Microhardness Enhancement in Polymer Composites via BaZrO₃-Based Ceramic Reinforcement

Featured Application

The developed BaZrO₃-Y₂O₃-SrTiO₃-reinforced polyetherimide composites provide a mechanically improved and thermally stable platform for advanced dental materials. The significant increase in microhardness and glass transition temperature indicates improved dimensional stability under masticatory loading and thermal cycling. This oxide-based reinforcement strategy offers a promising alternative to conventional ceramic fillers for high-performance biomedical and engineering polymer systems.

Abstract

Defect-tolerant oxide ceramics offer an alternative reinforcement strategy for high-performance polymer composites beyond conventional silica- and zirconia-based systems. In this work, a novel BaZrO₃-Y₂O₃-SrTiO₃ (BZYS) ceramic hybrid was introduced as a reinforcing phase in a polyetherimide (PEI) matrix to evaluate its effect on interphase formation, thermal stability and mechanical performance. BZYS powders were prepared by ball milling and incorporated at 1 and 3 wt% into solution-cast PEI films. X-ray diffraction confirmed the preservation of the BaZrO₃ perovskite structure after mechanical activation, with a slight lattice expansion, indicating partial ion incorporation and defect-mediated structural accommodation. SEM analysis revealed predominantly submicron agglomerates with homogeneous dispersion at low loading and controlled agglomeration at higher content. Differential scanning calorimetry demonstrated a systematic increase in glass transition temperature from 202.0 °C for neat PEI to 210.4 °C and 212.0 °C for 1 wt% and 3 wt% composites, respectively, evidencing restricted segmental mobility and interphase formation. Instrumented microindentation showed substantial hardness enhancement of 40% and 83% for 1 wt% and 3 wt% reinforcement, respectively (p < 0.05), with a strong linear dependence on filler content (R² = 0.9845). The results demonstrate that chemically stable, strain-tolerant BZYS ceramics effectively promote interphase-mediated reinforcement in PEI, establishing a novel oxide-based pathway for mechanically enhanced dental composite materials design.

1. Introduction

Dental restorative materials have an important role in modern dentistry, yet current composite systems continue to face limitations that compromise their long-term performance. The mechanical properties of dental materials often fall short of the requirements for high-stress-bearing applications, particularly in posterior restorations where masticatory forces are substantial [1]. Therefore, advanced composite materials are becoming very valuable, as being capable of maintaining mechanical integrity, dimensional stability and biological compatibility under long-term service conditions [2,3]. Dental composites are typically composed of a polymeric matrix reinforced with inorganic fillers, where the filler phase plays a decisive role in governing hardness, stiffness, wear resistance and resistance to crack initiation [4]. Ceramic reinforcement has emerged as a promising strategy to enhance the mechanical and thermal properties of polymer-based dental materials [5]. The incorporation of ceramic nanoparticles into polymer matrices can significantly improve hardness, flexural strength, elastic modulus and thermal stability through multiple mechanisms, including load transfer from the compliant matrix to rigid ceramic inclusions, crack deflection and bridging and restriction of polymer chain mobility at the filler-matrix interface [6,7]. The effectiveness of ceramic reinforcement is highly dependent on filler loading, particle size, dispersion quality and interfacial bonding between the ceramic phase and polymer matrix [8,9].

