Influence of B2O3 on Reactive and Non-Reactive Wetting Behavior of CaO-SiO2-MgO-Al2O3-B2O3 System

Academic Editor: Ludmila B. Boinovich Received: 29 July 2025 Revised: 15 August 2025 Accepted: 17 August 2025 Published: 19 August 2025 Citation: Novák, D.; ˇ Reháˇcková, L.; Novák, V.; Matýsek, D.; Peikertová, P. Influence of B2O3on Reactive and Non-Reactive Wetting Behavior of CaO-SiO2-MgO-Al2O3-B2O3System. Coatings 2025,15, 967. https:// doi.org/10.3390/coatings15080967 Copyright: © 2025 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/). Article Influence of B2O3on Reactive and Non-Reactive Wetting Behavior of CaO-SiO2-MgO-Al2O3-B2O3System Dalibor Novák 1,*, Lenka ˇ Reháˇcková 1, Vlastimil Novák 1, Dalibor Matýsek 2and Pavlína Peikertová 3 1Faculty of Materials Science and Technology, VSB-Technical University of Ostrava, 17. Listopadu 15, 708 00 Ostrava-Poruba, Czech Republic; lenka.r[email protected] (L. ˇ R.); [email protected] (V.N.) 2Faculty of Mining and Geology, VSB-Technical University of Ostrava, 17. Listopadu 15, 708 00 Ostrava-Poruba, Czech Republic; dalibor[email protected] 3Nanotechnology Centre, CEET, VSB-Technical University of Ostrava, 17. Listopadu 15/2172, 708 00 Ostrava-Poruba, Czech Republic; [email protected] *Correspondence: dalibor[email protected] Abstract Boron oxide is introduced into slag as a flux, significantly lowering the liquidus temperature; however, this advantage is accompanied by several undesirable consequences. This study aims to evaluate the impact of boron oxide addition on the wetting reactivity of the CaO-SiO 2 -MgO-Al 2 O 3 -B 2 O 3 slag system, particularly on platinum and graphite substrates, which are commonly utilized for wettability investigations of such systems. The slag system was modified to incorporate varying concentrations of B 2 O 3 , reaching up to 30 wt% , with the addition of this oxide at the expense of CaO and SiO 2 in a constant ratio, while the contents of Al 2 O 3 and MgO remained unchanged. High-temperature wettability tests were conducted at temperatures up to 1550 ◦ C under a flow of high-purity argon atmosphere (99.9999%). For the platinum substrate, the results indicated non-reactive wetting, characterized by a decrease in wetting angles with increasing temperature and boron oxide content. Conversely, for the graphite substrate, the nature of wetting varied, resulting in either reactive or non-reactive behavior depending on the B 2 O 3 content. Following the high-temperature experiments, additional analyses were performed using scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS). Furthermore, the powdered oxide systems underwent characterization through Fourier transform infrared spectroscopy (FTIR) and X-ray powder diffraction (XRPD). Keywords: slag; reactive wetting; boron oxide; liquidus temperature; wetting angle; sessile drop method 1. Introduction Boroaluminosilicate oxide systems are utilized in diverse applications, notably in lithium battery production as sealing glasses, heat-resistant materials, and for nuclear waste immobilization [ 1 – 3 ]. They serve essential roles in liquid crystal display substrates and the glass fiber industry, particularly with E-glass fibers that feature elevated boron oxide content and superior dielectric properties [ 4 – 7 ]. Key performance attributes include favorable chemical and mechanical properties, a low thermal expansion coefficient, and a high strain point [ 8 – 10 ]. The CaO–SiO 2 –MgO–Al 2 O 3 –B 2 O 3 oxide system is also extensively employed in steelmaking due to its customizable properties. This system provides precise control over crucial factors such as melting behavior, viscosity, crystallization, and chemical durability, thereby proving invaluable for applications in continuous casting, flux Coatings 2025,15, 967 https://doi.org/10.3390/coatings15080967 Coatings 2025,15, 967 2 of 20 design, and the development of various materials, including glass-ceramics and sustainable alternatives. Within this system, B 2 O 3 functions as a pivotal flux, significantly reducing both the melting point and viscosity by disrupting the silicate network and enhancing the glass-forming capability [ 11 – 13 ]. However, the influence of B 2 O 3 is both complex and concentration-dependent. While limited additions of B 2 O 3 can improve melting behavior and overall processability, excessive quantities may compromise desulfurization effectiveness by decreasing slag basicity, and they can variably impact MgO solubility. Specifically, in high-basicity slags, B 2 O 3 can enhance MgO solubility, whereas under different conditions, it may hinder it [ 14 – 17 ]. Current research efforts are focused on deepening the understanding of the thermophysical and structural roles of B 2 O 3 , facilitating the optimization of its performance in industrial applications [18–21]. The wettability of graphite by molten slags is a subject of investigation due to its significant implications in