We propose an innovative process coupling powder bed additive manufacturing by Electron Beam Melting (EBM) with Chemical Vapor Deposition (CVD) and Atomic Layer Deposition (ALD) to develop 3D Ti-6Al-4V structures covered with AlN coating. Despite of the high reactivity of Ti-6Al-4V with nitrogen, thick (similar to 10 mu m) and conformal AlN films are deposited by CVD on Ti-6Al-4V substrates with high surface roughness. An AlN underlayer deposited by ALD is necessary to mitigate the reaction between Ti-6Al-4V and the nitrogen precursor NH3(g) and to limit the formation of brittle titanium nitride phases. We have thus achieved an adherent coating without any modification of the Ti-6Al-4V microstructure at the core of the substrate. We show that a 7 mu m thick AlN coating is efficient in protecting Ti-6Al-4V against cyclic oxidation at 650 degrees C for at least 650 h. This study opens new opportunities for the design of coated 3D Ti-6Al-4V structures for use in high temperature oxidizing environments.

The oxidation resistance of receivers is a critical point in the development of plants using solar tower technology. Intermetallics in MoSiAl system present oxidation maximum temperature and creep resistance superior to the ones of the currently used Ni-based alloys. We followed here the oxidation resistance of such intermetallic exposed to different treatments in air: long-term oxidation (up to 700 h) at 1373 K, and for several cycles of 20 min in solar furnace. The normal spectral emissivity is measured up to 1640 K in air, and our main positive conclusion is this property is not degraded by the oxidation.

A comprehensive model to predict the stress evolution in a multilayer coating during its use in solar receivers is proposed. The model takes into account residual stress in the coatings, thermal gradient in the structure and high temperature phenomena like oxide scale growth and creep relaxation. The numerical tool developed in this work can help to understand the complex interplay of these phenomena occurring in all the materials involved. Additionally, the present model can be used to assess high temperature data like creep when it is compared with an experimental case.

There is an increasing interest for tower concentrated solar power (CSP) systems which can work at temperatures higher than 1073 K to optimize the efficiency. One of the challenges is to design the receiver that will be heated at high temperatures in air. On the contrary to coatings in gas turbine engine, the coating/substrate system must have a high thermal conductivity to ensure a good heat transfer to the fluid. Aluminum nitride (AlN) coating, deposited by chemical vapor deposition at 1373 K at a growth rate of 10-50 mu m h(-1), is selected for its high thermal conductivity, low thermal expansion coefficient, high temperature stability and its ability to develop stable alumina scales above 1273 K. Cast and ODS (Oxide Dispersion Strengthened) FeCrAl alloys, also alumina formers, are chosen as model substrates to reduce the influencing parameters in real-life receivers and to study the potential of these coatings. Accelerated cyclic oxidation tests and emissivity measurements allow the evaluation of AlN coatings as materials for high temperature CSP receivers. The multilayered systems show low degradation after hundreds of thermal cycles at 1073 K in air and can support higher temperatures (1373 K) for 100 to 500 h depending on the coating thickness. Nevertheless the fast cyclic oxidations in solar furnace generated cracks through the coatings. The measurement of the optical properties also revealed a decrease of the absorptivity after oxidation.

The origin of threading dislocations (TDs) in nitride films is not completely understood but it is well established that they degrade the film properties. This work investigates the assumption that they arise from the interface between the film and sapphire substrate owing to small in-plane rotations between nitride domains. Bollmann’s formalism is first used to determine the characteristics of dislocations at the nitride film/sapphire interface that compensate both for the parametric misfit and a small in-plane rotation of the film as frequently observed. It is shown that the dislocation density and line direction depend on the rotation angle. When islands grow and coalesce in the nucleation layer, some interfacial dislocations orientate along [0001] in the boundaries between domains and transform to so-called TDs. The amount of TDs lying in the boundaries between nitride domains is calculated as a function of the rotation angle. Estimations of TD density in the nucleation layer are deduced for a range of domain sizes and compared with experimental values of the literature.

