**Background:** Dental caries remain a major public health problem in children, with over 514 million children affected by primary teeth caries according to the 2022 WHO Global Oral Health Status Report. Stainless steel crowns (SSCs) are widely used for restoring primary molars due to their durability and high clinical success rates, but they have significant drawbacks including poor aesthetics and potential adverse effects from nickel content, such as local inflammation, allergic reactions, and cytotoxic or genotoxic effects. Alternative materials like pre-veneered stainless steel crowns, polymer crowns, and zirconia crowns each have limitations including nickel allergy risk, poor mechanical/wear resistance, or high cost. Polyetheretherketone (PEEK) composites, particularly those reinforced with TiO₂ nanoparticles, have emerged as a potential alternative due to their color, elasticity, non-cytotoxicity, and antibacterial properties. This study aimed to compare the tribological behavior of TiO₂-reinforced PEEK composite and 316L stainless steel under simulated oral conditions to assess PEEK's potential as an alternative material for pediatric crowns.
**Methods:** Samples of 316L stainless steel (composition: Fe-62.7%, Cr-16.2%, Ni-9.6%, C-2.4%, Si-0.7%, Mo-2.1%, Mn-1.6%) and PEEK composite reinforced with 20 wt% TiO₂ nanoparticles (composition: C-65.6%, Ti-14%) were cut into circular pieces (8 mm diameter × 0.2 mm thickness) from commercial blocks. The 0.2 mm thickness matched commercial SS pediatric crowns. All specimens were polished identically using SiC papers (800-4000 grit) and cleaned ultrasonically in isopropyl alcohol. Samples were cemented onto 2 mm thick zirconia substrates using Bifix Hybrid Abutment composite to simulate clinical crown conditions. Hardness was assessed using a nanoindenter with a Berkovich diamond indenter (9 indentations per specimen, 50 mN for SS, 100 mN for PEEK, 10 s dwell time). Color measurements were performed using a Vita Easyshade digital device. Reciprocating sliding wear tests were conducted using a Plint TE67/R ball-on-plate tribometer with 10 mm diameter alumina balls as counterface. Tests were performed under lubricated conditions at 37±3°C using Fusayama-Meyer's artificial saliva (pH corrected to 6.2-7.6). A normal load of 30 N was applied with a reciprocating frequency of 1 Hz and 4 mm stroke length. Each test lasted 1 hour (total sliding distance 28.8 m), and each condition was performed three times. Wear volume was calculated using profilometry measurements and specific wear rate using the equation k = ΔV/(W×L). SEM/EDS characterization was performed after gold sputter-coating.
**Key Results:** Color measurements showed the 20 wt% TiO₂ PEEK composite presented a shade in the lighter tooth shade zone of the Vita Classical guide with adequate whitening level, while conventional PEEK was significantly darker. The COF evolution differed markedly between materials: SS showed an initial low value (~0.075) up to 3 m sliding distance, then increased to ~0.35 between 3-5 m, stabilizing at 0.32±0.03 in steady-state. This initial low COF was attributed to oxide film formation that subsequently tore. PEEK composite achieved a well-defined stationary phase almost instantly, with COF remaining practically unchanged throughout testing at 0.094±0.004 — approximately 3.4 times lower than SS. Hardness values showed SS had the highest hardness, but no linear correlation between hardness and COF was established. Wear track analysis revealed the mean depth of SS wear track was 5.9±0.3 μm compared to 2.8±1.1 μm for PEEK (2.1 times greater). Mean wear track width was similar (SS: 610±79.4 μm; PEEK: 583.3±57.7 μm). PEEK showed significantly lower wear volume (0.0078±0.0125 mm³) compared to SS (0.0125±0.0029 mm³). The specific wear rate for PEEK was 9.07×10⁻⁶ mm³N⁻¹m⁻¹, approximately 1.6 times lower than SS (1.45×10⁻⁵ mm³N⁻¹m⁻¹). SEM analysis of SS worn surfaces revealed formation of a passive oxide layer with "puzzlelike" morphology and adhered tribo-layers containing artificial saliva components (Na, Ca, P, K) combined with wear debris. These tribo-layers became thick and unstable, leading to delamination and material loss — they did not provide a protective layer. PEEK surfaces showed parallel grooves from abrasive wear against the ceramic counterface with polished smooth areas, consistent with low COF values. EDS analysis of PEEK surfaces showed no artificial saliva components, indicating pure abrasion without tribological layer formation. Alumina ball analysis after SS testing showed adhered tribo-layers rich in saliva constituents and SS components, while after PEEK testing the ball exhibited a smooth surface with only slight abrasion and minimal material transfer.
**Clinical Implications:** The study demonstrates that TiO₂-reinforced PEEK composite has significantly superior tribological properties compared to 316L stainless steel under simulated oral conditions, including lower friction, lower wear volume, and higher wear resistance. Combined with its whitish color, high elasticity/adaptability, non-cytotoxicity, electrical non-conductivity, thermal insulation, and light weight, PEEK composite represents a promising alternative to stainless steel for pediatric crowns. However, the study has limitations: alumina balls were used as counterface rather than natural enamel or dentin, and factors such as varying saliva composition, pH levels, temperature, cyclic loads, and patient diets were not fully explored. Further testing accounting for these factors would complement these findings.