Characterization of mechanical properties of thin films using nanoindentation test

Using finite element modeling (FEM), this work investigates using finite element modeling (FEM) the mechanical behavior of film on substrate composites during the penetration of a rigid tip. In order to understand the magnitude of the substrate effect, the difference of strain gradient through the thickness of a given layer, deposited first on a softer substrate and then on an harder substrate can be observed. In this specific case, up to a critical ratio (h/t) = 0.35 (with h the indentation depth and t the film thickness), the mechanical behavior of the layer is quite similar. But, for h/t > 0.35, two different behaviors may be observed: (i) in the first case H_f/H_S  1 (with H_f and H_S, respectively, the film and substrate hardness values), the total strain remains contained within the film thickness up to a ratio h/t close to 1 and (ii) in the second case H_f/H_S  1, the total strain extends deeply into the substrate. These results show that the empirical 10% rule is not valid, even for a hard film on a softer substrate. The main error is caused by a wrong estimation of the contact depth between the indenter tip and the film surface. Indeed, the simulation runs exhibit the formation of pile-up depending on the ratios (h/t) and Y_f/Y_S (with Y_f and Y_S, respectively, the film and substrate yield stress values). As a function of the used model for calculating the contact depth, at least three variation of hardness may be found from load–displacement curves obtained by FEM. In these conditions, it seems ambiguous to try to determine a weighting function to extract meaningful mechanical properties of the thin film. Another way to determine film properties consists in using the loading phase. A relationship between the applied load (P) and the indentation depth (h) is studied during the loading phase. For the case of a soft film on harder substrate (H_f/H_S  1), it is possible to determine the yield stress of the film, from the previous relationship. This approach is applied to experimental amorphous Al₂O₃ films formed by electron beam evaporation on silicon substrate.     

Influence of in-plane fibre orientation on mode I interlaminar fracture toughness of stitched glass/polyester composites

The influence of in-plane fibre orientation on the mode I interlaminar fracture toughness, G_Ic of unstitched and stitched glass/polyester composites is investigated in this paper. The G_Ic of planar specimens depends on the fibre orientation, θ in the layers adjacent to the fracture plane, in addition to the property of matrix material. The mode I fracture toughness and fracture behavior of unstitched and stitched 0/0, 30/−30, 45/−45, 60/−60, 90/90 and 0/90 interfaces of unidirectional fibre mats (UD) and 30/−30, 45/−45 and 90/90 interfaces of woven roving mats (WRM) are studied. WRM layer orientation is represented by the direction of warp fibres. Stitching is done by untwisted Kevlar fibre roving of Tex 175 g/km at the stitch densities (number of stitches per unit area) of 10.24 and 20.48 stitches/inch². The specimens having same stitch density, but different stitch distributions are prepared, and the influence of stitch distribution on G_Ic is studied. Double cantilever beam (DCB) tests are carried out and the G_Ic is determined using modified beam theory. The G_Ic of both unstitched and stitched specimens increases with increase in orientation angle, θ upto 45° above which it decreases. The G_Ic values of unstitched 45/−45 delamination interface is around 2.4 times that of the unstitched 0/0 interfaces. The influence of fibre orientation on G_Ic is clearly observed in unstitched specimens, whereas in the stitched specimens, stitching plays an important role in improving the G_Ic and suppresses the influence of fibre orientation; degree of suppression increases with increasing stitch density. When the value of θ is above 45°, transverse cracks are observed in the delamination interface surrounded by UD layers; while in the delamination interface surrounded by WRM layers, transverse cracks are not initiated irrespective of the fibre orientation angle.     

Application of Merchant theory in machining particulate metal matrix composites

Particulate metal matrix composites (PMMC’s) are a category of engineering materials with growing applications in modern industry. Conventional machining processes are used to fabricate engineering PMMC’s components. This article presents a preliminary experimental study based on Merchant theory for PMMC’s (aluminium alloy reinforced with 20% of particulate silicon carbide-SiC). The experiments were carried out on PMMC’s special workpieces using cemented carbides K20 cutting tools in radial turning. The objective of this study is to evaluate the chip compression ratio (R_c), shear angle (Φ), shear strain (ε), shear strain rate (dε/dt), normal stress (σ) and shear stress (τ), under prefixed cutting parameters (cutting velocity and feed rate).     

