NANOSTRUCTURED PLASMA SPRAY COATINGS L. Górski (1) , I. Cieślik (1) , M.J. Woźniak (2) (1) Material Physics Department, National Center for Nuclear Research, A. Sołtana 7, 05-400 Otwock, Poland (2) Faculty of Materials Science and Engineering, University of Technology 02-507 Warsaw, Poland
Introduction The presented studies are connected with the works on high temperature resistant protective coatings especially thermal barrier coatings (TBC).These coatings are the result of advanced technology combining suitable choice of the substrate, metallic bond coat based on Ni and Fe (e.g. NiCrAlY) and essential external ceramic layer based on Al2O3 and ZrO2. TBC due to insulated ceramic properties reduce the temperature attained by metal base components and act as a barrier for erosion and corrosion effects caused by hot liquid and gaseous steams. Therefore these coatings have numerous applications among others in heat and gaseous turbines, aircraft and high compression engines. TBC coatings are deposited mainly with the use of plasma spraying technology. Plasma spraying process is characterized by short time of temperature action and high cooling rate.
Experimental The time of material flight by the plasma arc with temperature 104K range is about few ms and thus cooling rate of solidified material in the contact with cold substrate is 105-106 K/s. Schematic image of plasma spraying is shown in Fig. 1. Coatings structure are studied by X-ray diffraction (XRD) and microscopic methods (electron microscopy- EM and scanning probe microscopy-SPM). X-ray diffraction patterns show polycrystalline coatings structure with coexistence of few crystalline phases. For illustration two diffraction patterns are listed below. Fig. 1. Schematic image of plasma spraying process with the use of PN-120 plasmatrone 1-tungsten cathode, 2-copper anode cooled by water, 3-plasma arc, 4-place of sprayed material entering, 5-substrate to be sprayed
Results ZrO2 group
Results Al2O3 group
In the second pattern for ZrO2 stabilized by Y2O3 the main phase is tetragonal zirconia. Small amounts of monoclinic phase are also visible. The occurring of monoclinic phase due to martensite transformation in the limited range causes higher thermal shock resistance. Such materials are called partially stabilized zirconia (PSZ).
First pattern was made for Al2O3-TiO2 coating (Fig. 2). Besides peaks of separate oxides (Al2O3 in the α and γ forms, TiO2 in the form of rutile) several peaks belonging to common phase Al2TiO5 are registered. This latter phase has very attractive properties as high temperature resistant material due to low value of thermal conductivity and considerable thermal expansion anisotropy . Therefore it shows high resistance for rapid temperature and particularly for thermal shocks.
Fig.3 XRD pattern of ZrO2 – Y2O3 coating (Cu radiation) Fig.2 XRD pattern of Al2O3 – TiO2 coating (Cu radiation)
Fig. 4 AFM images of ZrO2 - Y2O3 sample a)Two-dimensional image, b)Three dimensional image Fig. 5 AFM images of Al2O3 – TiO2 sample a) Two-dimensional image, b) Three dimensional image T [°C]
Rapid coatings solidification causes specific microstructure observed earlier by SEM methods. Using of high resolving power microscope gives more and some new data in nanometric range which is shown in Figs. 4,5. Further works directing to observed effects full elucidation particularly by SPM methods are still in progress.