Standalone Tensile Testing of Thin Film Materials for MEMS/NEMS Applications

The microelectronics industry has been consistently driven by the scaling roadmap, colloquially referred to as the Moore’s law. Consequently, during the past decades, integrated circuits have scaled down further. This shrinkage could have never been possible without the efficient integration and exploitation of thin film materials. Thin film materials, on the other hand, are the essential building blocks of the micro- and nano-electromechanical systems (MEMS and NEMS). Utilization of thin film materials provides a unique capability of further miniaturizing electromechanical devices in micro- and nano-scale. These devices are the main components of many sensors and actuators that perform electrical, mechanical, chemical, and biological functions. In addition to the wide application of thin film materials in micro- and nano-systems, this class of materials has been historically utilized in optical components, wear resistant coatings, protective and decorative coatings, as well as thermal barrier coatings on gas turbine blades. In some applications, thin film materials are used mainly as the load-bearing component of the device. Microelectromechanical systems (MEMS) are the example of these applications. Thin film materials carry mechanical loads in thermal actuators, switches and capacitors in RF MEMS, optical switches, micro-mirror hinges, micro-motors, and many other miniaturized devices. In these applications, one of the main criteria to choose a specific material is its ability to perform the mechanical requirements. Therefore, a clear understanding of the mechanical behavior of thin film materials is of great importance in these applications. This understanding helps better analyze the creep in thermal actuators (Tuck et al., 2005; Paryab et al., 2006), to investigate the fatigue of polysilicon (Mulhstein et al., 2001; Shrotriya et al., 2004) and metallic micro-structures (Eberl et al., 2006; Larsen et al., 2003), to scrutinize the relaxation and creep behavior of switches made of aluminum (Park et al., 2006; Modlinski et al., 2004) and gold films (Gall et al., 2004), to study the hinge memory effect (creep) in micro-mirrors (Sontheimer, 2002), and to address the wear issues in micro-motors. (van Spengen, 2003) In some other applications, the thin film material is not necessarily performing a mechanical function. However, during the fabrication process or over the normal life, the device experiences mechanical loads and hence may suffer from any of the mechanical failure issues. Examples of these cases are the thermal fatigue in IC interconnects (Gudmundson & Wikstrom, 2002), strain ratcheting in passivated films (Huang et al., 2002; He et al., 2000), the fracture and delamination of thin films on flexible substrates (Li & Suo, 2006), the fracture of porous low-k dielectrics (Tsui et al., 2005), electromigration (He et al., 2004), the chip-package-interaction (CPI) (Wang & Ho, 2005), and thin film buckling and delamination (Sridhar et al., 2001). In order to address the above-mentioned failure issues and to design a device that has mechanical integrity and material reliability, an in-depth knowledge of the mechanical behavior of thin film materials is required. This information will help engineers integrate materials and design devices that are mechanically reliable and can perform their specific functions during their life-time without any mechanical failure. In addition to the tremendous industrial and technological driving force that was mentioned earlier, there is a strong scientific motivation to study the mechanical behavior of thin film materials. The mechanical behavior of thin film structures have been known to drastically differ from their bulk counterparts. (Xiang, 2005) This discrepancy that has been referred to as the length-scale effect has been one of the main motivations in the scientific society to study the mechanical behavior of thin film materials. In order to provide fundamental mechanistic understanding of this class of materials, old problems and many of the known physical laws in materials science and mechanical engineering have to be revisited from a different and multidisciplinary prospective. These investigations will not be possible unless a concrete understanding of the mechanical behavior of thin film materials is achieved through rigorous experimental and theoretical research in this area.Natural Sciences and Engineering Research Council (NSERC) of Canad

A Two-Stage Conversion Process For Az31B Corrosion

Permanganate phosphate coating is considered as one of the most promising alternative coatings to chromate conversion coating because of the similarity between permanganate and chromate. However, the permanganate phosphate coating always has net-work cracks. In this paper, a thick and crack-free permanganate phosphate coating was developed by a two-stage conversion process. Results of scanning electron microscope (SEM) with X-ray diffraction (XRD) analysis showed that some more corrosion resistant compounds appeared in the coating after a two-stage conversion treatment, which were not found in the single conversion coating. Moreover, the two-stage conversion coating has a higher thickness with few cracks. Furthermore, electrochemical tests and salt spray test further exhibited that the two-stage conversion coating had a better corrosion performance than either of the two single conversion coatings

