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Ceramic Coatings
Chapter · February 2019
DOI: 10.1201/9781315145808-5
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Sarath Chandra Debasish Sarkar
National Institute of Technology Rourkela National Institute of Technology Rourkela
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5
Ceramic Coatings
K. Sarath Chandra and Debasish Sarkar
CONTENTS
5.1 Introduction......................................................................................................................... 133
5.2 Properties and Testing Protocols...................................................................................... 135
5.3 Engineering Considerations.............................................................................................. 135
5.4 Principles of Coating Technology.................................................................................... 139
5.4.1 Gaseous State........................................................................................................... 139
5.4.1.1 Chemical Vapor Deposition.................................................................... 140
5.4.1.2 Physical Vapor Deposition...................................................................... 140
5.4.1.3 Ion Beam-Assisted Deposition............................................................... 142
5.4.2 Solution State........................................................................................................... 144
5.4.2.1 Chemical Conversion Coatings.............................................................. 144
5.4.2.2 Electroplating........................................................................................... 144
5.4.2.3 Sol-Gel Deposition Procedure................................................................ 144
5.4.3 Molten and Semi-Molten State.............................................................................. 144
5.4.3.1 Thermal Spraying.................................................................................... 145
5.4.4 Growth Mechanisms of the Ceramic Coatings.................................................. 145
5.5 Processing and Failure Analysis...................................................................................... 151
5.5.1 Abrasion Resistant Coatings................................................................................. 151
5.5.2 Corrosion Resistant Coatings................................................................................ 153
5.5.3 Thermal Barrier Coatings...................................................................................... 155
5.5.4 Emissive Refractory Coatings............................................................................... 157
5.5.5 Electric Insulation Coatings.................................................................................. 159
5.5.6 Biomedical Coatings............................................................................................... 161
5.5.7 Aesthetic Appearance Coatings............................................................................ 162
5.6 Conclusions.......................................................................................................................... 164
References...................................................................................................................................... 164
5.1 Introduction
Ceramic coatings are the two-dimensional layered structures deposited on the surface
of the substrate or object by several deposition procedures (e.g., plasma spraying, vapor
deposition and sol-gel methods) to improve the performance and sustainability of the
engineering materials against various disciplines of disparate wear processes acting under
distinct service environments. In recent times, ceramic coatings have received a great deal
of industrial and technological importance attributed to their unusual properties, which
were recommended as potential candidates for their extensive usage in a wide variety
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134 Ceramic Processing
of several miscellaneous engineering applications. [1] For instance, the extreme hardness
coupled with the good toughening character of ceramic coating materials is the key prop-
erty in endurable abrasive-resistant ceramic coatings primarily used for tool materials in
machining and casting to provide excellent mechanical protection against abrasion and
erosion. In a similar manner, the insulation property of electric insulation ceramic coatings
is vital for applications in microelectronic circuits and heating elements, superior chemical
inertness of corrosion resistant ceramic coatings is a prerequisite property against hos-
tile corrosive environments and high temperature resistance properties are indispensable
for thermal barrier ceramic coatings employed in distinct zones of gas turbine engines,
nuclear reactors and so on. [2] The important features of the several classes of ceramic coat-
ings categorized according to their application area are listed in Table 5.1.
Failure of a ceramic coating often occurs in most aggressive service environments by the
combined attack of disparate wear processes, including foreign object damage, abrasion
and erosion at room and elevated temperatures, high temperature oxidation, hot corrosion,
thermomechanical fatigue and creep. [3] However, the degree of action of these in-service
active degradation mechanisms is typically dependent on the features of the structural
component and severity of the service conditions., Further, these degradation processes
may often lead to premature failure of the substrate material in the event of coating fail-
ure, challenges the ceramic engineers to develop durable ceramic coatings with extended
performance to meet the growing demands. For instance, it is a widely considerable prac-
tice that the fabrication of thermal barrier ceramic coatings with enhanced performance
should have the following important tailored properties: they are dense columnar grain
morphology, good adhesion and cohesion to the substrate, and a low coefficient of thermal
expansion with a close match to the substrate. These improved properties markedly limit
the aggravating corrosive and oxidative wear processes under service, thereby reducing
the origin of residual stresses and minimizing the thermal shock cracking and failure as a
result of the consequent spallation mechanism. [4] Therefore, it has been strongly insisted
TABLE 5.1
Features of the Various Classes of Ceramic Coatings
Ceramic Coatings Property Examples
Wear/Abrasion-resistant Resistance against abrasion, sliding wear, Al2O3, ZrO2, TiO2 and
coatings friction, fretting and erosion. Al2O3– TiO2.
