Surface Texture

Surface

It is the one touches when holding an object, such as manufactured part.

• Nominal Surfaces  ↣  intended surface contour of part (defined by engineering lines).
• Strength increases  ↣  adhesively bonded or joints + rough surfaces.

Surfaces are important for a number of reasons:

1. Aesthetic Reasons  ↣  smooth and fire of scratches to give favorable impression to customer.
2. Surfaces affect safety.
3. Friction and wear depend on surface characteristics.
4. Assembly of parts is affected by their surfaces.
5. Smooth surfaces make better electrical contacts.
6. Mechanical and physical properties.

Surface technology is concerned with:

1. Surface Characteristics 
2. Surface Texture
3. Surface Integrity
4. Relationship between manufacturing process and characteristics of resulting surface.

1. Surface Characteristics 

Surface characteristics are defined as:

• Substrate  ↣  main core (bulk of part) of material.
• Grain size depend on metal processing (casting, molding, annealing, etc.).
• Surface Texture  ↣  exterior of part which is visible to us.
• In highly magnified cross-section  ↣  surface has roughness, waviness, flaws.
• Pattern  ↣  produce due to the mechanical process.
• Altered Layer  ↣  just below the surface texture differs from that of substrate.
• Result from work hardening (mechanical energy), heating (thermal energy), chemical and electrical energy.

2. Surface Texture

Surface texture is defined by four features:

• Roughness  ↣  small, finely spaced deviations from the nominal surface that determine material characteristics.
• Waviness  ↣  deviations of much larger spacing occur because of work deflection, vibration, heat treatment.
• Lay  ↣  direction or pattern of surface texture.
• Flaws  ↣  irregularities occur occasionally on the surface (include cracks, scratches, inclusions, etc.).

Surface Roughness and Surface Finish

• Surface Texture  ↣  measurable characteristics based on the roughness deviations as defined in the preceding.
• Surface Finish  ↣  is more subjective term denoting smoothness and general quality of surface.
• Surface Roughness  ↣  Average of vertical deviations from the nominal surface over a specified surface length.

There are two methods to find roughness (or average deviations)

Method 01: Arithmetic Average

• For this method arithmetic average (AA) or center line average (CLA) is used.
• Most widely used for surface roughness.

Method 02: Root Mean Square (RMS) Average

It is defined as, "the square root of mean of squared deviations over the measuring length".

• RMS surface roughness deviation > AA because larger deviations will figure more prominently in the calculation of RMS value.

Deficiencies of RMS method:

• It fails to account for the lay of surface pattern, thus affect surface roughness.
• Waviness can be included in roughness computation by using cut-off length.
• Cut-off length  ↣  a filter that separates the waviness in a measured surface from the roughness deviation.

Symbol of Surface Texture:

Designers specify surface texture on an engineering drawing by means of symbols.

3. Surface Integrity

It is defined as, "study and control of this surface layer when its structure differs from the substrate".

• Surface may have metallurgical or other changes  ↣  during mechanical, thermal, chemical, electrical treatment, etc.

Evaluation Techniques of Surface Integrity

1. Surface Texture  ↣  by roughness, lay designation, etc.
2. Visual Examination  ↣  reveals surface flaws (crack, craters, laps and seams) using fluorescent and photographic techniques.
3. Microstructural Examination  ↣  involve metallographic techniques.
4. Microhardness Profile  ↣  detecting hardness near surface (Knoop & Vickers Hardness) and obtain hardness profile (Hardness vs Distance).
5. Residual Stress Profile  ↣  X-ray diffraction technique used to measure residual stresses.

4. Effect of Manufacturing Processes

• Ability to achieve a certain tolerance is a function of manufacturing processes.
• It describes capabilities of various process in terms of tolerance, roughness and integrity.
• Most machining process  ↣  quite accurate, capable of 土0.05 mm or better. 

Note:

• Material science
• How material science helps to build greener future

References:

• Material from Class Lectures + Book named Fundamentals of Modern Manufacturing Materials, Processes and Systems by Groover + My knowledge. 
• Photoshoped pics are developed. 
• Some pics and GIF from Google.  
• Videos from YouTube (Engineering Sights).