Abstrate Co1-XO polycrystals, prepared by sintering at 1600oC with or without Mg2+-dopant ,were thermally etched at 400-1500oC for 10 min-12hr and studied by scanning electron microscopy regard to the effects of temperature and Mg2+-dopant on etching development. In low etching temperature (400-800 oC), vacancy relaxation process and spinel precipitate produced on the surface of Co1-XO-MgO polycrystals. In high etching temperature (1500 oC), Co1-XO epitaxy was spread on the steps and ledges. In addition, Co1-XO was thermally etched at 1500 oC for 10min to 12hr, we found triangular to hexagonal etch pits were produced. However, Mg2+-dopant changed the geometric shape of the pit opening. Besides, growth hillocks with corrugated terrace steps at edge and a pit at center showed that growth and etching of crystal can happen in the same time.

Revealing the prior austenite grain boundaries from a martensitic structure is well known to be very difficult and dependent on the chemical composition and the thermomechanical processing of the steel. In the present study, four different chemical etching reagents and additional thermal etching have been conducted for thermomechanical simulated tool steels Orvar Supreme and Stavax ESR. The etching results have been characterized using light optical microscopy and electron backscattered diffraction. The obtained results show that saturated aqueous picric acid, oxalic and sodium bisulfite based acid reveals prior austenite grain boundaries well for Orvar Supreme. For Stavax ESR, only aqueous CrO3-NaOH-picric acid provides good results in revealing the prior austenite grain boundaries. Thermal etching shows good potential and if conducted properly, thermal etching is a good alternative to the chemical reagents from a health- and environmental perspective.