The extending market of concentrated solar power plants requires high-temperature materials for solar surface receivers that would ideally heat an air coolant beyond 1300 K. This work presents investigation on high-temperature alloys with ceramic coatings (AlN or SiC/AlN stacking) to combine the properties of the substrate (creep resistance, machinability) and coating (slow oxidation kinetics, high solar absorptivity). The first results showed that high-temperature oxidation resistance and optical properties of metallic alloys were improved by the different coatings. However, the fast thermal shocks led to high stress levels not compatible due to the differences in thermal expansion coefficients.

Direct liquid injection - metalorganic chemical vapor deposition (DLI-MOCVD) is the most advanced process dedicated to the internal protection of nuclear fuel cladding in accident conditions such as loss of coolant. It allows the deposition of an amorphous, glassy-like chromium carbide CrCx coating which is resistant against high-temperature oxidation in air and steam. Since the above-mentioned material characterizations demonstrated that coatings possessed the appropriate protection properties, the DLI-MOCVD process was scaled-up. First, a joint development between experimental and numerical studies led to a deposition inside a 1 m long cladding segment with a coating of sufficiently large and uniform thickness. Optimized reactor parameters consist in a combination of low temperature (similar to 600 K) and low pressure (similar to 600 Pa) with a high vapor flow rate of reactive species in the reactor ensuring a short residence time. The second phase of the scale-up consisted in coating simultaneously three, then sixteen segments in a single run. 3D computational simulations of the deposition process assisted the development of specific flanges designed to distribute homogeneously the reactive vapor into the three or sixteen cladding tubes. Experimental conditions have been extrapolated from one to three and to sixteen cladding segments, resulting in the deposition of the CrCx coating inside all segments with a relatively uniform partition. Overall, this paper demonstrates the feasibility of the deposition of CrCx coating in a bundle of several, up to sixteen, nuclear fuel cladding segments of 1 m in length (ID 8 mm), in order to protect them during accident conditions. This "batch demonstration" is a first step in the course of DLI-MOCVD technological transfer. Next step will be the deposition in a full-length cladding tube (4 m) that is already supported by numerical predictions.

Titanium nitride (TiN) films were grown by chemical vapor deposition (CVD) from titanium chlorides, ammonia (NH3) and hydrogen (H2) on single crystal c-plane sapphire, WC–Co, stainless steel and amorphous graphite substrates. The preferred orientation and color of TiN layer are studied by combining a simplified kinetic model with experiments.

The increased need for protecting cutting tools has led to the development of more and more efficient coatings. Ti-Al-N is one of the most studied systems in the hard coating industry due to the high hardness and good oxidation resistance of Ti1-xAlxN coatings. The development of LPCVD processes has led to the discovery of new microstructures and morphologies. In this study, we discuss the microstructural and morphological changes caused by varying the aluminum content in films deposited by low pressure thermal CVD in an industrial reactor at low carrier gas flow and relatively high pressure (> 4 kPa). Coatings were characterized using FE-SEM, XRD and TEM analysis, revealing the growth of nanolamellae with modulated Al and Ti contents for the lowest Al containing coatings. The coatings with the highest Al contents were also found to show particular cube-shaped grains with a micromodulation of composition. Experimental Al content values are higher than the calculated one and their evolutions with the AlCl3/(AlCl3 + TiCl4) molar ratio are similar. The hardness and oxidation resistance were characterized and compared with available data in literature. Higher hardness is obtained for coatings having an Al content up to x = 0.65 and a hardness drop is found for higher Al contents. The oxidation resistance of the coatings rises continuously with an increasing Al content.

A novel methodology combining CVD experiments, nanoscale characterisation and reaction–diffusion modelling demonstrates Ti1−xAlxN epitaxial growth on single crystalline AlN films.