On the applied stress dependence of the subgrain size

The present work studies the collection of experimental data from which Raj and Pharr (Mater. Sci. Eng., 81 (1986) 217) deduced a universal empirical dependence of the subgrain size on the applied stress. In accord with their result and some theoretical predictions the normalized subgrain size d_s/b was ssumed to be proportional to G/σ (G is the shear modulus, b the Burgers vector length, σ the applied stress). The evaluated factor of proportionality K₁, having the value within the interval from 0.76 to 180 in the inspected data sets, was discussed from the point of view of various factors which can influence the experimental data.     

Optical properties of Nd:NaLa (WO4)2 single crystal

Nd³⁺-doped NaLa(WO₄)₂ single crystal with a dimension of 20 mm × 40 mm and a good optical quality was grown by Czochralski method. The polarized absorption spectra and emission spectra were measured at room temperature. The absorption cross-section and emission cross-section were presented. The Judd–Ofelt theory, extended to anisotropic system, has been applied to evaluate the intensity parameters Ω_t (t = 2, 4, 6), radiative transition rates A, radiative lifetimes τ_R and fluorescent branching ratios β. The calculated radiative lifetime was compared with the experiment data for the ⁴F_3/2 emitting level. All spectral features are strongly affected by an inhomogeneous broadening connected with the ‘disordered crystal’ character of the title compound.     

Orientation-dependent mobilities from analyses of two-dimensional TiN(111) island decay kinetics

We present a method for the determination of orientation-dependent mobilities Γ_eff(φ) based upon analyses of the detachment-limited coarsening/decay kinetics of equilibrium-shaped two-dimensional islands. An exact analytical expression relating the orientation-dependence of Γ_eff(φ) to that of the anisotropic step energies β(φ) is derived. This provides relative values of Γ_eff(φ) to within an orientation-independent scale factor that is proportional to the decay rate of the island area. Using in situ high temperature (T = 1550–1700 K) low-energy electron microscopy measurements of two-dimensional TiN island coarsening/decay kinetics on TiN(111) terraces for which β(φ) values are known [Phys. Rev. B 67 (2003) 35409], we demonstrate the applicability of our analytic formulation for the determination of absolute Γ_eff(φ) values.     

Grain boundary statistics in nano-structured iron produced by high pressure torsion

The microstructure and the spectrum of grain boundary misorientations were studied in Armco iron, following high pressure torsion (HPT) deformation, by means of transmission electron microscopy (TEM) and orientation imaging microscopy (OIM). It was found that HPT deformation results in the formation of an equiaxed grain structure with a mean grain size of 270 and 130 nm using a shear strain of γ = 210 and 420, respectively. The misorientation spectra in HPT iron have a bimodal character with maxima in low (at 1–2°) as well as in high misorientation angle ranges. A marked increase in the fraction of special boundaries (Σ3–Σ45) was revealed as a result of HPT. The microstructural changes due to HPT are discussed and compared with those obtained during conventional deformation modes.     

The effect of single-step low strain and annealing of nickel on grain boundary character

Combinations of a low tensile strain and annealing temperature have been successfully processed to double the fraction of special boundaries, Σ_sp, in commercially pure (C.P.) nickel. Processing sequence of one step of 6% strain followed by a 900 °C anneal for 10 min, has resulted a special boundary fraction (3 ≤ Σ ≤ 29) of 74.7%, compared to the as-received material of 36.5%. A large proportion of this increased in special boundaries were found to be made up of annealing twins Σ3 and its twin variants Σ3ⁿ. Deformation applied to the sample alone without heat treatment, caused the fraction of special boundaries, to decrease, the Σ_sp value falling, for example to 25.6% at 12% strain. Energy minimization processes such as grain boundary migration and grain growth were suggested as means of enhancing the formation of annealing twins.     

