342 Research article Received: 3 April 2011 Revised: 5 June 2011 Accepted: 10 June 2011 Published online in Wiley Online Library: 7 July 2011 (wileyonlinelibrary.com) DOI 10.1002/sia.3808 Surface modiﬁcation and wear test of carbon steel by plasma electrolytic nitrocarburizing Ahmad Reza Rastkar * and Babak Shokri Plasma electrolytic nitrocarburizing (PEN/C) was applied to the surface of carbon steel under the boiling condition of saturated urea electrolyte. In addition to the general effect of the bath temperature, different applied voltages and processing times were also considered in this new process. Optical and scanning electron microscopy, X-ray diffraction, microhardness and pin-on-disc wear tests were used to characterize the PEN/C-treated surfaces. A mixture of θ -(Fe 3 C) and ε-(Fe 2–3 N) was found in the compound layers. At certain conditions, dense surface layers with minimum porosity were observed at the top of the samples. The boiling condition resulted in special character of the compound layers on the surface. The layers consisted of some irregularities grown inward the samples andaffected the characteristics of the surface layers. The microhardness of the PEN/C-treated layers increased up to 1280 HV0.1, which was 3 to 4 times higher than that for untreated material and higher than that obtained by other investigators (750 HV0.1). PEN/C decreased the wear loss of carbon steel signiﬁcantly due to the change of the adhesive wear of untreated material to the abrasive mode of treated surfaces. The major advantage of this technique was a higher growth rate of the nitrocarburized layers and a more signiﬁcant improvement in the tribological performance of the treated samples if compared to similarly oriented surface treatments. Copyright c  2011 John Wiley & Sons, Ltd. Keywords: plasma electrolytic nitrocarburizing; Fe 3 C; adhesive wear; abrasive wear Introduction Steel materials have been widely used in industry as a structural material for their high strength, high toughness, good machin- ability and low cost in engine parts, water/oil supply valves, pipes and stamping die/mould materials. These components are usually worn on the surfaces and need surface modiﬁcation for pro- tection. Since two previous decades surface treatments of gas and plasma nitriding, nitrocarburizing or carbonitriding were em- ployed to improve the mechanical properties and wear resistance of carbon and alloyed steels. [1–3] Plasma electrolytic treatment is also being used as an emerging surface engineering tech- nique for hardening and/or commercial application of hard nitride and carbide layers [4–7] and low friction and corrosion-resistant oxide layers on carbon steels. [8] Plasma electrolytic treatments are a combination of conventional electrolysis and atmospheric plasma processes that have been an important ﬁeld of research for recent years. Therefore, this method can be developed as an efﬁcient technique for surface thermochemical treatments and improving the wear performance of carbon steels. This method has less environmental impact than that of conventional pro- cesses and is carried out at very lower bath temperatures. [5] However, the surface temperature of the treated material is high enough to receive the interstitial elements of nitrogen and carbon. Most investigations in plasma electrolytic processes have been carried out at temperatures below the boiling point of electrolyte. In this paper, we have studied the nitrocarburizing of carbon steel at the boiling temperature of urea solution. It was found that hard and wear-resistant compound layers can be produced on the surface without the necessity of cooling system at short times and maximum solubility of urea can be achieved in this condition. The results from this work are useful for producing less expensive coatings at short times. Experimental methods and material The electrolytic processes were carried out at the boiling point of the urea solution which was different from that of conventional plasma electrolytic processes. [9] The concentrations of the solutions were kept constant during the operation by adding the required amount of solution. The maximum soluble amount of urea (10 g/l) was used in the electrolytes or in fact the solution was saturated. It was observed that less amperages or in fact less energy was consumed during the processes [9] because the cooling system was cut off. The cooling system in plasma electrolytic processes reduces the heat applied to the samples and therefore wastes the energy. The treatments were carried out at different voltages of 110 to 220 V for 6 to 9 min. At voltages less than 110 V the thickness of the compound layers was not considerable and at voltages more than 220 V very porous surfaces were created. The thickness of layers after 5 to 6 min was considerable and after 9 to 10 min the layers did not grow signiﬁcantly. After ﬁnishing the treatments the process completed by shutting down the power supplied and the samples were extracted in the air after a short time. Therefore, there has been some kind of hardening by air cooling or incomplete quenching. The substrate sample was AISI 1045 (CK45) carbon steel in the form of disks with the dimensions of 20 mm in diameter and 2 mm thickness. The composition of the major elements in the material was 0.47% C; 0.24% Si and 0.65% Mn and the rest Fe. The substrates were ground to 1200 grit using SiC emery papers and cleaned by acetone. ∗ Correspondence to: Ahmad Reza Rastkar, Plasma Engineering Group, Laser & Plasma Research Institute, Shahid Beheshti University, G. C., Evin, Tehran 1983963113, Iran. E-mail: a rastkar@sbu.ac.ir Plasma Engineering Group, Laser & Plasma Research Institute, Shahid Beheshti University, G. C., Evin, Tehran 1983963113, Iran Surf. Interface Anal. 2012, 44, 342–351 Copyright c  2011 John Wiley & Sons, Ltd.