Multiaxial cyclic behaviour and fatigue modelling of AM30 Mg alloy extrusion

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.ijfatigue.2016.12.037 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Multiaxial fatigue characteristics of AM30 Mg alloy extrusion are studied through fully-reversed strain controlled cyclic experiments including pure torsional and combined axial-torsional at 0, 45 and 90 phase angle shifts. Under pure torsional cyclic loading, AM30 extrusion is realized to exhibit better fatigue properties than AZ31B and AZ61A extrusions, especially in low-cycle fatigue regime. Under proportional axial-torsional cyclic loading, twinning/de-twinning in axial mode results in asymmetric shear hysteresis loop. The effect of non-proportionality of biaxial loading on various aspects of material response is also examined and observed to be depending on the magnitude of axial strain amplitude. Finally, the life prediction capabilities of two critical plane models, i.e., modified Smith-Watson-Topper (SWT) and Fatemi-Socie (FS), as well as Jahed-Varvani (JV) energy-based approach are assessed, employing fatigue life data of AM30 extrusion. Correlation data between experimental and estimated lives are found to lie within narrow scatter band.Natural Sciences and Engineering Research Council (NSERC) under Automotive Partnership of Canada (APC) program [APCPJ 459269-13]Government of Ontario through Ontario Trillium Scholarship (OTS) progra

Effect Of Loading Strain Rates On Unloading Behavior Of Shot Peened Materials

Shot peening is a process widely used in industry to improve the fatigue life of materials through induced compressive residual stresses that retard crack initiation and growth. In the peening process, there are two stages: 1) loading: shot penetrating into target; and 2) unloading: shot rebounding from the target. The strain rates in the loading process are known to be in 105-106 1/s range, having heavy impact on the materials’ properties. However, the effect of the loading strain rates on the rebounding stage is not well studied. This paper aims to determine the effects of the loading strain rates on the unloading behavior of a material using FEM method. First, to better understand the material behavior, this study evaluates the loading-unloading responses of one element at high strain rates in different scenarios. Then, it obtains the strain rates during the loading and unloading for the different elements of a material being impinged by one shot. The results show that the unloading behavior of a material depends only on the loading equivalent plastic strain and the strain rate of the unloading step

Role of loading direction on cyclic behaviour characteristics of AM30 extrusion and its fatigue damage modelling

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.msea.2016.05.116 © 2016. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Anisotropic fatigue and cyclic behaviour of AM30 Mg alloy extrusion is investigated by performing fully-reversed strain-controlled tension-compression cyclic tests at strain amplitudes between 0.3% and 2.3%, along extrusion (ED) and transverse (TD) directions. The shapes of half-life hysteresis loops suggest the predominance of slip and twinning/de-twinning mechanisms below and above the strain amplitude of 0.5%, respectively. The twinning/de-twinning occurrence is found to be more extensive during straining along ED, which results in higher asymmetry of hysteresis loops, and thereby, higher induced mean stress. This adversely affects the fatigue resistance and yields to less number of cycles before failure in ED. Optical microscopy and texture analysis are employed to validate the findings. In addition, fracture surfaces are studied by scanning electron microscopy to identify the sources of fatigue crack initiation. Persistent slip bands (PSBs) and twin lamellae interfaces are evidenced as crack initiation sites at low and high strain amplitudes, respectively. Cracks emanated from debonded inclusion interface are also observed. Lastly, estimated fatigue life by Smith-Watson-Topper (SWT) and Jahed-Varvani (JV) fatigue models are compared with experimental life obtained through this study as well as the ones reported in the literature. The JV energy model is proven to yield better life predictions.Natural Sciences and Engineering Research Council (NSERC) under Automotive Partnership of Canada (APC) program [APCPJ 459269-13]Government of Ontario through Ontario Trillium Scholarship (OTS) progra

Application Of Fiber Bragg Grating Sensor For Strain Measurement At The Notch Tip Under Cyclic Loading

Notches are inevitable in many components and structures due to design limitations. In addition, they are the locations for stress concentration and are susceptible to fatigue failure. As a result, the cyclic stress/strain response at a notch is of key importance. Fiber Bragg Grating (FBG) sensors have been successfully utilized for mechanical and thermo-mechanical strain measurement in many cases; nevertheless, their capability of measuring strain at spots with intensive stress/strain has not yet been explored. In this research, FBG sensors are employed for strain measurement at the notch tip. A verification test was designed to substantiate the FBG measurements. The test involves a rectangular magnesium sheet with a center hole, subjected to uniaxial cyclic loading while the strain was measured at the notch tip using three different methods: strain gage, digital image correlation (DIC), and FBG. There were good agreements between the three measurements

