*2.2. Methods*

The roughness tests were performed using a non-contact 3D surface profiler Slynx Sensofar (Sensofar Metrology, Terrassa, Spain). Two parameters were determined: the Ra parameter, i.e., the arithmetic mean height of the surface, and the Rz parameter, i.e., the height of the highest point on the surface. The final values are the average of three different measuring points on the surfaces.

The wettability of the ceramic surfaces was tested by measuring the water contact angles (WCA) and the contact angles hysteresis (CAH) using an OCA15 (DataPhysics Instruments, Germany) goniometer equipped with OCA software. The contact angle hysteresis was determined by calculating the difference between the advancing and receding contact angles. The test was performed on rectangular tiles with dimensions of 50 × 40 × 10 mm using a 5 μL droplet. The final WCA values are the average of three different measuring points on the surfaces.

To examine the strength of the adhesive ceramic–elastomer joints, a shear test was conducted. The Lloyd LR 10K (AMETEK, Berwyn, IL, USA) strength testing machine connected with the Nexygen 3.0 computer program (AMETEK, Berwyn, IL, USA) was used for the tests. Shear strength was calculated from the force registered by the computer divided by the joint cross-section according to the formula (1). The tests were carried out at a constant velocity of 1 mm/min according to the ASTM D3163-01 standard:

$$
\pi\_{\text{max}} = \frac{F\_{\text{max}}}{\text{A}} \text{ (MPa)} \tag{1}
$$

where: τmax is the maximum shear stress [MPa], Fmax is the force causing damage to the joint (N), and the A is the cross-sectional area of the joint (mm2).

Adhesive joints were made for the test according to the scheme shown in Figure 2. Two ceramic plates were placed in a Teflon mold. A layer of elastomer was placed between them. The thickness of the elastomer layer corresponded to the thickness of the spacer plate. The ceramic specimens with dimensions of 5 × 7 × 37 mm were first placed in a molder, and then the reactive mixture was poured.

**Figure 2.** Graph showing the construction of the adhesive joint, 1—dense ceramic substrate, 2— elastomer, 3—mold, 4—spacer.

The character of joint failure (adhesive or cohesive) and the microstructure of composites were characterized using Scanning Electron Microscopy (SEM) TM3000 (HITACHI High-Technologies Corporation, Tokyo, Japan) operating at an applied voltage of 5 kV. Before observations with the SEM, the surfaces of the specimens were sputtered with a gold-palladium layer for 90 s at a current of 10 mA and voltage of 2 kV. SkyScan 1174 X-ray tomography (SkyScan, Aartselaar, Belgium) was used for testing of the ceramic preform and composites. Before scanning, samples in the shape of a cuboid with dimensions 10 × 10 × 15 mm did not require any special preparation. Scanning was performed using an X-ray tube with the following parameters: 100 kV voltage, 100 kA, no filter material, 0.5◦ rotation step in an angle interval of 180◦. The obtained cross-sections of the ceramic preforms and composites were studied using CTAn software v.1.18 (Bruker, Kontich, Belgium) and as a result, porosity of the ceramic preform as well as the residual porosity of composites were determined. The application of CTAn software enabled the investigation of the weight fraction of each phase, including voids in ceramic preforms as ceramics porosity and in composites as residual porosity.

The compressive test was carried out using the MTS Q/Test 10 testing machine test machine (MTS Testing Systems, Toronto, ON, Canada) according to the ISO 20504:2019 standard with 1 mm/min velocity. Based on the obtained stress–strain curves, compressive strength and stress at the plateau were calculated.

### **3. Results and Discussion**

### *3.1. Surface Roughness Test of Dense Alumina Ceramics*

The topography of the ceramic surface was analyzed by the results of the roughness test. Results of Ra and Rz roughness parameters, as well as a surface profile observation, are displayed in Figures 3–5. The surface roughness test was conducted for dense Al2O3 ceramics fabricated by two methods; for Al2O3-SS samples, pressureless sintering was the final process, whereas for Al2O3-HIP200 samples hot isostatic pressure sintering was applied additionally.