Yttrium oxide (Y₂O₃) has demonstrated considerable promise as a reinforcing agent in polymer composites for dental and biomedical applications [10]. These improvements are attributed to the high stiffness of Y₂O₃ particles, which act as load-bearing inclusions and restrict polymer chain mobility, thereby enhancing both mechanical strength and dimensional stability [11]. Beyond mechanical reinforcement, Y₂O₃ nanoparticles have exhibited excellent biocompatibility and have been shown to enhance cell adhesion, proliferation, and angiogenic properties in tissue engineering scaffolds [12]. Yttria-stabilized zirconia (YSZ) has also shown promising biocompatibility, with 5 mol% YSZ coatings exhibiting hemolytic ratios below 5%, antimicrobial activity against common pathogens, and non-toxic cytocompatibility, suggesting suitability for dental and orthopedic implant applications [13]. Strontium titanate (SrTiO₃) is a biocompatible perovskite proven as an efficient reinforcement for dental polymers [14,15]. Recent work on denture base materials reinforced with hybrid SrTiO₃/Y₂O₃ nanoparticles demonstrated remarkable improvements, with 1 wt% loading increasing microhardness three times [16]. Barium zirconate (BaZrO₃) represents another ceramic oxide with significant potential for polymer reinforcement, though its application in dental composites remains largely unexplored [17]. Its cubic perovskite structure is characterized by a rigid Zr–O framework capable of accommodating lattice distortions without phase transformation, making BaZrO₃ structurally stable across a wide range of processing and service conditions [18,19]. Importantly, BaZrO₃ exhibits low chemical reactivity and minimal susceptibility to moisture-induced degradation, properties that are highly desirable for dental applications [20,21]. In addition to its intrinsic stability, BaZrO₃ displays a remarkable capacity to tolerate lattice strain and crystallographic defects without catastrophic loss of structural coherence [22]. Furthermore, it has been shown that the structure and functional properties of BaZrO₃ could be modified with incorporation in polystyrene [23]. Research on polyimide/BaZrO₃ nanocomposites has demonstrated that BaZrO₃ nanoparticles can be homogeneously dispersed within polymer matrices and significantly enhance thermal properties [24]. Specifically, the incorporation of BaZrO₃ nanoparticles at loadings of 5, 10 and 15 vol% resulted in increased glass transition temperature (T_g) and heat capacity compared to neat polyimide, indicating restriction of polymer chain mobility and enhanced thermal stability. The mechanism underlying this thermal property enhancement involves the formation of a rigid interphase region at the ceramic-polymer interface, where polymer chain segments experience restricted mobility due to physical and chemical interactions with the nanoparticle surface. While BaZrO₃ has been investigated in high-temperature ceramic applications for its excellent chemical stability and low thermal conductivity, its potential as a reinforcing phase in biomedical polymer composites has received limited attention, representing a research gap [25].

Polyetherimide (PEI) is a high-performance thermoplastic polymer that offers several advantages over conventional dental resin matrices. PEI exhibits inherently high glass transition temperature, excellent mechanical properties, superior chemical resistance and the ability to withstand repeated sterilization cycles, properties that are highly desirable for biomedical applications [26,27]. It has been successfully employed in biomedical devices, including membrane filtration systems and implantable components, demonstrating good biocompatibility and processability [28]. Early in vitro and in vivo investigations demonstrated that PEI exhibits favorable biocompatibility profiles, including minimal inflammatory response and acceptable tissue integration when used as a membrane-forming polymer or scaffolds [29,30]. More recently, PEI has been proposed for additively manufactured orthopedic prosthetic components, where it demonstrated suitable cytocompatibility and mechanical compatibility with bone-mimetic environments [31]. The combination of PEI’s high-performance with ceramic oxide reinforcement presents an opportunity to develop dental composites with superior mechanical and thermal properties compared to conventional systems. Microhardness and glass transition temperature are critical properties for dental restorative materials, directly influencing clinical performance and longevity. Microhardness correlates strongly with wear resistance and the ability to withstand masticatory forces, which are essential for maintaining restoration integrity in the oral environment [32,33]. Materials with higher microhardness values exhibit superior resistance to surface degradation, scratching and abrasive wear from food particles and opposing dentition [34]. Glass transition temperature is equally important, as it defines the temperature range over which the polymer transitions from a rigid, glassy state to a more compliant, rubbery state. Dental materials with elevated T_g values maintain their mechanical properties and dimensional stability across the range of temperatures encountered in the oral cavity (typically 0–60 °C during consumption of cold and hot foods and beverages) [35]. Furthermore, higher T_g values indicate stronger intermolecular interactions and more restricted chain mobility, which translate to improved creep resistance and long-term dimensional stability under sustained occlusal loading [36]. The simultaneous enhancement of both microhardness and T_g through ceramic reinforcement represents a synergistic approach to improving the overall performance of dental composites.

The present work addresses a gap in the literature by investigating a novel dental composite system consisting of a polyetherimide matrix reinforced with BaZrO₃-SrTiO₃-Y₂O₃ (BZYS) ceramic hybrid oxides. Despite extensive research on BaZrO₃, SrTiO₃ and Y₂O₃ in electronic, structural and coating applications, their combined use as reinforcing ceramic fillers in dental polymer composites remains insufficiently explored. While previous studies have independently examined Y₂O₃ reinforcement in conventional dental polymers, no prior research has explored the combination of these two ceramic oxides in a PEI matrix for dental applications. By combining the complementary properties of BaZrO₃, SrTiO₃ and Y₂O₃ (mechanical reinforcement and thermal stability), combined with the inherent high-performance properties of PEI, this research aims to establish a new class of dental composites with superior mechanical and thermal properties for demanding clinical applications.

