Microhardness Testing Explained: Vickers & Knoop Testing
When you need to know just how tough a sample is and how well it will resist penetration, microhardness testing is the obvious choice. Dive in to find out everything you need to know about microhardness testing and how it could help you make sure your materials are up the job.
Microhardness testing: the basics
Microhardness testing is a method of determining a material’s hardness or resistance to deformation that can be used on samples where macrohardness testing isn’t suitable.
Macrohardness testing is very common, but it typically uses high applied loads of over 10 Newton (N). This can be too much for some materials, such as films, thin metals or ceramics, causing failure or destruction. Rockwell testing and Brinell testing are two great examples of macrohardness testing.
Microhardness testing uses loads of up to 10 N and is suitable for thin materials, smaller samples, thin films and plated surfaces.
Type of microhardness testing
Typically, microhardness testing uses an indenter probe which is pushed into a surface under a defined load for a determined amount of time. This process will generally leave an indentation in the surface, and this is then measured to determine hardness.
Sample size, preparation and environment all have to be taken into account when testing microhardness. For example, the microhardness testing machine must be placed away from vibrations to avoid affecting the results.
Rough surfaces can also cause variation and it is recommended to use a standard polishing method before testing. It is also often necessary to cut samples to a suitable size beforehand.
There are two main methods of microhardness testing, both offering repeatable and accurate results: Vickers hardness testing and Knoop hardness testing.
Vickers hardness testing
The Vickers hardness test uses a Vickers indenter, which is shaped like a squat pyramid and is typically pressed into the surface of the material for 10 seconds. The amount of force used is of a pre-determined amount and varies depending on the materials being tested.
Once the time is up, the indentation left in the sample is analyzed optically to measure the lengths of the diagonals, these are then used to calculate the size of the impression. Vickers testing isn’t suitable for coating thicknesses under 60 microns.
Knoop hardness testing
Knoop hardness testing is similar to Vickers, but it uses a differently shaped indenter. The Knoop indenter is more elongated and rectangular in shape. It is also pushed into the surface of a sample to produce an indentation which can then be measured. Unlike the Vickers method, Knoop uses only the long axis when calculating the indentation as a hardness indicator.
The Knoop method is often used done with lighter loads and the shape of the probe means it is suitable for samples needing indentations close together or on the edge of a sample. It can also offer higher levels of precision than the Vickers method, due to the fact the longitudinal diagonal of the indenter is greater. That said, careful sample preparation is necessary to ensure that the results remain accurate.
Key differences between Vickers and Knoop hardness testing
The biggest difference between Vickers and Knoop hardness testing is the design of the indenters. The Knoop test also tends to cause less damage to samples, due to the fact it’s indenter is shallower. The Knoop method only penetrates the specimen half as deep as the Vickers pyramid indenter.
The variation in the shape of the indenters also means that the Vickers method is more suitable for small, rounded samples while the Knoop method suits small, long samples.
Microhardness testing with Tru-Blue micro VT
Our Tru-Blue range of microhardness testing machines has been designed to help you determine just how durable your samples are. For repeatable and accurate testing results, look no further than the Tru-Blue system.
Offering both Vickers and Knoop testing in one handy machine, the Tru-Blue Micro VT microhardness tester is ideal for metal or alloy parts. It has a single, interchangeable turret and an easy to use interface. It allows you to test to industry standards, including and not limited to ASTM E834, ASTM E92, ISO 4546, ISO 6507 and ISO 9385.
We also offer a selection of useful accessories for preparing your samples prior to testing, including the stand-alone Tru-Blue Grinder & Polisher, which is easy to plug in and even easier to operate.
There is also the Tru-Blue Sample Cutter, perfect for cutting metal and alloy in preparation for Vickers or Knoop testing. It’s a sturdy machine with stainless steel components and a robust base, to ensure a sturdy, vibration free environment.
Microhardness testing with Industrial Physics
We know how important it is to have your lab running smoothly and for your testing processes to be fast, accurate and repeatable – that’s why every one of our testing machines is manufactured by skilled experts using the highest quality components possible.
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