P Pallav, PhD

Surface Hardness

 Surface Hardness is assessed as BHN, VHN, or KHN, referring to Brinell, Vickers, and Knoop Hardness Number respectively. Standardized steel balls (Brinell) or diamond indenters (Vickers, Knoop) are used to make indentations as shown in the lower part of the figure. 

 These are measured with a microscope, which is generally part of the testing device.

The surface hardness is a measure of the so-called plastic load carrying capacity. The deformations around the indenter should be (nearly) fully plastic.

Theoretically, the hardness would then be obtained by calculation of the average pressure; the force divided by the area of the indentation. The formulas associated with each method are a bit more complex than this to compensate for an average and small amount of elastic deformation, which is also present.

The requirement of nearly fully plastic deformation puts two categories of materials in a special position.

1. Very brittle materials cannot deform plastically. Consequently numerous fractures, rather than a plastic zone, will occur around the indenter. In spite of this, hardness measurements on brittle surfaces are generally considered acceptable.
2. Very flexible materials, such as soft rubbers yield mainly elastically. Of course the indentation will be large during loading, but afterwards there will be no indentation, erroneously suggesting a very hard material.

Penetration Hardness

Mainly Rockwell (A, B, or C) and Shore (I, II) occur in literature. With the Rockwell method a standardized cone or sphere indents the surface with a standardized force. This is similar to the surface hardness measurements described above.
The hardness is related to the size and shape of the indenter and the depth of penetration. 

The Shore method uses a small gadget with a standardized stamp, out of which a small steel pin emerges. This apparatus is pushed against the surface hard enough and the Shore-hardness can be read directly from a dial ranging from zero to one hundred.

Both penetration methods, especially Shore, are more popular with softer materials.

Roughness

Roughness is generally measured with surface tracing equipment. A steel or diamond tip traces a straight line over the surface, while the exact position (height) of the tip is recorded continuously. In this way many surface parameters may be assessed.

The roughness or Ra-value is defined as the average distance of the traced line to a straight line, midway through the surface profile. This is identical to the c.l.a. value. The lower part of the figure shows the traced surface profile. Normally the vertical magnification is much greater than the horizontal.

Mirror finish is obtained when the average distance between the peaks is less than twice the shortest wavelength of visible light, about 400nm (blue).
With ordinary surfaces the average distance between roughness peaks is usually some ten times as great as their height.
Luster therefore requires an Ra-value < 0.1µm.