metallurgical processes, particularly within blast furnaces. The injection of pulverized coal as a partial substitute for metallurgical coke can result in incomplete combustion, leading to the accumulation of unburnt char within the furnace environment. An in-depth understanding of the wettability of graphite by molten slag is essential for effective prediction and control of unburnt char consumption. This knowledge plays a critical role in influencing the efficiency and stability of blast furnace operations, ultimately contributing to enhanced overall productivity [ 22 – 26 ]. Other reasons include testing the corrosion and dissolution of graphite, which serves as a refractory lining in blast furnaces, and the interaction between slag and carbonaceous materials, which can cause slag foaming and the formation of gases that may disrupt furnace operations [ 22 , 24 , 25 , 27 ]. Platinum is frequently selected as a substrate in experimental studies due to its high melting point and relatively inert characteristics compared to other metals, such as iron or nickel. These properties enable researchers to investigate the wetting behavior of slags with minimal or absent chemical reactivity. This is essential for elucidating the purely physical aspects of wetting and adhesion, as well as for accurately determining the intrinsic surface tension of molten slag, free from the confounding influences of significant interfacial reactions [27–29]. Graphite exhibits relatively poor wettability when exposed to molten slags, attributed to the weak van der Waals forces present at the solid–liquid interface [ 30 ]. However, this wettability is enhanced at elevated temperatures, primarily due to interfacial chemical reactions, such as slag reduction and carbide formation [ 31 ]. Notably, research indicates that certain slag systems, particularly those comprising CaO-SiO 2 -Al 2 O 3 -MgO with iron oxides, can significantly reduce the contact angle to below 90 ◦ over time as the iron oxide content and temperature increase [ 22 , 24 – 26 ]. In addition, exploration of CaO-SiO 2 - Al 2 O 3 -FeO-MgO slag systems has revealed that the initial concentrations of Fe and Mg oxides influence the wetting behavior by facilitating oxide penetration and reduction at the graphite interface [ 32 ]. Additional examples concerning the wettability of graphite substrates with various oxide systems can be referenced in the literature [33,34]. In contrast, platinum generally demonstrates good wettability with slags, especially at elevated temperatures, where the wetting angles tend to decrease. For instance, CaO-SiO 2 - Al 2 O 3 slag exhibits significantly improved wettability on platinum compared to MnO-SiO 2 slag, with temperature enhancing the wetting process. In the context of the CaO-based system, factors such as oxygen desorption and interfacial reactions—occurring without the dissolution of silicon—contribute to the dynamic wetting behavior accompanied by bubble formation [ 18 ]. Further investigations have indicated that oxide reduction and manganese dissolution at the metal–oxide interface enhance wettability. However, discrepancies in thermal expansion between platinum and slag result in separation during the cooling process [ 17 , 19 ]. These findings underscore the significance of both chemical interactions Coatings 2025,15, 967 3 of 20 and temperature-dependent phenomena in determining the wettability of platinum in the presence of complex oxide systems. The objective of this study was to investigate the effect of boron oxide on the wettability characteristics and interaction intensity of the CaO-SiO 2 -MgO-Al 2 O 3 -B 2 O 3 oxide system when in contact with platinum and graphite substrates. To achieve this, high-temperature wettability tests were conducted, followed by a comprehensive characterization of the phase interface utilizing Scanning Electron Microscopy coupled with Energy Dispersive Spectroscopy (SEM/EDS) to elucidate the microstructural changes, while Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Diffraction (XRD) techniques were employed to assess the phase composition and crystallographic features of the oxide system. To the best of our knowledge, this work provides new insights into the wettability of platinum and graphite substrates by a molten oxide system that closely resembles industrial slag, specifically incorporating a variable boron oxide content ranging from 0 to 30 wt%. This work may contribute to the optimization of processes in industries related to glass production and metallurgical applications. 