The roughness parameters determined with the use of a laser profilometer allowed the assessment of the degree of surface development of both types of ceramic materials. It was demonstrated that the use of hot isostatic pressure during the sintering of alumina ceramics modified their surface condition. It should be noted that the application of 200 MPa pressure during ceramic sintering caused a decrease in roughness parameters in comparison to the pressureless sintered material. Looking into the ceramic profiles, in the Al2O3-HIP200 sample, numerous structural defects with a size of about 50 μm and a depth of 9 μm were eliminated. Other researchers have previously observed a similar relationship in alumina ceramic sintering [24]. In comparison, Al-Jawoosh et al. indicated that the Ra roughness parameter for the densely sintered alumina ceramic specimens was 0.6 μm [25]. As a consequence of the use of high pressure, the pores and cracks closing occurred and the material's compaction increased. Furthermore, densification was due to particle rearrangement, plastic deformation, grain boundary diffusion, and structural

defect elimination. During the sintering of a powder compact, both densification and grain growth occur simultaneously [26].

**Figure 3.** The Ra and Rz roughness parameters of dense alumina ceramics depend on fabrication methods: Al2O3-SS samples fabricated by pressureless sintering and Al2O3-HIP200 specimens obtained using hot isostatic pressure sintering.

**Figure 4.** The surface topography map of dense alumina ceramics depends on fabrication methods: (**a**) Al2O3-SS samples fabricated by pressureless sintering and (**b**) Al2O3-HIP200 specimens obtained using hot isostatic pressure sintering.

**Figure 5.** 3D Profilometry images illustrating dense alumina ceramics depending on fabrication methods: (**a**) Al2O3-SS samples fabricated by pressureless sintering and (**b**) Al2O3-HIP200 specimens obtained using hot isostatic pressure sintering.

### *3.2. The Contact Angle Measurements of Dense Alumina Ceramics*

The contact angle measurements were performed to examine the effect of the HIP sintering application as well as the silane coupling agen<sup>t</sup> on ceramic wettability. Furthermore, the influence of the coupling agen<sup>t</sup> content in the toluene solution on the contact angle was investigated. The results of the tests are presented in Figure 6.

Wettability is the ability to spread material in a liquid form on a solid surface. Depending on the contact angle value, surfaces are of a hydrophilic character (θ < 90◦) or hydrophobic character (θ > 90◦). In the case of IPC composites, the value of the contact angle affects the degree of filling of the porous preform by the liquid material, and thus affects the mechanical strength of the composite [27]. Reducing the contact angle value, i.e., improving wettability, can ensure better filling of pores in the ceramic preform, and elimination of gas bubbles from the interface. In addition, the elimination of structural discontinuities increases the contact area between the materials, which in turn leads to an improvement of the durability of the interface connection [28].

The obtained results show that the average water contact angle of the nontreated ceramic surface was about 80◦ (lack of differences between the fabrication methods of ceramic) but in the presence of a solution coat of 5 wt% U-15 coupling agen<sup>t</sup> in toluene, it was decreased to 60◦. This means that the wettability of ceramic was improved due to the U-15 agen<sup>t</sup> application. Although an angle value below 90◦ indicates the hydrophilic character of the ceramic surface uncoated by the solution of the U-15 agent, the contact angle was still close to 90◦. It decreased significantly only after applying the coupling agen<sup>t</sup> coat.

The solution concentration of the silane coupling agen<sup>t</sup> in toluene has been selected based on previous work [29]. For low concentrations of the U-15 agen<sup>t</sup> (<1%), a change in water contact angle was not observed. These observations were confirmed by other researchers [16,28]. It was found that the solution concentration, solution pH, and curing conditions can affect significantly the silane bonding to an inorganic surface. Silane solution baths are usually used in low concentrations (0.01–2%) because the formation of oligomers is suppressed in dilute solutions [28].

