Abstract: Key engineering components frequently operate under complex thermo-mechanical coupled conditions during service. The fracture toughness of the surface modification layer is a core indicator determining the service performance and structural integrity of these components. Core transmission components such as heavy-duty gears and wind turbine bearings are prone to surface microcrack initiation and propagation under thermo-mechanical coupled conditions, which can easily lead to sudden fracture failure and cause significant economic losses and safety hazards.Traditional standard testing methods for fracture toughness impose strict requirements on specimen dimensions, which not only require the preparation of large-size standard specimens and high-precision pre-cracks, but also can only obtain the macroscopic average properties of homogeneous materials, making it difficult to achieve micro-destructive, high-resolution quantitative characterization of micron-scale surface modification layers.To address these challenges, this study investigates surface modification layer of 18CrNiMo7-6 alloy steel by systematically conducting constant-load scratch tests at ambient temperatures ranging from 300 K to 673 K. And obtains key response parameters such as tangential force, scratch depth, scratch width and crack morphology of materials at different temperatures. Three fracture mechanics models are introduced to inversely determine the fracture toughness parameters of the material's gradient layer, which are then independently cross-validated using small punch test (SPT) results at corresponding temperatures. The research results show that under different ambient temperature conditions, the fracture toughness of materials at different depths of surface modified layer increases with the increase of depth.The inverse calculation results from scratch tests exhibit consistent trends with the results of small punch tests at 300 K and 473 K, with a maximum relative error of less than 20%. However, there are certain discrepancies in the variation trends at 673 K, which requires further in-depth investigation in subsequent studies.The research findings provide critical parameter support for the fracture resistance assessment and service life prediction of 18CrNiMo7-6 steel components under thermo-mechanical coupling conditions, offering significant reference value for ensuring the structural integrity of critical parts such as heavy-duty gears.