What is the difference between Mohs and Knoop hardness?

Difference Between Mohs and Knoop Hardness

The Mohs and Knoop hardness scales are two distinct methods used to measure the hardness of materials, each with its unique approach and applications. Understanding the differences between these scales is crucial for selecting the appropriate method for material testing.

Mohs Hardness

The Mohs hardness scale, developed by Friedrich Mohs in 1812, is a qualitative scale that ranks materials based on their ability to scratch or be scratched by other materials. It ranges from 1 (talc) to 10 (diamond), with each material capable of scratching those below it on the scale. The Mohs scale is primarily used for minerals and is less precise than other hardness scales. It is simple and does not require any special equipment, making it suitable for fieldwork or educational purposes.

Knoop Hardness

The Knoop hardness test, developed in 1939, is a microhardness test that measures the resistance of a material to indentation under a specific load. It uses a diamond indenter with a rhombic-based pyramid shape to make a small indentation on the material's surface. The Knoop hardness number (KHN) is calculated based on the indentation's long diagonal, allowing for the testing of brittle materials or thin layers. The Knoop test provides a quantitative measure of hardness and is more precise than the Mohs scale, making it suitable for laboratory analysis and quality control in manufacturing.

Comparison

• Mohs Hardness is qualitative and ranks materials based on their ability to scratch others, making it less precise.
• Knoop Hardness provides a quantitative measure of hardness based on indentation size, offering greater precision and suitability for detailed analysis.
• Mohs scale is easier to use and requires no special equipment, whereas Knoop testing requires specialized tools and is more complex.
• Knoop hardness testing is more versatile, allowing for the measurement of brittle materials and thin layers, which is not feasible with the Mohs scale.

In conclusion, while both Mohs and Knoop hardness scales serve to measure the hardness of materials, they do so in fundamentally different ways and are suited to different applications. The choice between them depends on the specific requirements of the material testing, including the level of precision needed and the nature of the material being tested.