Hardness Test

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Hardness in simple term is the property of material by which it is able to hold all its constituents in an intact form. We will deep ourselves some more, you may be familiar with one property of material called plastic deformation, during compression of powder/granules undergoes plastic deformation lead to form good tablet and when tablet undergoes plastic deformation1 then it will break into pieces.

Don't get confuse, Similar thing is happening in both cases i.e. material losing its internal strength but just it looks different...... how?

One tablet under applied external (compression) force will break. Consider, 1000 such tablets undergo similar force for several times then it will also give a BIGGGGG tablet. Similar thing happen with powder/ granule, during compression powder/ granules undergoes several such compression cycles by which one surface of particle will come in front of other, new surface will form bond and ultimately tablet form (under plastic deformation).

The force responsible for plastic deformation of any material 2-4 may be due to cutting, abrasion, scratching or bending.

Measurement of hardness5-7

In general case, hardness of any metals measured in five different scales viz,. Brinell (HB), Knoop (HK), Rockwell (HR), Shore (HS) and Vickers (HV). It can be done by using different shapes of diamonds, hard steel or carbide which pressed in the material with known amount of force and resulting depth of penetration or size of indent is measured. But, if we consider a tablet; it is brittle and can't able to determine hardness with these methods. Tablet hardness means compressive strength of tablet and not tablet strength.

Hardness of tablet can be measured in different units7,8 like

Kilogram (kg)

Newton (N) - 9.807 N= 1 kg/ 1N= 0.1020kg

Pound (lb) - 1 lb= 0.4536Kg = 4.4482N

Strong Cobb (SC)- 1.4 SC ~ 1kg

Kilo pond (kp)- 1 kp= 1kgf

Last two units are not used in recent time.

History of hardness test3,7

When we look into the history of the hardness test, we find it is around 80 years old test. Initially, it was measured by mechanical means simply by applying force using threaded screw with spring until tablet crack and that hardness was obtained on sliding scale. The same principle was followed in Strong Cobb tester which was designed first in 1950's. But, this tester was unable to give precise readings as testing force applied by this tester was provided by manual air pump. After this many improved instruments were introduced which shows more precise readings and recently many electro-mechanical devices available in market with high range of sensitivity and accuracy.

Importance of hardness test

Hardness test of material is indicative of its strength. On the other hand, one can say it also indicates resistance power to damage its intactness. For tablet, it reflects the internal bonding strength of granules/powder which can able to hold composite structure under applied external force.

How hardness of oral formulation related to pharmacokinetics is shown by figure 1:

Advantages of hardness test3

  • Hardness test give an idea about the amount of force which can able to fracture as well as it will also help to access compatibility of formulation.
  • It will also serve as guideline in handling, packaging and storage of formulation.

Disadvantages of hardness test3

It is unable to give idea about capping and lamination behavior of formulation especially in case of tablet.

Guidelines in Pharmacopoeias

There is no such official specification given in any pharmacopoeia about hardness of material and hardness test. Formulator should have to decide and maintain the range of hardness during preparation based upon requirement as well as type of formulation.

e.g. effervescent or mouth dissolving tablet should have low hardness as compared to conventional immediate release tablet or enteric coated tablet.

Factors affecting strength (hardness) of tablet3,9-12

  • Speed of compression
  • Solid state structure
  • Particle size
  • Mechanical interlocking
  • Solid bridges
  • Distance forces
  • Bonding mechanisms
  • Volume reduction mechanisms of powder during the process of compression
  • Surface area change during compression

We discuss details about various hardness testers in our next blog........

Anything if you have other than this, please contribute in the blog so that reader can get complete information. Any details required, please feel free and comment or please refer some resources given below:


