3D multiscale segmentation and morphological analysis of x-ray microtomography from cold-sprayed coatings

International audienceX-ray microtomography from cold-sprayed coatings brings a new insight on this deposition process. A noise-tolerant segmentation algorithm is introduced, based on the combination of two segmentations: a deterministic multiscale segmentation and a stochastic segmentation. The stochastic approach uses random Poisson lines as markers. Results on a X-ray microtomographic image of aluminium particles are presented and validated

A composite approach to Al2O3 based plasma-sprayed coatings

25-29 septembreInternational audienceThermally-sprayed ceramic coatings such as plasma-sprayed alumina show a composite microstructure actually due to the presence of defects such as pores, inter-lamellar and intralamellar cracks. These 2nd phase-typed features influence the mechanical behavior and electrical insulation of the coating dramatically. In this study, a composite approach to the microstructure of plasma-sprayed alumina was developed for the optimising of component properties such as electrical gaps used in the oil industry. This approach consisted of a Finite Element Analysis (FEA) of thermo-mechanical and electrical properties from simulated microstructures. Series of composite microstructures were tested, i.e that of air plasma-sprayed (APS) alumina basically plus those obtained by addition of glass or resin using co-spraying and impregnation respectively. Various degrees of porosity and cracks could be obtained from different spraying conditions and by subsequent laser surface remelting. Every composite microstructure was studied using quantitative image analysis of series of SEM cross-sections. Electrochemical Impedance Spectroscopy (EIS) in NaCl solution was also performed to characterize the level of connected pores and the resulting electrical insulating properties. From experiments, a Finite Element Model (FEM) based on the actual microstructure was developed. The latter was simulated with involving of all significant features, such as phase distribution, porosity and defects. This simulation was developed to optimize the composite microstructure to meet industrial applications

