Film Stress Research Articles

Temperature-Dependent Residual Stresses and Thermal Expansion Coefficient of VO2 Thin Films

This study aims to investigate the thermomechanical properties of vanadium dioxide (VO2) thin films. A VO2 thin film was simultaneously deposited on B270 and H-K9L glass substrates by electron-beam evaporation with ion-assisted deposition. Based on optical interferometric methods, the thermal–mechanical behavior of and thermal stresses in VO2 films can be determined. An improved Twyman–Green interferometer was used to measure the temperature-dependent residual stress variations of VO2 thin films at different temperatures. This study found that the substrate has a great impact on thermal stress, which is mainly caused by the mismatch in the coefficient of thermal expansion (CTE) of the film and the substrate. By using the dual-substrate method, thermal stresses in VO2 thin films from room temperature to 120 °C can be evaluated. The thermal expansion coefficient is 3.21 × 10−5 °C−1, and the biaxial modulus is 517 GPa.

Optical, Electrical, Structural, and Thermo-Mechanical Properties of Undoped and Tungsten-Doped Vanadium Dioxide Thin Films.

The undoped and tungsten (W)-doped vanadium dioxide (VO2) thin films were prepared by electron beam evaporation associated with ion-beam-assisted deposition (IAD). The influence of different W-doped contents (3-5%) on the electrical, optical, structural, and thermo-mechanical properties of VO2 thin films was investigated experimentally. Spectral transmittance results showed that with the increase in W-doped contents, the transmittance in the visible light range (400-750 nm) decreases from 60.2% to 53.9%, and the transmittance in the infrared wavelength range (2.5 μm to 5.5 μm) drops from 55.8% to 15.4%. As the W-doped content increases, the residual stress in the VO2 thin film decreases from -0.276 GPa to -0.238 GPa, but the surface roughness increases. For temperature-dependent spectroscopic measurements, heating the VO2 thin films from 30 °C to 100 °C showed the most significant change in transmittance for the 5% W-doped VO2 thin film. When the heating temperature exceeds 55 °C, the optical transmittance drops significantly, and the visible light transmittance drops by about 11%. Finally, X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to evaluate the microstructure characteristics of VO2 thin films.

Effect of YF3 Coating Process on Film Stress

YF3 rare-earth fluoride coating material has received wide attention as an alternative to ThF4 low refractive index material, but the film layer can be seriously affected due to the different coating processes. Therefore, to investigate the rules between the coating processes of YF3 rare-earth fluoride coating materials, YF3 thin films were coated with different deposition rates and substrate temperatures. The stresses of YF3 thin films under different processes were obtained by a thin film stress meter, and the microscopic morphology of the thin films was characterized by atomic force microscopy. The results show that: YF3 monolayer film is tensile stress; in the deposition rate between 0.5nm/s~1.5nm/s, with the increase of the coating deposition rate YF3 monolayer film stress tends to decrease, the roughness of the film is also gradually increased, and the overall thickness of the film surface decreases; in the substrate temperature is greater than 50 ℃, the film stress is significantly reduced.

Micro-structural, electrical, and tribological properties of SiC/DLC composite coating grown by PLD on steel

Protective diamond-like carbon (DLC) composite coating was grown on cylindrical steel surface in a simple inclined pulsed laser deposition (PLD) process. Based on this kind of flexible film-synthesis method, mixed deposition was used to generate the gradient interface that could enhance adhesive strength in Ti/SiC interface, and periodic SiC/DLC layers were grown to reduce residual stress of the pure DLC film. This kind of PLD-grown SiC/DLC composite coating on the curved thin steel that was imitated as piston cylinder showed excellent adhesive property and toughness. Compared with the bare steel, DLC-filmed steel strip had a much lower friction coefficient, higher nano-hardness, and enhanced elastic modulus. Raman spectroscopies showed transform of the micro-structure in DLC layers during friction process, revealing the important reason for reducing friction coefficient. Electrical conductivity test displayed the DLC-filmed steel strip was in high resistance state, although its thickness was only 1.27 μm. Further on, good adhesive strength, excellent toughness and high resistance state of SiC/DLC composite coating could be applied in the MEMS advices.

