Hardness in the Science of Dental Materials

1. Definition of Hardness

• Predicts wear resistance of restorations
• Indicates the ability to be polished or finished
• Reflects the brittleness or toughness of a material
• Helps compare restorative materials to natural tooth structure

2. Methods of Measuring Hardness

A. Brinell Hardness Test (BHN - Brinell Hardness Number)

• Indenter: Hardened steel or tungsten carbide ball (diameter 1-10 mm)
• Principle: A known load is applied, pressing the ball into the material surface. After removing the load, the diameter of the indentation is measured.
• Formula:

BHN = Load (kg) / Surface area of indentation (mm²)

• Best for: Softer metals and alloys (e.g., dental gold alloys, amalgam)
• Limitation: Not suitable for very hard or very thin materials; the ball may deform on harder materials
• Reference: Phillips' Science of Dental Materials, 12th ed., Chapter 4

B. Vickers Hardness Test (VHN or VHN - Vickers Hardness Number)

• Indenter: A square-based diamond pyramid with a 136° apex angle
• Principle: A light load is applied, and after removal, the diagonals of the square indentation are measured under a microscope.
• Formula:

VHN = 1.854 × Load (kgf) / (diagonal length in mm)²

• Best for: Wide range of materials - metals, ceramics, enamel, dentin, composite resins
• Advantages:
  • Continuous scale (no change of indenter needed)
  • Very small indentation - can test microhardness (microVickers)
  • Useful for thin sections and surface layers
• Enamel VHN ≈ 340-360, Dentin ≈ 68, Amalgam ≈ 90-100
• This is the most widely used test in dental materials research
• Reference: Anusavice, Phillips' 12th ed., p. 62

C. Knoop Hardness Test (KHN - Knoop Hardness Number)

• Indenter: An elongated rhombic-based diamond pyramid (longitudinal/transverse diagonal ratio = 7:1)
• Principle: The indentation is very shallow - only the longer diagonal is measured (the shorter diagonal recovers elastically).
• Formula:

KHN = 14.23 × Load (gf) / (long diagonal in mm)²

• Best for: Very hard, brittle, or thin materials - especially ceramics, enamel, and thin coatings
• Advantages:
  • Shallower indentation than Vickers - ideal for brittle materials that crack with deeper indentation
  • More sensitive at low loads
  • Can distinguish hardness in very small areas (e.g., different zones of a tooth or a restoration margin)
• Enamel KHN ≈ 343, Dentin ≈ 65, Feldspathic porcelain ≈ 460
• Reference: Anusavice, Phillips' 12th ed., p. 63-64; Craig's Dental Materials, 9th ed., p. 26

D. Rockwell Hardness Test (RHN)

• Indenter: A steel ball or diamond cone (Brale indenter)
• Principle: Measures the depth of penetration directly (rather than the area or diagonal), using a dial gauge. A minor pre-load is first applied, then a major load, and the difference in depth is the hardness value.
• Best for: Metals and polymers; less used in research but common in industrial/quality control settings
• Advantage: Fast and does not require microscopic measurement
• Limitation: Less precise for small specimens or brittle dental ceramics
• Reference: Craig's Dental Materials, 9th ed., p. 27

E. Shore A Hardness (for Soft/Elastic Materials)

• Used specifically for elastomers and resilient lining materials (e.g., impression materials, soft denture liners)
• A blunt probe is pressed into the material under a spring load; the degree of penetration is read on a 0-100 scale
• Not applicable to hard dental materials

3. Summary Comparison Table

4. Factors Responsible for Determining Hardness in Dental Materials

1. Composition - The elemental or molecular makeup of the material (e.g., addition of noble metals increases hardness of gold alloys; feldspathic porcelain is harder than glass-ionomer due to crystalline content)
2. Crystal structure and grain size - Finer grain structures and denser crystal packing increase hardness
3. Degree of polymerization - In polymers and composites, higher conversion of monomer to polymer increases hardness (important for resin composites and denture acrylics)
4. Heat treatment / annealing - Age-hardening (precipitation hardening) in dental alloys (e.g., gold alloys) significantly increases hardness
5. Porosity - Internal pores reduce measured hardness
6. Filler content and particle size - In composites, higher filler loading and harder filler particles (e.g., quartz, zirconia) increase hardness
7. Water sorption - Absorbed water acts as a plasticizer in polymers and glass-ionomers, reducing hardness over time
8. Setting or curing time - Hardness increases as materials fully set or cure

5. Hardness Values of Common Dental Materials (for Context)

Standard References

1. Anusavice KJ, Shen C, Rawls HR. Phillips' Science of Dental Materials, 12th edition. Elsevier Saunders, 2013. (Chapters 4 and 5 - Mechanical Properties and Testing Methods) - This is the primary standard textbook.
2. Craig RG, Powers JM, Wataha JC. Dental Materials: Properties and Manipulation, 9th edition. Mosby/Elsevier, 2012. (Chapter 2 - Physical Properties)
3. O'Brien WJ. Dental Materials and Their Selection, 4th edition. Quintessence Publishing, 2008.
4. McCabe JF, Walls AWG. Applied Dental Materials, 9th edition. Blackwell Publishing, 2008.