SURFACE TREATMENTS that change the surface chemistry of a metal or alloy, but that do not involve intentional buildup or increase in part dimension, include:

1. Surface Engineering to
Change the Surface
Chemistry
CHAPTER V
1

2. SURFACE TREATMENTS that change the surface chemistry of a
metal or alloy, but that do not involve intentional buildup or
increase in part dimension, include:
 Chemical or electrochemical conversion treatments that produce
complex phosphates, chromates, or oxides on the metal surface.
 Thermochemical diffusion heat treatments that involve the
introduction of interstitial elements, such as carbon, nitrogen, or
boron, into a ferrous alloy surface at elevated temperatures.
 Pack cementation diffusion treatments that involve the
introduction of aluminum, chromium, or silicon into an alloy
surface.
 Surface modification by ion implantation, which involves the
introduction of ionized species (virtually any element) into the
substrate using a beam of high-velocity ions.
 Surface modification by a combination of laser-beam melting and
alloying. 2

3. Phosphate Chemical Conversion Coatings
Phosphate coating is the treatment of iron, steel, galvanized steel,
or aluminum with a dilute solution of phosphoric acid and other
chemicals in which the surface of the metal, reacting chemically
with the phosphoric acid media, is converted to an integral,
mildly protective layer of insoluble crystalline phosphate. The
weight and crystalline structure of the coating and the extent of
penetration of the coating into the base metal can be controlled
by :
 Method of cleaning before treatment .
 Use of activating rinses containing titanium and other metals of
compounds.
 Method of applying the solutionTemperature, concentration,
and duration of treatment.
 Modification of the chemical composition of phosphating
solution
3

4. The method of applying phosphate coatings is usually
determined by the size and shape of the article to be coated.
Small items, such as nuts, bolts, screws, and stampings, are
coated in tumbling barrels immersed in phosphating solution.
Large fabricated articles, such as refrigerator cabinets, are
spray coated with solution while on conveyors. Automobile
bodies are sprayed with or immersed in phosphating solution.
Steel sheet and strip can be passed continuously through the
phosphating solution or can be sprayed.
Phosphate coatings range in thickness from less than 3 to 50
μm (0.1- 2 mils). Coating weight (grams per square meter of
coated area), rather than coating thickness, has been adopted
as the basis for expressing the amount of coating deposited.
4

5. 5
CABINET: Constructed of high quality 12 gauge steel.
BARREL: A six side barrel, constructed of 11 gauge steel. Access to parts loading and unloading is through a removable door, held in place by
cam type clamps for security.
CABINET DOOR: One, full opening, swing type mounted on heavy duty hinges.
CONTROLS: A start button controls reclaimer blower. Blast and barrel rotation is controlled by an adjustable timer for cycle duration. Barrel
rotation is by a fixed speed gear reducer. A jog button is provided to position the barrel for parts unloading. Blast pressure may be adjusted by a
a regulator located on the air header. A safety door switch allows blasting only when the door is closed.
GUNS: The TBS uses stationary production, suction feed style guns. A number five air orifice and carbide nozzle are installed as standard, other
sizes and types are available depending upon compressed air supply (see chart for number of guns and air consumption). All hoses are 1/2" ID,
lightweight, flexible and abrasive resistant. Media / Air blast ration is obtained by a fixed media feed valve.
RECLAIM SYSTEM: High efficiency centrifugal type, with an adjustable air wash and trash screen to trap large debris. A high performance
blower is used to convey media and blasting by-products to the reclaim for separation.
DUST COLLECTION: (see chart for size and type collector).
ELECTRICAL: A 220 or 440 VOLT, 60 HZ, 3 phase, service is required.
PAINT: A durable, corrosion resistant enamel, gray in color, is used internally and externally

6. 6
MODEL
#
MODEL
BAR.
SIZE
BAR.
CAP
CU
FT
BARREL
DOOR
OPEN
MAX
LOAD
MAX
RPM
DRIVE
HP
REC.
SIZE
DUST
BAG
DUST
CLT.
NOZ.
SIZE
NO.
GUNS
REQ
AIR
@ 80
PSI
ELECT.
AIR LINE
SIZE
CAB
SIZE
007526
TBS
1 - 7
12 W
24 D
1
12 W
10 D
125
LBS
4 1 / 2 700 11-715 5 2
66
SCFM
120
1 PH
1"
26 W
30 D
71 H
007527
1 - 7
DC
12 W
24 D
1
12 W
10 D
125
LBS
4 1 / 2 700 D10 5 2
66
SCFM
120
1 PH
1"
26 W
30 D
71 H
007535
TBS
2 - 7
24 W
24 D
2
24 W
10 D
225
LBS
4 1 / 2 700 11-715 5 4
132
SCFM
120
1 PH
1"
38 W
36 D
71 H
007536
2 - 7
DC
24 W
24 D
2
24 W
10 D
225
LBS
4 1 / 2 700 D10 5 4
132
SCFM
120
1 PH
1"
38 W
36 D
71 H
007540 2 - 9
24 W
24 D
2
24 W
10 D
225
LBS
4 1 / 2 900
(2)
11-715
5 4
132
SCFM
230
460
3 PH
1"
38 W
36 D
71 H
007541
2 - 9
DC
24 W
24 D
2
24 W
10 D
225
LBS
4 1 / 2 900 D20 5 4
132
SCFM
230
460
3 PH
1"
38 W
36 D
71 H
007546
TBS
3 - 9
36 W
24 D
3
36 W
10 D
325
LBS
4 1 / 2 900
(2)
11-715
5 6
198
SCFM
230
460
3 PH
1 1/2"
38 W
36 D
71 H
007547
3 - 9
DC
36 W
24 D
3
36 W
10 D
325
LBS
4 1 / 2 900 D20 5 6
198
SCFM
230
460
3 PH
1 1/2"
38 W
36 D
71
TUMBLE BARREL SYSTEMS

7. 7

8. 8

9. 9
Types of Phosphate Coatings
Three principal types of phosphate coatings are in general use: zinc, iron, and
manganese.
Zinc phosphate coatings encompass a wide range of weights and crystal
characteristics, ranging from heavy films with coarse crystals to ultrathin
microcrystalline deposits. Zinc phosphate coatings vary from light to dark gray in
color. Coatings are darker as the carbon content of the underlying steel increases,
as the ferrous content of the coating increases, as heavy metal ions are
incorporated into the phosphating solution, or as the substrate metal is acid
pickled prior to phosphating. Zinc phosphating solutions containing active
oxidizers usually produce lighter-colored coatings than do solutions using milder
accelerators.
Zinc phosphate coatings can be applied by spray, immersion, or a combination of
the two. Coatings can be used for any of the following applications of
phosphating: base for paint or oil; aid to cold forming, tube drawing, and wire
drawing; increasing wear resistance; or rustproofing. Spray coatings on steel
surfaces range in weight from 1.08 to 10.8 g/m2 (3.5 x 10~3 to 3.5 x 10-2 oz/ft2);
immersion coatings, from 1.61 to 43.0 g/m2 (5.28 x 10~3 to 1.41 oz/ft2).

