coating processes

7/27/2019 Coating Processes

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P7-1

Surface Treatment,Coating, and Cleaning

Processes


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CLEANING AND SURFACE TREATMENTS

Cleaning involves the removal of solid, semisolid

or liquid contaminants from a surface.

Chemical Cleaning

Mechanical Cleaning and Surface Preparation

Diffusion and Ion Implantation


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Surface Treatments for Various Metals

TABLE 33.1

Metal Treatment

Aluminum Chrome plate; anodic coating, phosphate; chromateconversion coating

Beryllium Anodic coating; chromate conversion coating

Cadmium Phosphate; chromate conversion coating

Die steels Boronizing; ion nitriding; liquid nitriding

High-temperature steels Diffusion

Magnesium Anodic coating; chromate conversion coating

Mild steel Boronizing; phosphate; carburizing; liquid nitriding;carbonitriding; cyaniding

Molybdenum Chrome plate

Nickel- and cobalt-base alloys Boronizing; diffusion

Refractory metals Boronizing

Stainless steel Vapor deposition; ion nitriding; diffusion; liquid nitriding;

nitriding

Steel Vapor deposition; chrome plate; phosphate; ion nitriding;induction hardening; flame hardening; liquid nitriding

Titanium Chrome plate; anodic coating; ion nitriding

Tool steel Boronizing; ion nitriding; diffusion; nitriding; liquid nitriding

Zinc Vapor deposition; anodic coating; phosphate; chromatechemical conversion coating

Source: After M. K. Gabel and D. M. Doorman in Wear Control Handbook, New York, ASME, 1980 p. 248.


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Overview of Industrial Cleaning

Almost all workparts must be cleaned one or more timesduring their manufacturing sequence

Chemical and/or mechanical processes are used to

accomplish this cleaning

- Chemical cleaning methods use chemicals to removeunwanted contaminants from the work surface

- Mechanical cleaning involves removal of contaminants froma surface by various mechanical operations


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Reasons Why Manufactured Parts (andProducts) Must be Cleaned

Prepare the surface for subsequent processing,such as a coating application or adhesive bonding

Improve hygiene conditions for workers andcustomers

Remove contaminants that might chemically reactwith the surface

Enhance appearance and performance of theproduct


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Important Factors in Selecting aCleaning Method

Contaminant to be removed

Degree of cleanliness required

Substrate material to be cleaned

Purpose of the cleaning

Environmental and safety factors

Size and geometry of the part

Production and cost requirements


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Contaminant to be Removed

Various contaminants build up on part surfaces,either due to previous processing or factoryenvironment

Principal surface contaminants found in thefactory:

- Oil and grease, e.g., lubricants in metalworking

- Solid particles such as metal chips, abrasive grits,

shop dirt, dust, etc.- Polishing compounds.

- Oxide films & rust


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Degree of Cleanliness

Refers to the amount of contaminantremaining after a given cleaning operation

A simple test is a wiping method, in whichthe surface is wiped with a clean cloth

- Amount of soil absorbed by the cloth isobserved

Non-quantitative but easy to use


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Other Factors in Selection of

Cleaning MethodThe substrate material must be considered, so that

damaging reactions are not caused by the cleaningchemicals.

Example:- Steels are resistant to alkalis but react with virtually all acids

Cleaning methods and associated chemicals shouldbe selected to avoid pollution and health hazards.


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P7-10

Chemical Cleaning Processes

Alkaline cleaning

Emulsion cleaning

Solvent cleaning

Acid cleaning

Ultrasonic cleaning

In some cases, chemical action is augmented by otherenergy forms

Example: ultrasonic cleaning uses high-frequency mechanicalvibrations combined with chemical cleaning


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P7-11

Alkaline CleaningUses an alkali to remove oils, grease, wax, and

various types of particles (metal chips, silica, lightscale) from a metallic surface

Most widely used industrial cleaning method.

Alkaline solutions include sodium and potassiumhydroxide (NaOH, KOH), sodium carbonate(Na2CO3), borax (Na2B4O7).

Cleaning methods: immersion or spraying, usually

at temperatures of 50-95C (120-200F), followedby water rinse to remove residue.


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Emulsion Cleaning

Uses organic solvents (oils) dispersed in an aqueoussolution.

The use of suitable emulsifiers (soaps) results in a

two-phase cleaning fluid (oil-in-water), whichfunctions by dissolving or emulsifying the soils on the

part surface

Can be used on either metal or nonmetallic parts

Must be followed by alkaline cleaning to eliminate allresidues of the organic solvent prior to plating.


