SCRATCH RESISTANCE
Glazed tiles intended for flooring are also subjected to mechanical aggressions in
the form of gouging owing to the movement, under pressure, of materials with sharp
edges that are harder than the glaze that forms the tile surface. Hence the consideration
of scratch resistance in the past as a fundamental characteristic for all glazed tiles
intended for flooring.
However, the subjectivity of the test method set out in standard UNE 67-101
(1992) has resulted in the elimination of this characteristic in standard EN 14411.
Since it is a characteristic that is still provided by most manufacturers, it is
interesting to have it as complementary information to the resistance to surface
abrasion. A scratch hardness of 6 provides a certain assurance of scratch resistance in
flooring subject to intermediate and high traffic.
MOHS SCALE OF HARDNESS
STANDARD EN 67-101-92
Mineral
Mohs hardness
TALC
1
GYPSUM
2
CALCITE
3
FLUORITE
4
APATITE
5
ORTHOCLASE
6
QUARTZ
7
TOPAZ
8
CORUNDUM
9
DIAMOND
10
Scientific-technical aspects
Hardness is defined as the resistance that a material opposes to being penetrated
by another material, this being a property innate to the nature of the material. To
measure this, in homogeneous materials, the Vickers microhardness test has been
used, which consists of indentation by an inverted diamond pyramid, of standard
dimensions, in the surface of a homogeneous material, applying a given load for a
period of time.
Physico-chemical characteristics
Scratch resistance
1
The effect of the indentation is then evaluated because, since a track is produced
by pressure, there is an elastic strain and a plastic deformation, though the physical
mechanisms underlying such behaviour are still insufficiently understood.
Some authors interpret the phenomenon by
assuming that the load that initially lies on a single
point creates such a high stress that it originates a
plastic flow. When the glassy system flows, a tiny
cavity opens whose walls progressively deform
owing to the extracted material. As this small
excavation becomes deeper, the contact surface
between the tip of the material that produces the
penetration and the glassy system increases while,
at the same time, the stress decreases until this
falls below the yield stress. From this moment on,
the elastic strain mechanism starts acting, it being
maintained provided the elastic limit is not
exceeded and rupture occurs.
The stresses that do not succeed in relaxing
by plastic flow or rupture become permanent and,
under polarised light, appear around the track
imprinted in the glaze.
M-400 microhardness tester
Although Vickers microhardness is an objective and reproducible determination,
it has the disadvantage of being a point test, so that in order to extrapolate data that will
allow interpretation of the characteristics of a tile, it is necessary to make numerous
measurements on the tile surface. In the case of heterogeneous surfaces, like those of a
large number of products, the scatter in values may become considerable.
Another interesting parameter is toughness, whose interpretation is related to the
capacity that a material has to resist sudden crack propagation.
In order to be able to calculate glaze toughness it is necessary previously to
measure microhardness and the modulus of elasticity, calculated from the data provided
by a microhardness tester, which records, during the entry and exit of the indenter, the
depths reached by the indenter as a function of the applied load at every moment, up to
a maximum load of 100 g. In this case, important loads are induced during the
indentation, capable of causing cracks to appear around the track, which are then
measured.
Physico-chemical characteristics
Scratch resistance
2
These data are used to calculate toughness from the following equation:
K1c = 0.061 (
E 0.5 -1.5
) Pc
H
where:
Klc: toughness (MPa m ½)
H: Vickers microhardness (GPa)
E: modulus of elasticity (GPa)
P: load applied by the indenter to generate cracks (N)
C: distance from the centre of the track to the end of the crack (microns)
The validity of this technique for measuring the toughness of the glaze coatings
has been confirmed by comparing the values obtained in experiments like those reported
in the literature surveyed for materials of a similar nature to that of the studied glazes.
Material
KIc (MPa m ½)
Experimental
KIc (MPa m ½)
Literature
Lead glass
-
0.4-0.7
Aluminosilicates
-
0.8-1.1
Glass ceramics
-
2.5
Transparent lead glaze
0.5 ± 0.1
-
Transparent glaze
1.0 ± 0.1
-
Glossy white glaze
1.6 ± 0.2
-
Glass-ceramic glaze
3.6 ± 0.3
-
State of the art
Different research lines have been initiated to characterise the scratch resistance
of ceramic tiles. The line of work proposed to measure hardness by the Vickers method,
using a microhardness tester, does not appear to be very promising owing to the already
noted lack of uniformity of the glazed surfaces. At present, the design of a method is
being investigated that allows scratching to be evaluated by a parameterised mechanical
operation, in a fully standardised mode, for example measuring the depth and crosssection of the scratch produced in one or more representative lines of the tile surface.
Since there is no direct correlation between Vickers microhardness and the
scratch hardness according to Mohs, it has also been attempted to establish a modified
method of standard EN 67-101-92, staining the scratched test piece and observing the
result. In this case a correlation is obtained between the results of the standard method
and the modified one, which brings us closer to daily reality in which an unseen scratch
can house dirt and subsequently become visible because of this. The following chart
provides an example of the correlation between both methods for several types of
glazed tiles.
Physico-chemical characteristics
Scratch resistance
3
MODEL
1
2
3
4
5
6
7
8
9
10
11
12
Scratch hardness according to Mohs
Method UNE 67-101-85
Modified method
5-4
5
4
5-4
6
5-4
5-4
5-4
9
5
5
6-5
4-3
3-4
3
4
6
3
4-5
4
6
4
4-5
4
It may be noted that the current method of determining the surface scratch
hardness according to Mohs is being quite skilfully interpreted by the designers of tile
models in order to achieve high hardness results by using decorations and protections
that, with the appropriate combination of relief and colour, camouflage the possible
scratches that the test method might produce on the surface. This fact, which partly
biases the results of the test, without adversely affecting the excellent properties of the
tiles, will possibly contribute further to the search for alternative methods of evaluation
of this parameter.
Physico-chemical characteristics
Scratch resistance
4