Characterization of AISI 4140 borided steels I. Campos-Silva a, *, M. Ortiz-Domı ´nguez a , N. Lo ´ pez-Perrusquia a , A. Meneses-Amador a , R. Escobar-Galindo b , J. Martı ´nez-Trinidad a a Instituto Polite ´cnico Nacional, Grupo Ingenierı´a de Superficies, SEPI-ESIME U.P. Adolfo Lo ´pez Mateos, Zacatenco, Me ´xico D.F., 07738, Mexico b Instituto de Ciencia de Materiales de Madrid (CSIC), E-28049 Cantoblanco, Madrid, Spain 1. Introduction Boriding is a thermochemical surface treatment, in which boron is diffused into, and combines with, the substrate forming a single or double phase metal boride layer at the surface. In industry, boriding is generally applied to ferrous alloys to enhance their surface hardness and wear resistance [1]. In addition to being a selective method, the paste-based treatment reduces manual work compared with the powder-based boriding process [2,3]. Depending on the boron potential that surrounds the material surface, the chemical composition of the substrate, temperature and treatment time, two phases can be identified in the surface layer, i.e. an outer phase, FeB, with a boron content of 16 wt.%, and an inner phase, Fe 2 B, with a boron content approximately of 8 wt.% [4,5]. Previously, it was found that the interfaces of FeB/Fe 2 B and Fe 2 B/substrate, which are present at the surface of different ferrous and nonferrous alloys in the boriding process, have a rough or saw-toothed morphology. However, when the alloying elements increase in concentration on the substrate, the formation and morphology of the growth interface at the surface of the sample tends to be flat [6]. The present study characterizes the surface of AISI 4140 steels hardened by the paste-boriding process. The evaluation of the growth kinetics of Fe 2 B layers at the material surface was done at different boride incubation times to estimate the boron diffusion coefficient at the hard coatings. The GDOES technique showed the concentration profile of alloying elements that diffused in the borided phase. The XRD technique was used to estimate the magnitude and distribution of residual stresses along the boride layer. Finally, the fracture toughness of the borided phase was evaluated under the experimental para- meters of 6 and 8 h of treatment for the different temperatures of the boriding process, considering the length of brittle cracks, parallel and perpendicular to the surface that originate at the tips of an indented impression. 2. Diffusion model The growth kinetics of borided layers has received significant attention during the last 20 years for the automation and optimization of the boriding process [7–10]. In this work, some assumptions were considered for the diffusion model: (i) The boron concentration C Fe 2 B ðxðtÞÞ at the Fe 2 B phase (Fig. 1) depended only on the position x(t). (ii) The growth kinetics was controlled by the boron diffusion in the Fe 2 B layer. (iii) The growth of the boride layer occurred as a consequence of the boron diffusion perpendicular to the specimen surface. Applied Surface Science 256 (2010) 2372–2379 ARTICLE INFO Article history: Received 2 October 2009 Accepted 20 October 2009 Available online 24 October 2009 Keywords: Boriding Growth kinetics Characterization Diffusion model Fracture toughness Hard coatings Residual stresses ABSTRACT The present study characterizes the surface of AISI 4140 steels exposed to the paste-boriding process. The formation of Fe 2 B hard coatings was obtained in the temperature range 1123–1273 K with different exposure times, using a 4 mm thick layer of boron carbide paste over the material surface. First, the growth kinetics of boride layers at the surface of AISI 4140 steels was evaluated. Second, the presence and distribution of alloying elements on the Fe 2 B phase was measured using the Glow Discharge Optical Emission Spectrometry (GDOES) technique. Further, thermal residual stresses produced on the borided phase were evaluated by X-ray diffraction (XRD) analysis. The fracture toughness of the iron boride layer of the AISI 4140 borided steels was estimated using a Vickers microindentation induced-fracture testing at a constant distance of 25 mm from the surface. The force criterion of fracture toughness was determined from the extent of brittle cracks, both parallel and perpendicular to the surface, originating at the tips of an indenter impression. The fracture toughness values obtained by the Palmqvist crack model are expressed in the form K C (p/2) > K C > K C (0) for the different applied loads and experimental parameters of the boriding process. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +52 55 57296000x54768; fax: +52 55 57296000x54589. E-mail address: icampos@ipn.mx (I. Campos-Silva). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.10.070