Abstract:Micro-motion wear is one of the primary factors limiting the service life of pressure tubes used in heavy-water reactors. To enhance the operational reliability of Zr-2.5Nb pressure tubes for heavy-water reactors, this study prepared a pre-formed film approximately 1 μm thick on the surface of Zr-2.5Nb alloy using three different processes and investigated the relationship between its mechanical properties and microstructure. The three pre-oxidation treatment processes were carried out for 24 hours at 400 °C: one in deoxidized superheated steam at 10.3 MPa; one in superheated steam with 300 μg/kg of dissolved oxygen; and one in low-pressure steam at 2 MPa.The results indicate that the Zr-2.5Nb alloy consists of α-Zr and β-Zr phases, with both α-Zr and β-Zr exhibiting elongated morphologies, and β-Zr continuously distributed at the α-Zr grain boundaries.The microstructures of the films formed under different pre-oxidation conditions exhibit differences: the pre-formed film under deoxidized conditions contains relatively more microcracks, with shorter and more randomly arranged columnar grains; the pre-formed film under dissolved oxygen conditions is the most dense with the fewest defects, while the pre-formed film under low-pressure conditions has the greatest thickness but relatively more pores and cracks.Compared with the original alloy, the pre-formed film increased the nano-hardness of the alloy by 50%–180%, improved the hardness-to-modulus ratio (H/E) by approximately 56%–81%, and reduced the wear rate by 31%–44%. The pre-formed film significantly enhanced the surface hardness and wear resistance of the alloy, transforming the wear mechanism from severe abrasive wear to mild adhesive wear.Among these, the pre-formed film produced under dissolved oxygen conditions is the most dense and exhibits the most pronounced hardness enhancement. This is because the strengthening effect of the pre-formed film is closely related to its microstructure. A dense and intact oxide film not only has higher hardness but also adheres more firmly to the metal substrate, making it less prone to peeling or cracking under localized stress.