Microstructure, Processing, Performance Relationships for High Temperature Coatings Page: 2 of 9
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MICROSTRUCTURE, PROCESSING, PERFORMANCE RELATIONSHIPS FOR HIGH
TEMPERATURE COATINGS
Thomas Lillo
Idaho National Laboratory
PO Box 1625, MS 2218 Idaho Falls, ID 83415
Thomas.Lillo(INL.gov, (209) 526-9746
Richard Wright
Idaho National Laboratory
PO Box 1625, MS 2218 Idaho Falls, ID 83415
Richard.WrightINL.gov, (208) 526-6127
ABSTRACT
High velocity oxy-fuel (HVOF)coating have shown high resistance to corrosion in fossil energy applications and it
is generally accepted that mechanical failure, e.g. cracking or spalling, ultimately will determine coating lifetime.
The use of HVOF thermal spray to apply coatings is one of the most commercially viable and allows the control of
various parameters including powder particle velocity and temperature which influence coating properties, such as
residual stress, bond coat strength and microstructure. Methods of assessing the mechanical durability of coatings
are being developed in order to explore the relationship between HVOF spraying parameters and the mechanical
properties of the coating and coating bond strength. The room temperature mechanical strength, as well as the
resistance of the coating to cracking/spalling during thermal transients, is of considerable importance. Eddy current,
acoustic emission and thermal imaging methods are being developed to detect coating failure during thermal cycling
tests and room temperature tensile tests. Preliminary results on coating failure of HVOF FeAl coatings on carbon
steel, as detected by eddy current measurements during thermal cycling, are presented. The influence of HVOF
coating parameters of iron aluminides - applied to more relevant structural steels, like 316 SS and Grade 91 steel, -
on coating durability will be explored once reliable methods for identification of coating failure have been
developed.
INTRODUCTION
Typically, materials with the high temperature strength and creep resistance required for fossil fuel boilers lack the
necessary corrosion resistance to provide a long service life. Conversely, materials with the needed corrosion
resistance often are not suitable for the high temperature structural requirements or they are difficult to
thermomechanically form into useful shapes. One way of satisfying all the requirements is to apply a coating with
the necessary corrosion resistance to a substrate material that will satisfy the high temperature structural
requirements. The functionality of the system is dependent on the coating being devoid of open porosity, which
would allow corrosive gases to reach the underlying substrate material, and the coating being resistant to cracking or
delamination, which would, again, allow corrosive gases to reach the underlying substrate. Due to these issues,
there has been reluctance to utilize coatings in high temperature, aggressive environments. However, the need to
increase operating temperatures to increase efficiency is driving research to understand the coating process and the
factors that contribute to coating failures. Therefore the current work is focused on understanding the relationship
between coating parameters and coating durability with the goal of optimizing those parameters to produce reliable
coatings with long operating lifetimes.
BACKGROUND
Thermally sprayed coatings as thermal barrier and/or corrosion resistant coatings have long been of interest.
Coating/substrate systems can be chosen to satisfy both the structural and corrosion resistance requirements of
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Lillo, Thomas M.; Wright, Richard N.; Swank, W. David; Haggard, D.C; Kunerth, Dennis C. & Clark, Denis E. Microstructure, Processing, Performance Relationships for High Temperature Coatings, article, July 1, 2008; [Idaho Falls, Idaho]. (https://digital.library.unt.edu/ark:/67531/metadc898732/m1/2/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.