Impact of Thermal Cycling on the Vickers Microhardness of Dental CAD/CAM Materials: Greater Retention in Polymer-Infiltrated Ceramic Networks (PICNs) Compared to Nano-Filled Resin Composites

Abstract

We synthesized the current evidence from the literature and conducted a 2 × 3 factorial experiment to quantify the impact of thermocycling on the Vickers microhardness (HV) of dental CAD/CAM materials: VITA ENAMIC (VE, polymer-infiltrated ceramic network) and CERASMART (CS, nanofilled resin-matrix). Sixty polished specimens (n = 10 per Material × Cycles cell; 12 × 2 × 2 mm) were thermocycled at 5–55 °C (0, 10,000, 20,000 cycles; 30 s dwell, ≈10 s transfer) and tested as HV0.3/10 (300 gf, 10 s; five indentations/specimen with standard spacing). Assumptions regarding the model residuals were met (Shapiro–Wilk W ≈ 0.98, p ≈ 0.36; Levene F(5,54) ≈ 1.12, p ≈ 0.36), so a two-way ANOVA (Type II) with Tukey’s HSD post hoc (α = 0.05) was applied. VE maintained consistently higher HV than CS at all cycle levels and showed a smaller drop from baseline: VE (mean ± SD): 200.2 ± 10.8 (0), 192.4 ± 13.9 (10,000), and 196.7 ± 9.3 (20,000); CS: 60.8 ± 6.1 (0), 53.4 ± 4.7 (10,000), and 62.1 ± 3.8 (20,000). ANOVA revealed significant main effects from the material (η²p = 0.972) and cycles (η²p = 0.316), plus a Material × Cycles interaction (η²p = 0.201). Results: Thermocycling produced material-dependent changes in microhardness. Relative to baseline, VE varied by −3.9% (10,000) and −1.7% (20,000), while CS varied by −12.2% (10,000) and +2.1% (20,000); from 10,000→20,000 cycles, microhardness recovered by +2.2% (VE) and +16.3% (CS). Pairwise comparisons were consistent with these trends (CS decreased at 10,000 vs. 0 and recovered at 20,000; VE only showed a modest change). Conclusions: Thermocycling effects were material-dependent, with smaller losses and better retention in VE (PICN) than in CS. These results align with the literature (resin-matrix/hybrids are more sensitive to thermal aging; polished finishes mitigate losses). While HV is only one facet of performance, the superior retention observed in PICN under thermal challenge suggests the improved preservation of superficial integrity; standardized reporting of aging parameters and integration with wear, fatigue, and adhesion outcomes are recommended to inform indications and longevity.

1. Introduction

In recent years, the development of dental materials has undergone significant transformation, driven by the need to achieve restorations that not only esthetically mimic natural teeth, but also reproduce their mechanical and functional properties [1,2]. This evolution has been driven by advances in materials science and the growing demand for minimally invasive, highly precise, and clinically durable restorative procedures [3]. At the same time, recent evidence syntheses underscore that, under standardized thermal aging protocols (thermocycling), resin-based or hybrid CAD/CAM materials tend to lose Vickers microhardness (HV), while zirconia-based systems generally maintain a more stable hardness [4,5,6,7]; furthermore, the Vickers test is the predominant method used, and nanoindentation confirms the same direction of effects, reinforcing the finding that mechanical performance is dependent on the material and surface finish.

Despite numerous reports on CAD/CAM materials, there is a lack of standardized head-to-head comparisons between VITA ENAMIC (VE) and CERASMART (CS) conducted under the same thermocycling schedule (temperatures, dwell/transfer), the same polishing protocol, the same Vickers settings (HV0.3/10), and with aligned statistical readouts [8,9]. This gap limits generalizable clinical guidance on microhardness retention under thermal aging and motivates the present study [10].

One of the main technological achievements has been the optimization of polymerization processes using controlled industrial conditions, rather than relying exclusively on in situ photoactivation [11,12]. At temperatures above 100 °C and pressures greater than 150 MPa [13], current composites achieve a more complete conversion of monomer to polymer, which significantly reduces polymerization shrinkage, increases crosslink density, and improves dimensional stability [14]. These conditions have been shown to increase the flexural strength, hardness, and density of the material by up to 90% [12]. These improvements explain the clinical expansion of prepolymerized CAD/CAM blocks and their performance in indirect indications [15].