The development of compression damage zones in fibrous composites

Recent experimental work (Narayanan S, Schadler LS. Mechanisms of kink band formation in graphite/epoxy compsites: a micromechanical experimental study. Comp Sci Technol 1999; 59:2201-13) suggests that kink bands in unidirectional continuous carbon fiber reinforced polymer composites initiate from damage zones formed under axial compressive loads. A damage zone consists of a cluster of locally crushed fibers and broken fibers, that are often fractured at an angle, θ > 0°, normal to the fiber axis. Typically, under compressive loads, fiber breaks in damage zones form roughly along a plane at an angle φ, normal to the fiber axis. These damage zones produce stress concentrations which can lead to instabilities in the nearby fiber and matrix and initiate microbuckling and kink bands. This paper extends a micromechanical influence function technique based on earlier shear lag fiber composite models. Our modified technique calculates the fiber axial and matrix shear stress concentrations due to multiple angled and crushed fibers in arbitrary configurations. Modeling reveals that angled or ‘shear’ breaks (θ > 0°) can lead to higher shear stress concentrations in the matrix than transverse breaks (θ=0°). Also we find that the damage zone is more likely to form at an angle φ, which is greater than that of its individual fiber breaks, θ. When φ is slightly greater than θ, the shear stress in the surrounding matrix regions within the damage zone achieves a maximum, potentially weakening the matrix and interface and consequently leading to kink band formation. Monte Carlo simulations incorporating this stress analysis predict that the initiation and propagation of crushed and angled breaks progress roughly along an angle, φ ≈ 17° in a linear elastic system. When possible, our model results are compared to strain measurements of fiber composites under compression obtained by Narayanan and Schadler using micro-Raman spectroscopy (MRS).     

Investigation of hydrogen induced cracking in 2205 duplex stainless steel in wet H2S environments after isothermal treatment at 675, 750 and 900 °C

The effect of isothermal treatment (at 675, 750 and 900 °C) on HIC (hydrogen induced cracking) in sour environments containing hydrogen sulphide of a 2205 duplex stainless steel has been investigated. The performance and microstructure of failed material were characterized by optical microscopy, scanning electron microscopy and also X-ray diffraction. Two kinds of Cr-, Mo-enriched intermetallic phases, σ and χ, were found to precipitate preferentially at /γ interfaces and within grains after different times of aging in the temperature range of 650–900 °C. After performing tests according to the NACE Standard TM 0284 (1987) the specimens were investigated by using quantitative metallography methods. The volume fraction of σ phase was changed with the time of aging and σ phase developed into coarse particles due to the high diffusibility of solute atoms at high temperatures. The variation of size and shape of σ phase particles was obtained by applying different heat treatment conditions to 2205 steel specimens. The results showed that 2205 duplex stainless steel containing nearly 12 vol.% of σ phase in dispersed conditions was resistant to step cracking in wet environments containing hydrogen sulphide. It was highly possible that a crack would propagate faster along the embrittled σ phase. However, very small cracks were found at austenite–ferrite boundaries where o phase particles were also present.     

Structural properties of swift heavy ion beam irradiated Fe/Si bilayers

The Fe/Si multilayers were prepared by electron beam evaporation in a cryo-pumped vacuum deposition system. Ag⁺ and Au⁺ ions of 100 MeV at two different fluencies such as 1 × 10¹² ions/cm² and 1 × 10¹³ ions/cm² at a pressure of 10^− 7 torr were used to irradiate the Fe/Si samples. The irradiated samples were analyzed by High-Resolution XRD and it reveals that the irradiated films are having polycrystalline nature and it confirms the formation of the β-FeSi₂. The structural parameters such as crystallite size (D), strain (ε) and dislocation density (δ) have been evaluated from the XRD spectrum. The role of the substrates and the influence of swift heavy ions on the formation of β-FeSi₂ have been discussed in this paper.     

