Compressive Stress In Films Research Articles

Enhanced adhesion and wear resistance of DLC films on AZ31 alloy achieved with high bias voltage

Diamond-like carbon (DLC) films exhibiting high adhesion and exceptional wear resistance were successfully applied onto the AZ31 surface without a adhesive layer. The structural features of the film were examined utilizing Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), and Raman spectroscopy. Mechanical properties were evaluated through Nano-indentation testing, while adhesion was evaluated using a scratch tester. Residual stress was determined by Stoney's equation. The results indicate that utilizing a high substrate bias ranging from −500 to −800 V, results in the presence of multiple craters and dislocations, which contribute to the enhancement of high residual compressive stress and hardness in the DLC film. The heightened compressive stress effectively embeds carbon atoms into the surface of the soft AZ31, creating an interfacial layer that significantly improves adhesion and wear resistance of the film. In contrast, at moderate bias levels with a range from −300 to −500 V, an increase in substrate temperature leads to the expansion and discontinuation of adjacent layers that decreases the mechanical properties, adhesion, and wear resistance of the film. When at low bias levels between 0 and -300 V, the hardness, crack resistance, residual stress, and wear resistance all increase with increasing of bias. Additionally, in this work, the mechanisms of adhesion and wear for the biased film are analyzed.

Comparative studies of the long-term corrosion behavior of Zr-Sn-Fe-Cr-Ni alloys in pure water at 360°C/18.6 MPa with high and low dissolved oxygen content

This study compared the uniform corrosion behavior of Zr-Sn-Fe-Cr-Ni alloys in deionized water at 360°C/18.6 MPa with high and low concentration of dissolved oxygen (DO). It was found that high concentration of DO accelerated the corrosion rate of the Zr-Sn-Fe-Cr-Ni alloys and led to an earlier corrosion transition. Increased DO concentration resulted in a higher content of t-ZrO2 near the metal-oxide interface, which induced greater in-plane compressive stress in the oxide film and a high-level phase transformation from t-ZrO2 to m-ZrO2. This, in turn, led to an earlier occurrence of corrosion transition.

Compressive stress reduction in sputter-deposited yttrium aluminum nitride (Y0.2Al0.8N) thin films for BAW resonators with high electromechanical coupling

In the past decade, rare earth elements have found increasing interest in enhancing the piezoelectric coefficient of aluminum nitride (AlN) thin films. Until now, scandium has been the most successful rare earth element to enhance the piezoelectric coefficient (d33) up to 600 %. In this research paper, we present a 250 % increase in d33 to 12.5 pC/N for sputter-deposited yttrium alloyed aluminum nitride (Y0.2Al0.8N) thin films compared to pure AlN. However, this increase in d33 comes with a substantial increase in compressive layer stress well above 1 GPa. Therefore, a chamber pressure sweep technique is introduced, resulting in a 40 % reduction in the stress level without significantly affecting the crystallographic film parameters to facilitate device integration. Even more, high compressive stress in thin films is a critical concern in piezoelectric MEMS devices such as bulk acoustic wave (BAW) resonators, as it can adversely affect the piezoelectric material properties and hence, the electromechanical performance. For this purpose, we demonstrate that the pressure sweep technique improves the electromechanical coupling factor of Y0.2Al0.8N to 7.02 %, being substantially above the reference value of 6.02 % set by pure AlN. In summary, the findings from this study may stimulate further effort to integrate YxAl1-xN thin films in BAW filter devices for future telecommunication applications.

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.

Electro-chemo-mechanical properties of anodic oxide (passive) films formed on Cu, Ni and Fe

Abstract Electro-chemo-mechanical properties of anodic oxide (passive) films formed on metals have been reviewed focusing on the results of stress variations caused by anodic oxidation of Cu, Ni, and Fe thin film electrodes in deaerated pH 8.4 borate buffer solution at 25 °C. The surface stress varies toward compressive direction due to adsorption of OH on Cu from aqueous solution as well as adsorption of oxygen on metals from gas phase. The stresses are generated with the growth of three-dimensional anodic oxide films on metals. The magnitude and sign (tensile or compressive) of the intrinsic film stress were determined by taking the residual stress of the substrate and the dielectrostriction into consideration. The tensile or compressive intrinsic film stress depends on p-type or n-type semiconductive properties of the anodic oxide films, which is explained in terms of the void formation or oxide formation in the metal side at the metal/film interface. Furthermore, the stress variation toward compressive direction during cathodic reduction of the anodic oxide films is explained in terms of the volume expansion due to the formation of intermediate species.

