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To develop an ultra-high-temperature resistant coating for a reusable thermal protection system, the preparation of a tantalum-based MoSi2-Mo coating by a new two-step process of multi-arc ion plating and halide activated pack cementation is presented. The coating has a dense structure and is well compatible with the tantalum substrate, which can be thermally shocked from room temperature to 1750 °C for 360 cycles without failure. The mechanism of the coating’s excellent resistance to high-temperature thermal shocks is that a strong-binding gradient interface and a dense SiO2 oxide scale with good oxygen resistance are formed by the high-temperature self-diffusion of Si.

Surface dents may occur on both rail head and wheel tread and have a deep influence on their microstructures. This investigation aims at exploring the microstructure evolution and RCF of wheel/rail materials with spherical dents. The experimental results, obtained through a suitable instrumented roller-rig, highlight the different microstructure evolutions not only between rail and wheel, but also between front and rear areas of dent. Electron backscattered diffraction data shows that the grains around dent on wheel have higher strain, misorientation and quantity of large angle grain boundaries than those on rail roller, leading to a weaker RCF resistance in wheel rollers. Meanwhile, cracks tend to propagate in front area of dent on rail and rear area of dent on wheel.

A new YSZ (yttria stabilized zirconia) modified silicide coating was prepared on Mo-based alloy through a two-step process, including spraying of Si-Mo-YSZ slurry followed by reaction sintering. Microstructure and oxidation behavior of the coatings with different YSZ content (0, 5, 10 and 15%) were studied. The coating consists of a MoSi2-ZrSi2-SiO2 outer layer and a MoSi2 inner layer. The 10% YSZ modified coating possessed the best oxidation protection at ultra-high temperatures and, for the coating, the mass gain was 0.96 mg/cm2 after being exposed at 1725 °C in air for 22 h, whereas the uncoated Mo-based alloy completely ablated for 0.35 h. The excellent oxidation resistance of the silicide coating was attributed to the compact and self-healing SiO2 protective scale, which contained the dispersed ZrO2 and ZrSiO4. Keywords: Molybdenum-based alloy, Silicide coating, Oxidation, YSZ, MoSi2, Ultra-high temperature

The cobalt coating with an average thickness of 15–20 nm was uniformly plated on the surface of ultra-fine WC powders by electroless plating in the present work. The effects of electroless plating condition parameters on the deposition rate were studied. It was found that the deposition rate increased with the increase of CoSO4·7H2O content, pH value, complexing agent concentration, and bath temperature, while decreased with the increase of reducing agent (NaH2PO2·H2O) concentration. Furthermore, the cobalt deposition rate empirical equation and activation energy were determined. The results show that the activation energy (Ea) for the electroless cobalt on WC substrates is approximately 54.65 kJ/mol.

Deformation twinning in a medium carbon steel containing ferrite and pearlite under shock compression is investigated below and above the α–e phase transition stress (σT), within a peak stress range of 10–20 GPa. Free surface velocity histories are measured to obtain the Hugoniot elastic limit (2.7 GPa), σT (13.3 GPa) and bulk plastic strain. Postmortem samples are characterized with electron backscatter diffraction and transmission electron microscopy. Below σT, all the {112}〈111〉 twinning activation in ferrite and pearlite follows the Schmid law, while above σT, approximately 1/6 of the activation deviates from the law. Propagation of the {112}〈111〉 twins in pearlite depends on the orientation of the activated twinning plane relative to cementite lamellae. With increasing peak stress, the {112}〈111〉 twin area fraction in ferrite/pearlite increases greatly/slightly; the {112}〈111〉 twin area fraction in ferrite is about 7 times that of pearlite. The {332}〈113〉 twins can only form above σT, favoring ferrite over pearlite, and are likely to be the secondary twins originated at the {112}〈111〉 twin boundaries.

The stress-aging method is proposed to improve the dimensional stability of Al-Cu-Mg (2024 aluminum) alloy. Both the reduction of residual stress and the acceleration of precipitates are investigated during the stress-aging process. It is observed that some S′ particles precipitate in the alloy during stress-aging. With the external stress increase to 100 MPa, the residual stress and thermal strain (ɛ c ) decrease, while the micro-yield strength increases significantly. This is attributed to that the dislocations are effectively pinned by the uniformly distributed precipitates and the stored elastic energy diminishes remarkably, which leads to a considerable improvement of the dimensional stability of 2024 aluminum alloy. However, when further increases the external stress to 150 MPa, due to the coarsened precipitates, the residual stress and thermal strain increase, while the micro-yield strength decreases slightly.

