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Oxidation behaviour of PACVD TiBN coating at elevated temperatures
Surface & Coatings Technology 204 (2009) 601–609
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Oxidation behaviour of PACVD TiBN coating at elevated temperatures Y. He a, J. Zhou a,⁎, T. Walstock b, J. Duszczyk a a b
Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands Adex B.V., Tjalkkade 2, 5928 PZ Venlo, The Netherlands
a r t i c l e
i n f o
Article history: Received 12 May 2009 Accepted in revised form 27 August 2009 Available online 4 September 2009 Keywords: PACVD TiBN Oxidation Thermal treatment
a b s t r a c t The elevated-temperature oxidation behaviour of a TiBN coating on a plasma-nitrided hot-work tool steel (DIN 1.2367) by means of plasma-assisted chemical vapour deposition (PACVD) was investigated under the condition where a coated die would be preheated prior to being mounted on the press for aluminium extrusion. The TiBN coating was found to possess good resistance to oxidation up to 400 °C. Rapid oxidation started to occur at 450 °C. Radio frequency glow discharge optical emission spectroscopy (rf-GDOES) indicated that the oxidised layer was thickened from 100 nm to 1.0 μm, as the soaking time at 500 °C was prolonged from 2 to 16 h, which was attributed to the high temperature that promoted the penetration of oxygen into the coating. rf-GDOES also showed that boron initially in the coating vanished from the oxidised layer when the temperature was 450 °C or higher. X-ray diffractometry confirmed that the oxidised layer was composed mainly of TiO2. SEM revealed that the TiO2 layer was pulverised, leaving many microcracks and cavities, as a result of the losses of boron oxide and nitrogen. The rapid oxidation at above 450 °C was attributed to the pulverised TiO2 layer that was unable to hinder the diffusion of oxygen into the coating. It is therefore recommended to apply a protective gas during the preheating of the TiBN-coated die for aluminium extrusion. Alternatively, an advanced TiBN coating with enhanced resistance to oxidation must be developed, which will be conducive to its application for aluminium extrusion dies. © 2009 Elsevier B.V. All rights reserved.
1. Introduction TiBN ternary coatings are of great industrial value, considering their prominent mechanical and tribological properties, such as superhardness, low intrinsic stress and high wear resistance [1–3]. These coatings have been successfully applied to cutting tools, die-casting moulds, forging dies, aluminium extrusion dies, etc. Marked increases in lifespan and productivity have been achieved [4–7]. TiBN coatings developed so far have different compositions and structures in connection with a variety of processes employed, such as magnetron sputtering [8,9], physical vapour deposition (PVD), chemical vapour deposition (CVD) and plasma-assisted chemical vapour deposition (PACVD) [10–12]. For any of TiBN coatings, one of the important criteria for industrial application remains the same, i.e. thermal stability and oxidation resistance. For coated cutting tools and die-casting moulds, for example, the working temperature may reach 600–800 °C [4]. For coated aluminium extrusion dies, the preheating temperature is typically at about 500 °C, soaking time may be as long as 4–5 h, and in most cases no protective gas is applied [13,14]. Therefore, for these applications, the thermal stability and oxidation resistance of TiBN coatings will have a profound influence on the performance of the coated tools, moulds and dies during service.
⁎ Corresponding author. Tel.: +31 15 2785357; fax: +31 15 2786730. E-mail address: [email protected] (J. Zhou). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.08.039
In recent years, many investigations on the thermal stability of TiBN coatings have been carried out. It is known that TiBN coatings are thermally stable at a post-deposition annealing temperature higher than 900 °C and post-annealing hardness higher than that in the asdeposited condition may be achieved [15–17]. However, few research efforts have been made to evaluate the oxidation resistance of TiBN coatings at elevated temperatures. Lu et al. studied two TiBN coatings with a difference in chemical composition and found significant differences in oxidation behaviour over a temperature range of 600 to 1000 °C [8]. Héau et al. noticed that the mass gain of a TiNx(B)y coating started at 627 °C. Over a temperature range of 727 to 977 °C, the coating was completely oxidised to TiO2 and the mass remained almost unchanged [9]. There is a lack of understanding of the oxidation resistance of TiBN coatings at the typical temperatures to which they are exposed prior to or during service, typically at 400– 550 °C where, for example, a coated die for aluminium extrusion is preheated in an air furnace. The objective of the present research was to understand the oxidation behaviour of a PACVD TiBN coating with compositional gradients deposited on a hot-work tool steel during thermal treatments at 400–550 °C in the ambient surrounding that resembled the preheating condition of the coated die for aluminium extrusion. It was hoped that the understanding of the oxidation behaviour and oxidation mechanisms would be of use for making recommendations as to the need of applying protective atmosphere during the preheating of the coated die.
