The microstructures and performance of Ni-based alloys and Ni-WC (nickel-tungsten carbide) composite overlays deposited by plasma transferred arc welding have been studied. The Ni-alloy overlays had similar microstructures consisting of Ni dendrites, with interdendritic Ni-based eutectics, borides and carbides. Low hardness alloy overlays contained a smaller fraction of interdendritic phases relative to the high hardness alloys. The interdendritic regions make a significant contribution to the hardness since they are more than twice as hard as primary dendrites. The Ni-WC composites contained similar phases, however WC dissolution was observed leading to the formation of other carbides. Ni-alloys with low carbon and Cr content exhibited the lowest WC dissolution. The Ni-WC overlays produced using these dilute alloys generally performed better in ASTM G65 wear tests. This was likely due to the reduced dissolution which avoided formation of brittle secondary phases, maintaining a short mean free path between WC particles and allowing increased impact energy absorption. / Materials Engineering

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Baigiamajame magistro darbe nagrinėjamos vibromechaniškai veikiamų perlydymo metu purkštinių NiCrBSi dangų savybės. Dangos pailgina mašinų, metalinių gaminių bei konstrukcijų eksploatacinį resursą, taupo medţiagas, apsaugo metalus nuo korozijos, agresyvios aplinkos poveikio. Terminis purškimas – vienas populiauriausių dangų dengimo būdų. Darbe pateikta terminio purškimo technologijų, purkštinių dangų savybių tyrimo metodų bei vibromechaninio apdorojimo taikymo apžvalga. Dangų purškimui bei perlydymui naudotas robotizuotas stendas. Atlikti dangos mikrostruktūros, porėtumo, kietumo, mikrokietumo bei atsparumo dilimui tyrimai, suformuluotos darbo išvados. Darbą sudaro šešios dalys: įvadas, literatūros analizė, eksperimentiniai tyrimai, tyrimų rezultatai, išvados, literatūros sąrašas. Darbo apimtis – 62 p. teksto be priedų, 24 iliustr., 15 lent., 35 bibliografiniai šaltiniai. / In the final master thesis the research of vibromechanical treatment of thermal sprayed NiCrBSi coatings during refuse process is completed. Coatings prolong machines, metal products and constructions worktime, save materials and protect metals from corrosion and impact of aggressive environment. Thermal spraying is one of the most common covering method. Review of thermal spraying techniques, methods of thermal sprayed coating properties analysis and application of vibromechanical treatment is given. Robot was used for covering and refusing the coating. Coatings microstructure, porosity, hardness, microhardness and wear test analysis were done. Conclusions are formulated. Structure: introduction, literature review, experimental researches, results, conclusions, references. Thesis consist of 62 p. text without appendixes, 31 pictures, 15 tables, 35 bibliographical entries.

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.

This thesis deals with surface re-melting by electron beam of thermal sprayed material NiCrBSi by method HVOF. The microstructure, chemical and phase composition, hardness and abrasive wear resistance were evaluated in depending on the electron beam parameters and modes of the electron beam 6-poinst and line. Observed microstructure was better after both methods of re-melting. The splats were not observed, coating porosity decreased and structure was more homogenous. Structure and phase analysis was acquired by light microscopy, scanning electron microscopy, energy dispersive spectroscopy and x-ray diffraction. A small change in hardness between modes of re-melting was observed, but measured hardness was higher than in sample without re-melting. Similar results were observed for abrasion resistance.

Fast deflection is one of the great advantages of an electron beam, making it possible to process a relatively large area at once and also to modify the distribution of the supplied energy. The dissertation deals with surface heat treatment with a focus on the effect of beam deflection on the process. Furthermore, the influence of processing speed and defocusing of the electron beam is studied. The principles of electron beam technology and the summary of surface heat treatment of various materials are presented in the literary part of the thesis. The experimental part is focused on the influence of process parameters on processing in the solid phase, carried out on steel 42CrMo4, and in in the liquid phase, carried out on alloy NiCrBSi. Especially changes in structure, hardness, shape of the processed area and wear resistance are evaluated on processed samples. The results are also evaluated from an energy perspective. Furthermore, an optimum combination of technological parameters is recommended for solid and liquid phase surface heat treatment.