Uğur Şen

Kinetics of Boride Layers Formed on the Surface of AISI 4140 Steel

The present study reports on boride layer growth kinetics of borided AISI 4140 steel. Steels were boronized in molten borax, boric acid and ferro-silicon bath at 1123°K 1173°K and 1223°K for 2, 4, 6 and 8 hours. Boride layer thickness ranged from 38.4 to 225μm. Layer growth kinetics were analysed by measuring the extent of penetration of FeB and Fe2B sublayers as a function of boronizing time and temperature in the range of 1123-1223°K. The depth of the tips of the most deeply penetrated FeB and Fe2B needles are taken as measures for diffusion in the fast directions. The kinetics of the reaction, K= Ko exp (-Q/RT) have also been determined by varying the boriding temperature and time. The results showed that K increase with boronizing temperature. Activation energy (Q) for present study was determined as 215 kj.mol -1 . The diffusion coefficient (K) ranged from 3x10 -9 cm 2 s -1 to 2x10 -8 cm 2 s -1 . Also temperature-dependent constant (Ko) at temperatures 1123°K, 1173°K and 1223°K was 179.4 cm 2 s -1 . Introduction Boronizing is a thermo-chemical surface treatment that involves diffusion of boron into a base metal at a high temperature. The boronizing is a diffusion process; similar to carburizing or nitriding, and conventional heat treating equipment may be used [1]. Industrial boronizing is mainly applied to steel and ferrous alloys, for process control in automated installations, knowledge of kinetic parameters of the boronizing process is essential [2]. It is well-known that in the borided layers the borides of FeB and Fe2B types, which are the most important constituents of these layers, present a characteristic texture [3]. In the boronizing process, boron atoms are introduced into the metal lattice at the surface of work piece through thermal energy to form borides with atoms of the substrate. When boron atoms diffuse into the surface zone of a ferrous work piece, a compound layer is formed that consist of either one iron boride (FeB) or two iron borides (Fe2B) [4-8]. However, the large difference between the thermal expansion coefficients of the duplex phases of the coated layer, i.e. the outer phase of FeB and the inner phase of Fe2B, makes the FeB layer very likely to be exfoliated when the work is cooled from the treatment temperature. As a consequence, efforts made to impart a single layer of Fe2B are of primary importance, although Fe2B is some what lower in hardness [9,10]. Experimental The experiments were conducted include boronizing, optical microscopy and scanning electron microscopy (SEM) studies of fractography, x-ray diffraction analysis (XRD) for borides and hardness tests. Specimen Preparation and Boronizing AISI 4140 steel test specimens were used for the present study. The test specimens were sized a rectangular shape with a nominal dimensions of 10x10x8mm and then polished progressively by hand finishing with 1000 grit emery paper to obtain the same surface condition. Ground specimens were embedded in the boronizing molten bath consisting of borax, boric acid and ferro-silicon. The boronizing experiments were conducted in a graphite crucible in an electrical resistant furnace at Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 565-568 doi:10.4028/www.scientific.net/KEM.264-268.565

The Influence of Composition and Calcination Temperature on the Phases in (Ba 1- x Sr x Zr)O 3 Perovskites

Several ceramic materials with a perovskite structure often exhibit interesting electronic properties and, therefore, are often called “electroceramics”. Electroceramics with high dielectric constants are used in capacitors and superconductors. The effect on calcination temperatures (1000°C–1400°C) and the changing of ions in A and B sites on the phase formation and microstructure of barium strontium zirconate (Ba1-xSrxZr)O3, BSZ, x = 0.2 and 0.4 ceramics were investigated. The BSZ powders were prepared using the solid state reaction method. The phase purity, crystal structure and microstructure of samples were examined using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD). We found out that the percentage phase purity of perovskite increases with an increase in the calcination temperatures as expected and it was found that BSZ powders adopts a perovskite structure with a cubic () unit cell as well. The purity of perovskite powders was obtained above 1300°C and the purity phase of ceramics was detected in all samples.

Dielectric Properties of Sr-Doped BaZrO 3 Ceramics

Perovskite compounds have received keen interest from academic research due to their unique properties such as ferroelectric, pyroelectric, piezoelectric, and dielectric properties. Nano sized oxides of strontium doped barium zirconate of general formula Ba1-xSrxZrO3 (x= 0.2, 0.4, 0.6, 0.8) have been prepared by a solid-state procedure. The surface morphology, microstructure, and phase analysis, of the sintered ceramics were characterized by field emission scanning electron microscopy (FE-SEM). Scanning electron microscopic studies showed the formation of cubic and hexagonal nanoparticles with an average grain size of 1.6μm±0.47 and 0.82±0.35μm. Dielectric studies were carried out as a function of frequency to explore the electrical properties of Sr-doped BaZrO3.