Xuwen Liu

Corrosion behaviour of Fe–Mn–Si based shape memory steels trained by cold rolling

Abstract Fe–Mn–Si based high nitrogen steels have been studied in recent years for potential industrial applications. These steels show good shape memory properties, high strength and excellent ductility. In the present study, the effects of training history on the corrosion properties of Fe–Mn–Si–Cr–Ni based high nitrogen steels were investigated. The corrosion behaviour of shape memory alloys was analyzed by implementing anodic polarisation measurements and immersion tests. The shape memory steels in annealed, deformed and recovered conditions were studied to examine the training effect on their corrosion behaviour. The features of the anodic polarisation curves indicated a general corrosion type of these steels. The experimental results showed that Cr and Mn had a marked influence on the corrosion behaviour of the steels, followed by Ni, N and V. It was also apparent that the deformation during the shape memory training by cold rolling decreased the corrosion stability, and the recovery heating reduced further their corrosion resistance. However, further studies are needed in order to better understand the corrosion behaviour of the investigated alloys.

Localized recrystallization and cracking behavior of lead-free solder interconnections under thermal cycling

The failure mechanism of lead-free solder interconnections under thermal cycling has been studied by cross-polarized light microscopy, scanning electronic microscopy (SEM), and nanoindentation test. From the results of finite element modeling (FEM), it was found that the critical solder interconnection was located at the chip corner, and the stress was concentrated at the outer neck region beneath the ball grid arrays (BGA) component. The FEM results were in good agreement with the experimental observation. Two failure modes of the interconnections were identified: one is the intergranular or transgranular cracking through many small equiaxed recrystallized grains and the other is the transgranular cracking in few large irregularly shaped recrystallized grains. The results show that the localized recrystallization makes the Ag3Sn intermetallic compounds (IMC) coalesce and distribute sparsely, which leads to the degradation of the recrystallized microstructure and easy propagation of the cracks.

Aluminum oxide/titanium dioxide nanolaminates grown by atomic layer deposition: Growth and mechanical properties

Atomic layer deposition (ALD) is based on self-limiting surface reactions. This and cyclic process enable the growth of conformal thin films with precise thickness control and sharp interfaces. A multilayered thin film, which is nanolaminate, can be grown using ALD with tuneable electrical and optical properties to be exploited, for example, in the microelectromechanical systems. In this work, the tunability of the residual stress, adhesion, and mechanical properties of the ALD nanolaminates composed of aluminum oxide (Al 2O3) and titanium dioxide (TiO2) films on silicon were explored as a function of growth temperature (110–300 °C), film thickness (20–300 nm), bilayer thickness (0.1–100 nm), and TiO2 content (0%–100%). Al 2O3 was grown from Me3 Al and H2O, and TiO2 from TiCl4 and H2O. According to wafer curvature measurements, Al 2O3/TiO2 nanolaminates were under tensile stress; bilayer thickness and growth temperature were the major parameters affecting the stress; the residual stress decreased with increasing bilayer thickness and ALD temperature. Hardness increased with increasing ALD temperature and decreased with increasing TiO2 fraction. Contact modulus remained approximately stable. The adhesion of the nanolaminate film was good on silicon.

Microstructural Evolution and Mechanical Properties of Au-20wt.%Sn|Ni Interconnection

In this paper, the microstructural evolution and properties of Au-20wt.%Sn|Ni reaction couples were investigated from two perspectives: (1) by analyzing the microstructure of the as-soldered and aged samples, as well as (2) by measuring the mechanical properties of the intermetallic compounds formed within the reaction zone. The evolution of interfacial reaction products for both the as-soldered and aged interconnections was rationalized by using the experimental results in combination with assessed thermodynamic data from the Au-Ni-Sn system. Moreover, nanoindentation tests were implemented to measure the indentation modulus and hardness of the compounds formed at the interface. It was found that aging had a negligible influence on the elastic modulus and hardness of AuSn and Au5Sn, while the solubility of the third element significantly changed the indentation modulus and hardness of the intermetallic compounds.

Interfacial mechanical testing of atomic layer deposited TiO2 and Al2O3 on a silicon substrate by the use of embedded SiO2 microspheres

In this paper the authors present a next generation measurement system for interfacial mechanical testing of especially atomic layer deposited (ALD) thin films. SiO2 microspheres were embedded in 100 and 300 nm thick ALD TiO2 and Al2O3, deposited at 110 °C, 200 °C and 300 °C on a silicon substrate. The embedded microspheres were detached using a fully programmable semi-automatic microrobotic assembly station employed to carry out the lateral pushing and detaching force F (μN) measurement. The area of interfacial fracture A (μm2) was measured using scanning electron microscopy and digital image analysis to calculate critical stress of interfacial fracture σ (MPa). Work W (J) and energy release rate G (J m−2) of interfacial fracture were also calculated from the measurement results. Interfacial fracture from the film-substrate interface occurred only for TiO2 deposited at 200 °C which had a crystalline structure with the biggest grain size, signifying that for all of the other samples, film adhesion was excellent, and significantly better than film cohesion. Quantitatively this means that thin film interfacial adhesion to the substrate was also higher than the values of the critical stresses and the measured energy release rates. Interfacial toughness seems to be related to film thickness and crystallinity in the case of TiO2, but with Al2O3 the interfacial toughness seems to increase with the deposition temperature. The method presented in this paper is generic, and can be applied for the evaluation of interfacial mechanical properties, such as adhesion, between any various film-substrate-sphere system of choice.

