**3. Results and Discussion**

*3.1. Microstructure Analysis of Geopolymer Coatings Using SEM and CLSM*

3.1.1. Geopolymer I Al

Figure 4 shows:


**Figure 4.** SEM and CLSM analysis of the surface of I geopolymer suspensions on the aluminum substrate.

Figure 4 shows the disparity in the surface structure between all of the analyzed samples. Even if they are coatings created from the same type of geopolymer suspension, which differ only in the number of created layers or in the curing method, the resulting surfaces are completely different. All coatings except for I S 5L Al exhibit some form of porosity. The I S 5L Al coating was granular and did not show any form of porosity. All

coatings are stable, the surface shows no flaking and the cracks are minimal and distributed homogeneously over the entire surface.

**Figure 5.** EDS mapping analysis of the surface of I geopolymer suspensions on the aluminum substrate.

The Figure 5 showed that except for the porosity positions, where, naturally, the presence of the elements analyzed using the EDS method was lower, it is evident that the elements Al, Si and P were distributed homogeneously, except for the sample I S 5L Al with a granular structure. It was observed that Al is more represented in the structure compared to Si, as the main elements of the geopolymer system.

### 3.1.2. Geopolymer I Fe

Figure 6 shows:


**Figure 6.** SEM and CLSM analysis of the surface of I geopolymer suspensions on the steel substrate.

In Figure 6, it was observed that no form of porosity occurs anymore in the coatings on the steel substrate. For the I S 5L Fe sample, a granular surface structure was again observed, just like the sample on the aluminum substrate in Figure 4. A lesser extent of the granular structure of the coating was also observed in the I S 5L Fe sample.

**Figure 7.** EDS mapping analysis of the surface of I geopolymer suspensions on the steel substrate.

Inhomogeneity was evident in all samples, even in samples without a granular structure. Figure 7 showed that the samples without a granular structure (I 1L Fe, I 3L Fe and I 5L Fe) have visible lumps on the surface, with an absence of P and a higher concentration of Al. The reason for this inhomogeneity was not studied, but it is probably a behavior of the geopolymerization and a property of the geopolymer. Application suspension by brush can also have an effect, since this phenomenon is also observed in the same suspension on the Al substrate, as described above. Si is distributed homogeneously.

#### 3.1.3. Geopolymer J Al

Figure 8 shows the following:


**Figure 8.** SEM and CLSM analysis of the surface of J geopolymer suspensions on the aluminum substrate.

The GP J coatings on the aluminum substrate show different surface structures compared to the GP I coatings in Figure 4. In the GP J coatings, there were not any signs of porosity or granular structures. The J S 5L Al sample showed a considerable number of cracks. The same J 5L Al multi-layer sample, where the layers were cured sequentially, showed fewer cracks, and spalling was present only in the sample J S 5L Al.

**Figure 9.** EDS mapping analysis of the surface of J geopolymer suspensions on the aluminum substrate.

The inhomogeneity of the distribution of Al and P elements which could be seen on Figure 9 was clearly visible on the J 1L Al and J 3L Al samples in the lines, which were created after the application of the geopolymer by brush. These lines were characteristic of a thinner layer of the geopolymer coating. The lines were enriched more with phosphorous, and a smaller presence of aluminum was evident.

#### 3.1.4. Geopolymer J Fe

Figure 10 shows:


**Figure 10.** SEM and CLSM analysis of the surface of J geopolymer suspensions on the steel substrate.

Here, in Figure 10, cracks were observed in all of the measured samples. Their intensity increased gradually with the increasing number of layers. No porosity or granular structure of the coating was visible. The surfaces for all samples are very similar to the surfaces on the aluminum substrate.

**Figure 11.** EDS mapping analysis of the surface of J geopolymer suspensions on the steel substrate.

EDS analysis seen on Figure 11 showed a homogeneous distribution of all elements. Homogeneity was the most pronounced of all the samples examined. No representation of elements was visible in the area of the cracks. The sample J S 3L Fe showed that, through the cracks, the steel substrate was visible and was in contact with atmospheric conditions. These coatings cannot provide full chemical protection of the substrate. This sample was chosen to present this phenomenon. The beam intensity of the SEM microscope could not penetrate the entire coating [37].

#### *3.2. Analysis of the Adhesion of the Geopolymer Layer by the Grid Test*

The achieved results of the grid test are shown in Figure 12, and the evaluation of the results is shown in Table 2. Table 2 shows the evaluated results of the grid test of various GP on Al and Fe substrates. GP I and J on the Al substrate achieved a rating of one, but only GP I S 5L had a rating of two, most likely due to too large of a layer of the GP suspension in combination with the method of application and curing. GP I on the Fe substrate achieved a worse rating of three, due to the presence of a natural oxide layer on the surface of the substrate and its peeling, together with the coating. GP J on the Al substrate achieved a rating of one for all samples and layers. GP J on the Fe substrate showed a better value of the grid test, with a rating of one for J 5L and a rating of two for J S 3L. The other GP J had a rating of three.

