Ceramic Coatings and Linings,Ceramic Coatings and Linings,Ceramic Coatings and Linings
CERAMIC COATINGS include glasses, with or without additions of refractory compounds; high-temperature coatings
based on oxides, carbides, silicides, borides, or nitrides; cermets; and other inorganic materials. Ceramic coatings are
applied to metals to protect them against oxidation and corrosion at room temperature and at elevated temperatures.
Special coatings have been developed for specific uses, including wear resistance, chemical resistance, high reflectivity,
electrical resistance, and prevention of hydrogen diffusion. Ceramic-coated metals are used for furnace components, heat
treating equipment, chemical processing equipment, heat exchangers, rocket motor nozzles, exhaust manifolds, jet engine
parts, and nuclear power plant components.
Selection Factors
Several factors must be considered when selecting a ceramic coating:
· Service environment to be encountered by the coated metal
· Mechanisms by which the coatings provide protection at elevated temperatures
· Compatibility of the coating with the substrate metal
· Method of applying the coating
· Quality control of the coating
· Ability of coating to be repaired
Service environment may involve a wide range of conditions. The intended operating life of a coated part may range
from a few seconds to several hundred hours. Conditions may involve exposure to atmospheric gases at various mass
flows with velocities up to, or even beyond, Mach 10. Components made of the refractory alloys may be subjected to very
high stresses, or they may be used as heat shields or furnace windings, for which the only load is the mass of the
component. Heating and cooling rates may be gradual or rapid, and one or more thermal cycles may be involved. For any
specific service environment, the coating selected must protect the metal from oxidation and the effects of hydrogen pickup by preventing or minimizing the diffusion of oxygen, nitrogen, and hydrogen from the atmosphere through the
coating into the substrate.
Mechanisms of Protection. Ceramic coatings have two mechanisms to protect metals at elevated temperatures. One
type of coating is applied as a layer of stable oxide on the surface of the metal, which prevents or delays contact between
metal and atmosphere. The other type of ceramic coating is an intermetallic compound that forms a thin oxide film on its
surface. The composition of the intermetallic is such that it provides the optimum combination of metallic elements for
forming a stable and adherent protective oxide film on its surface and for healing the oxide film in the event the film is
broken. Thus, this type of coating depends on the formation and preservation of the oxide film for protection of the
substrate material.
Chemical and Mechanical Compatibility. Chemical compatibility of the ceramic coating with the substrate metal is
important, especially when the coating is applied to refractory metals and nickel-base alloys for high-temperature service.
The so-called stable oxide coatings, such as alumina, are not stable in the presence of some of the refractory metals, such
as niobium (columbium) and tantalum, at temperatures above 1370 °C (2500 °F). Also, alumina reacts with metals such
as titanium and zirconium, and the protective characteristics of the coating are soon exhausted.
The coating must also be mechanically compatible with the underlying metal, so that undesirable mechanical stresses are
not induced in either material. Because most stable coatings are brittle at low temperatures, the coefficients of thermal
expansion of the coating and substrate should not be greatly different; however, the coefficient of expansion of the
coating should be somewhat less than that of the substrate, so that the coating will be in compression at low (room)
temperature. The mismatch in expansion should be greater for parts subjected to thermal cycling. The system must be
designed so the difference in the coefficients keeps the coating in compression at all temperatures below the softening
point of the coating. if the coating is in tension at low temperature, it will crack. Conversely, if it is under too much
compression, it will spall.
The effect of the coating on fatigue life and on the brittle transition temperature of the composite material should also be
considered. In general, the coating is more brittle than the substrate metal, and cracks that form in the coating during
service act as stress raisers on the substrate, thus reducing the low-temperature ductility and fatigue life.
Application Method. The method of applying the coating is restricted by the type of coating, the type of metal to be
coated, and the size and configuration of the work. Many of the coating processes include heat treatment to promote
bonding and sealing. The atmospheres used for spraying and heat treating must be closely controlled to prevent any
deterioration in properties of the substrate metal.
Control of Coating Quality. It is important to ensure that the coating is capable of protecting the substrate. Thickness
measurements and visual observations are two methods of determining coating quality. However, these methods are not
satisfactory for coatings on complex shapes and internal passages that are difficult to see or reach. A preliminary
oxidation test of a few minutes or hours in an oxidizing atmosphere at the operating temperature is also an acceptable
method for determining quality of the coating.
Ability of coating to be repaired is an important consideration in coating selection. The ideal coating should be
repairable if coverage is insufficient in the initial application or if the coating is damaged during handling or service.
Repair procedures and their effectiveness differ for the various coatings, methods of application, substrate metals, and
size and shape of work.
Coating Materials
The nonmetallic, inorganic materials used as ceramic coatings have several characteristics in common. Among these are
relatively good chemical stability at elevated temperatures, hardness, brittle behavior under load, and mechanical
continuity in thin cross section.
Silicate Glasses. Coatings prepared from glass powders, with or without additions of refractory compounds, have the
greatest industrial usage of all ceramic coatings. Glass is typically manufactured by mixing specific proportions of
minerals, heating the mixture to a molten or liquid state, and rapidly quenching. Quenching is normally accomplished by
discharging the melt into cold water or by pouring it through water-cooled steel rolls. The former case results in small
friable pieces of glass that can be ground into a powder with relative ease using a ball mill or other standard crushing
equipment. These friable pieces are called frit. The word refers to the physical condition of the particle, not its composition or properties. Quenching through water-cooled steel rolls results in flake-like particles, somewhat less friable
than those produced by water quenching but with less environmental nuisance.
Glass coatings are used for such long-duration elevated-temperature applications as aircraft combustion chambers,
turbines and exhaust manifolds, and heat exchangers. Variations in composition of the glass are virtually unlimited. They
range from alkali-alumina borosilicate glasses, which are relatively soft, low melting, and highly fluxed, to barium crown
glasses.
Crystallized glass coatings have been developed. In these coatings, crystallization of the glass is controlled by formulation
and heat treatment and by the presence of nucleating agents added to the glass during melting.
Several different refractory materials may be combined with glass to produce satisfactory coatings for elevatedtemperature
service. The addition of a refractory material depends on service requirements and on the compatibility of the
refractory material with the glass, other mill-added materials, and the substrate metal.

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