Effects of Electrochemical Boriding Process Parameters on the Formation of Titanium Borides

80 Effects of electrochemical boriding process parameters on the formation of titanium borides

A. Kilic, G. Kartal, M. Urgen, S. Timur

Istanbul Technical University, Department of Metallurgical and Materials Engineering, 34469 Maslak-Istanbul, Turkey, e-mail: [email protected]

In this study, the boriding of titanium via molten salt electrolysis was investigated in borax based electrolyte at various current densities (50–700 mA/cm2), temperatures (900–1200°C) and process durations (1 min to 4 h). Thin film XRD results revealed that two main titanium boride phases TiB2 and TiB was formed even after 1 minute of process time. Scanning electron microscopy (SEM) images conducted on the cross–sections demonstrated a bilayer boride structure composed of a continuous uniform TiB2 phase and TiB whiskers that grew below the TiB2 layer toward the substrate. Depend- ence of boride layer thickness and morphology on the process parameters was evaluated. The results of the study showed that temperature of boriding was the most critical parameter both on boriding rate and morphology of the boride layer. It was possible to grow 8 µm thick TiB2 layer on titanium in 30 minutes of boriding at 1200°C.

УДК 541.13

INTRODUCTION wastes and toxic gaseous emissions and long duration) of the technique mainly because of its long du- The demand for the development of new methods ration of pack boriding extending to 40 hours of and alternative materials have been increasing in pack boriding process [10, 11, 14, 15]. Basically, parallel to the industrial development and ever in- boriding is a thermal diffusion process in which bo- creasing concerns on energy and environment. Tita- ron atoms diffuse into the work piece to form a nium and titanium alloys with their exceptional range of metal boride phases [10, 11, 14–18]. strength-to-weight ratios (even at high tempera- Another alternative for boriding is electrochemi- tures), high stiffness, toughness, and excellent bio- cal boriding, which provides a considerably fast compatibility [1–4] became one of the most impor- growth rate for the formation of boride layers in an tant structural materials for a variety of demanding environmentally clean way without any ecological applications. However, poor friction and wear be- issues as experienced in the conventional pack havior of titanium and its alloys limits their usage boriding [16–22]. Potential of this technique has for tribological applications [5–9]. Different surface been clearly demonstrated with previous studies treatments have been applied to titanium for improv- conducted by our research group [16, 19–22]. ing their tribological behavior including thermo dif- A previous study conducted in our research fusion processes such as nitriding and pack boriding. group on electrochemical boriding of titanium has Pack boriding has been applied to iron and steel shown that the growth rate of titanium boride layers parts for a long time resulting in very positive con- on titanium substrate is higher than conventional tributions on the working efficiency and service life pack boriding [19]. In this study, the effects of the [10, 11]. Since transition metal borides have attrac- other critical parameters such as current density and tive properties such as high melting points and high electrolyte temperature were examined in a system- hardness at elevated temperatures, researches for the atic manner for evaluating their effects on the chem- boriding of transition metals have been accelerated istry, morphology and thickness of boride layers. in last decade; particularly for the applications in The knowledge gained with this work may offer a cutting tools, automotive, casting, textile, food, and different perspective on the unexplored part of the ceramic industries that friction dependent huge en- electrochemical boriding of titanium. ergy losses and intensive corrosion and wear occurs [12, 13]. At the moment pack boriding which uses 1. MATERIAL AND METHODS solid precursors in the form of powder or paste, is the most widely used industrial boriding technique. 1.1. Experimental procedure However, this process is not as extensively used as other thermo diffusion processes because of the limi- Mirror polished CP-Ti Grade 2 plates (size tations (generation of significant amounts of solid 10×50×1 mm) are used as a substrate material. Prior