**Figure 12.** Grid test of GP coatings on the Al and Fe substrates.



The results of the GP I 1L Al and GP J 1L Al + Fe coatings corresponded with our previous research [23,24], but GP I 1L Fe had worse ratings than those in previous research. Due to the fact that no pre-treatment of the surface before the application of GP suspensions took place on the substrates (except for surface degreasing), there are natural oxides on both types of substrates. As can be seen from the grid test, the natural Al2O3 oxide layer on the Al substrate does not have a negative effect on the adhesion of the GP coatings to the surface. The Fe substrate is also covered with a natural oxide layer that is thicker than that on the Al substrate. According to the XRD analysis, it was found that Fe3O4, Fe2O3 and FeO oxides are found on the surface of the Fe substrate. The lower adhesion of GP coatings on the steel substrate is apparently caused by this oxide layer, which does not have high adhesion to the substrate itself (the steel layer located below this layer) and thus peels off from the surface together with the GP coating.

#### *3.3. Analysis of the Thickness of the Geopolymer Layer*

A comparison of the measured values of the thicknesses of all layers for GP I on the aluminum and steel substrates can be seen in Figure 13. Single-layer coatings reach almost the same thickness for both types of substrates (11% difference). The resulting thickness of I 1L Al is approximately 2.7 times higher than the thickness achieved in previous research [23]. All samples on both types of substrates show an almost linear increase in the layer thickness with an increasing number of layers. It can be seen from the graph that overall thicker layers were achieved on the aluminum substrate than on the steel substrate (about 20%–30%). What is interesting is the comparison of the multi-layer sample I 3L with I S 5L and that of I 5L with I S 5L, where a clear trend can be observed, where the samples cured after each layer reach an overall lower thickness than the samples cured after the last-applied layer.

**Figure 13.** Comparison of the increase in the thickness of the resulting layer depending on the number of layers for suspension I.

The thicknesses of the GP J layers on both types of substrates are shown in Figure 14. As we can see, the J 1L Al single-layer sample reaches a thickness of 3.4 μm, which, as in the rare case of GP I, is an increase in thickness compared to previous research [23] by 2.3 times. Sample J 1L Fe has a greater thickness for this geopolymer, almost twice that of the aluminum substrate. All the layers on the Fe substrate reach greater thicknesses by about 70%–110% compared to the Al substrate, which is the opposite trend to that of GP I. The comparison of the multi-layer sample J 3L with J S 3L and that of J 5L with J S 5L again confirm the previous trend, where the samples cured after each layer reach a lower thickness than the samples cured after the last layer (except for J 3L and J S 3L on the Al substrate, where the thickness is almost identical).

**Figure 14.** Comparison of the increase in the thickness of the resulting layer depending on the number of layers for suspension J.

#### **4. Conclusions**

The basis of this research was the creation of geopolymer coatings on aluminum and construction steel substrates, which were applied to the surface using a brush. A thicker layer of the suspension was already created on the substrates during the application itself, which was supposed to verify the properties and surface of the thicker layers created in this way, which follows previous research that, on the contrary, was focused on creating very thin layers [23,24]. Two types of geopolymer suspensions, I and J, were selected from the previous research. Furthermore, GP suspensions were applied in multiple layers, three and five, and two different methods of curing were used (see Section 2.2).

The formed coatings showed a certain porosity, and, above all, they are prone to the formation of cracks. These cracks are created naturally during curing by the emission of water from the volume and the different thermal expansion of the geopolymeric suspension and the underlying substrate. Cracks generally increased with the increasing number of layers. However, in most cases, the cracks did not appear to affect the cohesion of the coating or the adhesion of the coatings to the substrate surface. Thus, the porosity, rough surface and cracks do not have to reduce the resulting properties or the use of the coating, and in some cases, they can be beneficial. A rough or cracked surface can show better adhesion, e.g., when using glue in glued joints, when a larger surface is needed for the good adhesion of the joint [37]. A rough surface can help improve part handling and increase safety [38]. A jagged, rough or cracked surface change the optical properties of the surface so that, for example, there are no reflections of light from the surface [39]. Self-lubricating systems appear to be a very suitable application for this type of surface, where the surface of the part is provided with this coating, which contains many cracks and capillaries into which the lubricant is applied, and then gets between the functional surfaces and thus affects the tribological properties, e.g., by reducing friction, which leads to an increase in the life of the component and a reduction in the need for maintenance [40–42].