2. Materials and Methods

2.1. Preparation of Composite Samples

BaZrO₃, SrTiO₃ and Y₂O₃ were ball-milled at 300 rpm for 30 min. The weight percent ratio BaZrO₃:Y₂O₃:SrTiO₃ was 87:10:3. The composition was chosen after a series of preparations, based on desirable shape and size. For film preparation, 20 wt% of PEI in chloroform was solution cast and kept at room temperature for 24 h. BaZrO₃-Y₂O₃-SrTiO₃ (BZYS) nanoparticles were added in 1 and 3 wt%. Samples were denoted: PEI, PEI-BZYS1 and PEI-BZYS3. Preparation of composites is presented schematically in Figure 1.

2.2. Characterization of Particles and Composites

The morphology, particle size distribution, and dispersion of BaZrO₃-Y₂O₃-SrTiO₃ ceramic fillers within the polyetherimide (PEI) composites were examined using field emission scanning electron microscopy (FESEM) (Tescan Mira 3, Brno, Czech Republic) and FEI Scios 2. Quantitative image analysis of ceramic particle size was carried out using Image-Pro Plus 7.0 software (Rockville, MD, USA), based on three representative micrographs for each sample. The X-ray diffraction (XRD) was used for the identification of crystalline phases and the calculation of unit cell parameter and crystallite size. The XRD pattern was collected over the range 20° < 2θ < 80° on an Ital Structures APD2000 X-ray diffractometer using CuKα radiation (λ = 1.5418 Å) with the step size of 0.02° and the counting time of 1 s/step. Fourier transform infrared spectroscopy (FTIR) was employed to investigate possible structural and chemical changes in the ceramic fillers and composites following thermal treatment. Spectra were recorded using a Thermo Scientific Nicolet iS35 spectrometer (Waltham, MA, USA) in the wavenumber range of 4000–400 cm⁻¹. Thermal properties of the PEI-based composites were evaluated by differential scanning calorimetry (DSC) using a TA Instruments Q10 calorimeter. Measurements were conducted at a heating rate of 10 °C min⁻¹ under a nitrogen atmosphere with a flow rate of 50 mL min⁻¹. The glass transition temperature (T_g) was determined from the midpoint of the inflection in the heat flow curve obtained during the first heating cycle. Microhardness measurements were carried out using an instrumented indentation method with a spherical indenter of 4 mm diameter. A load cell of 500 N was employed, with an indentation rate of 0.25 N s⁻¹. The maximum load of 5 N was maintained for 20 s before unloading at the same rate, while force, time and indenter displacement were continuously recorded. For each sample, three independent indentations were performed at different locations. The hardness was determined according to the Oliver–Pharr analytical approach [37]. Statistical analysis of the mechanical testing results was conducted using one-way analysis of variance (ANOVA) in Origin 9 software (OriginLab, Northampton, MA, USA) to evaluate the effect of BaZrO₃-Y₂O₃-SrTiO₃ ceramic reinforcement on the properties of the PEI composites.

3. Results and Discussion

3.1. Morphology and Size Distribution of Ball-Milled BZYS Powders

Quantitative image analysis of the ball-milled nanoparticles’ SEM micrographs (Figure 2) revealed a broad and right-skewed nanoparticle size distribution, indicating the coexistence of fine submicron particles and a limited number of larger secondary agglomerates. The diameter of the detected particles ranged from approximately 0.07 µm to 2.49 µm, with a median size of 0.17 µm and a mean value of 0.39 µm, reflecting the influence of occasional larger agglomerates on the average size. The relative frequency data (

Table 1. Particle size distribution.

Size Range (µm)	Relative Frequency (%)
<0.10	18.6
0.10–0.20	30.2
0.20–0.30	9.3
0.30–0.40	4.7
0.40–0.50	4.7
0.50–0.75	9.3
0.75–1.00	4.7
1.00–1.50	9.3
1.50–2.00	2.3
2.00–2.50	7.0

) show that the dominant fraction of agglomerates lies in the sub-0.5 µm range, accounting for approximately 67% of the total population. In particular, agglomerates smaller than 0.2 µm represent about 49% of all detected objects, confirming that ball milling effectively promotes fragmentation and refinement of particle assemblies. Furthermore, three-quarters of the agglomerates remain below the submicron scale.

