Characterizing Scratch Resistance of Ceramic Tiles

This article presents a new method for testing the scratch resistance of ceramic tiles and provides a better understanding of the mechanisms involved in material failure and removal. Based on a scratch test method, this novel approach is often used in other industrial domains. The principal reason for applying the scratch method for testing of ceramic tiles was the replacement of the outdated ‘EN 1071 Ceramic tiles - Determination of scratch hardness of surface in accordance with Mohs hardness scale’ standard, which was shown to be user subjective and highly inaccurate when applied to new advanced materials.

This article presents the testing conditions, experimental methods and results on both unglazed and glazed ceramic tiles. An all-inclusive test matrix was carried out with the aim to propose a replacement to the EN 1071 standard. The presented test method’s parameters were partly adapted from the ‘ISO 20502 Standard Fine ceramics – Determination of adhesion of ceramic coatings by Scratch testing’ [1].

Advantages of Scratch Tests on Bulk Materials

The scratch test method is routinely used to characterize the adhesion of surface coatings, but it can also be used to characterize bulk materials. In such situations, the scratch resistance is normally achieved instead of adhesion. Generally, the scratch tests are used for studying the mechanisms that occur during wear or abrasion. Moreover, these tests provide a unique possibility to study the impact of single particle abrasion, which is either difficult or impossible to replicate by other wear or abrasion tests.

During a scratch test, information such as elastic-plastic deformations, lateral and radial cracks, formation of median and material failure mechanisms that take place during scratch testing can be obtained. This information can considerably provide a better insight into the mechanisms involved in sliding abrasion wear and abrasion of fragile materials. Therefore, scratch testing is a perfect method for the simple, efficient assessment of scratch resistance of different types of materials, including ceramic tiles.

Aside from their decorative aspects, ceramic tiles are often used because they provide protection from severe abrasion and wear. This means these tiles should have surfaces that have excellent resistance against scratch and abrasion. Studies performed in the past [2, 3] have demonstrated that cracking and chipping of the tiles negatively affects aesthetic properties (discoloration effects, loss of brightness, etc.) and considerably decreases their wear resistance. In addition, surface roughness is increased due to the presence of open voids and cracks which favors inlay of dirt and dust, further decreasing the aesthetic aspects of the tiles.

Experimental Setup

For the study, three types of stoneware tiles with different surface finish and microstructure were selected.

The samples were categorized into two groups:

  • stoneware tiles with either as fired or polished surfaces
  • stoneware glazed tiles with different degrees of crystallinity

A summary of the stoneware samples with their main features is shown in Table 1. Unglazed stoneware tiles were used with the as fired surface (group A) or polished surface (group B).

Table 1. List of glazed tiles used in the study

Sample Color Surface finish Degree of crystallinity Hardness Hit [MPa]
T1 yellow glaze high 7944
T2 black glaze low 8604
T3 white glaze high 9488
T4 brown glaze low 9416
T5 red glaze very low 7621

The surface characteristics were measured by surface profilometry and the Ra and RM parameters (according to EN 623-4) were determined. An Anton Paar Micro Indentation Tester (MHT) fitted with a diamond Vickers indenter was used to measure sample hardness. The indentation hardness was determined at a maximum load of 1 N. Table 1 shows the hardness results for the glazed samples. The hardness of samples in groups A and B differed between 7230 MPa and 10120 MPa and no direct effect of the polishing on the hardness was observed. Next, the phase composition of the glaze layers was established by X-ray diffraction (XRD) analysis on the glazed surfaces. While the XRD analysis enabled qualitative comparison of the degree of crystallization of individual samples, a quantitative analysis could not be performed because no suitable standards are available.

The scratch tests were carried out on the top surface of the sample using an Anton Paar Revetest Scratch Tester with normal force range of 1 N to 200 N equipped with a 200 µm-radius Rockwell C diamond indenter. Progressive load scratching mode was employed in all the experimental methods. Table 2 shows a summary of the scratch parameters.

Table 2. Scratch test parameters.