Microwave dielectric properties of W-doped Ba0.6Sr0.4TiO3 thin films grown on (001) MgO by pulsed laser deposition with a variable oxygen deposition pressure

The microwave dielectric properties of Ba_0.6Sr_0.4TiO₃ 1 mol% W-doped thin films deposited using pulsed laser deposition, are improved by a novel oxygen deposition profile. The thin films were deposited onto (001) MgO substrates at a temperature of 720 °C. A comparison is made between three different oxygen ambient growth conditions. These include growth at a single oxygen pressure (6.7 Pa) and growth at two oxygen pressures, one low (6.7 Pa) and one high (46.7 Pa). Films were deposited in a sequence that includes both a low to high and a high to low transition in the oxygen deposition pressure. Following deposition, all films were post-annealed in 1 atm of oxygen at 1000 °C for 6 h. The dielectric Q (defined as 1 / tanδ) and the dielectric constant, ε_r, were measured at room temperature, at 2 GHz, using gap capacitors fabricated on top of the dielectric films. The percent dielectric tuning (defined as (ε_r(0 V) − ε_r(40 V)) / ε_r(0 V) × 100) and figure of merit (FOM) (defined as percent dielectric tuning × Q(0 V)) were calculated. The film deposited using the two-stage growth conditions, 6.7 / 46.7 Pa oxygen, showed a maximum Q(0 V) value with high percent dielectric tuning and gave rise to a microwave FOM twice as large as the single stage growth condition. The improved dielectric properties are due to initial formation of a film with reduced interfacial strain, due to the formation of defects at the film/ substrate interface resulting in a high Q(0 V) value, followed by the reduction of oxygen vacancies which increases the dielectric constant and tuning.     

Physical cutting model of Polyetheretherketone composites

This paper presents a study on the experimental physical model of the orthogonal cutting on the composite polyetheretherketone (PEEK) and PEEK CF30 (reinforced with 30% of carbon fiber). The objective of this study is evaluating the influence of the reinforcement on the chip thickness ratio (R), chip deformation (ε), friction angle (ρ), shear angle (), normal stress (σ) and shear stress (τ) under prefixed cutting parameters (cutting velocity and feed rate). The experimental physical model was compared with the Merchant equation.     

Electrical properties of sol-gel processed barium titanate films

The sol-gel technique has been used to prepare ferroelectric barium titanate (BaTiO₃) films. The electrical properties of the films have been investigated systematically. The room temperature dielectric constant (ε) and loss tangent (tanδ) at 1 kHz were respectively found to be 370 and 0.012. Both ε and tanδ showed anomaly peaks at 125°C. The room temperature remanant polarization (P_r) and coercive field (E_c) were found to be 3.2 μC/cm² and 30 kV/cm, respectively. The capacitance–voltage (C–V) and conductance–voltage (G–V) characteristics also showed hysteresis effect. The temperature variation of C–V and G–V characteristics also confirms the ferroelectric to paraelectric phase transition at 125°C.     

The closed form general solutions of 2-D curved laminated beams of variable curvatures

The closed form general solutions of in-plane static laminated curved beams of variable curvatures are derived. All the quantities of axial force, shear force, radial and tangential displacements are decoupled and expressed as harmonic functions in terms of angle of tangent slope. The first and second moments of laminated curved beams with respect to x- and y-axes are defined as fundamental geometric properties. The curved beams can be any curve. Some closed form solutions of non-circular laminated beams, rings are presented. Applications of elliptic, parabola, catenary, cycloid, and exponential spiral laminated curved beams are shown. The laminated circular ring under point load and distributed load are studied as well. Two kinds of laminate (0/θ) and (θ/−θ) are investigated in this paper. Effects of aspect ratio, thickness ratios, orthotropy ratios and stacking sequence on the laminated ring are presented. Results of isotropic curved beams are compared with existed solutions. The results are consistent.     

Ultrasonic studies of the binary mixtures of ethyl acetate and cresols—application of Kosower and Dimroth treatments

The ultrasonic velocity (ν) studies were carried out at a frequency of 2 MHz (transducer of x-cut quartz crystal) using ultrasonic pulse echo system (model UX4400-M) on cresols in ethyl acetate at constant temperature of 311 K. The values of internal pressure ( π_i) and molar free volume (V_f) were calculated from measured values of ultrasonic velocity (ν), viscosity (η) and density (ρ). An attempt is made to rationalize the ultrasonic velocity (ν), internal pressure ( π_i) and free volume (V_f) of binary mixtures using Kosower solvent parameter (Z), Dimroth solvent parameter (E_T) and Dielectric constant (). It is found that there is linear correlation between ultrasonic velocity and acidity constant pk⁻¹a, indicating the dependence of acidity. Correlation of Ksower and Dimroth parameters with ultrasonic velocity confirms that solvent polarity is an important factor in the variation of ultrasonic velocity in the present investigation.     