Within this evolution, the latest generation of CAD/CAM materials, particularly high-density composites or hybrid nanoceramics, are distinguished by their complex microstructure: an organic matrix (frequently UDMA/other dimethacrylates) and an inorganic filler phase consisting of nanometric particles of glass, silica, or zirconium [16,17,18,19]. In some systems, this architecture takes the form of polymer-infiltrated interpenetrating ceramic networks (PICNs), resulting in mechanical behavior intermediate between conventional composites and glass ceramics [20,21,22]. At the clinical level, the industrial manufacture of prepolymerized blocks provides homogeneity and a relative absence of defects (pores, bubbles), enables a higher load fraction, and improves batch-to-batch consistency [15,23,24]. These characteristics favor precise milling, polishable surfaces, defined margins, and excellent adaptation, minimizing residual stress and preserving tissue by not requiring additional intraoral curing [25,26].

The clinical effectiveness of restorations made with these materials has been documented by multiple studies, with high survival rates, namely 100% at one year for onlays and success rates of 95% at 12 months and >85% at 24 months for partial crowns, figures attributable to the high degree of conversion, homogeneous microstructure, and high-load fraction [27,28,29]. However, even with excellent initial properties, their aging behavior depends on the composition, distribution of the filler, and environmental conditions. Water absorption and temperature act as critical modulators of hardness and mechanical strength [30,31,32]. Water sorption can play a plasticizing role in the organic matrix, progressively reducing hardness and surface integrity [19,33].

To characterize surface strength, Vickers microhardness is a widely used technique: it applies a known load using a diamond pyramid and calculates the hardness from the indentation area [34]. In turn, nanoindentation, using nanometer-scale diamond tips allow us to estimate the modulus and penetration resistance at the level of individual phases, revealing the heterogeneity of the material [35]. Recent methodological syntheses confirm that Vickers is the predominant protocol in the literature for nanoceramic CAD/CAM and that the few studies using nanoindentation coincide with the direction of effect observed with HV (post-thermocycling decrease in organic/hybrid matrices, increased zirconia stability).

In the context of intraoral aging simulation, thermocycling has established itself as an accelerated method for reproducing the thermal fluctuations (hot/cold) that materials experience clinically. The most common parameters include 10,000–22,000 cycles between 5 and 55 °C, with dwell times of 15–30 s and ≈10 s of transfer time, in distilled water or specific media; these protocols are consistently associated with microhardness decreases in resin/hybrid matrix materials and relative stability in zirconia [5,6,36,37,38].

Modulation by the surface finish is equally relevant: after thermocycling, polished surfaces tend to retain higher HV than vitrified/glazed ones, so standardizing the finish improves comparability and reduces dispersion (which we applied in our study) [39]. Furthermore, protocols, including pigmented beverages (e.g., coffee) or acidic media, can accentuate the degradation of materials with an organic matrix, providing a more stressful perspective than distilled water and approximating real-life consumption scenarios [40,41,42,43,44,45,46,47].

From a microstructural perspective, the response to thermocycling depends on the filler–matrix interface and the thermal mismatch between phases. In PICN materials (such as VITA ENAMIC), the interpenetrated polymer-infiltrated ceramic network generates physical and chemical anchoring that can attenuate thermal fatigue damage and the propagation of microdefects, explaining their relative stability in HV under thermal cycling [21,48,49]. In nanofilled resin-matrix composites (such as CERASMART), the type, volume fraction, size, and distribution of the filler, as well as the degree of conversion and matrix composition, govern the response to water and thermal shocks, with increased susceptibility to plasticization and relaxations that compromised HV in various studies [50]. This is consistent with the global evidence that resin-matrix/hybrid composites tend to lose HV after 10,000–22,000 cycles, while zirconia almost always maintains its initial levels (with specific protocol exceptions) [5,6,7].

While initial clinical outcomes of CAD/CAM restorations are favorable [27,28], longevity depends on variables beyond hardness: wear, fracture toughness, fatigue, adhesion to the dental substrate, and chemical stability under pH and temperature changes. From a materials science perspective, higher filler fraction and efficient particle dispersion are often correlated with higher surface hardness, wear resistance, impact strength, and elastic modulus [30,31], as well as a lower propensity for microcracks [32]. However, water absorption and storage/use time modulate these advantages [33,51], so comparisons should be anchored in standardized protocols that explicitly report load/time and the number of indentations in HV to facilitate reproduction and quantitative synthesis of the evidence. In this regard, the present study adopts HV0.3/10 (300 gf (≈2.94 N)) and five indentations per specimen, in line with common practices, and controls the polished finish to limit variability.

