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Residual stress has a significant effect on the performance of thin films, in terms of adhesion, hardness, wear and fatigue resistance. Thus, when assessing innovative coatings or new deposition technologies, it is important to perform residual stress evaluation by means of a suitable and reliable technique. X-ray diffraction (XRD) is one of the commonly used techniques, because it is non-destructive, surface sensitive and phase selective. However, it is subject to certain limitations: X-ray diffraction allows stress evaluation (i.e. its indirect deduction from the measured diffraction profile) only in case of crystalline materials, and the results may be subject to aberrations in the presence of texture or stress gradients often occurring in thin films. Recently, a new class of methods for residual stress evaluation has been proposed, based on incremental focused ion beam (fib) milling, combined with high-resolution in situ scanning electron microscopy (SEM) imaging and full field strain analysis by digital image correlation (DIC). The aim of the present paper is to explore in some detail the significance of the stress values obtained for the same coating by X-ray diffraction and focused ion beam milling, and to demonstrate that the analysis of residual stress depth gradients is possible by using FIB-DIC techniques. Finally, a comparative assessment of the weaknesses and strengths of the two techniques will be carried out. For this purpose, a chromium nitride highly textured thin film sample was chosen. The residual stress state evaluation by the two methods was carried out for the coating deposited by the cathodic arc evaporation (CAE-PVD) technique. Although many significant differences were identified between the X-ray diffraction and focused ion beam milling methods, careful consideration of the gauge volumes and weighting demonstrated that satisfactory agreement was obtained. The analysis highlighted the importance of the issues related to (a) probe-to-sample interaction volume (gauge volume), (b) film texture, and (c) the elastic anisotropy. All these factors must be taken into account in order to enable valid comparisons to be drawn. [ABSTRACT FROM AUTHOR]

The present investigation has been carried out in order to study the erosion wear behaviour of WC-Co base thermal spray coatings. WC-12Co and WC-10Co-4Cr coatings were deposited by means of high velocity oxygen fuel (HVOF) thermal spraying. The erosion tests were conducted at impact angles of 30 and 90° using SiC particles of ∼50 μm in diameter as erodent, at a velocity of 83·4 m s-1. It has been found that the erosion rate for both coated systems was higher when the test was carried out at an angle of 90°. The through-thickness residual stresses of the coatings, as well as the microstructural characterisation, allowed an explanation of the results and the erosion mechanisms in each case. It has been found that, under the experimental conditions carried out in the present study, the WC-10Co-4Cr coating exhibited a higher erosive wear resistance as compared to the WC-12Co coating. [ABSTRACT FROM AUTHOR]

Abstract: In the present study, residual stress evaluation in thin films was achieved using a semi-destructive trench-cutting method. Focused Ion Beam (FIB) was employed to introduce the strain relief by ring-core milling, i.e. creating a trench around an “island”. Either SEM or FIB imaging can be used to record sequences of images for strain change evaluation by Digital Image Correlation (DIC) analysis of micrographs. A regular array of shallow holes was drilled on a thin overlayer of Pt (∼100nm) deposited on to the film prior to patterning and trenching, in order to reduce the damage introduced by the ion beam during imaging and to assist the DIC strain evaluation by adding traceable markers. Finite Element (FE) simulation was also carried out to predict the curves for strain relief as a function of milling depth, and compared with the experimental measurements, which show good agreement with each other. An empirical mathematical description of the curves was proposed that allows efficient residual stress evaluation in thin solid films. [Copyright &y& Elsevier]

We present a new procedure for the determination of surface elastic residual stress by instrumented sharp indentation, based on nanoindentation testing on focused ion beam (FIB) milled micro-pillars. Finite element modeling (FEM) of strain relief after FIB milling of annular trenches demonstrates that full relaxation of pre-existing residual stress state occurs when the depth of the trench approaches the diameter of the remaining pillar. Considering this, the average residual stress present in the sample material can be calculated by the comparison of two different sets of load-depth curves, the first one obtained at the center of stress relieved pillars, the second on the undisturbed (residually stressed) surface. Analytical modeling of the contact stress distribution in non-halfspace conditions was adopted to take into consideration the additional boundary conditions given by the edges of pillars and the elastic properties of the substrate (in case of coatings). The results are presented for residual stress evaluation of a 3.8-µm TiN coating on WC-Co substrate obtained by cathodic arc evaporation-physical vapor deposition (CAE-PVD) techniques, showing an average compressive stress state of -5.63 GPa. This result is in close agreement with the estimation obtained by XRD (sin2 ψ method) analysis of -5.84 GPa of the same sample, adopting the same elastic constants. [ABSTRACT FROM AUTHOR]

