The hardness of chromium-and titanium-doped sapphire crystals

The room temperature plasticity of sapphire crystals doped with chromium and titanium, respectively, is characterized by the hardness values derived from measurements of the penetration depths with the recording test equipment developed by Frohlich and Grau. The hardness measured under load as well as the “crack-free” hardness numbers increase at low dopant contents, whereby titanium is more effective than chromium influencing the hardness at concentrations by a factor of five higher than the titanium additives. At higher dopant contents the above mentioned hardness values decrease beneath the values of the undoped material. The influence of the dopants on the conventionally measured Vickers hardness depends on the applied load.