Metallurgist, Vol. 57, Nos. 3–4, July, 2013 (Russian Original Nos. 3–4, March–April, 2013)

FORMATION FEATURES OF HIGHLY RESISTANT CARBIDE AND BORIDE COATINGS PREPARED BY A TWO-STAGE METHOD G. V. Levchenko,1 A. B. Sychkov,2 A. M. Nesterenko,1 and V. L. Plyuta1

UDC 621.9.025(076)

Features of high resistance coatings in the surface area of a tool steel and hard-alloy inserts for a cutting tool with a two-stage preparation method are studied. Pure tantalum and niobium coatings on tool steel specimens and inserts are prepared by electrolysis of tantalum and niobium fluorides in a salt bath. After this, carburizing or boriding of a coating (tantalum or niobium) is performed in order to form carbide (TaC, NbC) or boride (TaB, NbB) claddings. It is established that formation within a surface area of test specimens of cladding layers of carbide (boride) phases with high abrasive and corrosion resistance is determined by external “outsourcing” of carbon (boron) atoms. Test evaluation for cutting tool life with hard-alloy inserts with a TaC-coating, prepared by the method developed, demonstrates their operating efficiency during machining with component cleaning (finishing) treatment. The promising nature of using the method developed for improving mining and metallurgical equipment component, cutting tool, and engine component life in mechanical engineering is demonstrated. Keywords: highly-resistant carbide and boride surface coatings, fluorides, tantalum, niobium, boriding, microstructure, abrasion and corrosion resistance, cutting tool.

Wear of components operating in many contemporary units and machines is aggravated by high loads and temperature. During wear there are simultaneously corrosion processes, diffusion of elements, fatigue, friction, etc. Wear of metal surfaces for many types of equipment and machines is a serious problem, for whose prevention many solutions have been proposed. In particular, with the aim of marked improvement in the operating life of cutting tools there is extensive use of coatings (TiC or TiN). In order to prepare these coatings it is normal to use recognized methods for their deposition, i.e., physical vapor deposition (PVD) or chemical vapor deposition (CVD). The higher deposition temperatures in the case of a CVD coating (compared with the PVD method) guarantees a good joint between base and coating, but unfavorable decarburization of the basic metal, since the strength properties of the intermediate layer between coating and base metal are somewhat reduced. The choice of carbide TiC as a basic strengthening phase for a coating adopted for PVD or CVD methods does not seem clear. It is well known that tantalum carbide TaC has higher wear resistance (relative wear resistance 1.50%) than TiC (0.61%), although its microhardness (16000 MN/m2) is lower than that (30000 MN/m2) for TiC [1]. In order to increase operating life under rapid wear and high temperature conditions, such carbides as NbC, HfC, etc. are quite promising. However, the higher melting point of Ta (2996°C) and Nb (2477°C) compared with Ti (1668°C) prevents preparation of high quality coatings of TaC or NbC by PVD or CVD methods. In view of this in order to prepare coatings of carbides and borides of these elements it is necessary to use an electrochemical method [2, 3]. 1 2

Institute of Ferrous Metallurgy, National Academy of Sciences of Ukraine, Dnepropetrovsk, Ukraine. Nosov Magnitogorsk State Technical University, Magnitogorsk, Russia; e-mail: [email protected].

Translated from Metallurg, No. 3, pp. 88–94, March, 2013. Original article submitted March 1, 2012.

0026-0894/13/0304-0237 ©2013 Springer Science+Business Media New York

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Fig. 1. Microstructure of original Nb-coating on tool steel, ×1000.

