Page 680 - Corrosion Engineering Principles and Practice

634 C h a p t e r 1 4 P r o t e c t i v e C o a t i n g s 635

Ion Plating/Ion Beam Enhanced Deposition (IBED)
Advantages include increased adhesion; increased coating density;
decreased coating porosity and prevalence of pinholes; and increased
control of internal stress, morphology, density, and composition.
Disadvantages include high equipment and processing costs; limited
coating thickness; part geometry and size are limited; and gas precursors
used for some implantation species are toxic. This technique can produce
a chromium deposit 10 µm thick with greater thickness attained by
layering. Such thickness is too thin for most hard chrome requirements
(25 to 75 µm with some dimensional restoration work requiring 750 µm)
and layering would significantly add to the cost of the process. IBED
provides some surface cleaning when the surface is initially illuminated
with a flux of high energy inert gas ions; however, the process will still
require precleaning (e.g., degreasing).
Ion Implantation
Ion implantation can be used for any element that can be vaporized and
ionized in a vacuum chamber. Since material is added to the surface,
rather than onto the surface, there is no significant dimensional change
or problems with adhesion. The process is easily controlled, offers high
reliability and reproducibility, requires no posttreatment, and generates
minimal waste. If exposed to high temperatures, however, implanted ions
may diffuse away from the surface due to limited depth of penetration and
penetration does not always withstand severe abrasive wear. Implantation
is used to alter surface properties, such as hardness, friction, wear
resistance, conductance, optical properties, corrosion resistance, and
catalysis. Commercial availability is limited by general unfamiliarity
with the technology, scarcity of equipment, lack of quality control and
assurance, and competition with other surface modification techniques.
Areas of research includes ion implantation of ceramic materials for high
temperature internal combustion engines, glass to reduce infrared radiation
transmission and reduce corrosion, as well as automotive parts (piston
rings, cylinder liners) to reduce wear.
Sputtering and Sputter Deposition
This technique is a versatile process for depositing coatings of metals,
alloys, compounds, and dielectrics on surfaces. The process has been
applied in industrial hard and protective coatings. Primarily TiN, as well as
other nitrides and carbides, has demonstrated high hardness, low porosity,
good chemical inertness, good conductivity, and attractive appearance.
Sputtering is capable of producing dense films, often with near bulk
quantities. Areas requiring future research and development include better
methods for in-situ process control; methods for removing deposited
TiN and other hard, ceramic-like coatings from poorly coated or worn
components without damage to the product; and improved understanding
of the factors the affect film properties.

TABLE 14.7 (continued)