A study on using a binder-free TiO2 paste with sodium dodecylbenzene sulfonate addition to improve the performance of low-temperature DSSCs

The utilization of binder-free TiO2 paste allows for the dye-sensitized solar cells (DSSCs) TiO2 electrode fabrication at low temperatures, thus making it a potential material for flexible solar cell devices. However, the physical robustness of formed electrodes using TiO2 paste at low temperatures remains challenging, including issues such as cracking due to film stress and challenges in adjusting the viscosity of the TiO2 paste. In this study, we enhanced the binder-free TiO2 paste performance by incorporating sodium dodecylbenzene sulfonate (SDBS) as an additive for TiO2 electrode precursor solution. It demonstrated that SDBS additives help to relieve internal stress and ultimately enhance the physical and structural characteristics of the films by improving the viscosity in the TiO2 paste and promoting the dispersed pinholes within the TiO2 films. Furthermore, implementing TiO2 paste with varying ratios of SDBS has enabled the fabrication of DSSCs with the highest achieved power conversion efficiency (PCE) of 4.53%, representing an enhancement of nearly 15% compared to TiO2 paste without SDBS. This research presents a practical and effective approach for preparing highly efficient binder-free TiO2 paste.

Silicon-tin thin-film anodes for low and high power-density lithium-ion batteries

Silicon (Si) is a promising replacement material for graphite, the commonly utilized anode material in lithium-ion batteries (LIBs). Si has a theoretical specific capacity up to 11 times higher than that of graphite. However, pure Si has two main issues: high electronic resistivity and significant volume change during charge/discharge cycling. To overcome these issues, Si was alloyed with high-conductivity materials. In this contribution, physical vapor deposition was utilized to produce binderless silicon-tin anodes for LIBs. Tin is a lithium-active alloying material that has high electrical conductivity and high specific capacity (i.e., up to three times that of graphite). Electronic resistivity, residual film stress, X-ray photoelectron spectroscopy, Raman spectroscopy, and cyclic voltammetry measurements were performed to thoroughly analyze the deposited Si-Sn films and help understand their charge/discharge cycling performance at low and high C-rates. At a low C-rate of 0.5 C, the alloyed films did not show any improvement compared to pure Si. However, the alloyed films showed better performance compared to pure Si when tested at a high C-rate of 10 C. Si-Sn films with (40 – 50 wt.% Sn) were able to retain a specific capacity of around 680 mAh g−1 after 1000 cycles.

Effects of residual stress and microstructure on extremely anisotropic grain growth in nanotwinned Cu films

In this study, the thermal stability and microstructural evolutions of nanotwinned copper (NT-Cu) films under thermal cycling are investigated. The microstructures and residual stresses were tuned by the electroplating current density. It was found that the thermal stability of the NT-Cu films decreased with an increase in current density. The texture transition from (111) to (100) suggests that the extremely anisotropic grain growth (EAGG) was driven by the minimization of strain energy. Bending beam technic was employed to measure the residual stress in the NT-Cu films. Large residual stress in the NT-Cu films is also considered a main driving force for the transition to the (100)-oriented texture. Additionally, strain energy, surface/interface energy, grain boundary energy, and twin boundary energy were calculated, and the results supported the experimental results.

