KINETICS OF IRON SILICIDE DEPOSITED ON AISI D2 STEEL BY PACK METHOD. Ugur SEN, Ozkan OZDEMIR, Senol YILMAZ, Saduman ŞEN

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1 KINETICS OF IRON SILICIDE DEPOSITED ON AISI D2 STEEL BY PACK METHOD Ugur SEN, Ozkan OZDEMIR, Senol YILMAZ, Saduman ŞEN Sakarya University, Sakarya, Turkey, Abstract: In this study, the growth kinetics of iron silicide layer deposited on AISI D2 steel samples by thermo-reactive diffusion (TRD) techniques in a solid medium was reported. Iron silicide was performed in a powder mixture consisting of silicon, ammonium chloride and alumina at 1323 K, 1373 K and 1423 K for 2 6 h. Iron silicide layer thickness of the AISI D2 steel ranged from to μm depending on the treatment time and temperature. Layer growth kinetics was analysed by measuring the depth of iron silicide layer as a function of time and temperature. The kinetics equation of the reaction has also been determined with Arrhenius' equation K=Ko exp (-Q/RT). The result showed that the diffusion coefficient (K) of the process increased with treatment temperature. Activation energy (Q) for the process was calculated as 540 kj/mol. The diffusion coefficients (K) are changing between 7.76x10-7 and mm2/s depending on the process temperature. Keywords: Kinetic, Iron silicide, Siliconizing, AISI D2 steel, Pack method 1. INTRODUCTION In industrial countries, the cost of corrosion is estimated to be a significant part in the range 2% to 4% of the Gross Domestic Product (GDP). Since steels are materials extensively used in a wide range of applications, preventing their corrosion is an important economic issue, and many strategies have been proposed to achieve this goal. For example, addition of silicon (Si) into stainless steel generally improves resistance to corrosion and erosion, but embritlement the material. Silicon coatings overcome this limitation because they improve the corrosion resistance of steel without changing the mechanical properties of the bulk material [1,2]. In addition that the addition of Si to steels could improve their high temperature oxidation resistance. The beneficial effects of Si on the high temperature oxidation resistance are two folds. First, with sufficient concentration, it can form a continuous vitreous silica layer between the metal and scale interface. This silica layer has a low concentration of defects, which becomes a good diffusion barrier and provides excellent oxidation resistance. Secondly, the preferentially formed silica acts as the nucleation site for the subsequent formation of chromia which renders oxidation protection [3-7]. Available processes for producing silicon diffusion coatings on the surfaces of metals include molten metal/salt baths, pack cementation, slurry/sinter, ion implantation, and/or chemical vapor deposition (CVD) [8]. Advantages of pack method include uniform coating of the substrate, minimum cleaning of parts after treatment, in situ treatment of fabricated parts, ease of surface cleaning, and capability for thermal treatment after deposition. It is known that siliconizing performed at high temperature is a diffusion-controlled process. It is very important to establish the process parameters that affect the siliconizing kinetics in order to select process parameters to attain the desired thickness of silicide layer and properties. In this study, the silicon diffusion on silicide layer is evaluated taking into account experimental data for the growth kinetics of the silicide layer during the pack siliconizing process on AISI D2 steel. The growth kinetics of the layer is analyzed by measuring the thickness of the layer as a function of the treatment time within the temperature range K.

2 2. EXPERIMENTAL STUDY The test material used in this study was AISI D2 steel essentially containing, 1.53%C, 12.2%Cr, 0.13% Mn, 0.13% Ni, 0.17%Si, 0.95%Mo, 0.34%V, 0.014%S and 0.020%P. AISI D2 steel was sectioned as cylindrical coupons that have a dimension of 20 mm in diameter and 5mm in height. Surface roughness of AISI D2 steel was 0.21 μm. Siliconizing of the steel was achieved using the pack method. In this technique, a silicon source (pure silicon), an activator (ammonium chloride) and filler material (Al 2 O 3 ) were thoroughly mixed to form the siliconizing powder mixture. After siliconizing for 2-6 h at K, siliconized samples were removed from the furnace and cooled in air. Siliconized samples were sectioned from one side and prepared metallographically up to 1-grid emery paper and then polished using 0.3 μm alumina pastes. Polished samples were etched by 4% Nital before tests. The thickness of silicide layers and their morphology were examined using optical microscopy and SEM on the cross sections of the siliconized samples. The phases formed in the coating layer was identified by X-ray diffraction (XRD) measurements that were performed using CuKα (λ= Å) radiation. The hardness of the siliconized steel was also measured using a digital microhardness tester fitted with a Vickers indenter under the loads of 10 g. 3. RESULTS AND DISCUSSION 3.1. Properties of silicide layer Both optical and SEM cross section examinations of the siliconized AISI D2 steel revealed that silicide formed on the surface of the substrate has a smooth morphology as shown in Fig. 1. On the cross sections of siliconized steel surfaces, there were three distinct regions at higher magnifications. These are: (i) a top surface layer which includes porous structure, (ii) compact coating layer and, (iii) matrix, which is not affected by silicon. XRD analysis showed that the silicide layer formed on the siliconized steel consisted of Fe 2 Si, FeSi and FeSi 2 phases (Fig. 2). EDS analysis showed that silicon atoms in the siliconized layer is more concentrated in the outer layer of siliconized layer than in the inner part. Silicon content close up the outer surface of the coating layer is higher than the inner part of the coating layer as shown in EDS analysis Fig 1.c. This might be indicated by FeSi 2 phase, determined by XRD pattern, in the outer layer of siliconized layer, FeSi and Fe 2 Si goes on to the inner parts of the coating layer. EDS analysis showed that iron concentration in the silicide layer is lower than the inner part. The highest iron concentration took place in the matrix while the lowest silicon concentration located in. The micro-hardness value of the siliconized AISI D2 steel from surface to the interior was measured in the range of 330 HV 0.01 and 610 HV Siliconized steel hardness is lower than that of the AISI D2 steel matrix. (a) (b)

