Materials Science, Vol. 43, No. 6, 2007 STUDY OF NANOCRYSTALLINE PLASMA ELECTROLYTIC CARBONITRIDING FOR CP–Ti M. Aliofkhazraei, 1 P. Taheri, 2 A. Sabour Rouhaghdam, 1 and C. Dehghanian 2 Commercially pure titanium is subjected to carbonitriding according to a new procedure called plasma electrolytic carbonitriding. The influence of voltage and time on the thickness of carbo- nitrided layers is investigated. The kinetics of growth of the layers is analyzed according to their thickness and microhardness. The coatings formed on the titanium substrate are multilayer. They are formed by a combination of Ti N, Ti O 2 , and the α-phase in the form of inclusions of high concentrations of nitrogen and carbon in a close-packed hexagonal crystal lattice of titani- um. All layers are very hard with hardnesses varying from 1100 (Vickers–Knoop hardness) for the Ti N layer and 900 for the α-phase to 250 for the nonmodified surface. There exist strong metallic bonds between the layers of the coating. The time dependence of the form t 1/2 is estab- lished for the diffusion of nitrogen and carbon in the outer layers. The results of wear tests re- veal that the mass losses of carbonitrided samples become 4–17 times lower than for the intact samples. Thus, the procedure of plasma electrolytic carbonitriding enables one to get highly ef- ficient stable coatings on titanium. Introduction Several processes are currently used to provide wear protection of metal surfaces in contact and to extend the life of critical components used in bearings, cutting tools, and aerospace industry. Titanium carbide/nitride films (such as TiC and TiN) have high hardness, low friction coefficients, and good corrosion resistance and biocompatibility [1–3]. They can improve the load-bearing capacity and wear resistance of titanium components extensively used in engineering and orthopedic applications. Up to now, various technologies have been deve- loped to deposit or form titanium carbide/nitride coatings, such as physical vapor deposition [4], chemical vapor deposition [5], ion-beam assisted deposition [6], and plasma carbiding/nitriding [2, 7]. Hard coatings that can withstand high stresses without plastic deformation and fracture are more efficient in reducing friction and wear than soft layers. Despite its attractive engineering properties, titanium appears to lack good wear resistance. However, the titanium carbide/nitride films on titanium produced by the indicated deposition methods are thin (generally less than 3 μm). Otherwise, the adhesion of the films is insufficient due to the increased internal stresses in the films and, thereby, delamination of the films is observed under the conditions of wear. Although the plasma carbiding/nitriding treatment of titanium at 700 – 900° C leads to the formation of firmly adhered tita- nium carbide/nitride layers, the process of formation of compound layers whose thickness is greater than 10 mm requires much higher temperature and longer treatment time [2], which results in a worse fatigue behavior and earlier fracture due to the aging of the substrate material [8]. It has been proved that the improvement of effici- ency of the wear resistance and load-bearing capacity strongly depends on the thickness and adhesion of the hard films [1, 9]. 1 Faculty of Engineering, Materials Engineering Department, Tarbiat Modares University, Tehran, Iran. 2 Faculty of Engineering, Materials Engineering Department, Tehran University, Tehran, Iran. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 6, pp. 36–42, November–December, 2007. Original article sub- mitted December 14, 2006. 1068–820X/07/4306–0791 © 2007 Springer Science+Business Media, Inc. 791