Surface Treatments Flashcards
What are the three major categories of surface processing operations?
Cleaning
Surface Treatments
Coating and thin film deposition
What are reasons that metals are commonly coated?
(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
Give examples of when a non-metallic material may be coated
(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
What are the main reason manufactured products should be cleaned?
(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
What are important factors to consider when selecting a cleaning method?
(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
What are the five main chemical cleaning methods?
(1) alkaline cleaning
(2) emulsion cleaning
(3) solvent cleaning
(4) acid cleaning
(5) ultrasonic cleaning
Describe blast finishing and give an example.
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.
What is shot peening?
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.
What are examples of mass finishing operations?
Tumbling / vibratory finishing
What do mass finishing operations achieve?
Deburring, descaling, deflashing, polishing, radiusing, burnishing, and cleaning
Describe the tumbling process.
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
What is plating?
Plating involves the coating of a thin metallic layer onto the surface of a substrate material
What is electrochemical plating?
An electrolytic process in which metal ions in an electrolyte solution are deposited onto a cathode workpart
What type of current is used in electroplating?
DC
What does the electrolyte consist of in electroplating?
An aqueous solution of acids, bases, or salts
What are faradays two physical laws that effect electroplating?
(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)
In the equation V=ECIt what does C and E stand for?
The plating constant and Efficiency
What is the equation for average plating thickness?
d = V/A
What is electroforming?
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.
What are applications of electroforming?
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.
What is electroless plating?
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.
What are advantages of electroless plating
(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
Give an overview of hot dipping
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.
Why is excellent cohesion achieved in hot dipping?
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.
