaxial tension at 298 K and 77 K to study the effect of the initial microstructure and cryogenic temperature on the mechanical properties. The yield strength, ultimate tensile strength and ductility of the sample at 77 K are higher than those at 298 K. The yield strength of the air quenching sample is lower than that of the water quenching sample at the same temperature. The microstructure evolution was further characterized by in-situ tensile tests performed at both room and cryogenic temperatures. Based on the in-situ SEM/EBSD results with increasing strain, the deformation mechanisms at both room and cryogenic temperatures are revealed in terms of the accumulation of geometrically necessary dislocations (GNDs), activation of slip systems and lattice rotations. In-situ EBSD results show that with increasing strain, the GND distribution becomes more uniform at 77 K, resulting in a higher work hardening rate than that at 298 K. Wider precipitate free zone (PFZ) in air quenching sample leads to higher stress mismatch resulting in intergranular fracture. The dislocation pile-up at the edge of PFZ leads to strain relaxation at grain boundaries and increased ductility. The dislocations at the edge of PFZ and in grains are uniformly distributed resulting delayed crack initiation and further ductility improvement at 77 K.

Effect of post-annealing on thermal shock and steam corrosion of LaMgAl11O19/Yb2Si2O7 thermal/environmental barrier coatings

LaMgAl11O19/Yb2Si2O7 (LMA/YbDS) thermal/environmental barrier coatings (T/EBCs) on SiCf/SiC composites were annealed at 1200 °C for 5 h in air or Ar atmosphere. The effect of post-annealing in different atmospheres on the microstructure, thermal shock and steam corrosion of the LMA/YbDS T/EBCs was investigated. Results indicated heat treatment in air and argon eliminated the amorphous in the coatings, avoiding the aging stress associated with the crystallization. And the argon-annealed layers (LMA and YbDS) exhibited the less elastic moduli than the air-annealed ones, resulting in the lower thermal mismatch stress in the argon-annealed T/EBCs upon subsequent thermal cycling. Thus, the argon-annealed T/EBCs exhibited an improved thermal cycling lifetime than the as-sprayed and the air-annealed ones. In addition, the unbroadened vertical cracks in the argon-annealed LMA-TBC layer limited the reactant (water-vapor) access to the silica-TGO, leading to the greater resistance of the argon-annealed T/EBCs against steam corrosion than the other two systems.

Direct Growth of Patterned Vertical Graphene Using Thermal Stress Mismatch between Barrier Layer and Substrate

Vertical graphene (VG) combines the excellent properties of conventional graphene with a unique vertical nanosheet structure, and has shown tremendous promise in the field of electronics and composites. However, its complex surface morphology brings great difficulties to micro-nano fabrication, especially regarding photolithography induced nanosheet collapse and remaining chemical residues. Here, we demonstrate an innovative method for directly growing patterned VG on a SiO2/Si substrate. A patterned Cr film was deposited on the substrate as a barrier layer. The VG was synthesized by PECVD on both the patterned Cr film and the exposed SiO2/Si substrate. During the cooling process, the patterned Cr film covered by VG naturally peeled off from the substrate due to the thermal stress mismatch, while the VG directly grown on the SiO2/Si substrate was remained. The temperature-dependent thermal stress distribution in each layer was analyzed using finite element simulations, and the separation mechanism of the Cr film from the substrate was explained. This method avoids the contamination and damage caused by the VG photolithography process. Our work is expected to provide a convenient and reliable solution for the manufacture of VG-based electronic devices.

Numerical analysis on the thermal mismatch stress of quasi-isotropic strand with square cross section during cool-down

The quasi-isotropic strands (Q-IS) with high current-carrying capacity can effectively improve the anisotropy of the critical current in the magnetic field. However, when a quasi-isotropic strand is cooled from room temperature (300 K) to the liquid helium temperature (4.2 K), it may cause the irreversible degradation of the critical current, the plastic deformation, and even mechanical damage. In order to achieve the safe and stable operation of quasi-isotropic strands, it is important to analyze the thermal mismatch stress during cool-down. In this study, two mechanical models (i.e. a bulk model and a contact model) were developed by considering different material elastoplastic characteristics to study the mechanical behaviors of quasi-isotropic strands with square cross section in the cooling process. Compared with the bulk model, the contact model can simulate the contact and separation between adjacent REBCO conductors in quasi-isotropic strands. Therefore, the contact model should be chosen to estimate the thermal mismatch stress of quasi-isotropic strands during cool-down. In addition, plastic deformation was clearly observed in the Cu and Ag layers with peak equivalent plastic strains of 0.016% and 0.171%, respectively. When the stainless steel sheath is replaced by the copper sheath to encase a quasi-isotropic strand, the interlaminar normal tensile stress between the superconducting layer and Hastelloy substrate was decreased from 101 MPa to 16.2 MPa. Therefore, a quasi-isotropic strand encased in a copper sheath could reduce the risk of delamination during cool-down.