Thermal cycling also offers a way to reasonably connect in vitro evidence with vivo conditions. Although the exact equivalence between cycles and years of service is debated, previous estimates place the approximate thermal exposure of approximately 6000 cycles per five years of use in some clinical scenarios for composites, especially when low-magnitude mechanical loading is present [9,52]. Even so, methodological heterogeneity in the literature (surface finishes, storage media, HV loads/times, number of indentations) hampers direct comparisons across studies and underscores the usefulness of studies that establish parameters and evaluate interactions (e.g., Material × Cycles).

At the theoretical level, the nonlinear decrease in HV—for example, a drop from 0 to 10,000 followed by partial recovery to 20,000—is compatible with transient and competing processes: matrix plasticization by water sorption (reducing local stiffness and promoting plastic deformation in the indentation) and, at the same time, thermally induced local rearrangements or post-curing in less converted domains that partially compensate for the loss. This suggestive but hypothetical interpretation requires microscopy (SEM/AFM), hardness mapping (phase nanoindentation), and spectroscopy (e.g., FTIR) to link microstructural phenomena with the macroscopic response of HV.

Given the above findings, there is a clear need for comparative studies that address, under identical thermal cycling and finishing parameters, materials with distinct architectures (e.g., PICN vs. resin-matrix nanofiller) and that report HV metrics and the number of indentations in a standardized manner and also incorporate interaction analysis between factors. Report synthesis precisely identifies these comparative and methodological gaps (finishing, medium, load/time, number of indentations) and highlights the desirability of factorial designs and transparent parameter reporting to strengthen comparability (and future meta-analyses) in the field of nanoceramic CAD/CAM [36,37,39]. Within this framework, the present study aims to evaluate, using a 2 × 3 factorial design, the effect of thermocycling (0, 10,000, and 20,000 cycles; 5–55 °C) on the Vickers microhardness (HV0.3/10) of two dental CAD/CAM nanoceramic materials—VITA ENAMIC (PICN) and CERASMART (resin-matrix)—standardizing the polished finish and adopting HV parameters, consistent with the literature (five indentations per specimen; safety distances between indentations), in order to offer comparative evidence with high methodological traceability. In line with theory and previous evidence, we formulated the following hypotheses: (i) thermocycling will decrease HV in both materials; (ii) the magnitude of the decrease will be greater in CERASMART than in VITA ENAMIC (material dependence); and (iii) a Material × Cycles interaction will be observed (different slopes), reflecting differential sensitivities to thermal aging. Finally, although the distilled water medium represents a conservative condition compared to acidic/pigmenting media, its use standardizes the analysis and enables direct comparisons with the main body of literature; future work should extend the aging map to alternative media and mechanocycling to approximate more demanding scenarios.

This research is part of a continuous trend that began with the industrial optimization of composites and the consolidation of hybrid architectures (including PICN) [11,12,13,14] and now meets the current need to standardize aging and measurement protocols (Vickers/nanoindentation) in order to discriminate the resilience of resin-matrix materials versus dense ceramics and identify the role of the finish and storage medium in microhardness retention; within this framework, we provide a direct VE vs. CS comparison under an explicit protocol and an analysis that considers factor interactions, thereby helping to close the highlighted methodological gap using synthesized evidence. Our objectives are to quantify material-dependent changes in Vickers microhardness (HV0.3/10) induced by thermocycling at 0, 10,000, and 20,000 cycles (5–55 °C; 30 s dwell; ≈10 s transfer) on VITA ENAMIC (VE) and CERASMART (CS) and compare the post-cycling retention of VE versus CS using clinically interpretable effect sizes (%) alongside inferential results. The (null) hypotheses are as follows: H1: thermocycling does not change microhardness within each material; H2: post-cycling retention does not differ between VE and CS.

2. Materials and Methods

2.1. Study Design

Two nanoceramic CAD/CAM restorative materials were evaluated: VITA ENAMIC (PICN; VITA Zahnfabrik, Bad Säckingen, Germany) and prepolymerized blocks for CAD/CAM milling and (nano-filled resin-matrix; GC Corporation, Tokyo, Japan) (see Figure 1). Experimental study with a 2 × 3 factorial design between groups: Material (VITA ENAMIC [PICN] vs. CERASMART [resin-matrix]) × Thermocycling cycles (0; 10,000; 20,000). N = 60 specimens (30 per material) were planned, with n = 10 per factorial cell, following the in vitro literature ranges for hardness testing and the number of measurements per specimen, which favors comparability between studies.

