Selection of Preparation Techniques Based on Material Problems and TEM Analyses - Sample Preparation Topic for Transmission Electron Microscopy - page 179

all the more significant depending on whether the material is ductile, soft, or resis-
tant. These are mechanical-thinning techniques that will present problems: either
the material is hard and brittle and sample breakage may occur or the material is
soft and strain hardening is likely. Between these two extremes, the technique must
be adapted to preserve the intrinsic structure of the material.
7.4.2 Soft-Ductile Materials
The softer a material, the more its structure will be sensitive to mechanical pressures
during preparation. Thus, the investigation will ruin a liquid crystal or mesophase
carbon structure if the mechanical-grinding technique is used, whereas the material
will be saved if the ultramicrotomy technique is used, as this technique minimizes
mechanical damage. Generally speaking, the best preparation technique for bulk
and fine particle soft samples is ultramicrotomy or cryo-ultramicrotomy. For bulk
soft materials, different thinning techniques can be used: chemical, electrochemi-
cal, ionic, and FIB thinning. If the sample is a thin layer or multilayer material,
the best result should be obtained using ultramicrotomy or cryo-ultramicrotomy, or
mechanical thinning followed by ionic or short chemical thinning.
7.4.3 Hard-Resistant Materials
For hard materials, ultramicrotomy will be ineffective because interfaces are not
kept complete due to the hardness of the material confronted with the hardness
of the diamond knife. In this case, mechanical polishing will be more favorable
because it helps to keep grain boundaries and interfaces together, even down to
small thicknesses.
For single-phase and hard materials, mechanical polishing combined with ionic
thinning can produce good results. Likewise, in this case, FIB ionic and electro-
chemical polishing also produces good results.
For multiphase materials, only gentle mechanical polishing can help to obtain
large observable areas, while maintaining all of the phases. In this case, FIB
produces very good results.
Preparing hard samples by means of mechanical polishing does not present any
particular limitation. However, the granular nature of a ceramic or composite can
result in a minimum critical thickness of the sample in order for it to maintain its
mechanical cohesion. If this thickness is not thin enough to be electron transparent,
final ionic or chemical thinning may then be necessary.
7.4.4 Materials of Intermediate Hardness and Ductility
For intermediate cases, pure mechanical thinning should be tried. If this is not pos-
sible, thickness should be reduced as much as possible with gentle tripod polishing
and a final ionic, chemical, or electrochemical thinning for just a few minutes, so
as to maintain the interfaces between phases or matrix and precipitates. The FIB
technique produces very good results.