Composite y-Ni+y’/Ni coatings on iron transformed from electrodeposited ni and sediment co-deposited Ni+Al films
Marek Konieczny
Quarterly No. 2, 2011 pages 168-173
DOI:
keywords: y-Ni/y’ composite coating, sediment co-deposition, annealing, oxidation
abstract Ni+Al/Ni composite coatings on an iron substrate were successfully developed via two-step technology: conventional electrodeposition (CED) and sediment co-deposition (SCD). The Al content produced in the composite coatings was about 25 vol. %. The coating morphology and composition were characterised using light optical microscopy, scanning electron microscopy (SEM - JEOL JMS 5400) and electron probe microanalysis (EDS - ISIS 300 Oxford Instruments). No cracking, porosity or other defects were observed in the Ni+Al/Ni coating microstructure. The hardness of the Ni+Al coating was about 240 HV, whereas the hardness of the pure Ni interlayer coating was only 220 HV. The effect of heat treatment under pressure on coating phase formation was studied. The Ni+Al/Ni coatings were converted into y-Ni+y’/Ni coatings by vacuum annealing at 600oC for 2 h and subsequently at 900oC for 3 h under uniaxial pressure of 1 MPa. Full-density composite coatings were obtained. The nickel interlayer was successfully employed to block the mutual diffusion between the iron substrate and aluminium and therefore hard and brittle Fe-Al intermetallics were not formed. The hardness of the reaction-formed y-Ni+y’ layer was about 280 HV. The hardness near the surface after oxidation tests decreased to about 250 HV, which was attributed to the depletion of the y’ phase. A hardness peak of about 320 HV appeared about 15 um within the outer layer, where the alumina continuos layer was revealed. The results of the microindentation hardness measurements were in accordance with the results of the SEM and EDS analysis.