2. Materials and Methods 2.1. Preparation of the Samples The investigation focused on the oxide system characterized by the composition of CaO-SiO 2 -MgO-Al 2 O 3 -B 2 O 3 , which incorporated varying amounts of boron oxide, specifically ranging from 0 to 30 wt% across four distinct sample types (samples 1–4). These samples were tested for high-temperature wettability on two different substrate materials: graphite, as detailed in Table 1, and platinum plates. The preparation of the oxide systems was conducted using high-purity chemicals in powder form, with a minimum purity level of 96.5%. The sources of these chemicals included calcium oxide (CaO), silicon oxide (SiO 2 ), and aluminium oxide (Al 2 O 3 ), which were obtained from Lach:ner (Lach-Ner, s.r.o., Neratovice, Czech Republic), boron oxide (B 2 O 3 ) sourced from Alfa Aesar (Alfa Aesar GmbH, Karlsruhe, Germany), and magnesium oxide (MgO) purchased from Mach chemikálie (Mach chemikálie, s.r.o., Ostrava-Hrušov, Czech Republic). The individual sample weights were calculated while maintaining a constant basicity ratio of 1.4 (as presented in Table 2). Following the weight determination, the pure oxides were meticulously blended, ground using a Retsch PM 100 laboratory mill (Retsch GmbH, Haan, Germany), and remixed to achieve consistent homogenization of the composite oxide system. Prior to the high-temperature wettability tests, approximately 0.7 g of the composite sample was accurately weighed and subsequently compressed into tablets with a diameter of 14 mm. To ensure the integrity of the test results, the surfaces of the substrates were thoroughly cleansed of any contaminants using acetone immediately before the experimental procedures commenced. Table 1. Details about the physical properties of the graphite plates used. Properties Value Unit Bulk density 1.78 g·cm−3 Resistivity 16 µΩ·m Flexural strength 52 MPa Compressive strength 110 MPa Thermal conductivity 82 W·(m·K)−1 Coefficient of thermal expansion (20–200 ◦C) 510−6K−1 Hardness 66 shore hardness scale Porosity 14 % Ash content 50 ppm Coatings 2025,15, 967 4 of 20 Table 2. Chemical composition of the CaO-SiO2-MgO-Al2O3-B2O3slag system in wt%. Sample CaO SiO2MgO Al2O3B2O3 1 42.8 37.7 10 9.5 0 2 39.9 35.6 10 9.5 5 3 34.0 31.5 10 9.5 15 4 25.3 25.2 10 9.5 30 2.2. Determination of Liquidus Temperatures The determination of liquidus temperatures is crucial in understanding the thermodynamic behavior of materials. This study employed two distinct methodologies to achieve accurate measurements: the rheological method and the optical method. The rheological approach was conducted using a high-temperature rheometer, specifically the Anton Paar FRS 1600 model, manufactured by Anton Paar GmbH, located in Graz, Austria. This device was instrumental in monitoring the vertical spindle’s position as it interacted with the sample surface under varying thermal conditions. The rheometer’s setup enables precise tracking of material flow behavior as the temperature increases, thereby facilitating the identification of the liquidus point. For a comprehensive overview of the instrumentation, including the specific heating scheme employed during the experiments, readers are directed to the detailed description provided in Article [ 35 ]. In conjunction with the rheological method, the optical method was implemented to perform high-temperature wettability tests. This method involved careful observation of the alterations in the sample’s silhouette as it was subjected to increased temperatures. Through these observations, it was possible to ascertain when the sample reached an optimal shape, allowing for the determination of surface and interphase properties, as reported in [36–38]. 2.3. High-Temperature Wettability Test The experimental determination of wetting angles at the interface between oxide melts and various substrates was conducted utilizing a sessile drop method within a high-temperature observation resistance furnace, specifically the CLASIC model provided by CLASIC CZ, s.r.o., located in ˇ Revnice, Czech Republic. Detailed descriptions of the apparatus and methodology have been previously documented in the literature [ 39 ]. The temperature range for sampling was selected between a temperature close to the liquidus temperature, as determined rheologically based on normal force measurements, and a maximum temperature of 1550 ◦ C. The oxide system was formed into a compact tablet, which was subsequently positioned within the furnace on either a polished graphite or platinum substrate. The furnace was hermetically sealed and evacuated to an approximate pressure of 1 Pa, followed by a flushing process with high-purity Argon gas (99.9999%) to create an inert atmosphere conducive to experimentation. The system was subjected to a controlled heating rate set at 5 ◦ C per minute. This rate was deemed appropriate, given the specific arrangement of the furnace and the dimensions of the sample, which ensured uniform heating throughout the experiment. During the thermal loading phase, the temperature was continuously monitored using a Pt-13% Rh/Pt thermocouple strategically placed near the sample to provide accurate temperature readings. During the high-temperature wettability assessments, the silhouettes of melted oxide droplets were meticulously captured using a Canon EOS 550D digital camera with