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

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-SiO₂-MgO-Al₂O₃-B₂O₃ 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 B₂O₃, reaching up to 30 wt%, with the addition of this oxide at the expense of CaO and SiO₂ in a constant ratio, while the contents of Al₂O₃ 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 B₂O₃ 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).

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 [1,2,3]. 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 [4,5,6,7]. Key performance attributes include favorable chemical and mechanical properties, a low thermal expansion coefficient, and a high strain point [8,9,10]. The CaO–SiO₂–MgO–Al₂O₃–B₂O₃ 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 design, and the development of various materials, including glass-ceramics and sustainable alternatives. Within this system, B₂O₃ functions as a pivotal flux, significantly reducing both the melting point and viscosity by disrupting the silicate network and enhancing the glass-forming capability [11,12,13]. However, the influence of B₂O₃ is both complex and concentration-dependent. While limited additions of B₂O₃ 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, B₂O₃ can enhance MgO solubility, whereas under different conditions, it may hinder it [14,15,16,17]. Current research efforts are focused on deepening the understanding of the thermophysical and structural roles of B₂O₃, facilitating the optimization of its performance in industrial applications [18,19,20,21].

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 [22,23,24,25,26]. 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 [27,28,29].

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-SiO₂-Al₂O₃-MgO with iron oxides, can significantly reduce the contact angle to below 90° over time as the iron oxide content and temperature increase [22,24,25,26]. In addition, exploration of CaO-SiO₂-Al₂O₃-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-SiO₂-Al₂O₃ slag exhibits significantly improved wettability on platinum compared to MnO-SiO₂ 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 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-SiO₂-MgO-Al₂O₃-B₂O₃ 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-SiO₂-MgO-Al₂O₃-B₂O₃, 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. 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)	5	10−6 K−1
Hardness	66	shore hardness scale
Porosity	14	%
Ash content	50	ppm

, 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 (SiO₂), and aluminium oxide (Al₂O₃), which were obtained from Lach:ner (Lach-Ner, s.r.o., Neratovice, Czech Republic), boron oxide (B₂O₃) 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. Chemical composition of the CaO-SiO₂-MgO-Al₂O₃-B₂O₃ slag system in wt%.

Sample	CaO	SiO2	MgO	Al2O3	B2O3
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

). 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.

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 [36,37,38].

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 Řevnice, 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].

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: CuKα 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° 2θ, a total duration of 25 s per step, and an angular range spanning from 5° to 80° 2θ. 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 fine-tuning 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].

The incorporation of boron oxide (B₂O₃) 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 B₂O₃ 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. 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

. 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% B₂O₃ demonstrated a reduction in liquidus temperature of approximately 420 °C compared to the B₂O₃-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 B₂O₃ [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.

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% B₂O₃; 1470 °C for sample 3 with 15 wt% B₂O₃; 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 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.

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 [50,51,52]. 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 B₂O₃ 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 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.

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.

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 (Figure 3 and Figure 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 Figure 4, Figure 5 and Figure 6 are listed in

Table 4. Results of SEM microanalysis of the oxide system/platinum and graphite substrate interaction.

Point	Caption	Ca	Si	Mg	Al	C	O	Pt
		(wt%)
1	Remains of the oxide system	25.1	18.2	6.0	5.9	―	44.8	―
2	Platinum substrate	―	―	―	―	―	―	100.0
3	Remains of the oxide system	21.6	17.9	5.8	10.4	―	44.3	―
4	Platinum substrate	―	―	―	―	―	―	100.0
5	Oxide system after evaporation	―	4.9	―	16.5	―	30.1	48.5
6	Oxide system after evaporation	―	5.5	―	19.6	―	30.1	44.8
7	Oxide system	17.6	12.8	7.1	6.2	―	56.3	―
8	Remains of the oxide system	39.6	4.3	1.4	2.1	―	52.6	―
9	Graphite substrate, Si, SiC, SiO2	―	39.3	―	―	44.6	16.1	―
10	Graphite substrate, Si, SiC, SiO2	―	4.9	―	―	92.1	3.0	―
11	Oxide system	19.7	17.1	8.3	8.0	―	46.9	―
12	Remains of the oxide system	19.7	14.7	6.8	11.3	―	47.5	―
13	Remains of the oxide system	19.9	15.8	7.8	8.9	―	47.6	―
14	Graphite with oxide remains	1.1	0.8	0.3	0.5	91.3	6.0	―

In the EDS point analysis, the influence of the surroundings must be considered for small particles and thin layers.