Analysis carried out in [2, 3] indicates that application of pure metal coatings on a metal base, which during subsequent operation form wear resistant carbides and borides, is accomplished by fluorides on molten eutectic mixtures of special salts. An electrolytic process proposed in [3] makes it possible to form at high temperature a layer of “cladding” metal stably joined with a metal base. A marked increase in strength of these deposited layers may be achieved by using such strengthening elements as carbon, nitrogen, boron, or silicon, whose atoms are normally arranged in interstitial positions at internodes or in substitution positions in a base metal solid solution. When the temperature of the base metal is quite high in order to provide thermal diffusion of these elements from the base metal to a shell layer of deposited metal, subsequently as a result of phase and structural transformations there is formation of a cladding layer in the form of carbides, borides, or silicides. In a number of works, in particular [4], a two-stage method has been analyzed for preparing cladding coatings of “pure” metals on a base metal by electrolytic deposition from salt solutions of specific metals, followed by carburizing or boriding operations. This provides formation of cladding coating layers at the surface of a base metal having a unique combination of high wear- and corrosion-resistance, high-temperature strength, hardness, and other properties. In developing assumptions made in [4], the authors have studied features of cladding carbide and boride coating formation on tool steel and hard alloy inserts for cutting tools, prepared by a two-stage method. This method includes electrochemical (electrolysis) deposition of such elements at Nb and Ta at the surface of test specimens, followed by carburizing or boriding the metal layers obtained. It should be noted that with the use of this method, taking account of operating conditions for specific objects and components, it is possible to accomplish electrolytic deposition of V, Co, W, Mo, Ti, Zr, Hf and formation of highly stable carbide or boride coatings of these elements on a base metal with appropriate carburizing or boriding operations. Use of the method developed is very promising for improving the life of a wide range of mining components, metallurgical equipment, and also machine components, and this defines the importance of carrying out this research. Materials and study procedure. Deposition of Nb and Ta on tool steel (M4 – SAE J438b) specimens and hard alloy (WC–Co composite alloy) billets selected for this study was carried out in a salt bath (eutectic mixture of lithium, calcium fluorides), whose housing was manufactured from nickel sheet. The “donor” agents of Nb and Ta in molten used were salts of these elements, i.e., potassium heptafluoroniobate and potassium heptafluorotantalate. The anode used was Nb or Ta plates. Cathodes were test specimens of tool steel and hard alloy billets. The salt bath temperature was 700–900°C; current density during electrolysis 5–100 mA/cm2; electrolysis duration 0.5–3 h. Electrolysis was performed in an argon protective atmosphere, whose pressure somewhat exceeded atmospheric. In order to prevent formation of course dendrites for deposited Nb and Ta and increase layer uniformity, deposition was carried out in the initial stage stepwise with a change in current density and deposition duration by a specific regime for each step: first step – 350 mA/cm2 (1 sec); second step – 200 mA/cm2 (5 sec); third step – 150 mA/cm2 (2 sec). Subsequent 238

Fig. 2. Microstructure of NbC–Nb coating on tool steel (arrow 1) and transition zone (arrow 2) between NbC–Nb coating and tool steel metal base. Arrow 3 indicates one of typical NbC carbide areas formed as a result of gas carburizing, ×500.

electrolysis in order to prepare a Nb and Ta surface layer of the required thickness was accomplished repeating these steps with an increase in duration of each step from 15 min to 2 h. During solid-phase carburizing, test specimens with a coating of pure Nb or Ta were enclosed within a container with crystalline powder graphite. Containers with specimens were placed in vacuum (residual pressure ~10–4 tor (1.5 kpsi)) furnace. Carburizing temperature was 1000°C, soaking duration 5 h. The duration of soaking during subsequent cooling at 700°C is within the limits ~45 min. For several series of specimens, a version of solid-phase carburizing in protective atmosphere (0–4.0% H2 + Ar) was used. Gas carburizing of specimens with pure Nb or Ta coatings, enclosed within a quartz tube, was carried out at 1000°C in a gas mixture of 0.5–2.0% CH4 + H2 in a CO2 atmosphere. The specimen exposure duration was 1–4 h. Specimen cooling to 700°C was carried out in the same atmosphere for not less than 1 h. Boriding of specimens with Nb and Ta surface layers was performed in an argon atmosphere in a salt bath consisting of anhydrous fuzed salt electrolyte, containing B, C, and a metal of the alkali halide group or alkaline-earth halides. The boriding regime was as follows: current density during electrolysis 200–30 mA/cm2; temperature 800°C; duration 1–5 h. Evaluation of resistance of the coatings obtained was carried out by accelerated testing of a cutting tool with a hard alloy insert coated with TaC during machining of automobile engine components of specific classes. Two types of test were used: 1) by a regime of rough (coarse) cutting of two typical components of gray cast iron, i.e., a stator fastening for a Ford automobile (cast iron G3500 with a hardness of 207–255 HB – SAE J431), and an oil pump fastening rod for the same automobile (cast iron G2500, hardness 170–229 HB – SAE J431); 2) by “finishing” (final) cutting of gear wheels of forged steel G 51500 (208–302 HB – SAE J404). In each case, results for cutting tests of these standard components with a cutting tool with an insert coated with TaC, prepared by the two-stage method in question, were compared with results obtained with machining of the same components with a tool with a normal coating (Ti(C, N) + aluminum oxide + TiN – outer layer). Features of coating structure formation of pure metals (Nb and Ta) after electrolytic deposition, carbide and boride coatings, and also the structure of a base metal in the surface area were analyzed by optical and scanning electron microscopy. x-Ray structural analysis was used in order to study phase composition of coatings. Distribution of elements in the surface area of specimens was accomplished by means of an Oxford (Great Britain) x-ray spectral energy dispersion attachment to a Carl Zeiss scanning electron microscope. Electrolysis of fluoride salts containing Nb or Ta was carried out at high temperature, and therefore during coating formation from these metals diffusion of their atoms occurred over a considerable distance with the base metal surface, there239

