Towards understanding the coupling interaction between thickness non-uniformity and flatness of wafer-scale electroformed Ni-Co mold: From grain structures to internal stress

Electroforming allows for the precise fabrication of Ni-Co alloy mold but encounters challenges with uneven thickness and persistent internal stress, which impact the mold's flatness and forming accuracy. This study firstly investigates the coupling interaction between the Ni-Co mold's thickness non-uniformity and internal stress based on internal stress simulation and microstructure analysis. The results indicate that increasing Co concentration and adjusting mold thickness can enhance thickness uniformity and stress controllability. A 1000 mu m thick mold with Ni: Co= 8.5:1.5 achieves a minimum thickness non-uniformity coefficient of 16 %, 5 % lower than that of Ni mold. This is because Co incorporation suppresses the Ni deposition at thicker locations, enhancing thickness uniformity. Co-incorporation worsens flatness due to internal stress accumulation and the transformation of columnar crystals to equiaxed crystals. However, increasing the mold thickness in a relatively high Co concentration significantly reduces the internal stress and improves the flatness in both thickness and area directions. At a thickness of 500 mu m and Ni: Co= 8.5:1.5, the thickness non-uniformity dominates internal stress. However, when the thickness is greater than 700 mu m, the area becomes the primary factor. This is due to a thickness-area coupling effect that causes uneven spatial stress distribution. Moreover, the hardness and tribological properties of the mold are improved with appropriate Co content, which is attributed to significant grain refinement, generating more dislocations and grain boundaries. This study fundamentally elucidates the intricate relationship between thickness non-uniformity and internal stress, offering valuable insights for reverseengineering high-performance Ni-Co material systems and low-stress Ni-Co mold electroforming.