Coating Microstructure-Property-Performance Issues Page: 3 of 9
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The overall goal of research on thermal-spray coatings for high-temperature environmental resistance at the Idaho
National Laboratory is to better understand the relationships between coating processes, coating characteristics, and
coating performance. Towards these ends, the effects of coating process variables (primarily thermal spray particle
temperature and velocity) on the characteristics of a number of different coating materials have been studied. These
include iron aluminides formed by high-velocity oxy-fuel (HVOF) spraying and Mo-Si-B coatings formed by air
plasma spraying (APS). [-3] The coatings have been applied to variety of structural substrates: low-alloy ferritic
steels, advanced ferritic-martensitic steels, austenitic stainless steels, and Ni-base alloys. A wide variety of coating
characteristics have been determined: microstructure, thermal expansion, elastic modulus, hardness, and residual
stress. The dependence of these characteristics on coating process parameters is reasonably well understood.
The next phase of research is to determine the influence of coating characteristics on performance. The purpose of
coating performance testing is to establish a baseline of performance-related properties and characteristics sufficient
to screen different coating types and processes and provide some level of confidence prior to service testing.
Laboratory-based performance testing can also help determine the contributions of different coating system
parameters to critical properties, enabling the selection of the ideal set of parameters for service testing and the
assessment of the effects of variations from the ideal parameters. Service testing will ultimately be required,
however, since it is nearly impossible to fully duplicate in a laboratory test the exact nature and range of conditions
encountered in service. Reference  describes some of the complexities encountered in service which would be
difficult to anticipate in a laboratory.
Laboratory-based coating performance tests used at the INL focus on two areas-environmental resistance and
durability. Environmental resistance, the essential function of the coating, is clearly of prime importance. Coatings
are typically created from feedstock materials which are known to be resistant to the specific environment of
interest, but it still must be demonstrated that that material in coating form possesses similar resistance. It is also
important to understand the effects of thermal cycling, which typically increases attack through spallation of
protective oxides. Finally, for coatings the influence of stresses and strains imposed by the substrate on
environmental resistance must be understood. These stresses may be residual from the coating process or may result
from thermal expansion mismatch strains.
Environmental resistance is studied by performing several different tests. First, gas-phase corrosion testing using
thermo-gravimetric analysis (TGA) is performed on free-standing coatings (i.e., with the substrate removed) to
measure intrinsic coating behavior. Free-standing coatings may also be tested with combined gas and ash or slag
corrodent. In this case tests are performed by long-term furnace exposure and metallographic examination of attack.
For both cases tests are performed under both isothermal and cycling conditions. Finally, coated substrate test
coupons are tested by furnace exposure and metallographic examination. Testing of this type is complicated by the
possibility of accelerated attack of the substrate or exposed coating-substrate interfaces, depending on the test
Coating durability testing is equally important, since the most environmentally-resistant coating will not protect the
substrate if it has spalled off or been severely cracked. Laboratory characterization of coating durability is difficult
again because of the difficulty in replicating the range of service conditions, but also because "durability" is not a
unique property of the coating or the coating-substrate system which can be unambiguously measured. This
problem is reflected in the wide variety of tests available for durability testing in the coating literature. Durability
can be roughly broken into two aspects: adherence and cracking resistance. Adherence of coatings is commonly
measured by the tensile adhesion test (TAT) according to the ASTM C 633 or other standard methods,"' but the
results obtained are dependent on the test method and do not provide a material or system property that can be used
in design. The resistance of the coating-substrate interface to crack propagation, assessed using fracture toughness
techniques, is a more fundamental measure of adhesion.  A range of test techniques are available for measuring
this parameter, but it does not appear to be as widely applied as the TAT, likely due to the difficulty in performing
the tests and the fact that the results are still difficult to apply in design conditions.
Resistance to cracking due to tensile loading prior to service is relatively straightforward to measure by simply
loading coated substrates until cracking is noted. Coating fracture tests may be performed in tension or bending and
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Totemeier, Terry C. & Wright, Richard N. Coating Microstructure-Property-Performance Issues, article, May 1, 2005; [Idaho Falls, Idaho]. (digital.library.unt.edu/ark:/67531/metadc877602/m1/3/: accessed January 20, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.