.

In the case of wetting the platinum substrate, the surface of the substrate was minimally affected. In the case of wetting with sample 1 (Figure 3A), residues of this oxide system were observed on the platinum substrate in the area under the droplet and in its vicinity. The situation was similar in the case of sample 4 (Figure 3B), except that there were more residues of the oxide system. The figures also show that no reactive wetting occurred; instead, platinum recrystallization and the presence of grain boundaries were observed.

Details of the areas under the droplet for the platinum substrate wetted with sample 1 and sample 4 are shown in Figure 4A and Figure 4B, respectively. The black contrasting circular areas at the interface between the oxide system and metal are not topographical depressions but can be interpreted as thin islands or layers of residual oxides adhering to the platinum surface. EDS analysis in these areas confirms the presence of elements that form oxides of the respective metals, i.e., Al, Si, Ca, and Mg. It is also apparent that the platinum substrate under the droplet was only minimally affected by interaction with the oxide system and that no reactive wetting occurred. In an even more distant area (Figure 4C shows sample 1, and Figure 4D shows sample 4), the situation was similar. However, the composition of the oxide system underwent significant changes, apparently due to the evaporation of the relevant phases in the form of suboxides [57,58,59].

Figure 5 presents a top view of the graphite plate following a high-temperature test, illustrating the effects of two oxide samples: sample 1 containing 0 wt% of B₂O₃ (Figure 5A) and sample 4 containing 30 wt% of B₂O₃ (Figure 5B,C). A comparative analysis reveals that the presence of boron oxide has a significant impact on the wetting process. In the case of sample 1, minimal surface alteration was observed alongside low-intensity reactive wetting. Conversely, the surface of the graphite plate associated with sample 4 exhibited pronounced alterations. Figure 5B depicts a droplet of the oxide system, characterized by a Ca/Al/Mg/Si ratio indicative of a complex glassy or partially crystalline slag. Furthermore, there is a reasonable assertion that this oxide system may facilitate the formation of aluminates, silicates, and spinel phases, such as MgAl₂O₄.

Surrounding the droplet, the surface displayed considerable modifications, as evidenced by changes in elemental composition, which suggest reactive wetting. In the transition zone, located further away from the droplet (Figure 5C), only carbon, oxygen, and silicon were detected, thereby implying a probable presence of metallic silicon and silicon carbide in this region. At elevated temperatures, metal oxides, including CaO, MgO, and B₂O₃, can transition into suboxides (e.g., Ca, Mg, B species) due to their significantly higher vapor pressures, resulting in potential losses to the gas phase [57,58,59]. However, given that the maximum temperature achieved during our experiment was 1550 °C, the evaporation reasoning may be effectively applied to boron oxide. In contrast, the relatively greater thermal stability of CaO and MgO likely resulted in significantly diminished volatility under given experimental conditions.

Figure 6 illustrates a cross-section of the oxide system/graphite substrate system corresponding to sample 4. The data presented in the figure indicate that reactive wetting occurred at the interface with the graphite substrate, resulting in the formation of a reactive transition zone. This zone is characterized by significant carbon infiltration and notable elemental redistribution, particularly among calcium, aluminum, magnesium, and silicon. Interestingly, boron was not detected within this zone. This absence may be attributed to the inherent volatility of the material or the limitations of energy-dispersive spectroscopy (EDS) in detecting elements with low atomic numbers [57,60].

The thickness of the reactive transition zone was observed to extend up to 450 µm, providing clear evidence of chemical interactions taking place at the interface. Notably, these interactions are believed to involve the formation of silicon carbide (SiC) as well as the reduction of various oxides through the action of carbon. Such chemical reactions can yield volatile byproducts that potentially lead to porosity or the generation of interfacial gases, both of which could adversely affect the integrity of the contact zone. Furthermore, the chemical erosion of the graphite substrate may contribute to an increase in interfacial roughness, thereby impacting the overall performance and stability of the material system.

In this study, droplets containing 0% B₂O₃ and 30% B₂O₃ were also subjected to Fourier Transform Infrared (FTIR) analysis following a high-temperature wettability test conducted on both platinum (refer to Figure 7A,B) and graphite substrates (illustrated in Figure 8A,B). The wavenumber regions associated with various structural units are catalogued in

Table 5. FTIR bands corresponding to structural units of oxide samples 1 and 4 after high-temperature wettability tests.

