Alain Billard

The development of protective coatings with simultaneously enhanced mechanical, tribological, and antioxidant properties remains a major challenge for micro-electro-mechanical systems operating under harsh environments. In this work, HfC_x/a-C:H coatings with varying carbon contents were deposited by magnetron sputtering. Increasing the C₂H₂ flow rate from 12 to 20 sccm drove the coating structure to undergo two-stage evolution, from a composite structure dominated by HfC nanograins with a-C:H distributed at triple junctions of HfC_x grain boundaries to a typical nanocomposite structure with ~8 nm HfC_x nanograins embedded in a continuous a-C:H matrix. The coating deposited at 18 sccm exhibited the highest hardness (31.3 GPa) and effective Young’s modulus (392.3 GPa), owing to enhanced interface-mediated strengthening effect induced by the optimized nanocomposite structure. The coating prepared at 20 sccm showed the lowest friction coefficient (0.28), the lowest wear rate (6.82 × 10⁻⁶ mm³/N·m), and the best oxidation resistance. These improvements were supported by the enhanced mechanical properties and a-C:H fraction, the increased interface density and tortuosity, and the regulation of oxidation kinetics by the a-C:H matrix. This work provides an effective strategy for designing multi-functional protective coatings with balanced mechanical, tribological, and oxidation performance.

This work focuses on the evolution of the morphology and structure of La₂NiO₄ (namely, LNO) coatings deposited by reactive magnetron sputtering (RMS) with subsequent annealing processes. The LNO coatings start to crystallize at 600 °C, and the LNO with K₂NiF₄ structure was formed at 700 °C. A small amount of La₃Ni₂O₇ appeared in the La₂NiO₄ coatings at 1100 °C. Interestingly, the LNO coatings realize the transformation from dense to different porous morphologies due to the annealing process. The LNO coating with abundant pores was formed after annealing treatment at 1000 °C for 2 h. This porous morphology can be stably maintained after short-term thermal stability experiments at 750 °C for 120 h. The electrochemical impedance spectroscopy (EIS) measurement of the LNO/YSZ/LNO symmetrical half-cells shows that the LNO cathode coating after annealing at 1000 °C for 2 h exhibits lower polarization resistance (R_pol) and activation energy.

Diamond grinding wheels (DGWs) have a central role in cutting-edge industries such as aeronautics or defense and spatial applications. Characterizations of DGWs are essential to optimize the design and machining performance of such cutting tools. Thus, the critical issue of DGW characterization lies in the detection of diamond grits. However, the traditional diamond detection methods rely on manual operations on DGW images. These methods are time-consuming, error-prone and inaccurate. In addition, the manual detection of diamond grits remains challenging even for a subject expert. To overcome these shortcomings, we introduce a deep learning approach for automatic diamond grit segmentation. Due to our small dataset of 153 images, the proposed approach leverages transfer learning techniques with pre-trained ResNet34 as an encoder of U-Net CNN architecture. Moreover, with more than 8600 hyperparameter combinations in our model, manually finding the best configuration is impossible. That is why we use a Bayesian optimization algorithm using Hyperband early stopping mechanisms to automatically explore the search space and find the best hyperparameter values. Moreover, considering our small dataset, we obtain overall satisfactory performance with over 53% IoU and 69% F1-score. Finally, this work provides a first step toward diamond grinding wheel characterization by using a data-driven approach for automatic semantic segmentation of diamond grits.

In this work, the compound of Bi_1.5Y_0.5O₃ was deposited from two metallic targets, respectively, Bi and Y; for a power fixed on the target of Y, the power on the target of Bi was adjusted in order to obtain the desired composition. The effects of atomic ratio of Bi to Y and annealing temperature on the film morphology and crystal structure were investigated. The X-ray diffraction (XRD) results showed different crystal structure as a function of the Y content in the film after annealing at 500 °C for 2 h. The Bi_1.5Y_0.5O₃ was obtained with the atomic ratio of Bi to Y adjusted to 3.1. An impurity phase (Bi_1.55Y_0.45O₃, Rhombohedral) appears in the cubic Bi_1.5Y_0.5O₃ after annealing at 600 °C. The field-emission scanning electron microscopy (FESEM) result showed that the Bi_1.5Y_0.5O₃ film after annealing at 800 °C for 2 h is denser than the as-deposited film, despite the presence of some holes. The ionic transport properties of Bi_1.5Y_0.5O₃ film was measured by electrochemical impedance spectroscopy (EIS), and the conductance activation energy was obtained on this basis. The synthesized Bi_1.5Y_0.5O₃ film with higher ionic conductivity (0.13 S/cm at 650 °C) is suitable for SOFC electrolyte.

Lanthanum manganite (LMO) thin films were deposited by co-sputtering La and Mn targets in an Ar and O₂ gas mixture. The films were synthesized on silicon and fused silica substrates. The influences of thermal annealing on the structure, optical and electrical properties of LMO films were investigated. The results exhibited a correlation between these properties. In the amorphous state, an increase in annealing temperature improved the optical transmission and decreased the electrical capacitance. The beginning of crystallization at 600 °C was manifested by a strong increase in the capacitance and a decrease in the optical transmission. At higher annealing temperature, polycrystalline films were obtained with different optical and electrical characteristics. On the other hand, the annealed LMO films showed a photocurrent effect during exposure to a weak LED light.

The LFPECVD (Low-Frequency Plasma-Enhanced Chemical Vapor Deposition) technique is now used on an industrial scale for the deposition of carbon-based coatings for several applications. This short review recalled the main principles of LFPECVD and provided examples of DLC-based films. The main differences between low-frequency (LF) and radio-frequency (RF) discharges were also recalled here and examples of deposition and characterization of carbon-based films were proposed. The influence of the bias voltage or the temperature of the active electrode on the deposition rate and the structure of a-C: H films obtained in cyclohexane/hydrogen mixtures was first discussed. Next, the properties of carbon-based films doped with silicon were described and, finally, it was shown that multilayer architectures make it possible to reduce the stresses without altering their tribological properties.

Gadolinia doped ceria coatings were elaborated by cathodic arc evaporation from a metallic Ce–Gd (90–10 at.%) target inserted into a conventional multiarc Ti evaporation target in the presence of a reactive argon–oxygen gas mixture. The structural and chemical features of these films were determined by x-ray diffraction and scanning electron microscopy. Their electrical properties were characterized using impedance spectroscopy measurements. It was shown that the as-deposited coatings crystallize in the fluorite type fcc structure of ceria and that their composition is the same as that of the target. The morphology of the coatings is influenced by the evaporation parameter (stress and droplet). The electrical measurements showed two contributions in Nyquist representation and the activation energy was slightly higher than that given in the literature data for the bulk material.

The physical vapour deposition processes, and in particular different variants of sputtering, have reached a maturity making possible industrial applications in numerous fields such as mechanics

Gadolinia-doped ceria (GDC) buffer layers were synthesized by reactive magnetron sputtering under different total pressures and different thickness. All as-deposited and after an annealing treatment during two hours under air at 1000 °C coating presents a face centered cubic (f.c.c) structure of ceria with dense and adhesive morphology. The cell synthesized under 0.1 Pa and 0.57 µm present the best performances. (open-circuit voltage (OCV): 1.133 eV and power density: 1650 mW·cm⁻² @ 800 mA·cm⁻² at 790 °C).