Minimal load
[N]
Maximal load
[N]
Scratch length
[mm]
Scratch speed
[mm/min]
Indenter radius
[µm]
1 80 8 10 200

Prescan / Postscan Procedure and Panorama Imaging

The scratch tests were carried out with the Prescan/Postscan procedure, which involves three stages: first, the surface is scanned by the tip with minimal load, then the scratch test is performed, while recording the penetration depth (Pd); and finally another surface scan is made along the scratch track to expose the true residual depth (Rd). For systematic analysis and archive purposes, panoramic imaging was also used. The Panorama imaging contains the entire scratch image and this image is synchronized to all recorded signals and saved with the data. The saved scratch results including the Panorama image can be recalled and users can work with them as with a real scratch. Figure 4 shows an example of this Panorama image.

Critical Load Determination

Loads corresponding to sudden events during the scratch are called critical loads (Lc) and are used to characterize the material’s scratch resistance. With the help of the Revetest Scratch Tester, critical loads can be directly determined by using the implemented optical microscope and video software, which, in turn, is synchronized with the scratch track.

For complete examination of the failure mechanisms, Scanning Electron Microscopy (SEM) was performed to inspect the scratch area corresponding to the critical load. The resultant output of the progressive load scratch tests on all the tested tiles were two critical loads Lc1 and Lc2, defined as follows:

  • Lc1: the load at which the first cracking takes place (Hertzian cracks)
  • Lc2 the load at which catastrophic failure begins

For each sample, critical load values were measured as an average from at least three tests made in various areas. Typical morphology of areas corresponding to critical loads Lc1 and Lc2 on a polished sample is illustrated in Figure 1.

Catastrophic failure area corresponding to Lc2

Both SEM images were taken on a polished sample.

Figure 1. Typical morphology of the first initiation of the Hertzian crack just before Lc1 (a). Catastrophic failure area corresponding to Lc2 (b). Both SEM images were taken on a polished sample.

Critical load Lc1: At loads below Lc1, the generated stresses led to tensile stresses which were not high enough to cause the formation of cracks. At such loads, the sample continued to remain fully compact and did not show any sign of permanent damage. At the first critical load, the stresses were adequately high to cause cracking of the material.

At loads between the first and the second critical load, the stresses in the material continued to increase: As cracking continued, the density of the cracks increased. However, until reaching the second critical load, there were no other failure mechanisms.

Critical load Lc2: At the second critical load, the stresses in the surrounding area of the indenter were high enough to cause chipping and spalling of the material, leading to catastrophic failure. This failure mechanism and the matching morphology were seen up to the end of the scratch. All tested samples, whether polished, glazed, or as fired, showed a similar failure behavior, but then the values of both critical loads were different for each set of samples. Hence, the tested ceramics were classified by the critical load value acquired on the given sample. Optical and SEM fractographic analysis was performed to complete the critical load value.

Difference in Scratch Resistance of as Fired and Polished Samples

One part of the unglazed stoneware samples was polished to assess how polishing affect the scratch resistance. The polishing reduced the surface roughness (Ra) from about 2.4 µm to as low as ~ 0.15 µm. While polishing can considerably increase the decorative properties of the tiles (gloss and coloring), it is known to negatively affect the abrasion and wear resistance. Therefore, scratch tests were carried out to find out whether this trend can be identified by this method. Figure 2 shows charts that demonstrate the variation between the as fired and polished samples for first critical load (Figure 2a) as well as second critical load (Figure 2b).

Comparison of the first critical load (a) and the second critical load for the as fired and polished stoneware samples.

Comparison of the first critical load (a) and the second critical load for the as fired and polished stoneware samples.

Figure 2. Comparison of the first critical load (a) and the second critical load for the as fired and polished stoneware samples.

A clear difference can be seen between the scratch behavior for the first and the second critical load: polishing had a major impact on most of the first critical loads, but the second critical load remained almost unchanged. In the majority of cases, the polishing decreased the first critical load, that is, a decrease in scratch resistance. This agrees well with industrial experience. The Lc2, on the other hand¸ was not affected by the surface treatment, indicating that the polishing process produces defects to only a certain depth (scratch depth at Lc1). However, above this depth, the material continues to remain unchanged and the critical load is extremely close for both as-fired and polished samples. This result demonstrates the utility of scratch tests for these types of materials and makes it possible to evaluate the effects of surface treatment in a rapid way.