Preparation and properties of anti-reflective and anti-thermal shock of Y2O3/AlN composite films on CVD diamond

Y2O3/AlN composite films with different thicknesses of AlN interlayer were prepared on CVD diamond by RF magnetron sputtering. The influence of AlN thickness on the microstructure, adhesion strength, film stress, thermal shock resistance and infrared transmittance of the films was investigated by X-ray diffractometer, scanning electron microscopy, scanning probe microscopy, UV–Vis spectrophotometer, Fourier transform infrared spectrometer, scratch tester and 3-D optical surface profiler. The results show that CVD diamond coated with Y2O3/AlN composite films with different thicknesses of AlN have different degrees of anti-reflective effects in the 8–10 μm infrared band. The adhesion strength and quality of the composite film were improved with the AlN thickness increased, and the anti-reflective and anti-thermal shock of the film showed a trend of first increasing and then decreasing. CVD diamond coated with Y2O3/AlN composite films with 700 nm AlN interlayer has an average transmittance of 77 % in the 8–10 μm band and a good adhesion between interfaces. It can withstand rapid temperature changes and effectively protect the diamond substrate with little or no degradation of infrared optical properties.

A review on flow instability in hydro-viscous drive

Hydro-viscous drive (HVD) plays a significant role in smoothly transferring torque and flexibly regulating the velocity of the disks. By hydro-viscous drive, we mean that the viscous shear stress of the thin oil film between a multi-layer assembly of rotating parallel disks is generated to transmit torque and power. The laminar-to-turbulent transition is an extremely complicated issue due to the combined effects of squeeze and shear on the oil film within the microscale friction pair system. Hence, a comprehensive and thorough analysis of flow instability in fluid-thermal-solid interaction of tribodynamic behavior is highly desirable. Following a brief introduction of fundamentals of HVD, this paper provides an overall review on the instability mechanisms for three types of canonical flow dynamic models, i.e., plane squeeze flow, plane shear flow, and rotating-disk flow. The effects of various aspects of wall conditions and working media, such as surface microstructure, and temperature-dependent viscosity, on flow instability are then summarized, which can serve as a reference and guidance for optimizing the design of friction pair systems. Based on the review of the former progress, this paper not only explores the in-depth mechanisms regarding the laminar-to-turbulent transition in microchannel flow, but also provides the possibility of bridging the gap between flow instability and tribodynamic behavior.

Hybrid periodic microstructures fabricated on chromium films by SLM-assisted nanosecond laser processing

An effective technique for preparing different hybrid periodic structures on chromium (Cr) films of different thicknesses using a 1064-nm nanosecond laser assisted by spatial light modulator (SLM) is proposed. For 1000-nm Cr films, by combining periodic modulated grating (MG) patterned by SLM with laser-induced periodic surface structures (LIPSS), regular dual-scale MG-LIPSS can be fabricated, with the morphological characteristics controlled by laser fluence, effective pulse number, and MG period. Due to diffraction effects of MG and LIPSS, MG-LIPSS patterned surfaces exhibit vivid anisotropic structural colors. In contract to MG-LIPSS, complex periodic structures consisting of MG and cracks (MGC) are fabricated on 200-nm Cr films due to the more significant thermal stress of a thinner film. Although the cracks of MGC distributed randomly, MGC show a long-range order characteristic with a certain transmittance, which can be utilized as a transmission grating with diffraction effects. These results demonstrate that optical field modulated laser machining based on SLM offers an efficient, cost-effective, and controllable method for fabricating large-area periodic structures on Cr films. Moreover, variations in film thickness can be employed to explore hybrid microstructures with specific properties, and thus utilized in diverse applications, such as optical components and anti-counterfeiting measures.

Structural evolution and self-destructive behavior of Mo/Ti transition layers during free-standing diamond-film preparation