Due to the excellent structural and functional properties, such as low density, high fracture toughness and fatigue strength, Al-Cu-Mg (2024 aluminum) alloy has become an attractive light weight material for inertial instruments and aerospace applications. However, it has relatively low dimensional stability. The aim of this investigation is to enhance the dimensional stability of 2024 aluminum alloy via two-stage aging method, and identify the precipitation behavior and the underlying stabilizing mechanisms of 2024 aluminum alloy. When the second aging time was 24 h, the uniformly distributed precipitates could effectively pin the dislocations and impede their movement. This leaded to a significant improvement of the micro-yield strength and the thermal strain in the 2024 aluminum alloy, and thus the dimensional stability of the alloy increased.

The Nb-30Ti-18Al alloys doped with the amounts of 0 at%, 1 at% and 5 at% carbon were prepared by arc melting and annealing. The effects of carbon addition on the microstructure and mechanical properties of Nb ss /Nb 3 Al in situ composites as well as the strengthening mechanisms were investigated. The results showed that during annealing at 1200 °C, supersaturated Nb ss decomposed and Nb 3 Al phase precipitated in C-free alloy, while carbide decomposed with the precipitation of Nb 3 Al in C-doped alloys. Furthermore, the orientation relationship of Nb 3 Al phase and the surrounding (Nb, Ti)C was observed in Nb-30Ti-18Al-5C (NA5) alloy. Additionally, with increasing the carbon content, the compressive strength of alloys at high temperature were improved significantly ascribed to the occurrence of solid solution and volume fraction increase of strengthen phase, while the fracture toughness decreased due to the volume fraction decrease of Nb ss .

The explosion of high density and high temperature electronic devices is intensifying the desire for high temperature Pb-free solders. Alloying high melting point elements with conventional solders is one of the most potential solutions drawing attention. In this paper, the relationship between cracking and microstructure evolution of SnAgCu and Ni, Sb, Bi alloyed SnAgCu solders during thermal cycling was systematically studied and compared by ultrasonic C-scanning, SEM, EDX, EBSD, etc. It turned out that alloying Ni, Sb, Bi with SnAgCu solder offered better crack resistance by strengthening and recrystallization energy consumption before ~600 cycles. However, its failure dramatically accelerated after those cycles because of the large amount of grain boundaries caused by severe early recrystallization, the coarsened IMCs, and the decrease of Schmid Factor as SnAgCu were softened, while SnAgCuNiSbBi can be excessively hardened by thermal cycles.

The in-situ laminated Nb/Nb5Si3 composite was prepared by spark plasma sintering. The microstructures and mechanical properties of the laminated Nb/Nb5Si3 composite were investigated in present work. It could be found that, alternant distribution microstructures of Nbss and Nb5Si3 layers were observed after sintering. The Nb/Nb5Si3 composite exhibited fracture toughness of 11.2 MPa·m1/2, and the predicted fracture toughness was estimated by the Ashby model. Moreover, the deviation of the experimental value from the predicted value was discussed. In addition, the compressive strength of the in-situ laminated Nb/Nb5Si3 composite was measured at 1400 °C and compared with that of the unidirectionally solidified Nb-17.5Si alloy.

Aluminum foam sandwich (AFS) structure materials have drawn much attention due to their unique structural and functional properties. However, the use of AFS materials as an attractive candidate for some applications was limited. In this work, AFS was fabricated by the brazing method, using Al-Si-based alloy, aluminum foam and steel plates as filler, matrix and panels, respectively. The microstructures of the soldered interfacial region, elemental distributions and phase identification were determined by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The diffusion behavior of Si, Al, Fe and Cr was investigated. The effect of brazing time on the shear strengths of soldered joints was also analyzed. The results showed that the microstructures of joints were changed on increasing the brazing time. When the brazing time was 5 min, the solder started melting, which led to low shear strength. When increasing the brazing time to 10 min, the shear streng...

