Effect of Thermocycling on the Microhardness of Pre-Heated and Non-Heated Zirconium Composite Resin

Aim: To evaluate the effect of thermocycling on the surface microhardness of a zirconium-based nanocomposite resin when applied in preheated and nonheated conditions. Materials and Methods: An in vitro , experimental study was conducted using 8 disc-shaped specimens (8 mm × 4 mm) of a zirconium nanohybrid composite resin. Specimens were divided equally into two groups ( n = 4): Group I (nonheated) and Group II (preheated at 55°C for 10 min). Surface microhardness was measured for all samples using a Vickers hardness tester with a 100 g load applied for 20 s. Samples were then subjected to thermocycling (500 cycles, 5°C–55°C, 20 s dwell time) and post-cycling microhardness was recorded. Statistical analysis was performed using paired t tests and two-way analysis of variance to assess the effects of preheating and thermocycling, with significance set at P < 0.05. Results: Preheated specimens demonstrated significantly higher surface microhardness compared to nonheated specimens both before and after thermocycling ( P < 0.001). Thermocycling resulted in a statistically significant reduction in microhardness in both groups ( P < 0.001), although preheated specimens retained higher hardness values postcycling. The interaction between preheating and thermocycling was also significant ( P = 0.004), suggesting that preheating may mitigate the deleterious effects of thermal aging. Conclusion: Preheating zirconium nanocomposite resin enhances its initial surface microhardness and offers greater resistance to thermally induced degradation. These findings support the clinical use of preheated composites, particularly in stress-bearing restorations where long-term material integrity is critical.