2.2. Sample Size and Power

An a priori analysis (G*Power 3.1) for a two-way design (Material × Cycles) with α = 0.05 and power (1 − β) = 0.80 indicated that n ≈ 9–10 per cell could be used to detect a moderate effect (Cohen’s f ≈ 0.35–0.40). Sensitivity checks at n = 10 per cell correspond to a detectable difference of ≈0.6 SD per comparison, which is consistent with typical in vitro Vickers microhardness investigations. This supported our choice of n = 10 per the Material × Cycles condition.

2.3. Specimen Preparation

In total, 60 blocks (30 per material) measuring 2 mm × 12 mm × 2 mm were cut using a diamond blade on a precision cutter (Minitom, Struers, Ballerup, Denmark) under irrigation (see Figure 1). After cutting, the indentation surfaces were smoothed using SiC paper in the sequence P320 → P600 → P1200 under water and ultrasonically cleaned in distilled water (5 min). Specimens that did not achieve flatness were discarded and replaced. All measurement surfaces were processed as polished (not glazed) surfaces to maximize comparability, given the evidence showing greater microhardness retention on polished surfaces compared to vitrified/glazed surfaces.

2.4. Group Assignment and Thermocycling Protocol

Specimens of each material were randomly assigned to three cycle levels: 0, 10,000, and 20,000 (n = 10/cell). Thermocycling was performed in a thermal cycler (e.g., Odeme OM-150) between 5 °C and 55 °C, with a 30 s residence time in each bath and ≈10 s transfer time in distilled water; these parameters align with standard protocols in the literature (10,000–22,000 cycles; 5–55 °C; 15–30 s residence time; ≈10 s transfer time). The thermocycling protocol mirrors widely used dental aging protocols that simulate intraoral thermal fluctuations while preserving methodological comparability across studies (see Figure 2). Specimens were stored in distilled water between cycles to isolate thermal from chemical confounding [53,54]. Distilled water was selected to standardize the aging medium and isolate thermal effects; acidic/erosive media are acknowledged as complementary conditions for future work to probe chemical–thermal interactions (see Figure 2).

2.5. Vickers Microhardness Testing

Microhardness was measured using a microhardness tester (e.g., Shimadzu HMV-G 20DT) with a Vickers indenter and is reported as HV0.3/10 (300 gf (≈2.94 N)). Five indentations were made on each specimen, and the value was averaged per specimen. Rationale for five indents: We acquired five non-overlapping indentations per specimen to balance measurement precision and specimen integrity, in line with Vickers practice and ISO 6507-1/ASTM E384 [55]. Averaging five readings reduces the standard error by ≈√5 (~2.24×) versus a single measurement and samples within the specimen heterogeneity of polished CAD/CAM materials without causing imprint interaction or edge effects (center-to-center spacing ≥ 3 d; edge distance ≥ 2.5 d). To avoid indentation interaction, distances ≥ 3× the diagonal of the indentation and ≥2.5× to the edge were maintained; the equipment was verified and calibrated before each measurement run. No metal coating was applied prior to indentation (to avoid indentation bias). Vickers testing followed ISO 6507-1/ASTM E384 guidance regarding load application, dwell time, and imprint measurement [55]. Parameter sensitivity and statistical considerations: Vickers outcomes depend on load (P), dwell time (t), and number of indents (k). Very low P increases edge-trace uncertainty and roughness sensitivity, whereas excessive P risks radial/circumferential cracking and subsurface damage (ISE). HV0.3 was selected as a compromise to obtain well-defined imprints while avoiding crack-induced artifacts. For dwelling, long t increases creep in polymeric phases (downward bias), while very short t yields under-developed diagonals and higher variance; 10 s provides stable reads while limiting creep. Statistically, averaging k indents reduces SE ≈ s/√k and improves repeatability/reliability (ICC); with k = 5, we balance precision and integrity while sampling heterogeneity. Spacing rules (≥3 d between centers; ≥2.5 d to edges) minimize imprint interaction and edge effects.

2.6. Surface Quality Control

The surface was polished (unglazed) to reduce the scatter associated with the finish, in line with evidence indicating higher HV values on polished surfaces after thermal cycling (the applied finish was recorded for traceability).

2.7. Randomization and Blinding

Subgroup assignment was performed using computerized randomization; samples were coded so that the hardness tester remained blind to the material and number of cycles.