The literature indicates that the contact angle is closely related to surface roughness [30]. It can be concluded that if the Ra parameter possesses a value lower than 0.5, the effect of roughness on the contact angle is insignificant. Higher Ra values indicate an increase in surface roughness, hence, an increase in the wetting surface (contact surface). Nevertheless, increasing roughness can affect wettability in two ways. Together with the improvement in surface roughness, the number of defects and pores on the surface into which liquid material can penetrate increases. As a result, the strength of such a joint can enhance. However, too many pores, especially narrow ones, become an obstacle, impede

wettability, and consequently, prevent the formation of a durable adhesive interface connection [30]. As shown in Figure 2, the achieved Ra roughness parameter is low independently of the fabrication method of dense alumina ceramic. This confirms the lack of a significant difference in water contact angle results for Al2O3-SS and Al2O3-HIP200 ceramic.

### *3.3. Shear Strength of an Interface Joint*

The shear strength results of the ceramic–elastomer interface joint are displayed in Figure 7. It can be seen that the application of the silane solution coat enhanced the shear strength of the ceramic–elastomer joints. The average shear strength value for Al2O3-SS ceramic–elastomer joint was 2.9 ± 0.7 MPa while using a silene solution coat increased the shear strength to 8.8 ± 0.2 MPa. In comparison, Chaijareenont et al. indicated that silane coupling agents affect polymethyl methacrylate (PMMA) bonding to alumina. The bond strength of PMMA on the alumina treated by a bath in a solution of N-2 (aminoethyl) 3-aminopropyltriethoxysilane) in ethanol was reached at 10.8 MPa, which is similar to obtained results [31]. The highest shear strength was achieved for the ceramic–elastomer joint in which dense ceramic was a fabrication by pressureless sintering. This is related to the slightly higher roughness parameters of the surface specimen. In other words, a higher surface roughness caused an increase in the contact area of the joint materials, while the use of a coupling agen<sup>t</sup> ensured the possibility to infiltrate the micropores on the ceramic surface by the elastomer reactive mixture. It is noteworthy that, in the case of dense ceramic fabricated by pressureless sintering, the pore size was not reduced as for Al2O3- 200HIP samples. Hence, the penetration of the reactive mixture into the micropores of the ceramic surface and the creation of additional mechanical bonds was easier. In addition, due to its bi-functionality, the U-15 agen<sup>t</sup> interacted with two materials, improving their mutual adhesion by creating molecular bridges, which connected the inorganic surface with the polymer through the available types of polar interaction. The bonding was covalent (siloxane bond) [32].

**Figure 7.** The shear strength on the alumina ceramic surface was treated using a silane solution of the coupling agen<sup>t</sup> with different concentrations; Al2O3-SS samples were fabricated by pressureless sintering, and Al2O3-HIP200 specimens were obtained using HIP sintering.

### *3.4. SEM Observations of Ceramic–Elastomer Joint*

The ceramic–elastomer bond quality was analyzed in terms of failure character joint after a shear test using a scanning electron microscope. Sample images are presented in Figure 8a,b. At first glance, it can be seen that the failure was adhesive for interfacial joints in which no coupling agen<sup>t</sup> was applied, as evidenced by the smooth surface of the joint after the shear test. In contrast, in samples in which the coupling agen<sup>t</sup> was applied

(Figure 8b), the failure was reached partially by elastomer decohesion, which indicated good adhesion of the elastomer to the ceramic surface.

**Figure 8.** SEM images illustrating failure after shear strength test of ceramic–elastomer joints: (**a**) alumina surface uncoated of silane solution, (**b**) alumina surface coated of silane solution.

### *3.5. Mechanical Properties of Porous Alumina Ceramic*

The main property of porous ceramic preforms is high mechanical strength. Therefore, the effect of the fabrication method as well as the degree of porosity of ceramic preform into their compressive strength were analyzed (Table 1). The results show that the average compressive strength of the sample fabricated by pressureless sintering was about 10 MPa, but with the addition of the HIP sintering stage, it was increased by 100%. This is related to the densification of alumina grains and pores size reduction.

