Biaxial Residual Stress Research Articles

Determination of proof stress and strain-hardening exponent for thin film with biaxial residual stresses by in-situ XRD stress analysis combined with tensile test

A new method is put forward to measure the proof stress and strain-hardening exponent of polycrystalline films (Cu, TiN) under a biaxial residual stresses state on the substrates. The Cu and TiN films were deposited on the steel substrates by ion beam-assisted magnetron sputtering and plasma-assisted chemical vapor deposition (PACVD), respectively. During the tensile test, the longitudinal stress ( σ 1) and transverse stress ( σ 2) of the films were determined by in-situ X-ray diffraction, and the applied strain ( ε a) of the films were measured by a strain gauge. Based on the experimental results, the equivalent stresses σ̄ and equivalent uniaxial strains ε t can be obtained. From the σ̄– ε t curves, the proof stresses σ 0.1 of the Cu and TiN films have been calculated. The results indicate that the values of σ 0.1 of the Cu and TiN films are 328 MPa and 4.2 GPa, respectively, and the values of the strain-hardening exponents for them are 0.62 and 0.36, respectively. In addition, evidence that the plastic flow of TiN film occurred under the tensile load is also obtained.

Effects of deposition temperature and bilayer thickness on the mechanical properties of AlN/TiN multilayer thin films

By controlling the thickness of individual layers (20–70 nm) and substrate temperature (∼50–800 °C), a number of AlN/TiN multilayer films were grown in a reactive magnetron sputtering on the Si(100) and WC–Co substrates. The films were characterized using grazing incidence X-ray diffraction (XRD), nanoindentation, and cross-sectional electron microscopy [transmission electron microscopy (TEM) and HRTEM]. The measured hardnesses of multilayer films were found to be always higher than the rule of mixture, but the magnitude of hardness enhancement was found to be dependant on the bilayer thickness and deposition temperature. The obtained results are discussed in terms of interface-related phenomena, as well as structural modifications, that occur within the individual layers of TiN and AlN, i.e., change of grain size, transition of preferred orientation from (111) to (200), localized epitaxial growth between TiN and AlN crystallites, and development of biaxial residual stresses.

Critical factors that determine face-centered cubic to body-centered cubic phase transformation in sputter-deposited austenitic stainless steel films

Bulk austenitic stainless steels (SS) have a face-centered cubic (fcc) structure. However, sputter deposited films synthesized using austenitic stainless steel targets usually exhibit body-centered cubic (bcc) structure or a mixture of fcc and bcc phases. This paper presents studies on the effect of processing parameters on the phase stability of 304 and 330 SS thin films. The 304 SS thin films with in-plane, biaxial residual stresses in the range of approximately 1 GPa (tensile) to approximately 300 MPa (compressive) exhibited only bcc structure. The retention of bcc 304 SS after high-temperature annealing followed by slow furnace cooling indicates depletion of Ni in as-sputtered 304 SS films. The 330 SS films sputtered at room temperature possess pure fcc phase. The Ni content and the substrate temperature during deposition are crucial factors in determining the phase stability in sputter deposited austenitic SS films.

Bending fatigue behaviour of differently shot peened Al 6082 T5 alloy

The fatigue behaviour of the Al 6082 T5 alloy subjected to two different shot peening treatments was studied. The fatigue improvements with respect to the unpeened condition and the influence of the peening intensity on fatigue were discussed accounting for the effects of surface modifications (roughness and strain hardening) and of residual stresses. In particular, the extent of the residual stress redistribution during loading was investigated by means of X-ray diffraction (XRD) measurements. An evolution to a stabilised value of the surface stress, depending on the applied loads, was found. The initial and the stabilised residual stress profiles were considered for discussing the improvement in the fatigue behaviour due to peening. To this purpose, a multiaxial fatigue criterion was adopted to account for the biaxial residual stress field. Accordingly, an attempt to separate the effect of residual stresses from that of the microstructural modifications induced by the plastic deformation during peening was carried out and, in particular, the influence of the surface hardening was highlighted.

Effects of biaxial loading and residual stresses on constraint

Finite element analyses have been carried out to determine the elastic stress intensity factors, the plastic limit loads, failure assessment diagrams (FADs) and the T- and Q-stresses for each of three postulated defects. The effect of biaxial loading and residual stresses on the stress intensity factor and T and Q-stress solutions, and the effect of weld-base metal mismatch on the plastic limit load were assessed. Results have also been derived for the reduction in constraint with increasing plasticity up to the limit load. For assessments of a narrow mismatched weld of overmatching strength using standard Option 1 or 2 FADs, up to L r =1, this parameter should be determined using the lower strength base material yield strength. The decrease in constraint with increasing L r varies at different positions around the crack front of semi elliptical surface defects. Although constraint is maintained highest just under the free surface towards the ends of the crack, this is generally the region where the stress intensity factor is lowest for tension loading and thus assessments need to take account of both crack tip driving force and constraint.

