Effect of the anisotropic growth on the fracture toughness measurements obtained in the Fe 2 B layer E. Hernández-Sanchez a , G. Rodriguez-Castro b , A. Meneses-Amador b , D. Bravo-Bárcenas b , I. Arzate-Vazquez c , H. Martínez-Gutiérrez c , M. Romero-Romo a , I. Campos-Silva b, ⁎ a Universidad Autónoma Metropolitana-Azcapotzalco, Materials Department, San Pablo 180, México D.F. 02200, Mexico b Instituto Politécnico Nacional, Grupo Ingeniería de Superficies, SEPI-ESIME, U.P. Adolfo López Mateos, Zacatenco, México D.F. 07738, Mexico c Instituto Politécnico Nacional, Centro de Nanociencias y Micro-Nano Tecnologías, U.P. Adolfo López Mateos, Zacatenco, México D.F. 07738, Mexico abstract article info Available online 11 October 2013 Keywords: Boriding Anisotropy growth Fracture toughness Cracking models Real hardness Brittleness In this study, the fracture toughness of the Fe 2 B layer was estimated using the Berkovich indentation technique. Boron diffusion at the surface of the AISI 1018 steel was conducted using a powder-pack boriding process at a temperature of 1273 K with 6 h of exposure. The mechanical characterization of the Fe 2 B layer was performed on three different distances from the surface using a range of indentation loads (10 to 500 mN) for each distance. The behavior of the hardness as a function of the indentation load showed the presence of the indentation size effect (ISE) at the different distances from the surface, in which the apparent or real hardness was estimated according to the concept of the Nix and Gao model. Finally, two indentation-cracking models were used to estimate the fracture toughness of the Fe 2 B layer on the different distances from the surface; the results were ranged from 1.5 to 4.2 MPa m 1/2 , which denoted the brittleness and the influence of the anisotropic nature of the boride layer. © 2013 Published by Elsevier B.V. 1. Introduction Boriding is a thermochemical surface treatment, whereby boron is diffused into, and combines with, the substrate material forming a single or double phase boride layer at the surface. The boriding of ferrous materials results in the formation of the Fe 2 B layer or double- layer (FeB/Fe 2 B) with definite composition [1–6]. The most relevant element of the method is the production of very hard layers that can exceed 20 GPa; this allows for better wear strength and abrasion than other thermochemical processes, such as carburising and nitriding [4–6]. In borided low-carbon steels, the morphology of the boride layers at the surface of the material displays a saw-toothed shape that reflects the anisotropic nature of the layers. Martini et al. [7], found that the crystallographic order of the boride layers is considerably different for regions located at different depths, in which both FeB and Fe 2 B display a (002) preferred orientation. This orientation increases through the boride layer (especially for the Fe 2 B) causing that the mechanical properties depend on the crystallographic order along the depth of the boride layers. Moreover, the use of boride layers is limited by their high sensibility to cracking under mechanical stresses. Therefore, the evaluation of the fracture toughness (K c ) on the boride layers is of great importance in the application of the borided steel [8]. The radial-median and Palmqvist crack geometry models have been used to determine the fracture toughness in boride coatings by means of Vickers indentation technique. The results showed that the fracture toughness of the boride layers depends on the type of boride formed on the surface of the material and the chemical composition of the steel [9–12]; the fracture resistance of the Fe 2 B layer is nearly four-fold greater than that of the FeB [12]. In recent years, the fracture toughness on the needles of the Fe 2 B layer formed at the surface of the AISI 1018 steel was evaluated using Berkovich indentation technique [13]. The fracture resistance obtained on the tips of the boride layer was established with applied loads in the range of 300–500 mN, in which the values remained constant as a function of the boriding temperatures (2.4 to 2.7 MPa ffiffiffiffi m p ). Likewise, Rodríguez-Castro et al. [14] evaluated the fracture resistance of the FeB and Fe 2 B layers formed on the surface of an AISI D2 borided steel. The fracture toughness of the boride layers was evaluated using the length of the cracks that originated from the corners of the Berkovich indentations site after the application of loads ranging from 300 to 500 mN; the estimated values fell within the range of 1.48–3.02 MPa ffiffiffiffi m p and 2.01–4.65 MPa ffiffiffiffi m p for the FeB and Fe 2 B layers, respectively. In this study, the effect of the anisotropy growth on the fracture toughness (K c ) of the Fe 2 B layer was estimated. First, Berkovich indentations were performed on three different distances (25, 50 and 75 μm) from the surface of the AISI 1018 borided steel. The range of applied loads was 10 to 500 mN for each indentation distance in order to evaluate the behavior of the hardness and Young´s modulus along the depth of the layer. The presence of the indentation size effect was Surface & Coatings Technology 237 (2013) 292–298 ⁎ Corresponding author. Tel.: +52 55 57296000x54768; fax: +52 55 57296000x54589. E-mail address: icampos@ipn.mx (I. Campos-Silva). 0257-8972/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.surfcoat.2013.09.064 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat