Type 316L stainless steel rod was coated with 300 μm thick yttria stabilised zirconia (≈8%Y
2
O
3
) by plasma spraying over a 50 μm metallic bond coating of NiCrAlY. However, the presence of porosity ...and microcracks in the as sprayed coating is a drawback. In the present work an attempt was made to study the plasma spraying of such coatings and to enhance their properties through laser surface treatment. A continuous wave CO
2
laser of 10·6 μm wavelength with a beam diameter of 1·5 mm was used to eliminate porosity and microcracks. Powers of 50, 75 and 100 W with 2500 μm s
&minus1
scan rate and 500 μm step size were applied to melt the surface. The as coated and laser treated samples were characterised using X-ray diffraction, optical microscopy and SEM. The porosity was decreased from 10% in the as coated sample to 0·5% in the laser treated samples. Segmented cracks were formed due to residual stresses developed during laser remelting and solidification. Delamination occured near the bond coat/ceramic interface in the coating as the melted thickness was higher for higher laser power. Microhardness measurements indicated higher hardness in the laser treated region. Optical and SEM examination revealed polygonal grains of zirconia formed during laser remelting in all the laser treated samples. The X-ray diffraction analysis of laser treated samples revealed the formation of tetragonal, non-transformable tetragonal and cubic phases. With an increase of laser power, the non-transformable tetragonal phase increased marginally.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Laser treatment of alumina surfaces in a nitrogen gas environment was carried out and the corrosion response of the resulting surfaces is examined. The corrosion tests were carried out using 5% NaCl ...solution for seven days, with continuous steering by a magnetic stirrer. Metallurgical and morphological changes in the laser irradiated region are examined using the scanning electron microscope, optical microscope, X-ray diffraction. It was found that formation of nitride (AlN) compounds was evident with a compact and dense structure formed in the laser irradiated region. Moreover, a few shallow pit sites are evident around the crack tips after the static corrosion tests, especially at a high laser output power.
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Boriding was carried out on mild steel (C15) plates. Under the experimental conditions used in the present study, a minimum laser power of 200 W was found essential to break down the needle ...morphology of the boride case. Laser surface modification using a 250 W beam results in the formation of equiaxed globular borides at the surface along with a significant breakdown of the boride needles away from the surface. Although, laser surface modification is found to significantly reduce the surface hardness, it also reduces the hardness gradient across the case and results in break down of directional structure of the case and thereby results in a significant enhancement in the ductility and toughness of the case. Laser surface treatment of borided steels with a power of 150 W or lesser results in the deterioration of the case microstructure and properties.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Afin de protéger les composants hydrauliques des phénomènes d’érosion par cavitation, les pièces sont très souvent revêtues par projection thermique. Parmi les matériaux qui présentent de bonnes ...performances contre cette forme d’érosion, certaines recherches, trop rares encore, relèvent les excellentes propriétés de la zircone stabilisée à 8 wt% yttrium (YSZ). C’est pourquoi, dans cette étude, des revêtements YSZ à 8 wt% ont été fabriqués avec différents procédés de projection thermique et post-traités par refusion laser puis ils ont été soumis aux tests de cavitation référencés selon la norme ASTMG32.Le revêtement YSZ a tout d’abord été élaboré par projection plasma atmosphérique (APS). Différentes tailles de poudre YSZ et différentes températures de préchauffage du substrat