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2. Experimental details A TiBN ternary coating with compositional gradients was deposited on a DIN 1.2367 (X40CrMoV53) hot-work tool steel substrate using a PACVD-coating system with a bipolar DC voltage-controlled pulse generator. Prior to coating deposition, the steel substrate was subjected to a tempering heat treatment to achieve a hardness value of 50 HRC. The substrate was then plasma-nitrided in the PACVD equipment to create a nitrided diffusion layer with a thickness of about 120 μm without a compound layer. Plasma nitriding was performed in a mixture of H2, Ar and N2 under a total pressure of 300–600 Pa. Substrate temperature was kept at 400–530 °C so as to retain the hardness of the as-tempered tool steel. During coating deposition, a very thin TiN layer was first deposited for the purpose of enhancing adhesion to the substrate before TiBN coating deposition started. During subsequent coating, the BCl3 partial pressure was increased in order to increase the boron concentration in the coating toward the coating surface where high hardness and wear resistance were desired. Process parameters such as gas flow, wall heat, voltage, pulse-on and pulse-off times, and chamber pressure were closely controlled with a programmable logic controller (PLC). H2, Ar, N2, BCl3 and TiCl4 vapour were used as process gases for coating deposition. Pressure was kept at 70–150 Pa and substrate temperature at 530 °C in order to avoid exceeding the tempering temperature of the hot-work tool steel. The TiBN coating thickness was controlled at approximately 3 μm. To simulate the preheating condition of PACVD coated aluminium extrusion dies physically, TiBN-coated samples with a cubic shape and sizes of 8 × 8 × 8 mm were subjected to thermal treatments at 400–
550 °C for 2–16 h in the ambient atmosphere in a chamber furnace, followed by air cooling. The samples were cleaned using ethanol in an ultrasonic bath, prior to microstructure examination and chemical analysis. The surface morphology and cross-section microstructure of the TiBN coating in the as-deposited state were determined by using a Jeol JSM 6500F scanning electron microscope (SEM) for comparison with those after the thermal treatments. The phases and crystallographic structures of the as-deposited and as-oxidised coating were examined by means of a Bragg–Brentano X-ray diffractometer (XRD) where the Co Kα line at 0.179026 nm was used as the source for diffraction. Chemical composition profiles on the cross section of the coating in both the asdeposited and as-oxidised states were quantitatively analysed using LECO's GDS-750A radio frequency glow discharge optical emission spectroscopy (rf-GDOES). The sputtering diameter on the sample was 4 mm. RF-mode was run at 700 V plus 14 W as the true power and Ar pressure was typically 9 Torr (120 Pa). The sputter rate was 4–5 µg/s. Spectral wavelengths were as follows: Ti: 365.350, N: 149.262, B: 182.641, O: 130.217, C: 165.701, Cl: 134.724, Cr: 2425.433, and Fe: 2371.994 nm. 3. Results 3.1. Colour change The PACVD TiBN coating in the as-deposited condition was silvergolden in colour (Fig. 1a). After thermal treatment at 400 °C for 4 h, the colour remained essentially unchanged. However, after thermal
Fig. 1. Colour changes of the TiBN coating surface from (a) the initial as-deposited state as a result of thermal treatments at (b) 450, (c) 500 and (d) 550 °C for 4 h.
