Knoop hardness test

Knoop Hardness Test

The Knoop hardness test is a microhardness testing method developed in 1939 by Frederick Knoop and colleagues at the National Bureau of Standards (now known as the National Institute of Standards and Technology, or NIST) in the United States. This method is particularly useful for measuring the hardness of brittle materials or thin sections of materials. Unlike other hardness tests, the Knoop test applies a lower load, making it ideal for a variety of materials without causing significant damage to the test specimen.

Overview

The Knoop hardness test involves pressing a diamond-shaped indenter into the surface of a material under a specific load for a predetermined amount of time. The indenter has a long, narrow rhombic shape, which distinguishes it from the more commonly known Vickers hardness test which uses a square-based pyramid indenter. After the load is applied and removed, the indentation left on the material's surface is measured using a microscope. The Knoop hardness number (KHN) is calculated based on the indentation's longest diagonal length, the applied load, and a constant that factors in the indenter's geometric properties.

Procedure

The procedure for conducting a Knoop hardness test is standardized under ASTM E384. The main steps include:

1. Preparing the specimen by polishing its surface to ensure accurate measurements.
2. Selecting an appropriate load for the material being tested. The load can vary but is typically much lower than that used in other hardness tests, making the Knoop test suitable for delicate materials.
3. Placing the indenter perpendicular to the test surface and applying the load for a specified time.
4. Measuring the long diagonal of the indentation left by the indenter using a microscope.
5. Calculating the Knoop hardness number using the formula that incorporates the indentation's diagonal length, the applied load, and the indenter's constant.

Applications

The Knoop hardness test is widely used in material science for evaluating the hardness of very brittle materials or thin layers where other types of hardness tests might cause too much damage or not be feasible. It is particularly useful in the following areas:

• Testing the surface hardness of ceramics, glass, and metals.
• Evaluating the hardness of thin coatings, such as paint or plated layers.
• Measuring the hardness of different phases within a microstructure without affecting the surrounding material.

Advantages and Limitations

Advantages:

• Minimal damage to the specimen due to low test loads.
• Suitable for a wide range of materials, including very hard and brittle substances.
• Capable of testing small areas and thin layers.

Limitations:

• Requires a polished surface for accurate measurement.
• More susceptible to operator error in measuring the indentation compared to some other hardness tests.
• Not as widely applicable for testing the bulk hardness of materials as some other methods.

Comparison with Other Hardness Tests

The Knoop hardness test is often compared to the Vickers hardness test and the Brinell hardness test. While the Vickers test can also measure microhardness using a diamond indenter, its square base makes it less suited for evaluating brittle materials. The Brinell test, which uses a spherical indenter, applies higher loads and is more suited for testing the bulk hardness of materials, but it is not suitable for brittle materials or thin layers.

Knoop hardness test Resources
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