In the present investigation the microstructure and corrosion behavior of Zn-Cu-Ti and Zn-Cu-Ti-Mg alloys in 3.5 wt% NaCl solution were studied through scanning electron microscope, electron probe microanalysis, X-ray diffraction, polarization measurements and immersion tests. The Zn-Cu-Ti exhibited eutectic structure with rod-shaped TiZn15 phase, while the Zn-Cu-Ti-Mg mainly consisted of Zn matrix, TiZn15, τ2-Cu2TiZn22 ternary phase and Zn-Mg intermetallics along the grain boundaries. It was observed that in the corrosion products of both Zn-Cu-Ti and Zn-Cu-Ti-Mg alloys, the main phases were ZnO and Zn5(OH)8Cl2 · H2O. The corrosion tests showed that the cathodic process was suppressed and the weight gain decreased for Zn-Cu-Ti-Mg alloy in 3.5 wt% NaCl solution. The better corrosion properties of Zn-Cu-Ti-Mg was owed to the formation of a more compact and protective corrosion layer arising from the increased intensity of Zn5(OH)8Cl2 · H2O.

The present work used hydrothermal synthesis method utilizing glucose as carbon source to obtain homogeneous nano-sized carbon spheres. The holding time and the concentration of glucose have a remarkable effect on the microstructure of nanometer carbon spheres. When synthesized at a constant temperature and kept the same in the productivity, the lower concentration of glucose or the shorter holding time, the easier to acquired homogeneous and tiny nanometer carbon spheres

Aluminum oxynitride (Alon) has been considered as a potential ceramic material due to excellent stability, chemical and mechanical properties such as high rigidity and good chemical stability, but it has relatively low strength and poor fracture toughness. The aim of this study was to investigate a type of silicon carbide (SiC) nano-particulate reinforced Alon composites with improved mechanical properties and fracture resistance via hot-press sintering. The addition of SiC nano-particles resulted in a reduction of both porosity and grain size, and a change of fracture mode from intergranular cracking in the monolithic Alon to intragranular cracking in the composites due to the pinning effect of SiC nano-particles positioned at grain boundaries or triple junctions of micro-sized Alon particles. With 8 wt% SiC nano-particles addition, the relative density, microhardness, Young's modulus, flexural strength, and fracture toughness all increased. Different toughening mechanisms including crack bridging, crack branching and crack deflection were observed, thus effectively increasing the crack propagation resistance and leading to a considerable improvement in the flexural strength and fracture toughness of the composites.

Aluminum oxynitride (AlON) has been considered as a potential ceramic material for high-performance structural and advanced refractory applications. Thermal shock resistance is a major concern and an important performance index of high-temperature ceramics. While silicon carbide (SiC) particles have been proven to improve mechanical properties of AlON ceramic, the high-temperature thermal shock behavior was unknown. The aim of this investigation was to identify the thermal shock resistance and underlying mechanisms of AlON ceramic and 8 wt% SiC–AlON composites over a temperature range between 175 °C and 275 °C. The residual strength and Young's modulus after thermal shock decreased with increasing quenching temperature and thermal shock times due to large temperature gradients and thermal stresses caused by abrupt water-quenching. A linear relationship between the residual strength and thermal shock times was observed in both pure AlON and SiC–AlON composites. The addition of nano-sized SiC particles increased both residual strength and critical temperature from 200 °C in the monolithic AlON to 225 °C in the SiC–AlON composites due to the toughening effect, the lower coefficient of thermal expansion and higher thermal conductivity of SiC. The enhancement of the thermal shock resistance in the SiC–AlON composites was directly related to the change of fracture mode from intergranular cracking along with cleavage-type fracture in the AlON to a rougher fracture surface with ridge-like characteristics, crack deflection, and crack branching in the SiC–AlON composites.

Aluminum oxynitride (AlON) ceramic was synthesized by one-step pressureless sintering technology using low cost and easily available α-Al 2 O 3 and aluminum powders as raw materials. The sintering temperature was reduced because aluminum powders were nitridized into high activity AlN under the flowing nitrogen atmosphere. The curves of thermal analysis, microstructure and atomic distribution were investigated. The influence of sintering temperatures on phase composition, sintering densification and flexural strength was also explored. The experimental results showed that α-Al 2 O 3 and aluminum powders were acceptable substitutes for more expensive AlN ultrafine powders. Under the optimum sintering process at 1750 °C for 2 h, the sintered density and flexural strength of AlON ceramic were higher, 3.62 g/cm 3 and 321 MPa, respectively. The sintering temperature was decreased by 50 °C because newly formed high activity AlN in situ reacted with Al 2 O 3 into Al 23 O 27 N 5 , enhancing flexural strength by 29.4%. However, the sintering temperature could not be too high because grain growth and displacement of oxygen atoms from AlON ceramics by nitrogen atoms caused a decline in sintering densification and flexural strength.