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Acid Cleaning

Removes oils and light oxides from metalsurfaces using acid solutions combined withwater-miscible solvents, wetting and emulsifying

agentsCommon application techniques: soaking,spraying, or manual brushing or wiping carriedout at ambient or elevated temperatures

Cleaning acids include hydrochloric (HCl), nitric

(HNO3), phosphoric (H3PO4), and sulfuric(H2SO4)


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Ultrasonic Cleaning

Mechanical agitation of cleaning fluid byhigh-frequency vibrations (between 20 and 45kHz) to cause cavitations - formation of low

pressure vapor bubbles that scrub the surface.

Combines chemical cleaning and mechanicalagitation of the cleaning fluid.

Cleaning fluid is generally an aqueous solutioncontaining alkaline detergents.

Highly effective for removing surfacecontaminants


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Mechanical Cleaning and SurfacePreparation

Physical removal of soils, scales, or films from the worksurface by means of abrasives or similar mechanicalaction ( Fibre Brushing)

Often serves other functions such as improving surfacefinish and surface hardening.

Processes:1) Blast finishing

2) Shot peening

3) Mass finishing processes


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Blast Finishing

High velocity impact of particulate media toclean and finish a surface

The media is propelled at the target surface bypressurized air or centrifugal force

Most well-known method issand blasting, whichuses grits of sand as the blasting media

Other blasting media:Hard abrasives such as aluminum oxide (Al

2

O3

) andsilicon carbide (SiC)

Soft media such as nylon beads


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Shot Peening

High velocity stream of small cast steel pellets(calledshot) is directed at a metallic surface tocold work and induce compressive stressesinto surface layers

Used primarily to improve fatigue strength ofmetal parts

Purpose is therefore different from blast

finishing, although surface cleaning isaccomplished as a byproduct of the operation


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Mass Finishing

Finishing parts in bulk by a mixing action in a container,usually in the presence of an abrasive media

The mixing causes the parts to rub against the media and

each other to achieve the desired finishing action Parts are usually small and therefore uneconomical to

finish individually

Processes include:

1) Tumbling

2) Vibratory finishing


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Figure 28.1 - Diagram of tumbling (barrel finishing)operation showing "landslide" action of parts and

abrasive media to finish the parts


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Tumbling Equipment


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Vibratory Finishing

An alternative to tumbling

Vibrating vessel subjects all parts to agitationwith the abrasive media, as opposed to only thetop layer as in barrel finishing

Consequently, processing times for vibratoryfinishing are significantly reduced

The open tubs in this method permit inspectionof the parts during processing, and noise isreduced


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Mass Finishing Media

Natural media (corundum, granite,limestone) - generally softer andnonuniform in size

Synthetic media (Al2O3 and SiC) - greaterconsistency in size, shape, and and hardness

Steel - used for burnishing,surface-hardening, and light deburringoperations


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Figure 28.2 - Typical preformed media shapes usedin mass finishing operations:

(a) abrasive media for finishing, and (b) steelmedia for burnishing


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Roller Burnishing

In roller burnishing the surface of the componentis cold worked by a hard & highly polishedrollers.

Advantages:

1) It improves surface finish by removing scratches,tool marks & non uniformity.

2) Improvement in corrosion resistance.


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Roller Burnishing

Figure 33.1 Roller burnishing of the fillet of a stepped shaft to induce

compressive surface residual stresses for improved fatigue life.

Figure 33.2 Examples of roller burnishing of(a) a conical surface and (b) a flat surfaceand the burnishing tools used. Source:Sandvik, Inc.


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COATING AND DEPOSITIONPROCESSES

Plating and Related Processes

Conversion Coatings

Vapor DepositionPhysical Vapor Deposition

Chemical Vapor Deposition

Thermal Spray Coating Processes

Th l S i


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Thermal SprayingSpraying of molten coating materials onto a substrate,

where they solidify and adhere to the surfaceCoating materials: It can be in the form of wire, rod or

powder.

pure metals and metal alloys; ceramics (oxides, carbides, and

certain glasses); other metallic compounds (sulfides, silicates);cermet composites; and certain plastics (epoxy, nylon, Teflon,and others).

Substrates:metals, ceramics, glass, some plastics, wood, and

paperHeating technologies:oxyfuel flame & plasma arc.


