Scratch Resistance Testing of Ceramic Tiles

In this application note, a new method for testing the scratch resistance of ceramic tiles and understanding the mechanisms behind material failure are discussed in detail. The latest technique is based on a scratch test method, which is commonly employed in industrial sectors.

The key reason for using the scratch technique for ceramic tile testing is to replace the archaic 'EN 101 Ceramic tiles - Determination of scratch hardness of surface per Mohs hardness scale' standard. When applied to advanced materials, this standard tends to be subjective and incorrect. Testing conditions, experimental techniques and results on glazed and unglazed ceramic tiles are discussed in the subsequent paragraphs.

In order to propose a suitable replacement to the EN 101 standard, a detailed test matrix was carried out. The parameters of this test technique were partly modified from the 'ISO 20502 Standard Fine ceramics - Determination of adhesion of ceramic coatings by Scratch testing.

Ceramic tiles.

Figure 1. Ceramic tiles.

Benefits of Scratch Tests on Bulk Materials

The scratch test method can be used for characterization of bulk materials. This technique is mostly utilized for determining the adhesion of surface coatings. In such instances, the scratch resistance is normally achieved, instead of determining the adhesion.

The scratch tests are employed to assess the mechanisms that occur during wear or abrasion. During a scratch test, information like median formation, elastic-plastic deformations, lateral and radial cracks, and other material failure mechanisms that take place during scratch testing can be obtained.

This information can help in understanding the mechanisms involved in abrasion and wear of brittle materials. Hence, scratch testing is effective for investigating the scratch resistance of a wide range of materials.

Experimental Framework

For the experiments process, three types of stoneware tiles having different surface finish and microstructure were selected. The samples were divided into two groups, such as stoneware tiles with fired or polished surfaces and stoneware glazed tiles with different degrees of crystalinity.

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 above table displays a summary of the stoneware samples with their key features. Unglazed stoneware tiles were utilized with the as fired surface (group A) or polished surface (group B).

Through surface profilometry, the surface characteristics were determined and the parameters, RM and Ra, were measured. A CSM Instruments Micro Indentation Tester (MHT) equipped with a diamond Vickers indenter was used to measure the sample’s hardness. The results of the hardness for the glazed samples are displayed in the above table.

The hardness of samples in groups A and B differed between 7230 MPa and 10120 MPa, with no significant impact of the polishing on the hardness.

For the scratch tests, a CSM Instruments Revetest Scratch Tester integrated with a Rockwell C diamond indenter of 200 µm radius was used.

Prescan/Postscan Process

The Prescan/Postscan procedure was used to perform the scratch tests. In this procedure, the surface was scanned by the tip with nominal load. Then, the scratch test was performed and the penetration depth (Pd) was noted simultaneously. Finally, another surface scan was made along the scratch track in order to expose the actual residual depth (Rd).

Additionally, panorama imaging was utilized for a systematic analysis and also for archive purposes. In panorama imaging, the entire scratch image was recorded and this was coordinated with all the recorded signals and then saved with the data.

The saved scratch results, including the panorama image can be easily recalled by users, who can work with them as they do with a real scratch. An example of such Panorama image is shown below.

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

Figure 2. Panorama image of a scratch on sample T5; vertical lines indicate areas corresponding to the critical loads.

Determination of Critical Load

Loads relating to abrupt events during the scratch are dubbed as critical loads (Lc) and are employed for determining the material’s scratch resistance. The Revetest Scratch Tester can be used to determine critical loads by utilizing the video software, which is coordinated with the scratch track, and an optical microscope.

The output of the load scratch tests on all tested tiles were two critical loads Lc1 and Lc2. Lc1 is the load at which the initial cracking occurs, while Lc2 is the load at which significant failure begins. For each sample, critical load values were measured as an average from a minimum of three tests were performed in different areas.

Typical morphology of the first initiation of the Hertzian crack just before Lc1 (a).

Figure 3. Typical morphology of the first initiation of the Hertzian crack just before Lc1 (a).

Catastrophic failure area relating to Lc2.

Figure 4. Catastrophic failure area relating to Lc2.

Difference in Scratch Resistance for as Fired and Polished Samples

In order to examine the effect of polishing on the scratch resistance, a certain area of the unglazed stoneware samples was polished. The polishing reduced the surface roughness, but categorically increased the decorative properties of the tiles.

However, polishing tends to affect the abrasion and wear resistance of materials. Therefore, the scratch tests were carried out to determine whether such a development can be identified through this technique.

A noticeable difference was observed between the scratch behavior for the first as well as the second critical load. Polishing had a major effect on most of the first critical loads, while the second critical load remained unchanged.

However, the Lc2 was not affected by the surface treatment, thus denoting that the polishing process produces defects to only a certain depth. This output validates that scratch tests are effective for such materials and allows rapid assessment of the effects of surface treatment.

Conclusion

The scratch test method allows quick and efficient characterization of the scratch resistance of different types of tiles. This innovative technique clearly proves that the critical load depends on the surface finish and the extent of crystallinity of the material.

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.

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