Microstructure evolution and mechanical properties of refractory molybdenum-tungsten nitride coatings

This study focused on the microstructure and the mechanical properties of MoWN coatings fabricated by radio frequency reactive co-sputtering technique. The modulation of input power, which was kept at 150 W for Mo and adjusted from 25 to 125 W for W target, was manipulated. The Ar/N2 inlet gas flow and the background temperature were set at 12/8 sccm/sccm and 350 °C, respectively. The MoWN possessed a multiphase polycrystalline structure featured with Mo(110), MoN2(200), B1-MoN(111) and δ-WN(100) phases. With the increase of W input power, the content of W rose from 4.1 to 24.2 at. %, and a gradual growth of the δ-WN(100) phase was observed. The 150/100 W/W deposited MoWN coating possessed 20.1 at. % W and was dominated by δ-WN(100) phase. From TEM analysis, the longer and wider nano columnar WN(100) grains could be confirmed. The growth of δ-WN(100) phase elevated the compressive residual stress in MoWN films from −4.25 to −7.18 GPa and enhanced the hardness of coatings from 19.7 to 25.7 GPa. This phenomenon also improved the wear resistance of thin film. The wear track maintained smooth and intact after a tribological test of 100 m wear length for MoWN films. Both mechanical and tribological properties were significantly improved, as compared to the MoN film. This could be attributed to the addition of W, which caused the phase transformation and competition between B1-MoN(111) and δ-WN(100) phases. The 150/100 W/W manufactured MoWN exhibited a highest hardness of 25.7 GPa and showed a superior anti-wear behavior with least damage on wear track.

Effect of Ion Flux Variation in DC Magnetron Sputtering on the Deposition of Cubic Boron Nitride Film

Cubic boron nitride (cBN) thin films were deposited using DC magnetron sputtering, and the effect of the ion flux on the deposition behavior and residual stress of the cBN thin films was investigated. To increase the ion flux, the magnetic force ratio of the central/peripheral permanent magnets inserted in the magnetron sputtering source was reduced. Due to the complementary relationship between ion flux and energy for cBN deposition, the critical bias voltage required for cBN nucleation decreased as the ion flux increased. The cBN content of the films was relatively higher under the deposition condition of the increased ion flux. This was interpreted to indicate the thinning of the intervening hexagonal boron nitride (hBN) layer formed prior to cBN nucleation. Comparing the compressive residual stress of the cBN films, the residual stress was relieved as the bias voltage decreased regardless of the ion flux. The increase in ion flux made it possible to deposit the cBN films at a low bias voltage, thereby depositing cBN films with lower residual stress. The results showed that reducing ion energy by increasing ion flux for cBN deposition is a promising method for depositing low-stress cBN thin film having thin intervening hBN layer.

Effect of SiO2 capping on the solid-phase-crystallized Ge thin films

We investigated the impact of SiO2 capping layers with a thickness of 70 nm, deposited through plasma-enhanced chemical vapor deposition, on the crystalline quality and electrical properties of 300-nm-thick, solid-phase crystallized Ge thin films on SiO2/Si substrates. After the solid-phase crystallization at temperatures ranging from 425 to 500 °C, Ge thin films with SiO2 capping layers showed a faster crystallization rate, larger grains, and reduced energy barriers at grain boundaries compared to uncapped Ge films, resulting in enhanced carrier mobility. The Raman spectra and X-ray diffraction measurements suggested that the compressive stress in Ge thin films sandwiched with SiO2 layers is responsible for these enhancements. These findings demonstrate the substantial influence of SiO2 capping layers on the solid-phase-crystallized Ge thin films, which has far-reaching implications for the use of Ge-based materials in various technological applications.

0D Additive for Flexible All-inorganic Perovskite Solar Cells to Go Beyond 60,000 Flexible Cycles.

