**1. Introduction**

Synthetic inorganic polymers, also known as geopolymers (GP), consist of chains or networks of mineral molecules linked by covalent bonds [1].

GP are considered to be environmentally friendly materials that have the potential to be used as substitutes for ordinary Portland cement [2,3]. Geopolymers are produced from natural sources such as kaolinite or clays. Modern approaches also try to use industrial

**Citation:** Jaskevic, M.; Novotny, J.; Mamon, F.; Mares, J.; Markopoulos, A. Thickness, Adhesion and Microscopic Analysis of the Surface Structure of Single-Layer and Multi-Layer Metakaolin-Based Geopolymer Coatings. *Coatings* **2023**, *13*, 1731. https://doi.org/10.3390/ coatings13101731

Academic Editor: Mohor Mihelˇciˇc

Received: 26 August 2023 Revised: 19 September 2023 Accepted: 29 September 2023 Published: 4 October 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

sources including fly ash, waste paper sludge or granulated blast furnace slag. Recycling these resources can have positive environmental impacts and reduce CO2 production compared to Portland cement production [4].

GP, as a group of alkaline-activated materials, have a number of exceptional properties such as strength, resistance to acids and bases, fire resistance, good thermal stability and good adhesion to the underlying substrate [5].

Also, there are some possibilities to use GP as a coating material for metal and nonmetal substrates. GP coating properties rely on the chemical composition of raw materials, followed by the roughness of the substrate or the Si/Al ratio [6,7]. Different types and concentrations of the acid could change the behavior and properties of the final GP coatings. For example, Shamala et. al. prepared GP coatings on wood substrates with various NaOH concentrations to find the best concentration for GP coatings [8]. The next factor for GP coatings depends on the water content, which affects the results of the GP coating thickness [9].

The preparation of geopolymers includes three basic phases. Dissolution is the first phase, in which Si and Al atoms transition from the basic raw material to the solution and complexes with hydroxide ions are formed. In the second phase, the condensation of monomers with mobile precursors follows, with a partial internal restructuring of the alkaline polysilicates. The third stage of the geopolymer formation process is the polycondensation or the polymerization of monomers. Here, the polymer structure is formed, and the whole system solidifies. The product is an inorganic polymer structure. It is very complicated to analyze ongoing processes because they occur almost simultaneously [10].

The geopolymerization process is based on the reaction of reactive aluminosilicates supplemented with metakaolin or fly ash, which quickly dissolve in alkaline solutions in the presence of alkali hydroxides (NaOH/KOH). This creates tetrahedral units-connected polymeric precursors (–SiO4–AlO4– or –SiO4–AlO4–SiO4– or –SiO4–AlO4–SiO4–SiO4–) forming amorphous geopolymer products with a 3D network structure [11,12]. Nergis reported that geopolymers also contain three types of pores formed by the arrangement of the OH– and Si groups (Si–OH), Si–O–Si groups, Si–O–Al groups and Si–O rings [13].

Some geopolymers are activated by acidic activators. The metakaolin-based geopolymer produced by using a phosphoric acid solution as an activator has a high compressive strength up to 93.8 MPa [14,15]. Another study also showed that acid-based geopolymers have a higher temperature resistance (up to 1450 ◦C) and better mechanical properties than alkali-based geopolymers [16]. However, GP have extremely good thermal stability and adhesion to the surfaces thought to be a kind of all-purpose material and potential tribological material [17].

Fire resistance is an interesting property of GP if the suspension contains a flame retardant. In our study, the stability of coatings with Al(OH)3 is monitored for possible future fire protection applications on Fe and Al substrates. Al(OH)3 as a flame retardant in a GP suspension was described on polystyrene and chipboard underlying substrates, and its positive influence has been proven [18].

The adhesion of the coating to the underlying substrate is an essential factor which is controlled by the surface treatments of the base metal as an underlying substrate [18]. Metal with a high surface roughness will have a higher adhesion strength with GP coating material compared to the polished metal substrate [9].

A general theory covering all relevant properties and parameters involved in the design and application of tribological coating composites is very far from being realized. Such a theory would have to treat the long chain of relations ranging from the coating deposition parameters to the tribological response of the coated component [19,20].