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exposure at raised temperatures, i.e. 450, 500 and 550 °C, the colour of the coating surface altered, as shown in Fig. 1, indicating oxidation to varying extents. After thermal treatment at 450 °C for 4 h, for example, the coating surface became reddish in some areas and bluish in other areas, while the original silver-golden colour was retained on the rest of
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the surface (Fig. 1b). As the temperature was increased to 500 °C, the silver-golden colour disappeared completely and the light blue-gray colour together with the light red colour characterised the coating surface (Fig. 1c). At 550 °C, the light blue colour dominated the coating surface (Fig. 1d). No thermal cracking, flaking or blistering occurred on
Fig. 2. Chemical composition profiles on the cross section of the TiBN coating determined by rf-GDOES: (a) in the as-deposited state and after thermal treatments at (b) 400, (c) 450, (d) 500 and (e) 550 °C for 4 h.
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the coating during the thermal treatments at these temperatures. The progressive changes of the coating colour with temperature indicate not only the onset temperature of rapid oxidisation but also the changes of oxidised layer thickness and the formation of different crystals of titanium oxides. 3.2. Compositional changes The as-deposited PACVD TiBN coating was composed of three layers with compositional gradients, as shown in Fig. 2a. The average concentration of titanium was about 50 at.% over the whole coating depth of about 3 µm. The concentration of nitrogen over the coating depth ranges of 0–1.0 µm, 1.0–2.0 µm and 2.0–3.0 µm step-wisely increased from about 36 at.% to 40 at.%, and to 42 at.%, respectively. In parallel with the changes of the nitrogen concentration, the concentration of boron decreased from 7 at.% to 5 at.%, and to 3 at.% over these depth ranges. In accordance with these compositional variations, the TiBN coating was composed of the mixtures of TiN0.72B0.14, TiN0.80B0.10 and TiN0.84B0.06 from the surface inwards. The observed compositional gradients of the TiBN coating were related to the gradual increase of the BCl3 gas pressure and corresponding decrease of the N2 gas pressure during the coating deposition process. By varying these partial pressures, leading to compositional gradients, a superhard surface layer with a relatively high boron concentration could be created and in the meantime the adhesion of TiN with the plasmanitrided substrate assured. In addition, about 20 at.% oxygen was present over a depth range from null to about 70 nm in the asdeposited coating. With increasing coating depth, the concentration of oxygen decreased to a level of about 3.5 at.% over a depth range of 0.5– 3.0 µm. Furthermore, about 2.5 at.% trace chlorine was retained in the as-deposited coating. rf-GDOES revealed an oxidised layer with a thickness of about 70 nm at the TiBN coating surface after the thermal treatment at 400 °C for 4 h, as shown in Fig. 2b. The oxygen profile over this depth was similar to that in the as-deposited coating without the thermal treatment (compare Fig. 2a and b). It indicates that the oxidised layer did not grow much during this thermal treatment. In the oxidised layer, the titanium and nitrogen concentrations both decreased and the boron concentration even decreased nearly to zero. After thermal treatment at 450 °C for 4 h, however, further compositional changes occurred (Fig. 2c). A titanium oxide layer formed over a coating depth range from null to about 250 nm, in which the oxygen concentration increased up to about 77 at.%, while the concentrations of titanium and nitrogen decreased to about 20 at.% and 2 at.%, respectively. Furthermore, boron vanished completely at a depth of 250 nm in the titanium oxide layer. Over the remaining coating depth ranging from 250 nm to the interface between the coating and substrate, the concentrations of all the elements remained at the same levels as in the as-deposited state and boron still displayed a gradient distribution. As the temperature was increased to 500 °C and even to 550 °C while the soaking time remained to be 4 h, the depth of the oxidised layer in the coating reached about 350 and 750 nm, respectively, as shown in Fig. 2d and e. In the oxidised layers, the concentration of oxygen increased to about 65–70 at.%, while the concentrations of titanium and nitrogen fell into the ranges of 15– 30 at.% and 5–15 at.%, respectively. As in the case of thermal treatment at 450 °C, boron completely escaped from the oxidised layer after thermal treatments at 500 and 550 °C. The trace element of chlorine also disappeared from the oxidised layer at these elevated temperatures. Fig. 3 presents the development of the oxidised layer thickness in the TiBN coating during thermal treatments in a temperature range of 400 to 550 °C with soaking time increasing from 2 to 16 h. rf-GDOES clearly showed good resistance of the TiBN coating to oxidation up to 400 °C. Higher temperature and long soaking time caused severer oxidation in the coating. The thickness of the oxidised layer at 450 and 500 °C for 2 h were 70 and 100 nm, respectively, being similar to that at 400 °C for 4 h, however, after 16 h the thickness became about 500 nm and 1.0 µm,
Fig. 3. Depth of the oxidised layer in the TiBN coating as a function of soaking time at 400, 450, 500 and 550 °C.