IR Assignment	Wavenumber (cm−1)	Ref.
T–O–T bond bending vibrations; T denotes a Si or Al atom	400–600	[61]
stretching vibrations of fourfold coordinated Al3+ ions, Al–O–Si bending modes	600–800	[61]
stretching vibrations of [SiO4] tetrahedra	800–1200	[62]
stretching vibration of Si-O-B in [SiO4]	1070	[59]
stretching vibrations of boroxol rings	1225	[59]
B–O stretching vibration of varied borate groups in [BO3] units	1400	[61]

and taken from relevant literature.

FTIR spectrum analysis reveals that the addition of boron oxide results in a significant increase in the number of spectral bands. Nevertheless, it is noteworthy that there are no significant shifts in the positions of the fitted bands between the spectra with and without boron oxide; these differences remain within the spectral resolution limit of 4 cm⁻¹. Additionally, the Full Width at Half Maximum (FWHM) values of the bands in the boron oxide-containing samples are broader, implying a more amorphous nature of these materials.

The spectral data can be systematically categorized into distinct regions: the range of 400–600 cm⁻¹ corresponds to T-O-T bond bending vibrations; the range of 600–800 cm⁻¹ is associated with stretching vibrations of fourfold coordinated Al³⁺ ions and Al–O–Si bending modes, suggesting the presence of mixed alumino-borosilicate networks; and the range of 800–1200 cm⁻¹ pertains to the stretching vibrations of [SiO₄] tetrahedra, whereby vibrations in the range 800–950 cm⁻¹ may indicate the presence of NBO (Non-Bridging Oxygens) [61,62]. It is important to note that the region indicative of stretching vibrations of [BO₄] tetrahedra overlaps with the aforementioned region associated with silicon [61]. Furthermore, new bands have emerged in the FTIR spectra upon the addition of boron oxide, observed at approximately 1070 cm⁻¹, 1225 cm⁻¹, and 1400 cm⁻¹. These new bands have been assigned to the stretching vibrations of Si-O-B in [SiO₄], the stretching vibrations associated with boroxol rings, and the B-O-B bonds in [BO₃], respectively [59].

The identification of planar boroxol rings indicates that the structural network established before the addition of boron oxide has been disrupted. This transformation occurs alongside other structural units derived from B₂O₃, which likely exhibit weaker intermolecular interactions than those present in the original network [63]. Consequently, this weakening of intermolecular forces at the surface plays a significant role in diminishing the overall surface tension of the material and may lead to a decrease in wetting angles.

The outcomes of X-ray diffraction (XRD) analyses revealed similar patterns across all examined samples. As illustrated in Figure 9, the diffraction patterns of selected oxide systems, specifically sample 1 (containing 0 wt% B₂O₃) and sample 4 (with 30 wt% B₂O₃), were studied in relation to their interactions with platinum and graphite substrates.

Notably, the samples demonstrated a predominantly amorphous character, accompanied by a minor presence of a crystalline phase. Specifically, sample 1 exhibited a crystalline phase of 3.11% when in contact with the platinum substrate, which increased to 8.81% when in contact with graphite. Conversely, for sample 4, a significant reduction in the crystalline phase was observed—an approximate decrease of 60% on the platinum substrate. On the other hand, a slightly lesser reduction of around 30% was noted for the graphite substrate.

The incorporation of boric oxide plays a crucial role in diminishing the degree of crystallinity, particularly at medium to high concentrations. This effect is attributed to the ability of boric oxide to lower both the glass transition and crystallization temperatures, subsequently extending the duration required for nucleation and growth processes. Moreover, it enhances the conditions necessary for these processes and stabilizes the amorphous glass state, as supported by previous studies [64,65].

The oxide system samples that interacted with graphite were found to contain carbon. In sample 1, the carbon content within the crystalline phase was lower compared to that of sample 4. This observation correlates with the weak adhesion characteristics of oxide sample 1, suggesting a phenomenon of non-reactive wetting. In addition, quartz emerged as the predominant crystalline phase identified within the samples analyzed. Aside from quartz and graphite, no crystalline peaks indicative of silicon carbide or magnesium aluminate spinel were observed; however, the formation of these compounds remains highly plausible [64,66,67,68,69].

