Hardness of Dental Materials

Hardness
–– Hardness is defined as the ability of a material to resist scratching.
–– Can be divided into microhardness and macrohardness.

Tests for Microhardness:
- Knoop Hardness Test
- Vickers Hardness Test

Tests for Macrohardness:
- Brinell hardness
- Rockwell hardness

1. Brinell Hardness Test

• Oldest test employed for determining the hardness of metals
• A hardened steel ball is pressed under a specified load into the polished surface of a material. The load is divided by the area of the projected surface of the indentation, and the quotient is referred to as the Brinell hardness number, usually abbreviated as BHN.
• For a given load, the smaller the indentation, the larger is the number and the harder is the material.
• Used extensively for determining the hardness of metals and metallic materials used in dentistry
• It is related to the proportional limit and ultimate tensile strength of dental gold alloys.

2. Rockwell Hardness Number 

• Is somewhat similar to Brinell test in that a steel ball or a conical diamond point is used
• Instead of measuring the diameter of the impression, the depth of penetration is measured directly by a dial gauge on the instrument
• Neither the Brinell test for the Rockwell test is suitable for brittle materials.

3. Vickers Hardness Test

• Employs the same principle of hardness testing as Brinell test
• Instead of a steel ball, a diamond in the shape of a square based pyramidal is used
• The lengths of the diagonals of the indentation are measured and averaged
• The Vickers test is employed in the ADA specifications for dental casting gold alloys
• Suitable for determining the hardness of brittle materials
• Used for measuring the hardness of tooth structure

4. Knoop Hardness Test

• Employs a diamond indenting tool that is cut in the geometric configuration
• The impression is rhombic in outline, and the length of the largest diagonal is measured
• The hardness value is virtually independent of the ductility of the material tested.

5. Shore and Barcol Test

• Are employed for measuring the hardness of dental materials, particularly rubbers and plastics