In contrast, micron-scale secondary agglomerates (>1 µm) constitute a minor population (~19%), with only ~7% exceeding 1.5 µm. These larger features are attributed to partial cold welding and mechanical interlocking during milling. The presence of a small tail extending toward larger sizes is therefore consistent with mechanically induced agglomeration accompanying severe plastic deformation.

Overall, the agglomerate size distribution reflects a heterogeneous but predominantly fine morphology, characteristic of ball-milled ceramic powders, where intensive milling leads to crystallite refinement and submicron agglomeration while preserving a limited fraction of larger, loosely bound secondary clusters [38].

3.2. XRD Analysis of BaZrO₃-Y₂O₃-SrTiO₃ Nanoparticles

X-ray diffraction patterns of the commercial BaZrO₃ powder and the mechanically activated BaZrO₃-Y₂O₃-SrTiO₃ mixture confirmed that the perovskite BaZrO₃ phase remains structurally stable after ball milling at 300 rpm for 30 min (Figure 3). The dominant plane reflections (110), (111), (200), (211), (220) and (310) were observed at approximately 2θ ≈ 30°, 37°, 43°, 54°, 63° and 71° (Cu Kα), corresponding to the characteristic reflections of cubic BaZrO₃, which agrees well with the PDF card number 06-0399 (a = 4.193, V = 73.7) [39]. The preservation of these reflections without the appearance of new dominant peaks indicates that the crystal structure of BaZrO₃ is retained during mechanical activation.

Distinct diffraction peaks corresponding to crystalline cubic Y₂O₃ (2θ ≈ 29°, 33.8°, 48.5°, and 58°) were not detected within the instrumental detection limit, suggesting at least partial incorporation of Y³⁺ [5]. This indicates that a (Ba, Sr) (Zr, Ti, Y)O₃ solid solution was probably formed. Sr²⁺ ions can occupy the places of Ba²⁺ ions in the lattice due to the similar size of the ionic radii of Ba²⁺ (1.61 Å, CN = 12) and Sr²⁺ (1.44 Å, CN = 12) ions. Additionally, Ti⁴⁺ (r = 0.605 Å, CN = 6) and Y³⁺ (r = 0.9 Å, CN = 6) can be incorporated into the BaZrO₃ lattice on B site in the octahedral arrangement instead Zr⁴⁺ (r = 0.72 Å, CN = 6) [40]. The unit cell parameter was not determined from a single plane. Instead, it was calculated using software-assisted refinement of multiple BaZrO₃ reflections, assuming cubic symmetry (space group Pm-3m). The refinement included the following reflections: (110), (111), (200), (211), (220) and (310), yielding unit cell parameter of a = 4.192(2) Å (V = 73.68(3) ų) for the commercial BaZrO₃ powder and a = 4.196(2) Å (V = 73.88(2) ų) for the milled powder. The incorporation of Y causes a slight shift of (110) reflection of samples towards lower values of 2θ (Figure 3b), thus causing the slight increase in the BaZrO₃ unit cell parameter and volume due to the partial incorporation of the Y³⁺ ions, with an ionic radius of 0.9 Å, on the Zr⁴⁺-sites (with a radius of 0.72 Å) [41]. The observed increase in lattice parameter (Δa ≈ 0.004 Å, ~0.1%) is consistent with this expectation. It is noteworthy that Sr²⁺ (1.44 Å vs. 1.61 Å for Ba²⁺) and Ti⁴⁺ (0.605 Å vs. 0.72 Å for Zr⁴⁺) would be expected to induce lattice contraction. The experimentally observed expansion, therefore, suggests that the effect of Y³⁺ substitution is dominant. The absence of detectable Y₂O₃ reflections suggests possible partial incorporation of Y³⁺ into the BaZrO₃ lattice; however, the presence of nanoscale or poorly crystalline secondary phases below the detection limit of XRD cannot be excluded.

The average crystallite size was calculated using the Scherrer equation (Equation (1)) as implemented in the software package:

$$D=K\times \lambda /(\beta \times \cos \mathsf{\Theta })$$

(1)

where

• D represents the average crystallite size;
• K = 0.9 (shape factor, assuming approximately spherical crystallites);
• λ is the wavelength of Cu Kα radiation;
• β is the full width at half maximum (FWHM) of the diffraction peak;
• θ is the Bragg angle.