high resolution. The analysis of the wetting angles was performed using the Axisymmetric Drop Shape Analysis (ADSA) method. This technique involves fitting the profiles of the droplet to a Laplacian curve through a nonlinear regression procedure, as documented in the article [40]. Coatings 2025,15, 967 5 of 20 2.4. SEM, EDS, FTIR, and XRD Methods A comprehensive examination of the interaction between the oxide system and the corresponding substrate was conducted utilizing scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) analyses. The substrate surface was characterized using a Quanta 650 field-emission gun (FEG) electron microscope (Thermo Fisher Scientific, Waltham, MA, USA), which was equipped with an energy-dispersive detector (EDS, EDAX Elect Plus). The microscopy was performed under specific operational parameters: an accelerating voltage of 20 kV, a current range of 8–10 nA , a beam diameter of 4 mm, and in a high vacuum environment. It is noteworthy that the samples were analyzed without any metallic coating. Fourier-transform infrared (FTIR) spectroscopy was conducted utilizing a Nicolet 6700 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) to investigate molecular vibrations in the mid-infrared spectral region, specifically between 400 and 4000 cm −1 . For the purpose of this study, however, only the spectral range from 400 to 1800 cm −1 is presented in the accompanying images, as this interval encompasses the most significant absorption bands relevant to our analysis. The spectral resolution of the measurements was meticulously set at 4 cm −1 , ensuring a detailed representation of the spectral features. A total of 32 scans were performed to enhance the signal-to-noise ratio, thereby increasing the reliability of the obtained spectra. The Attenuated Total Reflectance (ATR) technique was employed, utilizing a diamond crystal, known for its robustness and low absorption in the infrared range, which facilitates high-quality spectral acquisition. Following data collection, the spectra underwent treatment using ATR and baseline correction with OMNIC software (ver. 9.12). Following this initial processing, the spectra were normalized to standardize the intensity of the absorption bands, allowing for more straightforward comparisons among the samples. Finally, fitting of the spectra was performed using the Origin software (ver. 9.8). The phase composition of oxide samples following their interaction with graphite or platinum substrates was analyzed using a Bruker AXS D8 Advance X-ray diffractometer (Bruker AXS GmbH, Karlsruhe, Germany). This apparatus, equipped with a LynxEye position-sensitive silicon strip detector, operated under the following parameters: CuK α radiation with a Ni filter, a voltage of 40 kV, and a current of 40 mA. The analysis was conducted in step mode with an angular increment of 0.014 ◦ 2 θ , a total duration of 25 s per step, and an angular range spanning from 5 ◦ to 80 ◦ 2 θ . The phase composition was assessed utilizing the Rietveld method, as implemented in the Bruker Topas software (version 4.2). Data processing was performed utilizing Bruker AXS Diffrac and Bruker EVA software (ver. 4.2), while phase identification was facilitated through the PDF-2 database as provided by the International Centre for Diffraction Data. 3. Results and Discussion 3.1. Determination of Liquidus Temperatures Determining the liquidus temperature of oxide melts, particularly those encountered in slag, is of importance across various industrial applications, particularly within the field of metallurgy. This critical temperature provides essential insights that facilitate enhanced control during smelting operations. A thorough understanding of the liquidus temperature is instrumental in optimizing operational efficiency, regulating slag viscosity, and finetuning the chemical composition of the melt. Furthermore, knowledge of this temperature aids in predicting phase transformations that occur during metallurgical processes, thereby significantly influencing both the efficiency of production and the overall quality of the final product [41,42]. Coatings 2025,15, 967 6 of 20 The incorporation of boron oxide (B 2 O 3 ) into the oxide system serves a pivotal role as a flux, a material that effectively reduces the melting point of the mixture. This characteristic renders B 2 O 3 an environmentally friendly alternative to the traditionally employed and more toxic calcium fluoride [ 43 , 44 ]. In the evaluation of melting temperatures, as detailed in Section 2.2, the findings are summarized in Table 3. Notably, it was observed that the liquidus temperature—defined as the maximum temperature at which a thermodynamic equilibrium exists between the glassy state and the primary crystalline phase [ 45 ]—exhibited a marked decrease with an increase in the boron oxide content. Specifically, the sample with 30 wt% B 2 O 3 demonstrated a reduction in liquidus temperature of approximately 