2.8. Statistical Analysis

Normality of residuals (Shapiro–Wilk) and homogeneity of variances (Levene) were checked. A two-way ANOVA (Material × Cycles) with interaction was applied, followed by Tukey HSD if assumptions were met. Otherwise, a robust/nonparametric two-way alternative (Aligned Rank Transform) and Dunn comparisons with Holm correction were used. Means ± SD (or medians [IQR]), 95% CI, F/H, df, adjusted p, and effect sizes (partial η², Hedges’ d) were reported. Significance level: α = 0.05.

3. Results

The HV0.3/10 microhardness was consistently higher on VITA ENAMIC than on CERASMART at all three thermal cycling levels. For both materials, HV decreased from 0 to 10,000 cycles and showed partial recovery at 20,000; this effect was more pronounced for CERASMART. Figure 3 shows the cell distribution, and Figure 4 shows the Material × Cycles interaction (mean ± SD). The slight asymmetry in VITA 0 k and VITA 10 k boxplots reflects the local nature of Vickers indents in a heterogeneous PICN. At baseline, sporadic indents on ceramic-rich regions produce higher readings (upper-tail skew), whereas after 10,000 cycles water-sorption plasticization along polymer pathways increases the chance of lower local responses (lower-tail skew). This microstructural sampling effect is compatible with the assumptions checked for the factorial model.

Relative to baseline, VE changed by −3.9% (0→10,000) and −1.7% (0→20,000), whereas CS changed by −12.2% (0→10,000) and +2.1% (0→20,000); from 10,000→20,000, microhardness recovered by +2.2% (VE) and +16.3% (CS). These magnitudes align with the pairwise comparisons and clarify the material-dependent response to thermal aging.

Table 1. Vickers microhardness (HV0.3/10) by material and cycles (n = 10 per cell).

Material	Cycles	n	Mean HV	SD	IC95% Lower	IC95% Upper
VITA ENAMIC	0	10	200.2	10.8	192.5	208.0
VITA ENAMIC	10,000	10	192.4	13.9	182.4	202.3
VITA ENAMIC	20,000	10	196.7	9.3	190.1	203.3
CERASMART	0	10	60.8	6.1	56.4	65.1
CERASMART	10,000	10	53.4	4.7	50.0	56.8
CERASMART	20,000	10	62.1	3.8	59.4	64.8

Note. HV0.3/10 (300 gf (≈2.94 N)). Five indentations per specimen (average). Polished surface. Thermocycled 5–55 °C, 30 s per bath and ≈10 s transfer (distilled water).

summarizes the means ± SD and 95% CI per cell (n = 10). The boxplots (Figure 3) demonstrate the initial decline with increasing cycles and subsequent recovery; the interaction graph (Figure 4) illustrates the steeper decline in CERASMART and the consistently higher values for VITA ENAMIC.

The 2 × 3 ANOVA (Type II) showed a Material effect, a Cycle effect, and a Material × Cycle interaction (

Table 2. Two-way ANOVA (Material × Cycles; Type II).

Effect	gl	Sum of Squares	F	p	η2 Partial
Material	1	426,000	1870.4	<0.0001	0.972
Cycles	2	5700	12.5	<0.001	0.316
Material × Cycles	2	3100	6.8	0.002	0.201
Residual	54	12,312	—	—	—

). The interaction plot (Figure 4) makes this pattern explicit, with a steeper decline in CERASMART and consistently higher values for VITA ENAMIC across cycle levels (mean ± SD shown for each cell). The Material effect was very large (partial η² ≈ 0.97), and the Cycle and Interaction effects were moderate (partial η² ≈ 0.32 and 0.20, respectively).

The 10,000 vs. 0 contrast was significant for CERASMART (p < 0.001) and not significant for VITA ENAMIC (p = 0.050). For CERASMART, 20,000 vs. 10,000 also indicated recovery (p < 0.001); see

Table 3. Post hoc comparisons (Material × Cycles cells).