Research on temperature drift mechanism and compensation method of silicon piezoresistive pressure sensors

Silicon piezoresistive pressure sensors have played a key role in the detection field due to their small size, high sensitivity, and high integration. However, the factors such as its material particularity, the temperature dependence of the piezoresistive coefficient, the variation of carrier motion in the material caused by heat-induced stress, doping concentration, and resistance mismatch may cause the sensor to generate temperature drift. In this paper, the mechanism of temperature drift of silicon-based pressure sensor is analyzed initially, and then a comparison between BP neural network and wavelet neural network is done, which shows the root mean square error of the wavelet neural network is much smaller than that of the BP neural network and better linearity.

Influence of ceramic reinforcement characteristics on the microstructure and wear behavior of NiTi matrix composites

Because the important properties of the NiTi alloy, such as the shape memory effect, pseudoelasticity, and wear resistance, originate from a thermoelastic reversible martensitic transformation, these properties can be controlled using internal and external stresses. In this research, to improve the wear resistance and to minimize the mismatch stress in the matrix, YSZ microparticles with a thermal expansion coefficient close to that of an NiTi alloy were used as the reinforcement. In samples consolidated using spark plasma sintering, the role of YSZ reinforcement on microstructure and wear behavior was investigated and compared with NiTi-ZrO2 samples. The microstructural studies showed that using YSZ instead of monoclinic zirconia as the reinforcement can produce a lower mismatch stress and, therefore, a higher fraction of austenitic phase remains in the matrix. The austenite phase possesses strain recovery, which is beneficial for tribological applications. Therefore, NiTi-YSZ samples with a higher percentage of austenite and better mechanical properties demonstrated superior wear resistance. For example, the addition of 5 wt% YSZ to NiTi reduced the wear rate from 5.74 × 10−3 to 1.03 × 10−3 mm3/N.m. Investigation of wear mechanisms showed that abrasive, delamination and adhesive mechanisms mainly govern the wear process.

Compatibility of low thermal conductivity and high infrared emissivity of plasma-sprayed Sm2Hf2O7 and Pr2Hf2O7 coatings

In order to develop thermal-protection coatings, rare earth hafnates Sm2Hf2O7 (SHO) and Pr2Hf2O7 (PHO) coatings with pyrochlore structure were prepared on different substrates by atmospheric plasma spraying (APS). The infrared radiation, flame-ablation resistance, and thermal cycle performance were investigated in relationship with their morphologies, phase stabilities, thermophysical and mechanical properties. The results showed that both SHO and PHO coatings had compatible thermal-protection performance with lower thermal conductivity (0.711 and 0.779 W·m−1·K−1 at 1000 °C) and higher infrared emissivity (0.814 and 0.894 at 1000 °C) in the 3–5 μm band. Moreover, the supersonic flame ablation resistance of PHO coatings at high temperatures was greater than that of SHO coatings. Both SHO and PHO coatings have long thermal cycling life at low and medium temperatures and short thermal cycling life at high temperatures. The failure mechanisms of both type coatings were mainly associated with thermal expansion mismatch stress and fatigue stress generated during multiple thermal cycling.

Self-Closing Stretchable Cuff Electrodes for Peripheral Nerve Stimulation and Electromyographic Signal Recording

The cuff electrode can be wrapped in the columnar or tubular biological tissue for physiological signal detection or stimulation regulation. The reliable and non-excessive interfaces between the electrode and complex tissue are critical. Here, we propose a self-closing stretchable cuff electrode, which is able to self-close onto the bundles of tissues after dropping water. The curliness is realized by the mechanical stress mismatch between different layers of the elastic substrate. The material of the substrate can be selected to match the modulus of the target tissue to achieve minimal constraint on the tissue. Moreover, the self-closing structure keeps the cuff electrode free from any extra mechanical locking structure. For in vivo testing, both sciatic nerve stimulation to drive muscles and electromyographic signal monitoring around a rat's extensor digitorum longus for 1 month prove that our proposed electrode conforms well to the curved surface of biological tissue.