Synthesis of B4C thin films by plasma-enhanced magnetron sputtering

Bor karbür, yüksek sertliği, mekanik, tribolojik, elektronik, optik özelliklerinin yanı sıra yüksek nötron absorblama özelliği ile de dikkat çeken bir malzemedir. Düşük yoğunluğu, yüksek Young modülü, çok yüksek termal ve kimyasal kararlılığı vb. özellikleri olan B4C, elmas ve c-BN’den sonra bilinen en sert malzeme olmasına karşın bu durum oda sıcaklığında geçerlidir. Elmas ve c-BN’ün sertliği artan sıcaklıkla kademeli olarak düşerken B4C termal kararlılığı sayesinde yüksek sıcaklıklarda sertliğini muhafaza etmektedir ve özellikle 1100 °C’nin üzerindeki sıcaklıklarda bilinen en sert malzemedir. Bu özellikleriyle B4C ince filmler, kesici takımların, fren balatalarının, sabit disklerin ve çeşitli makine parçalarının kaplanması gibi mekanik, tribolojik uygulamaların yanı sıra, yüksek sıcaklık ortamları gibi zorlayıcı şartlarda çalışacak transistörler vb. elektronik ve optik uygulamalarda da kullanılmaktadır. Bu çalışmada, bor karbür tozlarının sıcak preslenmesiyle elde edilmiş olan bor karbür hedef malzeme kullanılarak, plazma-destekli doğru akım manyetik alan sıçratma tekniğiyle 350–400 nm kalınlığında homojen ve taban malzemeye iyi yapışan bor karbür ince filmler üretilmiştir. Biriktirme sıcaklığı tüm kaplamalar için 250 °C olarak sabitlenmiş ve 0–250 V arasında uygulanan taban malzeme voltajının kaplama yapısına olan etkileri incelenmiştir. Elektron sondası mikro analizleri (EPMA), kaplamaların elementel bileşimlerinin üretim şartlarından bağımsız olduğunu ortaya koymuştur. Kesitten gerçekleştirilen taramalı elektron mikroskobu (SEM) incelemeleri sonucunda, bu çalışma şartlarında bor karbür filmlerin kolonsuz yapıda biriktiği tespit edilmiştir. Nanosertlik testleri neticesinde, taban malzeme voltajının arttırılmasıyla sertlik ve Young modülü değerlerinde önemli bir artışın meydana geldiği tespit edilmiştir. Anahtar Kelimeler: Bor karbür, ince film, sıçratma, iyon bombardımanı, nanomekanik.Boron carbide (B4C) is the third hardest material at room temperature, which has many other attractive properties such as good wear resistance, high modulus, high chemical and thermal stability. These properties make boron carbide a promising candidate as hard and protective coating for cutting tools, automobile parts, hard disk drives and other wear-resistant applications. Within the range of 8-20% of carbon, boron carbide is ideally described by a rhombohedral unit cell with icosahedral arrangement of 12 atoms, in addition to a three-atom chain along the crystallographic c-axis that interconnect the icosahedra. For amorphous boron carbide films deposited by sputtering, it is believed that the structure is still based on a random icosahedral network at a carbon content less than 50%. Several techniques have been used to synthesize boron carbide thin films, including chemical vapor deposition, plasma-enhanced chemical vapor deposition, cathodic arc, atmospheric plasma spraying, electromagnetically accelerated plasma spraying, RF magnetron sputtering, and DC magnetron sputtering. Among these, magnetron-sputtering techniques have been successfully commercialized in a large scale because of their high film-deposition rate and low-temperature features. In this study, boron carbide powders were obtained from the carbothermal reduction of boric acid in a graphite resistance furnace at 2000 ºC. The powder thus obtained was hot pressed in pure nitrogen atmosphere with 100 MPa applied force at 2100 °C for 15 minutes to obtain boron carbide target used in this study. The sputtering target was 15 cm in diameter and 7 mm in thickness. Boron carbide thin films were deposited by plasma-enhanced DC magnetron sputtering of hot-pressed boron carbide target. AISI M2 steel and Si (100) wafers were used as substrates in each deposition. High-purity (99.999%) Ar was used as precursor and was introduced into the vacuum chamber through a mass flow controller to establish the desired working pressure, which was 0.3 Pa. The cathode power was fixed at 500 W for all the experiments. Microstructural examinations revealed the presence of continuous and homogeneous B4C films with 350-400 nm thickness. Elemental composition of the films was measured by EPMA. Grazing-angle XRD of the coatings over showed no characteristic peaks for boron carbide, indicating that the coatings were amorphous. Infrared spectra presented two broad bands, one centered at ~1100 cm-1 and the other at ~1570 cm-1. The band at 1100 cm-1 is attributed to B?C bonds in the icosahedra and is characteristic of B4C thin film structure. The band at 1570 cm-1 has been attributed either to the presence of graphite or free carbon in boron carbide structure or to the stretching in the linear chains that interconnect the icosahedra. Nanoindentation results demonstrated that boron carbide films deposited are remarkably hard and the increase in the negative bias voltage led to an increase in the measured hardness from 32 GPa for the film deposited at floating potential to a maximum value of ~40 GPa for the coating deposited at 100 V bias voltage. Further increase in the bias voltage to 250 V resulted with a decrease in the hardness to 32 GPa. The same tendency of increase was observed for the Young's modulus, from 270 GPa for the films deposited at floating potential, it reached its maximum value for 300 GPa at 100 V bias voltage and then decreased thereafter to 265 GPa for 250 V bias voltage. Modified sputtering configuration led non-columnar, featureless microstructures with smooth surface morphologies. There was no significant effect of the deposition parameters on the thickness and elemental composition of the films deposited. Films were amorphous and exhibited remarkably high hardness and Young's modulus values with high elasticity. With the increase in the bias voltage, increases in the hardness and Young's modulus of boron carbide films were observed.  Keywords: Boron carbide, thin film, sputtering, ion bombardment, nanomechanics

Laser-Assisted Cold Spray (LACS)