______ Kilic A., Kartal G., Urgen M., Timur S., Электронная обработка материалов, 2013, 49(2), 80–86. 81 to the electrochemical boriding experiments, they periments were conducted at various current densi- were ultrasonically cleaned in acetone. ties (50–700 mA/cm2) at a constant temperature Electrochemical boriding experiments were car- (1000°C) and time (30 min.) in order to investigate ried out at current densities ranging between the effect of current density on the structure and 50–700 mA/cm2, at electrolyte temperatures be- thickness of titanium borides. tween 900–1200°C and for electrolysis durations of The cross sectional SEM images of borided tita- 1 min. to 4 h in a borax based electrolyte composed nium at various current densities revealed the uni- of 15% sodium carbonate (Na2CO3) and 85% borax formly thick TiB2 layer formations at all applied cur- (Na2B4O7). A high frequency induction furnace r ent densities (Fig. 2). A relatively low current den- (180 kHz, 5 kW, 7 A) was employed to heat electro- sity of 50 mA/cm2 was sufficient to grow a compact chemical cell consisting of graphite crucible acting TiB2 layer. As observed in previous studies as an anode where the titanium substrate was polar- [19, 23–25] a whisker like growth of TiB phase took ized as a cathode (Fig. 1). During electrolysis, the place below the TiB2 layer. The magnitude of the temperature of the molten electrolyte was both applied current did not alter the appearance of TiB measured by a Pt-PtRh13 thermocouple and an IR whiskers. Accordingly, the morphology of TiB ex- pyrometer. hibited the same needle structure at all current densi- Once electrochemical boriding was completed, ties used in this study. the Ti samples were drawn out of the electrolyte and The current density dependent thickness varia- left in air to cool down then washed with boiling tions of titanium boride layers (Fig. 3) showed that water to dissolve the solidified electrolyte on them. the applied current did not lead to a noteworthy al- teration in the thickness beyond 100 mA/cm2. The 1.2. Boride layer characterization reason of the unremarkable influence of current density on thickness higher than 100 mA/cm2 could be After the boriding treatment, titanium samples explained by the limiting effect of boron diffusion were sectioned and polished metallographically then into the structure. Even though we increased the etched by Kroll’s solution to reveal the microstruc- amount of reduced boron on the substrate surface ture. The cross sectional morphology, thickness and with increasing current density, only limited amount chemical analyses of boride layers were inspected of boron atoms was able to diffuse into the structure with SEM and EDS investigations (JSM-7000F that was controlled by kinetics of boron diffusion Model Field Emission SEM and Oxford instrument into the TiB2 layer. INCA x-sight EDS unit capable of analyzing light The results of thin film XRD analysis of the bor- elements). ided surfaces at different current densities were The phase composition of the boride layer was given in Fig. 4. Only TiB2 peaks were detected at all characterized by the thin film X-ray diffraction applied current densities due to the low penetration analysis using CuKα radiation and with the grazing depth of X-rays with a glancing angle 2°. angle of 2º (XRD-Philips Model PW3710, 40 kV– As expected, the cell potential, thus the energy 40 mA). consumption of electrochemical cell, was getting higher with rising current densities (Fig. 5). 2. RESULTS AND DISCUSSION Overall, it could be deduced that the increase in the current densities did not trigger any major modi- 2.1 Effect of current density fications in both the thickness and the morphology

of boride layers, TiB2-TiB. Contrary to conventional boriding processes in which reduction of boron oxides is achieved through 2.2 Effect of electrolyte temperature chemical reducing agents, in our case the reduction of boron oxides to atomic boron is realized through Boriding process temperature was investigated in electrons produced during cathodic polarization of a systematic manner to explore its effects on the the substrate [10, 11, 22]. In electrochemical sys- growth of titanium boride layers. Since boriding is a tems, in which reaction rate is controlled with charge diffusion based process, the temperature of boriding transfer step, the magnitude of applied current is the is one of the significant parameter to be considered. major parameter for determining the thickness of the Temperature dependent experiments were carried coated or modified layer. According to Faraday’s out for a constant duration of 30 minutes, a current first law, the amount of electrochemical decomposi- density of 300 mA/cm2 and at temperatures ranging tion is proportional to the quantity of electricity between 900–1200 °C. passed in electrolytic processes; hence the amount of The cross sectional SEM images of borided tita- reduced boron, and consequently the thickness of nium substrates at various process temperatures boride layer should be the function of the applied showed that the boriding temperature caused consid- current. Within this perspective, current density ex- erable effects in the thickness of TiB2 layer and the 82

Fig. 1. Schematic drawing of the electrochemical boriding setup. a – High frequency furnace; b – Direct current source (±0.001mA); c – Alumina protector crucible; d – Graphite crucible acting as an anode; e – Cathode; f –Thermocouple in alumina protection tube; g – Multimeter (for temperature measurement); h – Multimeter (for cell potential measurement).

(a) (b)

(c) (d) Fig. 2. Cross-sectional SEM images of boride layer produced at different current densities: (a) 50 mA/cm2 , (b) 100 mA/cm2, 2 2 (c) 200 mA/cm , (d) 700 mA/cm , [15% Na2CO3 and 85% Na2B4O7, 1000°C, 30 min.].

