Structural Investigation of Alumina Thin Films Deposited by Chemical Vapor Deposition
- PDF / 766,658 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 32 Downloads / 216 Views
Y3.10.1
Structural Investigation of Alumina Thin Films Deposited by Chemical Vapor Deposition Naoufal Bahlawane∗, Sabine Blittersdorf, Katharina Kohse-Höinghaus, Burak Atakan1, Jürgen Müller2 Physikalische Chemie I, Universität Bielefeld, Universitätsstr. 25, D-33615 Bielefeld, Germany 1 Thermodynamik, Institut für Verbrennung und Gasdynamik, Gerhard-Mercator-Universität Duisburg, D-47048 Duisburg, Germany 2 Lehrstuhl für Werkstoffchemie, RWTH Aachen, D-52056 Aachen, Germany ABSTRACT The present study concerns the deposition of α−Al2O3 for diffusion barrier applications on superalloy substrates. The growth of α−Al2O3 has been achieved by chemical vapor deposition (CVD) using an AlCl3/CO2/H2 gas mixture at 1080 °C. Among several growth-controlling parameters with potential importance for the whisker growth process, the reactor pressure during deposition seems to be highly influential on the resulting film structure. Deposited films at low pressure presented solely a fine whisker structure. This non-closed structure is not suitable as diffusion barrier; however, the observed high porosity makes the deposit a potential candidate as a catalysis support. An increase of the deposition pressure led to a competitive growth of whiskers and grains. A suitable microstructure was attained at relatively high pressure (100 mbar) where the surface was fully covered by 2 µm large alumina crystals that formed a closed structure. Further increase of the pressure led to an irregular and rough surface microstructure. INTRODUCTION Superalloys are the material of choice for gas turbine components; however, three conditions need to be met in their use. First, a metallic layer, typically of aluminide, chromide or MCrAlY has to be applied to withstand the hot oxidation and corrosion of alkali-metal containing gases [1,2]. The protection is achieved when an oxide layer grows on the metallic coatings at high temperature. The second condition is the insulation of the superalloys from the heat of the gas using external thermal barrier coatings (TBCs). In this context, aluminide and MCrAlY coatings are also being used as bond coats for this TBC layer, improving its adherence [3]. The TBC layer allows a beneficial reduction of the metal temperature by 170 K [2]. The third condition is the establishment of an efficient internal and external cooling, since the temperature at the turbine’s entrance can reach 1650 °C, exceeding the melting point of the superalloy [1]. Even protected in this way, superalloys cannot be used at temperatures above 1100 °C because of the material limits. Indeed, when the temperature at the surface of the superalloy exceeds 1000 °C, inward diffusion of Al and outward diffusion of Co, W, Re and Ti in the metallic coatings and the superalloy interface [4] leads to a brittle interdiffusion zone where cracks initiate, inducing spallation of the coating [5]. This effect is assumed to be the cause for the limited lifetime of the coating. Considering the strong dependence of the efficiency of gas turbine engines on the turbine i
Data Loading...