10. 10
Iron phosphate coatings were the first to be used commercially. Early iron
phosphating solutions consisted of ferrous phosphate/phosphoric acid used at
temperatures near boiling and produced dark gray coatings with coarse crystals.
The term iron phosphate coatings refers to coatings resulting from alkali-metal
phosphate solutions operated at pH in the range of 4.0 to 5.0, which produce
exceedingly fine crystals. The solutions produce an amorphous coating consisting
primarily of iron oxides and having an interference color range of iridescent blue
to reddish-blue color. A typical formulation for an iron phosphate bath is:
Component Composition, %
Phosphate salts 12-15
Phosphoric acid 3-4
Molybdate accelerator 0.25-0.50
Detergents (anionic/nonionic) 8-10
Basically, then, iron phosphate formulations consist of primary phosphate salts
and accelerators dissolved in a phosphoric acid solution. It is the acid that
initiates the formation of a coating on a metal surface. When acid attacks the
metal and begins to be consumed, solution pH at the metal surface rises slightly.
This is what causes the primary phosphate salts to drop out of solution and react
with the metal surface, forming a crystalline coating.

11. 11
Although iron phosphate coatings are applied to steel to provide a
receptive surface for the bonding of fabric, wood, and other materials,
their chief application is as a base for subsequent films of paint.
Processes that produce iron phosphate coatings are also available for
treatment of galvanized and aluminum surfaces. Iron phosphate coatings
have excellent adherence and provide good resistance to flaking from
impact or flexing when painted. Corrosion resistance, either through
film or scribe undercut, is usually less than that attained with zinc
phosphate. However, a good iron phosphate coating often outperforms a
poor zinc phosphate coating.
Spray application of iron phosphate coatings is most frequently used,
although immersion application also is practical. The accepted range of
coating weights is 0.21 to 0.86 g/m2 (6.9 X 10"4 to 0.26 oz/ft2). Little
benefit is derived from exceeding this range, and coatings of less than
0.21 g/m2 (6.9 X 10~4 oz/ft2) are likely to be nonuniform or
discontinuous.
Quality iron phosphate coatings are routinely deposited at temperatures
from 25 to 65 0C (80-150 0F) by either spray or immersion methods.

12. 12
Manganese phosphate coatings are applied to ferrous parts (bearings,
gears, and internal combustion engine parts, for example) for break-in and
to prevent galling. These coatings are usually dark gray. However, because
almost all manganese phosphate coatings are used as an oil base and the
oil intensifies the coloring, manganese phosphate coatings are usually
black in appearance. In some instances, a calcium-modified zinc
phosphate coating can be substituted for manganese phosphate to impart
break-in and antigalling properties.
Manganese phosphate coatings are applied only by immersion, requiring
times ranging from 5 to 30 min. Coating weights normally vary from 5.4
to 32.3 g/m2 (1.8 x 10~2 to 9.83 oz/ft2), but can be greater if required.
The manganese phosphate coating usually preferred is tight and fine grain,
rather than loose and coarse-grain. However, desired crystal size varies
with service requirements. In many instances, the crystal is refined as the
result of some pretreatment (certain types or cleaners and/or conditioning
agents based on manganese phosphate) of the metal surface. Manganese-
iron phosphate coatings are usually formed from high-temperature baths
from 90 to 95 0C (190-200 0F).

13. 13
Applications
On the basis of pounds of chemicals consumed or tons of
steel treated, the greatest use of phosphate coatings is as a
base for paint. Phosphate coatings are also used to provide:
 A base for oil or other rust-preventive material.
 Lubricity and resistance to wear, galling, or scoring of
parts moving in contact, with or without oil.
 A surface that facilitates cold forming.
 Temporary or short-time resistance to mild corrosion.
 A base for adhesives in plastic-metal laminations or
rubber-to-metal applications.

14. 14
Corrosion Protection. Conversion of a metal surface to an insoluble
phosphate coating provides a metal with a physical barrier against
moisture.
The degree of corrosion protection that phosphate coatings impart to
surfaces of ferrous metals depends on uniformity of coating coverage,
coating thickness, density, and crystal size, and the type of final seal
employed. Coatings can be produced with a wide range of thicknesses,
depending on the method of cleaning before treatment, composition of the
phosphating solution, temperature, and duration of treatment. In
phosphating, no electric current is used, and formation of the coating
depends primarily on contact between the phosphating solution and the
metal surface and on the temperature of the solution. Consequently,
uniform coatings are produced on irregularly shaped articles, in recessed
areas, and on threaded and flat surfaces, because of the chemical nature of
the coating process.

15. 15
Medium to heavy zinc phosphate coatings, and occasionally,
heavy manganese phosphate coatings are used for corrosion
resistance when supplemented by an oil or wax coating. Zinc
phosphate plus oil or wax is usually used to treat cast, forged,
and hot-rolled steel nuts, bolts, screws, cartridge clips, and many
similar items. Manganese phosphate plus oil or wax is also used
on cast iron and steel parts.
The affinity of heavy phosphate coatings for oil or wax is used
to increase the corrosion resistance of these coatings.
Frequently, phosphate coated articles are finished by a dip in
nondrying or drying oils that contain corrosion inhibitors. The
articles are then drained or centrifuged to remove the excess
oil.

16. 16
Phosphate Coating as an Aid in Forming Steel. The contact
pressure used in deep-drawing operations sets up a great
amount of friction between the steel surface and the die. The
phosphate coating of steel as a metalforming lubricant, before it
is drawn:
 Reduces friction.
 Increases speed of the drawing operations.
 Reduces consumption of power.
 Increases the life of tools and dies.

17. 17
Wear Resistance. Phosphating is a widely used method of
reducing wear on machine elements. The ability of phosphate
coating to reduce wear depends on uniformity of the phosphate
coating, penetration of the coating in to metal, and affinity of the
coating for oil. A phosphate coating permits new parts to be
broken in rapidly by permitting retention of an adequate film of
oil on surfaces at that critical time. In addition, the phosphate
coating itself functions as a lubricant during the high stress of
break-in.
Heavy manganese phosphate coatings (10.8 to 43.0 g/m2, or 3.5 x
10~2 to 0.14 oz/ft2), supplemented with proper lubrication, are
used for wear resistance applications. Parts that are manganese
phosphate coated for wear resistance are listed in Table 1.

18. 18
Table 1 Parts immersion coated with manganese phosphate for wear resistance
Part(a) Material
Components for small Cast iron or steel; forged steel 15-30 Oils, waxes
arms, threaded fasteners(b)
Bearing races High-alloy steel forgings or bar stock 7-15 Oils, colloidal graphite
Valve tappets, camshafts Low-alloy steel forgings or bar stock 7-15 Oils, colloidal graphite
Piston rings Forged steel, cast iron 15-30 Oils
Gears(c) Forged steel, cast iron 15-30 Oils
(a) Coating weights range from 10.8 to 43.0 g/m2 (3.5 X 10~2 to 0.14 oz/ft2). (b) Coating may be applied by barrel tumbling, (c) Coat-ing weights range from 5.4 to 43.0 g/m2 (1.8 X 10" 2 to 0.14 oz/ft2).
Coating time, Supplementary Coatings
(a) Coating weights range from 10.8 to 43.0 g/m2 (3.5 x 10~2 to 0.14 oz/ft2). (b) Coating may be applied
by barrel tumbling, (c) Coating weights range from 5.4 to 43.0 g/m2 (1.8 x 10-2 to 0.14 oz/ft2).