  1. Plastic deformation in solids. http://en.wikipedia.org/wiki/Plastic_deformation_in_solids (accessed on 10 July' 2010)
  2. E. Aulton. Pharmaceutics: The science of dosage form design, Second Ed, Churchill Livingstone, Edinburgh.
  3. Remington- The science and practice of pharmacy. Twenty-first Ed, Lipincott Williams and Wilkins, Philadelphia. USA.
  4. Lachman L, Liberman H, Kanig J. The theory and practice of industrial pharmacy, Third Ed. Varghese Publishing House, Bombay. India.
  5. Hardness testing. http://www.gordonengland.co.uk/hardness/ (accessed on 10 July' 2010)
  6. Cooper D, Carter S. Some information on hardness testing. Engineering systems, 2009.
  7. http://www.gordonengland.co.uk/conversion/index.htm [for all units conversion simultaneously] (accessed on 10 July' 2010)
  8. Nystrom C, Alderborn G, Duberg M, Karehill PG. Bonding surface area and bonding mechanism two important factors for the understanding of powder compactability. Drug Dev. Ind. Pharm. 1993; 19: 2143-2196.
  9. Armstrong NA, Palfrey LP. The effect of machine speed on the consolidation of four directly compressible tablet diluents. J. Pharm. Pharmacol. 1989; 41: 149-151.
  10. Sebhatu T, Alderborn G. Relationships between the effective interparticulate contact area and the tensile strength of tablets of amorphous and crystalline lactose of varying particle size. Eur. J. Pharm. Sci. 1999; 8: 235-242.
  11. Israelachvili JN. Intermolecular and surface forces. Second Ed. Academic Press, London.

Books for Further Reference

  • Banker GS, Rhodes CT. Modern Pharmaceutics, Third Ed. Marcel Dekker Inc, New York.
  • Swarbrick J, Encyclopedia of Pharmaceutical Technology, Third Ed. volume 1, Informa Health care, New York.
  • Lieberman HA, Lachman L, Schwartz JB. Pharmaceutical dosage forms-Tablets, second edition, vol1, Second Ed. Marcel Dekker Inc, New York.
  • Watt PR, Armstrong NA. Tablet and Capsule Machine Instrumentation, First Ed. Pharmaceutical Press, London.
  • Qiu Y, Chen Y, Zhang GGZ. Developing solid oral dosage forms-Pharmaceutical theory and practice, First Ed. Academic Press, Elsevier, New York.

Related Articles

  • Alderborn G. Granule properties of importance to tableting. Acta Pharm Seuc 1988; 25: 229-238.
  • Hassanpour A, Kwan CC, Ng BH, Rahmanian N, Ding YL, Antony SJ, Jia XD, Ghadiri M. Effect of granulation scale-up on the strength of granules. Powder Technology 2009; 189(2): 304-312.
  • Vojnovic D, Moneghini M, Rubessa F, Simultaneous optimization of several response variables in a granulation process. Drug Dev Ind Pharm 1993; 19: 1479.
  • Walker GM, Holland CR, Ahmad MN, Fox N, Kells AG. Effect of granulation process parameters on the hardness of fertiliser granules. Powder Technology 2003; 132: 81-84.

About the Author

Amol Malpani's picture
Author: Amol Malpani


criticone's picture

Nice attempt. Couple of quick requests 1)If you could include one hardness tester photo that give good impact for readers . May be you can take picture of hardness tester in your lab and include 2)If possible collect old model harndess testers picture and include that picture in your history section. 3) You might want to discuss as another section like Effect on Hardness on Tablets (ex :low vs high on dissolution etc) 4) You might want to included information on what kind of tablets need very high hardness ( like chewable) and very low (like mouth dissolving) . May be by each tablet type (uncoated,coated (film,sugar coated), chewable,dispersible, etc.. OR may be like ...3 column table type/suggested hadrness range /comments... I am just throwing ideas ... hope you don't feel bad
Amol Malpani's picture

First of all Thanks a lot for your valuable suggestions. Actually, in next session we wish to compare different hardness testers and methods and thats why I didn't include any hardness tester here. But, I also feel I should include hardness tester as well as old model of hardness tester and I will include it soon. I wish to put hardness range of different tablets but I am unable to get appropriate values for each categories of tablet (I got some). Please inform me if you know any source. Anticipating your active involvement and help Sir. Regards,
criticone's picture

Thanks for your reply. I don't have the information but if i can find, i will post here. You can also post whatever the information you have . For rest of the information, hopefully any one of the leaders might be able to help you.
Dr.S.Gunasakaran's picture

Nice informative article you posted

Dr.S.Gunasakaran,MBBS,MD. Head - Clinical Research & Medical Affairs www.clinicalresearchsociety.org/forum

Amol Malpani's picture

Dear Sir, Happy to see your comment after long time. Thanks a lot. Suggestions from your side are welcome Sir. Regards,

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