Graphite-based metal transition-layer substrates hold great potential for preparing large-area diamond films and achieving crack-free release. In this study, a five-inch crack-free free-standing diamond film was prepared on a graphite substrate with a sacrificial Mo/Ti double-layer transition layer using direct-current arc plasma jet chemical vapor deposition (DC jet CVD). The interfacial bonding and diffusion behavior between the diamond and the transition layers were investigated. The effect of abundant metal particles and elemental diffusion between layers on film adhesion and stress release are discussed and analyzed. The surface of Ti layer prepared by the multi-arc ion-plating contains numerous metal particles that provide a template for the fine-crystalline Mo layer. Metal particles are encapsulated and embedded in the diamond and act as anchors that ensure the stability of the substrate–diamond bond during high-temperature deposition. After deposition, those metal particles act as weak points and play crucial stress-release roles. Mutual elemental diffusion occurs between the Ti, Mo, and diamond layers. Ti penetrates the Mo layer to form a continuous 140-nm-thick thin layer during the initial deposition period, facilitating diamond nucleation. Carbon from the plasma and diamond diffuses into the metal transition layer. Thermal stress generated between the diamond and the graphite substrate is released by destroying the metal transition layer with high thermal expansion coefficient and low elastic modulus during cooling from the deposition temperature. Cracks rapidly propagate along the interface between destroyable transition layers and graphite substrate, which results in the liftoff of a fully crack-free diamond film.

A New Method for Evaluating the Influence of Coatings on the Strength and Fatigue Behavior of Flexible Glass

Flexible glass is an interesting substrate for a variety of displays, especially bendable or foldable ones, as it shows excellent surface properties and appealing haptics. With the necessary skill, flexible glass can be coated with thin films of different functionality, such as electrical or optical thin films, using plasma processes. In displays, thin film coatings such as transparent conductive electrodes and/or antireflective layer stacks are of major importance. Despite its attractive surface properties, however, flexible glass is still brittle, and its strength must be examined and monitored during any functionalization process, especially with regard to the fatigue behaviour. Currently, specific setups for cyclic fatigue testing of coated flexible glass are not available. Therefore, a new test method is presented herein for easy-to-handle rapid strength and fatigue testing using an endurance testing machine. This method overcomes two issues with the commonly used two-point bending test: the correct insertion of specimens is much easier, and both strength and fatigue testing using the same setup are now possible. Finite element method (FEM) simulation outcomes and first experimental simple fracture tests show that results comparable to those with a two-point bending test setup can be achieved with less effort. This makes it possible to analyze the fracture behaviour of flexible glass under cyclic loading and to evaluate the influence of thin film stress and other coating properties on its performance.Graphical

Simulation Analysis and Parameter Optimization of Residual Film Pickup Process Based on Finite Element Method

The extended duration of mulching in Xinjiang cotton fields leads to a significant decline in the tensile strength of plastic film. When recycling is in operation, the soil and the spring teeth of the machinery used can easily cause secondary damage and fracture the residual film. Establishing appropriate working parameters for recycling is essential to enhance the overall quality of collection efforts. By analyzing the motion process of a chain-tooth residual film pickup device, we identified key working parameters that significantly impact the efficiency of recycling. Employing the finite element method (FEM) and a coupled algorithm incorporating smooth particle hydrodynamics (SPH), we developed a coupled finite element model representing the interaction among spring teeth, soil, and residual film. Through simulation and analysis of the process of inserting the spring teeth into the soil to collect film, we derived the governing rules for residual film stress and deformation changes. Utilizing forward speed, rotational angular velocity, and angle of entry into the soil of the spring teeth as test factors and selecting the residual film stress and the residual film deformation as test indices, we conducted a multi-factor simulation test. We established a mathematical model correlating test factors with test indices, and the influence of each factor on the test index was analyzed. Subsequently, we optimized the working parameters of the spring teeth. The results indicated that the optimal working parameters are forward speed of 1111.11 mm/s, rotational angular velocity of 25 rad/s, and angle of entry into the soil of 30°. At these values, the average peak stress of residual film was 4.51 MPa and the height of residual film pickup was 84.48 mm. To validate the optimized the spring teeth impact on performance, field experiments were conducted with recovery rate and winding rate as test indices. The results demonstrated a 92.1% recovery rate and a 1.1% winding rate under the optimal combination of working parameters. The finite element model presented in this paper serves as a reference for designing and analyzing key components of residual film recycling machines.