The calculation was performed for all clearly resolved BaZrO₃ reflections, and the reported crystallite size represents the average value obtained from these reflections. The commercial BaZrO₃ exhibited an average crystallite size of 78(3) nm, while the milled powder showed a decreased average crystallite size of 69(2) nm. It should be emphasized that Scherrer-derived values represent the coherent diffraction domain size and are influenced by both size- and strain-related peak broadening. Previous studies have shown that BaZrO₃ maintains its cubic symmetry even after high-temperature treatments and chemical modification, highlighting its defect tolerance and chemical stability [42].

According to the analysis, the structural changes detected after milling are attributed to defect-induced distortion and limited cation substitution within the perovskite lattice, rather than the formation of a fully equilibrated, single-phase thermodynamic solid solution.

3.3. Filler Dispersion and Microstructure of PEI-BZYS Composites

Following powder characterization, SEM analysis was conducted on surfaces of the PEI-BZYS composites to evaluate filler dispersion, interfacial interaction and microstructural homogeneity after solution casting (Figure 4). At 1 wt% loading (PEI–BZYS1), the microstructure exhibits a uniform spatial distribution of ceramic domains within the polymer matrix. Filler-rich regions are well separated and embedded within a continuous PEI phase, with no evidence of macroscopic phase segregation or percolating particle networks. The dispersed features are predominantly submicron in size, consistent with the initial powder morphology described in Section 3.1. The agglomerate size distribution is right-skewed, with a median diameter of 0.39 µm and only a small fraction of micron-scale agglomerates. The low proportion of particles exceeding 2 µm indicates that severe agglomeration of ceramics in PEI is effectively suppressed at this filler concentration. The observed morphology suggests that the processing route enables good particle wetting and spatial separation within the polymer matrix, while the limited presence of larger agglomerates likely arises from localized particle clustering rather than systematic phase separation [43].

The SEM image analysis of the PEI composite containing 3 wt% particles reveals a broader and more right-skewed particle size distribution compared to the 1 wt% system. While submicron particles remain dominant, the median diameter increases to 0.60 µm, and the upper quartile exceeds 1 µm, indicating a pronounced contribution from micron-scale agglomerates. The relative frequency data showed a rise in the presence of agglomerates larger than 1 µm, which is attributed to enhanced particle-particle interactions and partial agglomeration at higher filler loading. Nevertheless, the absence of large, continuous aggregates or network-like structures suggests that dispersion remains reasonably homogeneous at the microscale, with agglomeration occurring locally rather than through macroscopic phase separation. The absence of large voids or interfacial debonding further suggests adequate matrix–filler adhesion.

3.4. FTIR Analysis

The FTIR spectrum of the PEI-BZYS3 composite (Figure 5) retains all characteristic absorption bands of polyetherimide, confirming that the incorporation of BaZrO₃-Y₂O₃-SrTiO₃ particles does not alter the chemical backbone of the polymer matrix. The strong imide carbonyl bands observed at approximately 1779 cm⁻¹ and 1715 cm⁻¹, corresponding to asymmetric and symmetric C=O stretching, respectively, are preserved in the composite, indicating the structural integrity of the PEI imide rings [44]. Additional PEI-related bands, including aromatic C=C stretching (~1597 cm⁻¹), C–N stretching (~1350 cm⁻¹), as well as ether C–O–C vibrations (~1232 cm⁻¹), remain clearly visible [45]. In the low-wavenumber region, new absorption features appear in the composite spectrum that are absent in neat PEI and correspond to the ceramic reinforcement.

In the spectrum of BZYS3, the band observed at 1450 cm⁻¹ corresponds to Ba–O, while the peak at 527 cm⁻¹ is attributed to Zr–O and mixed metal–oxygen lattice vibrations, which are also present in the composite [40]. These bands confirm the successful incorporation of the ceramic phase into the polymer matrix. Importantly, no additional peaks or significant shifts in the PEI characteristic bands are detected, suggesting that the interaction between PEI and BZYS is predominantly physical rather than chemical in nature.

3.5. DSC Analysis

Differential scanning calorimetry (DSC) (Figure 6) revealed a clear glass-transition step for neat PEI and PEI/BaZrO₃-Y₂O₃-SrTiO₃ (BZYS) composites during the first heating cycle, with no distinct melting/crystallization peak in the investigated temperature range, confirming the predominantly amorphous nature of the matrix. The glass transition temperature (T_g), determined from the midpoint of the inflection, increased from 202.0 °C for neat PEI to 210.4 °C and 212.0 °C for composites containing 1 wt% and 3 wt% BZYS, respectively. The pronounced T_g shift at 1 wt% indicates that even low filler additions significantly restrict PEI segmental motion, likely due to polymer–particle interfacial interactions and the formation of an immobilized interphase that reduces free volume and hinders cooperative chain rearrangements. [46]. Increasing the loading from 1 wt% to 3 wt% produced only a marginal additional T_g rise, suggesting diminishing returns associated with partial saturation of the constrained-polymer fraction and/or reduced effective interfacial area at higher filler contents.

