Evaluation of Surface Roughness, Color Stability, and FTIR Characterization of Different Inorganic-Filled PEEK Materials After Coffee Thermal Cycling

Inorganic fillers are incorporated into polyetheretherketone (PEEK) to enhance mechanical performance, yet their long-term stability in chromogenic media remains insufficiently explored. This study evaluated the color stability, surface roughness, and chemical changes in modified PEEK variants following coffee thermal cycling. In this study, four PEEK materials were tested: unfilled (P), glass fiber-reinforced (GFP), nanoceramic-filled (CR), and titanium dioxide-filled (TP). A total of 40 disk-shaped specimens (10 × 2 mm, n = 10) were prepared and subjected to 30,000 cycles (5–55 °C) in coffee. Color differences (ΔE) after 5000 and 30,000 cycles, roughness changes (ΔRa), and chemical alterations via qualitative Fourier-transform infrared (FTIR) spectroscopy were evaluated. Thermal cycling significantly affected all materials (p < 0.05). TP demonstrated superior stability with the lowest roughness increase (ΔRa = 0.017 ± 0.003 μm) and minimal discoloration (ΔE = 1.66 ± 0.25), remaining within the clinically acceptable threshold (ΔE < 3.3). Conversely, GFP showed the greatest degradation (ΔRa = 0.133 ± 0.008 μm; ΔE = 8.45 ± 0.34). FTIR confirmed no chemical backbone cleavage across groups. Filler type significantly dictates thermochemical resistance in PEEK. While titanium dioxide reinforcement preserves surface smoothness and color stability under in vitro aging, glass fiber reinforcement markedly accelerates staining and roughening. This study highlights the necessity of considering hydrothermal aging resistance and microstructural filler stability when selecting PEEK biomaterials for long-term clinical restorations.