International audienceRésumé du livre : Discovered almost fifty years ago at Bell Labs (1964), the Nd:YAG laser has undergone an enormous evolution in the years, being now widely used in both basic research and technological applications. Nd:YAG Laser covers a wide range of topics, from new systems (diode pumping, short pulse generation) and components (a new semiorganic nonlinear crystal) to applications in material processing (coating, welding, polishing, drilling, processing of metallic thin films), medicine (treatment, drug administration) and other various fields (semiconductor nanotechnology, plasma spectroscopy, laser induced breakdown spectroscopy)

Etude par choc laser de l'adhérence de barrières thermiques aéronautiques

National audienceL'estimation de la durée de vie des barrières thermiques aéronautiques (BT) déposées sur les aubes de turbines haute pression nécessite de comprendre les mécanismes induisant l'endommagement entre la sous-couche et la zircone. Le LASAT, LAser Shock Adhesion Test ou essai d'adhérence par choc laser, est une technique exploitant tout son potentiel dans le cas des BTs, en particulier par sa rapidité, simplicité et au comportement optique de la couche de zircone. Ce dernier avantage permet de dimensionner l'endommagement généré sans réaliser de coupes métallographiques. Un nouveau protocole de l'essai LASAT a été mis en place utilisant les ondes de choc bidimensionnelles et exploitant la dépendance entre l'intensité du choc appliqué, le niveau d'adhérence du dépôt, et la dimension de la fissure générée

Densification superficielle de matériaux poreux par choc laser

Laser-driven shock-waves are used as a surface treatement for compacting porous materials. The compaction depth is typically a few hundreds of microns. The behavior of the porous medium is described through a compaction model based on the α theory of Herrmann. This model has been introduced into a one-dimensional finite difference hydrodynamic code describing the behavior of a target under the action of a laser-generated shock-wave. The code enables us to compute the compaction depth as a function of irradiation conditions, nature and initial porosity of the material. Experiments are performed on aluminum powder. Samples are observed by optical microscopy. The residual porosity is estimated by image analysis. Experimental results and computed compaction profiles correlate well

LAser Shock Adhesion Test (LASAT), an innovation dedicated to industry

NonWOSAvailable online for free at http://www.ila.org.in/kiran/kiran_19_2.pdfInternational audienc

Laser Shock Adhesion Test (LASAT) of electron beam physical vapor deposited thermal barrier coatings (EB-PVD TBCs)

International audienceDamage prediction, adhesion strength and remaining lifetime of TBC are highly important data for understanding and preventing TBC spallation on blades. LAser Shock Adhesion Test (LASAT) is a powerful method to measure adhesion of coating due to its rapidity, simplicity and capabilities to distinguish different strength levels and the easy damage observation in case of TBCs. A new protocol of LASAT has been introduced in order to measure the adhesion level of the ceramic coating from the exploitation of the two-dimensional effects that promotes a shock wave pressure-dependent size of the damage. Finite element modeling, taking into account the TBCs dimensions, showed the edges effect on interfacial stress applied by laser shock

Residual Stress Analysis of Laser-Drilled Thermal Barrier Coatings Involving Various Bond Coats

The gas turbine combustion chamber of aero-engines requires a thermal barrier coating (TBC) by thermal spraying. Further heat protection is achieved by laser drilling of cooling holes. The residual stresses play an important role in the mechanical behaviour of TBC. It could also affect the TBC response to delamination during laser drilling. In this work, studies of the cracking behaviour after laser drilling and residual stress distribution have been achieved for different bond coats by plasma spray or cold spray. From interface crack length measured pulse-by-pulse after laser percussion drilling at 20° angle, the role of the various bond coats on crack initiation and propagation are investigated. It is shown that the bond coat drastically influences the cracking behaviour. The residual stresses profiles were also determined by the incremental hole-drilling method involving speckle interferometry. An original method was also developed to measure the residual stress profiles around a pre-drilled zone with a laser beam at 90°. The results are discussed to highlight the influence of TBCs interfaces on the resulting residual stresses distribution before laser drilling, and also to investigate the modification around the hole after laser drilling. It is shown that laser drilling could affect the residual stress state