Study on the Hole-Forming Performance and Opening of Mulching Film for a Dibble-Type Transplanting Device

In order to improve the quality of transplanting devices and solve the problems of the poor effect on soil moisture conservation and more weeds easily growing due to the high mulching-film damage rate with an excessive number of hole openings, we developed a dibble-type transplanting device consisting of a dibble-type transplanting unit, a transplanting disc, and a dibble axis. The ADAMS software Adams2020 (64bit) was used to simulate and analyze the kinematic track of the transplanting device. The results of the analysis show that, when the hole opening of the envelope in the longitudinal dimension was the smallest, the transplanting characteristic coefficient was 1.034, the transplanting angle was 95°, and the transplanting frequency had no influence. With the help of the ANSYS WORKBENCH software Ansys19.2 (64bit), an analysis of the process of the formation of an opening in the mulching film and a mechanical simulation of this process were completed. The results indicate that, when the maximum shear stress of the mulching film was the smallest, the transplanting characteristic coefficient was 1.000, the transplanting frequency was 36 plants·min−1, and the transplanting angle was 95°. In addition, the device was tested in a film-breaking experiment on a soil-tank test bench to verify the hole opening in the mulching film. The bench test showed that, when the longitudinal dimension was the smallest, the transplanting characteristic coefficient was 1.034, the transplanting frequency was 36 plants·min−1, and the transplanting angle was 95°. When the lateral dimension was the smallest, the transplanting characteristic coefficient was 1.034, the transplanting frequency was 36 plants·min−1, and the transplanting angle was 90°. The theoretical analysis, kinematic simulation, and soil-tank test results were consistent, verifying the validity and ensuring the feasibility of the transplanting device. This study provides a reference for the development of transplanting devices.

Impact of temperature on residual stress and bonding in diamond-like carbon film: A molecular dynamics study under various deposition conditions

Diamond-like carbon (DLC) films exhibit excellent frictional properties; however, the high residual stress within the film, which causes detachment issues, limits their application in mechanical sliding surfaces. We have previously clarified via molecular dynamics (MD) simulations that the growth of sp3 clusters after the transition from sp2 to sp3 bonds during film formation causes the in-plane compressive stress in DLC films. In this study, we investigated whether this stress generation mechanism holds under various deposition conditions in the MD simulations, wherein incident energy, atomic flux, and substrate temperature were systematically changed. The results of the MD simulations consistently revealed that the compressive stress generation mechanism remains unchanged across different deposition conditions. With increasing incident energy and atomic flux and decreasing substrate temperature, in-plane compressive stress in the film increased owing to increased sp3 content. Conversely, when the incident energy was further increased, both stress and sp3 content decreased. Based on a molecular statistical calculation, it was shown that the stable bonding state changed from sp3 to sp2 at high temperatures. Because the temperature monotonically increased with increasing incident energy, it was concluded that the change in the stable bonding state caused the non-monotonic change in the stress and sp3 content at high temperatures.

The influence of Pt islands on the failure of Pt thin film on a flexible PET substrate

Flexible electronic devices must adapt to compliant polymeric substrates, thus maintaining the mechanical integrity of the multilayer systems is crucial. This study investigates the mechanical failure caused by active islands, focusing on how Pt islands influence the failure mechanism of a thin Pt film on a flexible polyethylene terephthalate (PET) substrate under uniaxial tensile loading. Tensile testing of the Pt film/PET bilayer revealed a failure progression in the Pt blanket film, characterized by crack initiation, elongation and merging, eventually delamination, and buckling, with the increase in tensile strain. Pt islands induced early crack initiation at comparatively low strains due to increased stress near their vertical edges. The impact of island shape and gap on the crack formation in a Pt film was subsequently investigated. The gap between islands, oriented perpendicular to the loading direction, has minimal impact on crack number and density; the presence of Pt islands reduced the stress in the Pt film within the gap, thereby lowering the susceptibility of cracking in these areas. Variations in island shape and gap along loading direction alter the stress profile in the film between islands but did not significantly impact crack density. Crack density is believed to be primarily associated with pre-existing defects, with the formation of cracks serving as a stress relief mechanism that prevents further crack initiation. Our study sheds light on the impact of active islands on blanket film failure and offers practical recommendations to mitigate crack formation, which may contribute to the optimisation of flexible electronics design.