Glazed Tiles: Effect of the Degree of Crystallinity on the Scratch Resistance

The group of glazed tiles included materials with different degrees of crystallinity (also Table 1):

  • 1 and T3 - high degree of crystallinity
  • T2 and T4 - low degree of crystallinity
  • T5 almost entirely amorphous

The criteria for critical load determination were found to be identical to those for the stoneware samples: Lc1 and Lc2 corresponded to the appearance of the first Hertzian cracks and the onset of catastrophic failure, respectively. Figure 3 summarizes the results of the progressive load scratch tests. The scratch results demonstrate a strong association between the second critical load and the degree of crystallinity.

Comparison of the first and the second critical load for the glazed samples. Samples T1 and T3 contained a high fraction of crystalline phase, samples T2 and T4 a low fraction of crystalline phase. Sample T5 was almost entirely amorphous.

Figure 3. Comparison of the first and the second critical load for the glazed samples. Samples T1 and T3 contained a high fraction of crystalline phase, samples T2 and T4 a low fraction of crystalline phase. Sample T5 was almost entirely amorphous.

There is a clear difference between the Lc2 for the same samples, although the difference between the Lc1 for the less crystalline and for the more crystalline samples is less pronounced. A higher second critical load was shown by samples with a high degree of crystallinity (T1 and T3), i.e. higher resistance to scratching, while samples with a lower degree of crystallinity (T2 and T4) exhibited the second critical load that was almost 50% lower than the samples with a high degree of crystallinity.

Out of all the tested samples, the amorphous T5 sample exhibited the lowest values for the first and second critical load. In all cases, Panorama image and optical analysis were shown to be crucial for critical load determination. Figure 4 shows an example of a Panorama image with the two critical loads. Yet, certain recorded signals correlated well with the optical observations: the drop in residual depth (Rd) corresponded to the second critical load. Figure 5 shows a classic example of a residual depth plot, where arrows point to the drop of the Rd signal.

Panorama image of a scratch on sample T5. Vertical lines indicate areas corresponding to the critical loads.

Figure 4. Panorama image of a scratch on sample T5. Vertical lines indicate areas corresponding to the critical loads.

Typical record of the residual scratch depth (Rd) for a highly crystalline sample (T3) and a less crystalline sample (T2). Arrows indicate drop in the Rd signal which corresponds to Lc2.

Figure 5. Typical record of the residual scratch depth (Rd) for a highly crystalline sample (T3) and a less crystalline sample (T2). Arrows indicate drop in the Rd signal which corresponds to Lc2.

An additional analysis by the SEM demonstrated that the samples with a high degree of crystallinity contained crystals elongated along one axis, effectively blocking crack propagation. This resulted in a higher value of Lc2 and better resistance to scratch failure.

Conclusions

This article has described an innovative approach for characterization of the scratch resistance of ceramic tiles. Based on the progressive load scratch test method with progressive load increase, this method can be used to determine the scratch resistance of different types of tiles in a fast and efficient way. The scratch test method distinctly shows that the critical load relies on the surface finish and on the degree of crystallinity of the material.

  • Polishing results in decreased resistance of the tiles because of the formation of surface defects
  • An increased degree of crystallinity results in increased scratch resistance

The progressive load scratch test method with a diamond indenter has been shown to be a far superior approach to the EN 101 standard with much higher robustness and repeatability.

Acknowledgements

The Authors would like to thank Dr. Antonella Tucci and Dr. Leonardo Esposito of the Centro Ceramico Bologna (IT) for providing the samples and the SEM images.

References

1. ISO 20502 Standard: Fine ceramics – Determination of adhesion of ceramic coatings by scratch testing.

2. Tucci, A. Guion, J. B. Esposito, L.: Microstructure and scratch resistance of ceramic surfaces. 11th International conference and exhibition of the European ceramic society, Krakow (PL) 21. – 25. June 2009, p. 191.

3. Esposito, L. Tucci, A. Naldi, D.: The reliability of polished porcelain stoneware tiles. Journal of the European Ceramic Society Vol. 25 (2005), 1487-1498.

This information has been sourced, reviewed and adapted from materials provided by Anton Paar TriTec SA.

For more information on this source, please visit Anton Paar TriTec SA.

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