ont été étudiées pour observer leurs effets sur la tenue en cavitation des échantillons. La simulation de cavitation acoustique de type Caflisch a permis d’estimer la vitesse du jet d'eau et les contraintes exercées par l'effondrement des bulles à la surface des échantillons. Les résultats en cavitation des deux revêtements YSZ et 304SS ont été étudiés et leurs mécanismes discutés.Ensuite et afin densifier ces échantillons, des revêtements composites YSZ-NiCrBSi contenant de 5 à 25 wt% NiCrBSi dans le mélange ont été fabriqués avec le même procédé APS. Les résultats de l'érosion par cavitation montrent qu'une faible adhérence des particules de NiCrBSi vient contrebalancer l'effet positif de la diminution de la porosité dans ces revêtements. Le revêtement YSZ a été finalement post-traité par refusion laser et différents paramètres laser ont été testés pour optimiser leur densification. Il s’avère que la refusion au laser produit une couche refondue dense en surface mais aussi des fissures à l'intérieur des revêtements. Exposés au test d'érosion par cavitation, il a été observé des arrachements importants de la partie refondue du revêtement et un décollement à son interface, d’où une perte de masse rapide. C’est pourquoi, de l'époxy a été utilisée pour infiltrer ces fissures. Le revêtement refondu par laser et infiltré par époxy présente une amélioration significative de la résistance à la cavitation. Enfin, le procédé de projection de plasma sous vide (VPS) qui permet d’obtenir des revêtements très denses a été utilisé pour fabriquer le revêtement YSZ. Un revêtement de porosité inférieure à 1% a été obtenu. En raison de la température élevée du substrat pendant la projection VPS, les liaisons inter lamellaires sont améliorées, contribuant à augmenter significativement la résistance à la cavitation.
In order to protect the hydraulic components from cavitation erosion phenomena, the parts are often coated by thermal spraying. Buck YSZ shows an excellent performance against cavitation erosion. However, the cavitation erosion resistance of YSZ coatings have vaguely been studied. Therefore, in this study, YSZ were manufactured with different thermal spraying processes and post-treated by laser remelting, then they were subjected to cavitation tests according to ASTM G32.The YSZ coating was first manufactured by atmospheric plasma spraying (APS). Various sizes of YSZ powder and different preheating temperatures of the substrate were studied to observe their effect on the cavitation behavior of the samples. The acoustic cavitation simulation of the Caflisch type has been used to estimate the speed of the water jet and the stresses exerted by the collapse of the bubbles on the surface of the samples. The cavitation results of the two YSZ and 304SS coatings were aslo studied to discuss cavitation damage of a thermally sprayed coating.Then in order to densify these samples, YSZ-NiCrBSi composite coatings containing from 5 to 25 wt% NiCrBSi in the mixture were manufactured with the APS process. The results of cavitation erosion show that low adhesion of NiCrBSi particles counterbalances the positive effect of decreasing porosity in these coatings. The YSZ coating was then post-treated by laser remelting and various laser parameters were tested to optimize their densification. It turns out that laser remelting produces a dense surface layer but also cracks inside the coatings. Exposed to the cavitation erosion test, it was observed significant removal of the remelted parts of the coating and, resulting in rapid mass loss. This is why epoxy has been used to infiltrate these cracks. The laser remelted and epoxy infiltrated coating exhibits a significant improvement in cavitation resistance. Finally, the vacuum plasma spraying method (VPS), which provides very dense coatings, was used to make the YSZ coating. A porosity coating of less than 1% was obtained. Due to the high temperature of the substrate during VPS projection, the interlamellar bonds are improved, contributing to a significant increase of cavitation resistance.