3 (c) Fig. 1. (a) Optical micrograph of siliconized AISI D2 steel at 1473K for 4h and (b) SEM images and (c) EDS analysis of the siliconized layer of the steel at 1373 K for 6h Fig. 2. X-ray diffraction analysis of the siliconized AISI D2 steel at 1373 K for 6h porous zone Homogenious layer AISI D2 steel matris Hardness, HV Distance from surface to interior, µm Fig. 3. Micro-hardness distribution of the silicide layer formed on the siliconized steel sample at 1375 C for 2h 3.2. Silicide layer growth kinetics Depending on the siliconizing temperature and time, the thickness of silicide layers ranged from 85.21µm to μm. Previous studies by Blandin et al [9] The thickness of silicide layer, doubtless, is closely related to the alloying element of the base steel and siliconizing time and temperature. As known, thermochemical coatings like boronizing, aluminizing, chromizing, siliconizing etc can be affected from the substrate compositions and bath composition of the process. As it can be seen in Fig. 4, there is nearly a parabolic relationship between silicide layer thickness and siliconizing time [10]. By assuming that: (i) the rate of layer

4 growth is controlled by silicon diffusion in the FeSi 2, FeSi and Fe 2 Si sublayers and (ii) Silicide layer growth occurs as a consequence of silicon diffusion perpendicular to the specimen surface, Ref. [11] indicated that the silicide layer thickness varies with time, and a parabolic law as follows: d 2 K. t (1) Where d is the silicide layer thickness (μm), t is treatment time (second) and K is silicon growth rate constant. Silicon diffusion in the silicide layer is the primary factor affecting layer growth (Fig. 4). The graphical representation in Fig. 4 shows that the higher the siliconizing temperature, the longer the treatment time, the thicker the silicide layer became. Furthermore, a contour diagram derived from Fig. 4 facilitates the selection of process parameter by means of Siqmaplot 12 software in industrial applications. At the same time for a predetermined layer thickness, the treatment time and temperature can be obtained from the diagram very close to the experimental study as shown in Fig. 4. If the kinetics of layer progress for the periods between 2 and 6 h is considered, it can be recognized that the square of silicide layer thickness changes linearly with time as it can be seen in Fig. 5 (a). The relationship between the diffusion coefficients (growth rate constant), K (m 2 s 1 ), activation energy, (j.mol 1 ) and the process temperature in Kelvin, T, can be expressed as an Arrhenius equation. The relationship between the growth rate constant, K, activation energy,, and the process temperature in Kelvin, T, can be expressed as an Arhenius equation: 0 Silicide layer thickness, µm K 1373 K 1423 K Temperature, K Silicide layer thickness, µm Time, h Fig. 4. Changing of the silicide layer thickness with time and temperature. RT o K K e (2) Where K o is the frequency factor (pre-exponential constant) and R is the gas constant. Taking the natural logarithm of Eqs. (2) and (3) can be derived as follows: LnK LnK o (3) RT

5 The plot of Ln K versus reciprocal treatment temperature is linear as shown in Fig. 6. q was determined from the slopes of straight lines as shown in Fig. 5 (b). The results show that K increases with treatment temperature. Activation energy () for present study was determined as 540 kj.mol -1. The growth rate constant (K) ranged from 7.76x10 7 to cm 2.s -1. The derived formulas between the growth rate constant values (K) and reciprocal treatment temperature (1/T) is Eq. (4). Yang et al [12] reported that activation energy of silicon in the silicide layer at 1023 K is 242 kj mol T K x e (4) Where, K is growth rate constant and T, temperature. The practical formula for calculating the layer thickness at pre-determined time and temperature can be derived from Eq. (1) and Eq. (4): d( µm) tx9.67x10 T (5) Square of layer thickness, mm K 1373 K 1423 K T 1423 K =2.45x10-5 mm 2 /s T 1373 K =4.25x10-6 mm 2 /s Ln K, mm 2 /s /R= =540 kj/mol T 1323 K =7.76x10-7 mm 2 /s Time, s /T, K -1 (a) (b) Fig. 5. (a) Square of the silicide layer thickness of siliconized AISI D2 steel vs. treatment time, and (b) Growth rate constant vs. temperature of siliconized AISI D2 steel 4. CONCLUSIONS The following conclusions can be made: a) The silicide layers formed on siliconized AISI D2 steel are compact and smooth. b) The presence of Fe 2 Si, FeSi and FeSi 2 phases in the silicide layer were confirmed by XRD. The hardness of silicide formed on siliconized steel according to treatment time and temperature are changing between 330 and 610 HV c) In addition, contour diagrams for the prediction thickness of silicide layer was utilized depending on the treatment time and temperature. This diagram is to give good results like classical kinetic formula depending on same process parameters. d) Activation energy (q) for the present study was determined as 540 KJ. mol -1. The growth rate constant (K) ranged from 7.76x10 7 to cm 2.s -1. e) The prediction of thickness and hardness of coating layer depending on process time and temperature is important for technological and industrial applications. In this study, it is possible to claim that contour diagrams of thickness can be used for practical and industrial applications.

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