The application of suspensions with a brush is economical, but according to microscopic analysis, it is evident that it introduces a certain inhomogeneity into the coating, whether it is the fluctuating thickness of the layer, which then causes cracking, or the inhomogeneity in the distribution of Al, Si and P elements in the coating.

The grid test confirmed the very high adhesion of GP coatings on the aluminum substrate, independent of the thickness, the number of layers and the method of curing. On the steel substrate, the adhesion of the coatings was lower and partly dependent on the above-mentioned variables.

Observing the thickness of the layer on the underlying substrate is an important aspect. Thicker layers protect the underlying substrate better against corrosion and mechanical wear. As can be seen from the electron microscopy images, cracks on the surface increased in size with the increasing number of layers. Cracks of the underlying substrate are undesirable in the case of the application of GP anti-corrosion suspensions, as they reduce corrosion resistance (the corrosive environment with cracks can reach the underlying substrate). Large cracks are undesirable because they lead to the peeling of the GP layer from the underlying substrate, which was observed in some cases.

Finding a balance between the layer thickness and cracks is essential for future applications.

By applying a thicker layer of the GP suspension when applied with a brush, a final, thicker layer could be created, which, according to analyses, had no negative effect on the quality of the resulting surface. By applying additional layers, thicker layers could be created, which could affect other properties (mechanical, chemical) of the resulting surface. The joint thickness is further influenced by both the composition of the geopolymer suspension and the method of the application and curing of single layers in multi-layer samples.

To increase the homogeneity of the distribution of individual elements in the resulting layers and further improve the quality of the surface, in terms of, e.g., reducing the roughness or reducing the heterogeneity of the layer thickness in different places of the surface, which is caused by an application with a brush, and, thus, to reduce the formation, number and size of cracks and fissures, which are apparently caused by too thick of an

applied suspension (again, in certain places), it would be necessary to change the method of application of the suspension. As a suitable solution to these inhomogeneities during the application, the suspension could be applied by spraying (air brush) or possibly even by means of a roller. These application methods (mainly the air brush) are more complicated and complex. Other forms of applications, e.g., dipping, are not suitable due to the need to maintain a very small thickness of the applied suspension. Moreover, the search for another application method contradicts the basic assumption of cheap and simple painting.

This research shows that it is possible to create multi-layer geopolymer coatings that achieve a good surface quality and adhesion on various underlying substrates while maintaining a relatively small thickness. This can be advantageous, for example, for functional components, where after the application of several layers of GP suspensions, there will be no large dimensional change, and the possible functionality is thus not affected. To further increase the quality of the surface, it would be advisable, for example, to change the method of application of the suspension in order to achieve a more even coverage of the surface of the substrate, which will ultimately affect the homogeneity of the resulting layers. The lower adhesion of the layers on the steel substrate could probably be solved by a suitable mechanical (or even chemical) pre-treatment of the surface, when the surface oxide layer is removed. Adhesion is excellent with the aluminum substrate, and the oxide layer does not seem to negatively affect adhesion. Furthermore, it is possible to focus on the final temperature when curing the layers. In this research, it was applied at a curing temperature of 170 ◦C, but it is possible to change this temperature further, which can affect the resulting mechanical properties of the applied layers. Lowering the resulting temperature and shortening the holding time will have a positive effect on production costs, but at the same time, the temperature must not be too low in order for the applied layers to properly geopolymerize. On the contrary, a higher temperature can further positively affect the mechanical properties of the layers, e.g., by increasing the surface hardness (mainly in the case of GP suspensions with Al2O3 content). Continuing that research and the previous research [23,24], the focus may be on the analysis of the mechanical properties of multilayer coatings, such as the microhardness of the surface of the layers or the tribological properties [40,42] (possibly the corrosion resistance of the coatings [43]).

**Author Contributions:** Conceptualization, M.J. and F.M.; Data curation, M.J. and J.N.; Formal analysis, F.M., J.M. and A.M.; Funding acquisition, M.J. and J.N.; Investigation, M.J., F.M. and J.M.; Methodology, M.J., F.M. and J.M.; Project administration, M.J. and F.M.; Resources, M.J., F.M. and J.M.; Supervision, J.N.; Validation, M.J. and J.N.; Visualization, M.J., F.M. and J.M.; Writing—original draft, M.J., F.M. and J.M.; Writing—review and editing, M.J., F.M., J.M. and A.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by NANOTECH ITI II. No. CZ.02.1.01/0.0/0.0/18\_069/0010045 and the internal UJEP Grant Agency (UJEP-SGS-2022-48-002-2).

**Data Availability Statement:** Data sharing is not applicable to this article.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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