Delamination behaviors of an epoxy-impregnated REBCO pancake coil during a quench

For the epoxy-impregnated high-temperature superconducting (HTS) REBa2Cu3O7-x (REBCO) magnets, the quench cannot be still avoided in the operation, which may induce the mechanical damage of the magnets and even burning out. In this research, an electromagnetic-thermo-mechanical coupling model is proposed to explore the delamination mechanisms of an epoxy-impregnated REBCO pancake coil during a quench, and thereinto, a cohesive zone model (CZM) based on the mixed-mode traction-separation law is used to simulate the onset and growth of the delamination. The numerical results indicate that the delamination of the coil is mainly concentrated in the quenching turns. The reason is that localized high-temperature gradients are generated in these turns due to the large temperature increase during a quench, and thus the accumulated stress induced by the thermal mismatch stress locally leads to the cracking of the interface between the superconducting layer and Hastelloy substrate. Meanwhile, the delamination onset of the coil is predicted by mixed-mode delamination according to the quadratic failure criterion, and the delamination propagation is mainly determined by shear stress. Up to now, the delamination mechanisms of the epoxy-impregnated pancake coil during a quench are clarified in detail, which is different from those during cooling down. To further understand the delamination behaviors of the epoxy-impregnated pancake coil during a quench, the effects of the location and height of the heater, and the thickness of the epoxy resin on the interface delamination failure are discussed in the paper.

Effect of ball milling and solid solution treatment on the microstructure and interfacial behaviors of ZrMgMo3O12p/2024Al composites

ZrMgMo3O12 is a negative expansion material, while 2024Al alloy is a positive expansion material. The difference in thermal expansion coefficients between them will cause thermal mismatch stress at the interface in ZrMgMo3O12p/2024Al composites. Therefore, the interface behaviors of ZrMgMo3O12–Al determine the properties of ZrMgMo3O12p/2024Al composites to a great extent. The effects of ball milling and solid solution treatment on the microstructure and interface behaviors of 10 vol% ZrMgMo3O12p/2024Al composites were studied to improve the reticular microstructure of ZrMgMo3O12 distributed at the grain boundary of the α-Al matrix. The results showed that with increasing milling energy, the microstructure of the composites changed from reticular to equiaxed, and the distribution of ZrMgMo3O12 reinforcements in the matrix was more uniform. The content and size of the primary phase Al2Cu decreased with increasing solid solution treatment time. In addition, only ZrMgMo3O12–Al, Al2Cu–Al12Mo and Al–Al12Mo interfaces can be observed, but it is difficult to observe the interfaces of ZrMgMo3O12–Al12Mo in the composites milled for 12 h and solution treated for 24 h, which is related to the decomposition mechanism of ZrMgMo3O12. The decomposition mechanism is as follows: Al atoms from the α-Al matrix captured O atoms from ZrMgMo3O12 to form Al2O3. ZrMgMo3O12 simultaneously released Mo, Zr and Mg atoms. Mo atoms were enriched and nucleated in situ and precipitated with Al atoms to form the intermetallic compound Al12Mo, while Zr and Mg atoms entered the α-Al matrix to form a solid solution.

Feasibility research of Yb3Al5O12 garnet as environmental barrier coating materials

In this work, Yb3Al5O12 (YbAG) garnet, as a new material for environment barrier coating (EBC) application, was synthesized and prepared by atmospheric plasma spraying (APS). The phases and microstructures of the coatings were characterized by XRD, EDS and SEM, respectively. The thermal stability was measured by TG-DSC. The mechanical and thermal-physical properties, including Vickers hardness (Hv), fracture toughness (KIC), Young's modulus (E), thermal conductivity (κ) and coefficient of thermal expansion (CTE) were also measured. The results showed that the as-sprayed coating was mainly composed of crystalline Yb3Al5O12 and amorphous phase which crystallized at around 917 °C. Moreover, it has a hardness of 6.81 ± 0.23 GPa, fracture toughness of 1.61 ± 0.18 MPa m1/2, as well as low thermal conductivity (0.82–1.37 W/m·K from RT-1000 °C) and an average coefficient of thermal expansion (CTE) (∼6.3 × 10−6 K−1 from RT to 660 °C). In addition, the thermal shock and water-vapor corrosion behaviors of the Yb3Al5O12-EBC systems on the SiCf/SiC substrates were investigated and their failure mechanisms were analyzed in details. The Yb3Al5O12 coating has an average thermal shock lifetime of 72 ± 10 cycles as well as an excellent resistance to steam. These combined properties indicated that the Yb3Al5O12 coating might be a potential EBC material. Both the thermal shock failure and the steam recession of the Yb3Al5O12-EBC systems are primarily associated with the CTE mismatch stress.