Fig. 3. Thickness variations of boride layers as a function of applied current densities [15% Na2CO3 and 85% Na2B4O7, 1000°C, 30 min.]. 83

Fig. 4. XRD diffraction pattern of boride layers obtained at different current densities [15% Na2CO3 and 85% Na2B4O7, 1000°C, 30 min.].

Fig. 5. Variation of cell potential at different current densities [15% Na2CO3 and 85% Na2B4O7, 1000°C, 30 min.].

(a) (b)

(c) (d) Fig. 6. Cross-sectional SEM images of boride layer formed on titanium substrate at different process temperature: (a) 900°C, 2 (b) 1000°C, (c) 1100°C, (d) 1200°C [15% Na2CO3 and 85% Na2B4O7, 200 mA/cm , 30 min.]. 84

(a) (b) Fig. 7. (a) – variations of boride layers thicknesses with respect to boriding process temperature; (b) – the ratio of depth to width of 2 TiB whiskers as a function of temperature [15% Na2CO3 and 85% Na2B4O7, 200 mA/cm , 30 min.].

Fig. 8. Variation of cell potential at different process temperature during a 30 min long boriding experiment [15% Na2CO3 and 85% 2 Na2B4O7, 200 mA/cm , 30 min.].

(a) (b) Fig. 9. (a) – comparison of TiB2 thickness values obtained from the current study and the previous study; (b) – variations of boride 2 layers thickness [µm] with time [min] [15% Na2CO3 and 85% Na2B4O7, 200 mA/cm , 950°C]. 1

26.3µm

(a) (b) Fig. 10. Cross-sectional SEM images of borided titanium sample in 1 min. (a) and 4 h (b) [15% Na2CO3 and 85% Na2B4O7, 200 mA/cm2, 950°C]. 85 morphology of TiB structure. The growth of TiB2 thickness, the penetration depth of TiB was also in- was promoted with the increase of temperature as spected for all boriding durations. The previous and expected. Increasing boriding temperature from current results are consistent with each other 900°C to 1200°C resulted in 5 µm increase in the (Fig. 9a), which further verified the repeatability of thickness of TiB2 layer (Fig. 6). the experiments. The thickness variation of TiB2 layers and the The time dependent variation of the total boride penetration depth of TiB whiskers as a function of layers were given with height bars where the thick- electrolyte temperatures revealed that there was a nesses of TiB2 and penetration depth of TiB were linear correlation between the thickness of TiB2 and represented separately (Fig. 9b). The duration of process temperature (Fig. 7a) which is an expected boriding had a positive contribution on the growth of outcome of diffusion controlled processes [11, both TiB2 and TiB. After 4 hours of boriding, 16, 26]. 8.7 µm thick TiB2 layer was achieved, whereas the Moreover, the boriding temperature also showed length of TiB whiskers reached to 30 µm. an appreciable influence on the morphology of TiB At the beginning of the boriding treatment, the whiskers. The tiny-hairy TiB whiskers formed at formation rate of TiB2 was very fast, and exhibited 900°C started to thicken at 1100°C and eventually an almost linear correlation with electrolyses time the needle structure of TiB was converted into an until 1 hour. However, beyond one hour, the growth almost continuous layer at 1200°C (see Fig. 6a-d). rate started to follow a parabolic relation as in other This remarkable tendency observed in the structure diffusion controlled processes due to the thickening of TiB was presented in Fig. 7b. The average depth of TiB2 (Fig. 9c). The parabolic dependence of the to width ratios (d/w ratio) of TiB whiskers gradually boride layer thickness on time also revealed that the decreased with the increase in boriding temperature. boron adsorption of the surface is the actual limiting It is known that the dendritic-whisker like growth of stage of boron diffusion into the substrate or else the mono-boride form of titanium (TiB) is related to the growth of boride layer. preferential diffusion of boron in [010] direction in The cross sectional SEM image of one minute the crystal lattice of titanium [23, 24, 27], which borided titanium sample revealed that even such a means that the activation energy for boron diffusion short time was enough to grow a continuous and into interstitial lattice sites is low in this direction. homogenous TiB2 layer and whiskers of TiB under- By the increase of boriding temperature, energy neath (Fig. 10a). The increase in process durations supplied to the system, it is possible to reach the ac- caused a thicker and denser TiB2 on top and TiB tivation energy levels required for diffusion in other whiskers growing deeper into the core of the tita- directions in the titanium lattice. Hence the loose of nium substrate. The length of TiB needle was directionality and broadening of TiB whiskers can reached to 30 µm after a 4 hour of process time be attributed to the increase of activation energy for (Fig. 10b). diffusion in other directions in the lattice leading to a 3. CONCLUSIONS