respectively. Moreover, the thickness of the oxidised layer formed at 550 °C for 2 h only was about 500 nm, and after 8 h it reached 1.2 µm, being nearly one half of the thickness of the coating (3 µm). Apparently, temperature played a more important role than soaking time in promoting the penetration of oxygen into the coating and allowing boron to escape. 3.3. Surface morphology and microstructural changes The morphology of the as-deposited TiBN coating surface exhibited a cauliflower-like pattern instead of the end of epitaxial growth of crystallites, as revealed by SEM and shown in Fig. 4a. SEM on the cross section (Fig. 4b) showed that the TiBN ternary coating did not consist of columnar crystals, as in the case of TiN or TiB2 binary coating. The columnless structure was assumed to consist of nanocrystallites according to the findings of other researchers in TiBN coatings [18–20]. Mayhofer et al. give a clear explanation about the mechanism of the change from columnar crystals to nanocrystallites when TiN coating is doped with a small amount of boron [17]. After thermal treatment at 450 °C for 4 h, the cauliflower-like surface morphology and nanocrystallite structural characteristics on the cross section remained essentially the same as those in the as-deposited state, as shown in Fig. 5a and b. However, when the soaking time at 450 °C was prolonged to 16 h, the cauliflower-like pattern on the coating surface was retained and many microcracks, as indicated by arrows in Fig. 6a, appeared as a result of oxidisation, while on the cross section (Fig. 6b) the coating still had almost the same morphology as in the as-deposited condition except a very thin oxidised layer under the surface. When the temperature was raised to 500 and even to 550 °C, the surface of the TiBN coating appeared to be pulverised, as shown in Figs. 7 and 8, while the cauliflower pattern was preserved. On the cross section, SEM revealed that the thickness of the oxidised layer under the surface increased and, moreover, the microstructure of this layer changed from dense nanocrystallites to being porous, as shown in Figs. 7b and 8b. Below the oxidised layer, the microstructure of the coating remained the same as in the as-deposited state, i.e. being composed of nanocrystallites. 3.4. Constituent changes Fig. 9 compares the XRD patterns of the coating in the as-deposited state and after thermal treatments at 450 and 500 °C for 4 h. In the asdeposited state, the peaks of TiN (200), (111), (220) and (311) in the TiBN coating appeared at the 2-theta angles of 50, 43, 73 and 89°, respectively. The peaks of TiB were hardly visible, probably because
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Fig. 4. SEM micrographs of the TiBN coating in the as-deposited state: (a) top view showing the cauliflower pattern and (b) cross-section view showing the nanocrystallite microstructure.
the amount of boron was very small (only about 7 at.%) and the intensities of TiB were too weak for reliable identification. It is important to mention that the intensity peaks of TiN in this TiBN ternary coating were all broadened in comparison with those of the same compound in TiN binary coating. This is believed to be associated with the presence of boron that is known for having a grain refining effect. Assuming the broadening to be due to crystallite size only, the mean size of the crystallites in the Ti(B)N coating was
calculated to be about 7.5 nm, using Scherrer's formula. The lattice parameter of TiN increased from a = 4.242 Å in the TiN binary coating to 4.264 Å in the TiBN ternary coating, a trend being in agreement with ab initio obtained values for NaCl structure TiBxN1 − x where B substitutes for N [21]. Obviously, the change of the lattice parameter is associated with the incorporation of boron in the TiN compound, but other factors such as defects or residual stresses may affect the lattice parameter. As the temperature was increased from 450 to 500 °C, the
Fig. 5. SEM micrographs of the TiBN coating after thermal treatment at 450 °C for 4 h: (a) top view showing the retained cauliflower pattern and (b) cross-section view showing a very thin oxidised layer under the surface.