19. 19
When two parts, manganese phosphated to reduce friction by
providing lubricity, are put into service in contact with each other, the
manganese coating is smeared between the parts. The coating acts as
a buffer to prevent galling or, on heavily loaded gears, welding. The
phosphate coating need not stand up for an extended length of time,
because it is in initial movements that parts can be damaged and
require lubricity. For example, scoring of the mating surfaces of gears
usually takes place in the first few revolutions. During this time, the
phosphate coating prevents close contact of the faces. As the coating
is broken down in operation, some of it is packed into pits or small
cavities formed in gear surfaces by the etching action of the acid
during phosphating.
Long after break-in, the material packed into the pits or coating
that was originally formed in the pits prevents direct contact of mating
surfaces of gear teeth. In addition, it acts as a minute reservoir for oil,
providing continuing lubrication. As work hardening of the gear
surfaces takes place, the coating and the etched area may disappear
completely, but by this time scoring is unlikely to occur.

20. 20
Chromate Chemical Conversion Coatings
Chromate conversion coatings are formed by a chemical or an
electrochemical
treatment of metals or metallic coatings in solutions containing
hexavalent chromium (Cr6+) and, usually, other components. The
process results in the formation of an amorphous protective coating
composed of the substrate, complex chromium compounds, and
other components of the processing bath.
Chromate conversion coatings are applied primarily to enhance bare
or painted corrosion resistance, to improve the adhesion of paint or
other organic finishes, and to provide the metallic surface with a
decorative finish. Chromating processes are widely used to finish
aluminum, zinc, steel, magnesium, cadmium, copper, tin, nickel,
silver, and other substrates. Chromate conversion coatings are most
frequently applied by immersion or spraying, but other methods of
application, such as brushing, roll coating, dip and squeegee,
electrostatic spraying, or anodic deposition, are used in special
cases.

21. 21
Processing Steps. Chromate coatings are applied by contacting
the processed surfaces with a sequence of processing solutions.
The processing baths are arranged in a series of tanks, and the
surfaces to processed are transferred through the sequence of
stages by using manual, semiautomatic, or automatic control. The
chromate coatings are usually applied to metal parts or to a
continuous metal strip running at speeds to 5 m/s (lOOOft/min).
The basic processing sequence consists of the following six
steps: cleaning, rinsing, conversion coating rinsing, posttreatment
rinsing or decorative color rinsing, and drying. In many
applications, this sequence is expanded to accommodate pickling,
deoxidizing, dyeing, brightening, and other rinsing stages, or the
sequence can be shortened when cleaning or posttreatment
rinsing is not necessary. More detailed information on key
processing steps can be found in Ref 2.

22. 22
Corrosion Protection. Chromate conversion coatings provide excellent
bare or painted corrosion protection to the metal. The level of protection
depends on the substrate metal, the type of chromate coating used, and the
chromium coating weight. In unpainted applications, corrosion protection
for the different conversion coatings generally increases with coating
weight, and the upper limit of the coating weight is determined by the
process limitations or by the color requirement. Table 2 lists typical
corrosion data measured by the ASTM B 117 method. In painted
applications, the conversion coating must improve corrosion resistance
and provide for good paint adhesion. The upper limit of the coating
weight for the painted surfaces is normally defined by the onset of weaker
paint adhesion or of corrosion problems related to paint delamination.
Opinions differ widely regarding the mechanism of corrosion protection
provided by the chromate coatings. The most widely advanced concepts
suggest that the chromate coatings provide a barrier insulation from the
environment and inhibit the cathodic corrosion reactions.

23. 23
Substrate coating
Time to corrosion
stain, h
Electroplated zinc Untreated <4
Clear 24-48
Iridescent 100-200
Olive drab 100-400
Electrolytic 1000
Hot-dip zinc Untreated <4
Clear passivate 24-100
Aluminum alloy 3003 Untreated <24
Clear 60-120
Yellow-brown 250-800
Table 2 Typical salt-spray data for chromate coatings on zinc and
aluminum

24. 24
Hardness and Abrasion Resistance. The hardness of chromate
coatings depends strongly on the temperature during chromating and
drying.
Freshly made wet films are very soft and can be easily damaged by
abrasion. After drying, the films develop good hardness, which allows
for safe handling. However, even the dry films are susceptible to severe
scratching or abrasion.
Health and Safety Considerations. The disposal of spent solutions
and rinse waters requires waste treatment. Hexavalent chromium must
be reduced to Cr3+ before neutralizing and precipitation. Sodium
pyrosulfite (Na2S2O5) is usually used as the reducing agent in smaller
operations, while for larger plants, sulfur dioxide (SO2) is preferred for
economic reasons. Wastewater treatment sludges from chromating
operations are considered hazardous waste. As a result, the use and
disposal of chromium and chromium compounds have received much
regulatory attention because of the toxicity of chromium and
indications that it is a cancer-causing agent.

25. 25
Due to worker health and safety concerns, alternatives to chromate
conversion coatings are being sought. Unfortunately, there are
currently no drop-in substitutes to chromate conversion coatings
that adequately match their corrosion resistance, paint adhesion,
and so forth. Possible elimination of chromate conversion coatings
due to regulatory restrictions is particularly troublesome for the
aircraft industry. In applying the coating to the entire aircraft
aluminum structure, the subsequent rinse process can generate
large quantities of chromium-containing wastes. The challenges
of adequately maintaining aging aircraft will help drive the search
for effective chromate substitutes.

26. 26
Aluminum Anodizing
Aluminum anodizing is an electrochemical method of converting
aluminum into aluminum oxide (Al2O3) at the surface of the item being
coated. It is accomplished by making the workpiece the anode while
suspended in a suitable electrolytic cell. Although several metals can be
anodized, including aluminum, titanium, and magnesium, only aluminum
anodizing has found widespread use in industry. A more detailed
discussion
on anodizing can be found in Ref 3.
Because a wide variety of coating properties can be produced
through variations in the process, anodizing is used in almost every
industry in which aluminum can be used. The broadest classification of
types of anodize is according to the acid electrolyte used. Various acids
have been used to produce anodic coatings, but the most common ones in
current use are sulfuric (H2SO4) and chromic (CrO3) acids. There are
two types of H2SO4 anodizing. The first is a room-temperature H2SO4
process termed conventional anodizing, and the second is a low-
Temperature H2SO4 process termed hardcoat anodizing. In addition to
CrO3, conventional, and hardcoat anodizing, a process known as sealing
can be used to enhance certain characteristics.

