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.