All-inorganic cesium lead halide flexible perovskite solar cells (f-PSCs) exhibit superior thermal stability compared to their organic-inorganic hybrid counterparts. However, their flexibility and efficiency are still below-par for practical viability. Herein, we report our design to use a zero-dimensional (0D) Cs4 Pb(IBr)6 additive to transform tensile stress into compressive stress in the perovskite film, effectively preventing expansion of cracks for significantly improved mechanical durability. It is found that not only improved flexibility is harvested, but also the cell efficiency is increased for the all-inorganic flexible three-dimensional CsPbI3- x Brx solar cells. The CsPbI2.81 Br0.19 f-PSC retains over 97% of its initial efficiency even after 60,000 flexing cycles at a curvature radius of 5mm (R = 5mm). Simultaneously, 0D Cs4 Pb(IBr)6 enhances the crystallinity of the CsPbI2.81 Br0.19 film and passivates the defects along the grain boundaries, effectively improving the photovoltaic performance of the all-inorganic f-PSCs. The highest power-conversion efficiency obtained is 14.25% with a short-circuit current density of 18.47mA cm-2 , open-circuit voltage of 1.09V, and fill factor of 70.67%. This strategy paves the way for further improvement of the mechanical durability of all-inorganic f-PSCs. This article is protected by copyright. All rights reserved.

Ultrathick Boron Carbide Coatings for Nuclear Fusion Targets

Boron carbide is an attractive ablator for next-generation inertial confinement fusion (ICF) targets. Here we describe several aspects of our ongoing systematic studies of the deposition and processing of B4C coatings for ICF targets. We show that residual compressive stress in films can be reduced and the deposition rate increased by N-doping. We also demonstrate successful Si substrate etching and surface polishing and discuss remaining challenges and offer potential solutions to the buildup of particulates in the deposition chamber during prolonged coating runs, control of nodular growth defects, and lateral nonuniformity of film properties for deposition conditions with relatively low target-to-substrate distances.

Thermomechanical and Creep Behaviors of Multilayered Metallization Systems Developed for High‐Temperature Surface Acoustic Wave Sensors

AbstractThe structural stability and durability of a sensing device at high‐temperatures (HT) have important effects on its functionality and reliability. This becomes more critical in the devices that work based on the surface acoustic waves (SAW). In this study, the thermomechanical and creep behaviors of the metallization systems, RuAl, MoLa, and Mo suggested for the electrodes of HT‐SAW sensors are investigated. Several experiments are designed to obtain the state and the value of stress in the films deposited on a Si [100] substrate caused by the fabrication process and the operational conditions. Results show that after the film deposition, the intrinsic compressive stress in the MoLa film is significantly lower than the other films. However, after annealing the samples, the lowest residual stress is reached in the Mo. Linear stress variations during the thermal cycling of the annealed samples are obtained for all metallizations. To study the durability of the films at HT, the creep behavior of the samples is investigated by keeping them at a temperature of 600 °C for 10 h. The creep rate and its constitutive equation for the films are obtained. It is observed that the creep rate of MoLa and Mo films is much lower than RuAl.

The anisotropic oxidation behavior of Hastelloy X alloy fabricated by laser powder-bed fusion (LPBF) during the cyclic oxidation process

The Hastelloy X alloy manufactured via LPBF has anisotropic oxidation behavior at 900 °C. The vertical cross section (V) specimen has worse oxidation resistance and more severe spalling behavior than the horizontal cross section (H) specimen as its low grain boundary density. During the oxidation process, larger voids was generated at Cr2O3-spinel interface of the V specimen to weaken the adhesive strength of oxidation film, and α-NiMoO4 with high cracking susceptibility only occurred in spinel layer of the V specimen, which broke down under high compressive stress in oxidation film to promote the spalling of oxides.

Substitutional effect of Mg2+ on structural and magnetic properties of La2MgxNi1‐xMnO6 double‐perovskite thin films

AbstractBy doping different concentrations of Mg2+ at Ni site, the (00l)‐oriented La2MgxNi1‐xMnO6 (abbreviated as LMxNMO, x = 0, 0.1, 0.2, 0.3, 0.4) double‐perovskite thin films were epitaxially grown by pulsed laser deposition. The substitutional effect of Mg2+ on the structural and magnetic properties of the films is comprehensively investigated. It is found that with the increase of Mg‐doping concentration, the in‐plane and out‐of‐plane lattice constants as well as the cell volume of the LMxNMO thin films increase, which could be ascribed to the radius difference between Mg2+ and Ni2+/Ni3+ ions, resulting in the in‐plane compressive stress in LMxNMO films. When the Mg‐doping concentration is small (x ≤ 0.1), the doped Mg2+ tends to substitute Ni3+, which restrains the intensity of antiferromagnetic interaction between Ni3+‐O‐Mn3+, resulting in the reduced the exchange bias field as well as the increased the saturation magnetization. However, when the Mg‐doping concentration increases to x ≥ 0.2, Mg2+ becomes to mainly replace Ni2+ position, which could inhibit the super‐exchange ferromagnetic interaction between Ni2+‐O‐Mn4+ magnetic paths and thus reduce the saturation magnetization. The enhanced magnetic properties can be obtained in the LM0.1NMO double‐perovskite thin film, with a large saturation magnetization of 492.12 emu/cm3 and a high Curie temperature of 262.7 K.