420 ◦C compared to the B 2 O 3 -free sample. Several factors may contribute to this significant decrease. Among them are the inherently low liquidus temperature of boron oxide, the formation of eutectic mixtures with other oxides, such as calcium oxide (CaO) and magnesium oxide (MgO) when combined with B 2 O 3 [ 46 ], and the role of boron oxide as a network modifier that can alter the structural properties of the glass matrix [ 16 , 47 ]. These factors collectively enhance the melting behavior of the oxide system, underscoring the utility of boron oxide in glass/slag formulation processes. Table 3. Determination of liquidus temperatures in ◦ C using an Anton Paar FRS 1600 rheometer and a CLASIC heating microscope. Sample Rheometer Heating Microscope 1 1399 1400 2 1275 1278 3 1095 1096 4 977 980 The observed discrepancies between the liquidus temperatures obtained through optical and rheological methods can be attributed to the differing definitions and criteria used in each approach. Specifically, the liquidus temperature derived from optical measurements is defined as the temperature at which the droplet attains a perfect geometric shape. This is essential for accurately assessing surface and interphase properties, as highlighted in references [ 37 , 38 ]. In contrast, the rheological method employs alternative frameworks that could lead to variations in the liquidus temperature readings. 3.2. Results of High-Temperature Wettability Tests The influence of boron oxide content, ranging from 0 to 30 wt%, on the wettability of platinum and graphite substrates was examined through high-temperature wettability tests conducted at temperatures extending from the liquidus point of the oxide system up to 1550 ◦ C. As depicted in Figure 1, the relationship between the average wetting angle and temperature demonstrates a non-monotonic trend, with a noticeable reduction in contact angle as temperature increases for all examined samples. Specifically, when assessing the wettability on a graphite substrate, non-wetting behavior—characterized by a contact angle exceeding 90 degrees—was observed up to various critical temperatures: 1520 ◦ C for sample 2, which contained 5 wt% B 2 O 3 ; 1470 ◦ C for sample 3 with 15 wt% B 2 O 3 ; and 1410 ◦ C for sample 4, which had the highest boron oxide concentration of 30 wt%. Beyond these critical temperatures, a marked decrease in the contact angle was recorded, indicating a transition from non-wetting to wetting behavior. The most pronounced reduction in contact angles—up to 103 degrees—was identified for sample 4, showcasing the significant impact of maximum boron oxide addition on wettability. In the temperature range where wetting behavior was observed, there was a greater scatter in the experimental results, indicating reactive wetting. This type of wetting involves chemical reactions occurring Coatings 2025,15, 967 7 of 20 at the interface between the liquid and the solid phase. Additionally, the variability in results can be attributed to the heterogeneity of the substrate and the dynamic nature of the interphase reactions [ 48 ]. Conversely, with respect to the platinum substrate, the results indicated that the wetting angles remained consistently below 90 degrees throughout the entire temperature range under consideration. Furthermore, in both cases, a clear trend was observed wherein wettability increased as both temperature and boron oxide content increased. Figure 1. Average wetting angles of molten oxide systems as a function of temperature, wetting of platinum (A) and graphite (B) substrates, temperature increase by 10 ◦ C, error bars denote standard deviation. The relationship between wetting angles (also known as contact angles) and surface tensions is fundamentally described by Young’s equation. This equation delineates the equilibrium among interfacial tensions at the point of contact among liquid, solid, and gas phases. Wetting is defined as the extent to which a liquid spreads upon a solid surface, and the wetting angle is quantitatively represented as the angle formed between the liquid– vapour interface and the solid surface. Surface tension, conversely, is the physical property that drives liquid surfaces to minimize their area due to cohesive forces acting among liquid molecules. Typically, a reduction in surface tension correlates with a decrease in the wetting angle, suggesting enhanced wettability of the solid surface. Conversely, increased surface tension is associated with a larger wetting angle, indicating diminished wettability [49]. In the context of melts, boron exhibits a tendency to migrate preferentially to the surface, largely attributed to its lower surface energy. This surface migration alters the composition at the liquid’s interface, which subsequently contributes to a reduction in both surface tension and the wetting angle. It is understood that it requires less energy for a boron atom to reside at the surface compared