Niobium boride coating on AISI M2 steel by boro-niobizing treatment Ugur Sen a, ⁎ , Suleyman Serdar Pazarlıoglu b , Saduman Sen b a Sakarya University, Engineering Faculty, Department of Metallurgy and Materials Engineering, Esentepe Campus, 54187, Sakarya, Turkey b Sakarya University, Technical Education Faculty, Department of Metal Education, Esentepe Campus, 54187, Sakarya, Turkey Received 2 March 2007; accepted 13 December 2007 Available online 28 December 2007 Abstract In this study, niobium boride coating was applied on pre-boronized AISI M2 steel by the thermo-reactive deposition technique in a powder mixture consisting of ferro-niobium, ammonium chloride and alumina at 950 °C for 1–4 h. The coated samples were characterized by X-ray diffraction, scanning electron microscope and micro-hardness tests. Niobium boride layer formed on the pre-boronized AISI M2 steel was smooth, compact and homogeneous. X-ray studies showed that the phases formed on the steel surfaces are NbB, Nb 3 B 2 , FeB and Fe 2 B. The depth of the niobium boride layer ranged from 0.97 μm to 3.25 μm, depending on treatment time. The higher the treatment time the thicker the niobium boride layer observed. The hardness of the niobium boride layer was 2738 ± 353 HV 0.01 . © 2008 Elsevier B.V. All rights reserved. Keywords: Borides; Thermo-reactive deposition; Niobium boride; AISI M2 steel 1. Introduction The use of hard coatings obtained by the thermo-chemical treatments opens up the possibilities for materials designed, in which the specific properties are located where they are most needed. The substrate material can be designed for strength and toughness, while the coating is responsible for resistance to wear, corrosion and oxidation and thermal loads [1]. Hard coatings with boride, carbide, nitride or carbonitride of transition metals are common to improve the wear resistance of ferrous materials [2,3]. Transition metal boride coatings have attractive properties such as high melting points, high mechanical strength and chemical inertness [4]. Especially, borides, carbides and nitrides of niobium, vanadium, chromium and titanium are famous among the transition metals [5–8]. Niobium borides are recognized among the transition metal borides as potential candidates for high temperature structural applications, due primarily to their excellent properties such as high melting temperature, high strength, high thermal and electrical conductivity, good chemical stability and high wear resistance [9–12]. This type of coating can be produced by several methods along with CVD and PVD, both of which have their respective advantages [3]. Besides CVD and PVD techniques to deposit niobium boride coating, thermo-reactive deposition technique can also be utilized for this coating on pre-boronized steel base alloys. Multi-component boriding is a thermo-chemical treatment involving consecutive diffusion of boron and one or more metallic elements such as niobium, aluminum, chromium, vanadium and titanium on to the component surface. This process is carried out at 850 to 1100 °C and involves two steps: (i) boriding by conventional method and (ii) diffusing metallic elements Available online at www.sciencedirect.com Materials Letters 62 (2008) 2444 – 2446 www.elsevier.com/locate/matlet ⁎ Corresponding author. Tel.: +90 264 295 57 67. E-mail addresses: ugursen@sakarya.edu.tr (U. Sen), sdmnsen@sakarya.edu.tr (S.S. Pazarlıoglu). Fig. 1. SEM image of boro-niobized AISI M2 steel at 900 °C for 2 h. 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.12.042