Obviously, a good adhesion to the substrate is a crucial property of most applications of coated components. Any adhesion test must superimpose an external stress field over the coating/substrate interface to cause a measurable adhesive failure. Since this stress field will depend on the geometry and type of loading (indentation, scratching, sliding, abrasion, impact, etc.) as well as on the elastic and plastic parameters of the coating and

substrate, the resulting adhesion value will only be representative of the particular test from which it has been obtained [21].

There are many ways in which suspensions can be applied to a substrate surface. One of the simple methods of application, together with the satisfactory results of the final layer, is the application of the suspension with a brush. This method is very cheap, with minimal economic costs in creating a coating, and does not require deep knowledge and know-how in applying and creating a coating. The disadvantage is that it is not possible to accurately correct the achieved layer thickness, and also, the homogeneity of the resulting layer thickness fluctuates within a certain range. An airbrush appears to be another suitable method for applying the suspension. This method is also widely used, but its use is already economically and technologically more demanding. However, it can improve the homogeneity of the thickness of the resulting layer [22].

The aim of the work was to prepare and compare single- and multi-layer coatings on metal underlying substrates, aluminum and construction steel. It was important to explain what effect applying a thicker layer with a brush would have on the adhesion, overall surface quality and change in thickness compared to single-layer systems and various application/curing methods. The prepared GP coatings were characterized using confocal microscopy and SEM (scanning electron microscopy) to capture the microstructure and the visual quality of the coatings. These results were supplemented by a grid test and thickness measurement.

This work follows our previous research [23,24] and expands our knowledge about the selected single- and multi-layer GP coatings with different applications by brushes and some types of curing processes. We observed changes in the visual quality of the prepared GP coatings and changes in their mechanical properties.

#### **2. Materials and Methods**

The GP coating was selected based on our previous research [23,24]. The selected GP had good properties, which was a good starting point for the following complex research creating multi-layer GP coatings. In this work, we presented this advanced preparation of GP coatings compared with a multi-layered GP coating.

The chosen substrates were aluminum alloy EN-AW 6060 (AlMgSi0.5) [25,26] and construction steel 1.0038 (according to EN 10025-2). These underlying substrates have been chosen as the most common alloys in all sectors of industry. The preparation of the substrate before the application of the geopolymer suspension consists only in removing gross impurities and degreasing the surface with an organic solvent (acetone). No other pre-treatment of the surface was applied; therefore, their natural oxide layers are found on the surface of the substrates.

#### *2.1. Preparation of the Suspensions and Underlying Substrate*

The preparation of the geopolymer suspension consists in mixing basic raw materials that have different phases (liquid and solid). In this research, there is a liquid component, phosphoric acid (H3PO4) and isopropylalkohol (iPrOH), for both GP suspensions and a solid component, metakaolin with AlOH3 (for GP suspension I) and metakaolin with powder Al2O3 (for GP suspension J). The good homogenization of the resulting mixture after mixing the basic ingredients is very important. A laboratory homogenizer, AD300L-H, 10,000 RPM, was used for homogenization and mixing.

Geopolymer suspensions I and J were selected from previous research for their interesting properties on the Al substrate [23], where geopolymer I reached an average coating thickness of 2.7 μm and geopolymer J reached one of only 1.5 μm. Both suspensions had excellent adhesion to the Al substrate. Even the microhardness values of HV 0.1 achieved very good results (GP I 118.4 HV 0.1 and GP J 127.1 HV 0.1) compared to the underlying substrate Al 93.6 HV 0.1.

#### *2.2. Roughness of Al Substrate EN AW-6060 and Fe Substrate 1.0038*

For the application of conventional coatings with an organic or inorganic composition (most often for the anti-corrosion protection of metals) on metal surfaces, the roughness of the underlying substrate is very important [27,28]. For example, for aluminum alloys, the adhesion of such coatings is generally lower than that when applied to steel surfaces [29–33].