Spectroscopic properties of Yb in heavy metal contained fluorophosphate glasses

A new series of 20Bi(PO₃)₃–10Sr(PO₃)₂–35BaF₂–35MgF₂ doped with Yb³⁺ is introduced for fiber and waveguide laser applications. The stimulated emission cross-section σ_emi, which was found to be 1.37 pm² at the lasing wavelength of 996 nm, is the highest one among fluorophosphate glasses. It has been found that an extremely high gain coefficient of G = 1.65 ms pm⁴ and high quantum efficiency of η = 93% for 1 wt.% Yb₂O₃ doped systems. The various concentration effects on laser performance properties including minimum pumping intensity I_min, the minimum fraction of excited ions β_min and the saturation pumping intensity I_sat are analyzed as a function of Yb₂O₃ concentration. Those results obtained in current system had advantage over some fluorophosphate glasses reported.     

The effects of trace Sc and Zr on microstructure and internal friction of Zn–Al eutectoid alloy

The damping properties of Zn–22 wt.% Al alloys without and with Sc (0.55 wt.%) and Zr (0.26 wt.%) were investigated. The internal friction of the determined by the microstructure has been measured in terms of logarithmic decrement (δ) using a low frequency inverted torsion pendulum over the temperature region of 10–230 °C. An internal friction peak was separately observed at about 218 °C in the Zn–Al alloy and at about 195 °C in Zn–Al–Sc–Zr alloy. The shift of the δ peak was found to be directly attributed to the precipitation of Al₃(Sc, Zr) phases from the alloy matrix. We consider that the both internal friction peak in the alloy originates from grain boundary (GB) relaxation, but the grain boundary relaxation can also be affected by Al–Sc–Zr intermetallics at the grain boundaries, which will impede grain boundary sliding. In addition, Al–Sc–Zr intermetallics at the grain boundaries can pin grain boundaries, and inhibit the growth of grains in aging, which increases the damping stability of Zn–22 wt.% Al alloy.     

Research on low cycle fatigue properties of TA15 titanium alloy based on reliability theory

This paper introduces a method to determine the symmetric cycle fatigue limit of TA15 alloy at given confidence γ and survival probability P. It gives a general method to calculate the true survival probability of this material fatigue limit. The median and data of the LCF at γ = 95% and P = 99.9% are acquired after studying the LCF properties of aircraft construction material TA15 at the temperature of 25 and 250 °C. The strain–life curve, cyclic stress–strain curve and parameters of LCF are also achieved. These provide reference to analyze the reliability of aircraft construction and estimate the life.     

Underlying mechanisms for unique elastic properties of nanocrystalline Au

In order to get an insight into the grain boundaries (GBs) in nanocrystalline (n-) metal, we prepared the high-density n-Au with ρ/ρ₀>99% by the gas-deposition method and carried out the vibrating reed measurements, where ρ/ρ₀ is the relative density referring to the bulk density. The strain amplitude dependence (SAMD) of the resonant frequency (f) and the internal friction (Q⁻¹) was measured for the strain () amplitude between 10⁻⁶ and 2×10⁻³ and for temperature between 5 and 300 K. No plastic deformations are detected for the present strain range, where f decreases for up to 10⁻⁴ and then turns to increase, showing saturation for between 10⁻⁴ and 2×10⁻³. The low temperature irradiation by 2 MeV electrons or 20 MeV protons causes an increase in the Young’s modulus at 6 K, which is surmised to reflect a modification of the anelastic process in the GB regions. In contrast, the SAMD of f is hardly modified by irradiation, suggesting that it is indicative of a collective motion of atoms in n-Au.