27. 27
The three common types of anodize described above are usually
controlled and described through the use of military specification MIL-A-
8625 (Table 3). It has become standard in the industry to describe
anodic coatings with the type and class nomenclature outlined in this
specification.

28. 28
Chromic Anodizing
The CrO3 anodizing process produces a coating that is nominally 2 μm
(0.08 mil) thick. It is relatively soft and susceptible to damage through
abrasion or handling. The color of the class 1 coating ranges from clear
to gray, depending on whether the coating is sealed and on the alloy
coated.
The coating can be dyed to produce a class 2 coating; however, this is
not generally done, because the coating is thin and does not retain the
dye color well. About two-thirds of the coating thickness penetrates the
base metal; one-third of the coating builds above the original base metal
dimension.
Thus, for a coating thickness of 2 μm (0.08 mil) per side, the dimensional
change of the workpiece would be 0.7 μm (0.028 mil) per side.

29. 29
Although the industry has adopted the penetration/buildup terminology,
the terms are somewhat misleading. Actually, when the aluminum is
converted to Al2O3 it takes up more space—approximately 133% of
the space previously occupied by the aluminum converted. The
penetration/ buildup terms are used only as a convenience in predicting
dimensional change in a coated article. The corrosion resistance of this
coating is very good. The coating will pass in excess of 336 h in 5%
salt (NaCl) spray per ASTM B 117.
Advantages and Uses. Although CrO3 anodizing is the least used of
the three types of anodize, it has several advantages that make its use
desirable.
First, because CrO3 is much less aggressive toward aluminum
than H2SO4, it should be used whenever part design is such that rinsing
is difficult. Difficult rinsing designs would include welded assemblies,
riveted assemblies, and porous castings.
.

30. 30
Second, a typical CrO3 anodize buildup is 0.7 μm (0.028 mil) per side
with good repeatability. Therefore, it is a very good coating to use when
it is necessary to coat a precise dimension to size.
Third, because CrO3 anodize produces the least reduction in fatigue
strength of the three coatings, it should be used where fatigue strength
is a critical factor. Fourth, the color of CrO3 anodize will change with
different alloy compositions and the heat treat conditions; this makes it
useful as a test of the homogeneity of structural components. Lastly,
when properly applied, CrO3 anodize can be used as a mask for
subsequent hardcoat anodize operations
Suitable Alloys. Most alloys can be successfully coated by the
CrO3 process. Exceptions are high-silicon die-cast alloys and high-
copper alloys.
The rule for suitability is that any alloy containing more than 5% Cu,
7% Si, or total alloying elements of 7.5% should not be coated by
this process.
Relative Costs. Chromic anodize costs more than H2SO4 but less
than hardcoat anodize.

31. 31
SuIfuric Anodizing
The H2SO4 process produces a coating that is normally 8 μm (0.31 mil)
in minimum thickness. Although harder than type 1 coatings, H2SO4
anodize may still be damaged by moderate handling or abrasion. The
color of the class 1 coating is yellow-green because of the preferred
sealing method of immersion in sodium dichromate (Na2Cr2O7). Clear
coatings can also be produced by sealing in hot water. Clear coatings
should be specified by the notation "class 1, clear."
This coating can also be dyed to produce a class 2 coating. This type of
anodize produces the most pleasing colors of the three anodizing
methods. Dyed H2SO4 anodize coatings have deep colors with good
repeatability. Like CrO3 anodize, H2SO4 anodize coatings penetrate the
base metal for two-thirds of their thickness and build above the original
base metal dimension for one-third the total thickness. As with all types of
anodize, the corrosion resistance of H2SO4 anodize is very good; it has
an ASTM B 117 salt-spray resistance of at least 336 h.

32. 32
of anodize and has many desirable benefits. First, because it has a
fairly hard surface, it can be used in situations that require light to
moderate wear resistance. Applications include lubricated sliding
assemblies and items subject to handling wear, such as front panels.
Second, because it is the most aesthetically pleasing type of anodize, it
should be used where final appearance is important. It can be dyed
almost any color and produces deep, rich shades that make the item
appear to be made of a material bearing a color throughout, rather than
an applied coating. Lastly, because corrosion resistance is good, it
should be used whenever corrosion resistance is needed and the
specialized benefits of the other two anodize types are not required.
Suitable Alloys. With the exception of high-silicon die-cast alloys, all
alloys can be successfully coated with H2SO4 anodize. Clarity and
depth of color of the anodize increase with the purity of the alloy.
Therefore, alloys should be chosen for maximum purity consistent with
the physical requirements needed in the item.
Relative Costs. Sulfuric anodize is the least costly and most widely
available type of anodize.

33. 33
Hardcoat Anodizing
The hardcoat anodize process produces a coating that is normally 50
μm (2 mils) thick, although other thicknesses can be specified. The
coating is extremely hard. It is described as file hard (equal to about 60-
70 HRC). The color of the class 1 coating ranges from gray to bronze to
almost black, depending on the alloy coated, the coating thickness, and
the electrolyte temperature. The coating can be dyed to produce a class
2 coating. Because thick coatings are naturally very dark, only colors
darker than natural are possible. Generally, this limits the dying of
hardcoat to black in common processes. If a more extensive color
choice is required, there are several proprietary hardcoat processes
available to accomplish this. Hardcoat penetrates the base metal for
one-half of its thickness and builds above the original base metal
dimension for one-half of its thickness. Thus, for a thickness of 50 μm (2
mils) per side, the dimensional change of the workpiece would be 25 |jim
(1 mil) per side. Commercially available coating thickness tolerances are
the greater of ±5 |mm or ± 10% of the total targeted thickness. The
corrosion resistance of the unsealed class 1 coating is very good and
comparable to the other types of anodize. When the hardcoat anodize is
sealed, as in a class 2 coating, it becomes the most corrosion-resistant
type of anodize.

34. 34
Advantages and Uses. Hardcoat anodize, because of its variety of
desirable properties, has found widespread use in manufactured
products.
First, because of it extreme hardness, it is used in situations in which
wear resistance is required. Applications include valve/piston assemblies,
drive belt pulleys, tool holders and fixtures, and many other items
requiring wear resistance.
Second, because of its excellent resistance to corrosion, hardcoat is used
on aluminum components in harsh environments. These include outside
exposure in salt air, marine components, automobile wash equipment,
components for the aircraft and aerospace industries, and food
preparation machines.
Third, because hardcoat is an excellent electrical resistor, it can be used
to insulate heat sinks for direct mounting of electrical or electronic
equipment.
Also, it is used in welding fixtures where some areas may need to
be insulated from work.

