Surface Treatments

Question 1

What are the three major categories of surface processing operations?

Answer 1

Cleaning
Surface Treatments
Coating and thin film deposition

Question 2

What are reasons that metals are commonly coated?

Answer 2

(1) provide corrosion protection
(2) enhance product appearance (e.g., providing a specified color or texture)
(3) increase wear resistance and/or reduce friction of the surface
(4) increase electrical conductivity
(5) increase electrical resistance
(6) prepare a metallic surface for subsequent processing
(7) rebuild surfaces worn or eroded during service

Question 3

Give examples of when a non-metallic material may be coated

Answer 3

(1) plastic parts coated to give them a metallic appearance
(2) anti-reflection coatings on optical glass lenses
(3) certain coating and deposition processes used in the fabrication of semiconductor chips and printed circuit boards

Question 4

What are the main reason manufactured products should be cleaned?

Answer 4

(1) to prepare the surface for subsequent industrial processing, such as a coating application or adhesive bonding
(2) to improve hygiene conditions for workers and customers
(3) to remove contaminants that might chemically react with the surface
(4) to enhance appearance and performance of the product

Question 5

What are important factors to consider when selecting a cleaning method?

Answer 5

(1) the contaminant to be removed
(2) degree of cleanliness required
(3) substrate material to be cleaned
(4) purpose of the cleaning
(5) environmental and safety factors
(6) size and geometry of the part
(7) production and cost requirements

Question 6

What are the five main chemical cleaning methods?

Answer 6

(1) alkaline cleaning
(2) emulsion cleaning
(3) solvent cleaning
(4) acid cleaning
(5) ultrasonic cleaning

Question 7

Describe blast finishing and give an example.

Answer 7

Blast finishing uses the high-velocity impact of particulate media to clean and finish a surface. The most well known of these methods is sand blasting, which uses grits of sand (SiO2) as the blasting media. The media is propelled at the target surface by pressurized air or centrifugal force.

Question 8

What is shot peening?

Answer 8

In shot peening, a high-velocity stream of small cast steel pellets (called shot) is directed at a metallic surface with the effect of cold working and inducing compressive stresses into the surface layers. Shot peening is used primarily to improve fatigue strength of metal parts.

Question 9

What are examples of mass finishing operations?

Answer 9

Tumbling / vibratory finishing

Question 10

What do mass finishing operations achieve?

Answer 10

Deburring, descaling, deflashing, polishing, radiusing, burnishing, and cleaning

Question 11

Describe the tumbling process.

Answer 11

The parts to be processed are mixed with a media in a large barrel which is spun at 10 to 50 rev/min. The land sliding effect caused by the rotation and gravity finishes the parts. Drawbacks are the high noise level and space requirement

Question 12

What is plating?

Answer 12

Plating involves the coating of a thin metallic layer onto the surface of a substrate material

Question 13

What is electrochemical plating?

Answer 13

An electrolytic process in which metal ions in an electrolyte solution are deposited onto a cathode workpart

Question 14

What type of current is used in electroplating?

Answer 14

DC

Question 15

What does the electrolyte consist of in electroplating?

Answer 15

An aqueous solution of acids, bases, or salts

Question 16

What are faradays two physical laws that effect electroplating?

Answer 16

(1) the mass of a substance liberated in electrolysis is proportional to the quantity of electricity passed through the cell
(2) the mass of the material liberated is proportional to its electrochemical equivalent (ratio of atomic weight to valence)

Question 17

In the equation V=ECIt what does C and E stand for?

Answer 17

The plating constant and Efficiency

Question 18

What is the equation for average plating thickness?

Answer 18

d = V/A

Question 19

What is electroforming?

Answer 19

Electroforming involves electrolytic deposition of metal onto a pattern until the required thickness is achieved; the pattern is then removed to leave the formed part. The layer left is substantially thicker than electroplating so it is a longer process.

Question 20

What are applications of electroforming?

Answer 20

Electroformed parts are commonly fabricated of copper, nickel, and nickel cobalt alloys. Applications include fine molds for lenses, compact discs (CDs), and videodiscs (DVDs); copper foil used to produce blank printed circuit boards; and plates for embossing and printing.