NiAl (0 0 1) terminated surface effect on the growth of the Al thin film

In this work, NiAl alloy material was adopted as a substrate surface to investigate the formation process of Al thin film using molecular dynamics simulation. The NiAl (001) substrate with both Al-terminated and Ni-terminated surface was considered to identify their effect on the growth behavior of Al thin film. The results show that Al atoms follow initially bilayer-like growth mode for both terminated surfaces. At this stage, the film seemed to adopt the fourfold geometry with (001)-orientation. However, upon further deposition, the Al adatoms behavior on the Al-terminated surface becomes different from that on the Ni-terminated surface. The stress analysis along the growth direction indicates that the Al film on the Al-terminated surface has a compressive stress in the bulk film, while on the Ni-terminated surface, the Al film has a tensile stress. These results have a significant effect on the lattice parameter when the Al film exceeds four layers. By using the radial distribution function, we demonstrate that when Al film is deposited on Al-terminated surface, its lattice parameter is changed until 2.85Å, which close to the Al-Al bulk, but on the Ni-terminated surface this value becomes 2.49Å, which is near to the Ni-Ni bulk. These behaviors indicate that Al atoms on (Al or Ni)-terminated are arranged differently. For this, the Al film structure was characterized by common neighbor analysis. It is found that the Al atoms are arranged within the fcc structure for Al-terminated, but for Ni-terminated, the result shows the appearance of the fcc and hcp structures. On the other hand, the surface roughness investigated at 0.1 eV showed that the Al film has a rough surface morphology when deposited on Al-terminated compared to that obtained on Ni-terminated. However, by increasing the incident energy from 0.1 to 2 eV, the surface roughness decreases significantly for both terminated surfaces.

Microstructure and Mechanical Properties of TiN/Ti2AlN Multilayers

Titanium nitride (TiN) thin films deposited by high-power pulsed magnetron sputtering usually have a high compressive residual stress, which is not conducive for the adherence of TiN thin films. This study investigated the potential of Ti2AlN for releasing the compressive residual stress of HPPMS-deposited TiN thin films and evaluated the adherence strength and hardness of TiN/Ti2AlN multilayers by introducing the Ti2AlN MAX phase to form TiN/Ti2AlN multilayers. The results showed that smooth TiN/Ti2AlN multilayers with the TiN (111) and Ti2AlN (002) textures were successfully synthesized by HPPMS deposition and subsequent vacuum annealing. The compressive residual stress in TiN was released by Ti2AlN. The adherence strength of the TiN/Ti2AlN multilayers was improved after the release of the compressive residual stress, and the hardness of TiN/Ti2AlN multilayers was close to the annealed TiN. This study provides a novel approach for releasing the residual stress of hard ceramic thin films using the MAX phase.

Properties of CrN/TiN Multilayer Films With Nanoscale Layer Thickness

Chromium nitride (CrN)/titanium nitride (TiN) multilayer films at nanoscale layer thickness were prepared using a combination of reactive high-power impulse magnetron sputtering (HiPIMS) and pulsed magnetron sputtering. Each layer thickness was adjusted by controlling each deposition time. The hardness and Young’s modulus of the multilayer films, which were measured using nanoindentation technique, did not vary significantly with layer thickness. The film hardness ranged between 22 and 24.5 GPa, and the Young modulus was between 300 and 330 GPa. On the contrary, the compressive stress of the multilayer films gradually decreased with the increase in the TiN layer thickness. The friction coefficient of the multilayer film did not depend on the CrN layer thickness, but it reduced drastically to 0.2–0.3 with the increase in the TiN layer thickness in the range of the TiN layer thickness less than about 2 nm.

