Hardness
Describe briefly hardness.
Or
Write a short note on hardness.
Answer:
Hardness is defined as resistance to permanent indentation. Initially, an arbitrary hardness indexing scheme known as the Mohs scale is used to determine hardness. Hardness does not have any clinical relevance, though the measurement of hardness is an indicator of other properties.
Hardness values are used as an indicator of various other properties, i.e.
- Resistance to wear: The hardness of the restorative material plays a major role in its resistance to wear, though hardness cannot be taken as an absolute indicator of wear resistance. Softer substances essentially do not wear faster than harder substances
- Strength: Some materials such as gypsum display a relationship between hardness and strength
- Degree of polymerization: At times, hardness values are used to indicate the degree of monomer conversion in denture-based resins and resin composites; it is implied that higher hardness values indicate a greater degree of polymerization.
- Depth of cure of resins: The depth of cure of light-activated composite resins can also be evaluated using hardness tests. A known thickness of composite resin specimen is light-cured. The surface hardness is evaluated on the cured surface and the bottom surface. If the surface is completely polymerized, it can be expected that the bottom surface be at least 80% as hard as the top surface.
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