3 dimensional growth of the TiB phase. The main objective of the study was to system- In order to investigate the cell potential variation atically investigate the electrochemical boriding pa- depending on boriding temperature, cell potentials at rameters (time, electrolyte temperature and current different boriding temperatures were recorded density) on the thickness, morphology and phase (Fig. 8). As expected, cell potential decreased with composition of boride layers formed on the titanium increasing temperature. This effect could be ex- substrate. plained by the increase in the conductivity of the It was found that only the time and temperature electrolyte and the decrease in the activation energy of boriding process provide a major influence on the of ionic sodium reduction. growth rate of titanium boride phases, TiB and TiB2. Although an increase in boriding temperature The temperature of boriding media was the most provides positive contributions on the thickness of critical parameter among the others to control the TiB2 layers, the high temperature corrosion of metal- thickness and morphology of both titanium borides. lic parts in the cell assembly starts to become a seri- It is observed for the first time that the whisker like ous problem above 1000°C. Thus for practical pur- growth of TiB gradually converted into a layer type poses boriding at elevated temperatures may not be 3D structure with the increase of boriding tempera- preferred. ture. The magnitude of applied current density did 2.3 Effect of electrolysis duration not alter morphology of boride layers beyond a cur- The effect of electrochemical processing time on rent density of 100 mA/cm2. boriding of titanium was previously studied in our ACKNOWLEDGEMENTS research group [19]. In this work, the same experiments were repeated with the addition of two new This work was supported by State Planning Or- durations (1 min and 4 hours). Other than TiB2 ganization of Turkey through the “Advanced Tech- 86 nologies in Engineering”. The authors would like to 18. Matiasovský K., Chrenková-Paucírová M., Fellner P., Ma- thank Prof. Dr. Gultekin Goller and Talat Alpak for kyta M. Electrochemical and Thermochemical Boriding in Molten Salts. Surf. Coat. Technol. 1988, 35, 133–149. their contributions on SEM-EDS analysis. We would 19. Kartal G., Timur S., Urgen M., Erdemir A. Electrochemical like to thank Sevgin Türkeli for her help during thin Boriding of Titanium for Improved Mechanical Properties. film XRD analysis. Surf. Coat. Technol. 2010, 204, 3935–3939. REFERENCES 20. Kartal G., Eryilmaz O.L., Krumdick G., Erdemir A., Timur S. Kinetics of Electrochemical Boriding of Low Carbon 1. Boyer R., Welsch G., Collings E.W. Materials Properties Steel. App. Surf. Sci. 2011, 257, 6928–6934. Handbook: Titanium Alloys. ASM International, Ohio, 1994. pp. 1169–1176. 21. Kartal G., Timur S., Sista V., Eryilmaz O.L., Erdemir A. The Growth of Single Fe2B Phase on Low Carbon Steel via 2. Sha W., Malinov S. Titanium alloys: Modelling of Phase Homogenization in Electrochemical Boriding microstructure, properties and applications. Wood-head (PHEB). Surf. Coat. Technol. 2011, 206, 2005–2011. Publishing Limited, Cambridge, 2009. pp. 413–496. 22. Kartal G., Timur S., Arslan C. Effect of Process Current 3. Oshida Y. Bioscience and Bioengineering of Titanium Density and Temperature on Electrochemical Boriding of Materials. Elsevier, Oxford, 2007. Steel in Molten Salts. J. Electron. Mater. 2005, 34, 4. Lutjering G., Williams J.C. Titanium: Engineering Materials 1538–1542. and Processes. New York, Springer, 2007. 