Structure–property relationships in single- and dual-phase nanocrystalline hard coatings

The structure of hard coatings deposited by PVD (physical vapor deposition) strongly depends on the growth parameters. In this work, the influence of microstructure and chemical composition on mechanical properties was investigated in detail for several nanocrystalline hard coatings. Single-phase TiN, CrN and TiB 2 as well as dual-phase CrN–Cr 2N, TiN–TiB 2 and TiC–TiB 2 coatings were prepared by reactive or non-reactive unbalanced dc magnetron sputtering, respectively. The hardness of the coatings investigated will be discussed with respect to the average grain size and residual biaxial stresses. By optimizing both nanostructure and biaxial stresses of stoichiometric TiN coatings, their microhardness ( H) could be improved from 33 to 56 GPa, whereas the reduced Young's modulus ( E *) changed from 402 to 480 GPa. Thus, the ratio H 3/ E *2 which is representative for the resistance to plastic deformation could be increased from 0.222 to 0.806 GPa. For Cr–N coatings, H 3/ E *2 values up to 0.521 GPa were obtained, depending on their chemical composition and nanostructure. The nanocrystalline dual-phase coatings TiN–TiB 2 and TiC–TiB 2 even yielded H 3/ E *2 values up to 1.332 and 1.575 GPa, respectively. Comparing the different single- and dual-phase coatings investigated, the establishment of correlations between structure and mechanical properties enables the development of advanced coatings with high hardness and high resistance against plastic deformation. In addition, this work shows the tremendous importance of a controlled deposition process for optimizing coating properties.

Microstructure alterations at the surface of a heavily corrugated rail with strong ripple formation

The topography, morphology, structure and residual stress state at the surface of a rail with severe longitudinal ripples was characterized using a combination of spectrometry, microscopy, scattering and diffraction methods. The investigations reveal that white etching layers develop at the ripple hills, while the ripple valleys contain a severely plastic deformed layer. While the structure in the ripple valleys remains pearlite, the structure of the white etching layers on top of the ripple hills is nanocrystalline martensite containing cementite particles of a few nanometer size. The microhardness of the white etching layers on top of the ripple hills is about three times higher than the microhardness in the ripple valleys. The microhardness throughout the white etching layer is not homogeneous due to the non-uniform amount of carbon in solute solution and also due to differences in the amount and the size of the fine carbide particles. The whole running surface is under biaxial compressive residual stresses. In the ripple valleys the direction of the principal residual stresses nearly coincides with the longitudinal and the transverse direction on the rail. On the ripple hills the compressive residual stresses due to the martensite formation are significantly higher and the stress state is nearly homogeneous.

Fracture patterns in thin films and multilayers

ABSTRACTWhile there are many stress relief mechanisms observed in thin films, excessive residual and externally applied stresses cause film fracture. In the case of tensile stress a network of through-thickness cracks forms in the film. In the case of compressive stress thin film buckling is observed in the form of blisters. Thin film delamination is an inseparable phenomenon of buckling. The buckling delamination blisters can be either circular, straight, or form periodic buckling patterns commonly known as telephone cord delamination morphology.While excessive biaxial residual stress is the key for causing thin film fracture, either in tension, or compression, it is the influence of the external stress that can control the final fracture pattern. In this paper we consider phone cord buckling delamination observed in compressed W/Si and TiWN/GaAs thin film systems, as well as spiral and sinusoidal though-thickness cracks observed in Mo/Si multilayers under 3-point high-temperature bending in tension.

Influence of crystal properties on the absorption IR spectra of polycrystalline AlN thin films

In this paper Fourier transform infrared (FTIR) transmission spectroscopy is presented as a technique suitable for the routine morphological study of polycrystalline AlN thin films. We have compared FTIR spectra and X-ray diffraction (XRD) patterns of wurtzite polycrystalline AlN films deposited on Si(001) by RF reactive sputtering of an Al target in an N 2/Ar mixture. The preferred orientation and crystal quality of the films were investigated by XRD. The FTIR spectra showed absorption bands due to vibrational modes of AlN bonds, in particular, the A1(TO) mode at 612 cm −1 and the E1(TO) mode at 672 cm −1. The ratio of integrated areas of A1(TO) and E1(TO) absorption bands is related to the preferred orientation of the films, which ranges from films with purely (00.2)-oriented grains to films with grains tilted by approximately 60° to the surface normal. Energy shifts of the IR absorption modes were observed in relation to biaxial residual stress present in the films. As residual stress varied from tensile to compressive, E1(TO) peak energy increased, while A1(TO) peak energy decreased. It is concluded that FTIR transmission spectroscopy has the potential to be used as a routine characterization technique for in-line process control of polycrystalline AlN thin films.

