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Surface Treatement Technologies
ABSTRACT
These processes are sometimes referred to as post-processing. It plays a
very important role in the appearance, function and life of the product.
Broadly, this is processes that affect either a thin layer on the surface of the
part itself, or add a thin layer on top of the surface of the part.
There are different coating and surface treatments processes, with different
applications, uses, etc. The important uses include: Improving the
hardness, improving the wear resistance, Controlling friction, Reduction of
adhesion, improving the lubrication, etc., improving corrosion resistance,
improving aesthetics
TECHNIQUES
1. Mechanical hardening of the surface
These methods apply mechanical impulses (e.g. light hammering) on the
surface of a metallic part. This hammering action causes tiny amount of
plastic flow on the surface, resulting in the work-hardening of the surface
layer due to the introduction of compressive residual stresses. Examples
of these processes include Shot peening (uses tiny balls of metal or
ceramic), Water-jet peening (uses a jet of water at high pressures, e.g.
400 MPa), or Laser peening (surface is hit by tiny impulses from a laser) –
an expensive process used to improve fatigue strength of jet fan blades
and turbine impellers. Another method is explosive hardening, where a
layer of explosive coated on the surface is blasted – the resulting impact
results in tremendous increase in the surface hardness.
This method is used to harden the surface of train rails.
A. Shot peening
Is a cold work process used to finish metal parts to prevent fatigue and
stress corrosion failures and prolong product life for the part In shot
peening, small spherical shot bombards the surface of the part to be
finished. The shot acts like a peen hammer, dimpling the surface and
causing compression stresses under the dimple.
As the media continues to strike the part, it forms multiple overlapping
dimples throughout the metal surface being treated.
The surface compression stress strengthens the metal, ensuring that the
finished part will resist fatigue failures, corrosion fatigue and cracking, and
galling and erosion from cavitation
B. Case hardening
This is a very common process that is used to harden the outer surface of
parts such as gear teeth, cams, shafts, bearings, fasteners, pins, tools,
molds, dies etc. In most of these types of components, the use involves
dynamic forces, occasional impacts, and constant friction.
Therefore the surface needs to be hard to prevent wear, but the bulk of the
part should be tough (not brittle); this is achieved best by case hardening.
There are several types of case hardening: in most cases, the chemical
structure of the metal is changed by diffusing atoms of an alternate element
which results in alterations to the micro-structure on the crystals on the
surface. The duration and temperature control the concentration and depth
of the doping. Most of these processes are used to case harden steel and
other iron alloys, including low carbon steels, alloy steels, tool steels.
2. Oxide coating
In vacuum tubes, a hot cathode or thermionic cathode is
a cathode electrode which is heated to make it emit electrons due to
thermionic. The heating element is usually an electrical filament, heated by
a separate electric current passing through it. Hot cathodes typically
achieve much higher power density than cold cathodes, emitting
significantly more electrons from the same surface area. Cold
cathodes rely on field electron emission or secondary electron emission
from positive ion bombardment and do not require heating. There are two
types of hot cathode. In a directly-heated cathode, the filament is the
cathode and emits the electrons. In an indirectly-heated cathode, the
filament or heater heats a separate metal cathode electrode which emits
the electrons.
Types
A. Boride cathodes
Cerium boride cathodes have one and half times the lifetime of lanthanum
boride, due to its higher resistance to carbon contamination. Boride
cathodes are about ten times as "bright" as the tungsten ones and have 10-
15 times longer lifetime. They are used e.g. in electron
microscopes, microwave tubes, electron lithography, electron beam
welding, X-Ray tubes, and free electron lasers. However these materials
tend to be expensive.
B. Thoriated filaments
The most common type of directly heated cathode, used in most high
power transmitting tubes, is the thoriated tungsten filament; a small amount
of thorium is added to the tungsten of the filament. The filament is heated
white-hot, at about 2400 °C, and thorium atoms migrate to the surface of
the filament and form the emissive layer. Heating the filament in a
hydrocarbon atmosphere carburizes the surface and stabilizes the emissive
layer. Thoriated filaments can have very long lifetimes and are resistant to
the ion bombardment that occurs at high voltages, because fresh thorium
continually diffuses to the surface, renewing the layer. They are used in
nearly all high-power vacuum tubes for radio transmitters, and in some
tubes for hi-fi amplifiers. Their lifetimes tend to be longer than those of
oxide cathodes
C. Thorium alternatives
Due to concerns about thorium radioactivity and toxicity, efforts have been
made to find alternatives. One of them is zirconiated tungsten,
where zirconium dioxide is used instead of thorium dioxide.
3. Phosphate conversion coating
Phosphate coatings are used for corrosion resistance, lubricity, or as a
foundation for subsequent coatings or painting. It serves as a conversion
coating in which a dilute solution of phosphoric acid and phosphate salts is
applied via spraying or immersion and chemically reacts with the surface
of the part being coated to form a layer of insoluble, crystalline
phosphates. Phosphate conversion coatings can also be used
on aluminum, zinc, cadmium, silver and tin.
The main types of phosphate coatings are manganese, iron and zinc.
Manganese phosphates are used both for corrosion resistance and
lubricity and are applied only by immersion. Iron phosphates are typically
used as a base for further coatings or painting and are applied by
immersion or by spraying. Zinc phosphates are used for corrosion
resistance (phosphate and oil), a lubricant base layer, and as a
paint/coating base and can also be applied by immersion or spraying.
