Transition Metal Aluminide Coatings and Initial Steps on Additive Manufacturing

During the last decades, Fe-, Ni- and Ti-based aluminides have been studied in terms of bulk materials with an effort to develop alloying and processing strategies to overcome their low ductilities and toughness compared to conventional alloys. Whenever significant improvements can be addressed in this direction, they will be opened to an extensive range of industrial applications, especially those related to high temperature resistance requirements. In parallel, progressing interest has also been focused on their application as protective layers. This chapter is intended to provide a review of the evolution that has been made mainly during the last two decades of the several coating technologies devoted for this purpose. From thick to thin coatings are revised, with insight into coating microstructures and properties as well. Lack of space has forced the selection of those technologies arising most interest within last years; therefore, the content will follow this order: joining (laser cladding and electrospark deposition), thermal spraying (high velocity oxygen fuel and cold spraying) and physical vapor deposition (magnetron sputtering and cathodic arc deposition)

Estudi del tractament de nitruració iònica per aplicació en vàlvules de bola

Aquest projecte representa l’estudi per a la millora de les propietats en servei d’unes vàlvules de bola d’acer carbonitrurat que comercialitza una empresa de vàlvules ben acreditada a nivell internacional. Davant doncs, de l’interès sorgit per dita empresa d’augmentar les prestacions en quant a desgast i corrosió, s’ha avaluat la viabilitat de substituir el procediment emprat per ells de nitruració en banys de sals (procediment TENIFER) per un tractament superficial de nitruració iònica. Una altra empresa catalana acreditada en els tractament de superficies, ha estat l’encarregada de dur a terme aquest tipus de nitruració, provant inclús de modificar diferents paràmetres del procés. Finalment, ha estat la meva responsabilitat, des del departament de Ciència dels Materials de la Universitat de Barcelona, de realitzar tota la caracterització tant a nivell estructural com de propietats de les peces tractades. Tal caracterització s’ha realitzat primer a nivell d’una preparació metal·logràfica per tal de veure les possibles diferències entre les diferents capes; en aquesta etapa s’ha emprat tant microscòpia òptica com electrònica. Un primer anàlisis de les mostres rebudes també inclogué l’estudi de la rugositat superficial i adherència de la capa. En un segon nivell, s’han avaluat propietats com la microduresa Vickers, la resistència al desgast per lliscament i la resistència al desgast abrasiu, així com la resistència a corrosió a través d’assajos de laboratori electroquímics i assajos industrials en cambres de boira salina. A petició de l’empresa, també es varen realitzar en algunes mostres, estudis de resistència a temperatura. El què s’ha observat és que efectivament hi ha una millora substancial, tant en desgast com en corrosió. De fet, els primers clients que se’ls ha proveït amb vàlvules tractades per nitruració iònica han vist un augment en el temps efectiu de les peces. Per tant, podem concloure que l’empresa ha adaptat satisfactòriament el nou procediment donant una millor resposta als seus clients. Cal dir però, que no queda la porta tancada a una investigació contínua, pensant inclús en altres tipus de tractaments superficials

Estudi del tractament de nitruració iònica per aplicació en vàlvules de bola

Aquest projecte representa l’estudi per a la millora de les propietats en servei d’unes vàlvules de bola d’acer carbonitrurat que comercialitza una empresa de vàlvules ben acreditada a nivell internacional. Davant doncs, de l’interès sorgit per dita empresa d’augmentar les prestacions en quant a desgast i corrosió, s’ha avaluat la viabilitat de substituir el procediment emprat per ells de nitruració en banys de sals (procediment TENIFER) per un tractament superficial de nitruració iònica. Una altra empresa catalana acreditada en els tractament de superficies, ha estat l’encarregada de dur a terme aquest tipus de nitruració, provant inclús de modificar diferents paràmetres del procés. Finalment, ha estat la meva responsabilitat, des del departament de Ciència dels Materials de la Universitat de Barcelona, de realitzar tota la caracterització tant a nivell estructural com de propietats de les peces tractades. Tal caracterització s’ha realitzat primer a nivell d’una preparació metal·logràfica per tal de veure les possibles diferències entre les diferents capes; en aquesta etapa s’ha emprat tant microscòpia òptica com electrònica. Un primer anàlisis de les mostres rebudes també inclogué l’estudi de la rugositat superficial i adherència de la capa. En un segon nivell, s’han avaluat propietats com la microduresa Vickers, la resistència al desgast per lliscament i la resistència al desgast abrasiu, així com la resistència a corrosió a través d’assajos de laboratori electroquímics i assajos industrials en cambres de boira salina. A petició de l’empresa, també es varen realitzar en algunes mostres, estudis de resistència a temperatura. El què s’ha observat és que efectivament hi ha una millora substancial, tant en desgast com en corrosió. De fet, els primers clients que se’ls ha proveït amb vàlvules tractades per nitruració iònica han vist un augment en el temps efectiu de les peces. Per tant, podem concloure que l’empresa ha adaptat satisfactòriament el nou procediment donant una millor resposta als seus clients. Cal dir però, que no queda la porta tancada a una investigació contínua, pensant inclús en altres tipus de tractaments superficials

