Reactive DC Magnetron Sputtered NbN Films on Mild Steel with Electroless Nickel Interlayer

Aims: To explore the benefits of NbN coating on mild steel (MS) with electroless nickel (EN) interlayer.
Study Design: To deposit NbN coatings at various N2/Ar flow-ratios and substrate biasing; optimize the parameters and then deposit NbN on MS with EN interlayer.
Place and Duration of Study: Material Processing Division, Bhabha Atomic Research Centre, Trombay, Mumbai, between June 2010 and July 2012.
Methodology: NbN was deposited by reactive DC magnetron sputtering varying N2/Ar flow ratio between 5-70% and substrate biasing from zero to -150V. Coatings were studied for thickness, hardness, phase analysis and adhesion. Effect of EN interlayer was studied for improvement in hardness, adhesion and corrosion. Corrosion was studied using potentiodynamic polarization technique in 1N H2SO4 solution at room temperature.
Results: X-ray diffraction showed hexagonal β Nb2N, cubic δ NbN and hexagonal δ’NbN phases with increasing N2. EN layer was found to be amorphous. When NbN top-coat was deposited by sputtering, EN structure changed to crystalline Ni3P and Ni. Surface hardness increased from 1084HK25 for NbN/MS to 1436, 1564 and 1612HK25 with EN interlayer in 2, 4 and 10µm thickness respectively. In the scratch tests, critical load for cohesive failure varied between 6-8N, 8-15N and 8-18N and critical load for adhesive failure varied between 9-12N, 12-25N and 10-28N for NbN coating on MS, SS and MS/EN respectively. Coefficient of friction (µ) in the scratch adhesion tests for NbN/MS was 0.28 at 30N, 0.35 at 40N and 0.45 at 60N load; for NbN/EN/MS, µ decreased to 0.22, 0.28 and 0.37 at 30, 40 and 60N loads respectively, demonstrating the improvement. Corrosion studies showed that Icorr decreased from 150.2µA/cm2 for NbN/MS to 2.3, 0.37 and 0.35µA/cm2 with 2, 4 and 10µm thick EN interlayer respectively.
Conclusion: There was considerable improvement in the hardness, corrosion resistance and adhesion of NbN/MS, when incorporated with EN interlayer.