This paper describes a new spraying process that uses a special torch to inject aluminum oxide suspensions into a dc plasma jet. Based on the results obtained, a general mechanism of action is proposed in which factors such as acceleration, impact behavior, and heat transfer phenomena play a role. Paper includes a German-language abstract.

For many years, a new interest in nanomaterials, with grain sizes smaller than 100nm, has emerged. This is due to the enhanced properties of the resulting sintered materials or coatings compared to those with coarser-grained materials. This paper is devoted to the feasibility to produce nanomaterial coatings by a dc plasma spray process. Until now, only thick coatings (> 100µm) have been elaborated using this technique, by injecting, with a carrier gas, micrometric particles in the plasma flow. But, it is not possible to inject too small particles (<5µm) without perturbing drastically the plasma jet by the high carrier gas flow rate necessary to give them a high enough momentum. This work presents a new dc plasma spray process, designed to elaborate alumina nanocoatings. The most important step of the process is the control of the ceramic nanometric particle penetration in the plasma. Because of their small size, a liquid, which density made the momentum transfer more efficient, replaced the carrier gas with an injector creating calibrated droplets with controlled velocity and flow rate. To study the liquid-plasma interaction, the penetration of pure water in an Ar/H 2 plasma jet was investigated by means of emission spectroscopy. The modification of temperature field together with oxygen concentration was determined quantitatively. Emission spectra were treated with a new localization method, avoiding the use of Abel's inversion implying a cylindrical symmetry, destroyed by the liquid injection. Such measurements allowed optimizing the liquid penetration in the plasma jet. Alumina nanopowders were dispersed in a liquid to form a stable suspension, which was injected in the plasma. The layered particle morphology, collected on glass substrates at different distances downstream of the injection point, was then studied.

The aim of this study is to produce free-standing functionally-graded structures in which density varies continuously from 2.2 to 17.3 g/cm3 through a total thickness of 4.5 mm. In order to optimize material performance, it is necessary to account for the different combinations or ratios of materials (i.e., tungsten and aluminum) in the plasma jet when determining mixture laws. A relationship based on the deposition efficiency, powder feed rate, and density of individually sprayed materials has been established and was used to predict the density, thickness, and deposition efficiency of the combined materials. The mixture laws were found to be in good agreement with experimental results, making it possible to build up coatings with a parabolic density profile and uniform layer thickness.

Polymeric substrates must be effectively cooled during plasma spraying to limit detrimental effects due to heat flow from the plasma jet. Even with the use of auxiliary cooling systems, however, some polymer substrates can undergo superficial modifications caused by particle heating at impact. This paper deals with chemical modifications in PET as a function of surrounding atmospheres during plasma processing. It also explains how aluminum coating adhesion was determined using tensile tests.

Thermal spraying induces stresses, which strongly influence thermomechanical properties of the deposits. To study both generation and influence of these stresses, various techniques could be used separately and/or concurrently. "In-situ" curvature, neutron diffraction and incremental hole drilling methods are often presented as complementary techniques. In this study, partially stabilized zirconia coatings, performed onto steel substrates at various spraying temperatures, have allowed to compare these three different methods.

The thermomechanical properties of plasma-sprayed deposits strongly depend on residual stress distribution. This latter is mainly attributed to the relative torch/substrate velocity as well as to the cooling system location and efficiency. The determining of both quenching and thermal stresses, which are generated respectively during spraying stage and cooling stage, is then required to improve coatings quality. A rather simple apparatus, which consists in monitoring the curvature of a beam substrate during the whole deposition process, has been developed to work under industrial conditions. It has been applied to partially stabilized zirconia coatings performed onto stainless steel and cast iron substrates. Spraying temperature and plasma gun velocity have been selected as relevant parameters for this study about stress generation and mechanical release. Finally, four point bend tests have been performed on deposited samples to measure coating mechanical properties and to evaluate damage level.

This paper presents a method for optimizing plasma spraying parameters based on statistical analysis and related models. The models presented account for particle velocity and plasma mass enthalpy and make it possible to study the influence of plasma properties on the deposition process.

In order to determine residual stresses in industrial plasmasprayed coatings, a rather simple apparatus, which monitors the curvature of a beam substrate during the deposition process, has been developed. The experimental set-up consists of a water-cooled rotating cylinder, holding an initially plane substrate, whose curvature is continuously measured using a contacting displacement sensor disposed into the cylinder. The combination of the plasma gun translation and the cylinder rotation allows to reach industrial spraying velocities. Liquid argon cryogenic system is used to control the substrate temperature from about 50°C to more than 300°C independently from the process velocity. A typical recording is analyzed thoroughly and a theoretical approach to residual stress calculation discussed. This method is applied to partially stabilized zirconia coatings performed onto stainless steel substrates for spraying temperatures between 80°C and 210°C.

A P.S.Z. coating elaborated by r.f. plasma spraying was studied and compared to industrial arc plasma-sprayed P.S.Z. coatings to evaluate the quality of the corresponding thermal barrier coating system for gas turbine applications. One commercial ZrO 2 - 8% Y 2 O 3 powder was sprayed with two industrial d.c. torches (7MB and F4) and one r.f. plasma torch (Tekna PL50). Physical properties such as density, porosity and thermal diffusivity were measured on the three types of P.S.Z. coating. The microstructure and quantitative phase analysis were respectively investigated by S.E.M. and X-Ray diffraction. The burner rig tests on the T.B.C. systems showed that the thermal shocks resistance on the r.f. coating was at least the same as the others. Induction plasma spraying gave a high deposit efficiency (around 80%) and a P.S.Z. coating with very interesting thermal properties. All these facts demonstrate that r.f plasma spraying can be a competitive process to produce high quality thermal barrier coatings.