Hi Praveen,
Just to give you a quick insight that why friability is done so that you can have a better understanding.
Friable is a French word which means “easily crumbled or broken” including Latin origin of Friare which means “rub away, crumble into small pieces”. So from the definition it is clear that it is on the other way we are checking the hardness only, so Why Friability testing, why not hardness testing only?
This test is carried out in un-coated, compressed tablets which has the tendency of getting chip off, crumble or break off during handling and subsequent storage (which also include transportation of product). It is not only important for dispersible tablets where hardness cannot be increased, but to those tablet as well which are already have proper hardness maintained. This is due to avoid tablet to exhibit any unacceptable capping or excessive wear on the tablet edges or logo while undergoing Coating operations. The tablet hardness cannot be increased beyond the limit as there will be impact on dissolution and disintegration parameter and it will have larger impact on biological activity of the drug product.
Hope I answer your query, for more details on friability please refer following references.
USP General Chapters: <1216> TABLET FRIABILITY
USP General Chapters: <1217> TABLET BREAKING FORCE