Surface Technology Flashcards
What does surface quality directly influence?
Friction, wear, and lubrication (tribology), resistance to corrosion, fatigue life (initiation of cracks), appearance, thermal and electrical conductivity.
Tolerance
The difference between the maximum and minimum limits.
Bilateral tolerance
Variation is permitted in both positive and negatice directions from the nominal dimension.
It is possible to be unbalanced.
Unilateral tolerance
Variation from the specified dimensions is permitted in only one direction.
It can be positive or negative.
Limit dimensions
Consist of the maximum and minimum dimensions allowed.
Gauging
Determines if the part characteristics meet (or don’t meet) the design specifications.
Bulk metal/ Metal substrate
Exhibits the grain structure/properties of the bulk of the material
Work-hardened layer
Plastically deformed and hardened from manufacturing processes.
It posesses smaller grains and may also contain residual stress.
Beilby layer
Amorphous. Caused by polishing a crystalline structure. Is superficial.
Oxide layer
A thin coating of oxide on the surface. Varies depending on the metal.
Adsorbed film
Gas and moisture from the air on the surface.
Adsorption is the process in which atoms, ions or molecules from a substance adhere to a surface.
Typical surface structure order
- Contaminants
- Adsorbed film
- Oxide layer
- Beilby layer
- Work-hardened layer
- Bulk metal
Existence/thickness of layers is process dependent
Surface defects
Cracks, craters, folds, corrosion pits, machining striations, and more.
Play a role in the long term durability of a product by reducing fatigue and corrosion resistance.
Surface texture
Made up of the repetitive and/or random deviations from the nominal surface of an object.
It is defined by roughness, waviness, lay, and flaws.
Roughness
Small, finely spaces deviations from the nominal surface.
Is determined by material characteristics and the processes that formed the surface.
Waviness
Deviations with larger spacing. Occurs due to work deflection, vibration, tooling, etc.
Roughness is superimposed on waviness.
Lay
Predominant direction or pattern of surface texture.
See surface tech 1 notes for lay classification symbols.
Flaws
Irregularities that occur occasionally on the surface. This includes cracks, scratches, inclusions, etc.
Can also influence surface integrity.
Surface roughness
Measurable characteristic based on roughness deviations.
Surface finish
More subjective. relies on smoothness and general quality.
Surface texture
Geometry of a surface. Includes surface roughness and surface finish.
Surface integrity
Material charateristics immediately below the surface, as well as the subsurface changes they may have caused.
What are the three methods to measure surface roughness?
- Subjective comparison with standard test surfaces (Fingernail)
- Stylus electronic instruments
- Optical techniques
Stylus instruments
Cone shaped diamond stylus is moved across the surface horizontally, following the vertical deviations.
Can create the profile of the surface as well as the average roughness profile.
Effects of surface roughness.
Quality of fit between components, friction and wear, preparation for more processing (painting, welding, etc.), contamination resistance, corrosion, fatigue resistance, wettability
Wettability
The tendency of one fluid to spread on or adhere to a solid surface in the presence of other immiscible fluids.
Adhesive/sliding wear
Friction between two materials can cause bonding between the uneven surfaces. They then fracture off as the sliding motion continues.
Abrasive wear
Hard/rough surfaces sliding over soft surfaces.
The harder surface tears chips from the softer surface, leaving scratches.
Corrosive wear
Caused by chemical/electrochemical reactions. A corrosion by-product s formed as a layer on the surface.
Passivation
Chemical treatment process taht helpds the surfaces of steels and other alloys to become corrosion resistant.
Surface fatigue
Caused by mechanical or thermal cyclic loading.
Mechanical cyclic loading cuases spalling or pitting. Thermal cyclic loading induces cracks in the surface due to thermal stresses.
What are three ways to reduce fatigue wear?
Lowering contact stresses, reducing thermal cycling, excluding impurities/inclusions that may start cracks.
Case hardening
Heat treatment of surfaces increases surface hardness and induces compressive-surface residual stresses.
Delays faulire by holding back the start of fatigue cracks.
Shot peening
The workpiece surfae is hit repeatedly with a large number of balls of cast steel, glass, or ceramics.
Created compressive residual stresses in the surface, increasing fatigue life and crack resistance.
Can also cave water jet, laser, and ultrasonic peening,
Roller burnishing
The surface is cold worked by running it through hard/highly polished rollers.
Improves mechanical properties, surface finish, and corrosion resistance.
Electroplating
The workpiece (cathode) is plated with a different metal (anode).
Common plating materials are chromium, nickel, cadmium, copper, zinc, and tin.
Considerations when electroplating.
Jigs - Workpieces must be suspended in the electrolyte
Airlocks and Cavities - Cavities could trap some of the chemicals and cause corrosion later on. This includes blind holes and welds.
Sharp corners - May cause non-uniform coating with buildup at the corners
Hollow surfaces can’t be electroplated unless the anode is somehow placed inside as well.
Anodizing
The workpiece (anode) is emmersed in an acid bath. This results in the chemical absorption of oxygen and creads a hard/porous oxide layer.
Vapour deposition
Various metals, alloys, carbides, nitrides, ceramics, and oxides can be applied to a surface via evaporation and deposition.
Physical vapour deposition (PVD)
Vapour particales to be deposited are carried physically to the workpiece.
PVD is used for thin decorative coatings on plastic and metal parts.
Chemical vapour deposition
Coatings are produced after a chemical reaction at the surface.
Can complete multi-phase coatings to combine properties of different coating materials.
Thermal spray
Various metals, alloys, carbides, and ceramics are applied to a metal surface via a spray gun with a stream of flame, electric arc, or plasma.
Feed stock can be wire or powdered.
What are the benefits of thermal spray?
Increases wear resistance (e.g. carbides), corrosion resistance (e.g. ceramics), and thermal resistance (e.g. NiCrAlY or Zirconia).
High pressure/ Velocity oxygen flame spraying
Highest bond strength of any mechanical bond coating
Does not reduce fatigue life due to its compressive stress state
Very dense
Cold gas spraying
Produced without the significant heating of powders. High particle velocity plastically deforms them on impact - adhereinf ot the workpiece (metals only).
Corrosion resistant, original material properties retained, works on thermally sensitive materials, eliminates residual stresses, works with dissimilar materials, high deposition rate is efficient.
Metallurgical bonding (in thermal spray)
Occurs when a splat melts to the solid substrate. Has a high coating adhesion and is made up of an intermediary composition of the splat and substrate.
Mechanical bonding (in thermal spray)
The splat amterials is mechanically interlocked with the substrate. Has a lower adhesion but both the splat and the substrate maintain their original material properties.