For nuclear safety issues, there is an international effort to develop innovative “Enhanced Accident Tolerant Fuels” (EATF) materials. EATF cladding tubes are of particular interest because they constitute the first barrier against radioactive fission species dispersal in case of accidental scenario such as LOCA (LOss of Coolant Accident). Actual nuclear fuel claddings are made from Zr-based alloys and to increase safety margins, both mechanical strength and resistance to high-temperature oxidation have to be improved. Several alternatives using high-temperature oxidation resistant coatings for outer-wall protection have been proposed worldwide but there is currently no solution for the inner-wall protection. In order to resist to high temperature steam environment upon LOCA transients, internal Cr-based coatings deposited by DLI-MOCVD (Direct Liquid Injection of MetalOrganic precursors) were investigated. These hard metallurgical coatings could also be used in high-temperature corrosive environments as those encountered in aeronautics and other industries to protect 3D complex components. Thanks to a suitable chemistry of the liquid Cr precursor, bis(ethylbenzene)chromium, different coatings were deposited including: metal Cr, chromium carbides CrxCy and mixed carbides CrxSizCy. The high-temperature behavior of these Cr-based coatings under oxidizing atmospheres has been studied using several techniques and various oxidation tests including pure steam environment followed by water quenching down to room temperature to be representative of LOCA situations. Amorphous CrxCy coatings showed the most promising properties. For instance compared to uncoated substrate, they shift the catastrophic oxidation towards higher temperatures and delay the complete oxidation of the substrate at 1473K of >2h. The results are discussed in terms of oxidation mechanisms and protection of the fuel claddings inner surface deduced from fine characterizations of the samples before and after oxidation tests.

Internally protecting a full-length nuclear fuel cladding is still a challenge. Amorphous chromium carbide coatings deposited with DLI-MOCVD were previously investigated for this purpose. This paper explores several ways to optimize the DLI-MOCVD process in order to achieve a uniform thickness profile of the coating in 1 m long cladding segments. Simulated growth rate profiles are compared with experimental ones. It appears that except at low temperature and pressure, the model fits very well. It is finally possible to smoothen the profiles by decreasing the pressure and the temperature, increasing the total gaseous flow rate, applying a temperature gradient along the cladding or by considering a two-step process. While experimental works are currently focusing on the treatment of a bundle of 16 cladding segments of 1 m long, the conclusive simulation in a full-length cladding is comforting us to keep upscaling the process.

Plasma enhanced atomic layer deposition (PE-ALD) of aluminum nitride (AlN) thin films often utilizes NH3 or a mixture of N-2 and H-2 as a plasma source. However, the possibility of separating the activation step from the nitridation step by using H-2 alone as the plasma source has never been explored. In this paper, we study the deposition of MN by PE-ALD by using trimethylaluminum, H-2 plasma and NH3 for deposition temperatures below 400 degrees C. The self-limiting ALD growth was achieved between 325 degrees C and 350 degrees C. As a comparison, AIN was also deposited by thermal ALD (T-ALD), where surface reactions between TMA and NH3 occurred with reasonable growth rates only at temperatures above 400 degrees C. The PE-ALD films showed low oxygen (1.5 at.%) and carbon contaminations (1 at.%). The T-ALD films contained carbon (5 at.%) mainly attributed to the presence of C-Al bonds that was insignificant in PE-ALD films. The flow rate of H-2 used in H-2 plasma was found to have a significant impact on the preferred orientation of MN films, where higher H-2 flow rate promoted the (002) preferred orientation. Besides, the electrical resistivities were probed to be 1085 cm, as expected in an insulating material. As an example, AlN was used to infiltrate porous sintered silicon carbide (SiC). Both AIN deposited by PE-ALD and by T-ALD operating with exposure mode deposited at 400 degrees C were attempted. Even though, there is a greater risk for TMA precursor to decompose at 400 degrees C, infiltration of MN was more successful by T-ALD operating with exposure mode.

