In the literature, many works deal with the microstructural modification of glazed tiles in an acidic or basic environment [1–3]. The few works concerning the use of chemical etching as a means of anti-slip treatment consider the problem in terms of safety without studying more deeply the ceramic surface characteristics [4–5]. A deep knowledge of the tile surface microstructure, topography and textural parameters plays a key role in assessing if the acid treatment will give rise to an anti-slip surface without causing damage to the ceramic product.
In particular, surface topography is an important characteristic of a ceramic tile surface. To measure surface parameters, there are many different types of instruments. Generally, the measurement techniques can be divided into two categories: (a) contact types and (b) non-contact types. Contact type stylus profilers are the most popular, but, more recently, non-contact profilers, such as confocal and interferometric microscopes, have been developed and are now widely used. Nowadays, the availability of this new generation of measuring instruments has promoted qualitative and quantitative characterization of surface texture, not only for advanced ceramics, but also for traditional ceramics. Usually ceramic tile surfaces are analyzed using stylus 2D profilers, however, quite often, 2D measurements are not enough to give an accurate description of a surface.
In the present work, the surface of two commercial glazed porcelain stoneware tiles were studied and analyzed before and after anti-slip treatment. The goal is to find if there is any correlation between their slip resistance (anti-slip performance) and their surface microstructure and 3D topography, as well as, their surface textural parameters .
Two glazed porcelain tiles with similar chemical and mineralogical composition having different surface finishes were investigated: tile A characterized by a heterogeneous and textured surface and tile B with a homogeneous and smooth surface. The commercially available anti-slip surface treatment consists of a hydrofluoric acid based solution which was applied onto the ceramic surface by following the manufacturer’s instructions (by hand) and allowed to stand for 1 minute for tile A and 30 seconds for tile B. Surface characterization has been performed on untreated and surface treated samples of both tile A ("A UNTREATED" or "A TREATED") and tile B ("B UNTREATED" or "B TREATED").
The study of the effects caused by the treatment of the ceramic surface at the microscopic scale has been carried out with the following analytical methods:
- mineralogical and microstructural analysis:
- X-ray diffraction (XRD PW 3830; Philips, NL);
- scanning electron microscopy (SEM, Zeiss EVO 40, D) and X-ray energy dispersive spectroscopy microanalysis (EDS, Inca, Oxford Instruments, UK) operated with a typical electron accelerating voltage (EHT) of 25 kV;
- surface topography characterization and mapping:
- optical 3D microscope/profilometer (Leica DCM 3D) using the confocal mode.
As the purpose of the study is to assess the effect of the surface treatment on the anti-slip performance (slip resistance) of the tiles, the tests have been carried out in accordance with the following standards: DIN 51130  and DIN 51097 . These standards describe the methods commonly known as ramp tests where a person with shod feet (wearing shoes) or barefoot walks on an inclined surface. The inclined surface can also be coated with a lubricant, such as oil, soapy water, clean water, etc. According to these standards, a test person walks back and forth on a flat surface, covered with the tiles to be investigated, while it is progressively inclined. The surface can be clean or coated with a lubricant. The angle of inclination at which the person starts to slip is then determined.
Table 1 shows, in order of abundance, the mineralogical phases identified on the surface of the untreated tile samples "A UNTREATED" and "B UNTREATED". The XRD patterns of both samples also show the presence of a substantial amount of amorphous (non-crystalline) phase, typical of the composition of the ceramic tiles.
Tab. 1: Mineralogical composition of the untreated samples for tiles A and B.
|"A UNTREATED"||Quartz, Plagioclase, Zircon (traces)|
|"B UNTREATED"||Quartz, Plagioclase, Zircon, Corundum (traces)|
Figure 1 shows the results from the SEM-EDS elemental microanalysis performed on the surface of the samples "A UNTREATED" and "B UNTREATED". The same microanalysis carried out on the surface of the treated samples is not reported here, because no significant variations in elemental composition with respect to the untreated sample have been detected.