NiTi splat features during Vacuum Thermal Spraying onto several substrates

Vacuum Plasma Spraying (VPS) has been used to produce coatings onto steel substrates. This work deals with the study of splat morphology of the NiTi alloy sprayed by VPS onto different substrates (aluminium, copper, stainless steel, glass and alumina). All the previous characteristics are discussed in terms of wettability and thermal conductivities regarding the rapid cooling involved in the process. Although identical conditions were used during thermal spraying, a wide variety of splat formations were observed; commonly, slushy or splash/disc splats are formed depending on whether the particles have partially or fully melted. The thermal effusivity of the substrate material, which is a measure of its ability to exchange thermal energy with its surroundings, seems to play an important role promoting more or less spreading. The higher the thermal effusivity is, the more rapidly the splats are cooled, thus starting the solidification before they come to rest and, changing their morphology

FeAl and NbAl3 intermetallic- HVOF coatings: structure and properties

Transition metal aluminides in their coating form are currently being explored in terms of resistance to oxidation and mechanical behavior. This interest in transition metal aluminides is mainly due to the fact that their high Al content makes them attractive for high-temperature applications. This is also a reason to study their resistance to wear; they may be suitable for use in applications that produce a lot of wear in aggressive environments, thus replacing established coating materials. In this study, the microstructure, microhardness, and wear and oxidation performance of FeAl and NbAl3 coatings produced by highvelocity oxy-fuel spraying are evaluated with two main aims: (i) to compare these two coating systems¿a commonly studied aluminide (FeAl) and, NbAl3, an aluminide whose deposition by thermal spraying has not been attempted to date¿and (ii) to analyze the relationship between their microstructure, composition and properties, and so clarify their wear and oxidation mechanisms. In the present study, the higher hardness of niobium aluminide coatings did not correlate with a higher wear resistance and, finally, although pesting phenomena (disintegration in oxidizing environments) were already known of in bulk niobium aluminides, here their behavior in the coating form is examined. It was shown that such accelerated oxidation was inevitable with respect to the better resistance of FeAl, but further improvements are foreseen by addition of alloying elements in that alloy

Feasibility of using low pressure cold gas spray for the spraying of thick ceramic hydroxyapatite coatings

This article deals with the production of thick ceramic hydroxyapatite coatings obtained by Low Pressure Cold Gas Spray (LPCGS) system. Several factors such as powder microstructure, surface roughness and cold gas spray system are here discussed in the build‐up process. The use of nanocrystalline powder composed by fine agglomerates and needle‐like shape microstructure allows the realignment and compaction of individual crystallites to form thick deposits. In addition, the activation of the substrate surface results convenient for the first impinging particles anchored properly. Then, layer by layer, particles can remain attached leading to coating build‐up. Additionally, the use of low shock pressure as well as constant feeding system provided by LPCGS system lead to homogeneous coatings in comparison with High Pressure Cold Gas Spray (HPCGS) system. The successful coating build‐up has been achieved not only by the use of an agglomerated feedstock powder, but also by previous surface treatment and the use of the low pressure system. The obtaining of HA components by LPCGS is promising within biomedical field. An improvement of component strength is also suggested by means of thermomechanical analysis of the powder. The performance of a post heat‐treatment leads to an increase in HA strength, as well as crystal size

