Electrodeposition of Invar Fe-Ni Alloy/SiC Particle Composite

Abstract

Invar Fe–Ni alloys with Ni contents around 36 mass% have lower coefficients of thermal expansion (CTEs) at temperatures near room temperature than those of pure Fe and Ni.1 Electrodeposition of the Invar Fe–Ni alloys is expected to enable more precise processing with higher throughput and result in alloys with improved mechanical properties compared with those of Invar alloys prepared by conventional processes such as rolling, machining, and etching methods. In addition, Invar electroforming processes can provide freestanding micrometer-sized, 3-dimensional structures with good thermal dimension stabilities, e.g., MEMS (microelectromechanical systems).We used the Invar electroforming process (KEEPNEXTM) to prepare a fine-pitch Invar metal mask for large-size and fine-pitch OLED (organic light-emitting diode) displays.2 Annealing at 600°C decreased the CTE of the electroformed Invar metal mask to 3 ppm/°C, which is four times smaller than that of a conventional Ni mask (13 ppm/°C). However, it is possible to anticipate strength degradation due to grain coarsening by annealing at 600°C. Consequently, the low CTE electroformed Invar annealed at 600°C might be inadequate for use as a micro/nano-mold for MEMS applications.It is known that implantation of particles such as SiC and Al2O3 reinforces electrodeposited metal layers and improves mechanical properties such as hardness and wear resistance by forming a composite structure.3 In this study, we fabricated electrodeposited Invar Fe–Ni alloy/SiC composites to improve hardness of electrodeposited Invar Fe–Ni alloys.Electrodeposition of the Invar Fe–Ni alloy/SiC composite was carried out on a stainless steel substrate using sulfate/chloride bath, modified from Watts-type Ni plating bath, with additives at a current density of 40 mA/cm2. Bath compositions4 were as follows: FeSO4 (0.35 mol/L), NiSO4 (0.95 mol/L), NiCl2 (0.17 mol/L), boric acid (0.49 mol / L), sodium saccharin (0.008 mol/L) and malonic acid (0.05 mol/L). SiC particle (0~20 g/L) with a mean diameter of 0.5 µm was used. The bath temperature was maintained at 50 °C and the bath pH was adjusted to pH 2.3. Pure Fe and Ni sheets were used as anodes. Freestanding electrodeposited Invar Fe–Ni alloy/SiC composite was obtained as specimen by mechanical removal of the deposits from the stainless steel substrate. The thickness of the specimen was approximately 100 µm. The specimen was subjected to heat treatments at 600°C for 1 h under a vacuum of approximately 5 mPa.The amount of codeposited SiC is shown in Figure 1. The volume percent of codeposited SiC in the coatings increased with increasing concentration of particles in the plating solution. For all of the composites, Ni contents of Fe–Ni alloy matrix were between 35 and 37 mass%.Figure 2 shows effects of heat treatment at 600°C on Vickers hardness of the Invar Fe–Ni alloy/SiC composites with different SiC particle contents. Without implantation of SiC particles, a hardness of Fe–Ni alloy was degraded from 230HV to 140HV by annealing at 600°C. The hardness of the composite increased with increasing amount of SiC particles in the composite regardless of the heat treatment. The increasing rate of hardness of annealed composites was larger than that of as-deposited composites. Consequently, there was no drastically degradation of hardness of the composites with higher SiC content annealed at 600°C; these hardness higher than that of pyrometallurgically produced Invar alloy (approximately 120HV).The CTEs of the as-deposited Invar alloy and Invar Fe–Ni alloy/SiC composites were approximately 10 ppm/°C, which are larger than those of pyrometallurgically produced Invar alloys. After heat treatment at 600°C, their CTEs drastically decreased to approximately 1 to 2 ppm/°C; these CTEs are comparable with that of pyrometallurgically produced alloy.Consequently, we successfully produced the Invar Fe–Ni alloy/SiC composites with high hardness and low CTE by Composite plating and post-annealing at 600°C; these materials have potential applications in devices that require materials with sufficient strength and good thermal dimensional stability, e.g., MEMS. Acknowledgement This work was partially supported by Regional Innovation Strategy Support Program "Kyoto Environmental Nanotechnology Cluster" (MEXT) and Industry-Academia Collaborative R&D Programs “Super Cluster Program” (JST).