On the reliability of nanoindentation hardness of Al2O3 films grown on Si-wafer by atomic layer deposition

The interest in applying thin films on Si-wafer substrate for microelectromechanical systems devices by using atomic layer deposition (ALD) has raised the demand on reliable mechanical property data of the films. This study aims to find a quick method for obtaining nanoindentation hardness of thin films on silicon with improved reliability. This is achieved by ensuring that the film hardness is determined under the condition that no plastic deformation occurs in the substrate. In the study, ALD Al2O3 films having thickness varying from 10 to 600 nm were deposited on a single-side polished silicon wafer at 300 °C. A sharp cube-corner indenter was used for the nanoindentation measurements. A thorough study on the Si-wafer reference revealed that at a specific contact depth of about 8 nm the wafer deformation in loading transferred from elastic to elastic–plastic state. Furthermore, the occurrence of this transition was associated with a sharp increase of the power-law exponent, m, when the unloading data we...

Hardness, elastic modulus, and wear resistance of hafnium oxide-based films grown by atomic layer deposition

The investigation of mechanical properties of atomic layer deposition HfO2 films is important for implementing these layers in microdevices. The mechanical properties of films change as a function of composition and structure, which accordingly vary with deposition temperature and post-annealing. This work describes elastic modulus, hardness, and wear resistance of as-grown and annealed HfO2. From nanoindentation measurements, the elastic modulus and hardness remained relatively stable in the range of 163–165 GPa and 8.3–9.7 GPa as a function of deposition temperature. The annealing of HfO2 caused significant increase in hardness up to 14.4 GPa due to film crystallization and densification. The structural change also caused increase in the elastic modulus up to 197 GPa. Wear resistance did not change as a function of deposition temperature, but improved upon annealing.

Single walled carbon nanotube network—Tetrahedral amorphous carbon composite film

Single walled carbon nanotube network (SWCNTN) was coated by tetrahedral amorphous carbon (ta-C) using a pulsed Filtered Cathodic Vacuum Arc system to form a SWCNTN—ta-C composite film. The effects of SWCNTN areal coverage density and ta-C coating thickness on the composite film properties were investigated. X-Ray photoelectron spectroscopy measurements prove the presence of high quality sp3 bonded ta-C coating on the SWCNTN. Raman spectroscopy suggests that the single wall carbon nanotubes (SWCNTs) forming the network survived encapsulation in the ta-C coating. Nano-mechanical testing suggests that the ta-C coated SWCNTN has superior wear performance compared to uncoated SWCNTN.

Nanodiamond embedded ta-C composite film by pulsed filtered vacuum arc deposition from a single target

Detonation Nanodiamonds (DNDs) are known to have sp3 core, sp2 shell, small size (few nm) and are gaining importance as multi-functional nanoparticles. Diverse methods have been used to form composites, containing detonation nanodiamonds (DNDs) embedded in conductive and dielectric matrices for various applications. Here we show a method, wherein DND-ta-C composite film, consisting of DNDs embedded in ta-C matrix have been co-deposited from the same cathode by pulsed filtered cathodic vacuum arc method. Transmission Electron Microscope analysis of these films revel the presence of DNDs embedded in the matrix of amorphous carbon. Raman spectroscopy indicates that the presence of DNDs does not adversely affect the sp3 content of DND-ta-C composite film compared to ta-C film of same thickness. Nanoindentation and nanowear tests indicate that DND-ta-C composite films possess improved mechanical properties in comparison to ta-C films of similar thickness.

Galvanic corrosion of structural non-stoichiometric silicon nitride thin films and its implications on reliability of microelectromechanical devices

This paper describes a reliability assessment and failure analysis of a poly-Si/non-stoichiometric silicon nitride thin film composite structure. A set of poly-Si/SiNx thin film structures were exposed to a mixed flowing gas (MFG) environment, which simulates outdoor environments, for 90 days, and an elevated temperature and humidity (85 °C/95% R.H.) test for 140 days. The mechanical integrity of the thin films was observed to degrade during exposure to the chemically reactive atmospheres. The degree of degradation was analyzed with nanoindentation tests. Statistical analysis of the forces required to initiate a fracture in the thin films indicated degradation due to the exposure to the MFG environment in the SiNx part of the films. Scanning electron microscopy revealed a porous-like reaction layer on top of SiNx. The morphology of the reaction layer resembled that of galvanically corroded poly-Si. Transmission electron microscopy further clarified the microstructure of the reaction layer which had a comp...