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Application of Thermal Spraying

1. First applications were to rebuild wornareas on used machinery components hadbeen machined undersize.

2. Also used in manufacturing as a coatingprocess for corrosion resistance, hightemperature protection, wear resistance,electrical conductivity, electrical

resistance, electromagnetic interferenceshielding


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ThermalSpray

Operations

Figure 33.3Schematic

illustrations ofthermal sprayoperations. (a)Thermal wirespray. (b)

Thermal metal-powder spray. (c)Plasma spray.


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Thermal Spray Operations

i. Thermal wire spray.: In this process oxy-fuelflame melts the wire and deposits it on thesurface of substrate.

ii. Thermal metal-powder spray: It is similar tothe thermal wire spray process, but the coatingmetal is in state of powder instead of wire.

iii. Plasma spray: It produces temperatures at the

order of 8300 C. it results in very high Bondstrength.


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Vapor Deposition

Vapor deposition is a process in which thesubstrate (Work piece surface) is subjected tochemical reactions by gases that contain

chemical compounds of the metal to bedeposited.

The coating thickness is a few Micro meters.

Typical applications for vapor depositions are the

coating of cutting tools, drills, reamers, millingcutters, punches & dies.


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Physical Vapor Deposition (PVD)

A family of coating processes in which a material isconverted to its vapor phase in a vacuum chamber andcondensed onto a substrate surface as a very thin film

Can be used for a wide variety of coating materials: metals,

alloys, ceramics and other inorganic compounds, and evencertain polymers

Possible substrates: metals, glass, and plastics

PVD is a very versatile coating technology, applicable to analmost unlimited combination of coating substances andsubstrate materials


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Applications of PVD

1. Decorative coatings on plastic and metalparts such as trophies, toys, pens andpencils, watchcases, and interior trim in

automobiles2. Coatings of magnesium fluoride (MgF2)

onto optical lenses

3. Coating titanium nitride (TiN) ontocutting tools and plastic injection moldsfor wear resistance

Physical Deposition: Vacuum


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Physical Deposition: VacuumEvaporation

Figure 33.4 Schematic illustration

of the physical deposition process.Source: Cutting Tool Engineering.

In Vacuum Evaporation, the metal tobe Deposited is evaporated at high

temperatures in vacuum and isdeposited On the substrate.

The coating material (cathode) isevaporated by several arc evaporators.

The arc produces a highly reactive

Plasma which consists of ionized vaporof the coating material.

The vapor condenses on the substrate(anode) and coats it.

Physical Vapor deposition:


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Physical Vapor deposition:Sputtering

Figure 33.5 Schematic illustration ofthe sputtering process. Source: ASMInternational

In Sputtering, anelectric filed ionizes aninert gas (argon). The

positive ions

Bombard the coatingmaterial (cathode) andcause sputtering(ejecting) of its atoms.

These atoms then

condenses on the workpiece


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Chemical Vapor Deposition (CVD)

Involves the interaction between a mixture of gases andthe surface of a heated substrate, causing chemicaldecomposition of some of the gas constituents andformation of a solid film on the substrate

Reactions take place in enclosed reaction chamber

Reaction product (metal or compound) reacts and growson substrate surface to form the coating.

Most CVD reactions require heat.

Wide range of pressures and temperatures in CVD

Variety of coating and substrate materials possible


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Chemical Vapor Deposition

Figure 33.7 Schematic illustration of the chemical vapor depositionprocess.


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Electroplating

Electrolytic process in which metal ions in anelectrolyte solution are deposited onto a cathodeworkpart .

Also called electrochemical plating

The anode is generally made of the plating metaland thus serves as the source of the plate metal

Direct current from an external power supply ispassed between anode and cathode

The electrolyte is an aqueous solution of acids,bases, or salts

Electroplating


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Electroplating

Figure 33.8 Schematic illustration ofthe electroplating process.


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Common Coating Metals

Zinc - plated on steel products such as fasteners,wire goods, electric switch boxes, and sheetmetal

parts as a sacrificial barrier to corrosion

Nickel- for corrosion resistance and decorativepurposes on steel, brass, zinc die castings, andother metals; also used as a base coat for chrome

plate

Tin - widely used for corrosion protection in "tincans" and other food containers

P i ti g


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Painting

Painting is widely used as a coating processbecause of the following properties:

Decorative appearanceLow cost

Environmental protection

Range of available colorsRelative ease of application

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Painting

Common methods of applying paint are :

1) Dipping.

2) Brushing.

3) Spraying.

In electrostatic spraying paint particles arecharged and attracted to the surface being coated.

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Painting

Figure 33.11 Methods of paint application: (a) dip coating, (b) flow coating, and (c) electrostatic spraying.Source: Society of Manufacturing Engineers.

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