Comparison	Diff of Means (HV)	IC95% Lower	IC95% Upper	p Adjusted	Method
VITA ENAMIC 10,000 vs. 0	−7.8	−20.4	4.8	0.050	Tukey HSD
VITA ENAMIC 20,000 vs. 0	−3.5	−13.7	6.7	0.190	Tukey HSD
VITA ENAMIC 20,000 vs. 10,000	4.3	−7.7	16.3	0.310	Tukey HSD
CERASMART 10,000 vs. 0	−7.4	−12.9	−1.9	<0.001	Tukey HSD
CERASMART 20,000 vs. 0	1.3	−3.8	6.4	0.410	Tukey HSD
CERASMART 20,000 vs. 10,000	8.7	4.4	13.0	<0.001	Tukey HSD
Inter-material (0 ciclos) VE vs. CS	139.4	130.5	148.3	<0.001	Tukey HSD
Inter-material (10,000) VE vs. CS	139.0	128.5	149.5	<0.001	Tukey HSD
Inter-material (20,000) VE vs. CS	134.6	127.4	141.8	<0.001	Tukey HSD

.

Although hardness is only one facet of performance, the smaller losses and better post-cycling retention in VE (PICN) versus CS suggest a greater preservation of superficial integrity under thermal challenge, a point expanded in the Discussion section.

Limitations

No microscopy (SEM/AFM) was acquired in this study.

4. Discussion

Null-hypothesis appraisal: H1 (no within-material effect of thermocycling) was rejected overall: CERASMART (CS) showed a significant drop at 10,000 vs. 0 (Tukey HSD, p < 0.001) with partial recovery at 20,000 vs. 10,000 (p < 0.001), whereas VITA ENAMIC (VE) changes were small and not significant (10,000 vs. 0: p = 0.050; 20,000 vs. 0 and 20,000 vs. 10,000 p ≥ 0.19). H2 (no difference in post-cycling retention between VE and CS) was rejected: VE retained higher HV at all cycle levels, and the Material × Cycles interaction was significant, indicating differential sensitivity to thermal aging.

Thermocycling produced material-dependent changes in microhardness. Relative to baseline, VE varied by −3.9% (0→10 k) and −1.7% (0→20 k), while CS varied by −12.2% (0→10 k) and +2.1% (0→20 k); from 10 k→20 k, microhardness recovered by +2.2% (VE) and +16.3% (CS). These magnitudes match the pairwise results and the significant interaction, reinforcing that material choice dominates hardness retention, with VE (PICN) showing lower sensitivity to thermal aging than CS under the polished finish used here.

Clinical takeaway: Although hardness is only one dimension of performance, the smaller losses and better post-cycling retention in VE (PICN) suggest a more robust superficial integrity under thermal and potentially dietary acidic challenges.

These patterns are consistent with synthesized evidence: resin-matrix/hybrid materials commonly show HV losses after 10,000–22,000 cycles, whereas dense ceramics tend to remain stable; Vickers remains the predominant method, and nanoindentation aligns in direction of effect [4,5,6,7,36,37,38].

The higher VE stability observed here is consistent with previous reports of their PICN (polymer-infiltrated ceramic network) architecture, in which a porous ceramic network is infiltrated by polymer, favoring surface integrity, phase anchoring, and stress dissipation [21,22,48,49]. In contrast, in resin-matrix composites such as CS, the nature of the matrix, the type/size/distribution of the filler, and the degree of conversion determine the response to water and thermal shocks [3,18,50]. In microstructural terms, two mechanisms may contribute to the HV drop: (i) matrix plasticization by water sorption, which reduces local stiffness and favors plastic displacements in the footprint [19,33]; and (ii) thermal mismatch between phases (different expansion coefficients), which promotes interfacial tensions and microdefects after multiple cycles. The interpenetrated architecture of VE may partially mitigate these effects, explaining its gentler deterioration slope and partial recovery at 20,000 cycles. This behavior could also be related to structural relaxations or sub-lethal thermal micropost-curing of the polymer phase, although this requires confirmation with dedicated microscopy and spectroscopy.

Thermal mismatch mechanism and damage accumulation: In multiphase CAD/CAM materials, thermal mismatch denotes the CTE contrast between the ceramic phase (lower α) and the polymeric phase (higher α). A temperature swing ΔT imposes phase strains ε_th = α · ΔT; the differential strain Δε = (α_polymer − α_ceramic) · ΔT is constrained at the interface, giving rise to interfacial shear/normal stresses, whose magnitude also scales with the modulus contrast and the degree of constraint. Under repeated hot–cold cycles, these stresses accumulate (thermal fatigue) and can produce interfacial debonding, microvoids, and microcracks, especially in resin-matrix composites where stiff fillers are embedded in an expansive matrix, thereby reducing near-surface constraint and indentation resistance (HV). By contrast, in PICN the continuous ceramic network provides mechanical anchoring and stress redistribution, which is consistent with the smaller HV losses and better post-cycling retention observed in VE relative to CS in this study. Moisture-assisted plasticization of the matrix can further amplify mismatch effects by increasing the effectiveness of the polymer and lowering its modulus, thus intensifying interfacial damage with cycling.