3.4. Wetting Mechanism

Based on the analysis results presented in the preceding section, a comprehensive understanding of the wetting mechanism can be articulated. Regarding the wetting of the platinum substrate, it is pertinent to note that platinum does not dissolve elements such as silicon, magnesium, calcium, or aluminum from the slag. Consequently, the formation of intermetallic or alloying phases is not anticipated, and the modification of the interface appears to be confined to physisorption. Additionally, physical adsorption of oxygen or slag constituents may occur, as referenced in the literature [70]. It is also necessary to consider that in a reducing atmosphere, the reduction of SiO₂ can lead to the dissolution of silicon into the metallic substrate, such as nickel, consequently altering the interface and making the wetting process more dynamic. However, in the context of this study, no silicon dissolution was observed, and the interface remained chemically inert, consistent with findings reported in [28]. Since the slag included magnesium oxide and alumina, the formation of the MgAl₂O₄ spinel is plausible. This formation could potentially influence the viscosity of the slag as well [71,72]. Furthermore, the presence of calcium oxide (CaO) and alumina in the slag could lead to the emergence of calcium aluminates, such as calcium aluminate (CA), calcium dialuminate (CA2), or calcium hexaaluminate (CA6). The formation of these new phases can affect the viscosity and surface tension of the slag, thereby altering its wetting characteristics [73,74,75]. However, the occurrence of these phases has not been substantiated by our analysis. Regarding the impact of boron oxide, the introduction of B₂O₃ into oxide melts has been shown to lower surface tension by facilitating the transformation of four-coordinated [BO₄] structures to planar [BO₃] units, as evidenced through Fourier-transform infrared (FTIR) analysis. These units can segregate to the surface, further reducing surface tension [76]. Additionally, CaO may facilitate network depolymerization by disrupting silicate frameworks and diminishing melt viscosity, thereby promoting improved wetting. These combined effects further reduce the contact angles, enabling the liquid to spread more readily across the substrate. In summary, in the absence of significant chemical interactions at the Pt-slag interface, wetting is predominantly driven by the minimization of surface energy and physical adsorption, resulting in a gradual reduction in the contact angle towards equilibrium [28], while dissolution or alloying was not observed.

When the molten slag comes into contact with the graphite substrate, the silica undergoes a reaction with the carbon present in the graphite, resulting in the formation of gaseous carbon monoxide (CO). This reaction has the potential to incite bubble formation within the slag and at the interface; however, such phenomena were not observed during the wetting processes undertaken in this investigation. The complexity of the reaction system presents numerous possible reaction pathways, with the prevailing consensus acknowledging that silica is initially reduced to silicon monoxide vapor [32,74,77,78].

$$\mathrm{S}\mathrm{i}{\mathrm{O}}_2\left(\mathrm{s},\mathrm{l}\right)+\mathrm{C}\left(\mathrm{s}\right)=\mathrm{S}\mathrm{i}\mathrm{O}\left(\mathrm{g}\right)+\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)$$

(1)

This reaction can be described as follows:

$$\mathrm{S}\mathrm{i}{\mathrm{O}}_2\left(\mathrm{s},\mathrm{l}\right)=\mathrm{S}\mathrm{i}\mathrm{O}\left(\mathrm{g}\right)+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2\left(\mathrm{g}\right)$$

(2)

The oxygen potential is controlled by the following reaction:

$$\mathrm{C}\left(\mathrm{s}\right)+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2\left(\mathrm{g}\right)=\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)$$

(3)

Silicon carbide, identified through SEM microanalysis, readily forms under these conditions. Silicon monoxide vapors subsequently react with carbon to produce silicon carbide.

$$\mathrm{S}\mathrm{i}\mathrm{O}\left(\mathrm{g}\right)+2\mathrm{C}\left(\mathrm{s}\right)=\mathrm{S}\mathrm{i}\mathrm{C}\left(\mathrm{s}\right)+\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)$$

(4)

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 gas-phase 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 [81,82,83,84]. Moreover, the incorporation of B₂O₃ significantly affects the melting characteristics and viscosity of slag systems. Specifically, the melting temperature of these fluxes decreases with an increase in B₂O₃ 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-SiO₂-MgO-Al₂O₃-B₂O₃ 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-SiO₂-MgO-Al₂O₃-B₂O₃ oxide system, while maintaining a constant basicity of 1.4 and varying the B₂O₃ 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 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.