In this paper, the plasma sprayed coatings were treated by laser remelting. The morphologies, elements analysis and phases of both sprayed and remelted coatings were studied by means of electron ...probe microanalysis (EPMA), X-ray diffraction (XRD) and so on. The results show that the structure of the sprayed coatings is coarse, the amorphization of HA is tremendous, and the bonding state between the coating and the substrate is mechanical combination. After the sprayed coatings were treated by laser remelting in a proper conditions, the properties of the coatings are improved greatly. The microstructure of remelted coatings is columnar and cellular dendritic crystal which is homogeneous and compact, and the coating consists of HA, α-TCP and CaO phases, the Ca/P ratio of transition layer is close to 1.67, but the Ca/P ratio of surface layer is higher than that of HA because of the loss of P.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Flame spraying is frequently used for polyether ether ketone (PEEK) and PTFE coating deposition on metallic surfaces. However, this process has a certain number of limitations, particularly on the ...coating quality such as high porosity, low interfacial adherence, etc. For that reason a thermal post-processing step is often necessary. The objective of this study is to analyze the effects produced during a laser beam heat treatment on morphological structure (compactness) of PEEK coatings and their mechanical properties (adherence and tribology). The influence of the laser beam wavelength (by using a Nd:YAG, CO
2
or diode lasers) on compactness of the flame sprayed PEEK coating deposited on metallic substrate (304L) was analyzed. Since the value of laser light absorption coefficient of the PEEK coating depends on the laser wavelength, an optimization of the operational parameters for each laser has been carried out in order to achieve melting but not burning of the PEEK coating. Nevertheless, whatever the laser wavelengths used, the results showed a good effect of the laser treatment: improvement of both polymer coating compactness and its adherence to the substrate.
Laser remelting and rapid solidification were performed in preparing the high-performance Al2O3/Y3Al5O12(YAG) eutectic in situ composite. The microstructure characteristic and solidification behavior ...were studied using scanning electron microscopy (SEM), energy dispersive spectroscopy(EDS), X-ray diffractometry(XRD) and simultaneous thermal analysis(STA). The hardness and fracture toughness were obtained using an indentation technique. The results show that the laser remelted Al2O3/YAG composite has a homogeneous eutectic microstructure without microcrack and pore. The component phases of Al2O3 and YAG are three-dimensionally and continuously reticular connected, and finely coupled without grain boundaries, colonies and amorphous phases between interfaces. The eutectic interspacing is greatly refined with increasing the scanning rate and average is only 1 μm. The synthetically thermal analysis indicates that the eutectic temperature of Al2O3-YAG is 1 824 °C, well matching the phase diagram of Al2O3-Y2O3 system. The maximum hardness reaches 19.5 GPa and the room fracture toughness is 3.6 MPa.m½.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The boronizing layers of 45 steel of drilling pump liner have been remelted with laser, and the wear resistance properties, phase transition and microstructures of the boronizing layers before and ...after laser remelting were analyzed. The results show that laser remelting enhances surface abrasive wear resistance properties by 40% ~90%. Laser remelting achieves improvements mostly by means of alternating boronizing layer morphologies, eliminating FeB and defects within boronizing layer, improving the interface bonding intensity, fining the microstructures, redistributing boron concentration and generating lots of alloy phases. The strong support to material surfaces by martensites gained during remelting process within transition area is also beneficial to improve wear resistance properties. It is shown from the practice that the laser remelting compound technology followed boronizing shall be widely used in surface hardening of petrochemical key parts.
Thermal barrier coatings (TBCs) constituted by yttria partially stabilized zirconia (i.e. Y-PSZ, ZrO
2+7wt.% Y
2O
3) and a metallic bond layer (i.e. usually MCrAlY where M represents a combination of ...Ni and Co) are extensively used to improve the performance of hot-section components of gas turbines. Air plasma spray (APS) and in-situ laser irradiation by diode laser processes are combined to modify structural characteristics of TBCs. The existence of interconnected porosity and segmented cracks in APS·TBCs not only influences their mechanical properties, but also deteriorates the oxidation and corrosion resistance of the component. One of the purposes of combined laser treatments is so to architecture differently the pore networks. The dependence between the microstructure of TBCs, which was modified by laser treatment, and corrosion resistance has been evaluated by an electrochemical test. Results show that, for a laser energy density ranging from 1.7 to 1.9 J mm
−2, the pore network connectivity level decreases. Scanning electron microscopy observation revealed also that laser treatment induces a change of the microstructure from lamellar to columnar dendritic. Intensive image analysis was also performed to further quantify the pore characteristics (i.e. nature, orientation, percentage, etc.). Moreover, no phase transition was noticeable: the metastable tetragonal phase still remains the predominant phase after laser treatment.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP