Long-Term Thermo-Oil Conditioning of PA66-GF25: Non-Monotonic Tensile Response Under Combined Thermal and Lubricant Exposure

Polyamide 66 reinforced with 25 wt.% short glass fibres (PA66-GF25) is widely used in engineering applications requiring long-term operation under combined thermal and lubricated conditions, including polymer bearing cage applications. However, the long-term evolution of its mechanical behaviour under thermo-oil exposure remains insufficiently documented, particularly over extended exposure periods relevant for service-oriented durability assessment. This study investigates the tensile behaviour of PA66-GF25 after immersion in an industrial bearing lubricant for up to 12 months at conditioning temperatures of −30 °C, 24 °C, and 60 °C. Tensile tests were primarily performed at 24 °C to evaluate the influence of conditioning history, while selected specimens were additionally tested at 60 °C to assess the effect of testing temperature. The results indicate that conditioning temperature strongly influences the evolution of tensile behaviour. Specimens thermo-oil-conditioned at 24 °C and −30 °C exhibited a non-monotonic evolution of tensile strength, characterised by an initial reduction after 4 months followed by recovery and an apparent tendency towards stabilisation at longer exposure durations. In contrast, specimens thermo-oil-conditioned at 60 °C exhibited a continuous increase in tensile strength throughout the investigated period. Although elevated testing temperatures reduced the absolute tensile strength, the relative trends associated with thermo-oil conditioning remained observable. The findings indicate that long-term thermo-oil exposure of PA66-GF25 does not necessarily lead to continuous degradation of tensile performance under the investigated conditions. Instead, the material exhibits a transient response at intermediate exposure durations followed by recovery or an apparent tendency towards stabilisation of tensile performance. These results provide a long-term experimental tensile dataset for PA66-GF25 under combined thermal and lubricant exposure conditions and highlight the importance of extended conditioning when evaluating material performance for lubricated engineering applications. The findings show that intermediate exposure data may not be sufficient for assessing long-term tensile behaviour. However, the mechanisms responsible for the observed tensile strength evolution require further verification using complementary physicochemical and microstructural analyses.