1. Introduction
Boroaluminosilicate glasses, multicomponent boron oxide-containing oxide glasses, have a wide range of applications, from lithium battery-sealing materials and liquid crystal display substrates to nuclear waste disposal [
1,
2]. However, they are also crucial in the glass fiber industry, with E-type glass fibers exhibiting excellent dielectric properties due to their B
2O
3 content at the expense of significantly reduced mechanical properties [
1,
2,
3,
4]. It is worth noting that in its pure form, borate glass is not ordinarily usable for any application because it has meagre chemical resistance and a high affinity to water. Therefore, it is used in combination with other oxides, such as Al
2O
3 or SiO
2, which leads to improved chemical durability but at the cost of higher processing temperatures [
1]. Low processing temperatures are critical in applications such as sealing and the passivation of electronic devices [
5]. Thus, it is logical that boron oxide is added as a fluxing agent, as is the case with fluxes where they significantly reduce the melting temperature of the flux by replacing CaF
2 and improving the desulphurization capacity of the flux, whereby the sulfur content of the metal can be reduced to thousandths of a percent [
6,
7].
Wettability is the ability of a liquid to spread on a solid substrate and is measured by the angle of wetting between the tangent drawn at the triple point and the substrate. There are two types of wetting: non-reactive and reactive. The former does not involve a reaction or absorption between the spreading liquid and the substrate, while the latter does. It should be noted that Young’s equation provides the equilibrium contact angle, and its formulation is valid under the assumption of spreading a non-reactive liquid on an inert, smooth, homogeneous solid [
8,
9,
10]. Platinum and graphite are commonly used as measuring systems [
11,
12,
13,
14,
15]; however, substrates manufactured from these materials often interact with the sample. Carbon tends to reduce oxides. Platinum can form alloys with metals such as iron, lead, zinc, tin, and antimony, while the melting point of these alloys is lower than that of platinum. Under reducing conditions, non-metallic elements can also react with platinum, particularly arsenic, phosphorus, boron, bismuth, silicon, and sulfur.
A reliable and reproducible contact angle measurement is essential for assessing the wetting behavior of the system since understanding high-temperature wetting behavior is essential for improving industrial processes involving a liquid phase and the quality of the final product. Over the years, various methods have been developed to evaluate the wettability of solids by liquids [
16,
17]. Among these techniques is the viral sessile drop method. Wetting is influenced by many factors that relate to both the properties of the liquid and the properties of the substrate and the measurement conditions. A common cause of contact angle hysteresis is the physical properties of the substrate surface, such as its roughness, presence of cracks, microcavities, coatings, etc., with large values of roughness resulting in significant scatter in the measured angles [
18]. The type of gaseous atmosphere also plays an equally important role, with oxidizing conditions capable of significantly distorting the results of metal contact angle measurements [
19]. What is also of note are the physical and chemical interactions of the molten droplet with the substrate, such as the dissolution of the substrate forming craters under the droplet, the formation of interfacial reaction products requiring structural characterization of the phase interface after the experiment, and the formation of carbon whiskers on the droplet surface when tested under a high vacuum [
8,
9,
19].