Integrated measurement-modeling approaches for evaluating residual stress using micromachined fixed-fixed beams

Two methodologies have been developed to determine the biaxial residual stress value in thin films using electrostatically actuated fixed-fixed beam test structures. In the first, we determine the compliance matrix of the support posts using 3-D finite-element analysis. The residual stress value is then found from the best fit between the measured and modeled deflection curves, with the residual stress as the only free parameter in the model. An accuracy of /spl plusmn/0.5 MPa for the average biaxial residual stress level is evaluated from the reproducibility of independent measurements over a wide range of loadings. The key to the second methodology lies in the recognition that for a given value of residual stress, there exists a unique family of deflection curves associated with two adjacent beams of different lengths. Therefore, compliance information can be extracted directly from the deflection curves. We proceed to show that essentially the same values of residual stress are found by the two methodologies, while the latter allows much more rapid extraction of the residual stress. With the second methodology established, we find that residual stress values vary across a quarter of a six-inch diameter wafer by 2.5 MPa for three structural levels of polycrystalline silicon in our five-level surface micromachining technology.

Application of Barkhausen noise to biaxial residual stress measurements in welded steel tubes

The relationship between Barkhausen noise and biaxial stress was studied in mild steel specimens in as received and annealed conditions. In the as received condition the saturation stress of the Barkhausen noise was higher but the stress sensitivity was lower than in the annealed condition. In the as received condition the Barkhausen noise response was different in the rolling and transverse directions. In the annealed condition no difference was observed. The stress sensitivity was found to be independent of stress perpendicular to the magnetising direction in the annealed condition. In the as received condition the stress sensitivity depended on the perpendicular stress. The biaxially calibrated Barkhausen noise method was applied to residual stress measurements of welded steel tubes. The results were compared with X-ray diffraction and hole drilling results. In the axial direction of the tube all methods produced consistent results. In the hoop direction of the tube Barkhausen noise and X-ray diffraction provided similar results. They were different from those obtained with the hole drilling method.

Prediction of error due to eccentricity of hole in hole-drilling method using neural network

The measurement of residual stresses by the hole-drilling method has been used to evaluate residual stresses in structural members. In this method, eccentricity can usually occur between the hole center and rosette gage center. In this study, we obtained the magnitude of the error due to eccentricity of a hole through the finite element analysis. To predict the magnitude of the error due to eccentricity of a hole in the biaxial residual stress field, it could be learned through the backpropagation neural network. The prediction results of the error using the trained neural network showed good agreement with FE analyzed results.

Finite element simulation of spherical indentation in the elastic‑plastic transition

Abstract Finite element simulations were conducted to examine spherical indentation in the elastic–plastic transition regime. Various elastic-perfectly plastic materials were studied by varying the ratio of the elastic modulus to the yield stress in the range 25–1000. Special attention was given to the influence of residual stress and friction on the indentation load–displacement behavior and development of the plastic zone. A new method for measuring yield strengths from indentation load–displacement curves is proposed, and a recently developed experimental method for measuring biaxial residual stresses by nanoindentation methods is assessed. In the appropriate limits, the simulations show good agreement with the theoretical descriptions of spherical indentation given by Hertz and Tabor.

Yield Strength Determination of TiN Film by In-Situ XRD Stress Analysis Method

By X-ray stress analysis technique, a new approach has been applied to determining the yield strength of polycrystalline TiN film with biaxial residual stress adherent to metal substrate. By plasma-assisted chemical vapor deposition (PACVD), TiN film was deposited on a strip of spring steel 60Mn allowing the film to be subjected to tensile stress under loading of the film/substrate. The film thickness is 2.5μm and the steel substrate thickness is 1.1mm. Longitudinal and transverse stresses, σ 1 and σ 2 of the film were measured in situ by X-ray diffraction. On the basis of the experimental results, the effective stress o and the effective uniaxial strain e t were obtained. According to the σ-e t relation, the calculated proof stresses, σ 0.1 , and σ 0.2 , of TiN film are 4.2GPa and 4.4GPa, respectively.

Estimation of Biaxial Residual Stress in Welded Steel Tubes by Barkhausen Noise Measurements

Relation between biaxial stress and Barkhausen noise was evaluated in mild steel specimens by biaxial bending experiments. Residual stresses of welded steel tubes were evaluated by Barkhausen noise and X-ray diffraction. Barkhausen noise values were converted into residual stress values by interpolation from reference measurements made by X-ray diffraction and Barkhausen noise. Residual stress values measured by both methods were consistent.

