Lecture Notes: Hardness Testing Methods

Importance of Hardness Testing

• Essential for components in contact (e.g., gears, shafts, bolts, pins, guide rails)
• High wear resistance & strong surfaces
• Hardness: resistance to indentation by a standardized object
• Methods depend on the same principle: indenter pressed into material

General Principles

• Hardness value is determined from indentation left behind
• Different methods: Brinell, Vickers, Rockwell
• Testing is destructive due to surface indentation

Brinell Hardness Test

• Procedure:
  • Material placed under a microscope
  • Cemented carbide ball as indenter
  • Indenter pressed into the material
  • Indentation surface evaluated under a microscope
• Evaluation:
  • Diameter of indentation measured from two perpendicular values
  • Hardness value: ratio of applied force (in Newtons) to indentation surface
  • Formula and calculation explained in detail
• Considerations:
  • Indentations should be spaced appropriately
  • Sample thickness should be adequate
• Advantages:
  • Suitable for soft to medium-hard materials
  • Good for heterogeneous microstructures (e.g., cast iron)
• Disadvantages:
  • Not suitable for very hard materials or thin surface layers

Vickers Hardness Test

• Procedure:
  • Uses a diamond pyramid indenter
  • Indenter pressed into the material
  • Indentation surface analyzed using the diagonal of the square imprint
• Evaluation:
  • Calculation of hardness based on the indentation diagonal
  • Forces in Newtons convert from kilo ponds
  • Diagonal measured from two perpendicular lengths
• Advantages:
  • Suitable for all hardness ranges
  • Accurate for thin sheets and thin surface layers
  • Independent of test force for large loads
• Disadvantages:
  • Elastic deformation impacts accuracy at lower forces

Rockwell Hardness Test

• Procedure:
  • Measures indentation depth
  • Indenter types: cemented carbide ball or diamond cone
  • Steps: pre-load, application of test load, and removal
  • Measures remaining indentation depth
• Evaluation:
  • Hardness value calculated based on penetration depth
  • Formula varies depending on indenter type
  • Various scales: A, B, C, etc.
• Considerations:
  • Minimum sample thickness and distance from the edge required
• Advantages:
  • Short testing time
  • Suitable for automation
• Disadvantages:
  • Less accurate for very hard materials
  • Affected by impurities in the material

Summary and Comparison

• Brinell:
  • Advantages: Heterogeneous materials, soft-medium hard range
  • Disadvantages: Not for very hard/ thin layers
• Vickers:
  • Advantages: Accurate, suitable for all hardness ranges & thin materials
  • Disadvantages: Less precise at low forces
• Rockwell:
  • Advantages: Fast, good for automation
  • Disadvantages: Small depth range, less accurate for very hard materials