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Fig. 6. SEM micrographs of the TiBN coating after thermal treatment at 450 °C for 16 h: (a) top view showing microcracks and (b) cross-section view showing a thin oxidised layer under the surface.
peak widths of TiN remained unchanged, but the peak intensities were weakened. This indicates that no crystallite growth or phase transformation occurred in the unoxidised TiBN coating part during the thermal treatment. A very weak intensity peak of titanium oxide in the TiBN coating in the as-deposited state appeared at a 2-theta angle of 32°. After thermal treatment at 450 °C for 4 h, however, noticeable peaks of TiO2 (rutile) appeared at the 2-theta angles of 44 and 64°. When the temperature was increased up to 500 °C, TiO2 peaks emerged at 32, 42, 48 and 64° over a 2-theta angle range from 30 to 130°. 4. Discussion The as-deposited TiBN ternary coating studied consists of three layers with a gradient of the boron concentration over a total depth of about 3 μm, as shown in Fig. 2a. The concentrations of boron in these three layers are all lower than 10 at.%. According to the Ti–B–N ternary phase diagram [1], the compositions of these three layers in the TiBN coating are within the triphase field of TiN–TiB2–Ti2N close to the TiN– TiB2 quasi-biphase line. The cauliflower-like morphology on the surface and the amorphous-like structure on the cross section (Fig. 4) indicate the absence of the columnar crystal structure as in TiN binary coating [17]. The broadened peaks of TiN in the as-deposited TiBN coating, clearly visible in the XRD pattern, are the result of refined grains and/or
higher lattice strains, associated with the involvement of boron. With reference to the previous research on TiBN coatings [18–20], it is likely that dense nanocrystallites are present in the TiBN coating. During thermal treatments at 400–550 °C, the nano-sized crystallites in the unoxidised TiBN coating part survived without remarkable growth of the nanocrystallites, as confirmed by the SEM micrographs presented in Figs. 4–8 and XRD patterns (Fig. 9). It indicates that the TiBN coating indeed possesses good thermal stability over this temperature range for a long time, which is in agreement with the findings from the research on other TiBN coatings with high concentrations of boron [15–17,21] that the crystallite size and hardness of the TiBN coatings remained stable up to an annealing temperature of 1000 °C [16,22]. However, for the TiBN coating, high thermal stability does not necessarily correspond to high oxidation resistance. In the as-deposited TiBN coating studied, 20 at.% of oxygen was detected over a depth range from null to 70 nm (Fig. 2a). The large amount of oxygen in the very thin layer at the TiBN coating surface is consistent with the weak peak of TiO2 in the as-deposited TiBN coating at a 2-theta angle of 32° in the XRD pattern (Fig. 9). The presence of such a large amount of oxygen in the asdeposited TiBN coating is attributed to its reaction with oxygen during storage after PACVD. If the oxygen had been introduced due to the leakage of the PACVD equipment used or due to residual oxygen in the reactant gases, the concentration of oxygen in the coating would have
Fig. 7. SEM micrographs of the TiBN coating after thermal treatment at 500 °C for 4 h: (a) top view showing a pulverised surface and (b) cross-section view showing an oxidised layer with a thickness of 200 nm under the surface.
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Fig. 8. SEM micrographs of the TiBN coating after thermal treatment at 550 °C for 4 h: (a) top view showing a pulverised surface and (b) cross-section view showing an oxidised layer with a thickness of 600 nm and with cavities under the surface.