to being situated within the bulk of the liquid [ 50 – 52 ]. Additionally, boron oxide plays a significant role in modifying the network structure of oxide melts by promoting depolymerization [ 53 ]. This depolymerization process leads to a decrease in viscosity, which in turn facilitates lower surface tension and wetting angles. The introduction of B 2 O 3 disrupts the silicate network that characteristically dominates in these melts, thereby diminishing the intermolecular forces that are primarily responsible for elevated surface tension. Furthermore, boron oxide forms relatively weaker bonds when contrasted with the more robust silicate or aluminosilicate connections found in traditional oxide compositions. This weakening effect further contributes to the reduction Coatings 2025,15, 967 8 of 20 in both surface tension and wetting angles, ultimately enhancing the fluidity of the melt, as elaborated in [54]. The observed profiles of the tested molten oxide systems wetting platinum and graphite substrates are depicted in Figure 2. This illustration captures the alterations in droplet morphology and corresponding wetting angles at liquidus temperatures and a maximum recorded temperature of 1550 ◦ C. Notably, all solidified droplets remained in contact with the substrate surfaces post high-temperature wettability experiments, except sample 1, which demonstrated a distinct failure to remain in contact. This phenomenon of non-separation suggests a robust adhesion mechanism among the tested systems during the cooling process. A more pronounced adhesion was noted in the interactions with the graphite substrate, potentially attributed to the process of reactive wetting. The involvement of chemical reactions at the phase interface during such interactions may lead to the formation of specific chemical products, enhancing adhesion. Figure 2. Profiles of droplets of a molten oxide system wetting platinum (A–H) and graphite (I–P) substrates. The columns, arranged from left to right, represent samples 1 through 4. Conversely, the relatively lower adhesion observed with the platinum substrate supports the hypothesis of non-reactive wetting. This is further corroborated by the narrower scatter of wetting angles recorded on platinum, in comparison to those observed on graphite, which indicates a more uniform wetting behavior [ 55 ]. Furthermore, platinum, being a noble metal, is not anticipated to undergo any significant chemical interaction with an oxide system within a neutral atmospheric environment [ 56 ]. These observations underscore the contrasting mechanisms of reactive and non-reactive wetting, as well as the intricate nature of the wetting process, which will be elaborated upon in subsequent sections of this study. Coatings 2025,15, 967 9 of 20 3.3. Analysis of Interaction at the Phase Interface Following the high-temperature wettability tests, samples 1 and 4, which included their respective platinum and graphite substrates, underwent a detailed series of analyses using Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM/EDS), Fourier Transform Infrared Spectroscopy (FTIR), and X-ray Diffraction (XRD). The primary focus of these analyses was to examine the interaction at the phase interfaces and to determine the presence or absence of reactive wetting phenomena. SEM/EDS analyses were conducted both from a top–down perspective of the platinum substrate (Figures 3and 4) and the graphite substrate (Figure 5). In addition, a cross-sectional SEM/EDS examination was specifically performed on sample 4, which successfully achieved wetting on the graphite substrate (Figure 6). This approach was particularly necessary, as in the other experimental scenarios, the liquid droplets readily detached from the substrates following the completion of the wettability tests, precluding further cross-sectional analysis. The results of SEM microanalysis at the marked points in Figures 4–6are listed in Table 4. Figure 3. Top view of platinum substrate after high-temperature tests, sample 1 (A) and sample 4 (B). Figure 4. Details of the platinum substrate viewed from above. Area under the droplet—sample 1 (A) and sample 4 (B). Area distant from the droplet—sample 1 (C) and sample 4 (D). Coatings 2025,15, 967 16 of 20 The reduction reaction occurring at the interface facilitates the spreading and infiltration of slag into the graphite substrate. Silicon, derived from the reduced silica, has the potential to penetrate deeper into the graphite’s porous structure, possibly through gasphase transport as SiO(g) [ 74 , 75 , 79 ]. It is worth noting that the extent of slag penetration into the refractory is closely correlated with the pore size of the refractory material. As pore dimensions increase, the driving force for slag infiltration intensifies significantly, indicating a substantial penetration of slag into the refractory medium. Initial slag penetration primarily occurs through capillary channels, which include open