The effect of hot treatment on composition and microstructure of HVOF iron aluminide coatings in Na2SO4 molten salts

The paper deals with the hot corrosion performance of FeAl base intermetallic HVOF coatings in molten Na2SO4 at 850 C in an isothermal process over the span of 45 h under static conditions. The test was validated with electron microscopy and compositional analyses in the cross-section area, as well as x-ray diffraction techniques. All the coatings were characterized by Al-depleted regions, intersplat oxidation and different stoichiometric ratios of iron aluminides. The results were discussed in relation to the formation of oxide scales on the surface after exposition to corrosive media, as well as heterogeneity and defects of the sprayed coatings. The Fe40Al (at.%) powder showed quite uniform phase distribution after spraying and preserved its integrity after corrosion test; the FeCr25% ? FeAl-TiAl-Al2O3 (wt.%) and Fe46Al-6.55Si (at.%) powders exhibited interface oxidation, with localized corrosion attacks proceeding through particle boundaries and microcrack networks with no evidence of Na and S penetration. FexAly alloys are susceptible to accelerated damage and decohesion of the coating, whereas the formation of sulfides is observed at certain points

Influence of the substrate on the formation of metallic glass coatings by cold gas spraying

Cold gas spray technology has been used to build up coatings of Fe-base metallic glass onto different metallic substrates. In this work, the effect of the substrate properties on the viscoplastic response of metallic glass particles during their impact has been studied. Thick coatings with high deposition efficiencies have been built-up in conditions of homogeneous flow on substrates such as Mild Steel AISI 1040, Stainless Steel 316L, Inconel 625, Aluminum 7075-T6, and Copper (99.9%). Properties of the substrate have been identified to play an important role in the viscoplastic response of the metallic glass particles at impact. Depending on the process gas conditions, the impact morphologies show not only inhomogeneous deformation but also homogeneous plastic flow despite the high strain rates, 10 8 to 10(9) s(-1), involved in the technique. Interestingly, homogenous deformation of metallic glass particles is promoted depending on the hardness and the thermal diffusivity of the substrate and it is not exclusively a function of the kinetic energy and the temperature of the particle at impact. Coating formation is discussed in terms of fundamentals of dynamics of undercooled liquids, viscoplastic flow mechanisms of metallic glasses, and substrate properties. The findings presented in this work have been used to build up a detailed scheme of the deposition mechanism of metallic glass coatings by the cold gas spraying technology

Influence of Cold Gas Spray process conditions on the microstructure of Fe-based amorphous coatings

Fe-based amorphous metallic coatings were prepared by Cold Gas Spray process. Through this study, the effects of the process conditions such as spraying distance, gas pressure and temperature on the microstructure of as-sprayed coatings are evaluated. Microstructural studies show that the coatings can present a densely layered structure with porosity below 0.5% and thickness around 800 μm depending on the process conditions. Precipitation of nanocrystals in as-sprayed coatings is observed and present results show its dependence on the thermal and kinetic energy implicated in the process. In general, when gas temperature and pressure decreased, in the studied range, coatings displayed a dense and amorphous structure

Ordering kinetics evaluation of FeAl powders

In this study, time resolved X-ray diffraction experiments using synchrotron X-ray radiation have been performed to get insight on the time and temperature dependent atomic ordering of an intermetallic Fe-40Al (at.%) ball-milled powder. The target of the present study is to gain knowledge on the rapid heating processes occurring during Thermal Spray coating processes. Present results show that in the temperature range 400 °C - 550 °C, the evolution of the order can be followed and modelled by fitting the powder diffraction patterns collected within the first minutes after fast heating. Reasonable refinements have been obtained by assuming the presence of two domains corresponding to the ordered and disordered lattices. The lattice constant changes from 0.29165 nm in the ball-milled powder at room temperature to 0.29281 nm in the ordered phase after 3000 s at 550 °C. The growth of the ordered phase is proposed to be a vacancy-related process with an activation energy of 1.04 eV. Above 550 °C, the ordering kinetics appears too fast to be resolved using the few seconds time scale of the present experiments which is in agreement to thermal spray results conditions.Peer ReviewedPostprint (author's final draft