**Table 1.** Compressive strength of porous ceramic preform fabricated by pressureless and HIP sintering.


Ceramic preforms intended for pressure infiltration should exhibit mechanical strength. Otherwise, they may be damaged during the fabrication of composites. Looking into the compressive test result, it can be seen that porous preform fabricated using HIP sintering is characterized by higher mechanical strength.

Interestingly, the degree of porosity of ceramic preform achieved in the work is considerably lower in comparison to ceramic preform porosity that can be obtained using other methods. In comparison, Peng et al. indicated that the porosity of ceramic foams fabricated by the polymeric sponge method was higher than 70% [33]. Similarly, the application of gel casting of ceramic foams method allowed porosity to be reached between 50% and 90% [34]. However, it is noteworthy that the growth in porosity causes a decrease in mechanical strength.

### *3.6. SEM Observations of Composites*

After characterization of porous ceramic preforms' mechanical properties, a part of them was used for ceramic–elastomer composites fabrication. The mechanical test results of porous samples confirmed that the HIP sintering application allowed them to achieve

higher compressive strength. From this point of view, the porous ceramic preforms were utilized for ceramic–elastomer fabrication by the infiltration method. The Al2O3-HIP200 porous samples with 20% and 40% porosity were infiltrated by the liquid elastomer. Part of the samples was coated with 5 wt% silane solution in toluene. To assess the microstructure of the fabricated ceramic–elastomer composites and the effect of the coupling agen<sup>t</sup> on their microstructure, SEM observations were conducted. The obtained results are shown in Figures 9b and 10b. In contrast, observations of composite fabricated by using porous ceramic preform without a U-15 agen<sup>t</sup> solution coat are displayed in Figures 9a and 10a.

**Figure 9.** The microstructure of Al2O3/PU2.5 composite fabricated by infiltration of the ceramic preform with 20% porosity: (**a**) porous alumina surface uncoated of silane solution, (**b**) porous alumina surface coated of silane solution. (The lighter phase is alumina ceramics and the darker phase is polyurethane elastomer, the black circle show structural defects).

**Figure 10.** The microstructure of Al2O3/PU2.5 composite fabricated by infiltration of the ceramic preform with 40% porosity: (**a**) porous alumina surface uncoated of silane solution, (**b**) porous alumina surface coated of silane solution. (The lighter phase is alumina ceramics and the darker phase is polyurethane elastomer, the black circle show structural defects).

At first glance, it can be seen that the macro and micro-pores of the ceramic preforms have been filled by the reactive mixture of elastomer. The elastomer also infiltrated the channels formed between the ceramic grains, as well as in their cracks. It should be also noticed that the structure of interpenetrating phases was successfully obtained. Observations confirmed that the infiltration method allowed the elastomer to fill the ceramic pores. Looking into the SEM images, it can be seen that the adhesion between Al2O3 ceramics with the elastomer for each porosity was improved for composites fabricated by using porous ceramic preform with a U-15 agen<sup>t</sup> solution coat. Furthermore, the pores have been filled

better and the interface boundary between the ceramics and the elastomer was continuous. In addition, delamination on the ceramic–elastomer boundary was not observed, contrary to composites obtained using porous ceramic preform without a U-15 agen<sup>t</sup> solution coat. In the case of uncoated samples, the effect was revealed of weaker adhesion between the ceramic and polymer phases, as evident from the isolated debonds highlighted in Figures 9a and 10a. It can be concluded that the silane coupling agen<sup>t</sup> facilitates infiltration and improves adhesion between the phases; other researchers have previously observed a similar relationship by coupling agen<sup>t</sup> application [35–37].