Figures 2–5 show typical SEM micrographs of the untreated and treated samples of tiles A and B. By comparing these micrographs (untreated versus treated samples), it can be deduced that the surface treatment etches the amorphous phases and causes the mineralogical (crystalline) phases present on the surface to be highlighted in the micrographs [2–3].
An area (each 2.93 × 2.2 mm) of the surface of both samples for tile A ("A UNTREATED" and "A TREATED") have been imaged by confocal microscopy using a 10x objective lens. The images in Figure 6 taken of the samples "A UNTREATED" and "A TREATED" show 3D representations of their surface topographies with a color scale for the z-range calibrated in micrometers (µm).
Upon comparing the images, no significant morphological differences between the two samples of tile A are seen. The outline diagrams (refer to Figure 7) of the same two areas imaged in Figure 6 illustrate representative level curves for different points on the surface with the same height level at the micrometer scale. After observing the diagrams, it can be seen that the level curves at the different points have similar shapes for both samples. Also, there is no substantial difference in height values obtained for the roughness parameters which have been determined for each analyzed area (refer to Table 2).
Tab. 2: Values of some surface roughness parameters concerning height for the areas of the untreated or treated sample of tile A shown in Figure 6 (Gaussian cut-off filter: 0.25 mm).
|ISO 25178 Height parameters||"A UNTREATED"||"A TREATED"|
|Sp (µm)||Height of the highest peak||62.52||82.14|
|Sv (µm)||Deepest pit||74.41||65.34|
|Sz (µm)||Sum Sp + Sv||136.93||147.49|
|Sa (µm)||Arithmetic average of the absolute value of the heights||12.06||11.05|
At higher magnifications (using a 20x objective) the imaged area of the surface is 2.92 × 1.71 mm. The 3D images of the analyzed surfaces (refer to Figure 8), as well as the roughness parameters (refer to Table 3), reflect a similar texture for both samples of tile A (treated and untreated).
Tab. 3: Values of some surface roughness parameters concerning height for the areas of the untreated or treated sample of tile A shown in Figure 8 (Gaussian cut-off filter: 0.25 mm).
|ISO 25178 Height parameters||"A UNTREATED"||"A TREATED"|
|Sp (µm)||Height of the highest peak||36.71||39.83|
|Sv (µm)||Deepest pit||26.16||28.78|
|Sz (µm)||Sum Sp + Sv||62.87||68.61|
|Sa (µm)||Arithmetic average of the absolute value of the heights||7.82||7.94|
Operating the confocal microscope at the maximum magnification possible (150x objective), it is possible to observe morphological differences between the surfaces of the 2 samples for tile A (treated and untreated). On the surface of sample "A TREATED” (refer to Figure 9b), there is an area where crystals are present, in agreement with observations from the SEM data (refer to Figure 3b). These crystals on the surface of sample "A UNTREATED" (refer to Figure 9a) are not evident, because they are embedded in the amorphous phase.
Also for the samples of tile B, "B UNTREATED" and "B TREATED", an area (2.93 × 2.2 mm) has been imaged by confocal microscopy using a 10x objective. The 2 images illustrated in Figure 10 are of the samples "B UNTREATED" (refer to Figure 10a) and "B TREATED" (refer to Figure 10b). They show a 3D representation of the surface topography for each area with a color scale for the z-range calibrated in micrometers. Upon comparing the images, no significant morphological differences are seen between the two samples, although it can be observed that the treated surface shows a slight smoothing of the peaks, in other words, the peaks overall protrude less than those of the untreated surface.
Comparing the outlines diagrams for the two areas in Figure 10, it can be noticed that the boundaries of the detected peaks for sample "B UNTREATED" (refer to Figure 11a) are moderately isolated and located at rather high levels (at approximately 33 µm), while for sample "B TREATED" (refer to Figure 11b) there is a wide spread presence of very uneven small areas.
A comparison of the surface roughness parameter values for the areas of the 2 samples (refer to Table 4) shows agreement with the conclusions made from the 3D image data: all surface roughness parameters, in terms of height, show a slight decrease for the treated sample with respect to the untreated sample.