Chromium carbide layers were deposited using liquid-injection metal-organic chemical vapor deposition inside long (0.3 to 1 m) and narrow (8 to 24 mm in diameter) metallic tubes. The deposition was carried out using a molecular single-source, bis(benzene)chromium (BBC), as representative of the bis(arene)metal family diluted in toluene and injected with N2 as carrier gas. A multicomponent mass transport model for the simulation of the coupled fluid flow, heat transfer and chemistry was built. The kinetic mechanism of the growth of CrCx films was developed with the help of large-scale experiments to study the depletion of the precursors along the inner wall of the tube. The model fits well in the 400–550 °C temperature range and in the 1.3 × 102 to 7 × 103 Pa pressure range. The pressure is shown to have a pronounced effect on the deposition rate and thickness uniformity of the resulting coating. Below 525 °C the structure, composition and morphology of the films are not affected by changes of total pressure or deposition temperature. The coatings are amorphous and their Cr:C ratio is about 2:1, i.e., intermediate between Cr7C3 and Cr3C2. The model was applied to the design of a long reactor (1 m), with a double injection successively and alternatively undertaken at each end to ensure the best uniformity with sufficient thickness. This innovative concept can be used to optimize industrial deposition processes inside long and narrow tubes and channels.

Cr deposits are widely used as protective coatings but multifunctional performances are required in harsh environments motivating research on new processes. MOCVD of Cr metal coatings was carried out by direct liquid injection (DLI) of a unique solution containing bis(ethylbenzene) chromium as metal source and thiophenol as inhibitor of carbide formation. A low amount (<6%) of the metastable delta-Cr phase was found embedded in the stable alpha-Cr phase. The formation of this metastable phase originates from both the low deposition temperature (<723 K) and the use of thiophenol. It was not reported under other CVD conditions. Dense coatings were deposited by implementing a multilayer growth mode. Such coatings exhibit a high nanohardness of about 17 GPa. The delta-Cr metastable phase undergoes an irreversible structural transformation to bcc-Cr above 723 K. The mechanical properties of coatings are not affected by the structural transformation because of the similarity of their crystallographic structures (both cubic), their density very close (a volume contraction of only 0.4% during the transformation) and its low content. This metastable phase is a signature of the DLI-MOCVD process and it can be used as a tracer for Cr coatings operating in high temperature environment without loss of the basic properties. (C) 2017 Elsevier B.V. All rights reserved.

Niobium nitride (NbN) thin films are extensively used in superconducting devices such as single-photon detectors, hot electron bolometers, microwave resonators and kinetic inductance detectors. The operation of these devices is strongly influenced by the quality of the films, especially by their resistivity and superconducting transition temperatures (T c ). NbN films have rather high T c of ~16.5 K and high resistivity of few hundreds micro-Ohm cm, which is perfect for operation of many superconducting devices. However, at low temperatures films are vulnerable to thermomagnetic instabilities in form of dendritic avalanches promoted by high resistivity in the normal state. Recently, new production route for NbN films has been established using high-temperature chemical vapor deposition (HTCVD). Transport measurements show low resistivity in normal state and suggest low level of lattice disorder. The highest for NbN T c of 17.06 K was also reported in the films grown by HTCVD. According to previous study, these films should be thermo-magnetically stable. This work clarifies if it is the case and searches in one of them for dendritic flux avalanches. The nanoscale origin of avalanches is discussed.

We modeled and simulated the surface reaction of silicon precursor on different surfaces by thermodynamic analysis and density functional theory calculation. We considered SiH2Cl2 and argon as the silicon precursor and the carrier gas without etchant gas. First, the equilibrium composition of both gaseous and solid species was analyzed as a function of process temperature. SiCl4 is the dominant gaseous species at below 750°C, and SiCl2 and HCl are dominant at higher temperatures, and the yield of silicon decreases with increasing temperature over 700°C due to the etching of silicon by HCl. The yield of silicon for SiO2 substrate is lower than that for silicon substrate, especially at 1000°C or higher. Zero deposition yield and the etching of SiO2 substrate at higher temperatures leads to selective growth on silicon substrate. Next, the adsorption and the reaction of silicon precursor was simulated on H-terminated silicon (100) substrate and on OH-terminated β-cristobalite substrate. The adsorption and reaction of a SiH2Cl2 molecule are spontaneous for both Si and SiO2 substrates. However, the energy barrier for reaction is very small (6×10−4eV) for Si substrate, whereas the energy barrier is high (0.33eV) for SiO2 substrate. This makes the differences in growth rate, which also supports the experimental results in literature.