Bench Top Testers: Selecting the Right Mechanical Tester

2nd in 4 Part Series: Selecting the Right Mechanical Testing Instrument
In this second installment of our four part series we’ll examine what you need to consider when choosing a benchtop hardness tester instrument for your application.  

Technique

Benchtop hardness testers work under the principal of applying a load with a known diamond shape and measuring the size of the indent. These measurements often provide the basis of standard or benchmark measurements. However trends in thinner materials, plating, scratch and wear resistant coatings present some considerations that should be examined when evaluating testing techniques.  

Brinnel

Brinnel Hardness Testers base hardness on the size of the imprint resulting from the application of a hard ball measuring 1/8” to ¼” or larger. These testers are ideal for thick metallic or plastic materials.  Samples must be at least ½” thick for testing in this method.  

Rockwell

Rockwell Testers have similar limits of Brinnel testers and also use spherical indenters but hardness is calculated by measuring the displacement of the tip from a base load to a maximum load. This implies that measurements on Rockwell testers are easily influenced by compliance issues. Bowed samples can allow a gap of air to develop between the sample and the base which can result deformation during the test thereby artificially increasing the depth and decreasing the calculated hardness. An advantage of Rockwell testers is that it easily tests elastic materials since an observation of the indent is not necessary.

Vickers

Vickers Hardness Testers use a sharp four sided pyramidal tip to measure hardness, loads can vary from 1 gram to 30 kg and even higher. Hardness is calculated from the measurement of the size of the indent left on the surface. Of course, this prevents its use for materials that are very elastic such as glass. If the indent is big enough to be seen, optical cracks will appear around the indent. This problem is common in testing ceramic materials. At low loads the Vickers tester, also call a Micro Hardness Tester, experiences greater influence of surface information, sometimes within the first few microns.

Because the indent needs to be easily identifiable on the surface roughness can be an issue if the surface is significantly uneven. Surface errors from measuring the size of the indent will increase because of the optical limit, roughness, or structure of the materials.  

Micro Hardness

When using Micro Hardness Testers using the Vickers method it is important to understand that the depth of the indent must not be greater than 1/10 of the thickness of the platting, otherwise the measurement will be influenced by the substrate. For most hard materials a rule of 1/8” is acceptable. Because of optical limits it is not possible to measure coatings under 2 µm without some influence from the substrate with this type of instrument. Even over 2 µm, the variation from optical observation can easily increase the standard deviation between measurements. The error will decrease as the tip goes deeper. 

Next Insider we will discuss Instrumented Micro and Nano Indentation Testers which are used for higher precision and lower loads.