**1. Introduction**

Composite materials are a widely used group of materials in various industries. Traditional composites can be classified according to the shape and geometry of the reinforcement (such as particles, long fibers, short fibers, etc.). Interpenetrating phase composites (IPCs) have become a new group of materials that can be used as light materials with improved physicochemical, mechanical, and thermal properties. The IPCs are characterized by the microstructure with two three-dimensional and topologically interpenetrating phases. Furthermore, the absence of the preferred orientation of matrix and reinforcement in IPCs

**Citation:** Kozera, P.; Boczkowska, A.; Perkowski, K.; Małek, M.; Kluczy ´nski, J. Influence of Fabrication Method and Surface Modification of Alumina Ceramic on the Microstructure and Mechanical Properties of Ceramic–Elastomer Interpenetrating Phase Composites (IPCs). *Materials* **2022**, *15*, 7824. https://doi.org/10.3390/ ma15217824

Academic Editor: Alberto Ortona

Received: 11 October 2022 Accepted: 3 November 2022 Published: 6 November 2022

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makes them predominantly isotropic and with consequent properties that are unattainable by other materials [1,2].

The strength of the phase separation surface, called the interphase boundary, depends on the type of phase joint, the appropriate wettability of the solid material by the liquid phase, and the surface topography of the solid component. Regardless of the type of reinforcement, the adhesion between inorganic and organic components in composites is usually weak due to the poor compatibility of the polymer or metal with the mineral surface. The problem of the insufficient wetting of the ceramics by molten metal or polymer is one of surface tension and surface quality, including any contamination, or oxidation. Some basic ways can be used to improve wetting. Generally, increasing the surface energies of the solid, decreasing the surface tension of the liquid matrix, as well as decreasing the solid–liquid interfacial energy at the interface can be essential [3]. Several treatments could be performed to increase the wettability of IPCs. In the work [4,5] it was found that the pressure-assisted infiltration, as well as process temperature and gas atmosphere, overcomes insufficient wettability between liquid metal and ceramic preform compared to pressureless fabrication methods. Moreover, in the case of pressureless techniques, insufficient wettability of the ceramic preform by liquid metal causes the formation of intermetallics as a result of interfacial reactions. According to [6,7], the application of some additives to aluminum alloys used in pressureless infiltration to the fabrication of ceramic–metal interpenetrating phase composites improved the wettability between the metallic and ceramic phases. Wang et al. [6] studied the microstructure of interface in 3D-SiC/Al-Si-Mg interpenetrating composites. It was shown that to improve wettability; the optimum content of Mg addition to the Al alloy was 4–8 wt%. A similar effect was obtained for SiC/aluminum alloy composites fabricated by pressureless infiltration as a result of the addition of 6–12 wt% Si [7]. A new method of producing ceramic-metal IPCs is the focus of the work reported by Qi et al. [8]. In the ultrasonic infiltration, the generated ultrasonic waves cause a collapse of gases dissolved in the melt and those entrapped in the ceramic foam. As a result, a pressure wave provides filling of the porous ceramic preform by the molten alloy. Furthermore, it was observed that the contact angle ZrB2–SiC porous ceramic by aluminum alloy decreased, improving wettability.

An alternative possible technique to improve adhesion between composite phases is to modify the filler surface with a surfactant, a coupling agent, or other surface-active agents. They reduce surface tension at the interface, which has a positive effect on the adhesion between composite components [9,10]. In the literature, the application of an organosilicon coupling agent, especially for composite materials based on ceramics and polymers or metal and polymer, is widely described. When an organofunctional coupling agen<sup>t</sup> is added to ceramic–polymer composites during the polymerization or crosslinking process, it forms a phase at the boundary by reacting with the functional groups of the polymer [10–12]. The use of coupling agents can increase the degree of ceramic pore filling by the polymer in a liquid form up to 80 vol.%. In addition, improving the adhesion between the components of the composite increases its compressive and tensile strength. Moreover, it can also lead to increased abrasion and corrosive resistance of the composite [11,13,14].

In polymer ceramics composites, microcracks may induce fracture to occur during stressing at or near the components' boundary. Therefore, the endurance of the connection between two phases in a composite is one of the main factors determining the IPCs' properties. To improve the mechanical properties of inorganic–organic phases, coupling agents can be applied. In the work [15], it was found that using silane coupling agents may cause the formation of siloxane bridges at the interphase, and as a consequence, result in the improvement of adhesive strength of polymer to ceramics.

In the work of [16], the effect on the dielectric and mechanical properties of cocontinuous barium strontium titanate (BST)–polymer composites of surfactant introduction onto the ceramic filler surface was investigated. The result means that carboxylic acid with longer carbon chains enhances the relative permittivity of the composite more effectively. The feasibility of processing polymer syntactic–aluminum foam IPC and its compression

response was studied by Jhaver and Tippur [17,18]. They applied silane to increase adhesion between the metal scaffold and polymer foam. The IPC foam coated by silane showed improvement in elastic modulus, compression strength, and plateau stress values by 28–35%, 20–25%, and 37–42% respectively.

The adhesion of two phases in IPC composites depends on the development of the ceramic surface. The topography of the ceramics**'** surface is changed according to the method and parameters of fabrication. Generally, during the sintering of a powder compact, both densifications occur simultaneously. The most effective way to stop grain growth and, as a result, obtain a ceramic fine-grain structure is to support the sintering processes with high pressure. Despite the high cost of the Hot Isostatic Pressing (HIP) technique, most ceramics, sintered under isostatic pressure gas, compared to freely sintered materials, are characterized by better fracture toughness [19], increased compressive strength, and tearing [20,21], as well as greater Vickers hardness [22]. Taking into consideration the porous ceramic fabrication, the HIP technique is a relatively rarely used method. Polymeric sponge method or polymer replica technique, gel casting of foams of ceramic foams all well as porous structure printing are more commonly used methods. Regardless of the method used and the pore size achieved, fabrication of a preforms structure of open pores and joined canals is required in the case of IPC materials. This structure allows for the easy flow of the liquid material.

In work [23], the authors present a comprehensive description of the freeze casting method, also known as ice templating, which has been applied extensively for the fabrication of well-controlled biomimetic porous materials based on ceramics, as well as metals. Infiltration of freeze-cast skeletons with a secondary phase allows for fabrication of aligned composites, hydrogels, and nacre-mimetic hybrids, as well as materials resembling interpenetrating phase composites.

This paper presents the influence of the fabrication methods of ceramic and its surface modification by the coupling agen<sup>t</sup> application on the microstructure and mechanical properties of the ceramic–elastomer composite. Tests were carried out to obtain an accurate ceramic–elastomer joint characterization, measurements of a contact angle, surface roughness, and shear strength. The appropriate amount of coupling agen<sup>t</sup> was determined for ceramic–elastomer composite fabrication. The application of a silane coupling agen<sup>t</sup> improved the properties of the composites, as well as a ceramic–elastomer joint.

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