Surface Engineering

Surface Engineering

1. Prevents

   • Metal Fatigue
     • Crack growth and local plastic deformation
     • 90% of component failure
     • Avoided by
       • Strengthening surface
       • Putting surface layer under compression
         • Shot peening
   • Corrosion and Oxidation
     • Caused by reaction of surface with environment
     • Costs 4% of GNP
     • Reduced by modifying surface chemistry, e.g. galvanising
   • Wear
     • Abrasive
       • Cutting action
       • Hard particles between 2 surfaces
     • Adhesive
       • Rubbing of two smooth surfaces together
     • Erosion
       • Impact of a stream of liquid or gas on a surface
     • Fatigue
       • loss of material when surface fatigue cracks join up to allow surface particles to break away from the surface

2. Improves appearance

3. 2 Approaches

   • Heat treatment
     • Sometimes accompanied by changing surface chemistry
     • Mostly to harden surface of ferrous alloys
     • Surface layer is integral with substrate
     • 2 types
       • Thermal Hardening
         • Case hardening
           • Quenching
             • From austenite at 800-100 degrees C to 300-600 degrees C. (0.3-0.6wt%)
               • Rapid quenching gives
                 • Bainite
                 • Martensite
                   • Only if sufficient C 0.2wt%
             • Flame Hardened
             • Electromagnetic Induction
           • Tempering
           • Cheap
           • Limited to steel
           • No adhesion wear as surface layer is integral to substrate
             • May be under compressive stress which will reduce fatigue
           • Can select specific areas to be treated
           • Depth of case hardening can be altered
       • Thermochemical hardening
         • Carburising
         • Carbonitriding
         • Nitriding
   • Deposition of a hard material on the surface
     • Low temperature processes can be applied to a wide range of materials
     • High temperatures restrict the substrate and coating that can be used
     • 6 types
       • Electrochemical
       • Chemical treatments
       • Chemical Vapour deposition
       • Physical vapour deposition
       • Spraying
       • Welding

4. Tensile strength of steel increases with respect to C wt% up to 0.8wt%