What type of surface is required for test pieces in the Vickers hardness test?

In the Vickers hardness test, a polished surface is essential for obtaining accurate and reliable results. The testing process involves pressing a diamond indenter into the material's surface under a specific load. A polished surface ensures that the contact area between the indenter and the material is as uniform as possible, which is crucial for measuring the size of the indentation accurately. This precision is vital because the hardness value is calculated based on the diagonal length of the resulting indentation.

If the surface is textured, rough, or layered, it can introduce variability in the indentation size due to irregularities in the surface. Such imperfections can affect the pressure distribution and lead to inconsistent hardness readings. Therefore, having a polished surface minimizes these errors, enhancing the test's reliability and the credibility of the hardness measurement.