Hardness and elastic modulus of silicalite-1 crystal twins

Silicalite-1 crystal twins 220 × 40 × 40 μm3 in size were prepared by hydrothermal synthesis. Their hardnesses and elastic moduli were determined from depth sensing indentations using Berkovich tip. Measurements on all four lateral crystal faces did not show any significant difference, which is in agreement with the face equivalence given by form {1 0 0}. Also no difference in mechanical properties was found on those surface areas that appeared to be relatively resistant against etching with HF acid.As-synthesized crystals, calcined crystals (free of organic template by calcination) and fused silica bulk were compared. The calcined crystals were harder than as-synthesized ones but both much softer than fused silica. Elastic modulus of both as-synthesized and calcined crystals was practically the same while it was nearly doubled for fused silica. The ratios of hardness to reduced elastic modulus (H/E∗) were found to be 0.063, 0.103 and 0.120 for as-synthesized crystals, for the calcined ones and for fused silica, respectively. Elasticity and plasticity energies of samples were calculated. All three investigated materials showed elastic recovery greater than 50%. The experimental values were also compared with theoretical dependencies.