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. A pre-formed film with thickness of approximately 1 μm on the surface of Zr-2.5Nb alloy by three pre-oxidation treatment processes was prepared, and the relationship between its mechanical properties and microstructure was investigated. The three pre-oxidation treatment processes were conducted at 400 °C for 24 h: (1) in deoxidized superheated steam at 10.3 MPa; (2) in superheated steam with dissolved oxygen of 300 μg/kg; (3) 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 phases exhibiting elongated morphologies. And β-Zr phase continuously distributes at the α-Zr grain boundaries. Microstructures of the films formed under different pre-oxidation conditions exhibit differences. Among them, the pre-formed film prepared under deoxidized conditions contains relatively more microcracks, with shorter and more randomly arranged columnar grains; the pre-formed film prepared under dissolved oxygen conditions is the densest with the fewest defects, while the pre-formed film prepared under low-pressure conditions has the greatest thickness with relatively more pores and cracks. Compared with the original alloy, the pre-formed film increases the nano-hardness of the alloy by 50%–180%, improves the hardness-to-modulus ratio (H/E) by approximately 56%–81%, and reduces the wear rate by 31%–44%. The pre-formed film significantly enhances the surface hardness and wear resistance of the alloy, transforming the wear mechanism from severe abrasive wear to mild adhesive wear. Among three pre-formed films, the pre-formed film prepared under dissolved oxygen conditions is the densest 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.