What is the third type of hardness testing machine alongside Brinell and Rockwell?

The Vickers hardness test is indeed considered a third type of hardness testing method, alongside Brinell and Rockwell. This method employs a diamond pyramid indenter, which creates a specific pattern of indentation on the material being tested. The Vickers test is particularly versatile because it can be used on a wide range of materials, from soft metals to hard ceramics, providing a continuous scale of hardness values.

One of the key advantages of the Vickers test is that it uses the same indenter regardless of the material hardness, allowing for consistent comparison across different materials. The Vickers hardness number (HV) is determined by applying a known load to the indenter for a specified time, measuring the diagonal length of the indentation produced, and calculating the hardness based on the applied load and the size of the indentation.

The other methods listed, such as Knoop, Mohs, and Barcol, serve specific purposes or apply to certain materials but are not recognized as the third main type as per the established hardness testing criteria alongside Brinell and Rockwell. Each of these alternatives has unique characteristics and applications, making them more suitable for certain situations, but they do not have the same wide acceptance as the Vickers method in general hardness testing.