This paper presents a comprehensive methodology to model the static temperature-humidity (TH) aging test ( 85 degC/85%RH over 1000 h) of flip chip on flex interconnections with nonconductive adhesives (NCAs). NCAs, being a special form of conductive adhesives, are chosen, as they allow bringing the pitch further down. The methodology combines experimental techniques for material characterization, finite element modeling (FEM), and model validation. A NCA has been characterized using several techniques. The thermomechanical properties and the moisture absorption properties were obtained for the NCA. A temperature dependent viscoelastic constitutive model was also obtained for the NCA. The viscoelastic model was defined by the Prony series expansion. The shift factor was approximated by the Williams-Landel-Ferry (WLF) equation. Finite element modeling has been performed to analyze the flip chip interconnects on flex with the NCA under process condition and reliability aging conditions. The viscoelastic constitutive relation has been used to model the NCA in aging modeling. An integrated process-aging modeling methodology has been developed to combine the thermo-mechanical stress and hygro-mechanical stress, followed by stress relaxation analysis. To verify the finite element models, static TH aging tests (85 deg C/85%RH) were also performed. The contact resistance was monitored with high measuring resolution during the accelerated test. The simulation results are in good agreement with the experimental results. The approach developed in this paper can be used to provide guidelines with respect to adhesive material properties, assembly process parameters to achieve good reliability performance.

Drop impact reliability has become an important criterion when assessing the reliability of portable electronics. Finite element software such as ABAQUS is used to simulate solder joints during drop-impact tests and determine the drop-impact reliability of the product. This modelling requires the rate-dependent constitutive properties of the solder in the range of strain rates between 0.1 s− 1 to 300 s− 1. Unfortunately, the constitutive properties of most solder materials in this range of strain rates have not been characterized. This paper presents the mechanical properties of four solder alloys — 63Sn37Pb, Sn1.0Ag0.1, Sn3.5Ag, and Sn3.0Ag0.5Cu at strain rates between 0.005 s− 1 and 300 s− 1. The lead-free solder alloys tested exhibited greater strain rate sensitivity compared to 63Sn37Pb. In addition, both the lead-free solders with high Ag content (≥ 3%) exhibited much higher flow stresses compared to 63Sn37Pb. The higher flow stress could explain the lower reliability of these joints in drop tests as a result of shifting of the failure site from the ductile solder to the brittle intermetallic compound [Wong EH, Rajoo R, Mai Y-W, Seah SKW, Tsai KT, Yap LM. Proc. 55th Elec. Comp. & Tech. Conf. 2005;1202–9]. Based on the data obtained, Sn1.0Ag0.1 seems to be the most suitable lead-free alloy to replace eutectic tin-lead in products where drop-impact performance is required.

This paper presents a comprehensive methodology to model the assembly process of flip chip on flex interconnections with non-conductive adhesives (NCAs). The methodology combines experimental techniques for material characterization, finite element modeling, and model validation. A non-conductive adhesive has been characterized using several techniques. A unique experimental technique has been developed to measure the cure shrinkage. A 2D axisymmetric finite element model is used for analysis of flip chip on flex package with the non-conductive adhesive (NCA), which takes into account assembly force, cure shrinkage, adhesive modulus buildup, removal of assembly force, and cooling down to room temperature. The relationship between the bump contact resistance and the bump pressure has been established through the development of a dedicated experimental setup, which uses a micro-force tester combined with a digital multimeter and a nano-voltmeter. The process modeling has been validated by comparing the predicted bump contact resistance value and the measured bump contact resistance value after assembly process. The approach developed in this paper can be used to provide guidelines with respect to adhesive material properties, assembly process parameters, and good reliability performances.