The roughness of the used metal substrates EN-AW 6060 and 1.0038 was measured using a Hommel Tester t1000 according to ISO 4287. The input values of the measurement were as follows: probe type T1E 2 μm/90◦, compressive force 1.5 mN, traverse length 4.8 mm, traverse speed 0.5 mm/s and measurement range ±80 μm/0.01 μm. For the underlying metal substrates, a sheet with a thickness of 3 mm was used for both types. These sheets were processed by rolling and show a one-way orientation of the grooves that were created during the rolling process, as shown in the detail of the surface in Figure 1. This orientation is very well observed in the aluminum alloy. The roughness of the substrates was measured along the rolling direction A and also perpendicular to the rolling direction B, as shown in Figure 1. Table 1 shows the achieved surface roughness values in individual directions [23].

**Figure 1.** Surface detail of the EN-AW 6060 (Al) [23] and 1.0038 (Fe) underlying substrate with visible directional anisotropy and roughness measurement in the direction of rolling A and the perpendicular direction of rolling B.


**Table 1.** Surface roughness of the underlying substrate EN-AW 6060 and 1.0038.

Ra—arithmetic mean roughness; Rz—ten-point mean roughness; Rmax—maximum roughness depth; Rt—maximum height of the profile.

#### *2.3. Application of GP Suspensions*

Application by brush was chosen, which is the simplest possible application of geopolymers with a sufficient resulting coating quality [23].

The geopolymer suspension was applied to the substrate by a brush, which is designed for water-based coatings. This method was chosen as in previous research [23], but with a different approach regarding the thickness of the coatings. In this case, we tried to prepare thicker layers with a brush compared to the previous research [23]. In previous research, it was found that by applying geopolymer suspensions with different compositions in one layer using a brush, very thin coatings with a thickness of up to approx. 20 μm (depending on the type of GP) with good adhesion can be achieved on the Al and Fe substrate. These thickness sizes were conditioned by the application of a very thin layer of the suspension with a brush; when applying the suspension, care must be taken to spread it very well over the surface of the substrate. The result was coatings that have a very good surface quality and very good adhesion [23,24]. Such application of GP in a thin layer is not complicated, but it requires concentration, and when applying it to larger or more fragmented surfaces, this procedure may no longer be followed exactly. A failure to follow the procedure can be caused by, for example, the human factor or even the brush application method itself, when this method is simple but not very accurate. The following research therefore simulates a process where the application procedure of a very thin layer is not followed, but the GP layer applied with a brush is thicker and a comparison is made of the effect on the properties (adhesion) and the appearance of the surface that the application of a thicker layer/layers will have.

Labeling explanations in Figure 2: GP—geopolymer suspension, X—type of geopolymer suspension (I or J), -/S—application and curing method (—-curing after each applicated layer; S—curing after three or five layers; see Figure 3), 1 L, 3 L, 5 L—number of layers applied (1 L—one layer, 3 L—three layers, 5 L—five layers). Our geopolymers were divided into four series, where two different GP coatings (I or J) were applied by brush on two metal substrates (Al or Fe).

**Figure 2.** Scheme of the preparation of each GP coating.

In order for geopolymer suspensions to acquire their final properties after the application to the underlying substrate (Figure 2), chemical reactions, so-called geopolymerization, must occur in the mixture [24,34,35]. For the geopolymer mixture, geopolymerization occurs at elevated temperatures, in contrast to mixtures with a different composition, where geopolymerization can occur at lower temperatures [24,34,35]. For the selected geopolymer mixture, it is necessary to reach a certain minimum temperature, which was experimentally determined to be 170 ◦C, and to maintain the same conditions as in the previous research [23]. This temperature is an important parameter influencing the resulting quality of the coatings. This increase in the temperature is needed for the geopolymerization process [24].

Every series of GP has a different curing for the geopolymerization of these coatings shown in Figure 3. The geopolymers marked GP X 1L, GP X 3L and GP X 5L had a curing process with every layer immediately after the application. The GP marked GP X S 3L and GP X S 5L had a different approach to geopolymerization curing, with only single curing after the third or fifth layer of GP coatings. Between every layer, there were 24 h of drying, and after that, the next layer was applied. This approach changed the coatings' properties and their behavior (see the next chapters). This curing procedure was chosen in order to simplify the application of the suspensions to the substrate and at the same time reduce the resulting cost of creating multilayer coatings and analyze whether there is a difference between the layers when applying and curing after each layer compared to curing after applying the final layer.

**Figure 3.** Curing for the geopolymerization of coatings.