The performance of the phosphate coating is significantly dependent on
the crystal structure as well as the weight. For example,
a microcrystalline structure is usually optimal for corrosion resistance or
subsequent painting. A coarse grain structure impregnated with oil,
however, may be the most desirable for wear resistance. These factors are
controlled by selecting the appropriate phosphate solution, using various
additives, and controlling bath temperature, concentration, and phosphating
time. A widely used additive is to seed the metal surface with tiny particles
of titanium salts by adding these to the rinse bath preceding the
phosphating. This is known as activation
4. Chromate conversion coating
Aluminium and aluminium alloys are treated by a corrosion resistant
conversion coating that is called "chromate coating" or "chromating".
General method is to clean the aluminium surface and then apply an acidic
chromium composition on that clean surface. Chromium conversion
coatings are highly corrosion resistant and provide excellent retention of
subsequent coatings. Different type of subsequent coatings can be applied
to the chromate conversion coating to produce an acceptable surface.
Along with providing high corrosion resistance and paint adhesion
properties to aluminium surface.
Quality of surface pre-treatment prior to powder coating is the most
important factor that effects to stability of paintings. Properly pre-treated
aluminium surfaces become highly protected against corrosion even if the
surface is exposed to external impacts (damage, high temperature,
humidity…etc.).
Chromating is generally used as under paint protection
Provided by yellow chromating (Cr+6), green chromating (Cr+3),
transparent chromating (Cr+3).
Coating quality will be effected positively when surface treated with
deionized water after chromating. Refinishing of the rinsing baths also
improve the quality of the coating.
Chromated and rinsed aluminium workpieces should be dried in driers or
ovens but it is important not to set drying temperatures above 70°C. After
all these treatment workpieces are painted then cured 10 – 15 minutes at
200°C
5. Thermal spraying
Also commonly known as metal spraying is a surface engineering / coating
process where a wide range of metals and ceramics can be sprayed onto
the surface of another material.
Thermal spraying is widely used to provide corrosion protection to ferrous
metals or to change the surface properties of the sprayed items, such as
improve the wear resistance or thermal conductivity.
Thermal spraying can provide thick coatings (approx. thickness range is 20
micrometers to several mm, depending on the process and feedstock),
over a large area at high deposition rate as compared to other coating
processes such as electroplating, physical and chemical vapor deposition.
Several variations of thermal spraying are distinguished:
Plasma spraying
Detonation spraying
Wire arc spraying
Flame spraying
High velocity oxy-fuel coating spraying (HVOF)
Warm spraying
Cold spraying
Coating quality is usually assessed by measuring its oxide content, macro
and micro-hardness, bond strength and surface roughness. Generally, the
coating quality increases with increasing particle velocities.
6. Physical Vapor Deposition
Physical Vapor Deposition, or PVD, is a term used to describe a family of
coating processes. The most common of these PVD coating processes are
evaporation (typically using cathode arc or electron beam sources), and
sputtering (using magnetic enhanced sources or “magnetrons”, cylindrical
or hollow cathode sources).
All of these processes occur in vacuum at working pressure (typically 10-2
to 10-4 mbar) and generally involve bombardment of the substrate to be
coated with energetic positively charged ions during the coating process to
promote high density.
Additionally, reactive gases such as nitrogen, acetylene or oxygen may be
introduced into the vacuum chamber during metal deposition to create
various compound coating compositions.
It forms a compound with the metal vapor and is deposited on the tools or
components as a thin, highly adherent coating. In order to obtain a uniform
coating thickness, the parts are rotated at uniform speed about several
axes.
The properties of the coating (such as hardness, structure, chemical and
temperature resistance, adhesion) can be accurately controlled.
7. Chemical Vapor Deposition
Chemical Vapor Deposition (CVD) is an atmosphere controlled process
conducted at elevated temperatures (~1925° F) in a CVD reactor. During
this process, thin-film coatings are formed as the result of reactions
between various gaseous phases and the heated surface of substrates
within the CVD reactor.
As different gases are transported through the reactor, distinct coating
layers are formed on the tooling substrate. For example, TiN is formed as
a result of the following chemical reaction: Titanium carbide (TiC) is
formed as the result of the following chemical reaction.
The final product of these reactions is a hard, wear-resistant coating that
exhibits a chemical and metallurgical bond to the substrate. CVD coatings
provide excellent resistance to the types of wear and galling typically seen
during many metal-forming applications.
8. Thermo reactive Diffusion
Thermoreactive Diffusion (TD or TRD) is a high temperature coating
process for producing metal carbides (typically vanadium carbide) on the
surface of a carbon-containing substrate.
This is a multi-stage coating process which utilizes a pre-heat cycle, a
coating segment, ultra-sonic cleaning, heat-treating, and post-coating
polishing. The coating segment is performed in a molten bath [typically
consisting of a solute (Borax), a metal source, and a reducing agent]:
carbide-forming compounds in the bath react with carbon in the substrate
and produce metal carbides on the substrate surface. TD coatings exhibit a
diffusion type bond, thereby providing superb adhesion between the metal
carbide layer and the substrate. This bonding characteristic, combined with
the coating’s high micro-hardness, provides excellent resistance to the
types of wear and galling often seen in many metal-forming processes.