Thermal-etching development of α-Zn2SiO4 polycrystals: effects of lattice imperfections, Mn-dopant and capillary force

Willemite (α-Zn2SiO4) polycrystals, prepared by sintering at 1350°C with or without Mn-dopant, were thermally etched at 1250°C for 1 min to 12 h and studied by scanning electron microscopy with regard to the effects of lattice imperfections, Mn-dopant and capillary force on etching development. For α-Zn2SiO4 without dopant, a rather rapid grooving at grinding scratches, grain boundaries and {1120} faceted pores caused a hill-and-valley structure in the nascent etching stage. The grooves were subsequently obscured due to increasing deposition rate and decreasing sublimation rate with increasing negative curvature and hence, a prominent capillary effect. In the later stage, etching at {1120} faults, dislocations (parallel to 〈1010〉 and [0001]) and subgrain boundaries became prominent. Thermal grooving at these lattice imperfections was significantly suppressed by Mn-dopant. Under such circumstances, surface diffusion and anisotropic lattice plane etching, the etching rate being faster for (0001) than {1010} and {1120}, caused the formation of surface hillocks with terraces and ledges.