23. Tikekar N.M., Chandran K.S.R., Sanders A. Nature of Gro- 5. Zhecheva A., Sha W., Malinov S., Long A. Enhancing the wth of Dual Titanium Boride Layers with Nanostructured Microstructure and Properties of Titanium Alloys Through Titanium Whiskers on the Surface of Titanium. Scr. Mater. Nitriding and other Surface Engineering Methods. Surf. 2007, 57, 273–276. Coat. Technol. 2005, 200, 2192–2207. 24. Aich S., Chandran K.S. Ravi. TiB Whisker Coating on 6. Wilson A., Leyland A., Matthews A. A Comparative Study Titanium Surfaces by Solid-state Diffusion: Synthesis, of the Influence of Plasma Treatments, PVD Coatings and Microstructure, and Mechanical Properties. Metall. Mater. Ion Implantation on the Tribological Performance of Ti-6Al- Trans. 2002, 33A, 3489–3498. 4V. Surf. Coat. Technol. 1999, 114, 70–80. 25. Sanders A.P., Tikekar N., Lee C., Chandran K.S.R. Surface 7. Fei C., Hai Z., Chen C., Yangjian X. Study on the Tri- Hardening of Titanium Articles with Titanium Boride bological Performance of Ceramic Coatings on Titanium Layers and its Effect on Substrate Shape and Surface Alloy Surfaces Obtained Through Microarc Oxidation. Texture. J. Manuf. Sci. Eng. 2010, 131, 031001-1-8. Prog. Org. Coat. 2009, 64, 264–267. 26. Sen U. Kinetics of Titanium Nitride Coatings Deposited by 8. Itoh Y., Itoh A., Azuma H., Hioki T. Improving the Thermo-reactive Deposition. Vacuum. 2004, 75, 339–345. Tribological Properties of Ti-6Al-4V Alloy by Nitrogenion 27. Fan Z., Guo Z.X., Cantor B. The Kinetics and Mec-hanism Implantation. Surf. Coat. Technol. 1999, 111, 172–176. of Interfacial Reaction in Sigma Fibre-inforced Ti MMCs. 9. Liu L., Ernst F., Michal G.M., Heuer A.H. Surface Har- Composites Part A: Applied Science and Manufacturing. dening of Ti Alloys by Gas-phase Nitridation: Kinetic 1997, 28A, 131–140. Control of the Nitrogen Surface Activity. Metall. Mater. Received 29.03.12 Trans. A. 2005, 36A, 2429–2434. Accepted 29.05.12 10. Matuschka A.G. Boronizing. Philadelphia, Heyden and Son Реферат Inc., 1980. 11. Sinha A.K. Heat Treating: boriding (boronizing) of steels. В работе исследовался процесс борирования титана fourth ed., ASM Handbook, USA, 1991. путём электролиза солевого расплава в электролите на 2 12. Эпик А.П. Борирование тугоплавких переходных ме- основе бора при разных плотностях тока (50–700 мА/см ), таллов. Порошковая металлургия. 1963, 17(5), 21–27. температурах (900–1200°C) и длительности процесса (от 1 мин до 4 ч). Результаты XRD-анализа тонких пленок 13. Kaptay G., Kuznetsov S.A. Electrochemical Synthesis of показали, что две основные фазы борида титана (TiB и TiB) Plasmas & Ions 2 Refractory Borides from Molten Salts. . образовались уже спустя 1 мин после начала процесса. 1999, 2, 45–56. Анализ сечений с помощью сканирующей электронной 14. Spence T.W., Makhlouf M.M. Characterization of the Ope- микроскопии (SEM) показал двухслойную структуру rative Mechanism in Potassium Fluoborate Activated Pack борида, состоящую из сплошной однородной фазы TiB2 и Boriding of Steels. J. Mater. Process. Technol. 2005, 168, нитевидных кристаллов TiB, которые формировались под 127–136. слоем TiB2 по направлению к подложке. Была оценена 15. Jain V., Sundararajan G. Influence of the Pack Thickness of зависимость толщины и морфологии слоя борида от the Boronizing Mixture on the Boriding of Steel. Surf. Coat. параметров процесса. Результаты исследования показали, Technol. 2002, 149, 21–26. что температура процесса является наиболее критическим параметром, влияющим как на скорость борирования, так и 16. Kartal G., Timur S., Eryilmaz O.L., Erdemir A. Influence of на морфологию слоя борида. Слой TiB толщиной 8 µm на Process Duration on Structure and Chemistry of Borided 2 титане был получен за 30 мин борирования при температуре Low Carbon Steel. Surf. Coat. Technol. 2010, 205, 1200°C. 1578–1583. 17. Segers L., Fontana A., Winand R. Electrochemical Boriding of Iron in Molten Salts. Electrochim. Acta. 1991, 36, 41–47.