Thermal barrier coatings Resistance to oxidation, corrosion and ZrO2, Al2O3 and YSZ.
thermal insulation.
Electrical insulation coatings Good dielectric strength for effective SiO2–Al2O3, Al2O3, electric
insulation. porcelain, Al2O3 and
Al2O3– TiO2.
Biomedical coatings Biocompatibility in terms of tissuegrowth Hydroxyapatite, TiO2 and
propensity. porcelain.
Aesthetics Aesthetic qualities in terms of color, Porcelain, glass–ceramic,
smoothness and glossy appearance Al2O3 and ZrO2.
together with resistance against disparate
wear processes.
Corrosion resistant coatings Resistance against corrosion. Al2O3, ZrO2, MgO and Cr2O3.
Refractory emissive coatings Compactable, highly chemical inert and Cr2O3, CoO, Fe2O3 and NiO.
radiative-coated structure promotes
excellent resistance against in-service
wear attack, improves thermal efficiency
and service life.
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Ceramic Coatings 135
that the processing of sustainable ceramic coatings or coated structures with such afore-
mentioned desirable properties for enhanced performance in distinct applications is typi-
cally dependent upon the choice of coating system that comprises the coating process and
coating material selected on the basis of engineering considerations that certainly varies
with the substrate component and the kind of process environment. [4, 5] However, such
studies on ceramic coatings are comprehensively limited and utilized for the broad range
of applications.
This text explores the fundamental methodology behind selecting a coating system
which comprises the coating material and coating process compatible with the substrate
according to engineering considerations and depending upon the kind of application
needed to tailor the intrinsic properties of a coated structure for a high degree of per-
formance and a long service life. In addition, a greater emphasis has been placed on the
thermal barrier coatings due to their growing science and technological interest towards
processing and consequent in-service failure behavior under hostile conditions.
5.2 Properties and Testing Protocols
The intrinsic properties of a ceramic coating or coated structure, which most commonly
include chemical composition, morphology, wettability, adhesion, thickness, roughness,
residual stress and hardness, as listed in Table 5.2 are the characteristic features that
unambiguously reflect the degree of performance and consistency against failure caused
by the combined attack of disparate wear processes acting under diverse process environ-
ments. However, these properties are typically controlled by the coating system that has
been selected in accordance to the engineering considerations depending upon the kind
of application. [3, 6, 7] This module discusses the significance of the intrinsic properties
of a ceramic-coated structure and the associated characterization tools used to ensure the
coating selection criterion.
5.3 Engineering Considerations
Engineering considerations are the decisive factors for selecting a particular coating sys-
tem by ascertaining the compatibility of the coating process and coating material with
respect to the components’ base materials in order to develop a coated structure of requi-
site intrinsic properties. In this module, primary engineering considerations needed for
the evaluation of a specific coating system, which typically include chemical or metallurgi-
cal compatibility, process compatibility, mechanical compatibility, and component coating
efficacy are explained in detail as follows. [4, 8]
a. Chemical or metallurgical compatibility
This engineering consideration typically specifies that the deposited ceramic structure
must be relatively stable with respect to the substrate component in order to control inter-
diffusion kinetics and limit chemical reactions under service. The interdiffusion process is
indispensable for many coating systems to ensure improved adhesion characteristics with
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