The effect of pure aluminum cold spray coating on corrosion and corrosion fatigue of magnesium (3% Al-1% Zn) extrusion

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.surfcoat.2016.11.014 © 2017. This publisher's version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ open access optionPure aluminum powder was successfully sprayed on AZ31B extrusion flat and round coupons at low temperature. The corrosion and corrosion fatigue behavior of the coated and uncoated samples were examined by performing accelerated corrosion tests. The corrosion resistance of AZ31B samples with and without coating was investigated based on ASTM B117 standard salt spray with a concentration of 5% NaCl at 36 degrees C, 100% relative humidity. The corrosion fatigue of bare and coated round samples was examined by producing a thin film of 3.5% NaCl solution on the surface of the fatigue samples via integrating a corrosion chamber into a rotating bending fatigue testing machine. Pure Al coating provided significant corrosion protection for AZ31B in 5% NaCl fog environment by improving its corrosion resistance from 90% average weight loss in 33 days for bare samples to <10% average weight loss in 90 days of continuous corrosion cycles. However, pure Al coating did not improve the corrosion fatigue strength of magnesium and samples with and without coating showed similar corrosion fatigue trends. Test results in salt solution showed fatigue life reduction of 88% when compared with test results in air. The microstructure examination of samples failed under cyclic load showed early cracking of Al coat which allowed the electrolyte penetration into Mg substrate creating a localized corrosion and premature failure. The early cracking was attributed to the lower fatigue strength of pure Al compared to AZ31B.Saudi Basic Industries Corporation Natural Sciences and Engineering Research Council of Canada through Automotive Partnership Canada program || APCPJ 459269-1

A Lagrangian model for hardening behaviour of materials at finite deformation based on the right plastic stretch tensor

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.matdes.2009.12.002 © 2010. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/In this paper a modified multiplicative decomposition of the right stretch tensor is proposed and used for finite deformation elastoplastic analysis of hardening materials. The total symmetric right stretch tensor is decomposed into a symmetric elastic stretch tensor and a non-symmetric plastic deformation tensor. The plastic deformation tensor is further decomposed into an orthogonal transformation and a symmetric plastic stretch tensor. This plastic stretch tensor and its corresponding Hencky's plastic strain measure are then used for the evolution of the plastic internal variables. Furthermore, a new evolution equation for the back stress tensor is introduced based on the Hencky plastic strain. The proposed constitutive model is integrated on the Lagrangian axis of the plastic stretch tensor and does not make reference to any objective rate of stress. The classic problem of simple shear is solved using the proposed model. Results obtained for the problem of simple shear are identical to those of the self-consistent Eulerian rate model based on the logarithmic rate of stress. Furthermore, extension of the proposed model to the mixed nonlinear isotropic/kinematic hardening behaviour is presented. The model is used to predict the nonlinear hardening behaviour of SUS 304 stainless steel under fixed end finite torsional loading. Results obtained are in good agreement with the available experimental results reported for this material under fixed end finite torsional loading

Characterization of single- and multilayer cold-spray coating of Zn on AZ31B

Zinc, a soft material with a low melting point and high corrosion resistance, was coated onto AZ31B Mg alloy using different cold spraying process parameters. The physical and mechanical properties of the resulting Zn/AZ31B samples were then investigated to explore the effect of the process parameters on the microstructural and mechanical characteristics. The results obtained via X-ray diffraction show the formation of an intermetallic material at the interface of Zn/AZ31B even at low process temperatures. In addition, spherical droplets of Zn were observed at the surface, confirming the partial melting of Zn particles during the impact. This partial melting is believed to lead to the formation of intermetallic compounds during solidification. To engineer the residual stress induced in the cold spraying process, a thin layer of dense Zn was then used as an intermediate layer before coating with Al7075, forming a multilayered surface of Al7075/Zn/AZ31B. Because of the higher thermal expansion coefficient of Zn compared with those of Al7075 and AZ31B, beneficial compressive residual stress could be created in all three layers of this novel multilayer deposition. Without the Zn interlayer, Al7075/AZ31B under the same coating parameters exhibited undesirable tensile residual stress in the substrate.Financial support through funds from Natural Sciences and Engineering Research Council of Canada (NSERC) RTI program under EQPEQ458441-2014 grant, and NSERC through APC under APCPJ 459269–13 grant are gratefully acknowledged