35. 35
Fourth, because hardcoat is a naturally porous substance, it is used in
many areas in which the bonding or impregnation of other materials to
aluminum is needed. This coating bonds very well with paints and
adhesives. Also, it can be impregnated with teflon (polytetrafluoroethylene,
or PTFE) and many dry film lubricants to impart lubricating properties to
the coating. Lastly, because of its desirable properties and also because it
produces a buildup of coating, it is widely accepted as a salvage coating to
restore worn or improperly machined parts to usable dimensions. Coating
thicknesses in excess of 250 μm (10 mils) per side are possible on some
alloys with certain proprietary hardcoat processes.
Suitable Alloys. Although almost all alloys can be coated, the 6000-
series aluminum alloys produce the best hardcoat properties. As with the
other anodize types, high-silicon die castings produce the lowest-quality
coatings. Also, because the hardcoat process is sensitive to copper,
alloys in the 2000 series should be avoided if possible. Alloys containing
copper can be hardcoated, but only a relatively few commercial sources
have the ability to coat these alloys with reliability.

36. 36
Relative Costs. Hardcoat anodize is the most expensive type of anodize.
It is generally twice the cost of H2SO4 anodize and 50% more than
CrO3 anodize.
Sealing ofAnodized Coatings
Because all of the anodic processes produce porous Al2O3 coatings, it is
often desirable to seal the coating to close these pores and to eliminate
the path between the aluminum and the environment. Sealing involves
immersing the coating in hot water; this hydrates the Al2O3 and causes
the coating to swell in order to close the pores. Conventional sealing is
generally done at a minimum temperature of 95 oC (200 oF) for not less
than 15 min. There are also several proprietary nickel-base sealing
agents available that area said to produce sealing at low temperature
through catalytic action. Chromic and sulfuric anodizes are almost always
sealed. However, because sealing softens the coating somewhat,
hardcoat anodize is usually not sealed unless criteria other than hardness
have the maximum importance in the finished coating.

37. 37
Corrosion Resistance of Anodized Aluminum
In general, corrosion resistance of anodic coatings is greatest in
approximately neutral solutions, but such coatings are usually serviceable
and protective if the pH is between 4 and 8.5. More acidic and more
alkaline solutions attack anodic coatings.
Under atmospheric weathering, the number of pits developed in the base
metal decreases exponentially with increasing coating thickness (Fig. 1).
The pits may form at minute discontinuities or voids in the coating, some
of which result from large second-phase particles in the microstructure.
The pit density was determined by dissolving the anodic coating in a
stripping solution that does not attack the metal substrate. After the 8 1/2-
year exposure, the pits were of pinpoint size and had penetrated less than
50 μm (2.0 mils). Specimens with coatings at least 22 μm (0.9 mil) thick
were practically free of pitting. Weathering of anodic coatings involves
relatively uniform erosion of the coating by windborne solid particles,
rainfall, and some chemical reaction with pollutants. The available
information indicates that such erosion occurs at a reasonably constant
rate, which averaged 0.33 jxm/yr (0.013 mil/yr) for several alloys exposed
to an industrial atmosphere for 18 years (Fig. 2).

38. 38

39. 39
A three-year seacoast exposure of specimens of several alloys with 23 μm (0.9 mil)
thick sulfuric acid coatings caused no visible pitting except in several alloys of the
7xxx series and in a 2xxx alloy (Table 4). Alloys that exhibited pitting were not
protected any more effectively by 50 μm (2 mils) thick coatings. This confirms a
general observation that optimal protection against atmospheric corrosion is
achieved in the coating thickness range of 18 to 30 |xm (0.7-1.2 mils) and that
thicker coatings provide little additional protection.

40. 40

41. 41
Oxidation Treatments
Tool Steels. Oxidation is a well-established process used for high-
speed steel cutting tools. Increases in tool life of up to 100% are
achieved, mostly due to a decrease in friction, because of the hard
oxide coating and the ability of the porous oxide to entrap lubricant
and draw it to the tool/workpiece interface. Steam oxidation of a
finished tool is accomplished either by exposing it to steam at a
temperature of about 565 oC (1050 oF) or by treating it in liquid
sodium hydroxide and sodium nitrate salts at approximately 140 0C
(285 0F) for 5 to 20 min. These treatments result in a black oxidized
layer that is less than 5 |xm (0.2 mil) thick and will not peel, chip, or
crack, even when the tool is bent or cut. Tool life improvements due to
steam oxidation are listed in Table 5.

42. 42

43. 43
Steam Treating of Powder Metallurgy (P/M) Steels. Many P/M parts
have traditionally been steam treated for improved wear resistance,
corrosion resistance, and sealing capacity. In this process, P/M parts are
heated in a specific manner under a steam atmosphere at temperatures
between 510 and 570 0C (950 and 1060 0F) to form a layer of black iron
oxide, identified as magnetite, in the surface porosity. Magnetite has a
hardness equivalent to 50 HRC.
Spalling or flaking of the surface oxide layer can occur if the process
temperature exceeds 570 0C (1060 0F) and process times exceed 4 h.
The maximum thickness of the surface oxide layer should not exceed 7
μm (0.28 mil). Beyond this thickness, flaking can occur due to an
increase in surface tensile stress.
Sintered density is an important consideration when applying steam
treating for improved strength and hardness. Its ability to increase the
wear resistance of the substrate material depends on the available
porosity for oxidation. As density is increased, the amount of oxide
formed is decreased, which minimizes the improvement in apparent
hardness attributed to steam treating.

44. 44
This is shown in Fig. 3 for sintered steel. The increase in density and
apparent hardness produced by steam treating is illustrated in the
micrograph of a sintered steel (Fig. 4). By filling the porosity with a hard
second phase, the P/M steel offers a better support to the indentation
hardness tester. Figure 5 illustrates that the transverse rupture strength is
increased significantly by steam treatment for low-carbon P/M steels, but
only modestly for high-carbon (0.8% C) P/M steels.

45. 45

46. 46

47. 47
The diffusion coatings described in this section involve heat treating
processes that cause carbon, nitrogen, or a combination of the two to
diffuse into the surface of a ferrous part to alter the surface
chemistry/properties. As listed in Table 6, these processes include
carburizing, nitriding, and carbonitriding. Each of these depends on the
concentration gradient of the diffusing species, the diffusivity of the atomic
species in the host material, and the time and temperature at which the
process takes place.
All carburizing and nitriding processes increase the surface carbon or
nitrogen content of the alloy to allow the surface to respond to quench
hardening. The heat treater usually relies on empirical data to determine
how long to expose the part to achieve the desired carbon or nitrogen
diffusion. The term used for the entire field of surface-hardening
processes is case hardening, and the case indicates the depth of
hardening below the surface. Although the depth of hardening decreases
gradually because the diffused species does not stop abruptly, the
effective case depth is considered to be the depth at which the hardness
falls below 50 HRC. More detailed information on the case-hardening
procedures described in this section can be found in Heat Treating
Diffusion Heat Treatment Coatings

48. 48

49. 49

50. 50
Carburizing
Carburizing is the addition of carbon to the surface of low-carbon steels
at temperatures (generally between 850 and 950 0C, or 1560 and 1740
0F) at which austenite, with its high solubility for carbon, is the stable
crystal structure. Hardening of the component is accomplished by
removing the part and quenching or allowing the part to slowly cool and
then reheating to the austenitizing temperature to maintain the very
hard surface property.
On quenching, a good wear- and fatigue-resistant high-carbon
martensitic case is superimposed on a tough, low-carbon steel core.
Carburized steels used in case hardening usually have base carbon
contents of about 0.2 wt%, with the carbon content of the carburized
layer being fixed between 0.8 and 1.0 wt% (Ref 8)

51. 51
Carburizing methods include gas carburizing, vacuum carburizing,
plasma (ion) carburizing, salt-bath carburizing, and pack carburizing.
These methods introduce carbon by use of an atmosphere
(atmospheric gas, plasma, and vacuum), liquids (salt bath), or solid
compounds (pack). The vast majority of carburized parts are
processed by gas carburizing, using natural gas, propane, or butane.
Vacuum and plasma carburizing are useful because of the absence of
oxygen in the furnace atmosphere. Salt-bath and pack carburizing
have little commercial importance, but are still done occasionally.