Stresses in silicon dioxide films deposited from dielectric targets: results of atomistic modeling

The previously proposed method of molecular dynamics simulation of the deposition of thin films from metal targets is adapted to the case of dielectric targets and applied to silicon dioxide films. The possibility of not only silicon atoms leaving the target, but also clusters with oxygen atoms was taken into account by adding O=Si=O molecules to the flow of deposited atoms. Atomistic film clusters were obtained by high-energy and low-energy deposition at different percentages of molecules in the flow of deposited atoms. The values of the stress tensor components are calculated. With high-energy deposition, compressive stresses are observed, with low-energy deposition, tensile stresses are observed. The absolute values of the diagonal components of the stress tensor increase with increasing proportion of molecules in the flow of deposited atoms.

Dual interface strategies enable efficient wide bandgap perovskite solar cells

High performance wide bandgap perovskite solar cells (WB-PSCs) have found widespread applications in tandem solar cells. In WB-PSCs, achieving a high conversion efficiency relies on the effective utilization of light absorption and minimization of electronic defects. In this work, electronic defects at the surface and grain boundaries of perovskite materials have been passivated by n-butylammonium bromide (BABr) to suppress carrier non-radiative recombination. Confirmed through x-ray powder diffraction and x-ray photoelectron spectroscopy spectra, ultra-thin two-dimensional (2D) perovskite layers were successfully generated on a perovskite surface. The BABr-treated devices exhibited an increased fill factor and open circuit voltage (VOC) compared to the references. Furthermore, a nanotextured electrode with a roughness of 22.98 nm was employed to trap light. The nanotextured buried interface not only promoted light utilization but also alleviated residual stress and micro-strain in the perovskite film compared to the smooth substrate. Finally, the champion WB-PSC achieved a power conversion efficiency of 20.46% in the reverse scan. These findings pave a promising path for the development of solution-processed perovskite films on nanotextured silicon substrates to improve the performance of monolithic tandem solar cells.

The adhesion strength and stability of TiN films deposited on magnesium substrate with different substrate roughness

The adhesion stability of a film on its substrate is essential for enhancing and maintaining the protective performance of the film. In this study, titanium nitride (TiN) films were deposited on Mg alloy (ZM21) substrates with roughness values (Ra) of 200, 400, and 600 nm by magnetron sputtering. The effects of the substrate roughness on the film residual stress, adhesion strength, and wear resistance were studied. The results indicated that the residual stress of the film decreased as the substrate roughness increased. Adhesion is affected by a combination of the balance of film residual stresses and mechanical anchorage and the distribution of stresses on the rough surface. The TiN film on a substrate with a Ra of approximately 400 nm exhibited optimal adhesion strength. The wear resistance decreased with increasing Mg substrate roughness. Both the film residual stress and adhesion strength of the TiN film deteriorated after natural storage for 60 d, and the TiN film on the substrate with a Ra of 400 nm had optimal adhesion stability.

Deposition of SiN<sub>x</sub> Films with Controlled Residual Stress from SiH<sub>4</sub>-NH<sub>3</sub>-He Gaseous Mixture in Inductively Coupled Plasma

We have studied residual mechanical stresses of SiNx films deposited on silicon substrates from a SiH4-NH3-He gaseous mixture in an inductively coupled plasma reactor at a deposition temperature of 150 °C. By varying the flow rate ratio of the reacting gases, the power of the plasma source and the pressure in the reaction chamber, it is possible to obtain SiNx films with tensile or compressive residual stresses. The stress drift was estimated within four weeks after film deposition. It has been shown that for nitride films with residual stresses initially close to zero, an increase in the level of compressive stresses to (–300) MPa is observed during storage.