3.6. Microindentation Test

Microhardness testing results (Figure 7) demonstrate a pronounced strengthening effect induced by the incorporation of BaZrO₃-Y₂O₃-SrTiO₃ (BZYS) particles into the PEI matrix. Compared to neat PEI, the composite containing 1 wt% filler exhibited an increase in hardness of approximately 40%, while a further rise to 3 wt% resulted in an enhancement of about 83%, indicating a strong dependence of surface mechanical response on reinforcement loading [47]. This improvement can be directly correlated with the microstructural features revealed by SEM image analysis. At 1 wt% loading, particles are predominantly dispersed in the submicron range, providing effective reinforcement through localized restriction of polymer chain mobility and improved stress transfer at the particle-matrix interface. At 3 wt% loading, the highest microhardness values were achieved, which is consistent with the increased particle population and the higher contribution of submicron-to-micron-scale agglomerates observed in the SEM micrographs. The denser distribution of rigid ceramic domains enhances resistance to indenter penetration by increasing local constraint of the PEI matrix and promoting load sharing between the polymer and the reinforcing phase. Although a broader particle size distribution is observed at higher loading, the fraction of large agglomerates remains limited, indicating that the particle content value does not exceed the point at which severe aggregation or stress concentration would compromise load distribution.

The upward T_g shift and strong suppression of T_g-region enthalpic relaxation on first heating indicate reduced segmental mobility and diminished time-dependent rearrangement in the composites; combined with the intrinsic rigidity of BZYS ceramics, these effects limit indentation-induced chain flow and enhance load transfer, consistent with the measured hardness increases.

One-way ANOVA revealed statistically significant differences in hardness (p < 0.05) among the investigated compositions. Post hoc Tukey analysis demonstrated that the mean hardness values differ significantly between neat PEI and PEI-BZYS1, as well as between neat PEI and PEI-BZYS3 (p < 0.05), confirming the reinforcing effect of BaZrO₃-Y₂O₃-SrTiO₃ addition. The strong linear correlation between filler content and hardness, reflected by a high coefficient of determination (R² = 0.9845), further indicates that reinforcement loading is a dominant factor governing the mechanical response of the composites within the investigated range. The observed microhardness enhancement arises from a synergistic combination of increased filler content, effective dispersion and controlled agglomeration, which together promote efficient stress transfer and restrict localized plastic deformation without inducing detrimental structural heterogeneity.

Although the present investigation was conducted using a PEI matrix, the reinforcing mechanism observed here may be conceptually extended to other polymer systems. In a conventional dental resin matrix based on poly(methyl methacrylate), the successful integration of perovskite SrTiO₃-based particles was already achieved [48]. Similarly, in future research, reinforcement effects of perovskite BaZrO₃-based hybrid oxides could be investigated within different dental formulations.

4. Conclusions

Novel BaZrO₃-Y₂O₃-SrTiO₃ (BZYS) ceramic nanoparticles were successfully incorporated into a polyetherimide matrix, producing structurally stable and mechanically reinforced composites. XRD analysis confirmed preservation of the BaZrO₃ perovskite phase after mechanical activation, with a slight lattice expansion indicative of defect accommodation and ion incorporation, without phase degradation. SEM observations demonstrated predominantly nanoparticles and submicron agglomerates, as well as effective dispersion within the polymer matrix. Thermal analysis revealed a systematic increase in glass transition temperature, confirming restricted segmental mobility and the formation of a constrained polymer interphase. The most significant outcome was the pronounced microhardness enhancement, reaching 83% at 3 wt% reinforcement, with statistically significant differences compared to neat PEI. The strong linear relationship between filler content and hardness highlights efficient stress transfer and effective interfacial reinforcement. These findings establish BaZrO₃-based hybrid oxide systems as viable alternatives to conventional ceramic fillers for high-performance polymer composites. Future work will focus on wear resistance, long-term aging stability and application in conventional resin-based dental composites, to further validate the relevance of this reinforcement strategy.