Dielectric and Antiferroelectric Properties of AgNbO3 Films Deposited on Different Electrodes

AgNbO3 antiferroelectric materials have become a hot topic due to their typical double polarization–electric field loops. AgNbO3 films usually exhibit superior properties to bulks. In this work, AgNbO3 films were fabricated via the pulsed laser deposition on (001) SrTiO3 substrate with (La0.5Sr0.5)CoO3, LaNiO3 and SrRuO3 bottom electrodes, in which the (La0.5Sr0.5)CoO3, LaNiO3 and SrRuO3 bottom electrodes were used to regulate the in-plane compressive stress of AgNbO3 films. It is found that AgNbO3 films deposited on (La0.5Sr0.5)CoO3, LaNiO3 and SrRuO3 bottom electrodes are epitaxial with dense microstructure. In changing the bottom electrodes from (La0.5Sr0.5)CoO3, LaNiO3 to SrRuO3, the in-plane compressive stress of AgNbO3 thin films becomes weaker, which leads to increased relative dielectric permittivity and reduced antiferroelectric–ferroelectric phase transition electric field EF from 272 kV/cm to 190 kV/cm. The reduced EF implies weakened antiferroelectric stability in AgNbO3 films. It can be seen that the antiferroelectric stability of AgNbO3 films could be regulated by changing the bottom electrodes.

Features of the porous morphology of anodic alumina films at the initial stage of disordered growth

A characteristic feature of disordered porous anodic film growth at the initial stage of aluminum anodizing was revealed by varying the electrolyte type and anodizing voltage. The samples were obtained by the electrochemical oxidation of thin aluminum films (100 nm thick) on SiO2/Si substrates in a 0.3 M oxalic acid at 10–50 V and were studied by SEM. The ImageJ analysis of the images revealed the simultaneous development of two large groups of pores: major pores with a large diameter and minor pores with a smaller diameter. When anodizing in oxalic acid at 10–50 V, it has been shown that the ratio of the diameters of the major and minor pores remains constant and is about 1.17. Using a geometric model, we demonstrated that the centers of the minor pores are located inside the elementary hexagonal cell formed by the centers of the major pores. Moreover, our results are very close to the theoretical value of 2/√3. At the initial stage of disordered pore growth, the development of minor pores rather than major pores is not a random process and is determined by energy-efficient conditions for the development of pores inside the hexagonal cells formed by the major pores. The increase in compressive mechanical stress in the anodic film leads to an interruption in the development of such pores.

Residual stress and tribological behavior of hydrogen-free Al-DLC films prepared by HiPIMS under different bias voltages

Hydrogen-free Al-DLC films were prepared by HiPIMS technology under different bias voltages through an AlC composite target. Surface and cross-sectional morphologies, chemical composition, microphase structure and bonding state, residual stress, hardness and elastic modulus, friction coefficient and wear resistance of Al-DLC films were investigated to study the influences of substrate bias voltage. Results show that bias voltage decreased film roughness (Ra) from 3.83 nm to 1.16 nm through ion bombardment. The increase of negative bias voltage improved film disorder and altered the content of sp3 bonds. However, the existence of sp3 bonds should not be responsible for residual stress. Local distortions generated by film disorder together with the mismatch between film and substrate give rise to residual compressive stress in Al-DLC films. Film hardness is in the range of 10 GPa to 17 GPa which is affected by the content of sp3 bonds and film disorder. The lowest wear rate of Al-DLC film is 6.07 × 10−8 mm3/Nm. Besides graphitization, friction can also generate more sp3 bonds at wear interface between Al-DLC film and the counterpart ball.

Investigating residual stress evolution in the deposition process of diamond-like carbon film through molecular dynamics

Diamond-like carbon (DLC) films possess excellent tribological properties, such as low friction and high wear resistance. However, DLC films detach easily owing to the high residual stress, which limits their application on mechanical sliding surfaces. To overcome this problem, it is crucial to clarify the residual stress generation mechanism in DLC films. In this study, molecular dynamics simulations were conducted on the deposition process of a DLC film to clarify the residual stress generation mechanism. The results suggest that sp3 hybridization clusters are nucleated in the sp2 formed on the film surface, and sp3 clusters grow in the sp2 carbon through the sp2 → sp3 bond change during the film growth process. The bond length of sp3 was approximately 8.5% larger than that of sp2 under stress-free bulk conditions. Thus, when the sp2 → sp3 bond change occurs under construction by surrounding atoms, compressive stress is generated in the sp3 cluster, which increases with the growth of the sp3 cluster. Based on the results, we conclude that the nucleation and growth of sp3 clusters in sp2 result in residual compressive stress in the DLC film. We believe that the present mechanism clarification of the stress development process in DLC films will facilitate the development of new methods for residual stress reduction.