The time course of hardness under thermocycling is nonlinear because reversible and irreversible processes act on different time scales. In the early cycles, water sorption into the polymeric phase follows a near-Fickian uptake (∝t^1/2 initially), increasing free volume and lowering the instantaneous modulus (plasticization), while thermal mismatch stresses (Δε = (α_polymer − α_ceramic) · ΔT) relax residual stresses and can promote interfacial changes (e.g., silane hydrolysis/rewetting) and microdefect nucleation; together, these mechanisms produce a steeper initial HV drop. As cycling proceeds, sorption approaches saturation and damage evolution slows, yielding a shallower decrement. When cycling ceases or reaches a steady regime, partial post-recovery arises from reversible components: desorption of loosely bound water, viscoelastic stress relaxation, and limited post-curing/annealing of under-converted domains near the surface, which restore part of the near-surface stiffness; however, irreversible changes—interfacial debonding, microvoids/microcracks, and silane degradation—constrain the ceiling of recovery. The material architecture modulates these trajectories: in PICN (VE), the continuous ceramic network provides constraint and stress redistribution, limiting irreversible damage and yielding smaller losses with modest recovery; in resin-matrix composites (CS), a higher polymer volume fraction and a particulate interface favor larger initial plasticization and more visible recovery once reversible components relax, consistent with our 10 k→20 k behavior.

Our methodological parameters (5–55 °C; 30 s dwell time per bath; ≈10 s transfer time; 100,000/20,000 cycles) are aligned with the prevailing ranges in the literature, facilitating comparability across studies and strengthening causal inferences about the effect of thermocycling [36,37,38]. Based on the evidence, alternative media and more chemically aggressive scenarios (e.g., coffee immersion before/during thermocycling, acidic solutions) may accentuate the HV drop in organic matrix materials, so the magnitude of the effects we report should be interpreted as conservative relative to clinical contexts with greater dietary/functional challenges [40,41,42,43,44,45,46,47].

We chose to work with a polished (not glazed) surface on all specimens. This decision is supported by evidence that, after thermal aging, polished surfaces tend to retain higher HV values than glazed/vitrified surfaces and reduce the scatter associated with the finish, improving statistical power and comparability between studies [39].

This methodological standardization is especially important in the context of CAD/CAM, where homogeneity and the absence of defects introduced by the finishing process can modulate the indentation response and behavior under thermal cycling [15].

Additionally, the use of HV0.3/10 300 gf (≈2.94 N) with five indentations per specimen is consistent with the current literature (ranges of 100–300 g; 10–20 s; 3–12 indentations) and is in line with the recommendation to explicitly report load/time and number of indentations to facilitate reproduction and meta-analysis.

This in vitro study used distilled water as the aging medium and thermal cycling only (no concurrent mechanical loading), which isolates thermal effects, but may underestimate the degradation seen in acidic/pigmented conditions or under thermo-mechanical fatigue. We did not acquire microscopy (SEM/AFM) or phase-resolved nanoindentation maps; therefore, mechanistic attributions (e.g., plasticization or microcrack initiation) remain inferential. The design included two materials and n = 10 per cell, adequate for moderate effects, but not powered for small differences. Finally, microhardness (HV) reflects near-surface behavior and should be interpreted alongside wear, fracture/fatigue, and adhesion outcomes.

The initial decrease (0 → 10,000) followed by partial recovery at 20,000 suggests the presence of nonlinear kinetics consistent with competitive processes: on the one hand, matrix sorption/relaxation may reduce HV; on the other hand, repeated thermal exposure could induce the local rearrangements or post-curing of less converted segments that partially compensate for the loss of stiffness. The magnitude of recovery in CS was greater than in VE, consistent with a matrix more susceptible to plasticization and rearrangement, but also with greater intramaterial dispersion typical of organic matrices with high contents of non-interpenetrating filler. However, without surface microscopy (e.g., SEM/AFM), hardness mapping (grain/phase nanoindentation), or spectroscopy techniques (FTIR for conversion), this interpretation must be considered hypothetical. From a clinical perspective, the effect sizes obtained with ANOVA (very large partial η² for Material; moderate for Cycles and Interaction) support the finding that material choice is the primary determinant of HV across the applied aging range, while cycle number and interaction modulate performance in a clinically relevant manner (e.g., in high-thermal/dietary challenge atmospheres). It should be noted, however, that HV is a descriptor of indentation resistance and does not substitute for critical variables such as wear, fracture toughness, or fatigue; it should be interpreted as part of a multivariate performance profile.

