Thermal Mechanical Fatigue Behavior of Advanced Overlay Coatings

Abstract

Environmental protection and cost effectiveness lead to the demand to increase the efficiency of energy conversion. In the past decade gas turbines have become an important factor in efficient power generation. Gas turbine inlet temperatures are being increased continuously to decrease the specific fuel consumption. The development of new base materials and sophisticated processes like single crystal solidification allows higher metal temperatures. This causes higher oxidative stress to the MCrAlY overlay coatings. As shown in an earlier paper (I), an improvement of the high temperature properties is achievable by the addition of Rhenium. In this paper we report on properl ies of MCrAlY coatings containing 8 to 12% Al and 10% Rhenium by weight. The coatings were applied on Inco 738 LC by LPPS (Low Pressure Plasma Spraying). Data for static oxidation and cyclic oxidation data at 950°C, I000°C up to 10,000 hours are presented. However, the main aim of this report is to discuss the thermal-mechanical properties of LPPS-coatings with Re. As known from earlier work, the variation or addition of different elements to overlay coatings lead to a change in the ductile-brittle transition. Ductile-Brittle-Transition-Temperature (DBTT) measurements on 10% Re containing systems under the variation of Co, Cr, Al and Si have been done. In order to get more information about the thermal-mechanical properties of the coating under service like conditions, TCF (Thermal Cycle Fatigue) and TMF (Thermal Mechanical Fatigue) tests were carried out. On discus shaped samples, the TCF behavior of an MCrAlYRe coating in comparison to an MCrAlY coating has been evaluated up to 1,500 cycles. The samples were heated in a radiation oven up to 1000°C and cooled down by compressed air. By this method a differentiation in the thermal fatigue behavior is possible. However, this kind of test does not cover all possible relaxation processes. A more realistic investigation of the thermal mechanical fatigue behavior is given by TMF tests. A service life cycle was taken to study the thermal mechanical behavior. It is shown that Re improves the thermal mechanical properties of MCrAlY coating systems considerably.