5. Interpretation and discussion
5.1. Surface hardness related to porosity
In general coal is characterised by its thermal maturity or rank,
and its organic (type) and inorganic (grade) composition. Chemical
and physical changes occur, of which one is porosity, and this wil
influence the surface hardness. There are two types of porosity:
micro and macropore systems [2]. The macropore system is known
as the primary porosity and occurs primarily in lignites and subbituminous coals. The progressive coalification leads to increasing
rank accompanied with a change in geophysical factors, like compaction, loss of volatile matter and water expulsion. This reduces the
porosity in coal and reaches a minimum at the border from medium
volatile bituminous to low volatile bituminous coal [6], [18].
Henceforward the molecular structure through coals of a higher rank
(e.g. semi-anthracites) changes and a meso- and micropore system
starts to form. This development is termed secondary porosity [19].
The development of secondary porosity makes the material more
heterogeneous and thus increases the hardness of the coal. The
porosity acts like discontinuities in which the energy dissipates and
avoids the fractures to propagate through the material. This could
explain the increase of the SSRH values at the border from medium
volatile bituminous coal to low volatile bituminous coal (VR¼1.4%)
(Fig. 7).
5.2. Surface hardness related to cleats
Another explanation for the parabolic curvature could be the
presence of cleats, since the frequency of cleats is related to coal
rank [20–22]. Wolf et al. [23] described a parabolic trend for the
frequency of cleats decreasing from lignites (low frequency) to
bituminous coals (high frequency) and increasing again through
the antracite (low frequency) range. This trend suits the observed
trend as already described in this study (Fig. 7). In this case a low
frequency equals a high SSRH and contrariwise. Therefore this
could be used to predict the breakage behaviour of coal or to
determine the frequency of cleats.
5.3. Surface breakage behaviour of different lithotypes
After testing the SSRH on the confined samples, the impacts of the
diamond tipped hammer were analysed under the microscope to
determine how the impact affects the material and to investigate the
fracture pattern. The porosity of coal is dependent on the micropore
system [24] and maceral composition [2]. Vitrinite consists majorly of
a microporous structure, whereas inertinite has predominantly a
meso- and macroporous structure [25–28]. Fig. 8A shows the impact
of the diamond tipped hammer on a telovitrinite band, Fig. 8B on a
heterogeneous dull band and Fig. 8C close to a cleat. The impact
seems to be much more intense on the telovitrinite band (Fig. 8A) in
comparison to the impact on a heterogeneous dull band (Fig. 8B).
Fig. 8C shows that the impact affects the telovitrinite band but stops
at fractures or cleats filled with mineral matter. Due to the microporous structure of vitrinite, the material is more homogeneous.
Therefore the impact has a strong effect on the surface and breaks it
more easily. In comparison the impact on surfaces with a meso- and
macroporous structure is less intense (Fig. 8B). Inertinite consists
predominantly of a meso- and macroporous structure, which leads to
its heterogeneity. It is remarkable that the impact is not that intense
as already seen on the vitrain band (Fig. 8A). However, the radial
fracture pattern is still visible, but the impact does not tend to have a
great effect on the surrounding area. The energy seems to dissipate in
the pores of the meso- and macroporous structure.
Fig. 9 shows a close-up of the SSRH impact on an inertinite
band next to a vitrain band. The surface tends to break in a radial
fracture pattern (indicated by the white arrows). This fracture
pattern could be explained by the mechanisms of fatigue-crack
propagation and cyclic crack growth, which occur in brittle
material, such as intermetallics and ceramics [29]. The impact on
a material leads to an intrinsic microstructural damage, which
causes an extension of the crack growth departing from the impact
area. Further this is followed by an extrinsic crack-tip shielding
mechanism. This occurs behind the outgoing tip due to the
creation of an inelastic zone, which surrounds the crack, or from
physical contact between the crack surfaces resulting from wedging, bridging or sliding [30]. This shielding provides the crack
from breaking out and leads to microcrack toughening, which has
also been detected in rocks [29], [30]. This could be an explanation
of the radial structure pattern observed in the meso- and macroporous structure. Even though the radial breakage pattern can be
indicated in the microporous structure, the cracks tend to break