The aim of this work, devoted to yttria stabilised zirconia (YSZ) thermal barrier coatings, is to produce by DC plasma spraying a single thick pass macro cracked orthogonally to the substrate. YSZ was plasma sprayed in air atmosphere on Hastelloy X substrates, with a NiCrAlY bond coat. A three-zone microstructure is observed, where lamellae and columns are present. The measurements of deposition stresses during spraying allow explaining the macrocrack formation.

Coating production with reproducible properties within a range of values acceptable for the specific application requires a on-line control of the thermal spray process. Sprayed coatings present very often reproducibility problems due to spray parameter variations. In fact the quality of coatings is strongly linked to the temperature and velocity of particles at impact, and the temperature evolution of substrate and coating, before (preheating), during (spraying) and after (cooling). The particle temperature and velocity measurements require sophisticated devices rather expensive and not well adapted to work in the harsh environment of spray booths. Moreover in HVOF spraying, the particle surface temperature is generally below 2200K which make the measurements trickier than in plasma conditions. That is why, the Spray and Deposit Control (SDC) has been developed (collaboration between SPCTS laboratory of the University of Limoges and SNECMA Services), which allows to follow continuously both the light emitted by the hot particles (maximum intensity, mean trajectory and trajectory dispersion) and the substrate and coating temperature evolution during spraying. This simple and light system, fixed on the torch, has been tested in flame, plasma and HVOF spraying. The SDC measured parameters allow to develop easily process windows to obtain good working areas corresponding to specific coating parameters. In this study, the SDC on-line system is used in industrial conditions to optimize and control the HVOF spraying of carbide (WC-Co 17% wt) powders. A special attention is given to the possibilities of process parameters adjustment to regain the specific particle jet parameters and the good substrate temperature, in case of measured SDC parameters shifting. This study demonstrates the SDC system ability for an industrial on-line control of HVOF spraying, using process maps, to increase the consistency of coating properties. It is a step to a closed-loop control of the process through both the particle spray jet properties (mean trajectory and radiated flux distribution) and the target surface temperature.

The University of Limoges developed a compact sensing module that measures particle beam properties and substrate surface temperatures during plasma spraying. In this paper, the authors explain how they built and tested a closed-loop controller for APS processes using the sensing module. One of the key elements in the online control system is an empirical model that relates process inputs, in-flight particle parameters, substrate temperatures, and coating properties. The paper discusses the development and implementation of the model and the determination of the main input parameters. Paper includes a German-language abstract.

During last 10 years, it has been pointed out that the reproducibility and reliability of air plasma sprayed (APS) coatings depend, among other parameters, on the particle velocity and temperature distributions prior to their impacts to the target surface. On-line control systems have been designed to follow these parameters in the harsh environment of booths. However, in spite of significant strides, works have yet to be carried out to establish relationships between deposit properties and in-flight particle parameters and/or surface target temperature, roughness and oxidation stage. The SPCTS laboratory at the University of Limoges has developed the SDC (Spray and Deposit Control) system in collaboration with SNECMA Services. It controls the stability of the spray jet and the mean particle trajectory together with the target surface temperature. It has been used with a device allowing to measure the deflection of a rectangular beam during APS of WC-Co17wt% or ZrO 2 +8wt%Y 2 O 3 powder on Hastelloy X (Ni base alloy) substrate. The aim of this study was to determine which spray parameters influence the residual stresses, in order to achieve a mean compressive residual stress in the WC-Co17wt% coating on Hastelloy X and to control it with the deposit temperature.