Cathodoluminescence and Al Composition of Biaxially Stressed AlGaN/GaN Epitaxial Layers

We report on the cathodoluminescence (CL) of Al 0.25 Ga 0.75 N (x = 0.25) and underlying GaN films and on the Al composition of the AlGaN with the biaxial residual stress taken into account. Crack-free thick AlGaN layers were fabricated by introducing a thin AlN interlayer on a high quality GaN epitaxial layer. The CL band-edge emission of AlGaN from a non-cracked region appeared at a lower energy than that from the cracked region. This is the direct indication, that crack relaxes biaxial tensile stress in AlGaN layers. When the Al composition x was lower than 0.5, the Al compositions measured by X-ray diffraction fell between the values estimated by Vegard's law and the values of pseudomorphic assumption due to partially relaxed residual stress in the AlGaN films. When x > 0.5, they are in agreement with Vegard's law, indicating that the AlGaN layer is fully relaxed.

Waterjet and abrasive waterjet surface treatment of titanium: a comparison of surface texture and residual stress

In this study, commercially pure titanium (cpTi) and a titanium alloy (Ti6Al4V) were subjected to waterjet (WJ) peening and abrasive waterjet (AWJ) peening surface treatments. The texture and in-plane biaxial residual stress of the treated surfaces were quantified using contact profilometry and X-ray diffraction, respectively. Regardless of the specific process conditions, the surface residual stresses resulting from WJ and AWJ peening of both materials were compressive. Residual stresses in the Ti6Al4V ranging −400≤σ≤−30MPa, whereas stresses in the cpTi treated with the same conditions ranging −200≤σ≤−60MPa. Residual stresses resulting from WJ peening increased with the WJ pressure whereas those resulting from AWJ peening decreased with an increase in both jet pressure and abrasive size. The surface roughness of the metals did not change appreciably with WJ treatment, but AWJ peening resulted in a significant increase in roughness. Therefore, AWJ peening may serve as a new method for introducing compressive residual stresses in engineering components that also require rough surfaces.

Measurement of residual stress by load and depth sensing indentation with spherical indenters

A new experimental technique is presented for making measurements of biaxial residual stress using load and depth sensing indentation (nanoindentation). The technique is based on spherical indentation, which, in certain deformation regimes, can be much more sensitive to residual stress than indentation with sharp pyramidal indenters like the Berkovich. Two different methods of analysis were developed: one requiring an independent measure of the material's yield strength and the other a reference specimen in the unstressed state or other known reference condition. Experiments conducted on aluminum alloys to which controlled biaxial bending stresses were applied showed that the methods are capable of measuring the residual stress to within 10–20% of the specimen yield stress. Because the methods do not require imaging of the hardness impressions, they are potentially useful for making localized measurements of residual stress, as in thin films or small volumes, or for characterization of point-to-point spatial variations of the surface stress.

Microstructure, stress and mechanical properties of arc-evaporated Cr–C–N coatings

The relationships between coating microstructure and properties in the Cr–C–N system have been investigated as a function of composition and post-deposition annealing. Coatings of varying compositions were grown using arc-evaporation, by varying the reactive gas flow ratio fR=f(C2H4)/f(N2) from 0 to 0.2, and were found to consist primarily of the cubic δ-Cr(C,N) phase. Changes in both the unstressed lattice parameter, ao, and X-ray diffraction background intensity indicate that both the carbon concentration within the δ-phase and amorphous/crystalline content increases with fR. Increasing fR also decreases the magnitude of the compressive biaxial residual stress, from approximately 6 to 1 GPa, while increasing both the inhomogeneous stress and thermal stability. The elastic modulus and hardness of as-deposited coatings were determined from nanoindentation to be 320 and 23 GPa, respectively, for moderate carbon concentrations (fR≤0.05). Concurrent variations in microstructure and hardness with post-deposition annealing indicate that the as-deposited hardness is significantly enhanced by the microstructure, primarily by lattice defects and related stresses (microstresses) rather than average stresses (macrostresses).

Effect of support compliance and residual stress on the shape of doubly supported surface-micromachined beams

Doubly supported silicon-micromachined beams are increasingly used to study the mechanical properties of materials. Residual stresses in the beams and support compliance cause significant vertical deflections, which affect the performance of these micromachined devices. We present here both experimental results for doubly supported polysilicon surface-micromachined beams, and an elastic model of the devices that takes into account the compliance of the supports and the geometrical nonlinear dependence of the vertical deflections on the stress in the beam. An elastic one-dimensional model was used for the beams, and the response of the supports to forces and moments was obtained using finite-element method simulations. The model explains a previously observed gradual increase of the maximum vertical deflections of the beams with increasing length at a given constant residual stress and, in agreement with experimental observations, predicts two stable states for compressively stressed beams: one with the beam bent up, the other down. We introduce a critical biaxial residual stress /spl sigma/r/sub c/, above which there are significant deflections of the beams, /spl sigma//sub rc/ can be used in practice to determine the maximum allowable compressive biaxial residual stress as a function of the beam length. The effect of variations of the compliance of the supports is reported.