Tab. 4: Values of some surface roughness parameters concerning height for the areas of the untreated or treated sample of tile A shown in Figure 10 (Gaussian cut-off filter: 0.25 mm).
|ISO 25178 Height parameters||"B UNTREATED"||"B TREATED"|
|Sp (µm)||Height of the highest peak||29.06||18.53|
|Sv (µm)||Deepest pit||27.08||19.75|
|Sz (µm)||Sum Sp + Sv||56.14||38.28|
|Sa (µm)||Arithmetic average of the absolute value of the heights||1.85||1.69|
Again obtaining images at the maximum magnification of the confocal microscope (150x objective), it is possible to observe morphological differences between the two sample surfaces for tile B, treated and untreated, as was the case for the samples of tile A. On the surface of sample "B TREATED" (refer to Figure 12b), there is an area with crystals present. Such crystals are not evident on the surface of sample "B UNTREATED" (refer to Figure 12a), as they are embedded in the amorphous phase.
It can be assumed for tile B, which has a surface characterized by a smooth, homogeneous texture, that the anti-slip treatment causes a significant change in the surface roughness parameters, i.e., after treatment the average parameter values decrease).
For tile A, which has a surface characterized by a more structured and heterogeneous texture when compared to tile B, the effect of the acid treatment on the surface morphology can be seen at high magnification, but still the values for the surface roughness parameters do not change significantly.
The anti-slip performance (slip resistance) results (refer to Table 5) determined following the test methods defined in the standard DIN 51130  show no improvement for tile A (heterogeneous, textured surface) after anti-slip treatment, as it maintains the classification R9, even though the slip angle increases by 1°. The results for both samples of tile B (homogeneous, smoother surface) show it is not suitable for workrooms having a high risk of slipping, even after anti-slip treatment. For the conditions of barefoot people walking on wet surfaces, such as at swimming pool areas or in shower or bath facilities, the tests made according to the methods defined by the standard DIN 51097  show that the results for both tiles after treatment attain the best classification, A+B+C, while the untreated samples are unclassifiable (UC). However, the data seems to indicate that the tile anti-slip performance is more dependent on the lubricant used and the chemical surface composition of the tile rather than its surface roughness.
Tab. 5: Slip resistance (anti-slip performance) results from tests made on treated and untreated samples of tile A and B in accordance with DIN 51130 and DIN 51097.
|Samples||Test method||Average slip angle (°)||Group of anti-slip properties|
|"A UNTREATED"||DIN 51130||6||R9|
|"A UNTREATED"||DIN 51097||8||UC|
Two types of tiles were investigated for this report: tile A with a heterogeneous and textured surface and tile B with a homogeneous and smooth surface. Both types of glazed tiles have a similar chemical and mineralogical composition. The acid anti-slip treatment used to modify their surfaces seems suitable for this kind of glazed tile. Its effect was especially noticeable for the case of wet (water) surfaces, where the treated samples for both tile A and B showed a significant increase in slip resistance (anti-slip performance). Moreover, the cleanability of the tiles is not compromised by the anti-slip treatment. The data imply that the lubricant used and the chemical surface composition of the tile have a more significant effect on the anti-slip performance than the surface roughness, although further investigation must be done in order to have a better understanding of this phenomenon.
The topographical analysis of the tile surfaces shows that the changes in surface structure related to the anti-slip treatment are detectable at the microscopic scale. The variation of microscale surface roughness is significant only for tile B (smooth and homogeneous texture), where the etching of the amorphous phase in the glazed tile induces an overall smoothening of the surface morphology.
For this type of tile, from the microstructural point of view, the application of the treatment causes etching of the amorphous (non-crystalline) phase, while the crystalline phases (quartz, plagioclase, zircon and corundum) are not altered.
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- ISO 25178-6:2010: Geometrical product specifications (GPS) – Surface texture: Areal – Part 6: Classification of methods for measuring surface texture.http://www.iso.org/iso/home/store/catalogue_tc/catalogue_detail.htm?csnumber=42896
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The authors would like to express their gratitude to James DeRose of Leica Microsystems for useful discussion of the results and extensive review and proof-reading of the manuscript.