Abstract: Hard coatings like titanium nitride (TiN) normally contain a high degree of internal stress (usually compressive in-plane parallel with the surface) owing to growth defects developed during the deposition process and thermal mismatch effects after final cooling; it is, therefore, difficult to produce single-layer TiN coatings thicker than 6–7 μm, without adhesion problems. In the present study, thick coatings (i.e. >10 μm) have been achieved by alternate multilayering of TiN with Ti interlayers, leading to a tougher and less-stressed film. However, having a constant distribution of titanium interlayer thickness is not necessarily the best solution to achieve maximum performance in terms of wear resistance and hardness. The residual stress distribution along the thickness is unlikely to be constant with the inner layers being more stressed due to a greater amount of thermal differential strain. Following this guideline, a series of numerical simulations was performed in order to calculate the residual stress through thickness distribution due to the deposition process. Three sets of multilayered Ti/TiN coatings having both constant and variable Ti interlayer thickness were modelled and deposited, using a reactive arc PVD process. Mechanical and tribological properties were characterized using static and depth sensing Vickers micro-hardness indentation tests, rotating wheel (dimpling grinder) abrasive wear tests and Rockwell C adhesion tests. Coating interface characterizations were made by SEM-EDS. Results showed that adhesion can be significantly improved by adopting a titanium through thickness quantity increasing towards the interface: an optimized distribution allows also higher hardness and wear resistance to be obtained, as it requires a lower total amount of titanium to obtain good adhesion properties. [Copyright &y& Elsevier]

In this article authors present similarities between an article by J.G. Zhu and colleagues regarding measurement of the residual stress in thermal barrier coatings (TBCs) using a self developed deformation sensor and the article regarding the same by M. Sebastiani and colleagues published in the previous issue. Topics discussed include similarity in experimental implementation, use of scanning electron microscope images, and use of micro-ring core method for thermal spray coatings.

Abstract: A novel method is presented for the assessment of the Poisson's ratio and residual stress on the micron scale, based on focused ion beam (FIB) two-step four-slot micro-milling and in-situ digital image correlation (DIC) analysis of the induced relaxation strains at the specimen's surface. The methodology has been fully validated through modelling and experiments on three different materials, namely, physical vapour deposition (PVD) chromium nitride (CrN), and as-deposited and annealed Cu thin films. The cases of non-equibiaxial stress state and non-isotropic elastic behaviour are discussed in detail, and a quantitative evaluation of the accuracy, error sensitivity and the effects of microstructure and anisotropy is presented. This method represents a substantial improvement over the existing state-of-the-art. It is suitable for application on amorphous and nanostructured materials, and provides a breakthrough in micro-scale Poisson's ratio analysis of thin films and small structures and components. [Copyright &y& Elsevier]

Abstract: The effect of surface defects on the residual stress local distribution in thin coatings has been investigated by an innovative high resolution methodology, which mainly consists of incremental focused ion beam (FIB) micron-scale ring-core milling (IμRCM), combined with high-resolution in situ SEM-FEG imaging of the relaxing surface and in-plane strain analysis by digital image correlation (DIC). The depth profile of the residual stress can be obtained from the experimentally measured surface strain with sub-micron spatial resolution (both lateral and in-depth) by using a finite element modeling (FEM) reverse calculation procedure. A chromium nitride (CrN) coatings was produced by Cathodic Arc Evaporation Physical Vapor Deposition (CAE-PVD) on AISI 316L steel substrate. Preliminary characterization of coatings consisted of nanoindentation testing, FIB cross section microstructural analysis and XRD (sin2ψ) average residual stress measurement. Local residual stress was measured nearby to surface micro-droplets, with the main aim of investigating the effect of such defects on the residual stress in-depth distribution. A series of stress measurements were also performed on the homogeneous defect-free coating. Significant differences were found in terms of residual stress and stress gradient in proximity of surface defects, in comparison with the residual stress measured on the defect-free areas. In case of micro-droplets, such differences were attributed to significant modification of growth mechanisms and microstructure of the coating, that lead to a different competitive mechanism of residual stress build-up. This assumption was supported by the microstructural FIB-SEM observation, which showed anomalous grain growth and changes in crystal orientation in correspondence of a droplet. [Copyright &y& Elsevier]

Abstract To withstand oxidation at a temperature ≥ 850 °C, an optimized Ti 1−x Al x N coating was grown via reactive High Power Impulse Magnetron Sputtering (HiPIMS) technology on three identical Ti 0.48 Al 0.48 Cr 0.02 Nb 0.02 billets. Different substrate surface pre-treatments were designed to increase performance: i) mechanical polishing, ii) mechanical polishing combined with a strong plasma etching, and iii) mechanical polishing coupled to both a weak plasma etching and a Ti 1−y Al y metallic interlayer deposition. Then, all the specimens were cyclically heat treated up to 200 cycles at 950 °C, using a Burner Rig (BR) facility. The chosen Ti 1−x Al x N/substrate interface architecture considerably influenced average compressive residual stress (S res) and adhesion of just deposited films. Moreover, it was possible to identify a clear relationship between S res behavior and each coating comportment after BR tests. It became clear that the weak plasma etching/Ti 1−y Al y interlayer match helped improving the system stability (i.e. very low average residual stress thermal relaxation) thus guarantying high temperature oxidation resistance. Graphical abstract Unlabelled Image Highlights • Reactive HiPIMS allows depositing high T oxidation resistant Ti 1−x Al x N films. • Ti 1−x Al x N/γ-TiAl interface design allows tailoring adhesion and microstructure. • Ti 1−x Al x N/γ-TiAl high T oxidation resistance is related to the interface design. • Average residual stress and recovery are clearly affected by the interface design. • Residual stress tests let studying coated samples high T oxidation behaviors. [ABSTRACT FROM AUTHOR]

