Plasma-sprayed TiC Coatings Research Articles

Acoustic emission study of failure mechanisms in TiC thermal barrier coatings

The failure mechanisms of thick plasma-sprayed TiC coatings subjected to severe thermal shocks were investigated using acoustic emission (AE), detected during and after the thermal shocks. The AE signal was triggered at the onset of the thermal shock and was completely captured for time durations up to 3 s. Signals were then studied and quantified directly by integrating the square of the waveform signals ( i.e. calculating the electrical energy). This method was compared with the more classical ringdown counting and with the maximum-amplitude method. The electrical energy demonstrated the best results that could be correlated to physical events. Besides the coating segmentation, two modes of delamination were observed: a sudden delamination during heating due to the bending of the coating under compressive stresses and a progressive delamination during cooling after plastic deformation had taken place at a high temperature during the thermal shocks.

Enhanced resistance of plasma-sprayed TiC coatings to thermal shocks

In previous tests the maximum observed TiC coating thickness which showed good resistance to 0.7 s thermal shocks was 400 μm. For this thickness, only microcracks perpendicular to the surface (segmentation) were observed, whereas for thicker ones, spalling inside the coatings occurred. With a combination of substrate surface modification (macroroughening or spraying a bondcoat) and preheating to 375°C before the thermal shocks, it has been possible to completely avoid the delamination during heating and to promote the resistance to delamination on cooling. This allows a doubling of the thickness of thermal shock resistant coatings up to 1mm. Hemispherical coated limiters were tested in TdeV (Tokamak de Varennes) with plasma currents of 210 kA and have absorbed 20 kW during 1.2 s associated with a power deposition factor of about 10 MW m −2 s 0.5 .

Enhanced resistance of plasma-sprayed TiC coatings to thermal shocks

Enhanced resistance of plasma-sprayed TiC coatings to thermal shocks

Study of surface preparation for enhanced resistance to thermal shocks of plasma-sprayed TiC coatings

Plasma-sprayed TiC coatings are of interest for the protection of plasma limiters in fusion devices. In previous tests the maximum observed thickness which showed good resistance to thermal shocks (400 μm) is unfortunately not enough for long-term tokamak operation as erosion takes place. Consequently, in order to ensure the good behaviour of thicker coatings and to understand the failure mechanisms, a study has been made of the influence of different surface preparations of the metallic substrate (grit blasting, macroroughening or spraying a bond coat) combined with modified thermal shock parameters (shock duration and pre-heating of the material). The thermal shock resistance of the resulting samples was studied in an electron beam gun device where substrate and surface temperatures and video images have been monitored. Three failure modes were observed: delamination in the coating during the heating phase of the thermal shocks and segmentation or edge delamination during cooling. Coating bending and plastic deformation added to residual stresses and shear at the coating-substrate interface explain the observed failures. Hence, with a combination of surface modification and pre-heating, it has been possible to double the thickness of thermal shock-resistant coatings to up to 900 μm.

High emissivity TiC coatings for a first wall

Part of the First Wall of the conceptual design of the Next European Torus NET consists of radiation cooled carbon tiles. The tile temperature is determined by the optical properties of the facing surfaces. The heat transfer to the 316 stainless steel structure can be improved by applying a high emissivity coating. For this purpose ceramic coatings can be applied. This paper deals with the development and characterization of atmospheric and vacuum plasma sprayed titanium carbide as high emissivity coatings. Microstructural evaluation of these coatings includes X-ray diffraction and light microscopy of cross-sections. The total emissivities of vacuum and atmospheric plasma sprayed TiC coatings were measured at 525 K. Reflection measurements were performed using a YAG laser with a wavelength of 1.06 μm at room temperature. The effects of compositional differences on optical properties are discussed.

Hydrogen ion interaction behaviour of plasma-sprayed TiC coatings

Abstract The high melting point, low atomic number, and resistance to thermal fatigue and erosion are desirable attributes of TiC coatings for fusion reactor containment vessels and components. However, such coatings are subjected to hydrogen ion bombardment from circulating plasma. This can result in the formation and adsorption of various molecular complexes. Conventional characterization techniques involving low energy electron beams cause desorption of these complexes. The unenhanced surface Raman scattering technique was used to investigate the possibility of the formation of C-H and Ti-H complexes on the plasma-sprayed TiC surface.

Surface Raman characterization of plasma-sprayed TiC coatings from the tokamak fusion reactor at Varennes, Quebec

TiC coatings deposited by a plasma spraying process were placed on the internal wall of the tokamak fusion rector at Varennes, Quebec and were subjected to repeated ~1500 hydrogen plasma discharges. X-ray photoelectron spectroscopy and unenhanced surface Raman scattering were used to investigate the possibility of the formation of C–H and Ti–H complexes on the TiC plasma sprayed surfaces.

Plasma-sprayed TiC protective coatings for fusion devices: Coating fabrication criteria

Plasma-sprayed TiC coatings are of interest as protective coatings for surfaces exposed to high energy fluxes in fusion devices. Thick, dense coatings are required primarily to withstand erosion and sputtering for long times whereas less dense coatings seem preferable for preserving a high thermal shock resistance. This study was carried out to outline the fabrication criteria needed to obtain thick TiC coatings which can withstand severe thermal shocks.TiC coatings with different microstructures (porosities) and thicknesses, plasma-sprayed under atmospheric and inert atmosphere conditions, were exposed to pulsed electron beam thermal shocks. Different microstructures were obtained after spraying TiC powders of various particle size distribution and morphology. The porosity of sprayed coatings ranges from 17% to 29%.The results obtained after thermal exposure indicate that the thermal shock resistance decreases as the TiC coating porosity increases. They also indicate that coatings obtained by spraying a narrow band powder into an inert gas enclosure contain some porosity that could help them to withstand spallation.

Behaviour of plasma-sprayed TiC coatings under H and He irradiation

The effects of H and He irradiation on plasma-sprayed TiC coatings have been studied. The effects were studied by SEM and TEM microscopy, and by H depth-profiling with the ERD technique. Samples prepared by CVD were also implanted for comparison. In the case of H, bubbles have been observed during an in-situ implantation inside the TEM. In order to understand the absence of hydride, H depth-profiling was performed. The H saturation concentration is about 20 at.% or slightly more (the uncertainty is due to the effect of the porosity of the coatings on the ERD technique). Helium bubbles are observed above 10 15 He/cm 2. Blisters are formed on polished samples above 10 17 He/cm 2. In contrast, the high roughness of non-polished samples prevents blister formation. CVD samples behave essentially like the polished samples.

Transmission electron microscopy characterization of plasma sprayed TiC coatings

In plasma spraying of high melting point materials like TiC, complete fusion is difficult to obtain and also not always desirable. This paper reports the transmission electron microscopy (TEM) study of the microstructure relation between the melted and the unmelted material in the coating. TEM samples were prepared by ion beam milling of mechanically ground coatings. We found that, under our spraying conditions from which sound coatings were obtained, the original grain structure of the powder was partially retained in the deposited coatings. The average particle size of the powder was ∼40 μm and scanning electron microscopy (SEM) observations indicated that these particles had a grain size of ∼10 μm. TEM study of the coatings showed two families of grain sizes. The first one is similar to that of the starting powder, i.e., 30 μm aggregates of ∼10 μm grains. This indicates that the core of many of the particles did not melt during the plasma spraying process. The second family consists of regions of ∼0.2 μm grains surrounding the 30–40 μm aggregates. This region has crystallized from the melted layer around the particles or from melted small particles. In addition, the plasma spraying process caused an important lowering of C/Ti. This was found by crystal lattice measurements using x-ray diffraction.