Surface and Coatings Technology 180 –181 (2004) 566–569 0257-8972/04/$ - see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.10.120 Structure and properties of thin iron phosphide films on carburised layers Jerzy Nowacki* Institute of Materials Engineering, Technical University of Szczecin, Al. Piastow 19, PL 70-310 Szczecin, Poland Abstract Iron phosphate layers on the carbuised layer structure and properties on the carburised layer have been studied. The layers were generated as a result of a gas phosphorcarburising of Armco iron and 0.2% C steel. Calculations have been done on the Gibbs energy of phosphates Fe P, Fe P and FeP synthesis reactions. In terms of the value of the Gibbs energy, Fe P creation as 3 2 3 a result of the synthesis of iron and atomic phosphorus P is privileged. Fe P phosphides and a solid solution of phosphorus layer 1 3 in iron on the carburised layers generation possibility have been suggested. Thin iron phosphide layers increase hardness and decrease friction wear of carburised layers on iron and steels.  2003 Elsevier B.V. All rights reserved. Keywords: Phosphorcarburising; Iron phosphide layers; Iron phosphides 1. Introduction Profitable influence of phosphorus on iron alloy sur- face layer proprieties was not examined sufficiently and applied in practice w1–3x. That is why phosphorus decreases steel ductility, which did not encourage an attempt to use this element as an alloying steel constit- uent. However, it was observed that presence of phos- phorus in the iron alloy surface layer increases its hardness and friction wear resistance. It is a result of solid solutions hardening, precipitation processes and creation of phosphide layers of considerable hardness on iron on iron alloy surface w4–6x. 2. Gibbs free energy of reaction in Fe–P system Reactions of the synthesis of iron phosphides, Fe P, 3 Fe P, FeP, in temperatures from 700 to 1200 K have 2 been considered (Fig. 1). For the reaction, the values of the Gibbs energy (DG8 ) related to a molecule of P in T function of temperature (T) have been calculated using standard thermodynamical functions of the reactants w7x. A value of the Gibbs energy of the considered reaction shows that in temperatures from 700 to 1200 K synthesis of all considered iron phosphides is possible. Reaction of Fe P synthesis of iron and atomic phosphorus P has 3 the most profitable value of Gibbs energy DG8 (Fig. T *Tel.: q48-91-449-4751; fax: q48-91-449-4356. E-mail address: jnowacki@ps.pl (J. Nowacki). 2). Calculation results of iron phosphide dissociation pressure p according to the considered reaction in p temperatures of 700–1200 K under normal pressure are presented in Fig. 2. Iron phosphide Fe P has the largest 3 thermodynamical stability than the other iron phos- phides. Calculations of P , P and P phosphorus partial 4 2 pressure (p , p , p in function of temperature under P P P ) 4 2 1 normal and selected total pressure have been done. Dissociation of phosphorus molecule P can precede 4 according to Eq. (1). Partial pressure values in a state of thermodynamic equilibrium of the phosphorus P , 4 P and P have been calculated based on Eqs. (2)–(4). 2 Results of the calculations (Table 1) show that values of phosphorus P , P and P partial pressure p , p , p 4 2 P P P 4 2 1 are superior to iron phosphide dissociation pressure p p values. It provides evidence of possibilities of synthesis of iron phosphides in the considered temperature and pressure first of all as a result of a reaction of iron and atomic phosphorus P. As the thermodynamical criteria are considered, iron phosphide Fe P synthesis as a result 3 of reaction of iron with monatomic phosphorus P is privileged. P l2P and P l2P (1) 4 2 2 K s(p yp ) y(p yp ) (2) 1P P 0 P 0 2 4 2 K s(p yp ) y(p yp ) (3) 2P P 0 P 0 1 2 p qp qp sp (4) P P P 0 4 2 1