Ni-B coatings with low, increasing amount of Boron have been produced using a Ni sulfamate plating bath with the addition of dimethylaminborane. Coatings' characterization revealed that it is possible to produce crack-free coatings with a B content up to 0.12 wt%, with a thickness of about 40–50 μm. The introduction of an increasing amount of B changes progressively the coatings microstructure from columnar, to fine fibrous and then to lamellar. The presence of B as interstitial atom in the Ni elementary cell caused an increase of the residual stress, as measured by FIB-DIC method, which change from compressive to tensile as a function of B concentration. The microstructure refinement and the increase of the residual stresses caused a noticeable increase of the microhardness. On the other hand, the resistance to localized corrosion decreased by increasing the B content maybe due to the formation of micro-defects or micro-cracks on the Ni passive layer due to the residual stresses. To confirm this hypothesis, the Ni-B coatings have been annealed at 400 °C to achieve a complete stress relaxation. The stress relaxation caused a decrease of the hardness and a noticeable increase of the corrosion resistance. [ABSTRACT FROM AUTHOR]

Residual stresses play a crucial role in determining material properties and behaviour, in terms of structural integrity under monotonic and cyclic loading, and for functional performance, in terms of capacitance, conductivity, band gap, and other characteristics. The methods for experimental residual stress analysis at the macro- and micro-scales are well established, but residual stress evaluation at the nanoscale faces major challenges, e.g. the need for sample sectioning to prepare thin lamellae, by its very nature introducing major modifications to the quantity being evaluated. Residual stress analysis by micro-ring core Focused Ion Beam milling directly at sample surface offers lateral resolution better than 1 μm, and encodes information about residual stress depth variation. We report a new method for residual stress depth profiling at the resolution better than 50 nm by the application of a mathematically straightforward and robust approach based on the concept of eigenstrain. The results are validated by direct comparison with measurements by nano-focus synchrotron X-ray diffraction. [ABSTRACT FROM AUTHOR]

Abstract: Steel pipes used in the oil and gas industry undergo the action of cyclic loads that can cause their failure by fatigue. A consistent evaluation of the fatigue damage during the initiation phase should fundamentally be based on a nanoscale approach, i.e., at the scale of the dislocation network, in order to take into account the micromechanisms of fatigue damage that precede macrocrack initiation and propagation until the final fracture. In this work, microstructural changes related to fatigue damage initiation are investigated in the API 5L X60 grade steel, used in pipe manufacturing. Microdeformations and macro residual stress are evaluated using X-ray diffraction in real time during alternating bending fatigue tests performed on samples cut off from an X60 steel pipe. The aim of this ongoing work is to provide ground for further development of an indicator of fatigue damage initiation in X60 steel. This damage indicator could allow a good residual life prediction of steel pipes previously submitted to fatigue loading, before macroscopic cracking, and help to increase the reliability of these structures. [Copyright &y& Elsevier]

Abstract: The present investigation has been conducted in order to determine the residual stresses of an as-ground WC–12Co coating of two different thicknesses, by means of two different methods. Firstly, X-ray diffraction techniques, which allowed the determination of the surface residual stresses of the coating by means of the method called “sin2 ψ” method. Secondly, an incremental hole drilling technique together with the integral method, which allowed the analysis of the non-uniform through-thickness residual stresses present in the coatings. It has been determined that the surface residual stresses are of a compressive nature, which could be due to the grinding that was applied to the coatings in order to achieve the desired thicknesses. On the contrary, the results of the incremental hole drilling tests indicated that the through-thickness residual stress distributions are not uniform and are characterized by the presence of tensile peak stresses, at depths in the range of ~50–125 μm. Such stresses were observed to decrease towards the coating–substrate interface where the compressive component of the stress state becomes greater than the tensile component. It has been found that the mean residual von Mises stress is higher in the thinner coating than in the thicker one, of approximately 180 and 107 MPa, respectively. [Copyright &y& Elsevier]

Abstract: The experimental measurement of residual stresses originating within thick coatings deposited by thermal spray processes onto solid substrates plays a fundamental role in the preliminary stages of coating design and process parameters optimization. The main objective of the present investigation was to determine the residual stresses by means of the incremental hole drilling method in order to perform the measurement of the stress field through the thickness of two different HVOF Nickel-based coatings. The holes through the coatings were carried out by means of a high velocity drilling machine (Restan). A finite element calculation procedure was used to identify the calibration coefficients necessary to evaluate the stress field. The Integral method was used for the analysis of non-uniform through-thickness stresses. The results for both coatings indicate that the nature of the residual stresses is tensile and their values are between 150–300 MPa. [Copyright &y& Elsevier]