Fig. 3. Microstructure of metallic Ta-coating (electrolytic deposition) (a) and carbide TaC-coating (gas carburizing) (b) on hard-alloy tool surface, ×1000.

by preventing the formation of an interface of a continuous layer of deposited metal (Nb or Ta) with the base metal [4]. The data obtained by the use of optical and scanning electron microscopy on the example of Nb, deposited on tool steel by electrolysis, showed that in this case the Nb metal layer is joined to the base metal without boundary defects. A transition zone formed during electrolytic deposition between a solid layer of Nb and tool steel, containing (according to x-ray microanalysis) about 84 wt.% Nb and 10 wt.% Fe, does not have any defects (Fig. 1) and provides a high-strength joint of the cladding Nb coating and base metal. Analysis showed that as a result of secondary recrystallization development grains of an Nb coating at high electrolysis temperature are transformed into coarse formations. In order to prevent the formation of an Nb coating coarse grained structure, a special stepwise deposition regime was used as indicated above in all stages of electrolysis (up to the preparation of the coating thickness required). During gas carburizing of Nb metal coating on tool steel between the NbC–Nb coating and the base metal there was formation of a transition zone with a thickness of about 8 µm (Fig. 2) bonding the cladding NbC–Nb layer and base metal. During checking the gas carburizing regime according to x-ray spectrometric analysis data 80% of the Nb of a cladding layer is transformed into NbC carbide, i.e., a considerable part of it remains untransformed. In the case of a stepwise electrodeposition regime of Ta in the surface area of a hard alloy insert there is formation of a “metal” Ta-coating with a relatively smooth surface (Fig. 3a). During carburizing this is transformed into a carbide TaC coating (Fig. 3b). It is clearly identified by a gold color of the coating obtained, and x-ray diffraction analysis data. Carbide of another stoichiometric composition, for example, type Ta2C with lower stoichiometry with respect to carbon, did not form by the approved carburizing regime. The TaC cladding layer obtained has high density, but close joining with the metal base through a transition layer as a rule is not observed. It follows from this that a high-strength type joint of TaC coating and base metal is not realized in this case. In spite of this, the two-stage method has a certain advantage over other methods. One of them involves the absence of carburizing in the surface area of a base metal during imposition of a Ta layer and prevention of formation of brittle δ-phase as a result of intense external “outsourcing” of carbon atoms, both with solid-phase and also gas carburizing. It should be noted that carburizing of a surface area in the case of high-temperature processes, such as CVD or sintering high-hardness composite WC–Co-inserts, reducing strength of the surface area of base metal, is caused by insufficient “donor” flow of carbon atoms into this zone. A typical TiC coating structure, obtained by gas (CVD) deposition, with clear differences, distributed throughout its volume and adjacent to its surface with crystals of δ-phase (in the form of bands), is shown in Fig. 4. Accelerated tests for the rough (coarse) machining of a cutting with inserts with a TaC coating, prepared by gas deposition, showed a short life for this tool, i.e., 25 and 50% of that for a standard tool during machining of support components for a stator and an oil pump respectively. In a second test, i.e., in finishing (fine) machining of a gear wheel, a tool 240

Fig. 4. TiC-coating microstructure prepared by CVD-method, ×1000.

with a test insert prepared by gas carburizing had the same low life, i.e., 50% of that for a standard tool. The reduced tool life with TaC coating during rough machining of components, characterized by high cutting forces, was connected as analysis showed with their increased brittleness. At the same time, a cutting tool of hard alloy insert with a TaC coating prepared by solid-phase carburizing, showed a life equal to 128% of that for a standard tool. Consequently, use of a cutting tool with inserts with a TaC coating, prepared by solid-phase carburizing, is promising for finish machining of components with reduced cutting forces. A series of experiments performed in the course of research made it possible to establish that the method developed also provides high quality for NbB and TaB boride coatings on tool steel and hard alloy cutting tool inserts. Comparative analysis of the method developed with known CVD and PVD methods used extensively, and also with the so-called thermal diffusion method TD [5], showed the following. CVD and PVD methods, as indicated above, have been used for many years and have quite high productivity. They make it possible to prepare coatings with a very smooth surface and controlled thickness. The high temperature during deposition by the CVD method guarantees good joining between a coating and base metal, but there be decarburizing of the base metal surface [4]. The PVD method is accomplished with a quite low (<500°C) temperature, and therefore base metal surface decarburizing is prevented. However, this method does not provide preparation of a high-strength joint for a coating and base metal through an intermediate zone. The reason for this is the fact that a cladding carbide layer forms directly on a base metal surface, within which carbon and alloying elements are already bonded into carbides and other phases, and therefore mutual diffusion of elements with the contact zone of the base is not accomplished. At the same time, the TD method provides development of diffusion between a coating and base metal [5]. The TD process is accomplished by immersing specimens (components) in a salt bath at 870–1037°C and subsequent exposure for 1–8 h. Metal atoms released from molten salt combine as a result of diffusion with carbon and other elements, contained in a tool steel base, and form carbide layers in its surface zone. Experimental results indicate that carbide coatings, prepared by the TD method, have higher adhesion with a base metal than a coating prepared by the CVD and PVD methods. On the other hand, in the case if the TD method directional diffusion of carbon from a base metal causes partial decarburizing of the surface, and this reduces joint strength of a coating with a base metal. In the case of hard alloy inserts, decarburizing leads to the formation in their surface area of η-phase of the form (Co6W6)C and (Co3W3)C, having reduced hardness compared with WC [1]. In the case of the two-stage method developed not only high joint strength provided between a coating and base metal, but as a result of continuous external “outsourcing” of carbon atoms (in contrast to the TD method) decarburizing of the base metal surface and formation of η-phase are prevented. Carbide coatings prepared by this method are dense and are not porous, because intense diffusion of “external” carbon into the metal layer of a coating and into the transition zone during carburizing provides formation of carbide phase throughout the whole coating volume and expands its area in the direction of a base metal. In addition, in the case of using the method developed the increased probability of pore formation in base metal adjacent to the transition zone, arising as a result of massive expulsion of carbon atoms form base metal into the coating, inherent for the TD method, is prevented. 241

