Enhancement of hardness and corrosion resistance of low alloy high carbon steel by plasma immersion ion implantation K. Ram Mohan Rao 1 , S. Mukherjee 2 , S. K. Roy 1 and I. Manna* 1 Plasma immersion ion implantation (PIII) of nitrogen on low alloy high carbon steel was carried out at low energy negatively biased (–1 kV) pulses with 3?8 kHz repetition rate for 3, 4 and 5 h of cumulative duration at a high dose of 10 23 nitrogen ions per m 2 . Following PIII, the plasma ion implanted samples were subjected to microhardness measurements, potentiodynamic polarisa- tion tests in 1 wt-%NaCl solution and post-corrosion microstructural studies. PIII at 500uC for 3 h seems to yield the optimum combination of increase in both hardness and resistance to corrosion. Keywords: Plasma immersion ion implantation, Nitrogen, Hardness, Corrosion resistance, Steel Introduction Plasma immersion ion implantation (PIII) is a non-line- of-sight implantation process of ions from inductively coupled radio frequency plasma of gaseous species at low negatively biased pulsed potential (21 to 210 kV) and at ambient or an elevated temperature. 1–3 This process is particularly useful for developing a shallow depth nitride layer on metallic components of any dimension or geometry under finished or semifinished conditions. In the past, a number of studies have reported significant improvement in hardness and resistance to wear and corrosion following PIII of Fe, Al, Ti based substrates. 4–8 Recently, the authors have achieved more than three times improvement in hard- ness in AISI 52100 ball bearing steel compared to that under annealed condition by PIII of nitrogen at 300– 500uC for 1–5 h due to both solid solution hardening and precipitation of the nitrides. 9 Although precipita- tion of nitride enhances hardness, formation of a two phase microstructure usually deteriorates corrosion resistance. However, the authors have recently been successful in defining an optimum condition that offers an increase in both hardness and corrosion resistance after PIII of nitrogen in AISI 304L stainless steel. 10 Although hardness and wear resistance are of primary concern for materials used in bearing applications, corrosion is not totally unlikely under conditions with unfavourable combination of humidity, heat and lubricant. Usually, surface engineering treatments that increase hardness and wear resistance by formation of hard carbides or nitrides deteriorate corrosion resistance due to formation of local/galvanic cells between them and the matrix. Strengthening in PIII is both due to solid solution and precipitation hardening. However, an attempt to enhance both mechanical and electrochemi- cal property by PIII has not been undertaken. It is known that nitriding of low alloy steels improves corrosion resistance due to the formation of compound layer. 11 Earlier Misra and Kustas 12 pointed out that AISI 52100 steel is good for high load bearing capability except that this steel possessed poor pitting corrosion resistance in chloride medium, and hence would be ideal only in non-corrosive environment. The same work showed that beam line (line of sight) implantation of Cr significantly enhanced corrosion resistance. Similarly, Wang et al. 3 suggested improvement of corrosion resis- tance of inexpensive AISI 52100 steel would make it equivalent to more expensive AISI 440 stainless steel. Lubricants frequently contain aggressive ions like Cl 2 that cause gradual pitting and other forms of corrosion and/or tribo corrosion of ball bearing components. To extend the life of bearings or machine parts in corrosive environment along with scope of regular wear or tribological degradation, the present study explored the possibility of enhancing resistance to corrosion without deteriorating hardness and wear resistance of AISI 52100 steel. Here, AISI 52100 serves as a model low alloy high carbon steel not necessarily meant only for bearing applications (AISI 52100 steel finds wide ranging applications as a low alloy high carbon steel as sleeves, guides, spindles, bushings, collets, hydraulic parts, etc. Saline solution is a standard medium for corrosion studies. Furthermore, PIII, unlike gas or plasma nitriding is a plasma assisted implantation process (non-line of sight) that has not been explored with AISI 52100 steel for enhancement of both hardness and corrosion resistance. It may be pointed out that PIII offers better control over the process parameters (dose, temperature, pres- sure, implantation energy) and resultant microstructure than those offered by conventional gas nitriding or 1 Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, WB 721302, India 2 FCIPT, Institute for Plasma Research, B15-17/P, GIDC Electronics Estate, Gandhinagar 382044, Gujarat, India *Corresponding author, email imanna@metal.iitkgp.ernet.in ß 2008 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 29 June 2006; accepted 16 April 2007 4 Surface Engineering 2008 VOL 24 NO 1 DOI 10.1179/174329407X265875