Effects of bond coat misfit strains on the rumpling of thermally grown oxides
- PDF / 406,997 Bytes
- 11 Pages / 576 x 720 pts Page_size
- 0 Downloads / 205 Views
t factor governing the durability of thermal barrier systems is the concurrent thickening and elongation of the thermally grown oxide (TGO) upon temperature cycling.[1–5] The elongation, when it occurs, can cause cyclic ratcheting of the TGO, creating a stress in the adjacent thermal barrier coating (TBC) that promotes delamination.[2,6–8] When this process can be arrested, the durability of the system increases, and failure occurs by an alternative mechanism: usually by delamination along the interface between the TGO and the underlying bond coat alloy.[9,10] A search for approaches that suppress ratcheting has emerged as a major research focus in thermal barrier systems. Attainment of this objective has been facilitated by the development of models for undulation growth.[11–16] Thus far, the predictions of the models have been corroborated by three sets of measurements: (1) distortions of surface grooves on FeCrAlY substrates;[17–20] (2) groove distortions, grain boundary grooving, and intragrain undulations in a commercial thermal barrier system with a Pt-aluminide bond coat alloy;[21] and (c) trends in undulation growth with bond coat thickness on another system with a Pt-aluminide bond coat composition.[22,23] One of the remaining challenges is to contend effectively with the large number of constituent properties and geometric parameters that affect undulation growth, that is, down-selecting the features having the greatest influence. The present article uses simulations to examine some of the salient characteristics. To assure practical utility of the findings, the parameters are adjusted within ranges that might be achievable through modifications to the chemistry and microstructure of realistic bond coat alloys. A.W. DAVIS, formerly Graduate Student with the Mechanical and Aerospace Engineering Department, Princeton University, Princeton, NJ 08544, is Postdoctoral Researcher, Army Research Laboratory, Aberdeen Proving Ground, MD 21005-5069. Contact e-mail: [email protected] A.G. EVANS, Professor, is with the Materials Department, University of California, Santa Barbara, Santa Barbara, CA 93106. Manuscript submitted September 19, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
SYNOPSIS OF TRENDS
The following physical principles affecting undulation growth have emerged from the models.[11–21] (1) The TGO must experience an elongation strain, causing it to be in compression during growth. (2) The TGO must have sufficiently high creep strength (growth stress), relative to the bond coat, that it exerts a force on the bond coat large enough to plastically displace it as the undulation enlarges. (3) The bond coat must have a creep strength small enough (relative to the TGO) to enable it to flow plastically around the undulation in the TGO. (4) Misfit strain, emisfit, between the bond coat and the substrate plays a key role. Such strain facilitates undulation growth by subjecting it to generalized yielding during thermal cycling. This effectively ‘‘softens’’ the bond coat, allowing it to accommodate the pen
Data Loading...