pores and microcracks [ 80 ]. The rate at which silica is reduced is contingent upon the silica activity within the slag as well as the temperature conditions. Generally, elevated silica activity coupled with higher temperatures results in expedited reaction rates and enhanced slag penetration into the graphite [74]. Furthermore, the interaction between slag and graphite may engender the formation of novel phases at the interface, such as calcium silicates or aluminates [ 81 – 84 ]. Moreover, the incorporation of B 2 O 3 significantly affects the melting characteristics and viscosity of slag systems. Specifically, the melting temperature of these fluxes decreases with an increase in B 2 O 3 content, which subsequently affects the reactivity and fluidity of the molten slag in contact with graphite. This relationship is crucial, as a reduction in viscosity at elevated temperatures can facilitate improved infiltration of the molten material into the porous structure of graphite [54,85]. In conclusion, the interplay between CaO-SiO 2 -MgO-Al 2 O 3 -B 2 O 3 slag and graphite is inherently complex, characterized by reactions that modify the slag’s composition, microstructure, and wetting properties. Key factors in this intricate process include the reduction of silica by graphite, the subsequent formation of new interfacial phases, and the significant impact of boron oxide on slag behavior. 4. Conclusions This study investigates the impact of boron oxide on the wettability characteristics of platinum and graphite substrates when interacting with a CaO-SiO 2 -MgO-Al 2 O 3 -B 2 O 3 oxide system, while maintaining a constant basicity of 1.4 and varying the B 2 O 3 content. Additionally, the investigation assesses the extent of interaction at the phase interface between the oxide system and the substrates. The findings of this study can be summarized as follows: • The incorporation of boron oxide resulted in a reduction in contact angles on both platinum and graphite substrates. Furthermore, it was observed that contact angles decreased with an increase in temperature. • The findings from scanning electron microscopy (SEM) microanalysis revealed that the reactive wetting of the graphite substrate was influenced by the concentration of boron oxide, with a marked increase in intensity corresponding to higher concentrations of boron oxide. Conversely, no evidence of reactive wetting was observed in the case of platinum. • FTIR analysis confirmed that the addition of boron oxide altered the structural network of the oxide system, weakening the intermolecular forces at the surface and resulting in a decrease in the contact angles. • The X-ray diffraction (XRD) analysis results demonstrated the amorphous characteristics of all samples within the oxide system. Quartz was identified as the predominant crystalline phase, accompanied by graphite in cases where the graphite substrate was wet. The findings of this research complement our previous investigations focused on the wetting of platinum and graphite substrates when interacting with multi-component oxide Coatings 2025,15, 967 17 of 20 systems. Additionally, this study assesses the complexities of interphase interactions that occur within these systems. The implications of these results are particularly relevant to the metallurgical, ceramic, and glass industries, where experimental wetting data are essential not only for process control but also for optimizing various industrial operations that utilize oxide materials. Author Contributions: Conceptualization, D.N. and L. ˇ R.; methodology, D.N., L. ˇ R. and V.N.; software, D.N., L. ˇ R. and V.N.; formal analysis, D.N., L. ˇ R. and V.N.; investigation, D.N., L. ˇ R., D.M. and P.P.; resources, L. ˇ R.; writing—original draft, D.N., L. ˇ R. and P.P.; writing—review and editing, D.N., L. ˇ R. and V.N.; visualization, D.N. and L. ˇ R.; supervision, D.N.; project administration, L. ˇ R.; funding acquisition, D.N. All authors have read and agreed to the published version of the manuscript. Funding: This article has been produced with the financial support of the European Union under the REFRESH—Research Excellence For REgion Sustainability and High-Tech Industries project number CZ.10.03.01/00/22_003/0000048 via the Operational Programme Just Transition. This paper was also supported by the project No. CZ.02.01.01/00/22_008/0004631 Materials and technologies for sustainable development within the Jan Amos Komensky Operational Program financed by the European Union and from the state budget of the Czech Republic and by the student project SP2025/044. Data Availability Statement: The data presented in this study are available in ZENODO at 10.5281/zenodo.15837286, accessed on 8 July 2025. Acknowledgments: We would also like to thank Michaela Topinková for preparing the samples and grinding. Conflicts of Interest: The authors declare no conflicts of interest. 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