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Scratch Resistance 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 1 Scratch resistance 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 2 Scratch resistance These data are used to calculate toughness from the following equation: E ( 0.061 K = 0.061 ( Pc) 1.5-0.5 1c 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) KIc (MPa m ½) Experimental 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 cross- section 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 3 Scratch resistance MODEL Scratch hardness according to Mohs Method UNE 67-101-85 Modified method 1 5-4 4-3 2 5 3-4 3 4 3 4 5-4 4 5 6 6 6 5-4 3 7 5-4 4-5 8 5-4 4 9 9 6 10 5 4 11 5 4-5 12 6-5 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 4 Scratch resistance .
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