52. 52
Gas carburizing
can be run as a batch or a continuous process. Furnace atmospheres
consist of a carrier gas and an enriching gas. The carrier gas is
supplied at a high flow rate to ensure a positive furnace pressure,
minimizing air entry into the furnace. The type of carrier gas affects
the rate of carburization. Carburization by methane is slower than by
the decomposition of CO. The enriching gas provides the source of
carbon and is supplied at a rate necessary to satisfy the carbon
demand of the work load.
Most gas carburizing is done under conditions of controlled carbon
potential by measurement of the CO and CO2 content.

53. 53
The objective of the control is to maintain a constant carbon potential by
matching the loss in carbon to the workpiece with the supply of enriching
gas. The carburization process is complex, and a comprehensive model
of carburization requires algorithms that describe the various steps in the
process, including carbon diffusion, kinetics of the surface reaction,
kinetics of the reaction between the endogas and enriching gas, purging
(for batch processes), and the atmospheric control system. Possible
models of each of these steps have been outlined (Ref 9).
Vacuum carburizing is a nonequilibrium, boost-diffusion-type
carburizing process in which austenitizing takes place in a rough
vacuum, followed by carburization in a partial pressure of hydrocarbon
gas, diffusion in a rough vacuum, and then quenching in either oil or gas
(Ref 10). Vacuum carburizing offers the advantages of excellent
uniformity and reproducibility because of the improved process control
with vacuum furnaces, improved mechanical properties due to the lack
of intergranular oxidation, and reduced cycle time. The disadvantages
of vacuum carburizing are predominantly related to equipment costs
and throughput.

54. 54
Plasma (ion) carburizing is basically a vacuum process utilizing
glowdischarge technology to introduce carbon-bearing ions to the steel
surface for subsequent diffusion (Ref 11). This process is effective in
increasing carburization rates because the process bypasses several
dissociation steps that produce active soluble carbon. For example,
because of the ionizing effect of the plasmas, active carbon for
adsorption can be formed directly from methane (CH4) gas. High
temperatures can be used in plasma carburizing because the process
takes place in an oxygen-free vacuum, thus producing a greater
carburized case depth than both atmospheric gas and vacuum
carburizing (Fig. 6).

55. 55

56. 56
Nitriding
Nitriding is a process similar to carburizing, in which nitrogen is diffused
into the surface of a ferrous product to produce a hard case. Unlike
carburizing, nitrogen is introduced between 500 and 550 0C (930 and
1020 0F), which is below the austenite formation temperature (Ac1) for
ferritic steels, and quenching is not required. As a result of not
austenitizing
and quenching to form martensite, nitriding results in minimum distortion
and excellent control. The various nitriding processes (Table 6) include
gas nitriding, liquid nitriding, and plasma (ion) nitriding.
All hardenable steels must be quenched and tempered prior to nitriding.
The nitriding process is used to obtain a high surface hardness, improve
wear resistance, increase fatigue resistance, and improve corrosion resist
taining a diffusion zone with or without a compound zone (Fig. 7),
depends on the type and concentration of alloying elements and the
timetemperature exposure of a particular nitriding treatment (Ref 13).
The diffusion zone is the original core microstructure with the addition of
nitride precipitates and nitrogen solid solution.

57. 57
The compound zone is the region where y' (Fe4N) and 8 (Fe23N)
intermetallics are formed. Commercial steels containing aluminum,
chromium, vanadium, tungsten, and molybdenum are most suitable for
nitriding because they readily form nitrides that are stable at the nitriding
temperatures (Ref 14). The following steels can be nitrided for specific
applications:
• Aluminum-containing low-alloy steels: Nitralloys
• Medium-carbon, chromium-containing low-alloy steels: 4100, 4300,
5100, 6100, 8600, 8700, and 9800 series
• Low-carbon, chromium-containing low-alloy steels: 3300, 8600, and
9300 series
• Hot-working die steels containing 5% Cr: HIl, H12, and Hl3
• Air-hardenable tool steels: A2, A6, D2, D3, and S7
• High-speed tool steels: M2 and M4
• Nitronic stainless steels: 30, 40, 50, and 60
• Ferritic and martensitic stainless steels: 400 series
• Austenitic stainless steels: 200 and 300 series
• Precipitation-hardened stainless steels: 13-8 PH, 15-5 PH, 17-4 PH,
17-7 PH, A-286, AM 350 (Ref 14), and AM 355

58. 58
Gas nitriding (Ref 14) is a case-hardening process that takes place
in the presence of ammonia gas. Either a single-stage or a double-
stage process can be used when nitriding with anhydrous ammonia.
The singlestage process, in which a temperature of 495 to 525 oC
(925-975 oF) is used, produces the brittle nitrogen-rich compound
zone known as the white nitride layer at the surface of the nitrided
case. The double-stage process, or Floe process, has the advantage
of reducing the white nitrided layer thickness. After the first stage, a
second stage is added which either by continuing at the first-stage
temperature or increasing the temperature to 550 to 565 oC (1025-
1050 oF). The use of the higher-temperature second stage lowers
the case hardness and increases the case depth.

59. 59
Liquid nitriding (nitriding in a molten salt bath) uses similar temperatures as
in gas nitriding and a case-hardening medium of molten, nitrogenbearing, fused-
salt bath containing either cyanides or cyanates (Ref 15). Similar to salt-bath
carburizing, liquid nitriding has the advantage of processing finished parts
because dimensional stability can be maintained due to the subcritical
temperatures used in the process. Furthermore, at the lower nitriding
temperatures, liquid nitriding adds more nitrogen and less carbon to ferrous
materials than that obtained with high-temperature treatments because ferrite
has a much greater solubility for nitrogen (0.4% max) than carbon (0.02% max)
Plasma (ion) nitriding is a method of surface hardening using glow-
discharge technology to introduce nascent (elemental) nitrogen to the
surface of a metal part for subsequent diffusion into the material (Ref
13). The process is similar to plasma carburizing in that a plasma is
formed in a vacuum using high-voltage electrical energy and the
nitrogen ions are accelerated toward the workpiece. The ion
bombardment heats the part, cleans the surface, and provides active
nitrogen. The process provides better control of case chemistry, case
uniformity, and lower part distortion than gas nitriding. Properties of
plasma nitrided ferrous alloys are listed in Table 7.