It is noteworthy that after the ceramic pores are filled with elastomer in liquid form, polymerization of the monomer is started, and the unavoidable volume shrinkage can appear. In work [10], it was confirmed that the application of pressure during infiltration as well as a reduced polymerization speed can limit the occurring polymerization shrinkage, which leads to a decrease in the appearance of defects in the microstructure of IPCs. In this study, despite pressure infiltration application to fill ceramic pores with liquid elastomer, the polymerization shrinkage generated pores (defects) in the microstructure. This led to an interfacial boundary loss between polymer and ceramic. An optimized, defect-free microstructure was obtained for composites fabricated by infiltration of ceramic preforms coated by silane agent.

### *3.7. Residual Porosity Measurement of Composites*

Residual porosity, i.e., porosity, which is a result of insufficient pore filling by the elastomer, was determined by X-ray tomography. The results of residual porosity for composites fabricated by infiltration of porous ceramic pre-form coated with a U-15 agen<sup>t</sup> solution as well as uncoated ceramic preforms are shown in Figure 11.

The coupling agen<sup>t</sup> had a significant impact on the infiltration process and, consequently, the degree of pore filling. The smallest residual porosity after infiltration, approximately 2 vol.%, was evaluated for composites fabricated by infiltration of ceramic pre-form with 40 vol.% porosity coated with a U-15 agen<sup>t</sup> solution. Infiltration of preforms with higher porosity was easier, pores were better filled, and residual porosity was smaller than in the case of the preform with smaller porosity.

### *3.8. Mechanical Properties of Composites*

To analyze how the coupling agen<sup>t</sup> application affects the compressive strength and stress at the plateau of composites, the compression test was performed. The calculated compressive strength and stress at the plateau area are presented in Figures 12 and 13. For the composites fabricated by infiltration of the uncoated porous preform, the compressive strength was under 30 MPa. Using silane solution on porous ceramic preforms increased the compressive strength of the composite to about 35 MPa. A similar tendency of stress in the plateau area growth was found. The obtained results can be summarized: the application of the U-15 coupling agen<sup>t</sup> caused a significant increase in the mechanical properties of ceramic–elastomer interpenetrating phase composites. In addition, the higher score of the area under the stress-strain curve, i.e., the plateau area, as well as stress at the plateau, was achieved. Most likely, the ability to absorb the energy of composites fabricated with a U-15 promoter application was also improved. Moreover, the samples were not destroyed during the compressive test. After removing the load, the composite specimens almost returned to their original shape, because of the highly elastic deformations of the elastomer. Similar results were obtained in [18], where the increase in elastic modulus and compressive strength of silane-coated preform can be attributed to improved wettability, which in turn enhances adhesion between the metal and polymer phases.

**Figure 12.** Comparison between compressive strength of composites fabricated by using porous alumina uncoated and coated of silane solution; Al2O3/PU2.5 composites fabricated by infiltration of the ceramic preform with 20% and 40% porosity.

Figure 14 shows the typical stress–strain response at static compressions of IPCs. For all composites coated and untreated with silane solution, a linear elastic deformation followed by a protracted nonlinear behavior was observed. After reaching the elastic limit, the inelastic stage includes a distinct softening response due to the onset of ceramic foam failure. It is important to note that the plateau stage is characterized by a long duration of slightly increased stress and quickly increased strain. The last stage is concerned with the densification of the composite structure due to load impact. As noted earlier, the increase in compressive strength and stress of plateau of composites fabricated by using silane-coated preform can be attributed to improved wettability, which in turn enhances adhesion between the ceramic and polymer phases. The improvement in the silane-coated composites' mechanical characteristics relative to the uncoated one is caused by the stronger bond between the ceramic and polymer matrix. Application of silane coupling agen<sup>t</sup> delays failure of interfacial bonds during the deformation process [17,18].

**Figure 13.** Comparison between stress at a plateau of composites fabricated by using porous alumina, uncoated and coated, of silane solution; Al2O3/PU2.5 composites fabricated by infiltration of the ceramic preform with 20% and 40% porosity.

**Figure 14.** Stress–strain response in uniaxial compression for composites with uncoated (Al2O3/PU2.5) and silane coated performs (Al2O3/PU2.5+U-15); ceramic foam with 20% porosity.
