Lepao

Available online at www.sciencedirect.com

Acta Materialia 56 (2008) 4080–4090
www.elsevier.com/locate/actamat

On cracks and delaminations of thermal barrier coatings due
to indentation testing: Experimental investigations
Jin Yan a, Thorsten Leist b, Marion Bartsch c, Anette M. Karlsson a,*
a

Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, USADarmstadt University of Technology, Materials Science, NAW, 64287 Darmstadt, Germany
c
Institute of Materials Research, German Aerospace Center (DLR), Linder Hoehe, D-51147 Cologne, Germany
b

Received 20 September 2007; received in revised form 15 April 2008; accepted 16 April 2008
Available online 24 May 2008

Abstract
Rockwell indentation testing as a method of establishing the interfacialfracture toughness of thermal barrier coatings is investigated.
To this end, indentation tests have been systematically performed on coatings with yttria-stabilized zirconia top coat deposited by electron beam physical vapor deposition. Specimens in ‘‘as-coated” condition and after heat treatment in air have been studied. Unexpectedly, indentation of the heat-treated samples resulted in smallerdelaminations than the as-coated samples, suggesting an increase in
fracture toughness for coatings subjected to elevated temperatures. Careful image analyses of the cross-section of the indented area show
that the ceramic top coat undergoes a complex damage evolution during indentation that is altered by thermal treatment. The consequences of this are discussed and we note that care must be takenwhen evaluating fracture parameters for multilayered structures based
on indentation testing.
Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Indentation; Thermal barrier coatings (TBC); Interfacial fracture toughness

1. Introduction
Thermal barrier coatings (TBCs) are used in a range of
industrial applications, e.g. on cooled gas turbine blades orvanes, in order to reduce the temperature of these parts.
Unfortunately, design engineers are not able to take full
advantage of TBCs since reliable lifetime assessment methods for the coating system are not available. Even though
significant efforts have been dedicated towards improving
the reliability of TBCs, the failures are not completely
understood. The lack of understanding of the failuresin
TBCs stems from the complex structure of the coatings.
A TBC is a multilayered system, where an aluminum-rich
metallic bond coat is deposited on the superalloy, followed
by a ceramic top coat deposited on the bond coat. During
processing and further thermal exposure, a third layer is
*

Corresponding author. Tel.: +1 302 831 6437; fax: +1 302 831 3619.
E-mail address:karlsson@udel.edu (A.M. Karlsson).

formed between the bond coat and the top coat. This layer
is commonly referred to as the thermally grown oxide
(TGO), primarily made up by alumina (Al2O3). Yttria-stabilized zirconia (YSZ) is currently the most commonly used
top coat material. The ceramic top coat is by design porous, to allow for strain tolerance during thermal cycling,
thereby preventing instantaneousspallation. Two major
groups of bond coat have emerged: platinum aluminide
and MCrAlY (M = Fe or NiCo). The in-service failures
of the coating systems occur in most cases by debonding
(delamination) of the coating, followed by subsequent
spallation. The delamination is normally associated with
the TGO [1,2]. The cracks are not necessarily seen at one
particular interface but may occur at theinterface between
either the top coat and TGO, or the bond coat and TGO,
and may even occur as cohesive failure in one of the layers.
The failure process is highly sensitive to the system parameters such as processing technique and pattern of use (e.g.
power generation vs. propulsion). In addition, the thermal

1359-6454/$34.00 Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All...