60. 60
Carbonitriding and Ferritic Nitrocarburizing
Carbonitriding introduces both carbon and nitrogen into the austenite of
the steel. The process is similar to carburizing in that the austenite
composition is enhanced and the high surface hardness is produced by
quenching to form martensite. This process is a modified form of gas
carburizing in which ammonia is introduced into the gas-carburizing
atmosphere (Ref 16). As in gas nitriding, elemental nitrogen forms at the
workpiece surface and diffuses along with carbon into the steel.

61. 61
Typically, carbonitriding takes place at a lower temperature and a shorter
time than gas carburizing, producing a shallower case. Steels with carbon
contents up to 0.2% are commonly carbonitrided; these include 1000,
1100, 1200, 1300, 1500, 4000, 4100, 4600, 5100, 6100, 8600, and 8700
series.
Ferritic nitrocarburizing is a subcritical heat treatment process, carried
out by either gaseous or plasma techniques, and involves the diffusion of
carbon and nitrogen into the ferritic phase. The process results in the
formation of a thin white layer or compound layer, with an underlying
diffusion zone of dissolved nitrogen in iron, or alloy nitrides (Ref 17). The
white layer improves surface resistance to wear, and the diffusion zone
increases the fatigue endurance limit, especially in carbon and low-alloy
steels. Alloy steels, cast irons, and some stainless steels can be treated.
The process is used to produce a thin, hard skin, usually less than 25 μm
(1 mil) thick, on low-carbon steels in the form of sheet metal parts, powder
metallurgy parts, small shaft sprockets, and so forth.

62. 62
Pack-Cementation Diffusion Coatings
The pack-cementation process originally involved pack carburizing,
which is the process of diffusing carbon into the surface of iron or low-
carbon steel by heating in a closed container filled with activated
charcoal. The simplest and oldest carburizing process involves filling a
welded sheet metal or plate box with granular charcoal that is activated
with chemicals such as barium carbonate to assist the formation of
carbon monoxide (CO).
In the heated box, charcoal forms carbon dioxide (CO2), which
converts to CO in an environment with an excess of carbon. The CO
then forms atomic carbon at the component surface and diffuses into
the part. The packcarburization process is of little commercial
importance, although it is still done occasionally. It has given rise to
other pack-diffusion processes including aluminizing, siliconizing,
chromizing, and boronizing.

63. 63
Basic Principles. Pack cementation is a batch vapor-phase process that
involves heating a closed/vented pack to an elevated temperature (e.g.,
1050 0C, or 1920 0F) for a given time (e.g., 16 h) during which a
diffusional coating is produced (Ref 18). The traditional pack consists of
four components: the substrate or part to be coated, the master alloy (i.e.,
a powder of the element or elements to be deposited on the surface of the
part), a halide salt activator, and relatively inert filler powder. The master
alloy, the filler, and halide activator are thoroughly mixed together, and the
part to be coated is buried in this mixture in a retort (Ref 19). When the
mixture is heated, the activator reacts to produce an atmosphere of source
element(s) halides that diffuse into the pack and transfer the source
elements) to the substrate on which the coating is formed (Ref 20).
Aluminizing. An aluminizing pack-cementation process is commercially
practiced for a range of alloys, including nickel- and cobalt-base
superalloys, and carbon, low-alloy, and stainless steels. Simple aluminide
coatings resist high-temperature oxidation by the formation of an alumina
protective layer and can be used up to about 1150 0C (2100 0F), but the
coating can degrade by spallation of the oxide during thermal cycling.

64. 64
For extended periods of time at temperatures in excess of 1000 oC
(1830 oF), interdiffusion of the coating will cause further degradation,
and therefore practical coating life is limited to operating
temperatures of 870 to 980 oC (1600 to 1800 oF). Pack
compositions, process temperatures, and process times depend on
the type of base material to be aluminized and fall into the following
classifications (Ref 21):
Class Alloy
I Carbon and low-alloy steels
II Ferritic and martensitic stainless steels
III Austenitic stainless steels with 21^0% Ni and iron-
base superalloys
IV Nickel- and cobalt-base superalloys

65. 65
As a general rule, overall aluminum diffusion is slowed as the nickel,
chromium, and cobalt contents increase. Thus, higher temperatures and
longer processing times are required to produce greater aluminum
diffusion thicknesses as the base material increases in alloy content.
Stainless steels are oxidation resistant as a result of the formation of a
thin chromium-rich oxide on the component surface. A similar reaction
occurs in aluminized steels in which a thin, slower-growing aluminum
richoxide forms. Unlike chromium oxide, Al2O3 does not exhibit volatility
in the presence of oxygen above 927 0C (1700 0F). Figure 8 compares
an aluminized carbon steel with several alloys at a temperature in which
scaling remains less than 10 mg/cm2 for oxidation in air. In sulfidizing
environments, pack-aluminized coatings have excellent resistance to
corrosive attack. In contrast to stainless steels, the aluminum-rich surface
(50% Al) and diffusion zone (20% Al min) of the coating is far more
resistant than chromium to sulfidation corrosion. Figure 9 compares the
corrosion rates of bare and aluminized 9Cr-IMo steel in a hydrogen
sulfide (H2S) environment.

66. 66

67. 67

68. 68
As mentioned in the preceding text, pack aluminizing is commonly
carried out on nickel- and cobalt-base superalloys. Diffusion-coated
superalloys develop an aluminide (NiAl or CoAl) outer layer with
enhanced corrosion resistance. It is estimated that more than 90% of
all coated gas turbine engine hot section blades and vanes made from
superalloys are coated by pack cementation and related processes.
Detailed information on protective diffusion coatings for superalloys can
be found in Ref 24.
Siliconizing, the diffusion of silicon into steel, occurs similarly to
aluminizing.
There are pack and retort processes in which parts are subjected
to gas atmospheres that react with the heated part surface to produce
nascent silicon that diffuses into the substrate to be coated. In a pure
silicon pack that is activated with NH4Cl, SiCl4 and SiHCl3 gases
form, which are reduced by hydrogen gas to deposit elemental silicon
on the surface of the parts (Ref 20). Another process involves
tumbling parts in a retort with SiC. When a temperature of 1010 oC
(1850 oF) is reached, silicon tetrachloride gas is introduced,

69. 69
which reacts with the part and the SiC particles to produce a
concentration gradient of silicon on the part surface as the silicon
diffuses into the substrate. The process normally takes place on low
carbon steels, and these steels develop case depths up to 1 mm (0.040
in.) with a silicon content of 13 wt% (Ref 25). Case depths developed on
these siliconized steels have hardnesses of about 50 HRC and therefore
can be used for wear resistance. The presence of silicon on the surface
allows for the formation of a stable silicon dioxide (SiO2) phase in
oxidizing environments and excellent corrosion resistance.
Chromizing. Chromium can be applied in the same manner as
aluminum and silicon to produce a chromium-rich coating, and many of
the same principles of aluminizing packs apply to chromizing packs.
Parts are packed in chromium powder with an inert filler such as
aluminum oxide.
A halide salt activator is added that changes to the vapor phase at the
processing temperature and serves as a carrier gas to bring chromium
to the surface of the part. Diffusion coatings can be formed on nickel-
base superalloys by pack cementation using ammonium chloride as a
chromiumalumina activator. These coatings usually contain 20 to 25 wt%
Cr at the outer surface and involve approximately equal rates of
interdiffusion of chromium and nickel.