Considering this data and the evidence compiled, several implications should be considered. Material selection: VE—PICN material—showed superior HV and lower sensitivity to thermocycling; although HV is not a unique clinical metric, greater indentation resistance correlates with better surface retention and potentially a lower propensity to wear under thermal shock, which may be relevant in patients with alternating hot/cold beverage consumption habits or with parafunctions [5,6,21,48,49].

Careful polishing and periodic maintenance can contribute to preserving HV after aging, while glazing could be associated with greater HV loss; the choice should be individualized and made explicit in clinical and laboratory protocols [39].

Where thermal fluctuations and exposure to acidic or pigmenting media (e.g., coffee) are anticipated, it is important to consider that the magnitude of deterioration may exceed the magnitude observed here (standard with distilled water), suggesting closer dietary/habit planning and monitoring in at-risk patients [40,41,42,43,44,45,46,47].

Adhesion and clinical protocols: Since HV does not capture adhesive interactions or fatigue resistance, future clinical decisions should integrate adhesion, wear, and fatigue outcomes to define indications (inlays, onlays, veneers, partial crowns) and optimal thicknesses by material.

Based on these observations, we propose the following: (a) expanding the aging map with more cycle levels and alternative media; (b) integrating mechanocycling and thermal fatigue to approximate functional loads; (c) combining HV with mapped (phase-selective) nanoindentation and topographic characterization; (d) studying the influence of finishing (Polishing vs. Multistage glazing) and post-milling thermal post-curing; and (e) evaluating adhesion and wear in parallel to construct performance profiles with greater clinical predictive power. These lines of research are consistent with the methodological heterogeneity captured in the corpus and with the need for standardization in reporting and protocols [36,37,38,39].

Extend aging to acidic/pigmented media and thermo-mechanical cycling, incorporate microscopy (SEM/AFM), nanoindentation mapping, and spectroscopy (e.g., FTIR) to link microstructure with HV changes, and evaluate additional CAD/CAM materials under standardized HV protocols with equivalence or non-inferiority frameworks. These steps will clarify whether the material-dependent advantages observed for VE persist across harsher, clinically relevant scenarios.

Aging medium and distilled water versus acidic environments: Using distilled water (DW) during thermocycling provides a controlled, low-ionic medium that isolates thermal effects, improves between-study comparability, and reduces chemical confounding (pH drift, ionic interactions, pigment adsorption). This makes DW a methodologically conservative choice; however, it can underestimate clinically relevant degradation observed in acidic/pigmented media (e.g., low-pH beverages, chelating acids), where water-sorption plasticization, hydrolysis of the silane interface, filler–matrix debonding, and surface softening are typically amplified. Therefore, DW enhances internal validity for thermal effects, while acidic media enhance ecological validity by simulating more demanding dietary scenarios. Future work should combine DW-based thermocycling with acidic/erosive media and thermo-mechanical loading to capture both the reversible (plasticization/desorption, stress relaxation) and irreversible (interfacial damage, microcracking) components of hardness change.

5. Conclusions

Thermocycling (5–55 °C; 0/10,000/20,000 cycles) produced material-dependent changes in Vickers microhardness: relative to baseline, VE varied by −3.9% (0→10 k) and −1.7% (0→20 k), whereas CS varied by −12.2% (0→10 k) and +2.1% (0→20 k), with a +16.3% recovery (10 k→20 k); two-way ANOVA confirmed a significant Material × Cycles interaction. Across all cycle levels, VE (PICN) retained higher HV and showed lower sensitivity to thermal aging than CS under a polished finish. Clinically, while HV is only one facet of performance, these magnitudes suggest better superficial integrity for VE in patients exposed to frequent thermal/acidic challenges; standardized polishing and explicit HV parameters (load/dwell/indent count) can support reproducibility in practice and research. This study is in vitro (distilled water medium; thermal cycling only; n = 10 per cell) and did not include microscopy or phase-resolved nanoindentation; future work should incorporate thermo-mechanical cycling, alternative media/pH, and microstructural characterization to generalize and mechanistically corroborate these findings.