Question 21

What is electroless plating?

Answer 21

Electroless plating is a plating process driven entirely by chemical reactions-no external source of electric current is required. Deposition of metal onto a part surface occurs in an aqueous solution containing ions of the desired plating metal. The process uses a reducing agent, and the workpart surface acts as a catalyst for the reaction.

Question 22

What are advantages of electroless plating

Answer 22

(1) uniform plate thickness on complex part geometries (a problem with electroplating)
(2) the process can be used on both metallic and nonmetallic substrates
(3) no need for a DC power supply to drive the process

Question 23

Give an overview of hot dipping

Answer 23

Hot dipping is a process in which a metal substrate is immersed in a molten bath of a second metal; upon removal, the second metal is coated onto the first. The first metal must possess a higher melting temperature than the second. The most common substrate metals are steel and iron. Zinc, aluminum, tin, and lead are the common coating metals.

Question 24

Why is excellent cohesion achieved in hot dipping?

Answer 24

Hot dipping works by forming transition layers of varying alloy compositions. Next to the substrate are normally intermetallic compounds of the two metals; at the exterior are solid solution alloys consisting predominantly of the coating metal. The transition layers provide excellent adhesion of the coating.

Question 25

What two mechanism operate after hot dipping to provide corrosion resistance?

Answer 25

(1) barrier protection-the coating simply serves as a shield for the metal beneath
(2) sacrificial protection-the coating corrodes by a slow electrochemical process to preserve the substrate

Question 26

What are the different names for hot dipping?

Answer 26

Galvanizing is when zinc (Zn) is coated onto steel or iron; aluminizing refers to coating of aluminium (Al) onto a substrate; tinning is coating of tin (Sn); and terneplate describes the plating of lead–tin alloy onto steel

Question 27

What are uses of tin plated materials?

Answer 27

Tin plating by hot dipping provides a non toxic corrosion protection for steel in applications for food containers, dairy equipment, and soldering applications

Question 28

What are the two main differences between anodizing and electroplating?

Answer 28

(1) In electrochemical plating, the workpart to be coated is the cathode in the reaction. By contrast, in anodizing, the work is the anode, whereas the processing tank is cathodic.
(2) In electroplating, the coating is grown by adhesion of ions of a second metal to the base metal surface. In anodizing, the surface coating is formed through chemical reaction of the substrate metal into an oxide layer.

Question 29

What are organic coatings?

Answer 29

Organic coatings are polymers and resins, produced either naturally or synthetically, usually formulated to be applied as liquids that dry or harden as thin surface films on substrate materials.

Question 30

What are the four standard ingredients of an organic coating?

Answer 30

(1) binders, which give the coating its properties
(2) dyes or pigments, which lend color to the coating
(3) solvents, to dissolve the polymers and resins and add proper fluidity to the liquid
(4) additives

Question 31

What is transfer efficiency?

Answer 31

Transfer efficiency is the proportion of paint supplied to the process that is actually deposited onto the work surface. Some methods yield as low as a 30% transfer efficiency (meaning that 70% of the paint is wasted and cannot be recovered).

Question 32

What are the four main transfer methods?

Answer 32

Brushing/Rolling
Spray Coating
Immersion
In Flow Coating

Question 33

Spray coating has a transfer efficiency of about 30%, how can this be improved?

Answer 33

Electrostatic Spraying (90%)

Question 34

What is curing?

Answer 34

Curing involves a chemical change in the organic resin in which polymerization or cross-linking occurs to harden the coating.

Question 35

What are the four types of curing?

Answer 35

(1) ambient temperature curing, which involves evaporation of the solvent and oxidation of the resin
(2) elevated temperature curing, in which elevated temperatures are used to accelerate solvent evaporation, as well as polymerization and cross-linking of the resin
(3) catalytic curing, in which the starting resins require reactive agents mixed immediately prior to application to bring about polymerization and cross-linking (epoxy and polyurethane paints are examples)
(4) radiation curing, in which various forms of radiation, such as microwaves, ultraviolet light, and electron beams, are required to cure the resin.