A marked disadvantage of the TD method compared with the PVD and CVD methods is difficulties connected with accomplishing purposeful control of carbide coating surface thickness and quality. Therefore, apart from careful regulation of electrolytic deposition process parameters in the TD method, subsequent manual cleaning method for the carbide coating obtained from salt surface formations and subsequent polishing is prescribed in the TD. Use of the two-stage method developed guarantees a smooth carbide coating surface of required thickness and other quality indices by the stepwise control of electrochemical deposition of layers in the first stage of this process, and also introduction of special additions to a salt bath during electrolysis. Coating thickness is regulated by careful control of carbide-forming element deposition duration in electrolysis. In addition, for cases connected with disturbance of process technology (“elemental depletion of electrolytes,” uncontrolled change in deposition regimes, etc.), when coating surface, obtained by a two stage process develops, is low in quality (rough), operations of electrolytic polishing are provided for a deposited metal coating. Compared with manual polishing operations for a finished coating in the TD method, electrolytic polishing of metal coatings on specimens (components) of carbide-forming elements in the procedure developed has apparent advantages in the form of a reduction in material and time delays. This analysis has shown that use of the method developed, apart from tool steels and cutting tools, has significant promise for improving the operating properties of renewable components for equipment of mining, metallurgical, and mechanical engineering enterprises. Currently in drum mills for crushing iron ore combined rubber and metal linings are being used [6]. Use of the method developed for surface strengthening of metal inserts (wear-resistant steel and cast iron) of a rubber and metal lining makes it possible to increase considerably the resistance to abrasive-shock-corrosive wear, and thereby to increase the service life of these linings and other rubber and metal renewable components of mining enterprise equipment. The problem of wear for shafts and drive fittings for rolling mills [7, 8] remains acute. In view of this use of the method developed, for example, for strengthening the working surface of shafts and reduction and shaving machines, operating at high temperature and with specific loads, combined with traditional facing methods, may make a contribution in resolving this problem. Use of the coatings developed is also possible for strengthening internal combustion engine (ICE) components. Traditionally for a whole series of ICE components (gear and distributor shafts, piston liners, pins, valves, cylinder sleeves, rockers, plungers, etc.) with the aim of improving working surface wear resistance, various methods of gas and solid-phase carburizing, nitriding, cyaniding, combined with heat treatment are used, and in a number of cases with additional laser melting [9]. Considering the requirement for improving operating life, precision accuracy with respect to thickness of a strengthened coating layer for working surfaces, high toughness and resistance to crack formation for a base metal of ICE components, use of the method developed is promising. Conclusions. Studies of coating formation with high wear and corrosion resistance properties in the surface area of tool steel and hard alloy inserts for cutting tools with different operating regimes by a developed two-stage method and analysis of prospects for its use have shown that: 1) formation of cladding layers of carbide (boride) phases in the surface area of a test specimen is determined by external “outsourcing” of carbon (boron) atoms, and not their massive diffusion from a metal base; 2) carbide (boride) coatings prepared by this method have no pores or other defects, and also high joint strength with a base metal; 3) rapid external “outsourcing” of carbon (boron) atoms prevents decarburizing of the surface area of test materials, i.e., tool steels and hard alloy inserts for cutting tools; 4) the required thickness and high quality of coating surface in this method is provided by controlled electrochemical deposition regimes, including in the initial stage of the process; and 5) a promising area for use of the method developed is strengthening working surfaces of renewable components of mining equipment, shafts and drive fittings for reduction and rolling mills, cutting tools, and internal combustion engine components.

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