Over the last few decades, many authors and research groups have studied the wettability of refractory materials, such as platinum and graphite, by oxide systems. Using the sessile-drop method, the wettability of platinum and other metal substrates and graphite by slag was measured. The slag wets Pt relatively well, while increasing the temperature promotes wettability. Parry and Ostrovski studied the wetting of solid metals (iron, nickel, and platinum) with molten CaO–SiO
2–Al
2O
3 [
13] and MnO–SiO
2 [
20] slag in a graphite furnace with a reducing atmosphere containing trace oxygen. In the case of the first oxide system, silica reduction, silicon dissolution (which did not occur for the platinum substrate), and oxygen adsorption substantially changed the phase interface, making the conditions dynamic. The platinum substrate was wetted the most, and the wetting angles decreased with increasing temperatures. Oxygen desorption led to the development of gas bubbles, as observed in experiments with platinum substrates. In contrast, the wetting of the platinum substrate was the lowest for the second oxide system. Modifying the metal–oxide interface due to oxide reduction and oxygen adsorption created conditions for dynamic wetting behavior and affected the interface properties. The reaction at the phase interface aided wetting, and the dissolution of the reduced manganese occurred for all three substrates. In paper [
14], the difference in the thermal expansion coefficient of platinum and slag caused a facile separation of these systems during cooling. On the other hand, the slag did not wet the graphite. However, the increase in this wettability at high temperatures was due to a chemical reaction that also caused the drop to foam [
14]. Obviously, graphite as a substrate can only be used in a reducing atmosphere and may react with the oxide system [
14,
21]. The wetting of platinum by the MnO–SiO
2 and CaO–AI
2O
3–SiO
2 oxide system was analyzed in [
22], where oxide reduction and the dissolution of Mn into the substrate occurred, and the reaction at the phase interface promoted wetting, resulting in dynamic wetting behavior. In the case of the graphite substrate, the absence or minimal wetting by molten oxide mixtures is due to the weak van der Waals forces underlying interfacial bonding [
23]. However, for some oxide systems, the contact angle decreases with time to values below 90° due to interfacial reaction [
24,
25,
26]. This has been addressed in several works on the wettability of carbonaceous materials by CaO–SiO
2–Al
2O
3–MgO–FeO
x slags in the 200 K range from 1673 K [
24,
25,
26,
27,
28], and it has been found that wettability can be improved by increasing the iron oxide content and temperature. In the wettability test measurements of the CaO–SiO
2–Al
2O
3–MgO system and carbon substrate [
29], it was found that the reduction of SiO
2 in the slag and the formation of SiC on the substrate surface are the dominant factors improving the wettability. Furthermore, the reduction of MgO proceeded preferentially and inhibited the spreading of molten slag. The wettability of graphite by the CaO–SiO
2–Al
2O
3–FeO–MgO oxide system was investigated with a focus on interfacial phenomena using the sessile drop method, and it was found that the wettability was mainly influenced by the initial magnesium and iron oxide content, and that iron reduction occurred due to the penetration of the oxide system into the graphite [
30]. Duchesne studied the temperature dependence of wetting angles, density, and surface tension of slag on graphite, molybdenum, and alumina substrates. Although alumina and molybdenum provided lower contact angles, they interacted less with the slag and were stable under oxidizing conditions [
31]. Using the dispensed drop method, Liu et al. investigated the wettability of graphite by TiO
2–CaO–SiO
2–Al
2O
3–MgO slag in an argon atmosphere and found that the TiO
2 content significantly aggravated the wettability in contrast to the CaO/SiO
2 ratio. Interfacial reaction and reduction of the corresponding metal oxides also occurred during the high-temperature test [
32].
The aim of this paper was to describe the interaction at the interface between the CaO–MgO–SiO2–Al2O3–B2O3 oxide system and graphite and platinum substrates using a wettability test. The phase interface after high-temperature tests was characterized by SEM and EDX analyses. The phase composition of the oxide system was analyzed by XRD. Attention has been paid to the effect of B2O3 on wettability due to the frequent use of B2O3 as a component that reduces the liquidus temperature. Our literature survey revealed that no work has been published to date on wetting platinum and graphite substrates with a molten oxide system similar in composition to E-glasses and with a graded boron oxide content. This provides this work with practical significance because a material with low interaction at high temperatures can be used as a measuring system, e.g., crucible, mold, or substrate, to measure critical thermophysical properties such as surface tension, density, and viscosity of this oxide system. Furthermore, the description of the phase behavior aids in understanding the interfacial processes for the widely used graphite substrate and the minimally reactive platinum substrate. This work can also contribute to optimizing ironmaking processes when contact between molten slag and coke occurs, and it can also increase the understanding of glass container manufacturing processes when glass is stuck to metal molds.