Light Emitting Diodes (LED) are being implemented more and more into demanding applications like automotive and high-brightness general lighting. From the reliability point of view, the automotive environment is extremely harsh and challenging. Automotive electronics have to withstand exposure to high temperature fluctuations, mechanical shock impacts and vibration. The cyclic thermal load of solder interconnections can be up to 150°C for several thousand cycles. For harsh environments, high-Pb and eutectic AuSn solders are currently being used as the interconnection material. Recent developments of SAC-based solders provide alternative lead-free solutions with lower processing temperature. However, little is known about the reliability and failure mechanism of these solder interconnections, especially under mechanical impact loadings. For a successful reliability test of the solder interconnection under the impact, it is important to find a widely accepted mechanical test method of measuring the degradation of the interconnection. In this study a newly designed high-speed impact tester that is based on the use of a pendulum was used to achieve this target. The purpose was to investigate the mechanical behaviour of lead-free solder interconnections under different loading conditions. The studied carrier had multi dies mounted onto a ceramic sub-mount, which was soldered to a Cu substrate. In total 60 samples were tested with five different test velocities (0.7, 1.0, 1.4, 1.8 and 2.2 m/s). The test results showed that the primary failure mode was the ductile failure in the solder bulk and only some local fractures at the solder interfaces were observed. This indicates that there was no dependency of the test velocity, since all the test velocities induced similar failures in the bulk solder of the solder joints.

The feasibility to assemble the NiPdAu pre-plated lead frames was studied with TSSOP components as a carrier. The wetting behavior of TSSOP components from different suppliers has been evaluated. The components have been assembled on a dedicated test board. Several assembly runs were done under different conditions of pre-aging, applied solder volume, component placement force and reflow profiles. Both eutectic SnPb and Pb-free SAC (SnAg4.0Cu0.5) solder were used. Evaluation was done using visual inspection and lead pull testing. A pre-study was done to define the board assembly conditions and to get some basic understanding of the parameters that define the wetting behavior. Results of the study indicate that the larger pitch components always show evidence of good wetting of the leads and the wetting of PPF components was comparable with the SnPb reference components. For the smaller pitch components, the wetting behavior was different for different suppliers and some lead frames showed higher reject rate with Pb-free SAC solder than with eutectic solder. Based on the outcome of the study, a guideline is proposed to evaluate the wetting behavior of PPF lead frames

This paper presents a comprehensive methodology to model the static temperature-humidity (TH) ageing test (85°C/85%RH over 1000 hours) of flip chip on flex interconnections with non-conductive adhesives (NCAs). Nonconductive adhesives, being a special form of conductive adhesives, are chosen, as they allow bringing the pitch further down. The methodology combines experimental techniques for material characterization, finite element modeling (FEM) and model validation. A non-conductive adhesive (NCA) has been characterized using several techniques. The thermomechanical properties and the moisture properties were obtained for the NCA. A temperature dependent viscoelastic constitutive model was also obtained for the NCA. The viscoelastic model was defined by the Prony series expansion. The shift factor was approximated by the Williams-Landel-Ferry (WLF) equation. Finite element modeling has been performed to analyze the flip chip interconnects on flex with the NCA under process condition and reliability ageing conditions. The viscoelastic constitutive relation has been used to model the NCA in ageing modeling. An integrated process-ageing modeling methodology has been developed to combine the thermo-mechanical stress and hygro-mechanical stress, followed by stress relaxation analysis. To verify the finite element models, the static TH ageing test (85°C/85%RH) were also performed. The contact resistance was monitored with high measuring resolution during the accelerated test. The simulation results are good agreement with the experimental results. The approach developed in this paper can be used to provide guidelines with respect to adhesive material properties, assembly process parameters and good reliability performances.Copyright © 2005 by ASME

lbis study focuses on moisture induced failure mechanisms of flip chip on flex interconnections with a nonconductive adhesive. It involves a comprehensive methodology that combines f~te element simulations and experimental techniques for material characterization and model validation. Finite element simulation is used to calculate the contact pressure between the bump and the copper track. An integrated modeling methodology has been developed that simulates the evolution of contact pressure from the bonding process to subsequent stress relaxation due to exposure to steady state temperature-humidity conditions. The mechanical contact pressure obtained in finite element modelmg has to be translated to the electrical contact resistance for practical interpretation and validation with the reliability ageing test. The relation has been established with ultimate resolution, through development of a dedicated experimental set-up using microforce-tester and combined Dh4M and nano-voltmeter. To verify the models, damp heat tests are performed. The contact resistance is taken as quality criterion and is monitored continuously with high measuring resolution during accelerated testing at 85"C/85%RH conditions. The on-line resistance measurement turns out to he a very powerful tool, making visible how the contact resistance changes during the accelerated test. The approach can be used to generate guidelines with respect to adhesive material properties and assembly process parameters. This study is an important step towards realistic and dedicated reliability assessment for anisotropic conductive adhesive interconnects.