70. 70
Significant depletion of aluminum and titanium from the alloy surface
occurs, thus producing a coating that is a solid solution of the chromium in
the remaining nickel-base superalloy. The deposited coating is usually
overlaid with a thin layer of a-chromium, which must be removed
chemically (Ref 20).
In low-alloy steels, it has been shown that chromizing is much more
complex, leading to microstructures that may behave detrimentally in
some environments (Ref 26). In a chromized 2.25Cr-IMo alloy, the coating
contains a thin outer layer (~5 |xm, or 0.2 mil) of mostly chromium (>80
wt%), which is essential for corrosion protection. Large columnar ferritic
grains, containing between 30 and 15 wt% Cr, are found beneath the
outer layer. The columnar grain boundaries, as well as the boundary
between the outer chromium-rich layer and the columnar grains, are
decorated with chromium carbides that were found to contribute to
coating degradation. A layer of Kirkendall voids (also decorated by
carbides), iron carbides at the coating/substrate interface, and a large
decarburized zone in the substrate are also produced by the process.

71. 71
Evaluation of samples exposed to a fossil-fired boiler up to two years
revealed two degradation mechanisms: cracking and sulfidation
corrosion.
Cracking of the outer coating layer allowed ingress of sulfur, resulting in
intergranular sulfidation corrosion attack. Once the outer protective
chromium layer has been breached, the columnar grain-boundary
orientation promotes crack initiation and propagation along the carbides
when the tube is subjected to axial thermal loading. However, it should
be noted that chromized coatings have been used up to 10 years in
some fossil-fired boilers. Therefore, stress and environmental conditions
are critical to the successful use of these pack-cementation coatings, as
long as the effect of the processing thermal cycle on the coating and
substrate morphology is understood.

72. 72
Boriding, or boronizing, is a thermochemical surface-hardening process
that can be applied to a wide variety of ferrous, nonferrous, and cermet
materials. The boronizing pack process is similar to pack carburizing with
the parts to be coated being packed with a boron-containing compound
such as boron powder or ferroboron. Activators such as chlorine and
fluorine compounds are added to enhance the production of the boron-rich
gas at the part surface. Processing of high-speed tool steels that were
previously quenched hardened is accomplished at 540 0C (1000 0F).
Boronizing at higher temperature up to 1090 0C (2000 0F) causes
diffusion rates to increase, thus reducing the process time. The boron case
does not have to be quenched to obtain its high hardness, but tool steels
processed in the austenitizing temperature range need to be quenched
from the coating temperature to harden the substrate.

73. 73
Boronizing is most often applied to tool steels or other substrates that
are already hardened by heat treatment. The thin (12-15 jjim, or 0.48-0.6
mil) boride compound surfaces provide even greater hardness,
improving wear service life. Distortion from the high processing
temperatures is a major problem for boronized coatings. Finished parts
that are able to tolerate a few thousandths of an inch (75 fim) distortion
are better suited for this process sequence because the thin coating
cannot be finish ground (Ref 25).
Although pack boriding is the most widely used boriding process, it is
important to note that other thermochemical boriding techniques are also
used. These include paste boriding, liquid (salt-bath) boriding, gas
boriding,plasma boriding, and fluidized-bed boriding. These alternative
techniques are described in Ref 27.

74. 74
Ion Implantation
Ion implantation involves the bombardment of a solid material with
medium- to high-energy ionized atoms and offers the ability to alloy
virtually any elemental species into the near-surface region of any
substrate.
The advantage of such a process is that it produces improved surface
properties without the limitations of dimensional changes or delamination
found in conventional coatings. During implantation, ions come to rest
beneath the surface in less than 10 to 12 s, producing a very fast quench
rate and allowing the development of nonequilibrium surface alloys or
compounds.
In almost all cases the modified region is within the outermost micrometer
of the substrate, often only within the first few hundred angstroms (i.e.,
microinches) of the surface. Details of the process and associated
equipment are documented in Ref 28. Ion implantation is commercially
applied to various steels, tungsten carbide/cobalt materials, and alloys of
titanium, nickel, cobalt, aluminum, and chromium, although applications
are restricted to temperatures below 250 0C (480 0F) for steels and 450
oC (840 0F) for carbides. Advantages and limitations of the
ionimplantation process are outlined in Table 8.

75. 75
Applications. Table 9 lists some of the applications for the ion-
implantation process. Ion-implantation surfaces produce exceptional
results in reducing wear, friction, and corrosion (Ref 28, 29). Commercial a
pplications involve tooling, bearings, and biomedical components.
Nitrogen implantation, especially in alloy surfaces containing elements
forming stable nitrides, has found use in tools and dies such as cobalt-
cemented tungsten carbide wire-drawing inserts. Nitrogen implantation
has been especially successful in increasing the life (up to 20 times) of
tools and parts used in the manufacture of injection-molded plastics. Ion
implantation with nitrogen or titanium and carbon has provided increased
tool life for stamping and other forming tools. For example, the life of
punches and dies for the manufacturing of aluminum beverage cans has
increased to 6 to 10 times that of untreated tooling. Table 10 lists
examples of extending tool life with ion implantation.

76. 76

77. 77
Titanium and cobalt-chromium alloy orthopedic prostheses for hip and
knee joints are among the most successful commercial applications for
ion-implantation components for wear resistance.
Laser Alloying
Processing. A technique of localized alloy formation is laser surface
melting with the simultaneous, controlled addition of alloying elements.
These alloying elements diffuse rapidly into the melt pool, and the
desired depth of alloying can be obtained in a short period of time. By
this means, a desired alloy chemistry and microstructure can be
generated on the sample surface; the degree of microstructural
refinement will depend on the solidification rate. The surface of a low-
cost alloy, such as mild steel, can be selectively alloyed to enhance
properties, such as resistance to wear, in such a way that only the locally
modified surface possesses properties typical of tribological alloys. This
results in substantial cost savings and reduces the dependence on
strategic materials.

78. 78
One method of alloying is to apply appropriate mixtures of powders on the
sample surface, either by spraying the powder mixture suspended in
alcohol to form a loosely packed coating, or by coating a slurry
suspended in organic binders (Ref 31). The use of metal powders in laser
alloying is the least expensive, but, with appropriate process
modifications, alloys in the form of rods, wires, ribbons, and sheets can
also be added.
Applications. Laser alloying has been primarily applied to improve
corrosion resistance. A very common technique is to alloy steels with
chromium. An example of laser alloying to improve wear resistance is
exhaust valves fabricated by Fiat Research Laboratory.

79. 79