been at a constant level, for example, at 4 at.% over a depth range of 0.5– 3.5 µm in Fig. 2a. Furthermore, the possibility of the leakage of the rfGDOES equipment was excluded by analysis of other coatings, such as Cr7C3 + TiN coating, which showed a significantly smaller amount of oxygen across a depth of a hundred nanometers under the surface. At room temperature, the main phase TiN in the TiBN coating may react with oxygen in the ambient atmosphere, because of the following thermodynamically favourable reaction: TiN þ O2 ðgÞ→TiO2 þ 1=2N2 ðgÞ;
ΔG ¼ −576:95kJ=mol
ð1Þ
The change in the Gibbs free energy of the reaction ΔG was calculated from the thermochemical data [23]. From the kinetic point of view, the oxidising rate at room temperature must be very low and the diffusion of oxygen into the coating must be very slow, as the TiBN coating studied was kept for about one year after PACVD. Over a depth range of 0.5–3.0 µm in the as-deposited TiBN coating, about 3.5 at.%
oxygen might stem from residual oxygen in the deposition chamber. Such a small amount of oxygen was also found in other TiBN and TiN coatings [8,24,25]. XRD and rf-GDOES analyses confirmed the formation of TiO2 over a certain depth and disappearance of boron at an elevated temperature of 450 °C or higher. The loss of boron also occurred in other TiBN coatings at higher temperatures (above 600 °C) [8,15]. It is likely that the following reaction occurred as the temperature was increased to 450 °C in an oxidising atmosphere: TiBNðsÞ þ O2 ðgÞ→TiO2 ðsÞ þ N2 ðgÞ þ Bx Oy ðzÞ
ð2Þ
where boron oxide BxOy(z) may be of different types: B2O2(g), B2O3(l), B2O3(g), B2O3(s), B2O3(s, amorphous). These types of boron oxides are all possible products of the reaction, because the Gibbs free energy changes (ΔGθf ) are all very low at 450 °C, namely, −463.8, −1325.2, − 846.56, − 1351.0 and − 1326.0 kJ/mol, respectively, calculated
Fig. 9. XRD patterns of the TiBN coating in the as-deposited state and after thermal treatments at 450 and 500 °C for 4 h.
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according to the thermochemical data [23]. It was thought that boron in the TiBN coating might be prone to form B2O3 (l), B2O3 (s) and B2O3 (s, amorphous) due to their low values of ΔGfθ. The boron oxide of B2O3 (l), B2O3(s) and B2O3(s, amorphous) might be evaporated from the TiBN coating during thermal treatments at 450–550 °C, because of the lowmelting temperatures of both B2O3(s) and B2O3(s, amorphous) at 450– 460 °C. This explains the disappearance of boron in the oxidised layer, as found from the rf-GDOES analysis. The pulverised oxidised layer with microcracks and porosity might result from the disappearance of boron oxide. In addition, the nitrogen gas as a product of the reaction (2) and the entrapped trace element of chlorine escaped from the oxidised layer at elevated temperatures, which resulted in the low concentration of nitrogen and depletion of chlorine. The residual nitrogen in the oxidised layer might form the TiNxOy phase which was found by the X-ray photoelectron spectroscopy (XPS) of TiN and magnetron sputtering TiBN coatings when temperature exceeded 600 °C [8,26]. The rapid oxidation of the TiBN coating occurring at a temperature of 450 °C, as shown in Fig. 3, is partly in connection with the evaporation of boron oxide. Obviously, the oxidation rate must have been further accelerated at 500 and 550 °C, as the thickness of the oxidised layer rapidly increased. It may be farfetched to calculate the activation energy of oxidation according to the titanium oxide layer thickness, because it is clear that the thickness and microstructure of the TiBN coating have significant influences on the diffusion of oxygen and the thickness of the titanium oxide layer. This is ascertained by comparing the presently investigated PACVD TiBN coating (being 3 µm thick and containing 7.5 at.% boron), Fig. 2, with a much thicker titanium oxide layer (being 1 µm thick) formed in a thinner PACVD TiBN coating (being 1.5 µm thick and containing 7.5 at.% boron), Fig. 10, subjected to the same heat treatment at 500 °C for 4 h. It indicates that the diffusion of oxygen through the already grown titanium oxide layer in the PACVD TiBN coatings is not a rate-limiting process as in the case of TiN coating [27]. The pulverised titanium oxide in the oxidised TiBN coating was difficult to act as an effective barrier to the diffusion of oxygen, as can be seen in Figs. 7b and 8b. Over the temperature range of 400 to 600 °C, oxidation kinetics follows cubic rate law, depending on the dissolution and diffusion of oxygen and thus on temperature [28]. A pulverised TiO2 layer makes a large specific surface available for mass diffusion of oxygen such that oxygen can easily penetrate through the pulverised TiO2 layer and react with boron and titanium in the TiBN coating with a total depth of 3 µm. The results of the present research indicate that, in order to avoid further oxidation of the TiBN coating on the aluminium extrusion die, gas protection should be employed during preheating in a single-cell die furnace [14]. An alternative approach is to improve the resistance of the TiBN coating to oxidation. In recent years, the amorphous Si3N4 phase has been successfully introduced into TiN coatings [29,30], which significantly improves the resistance to oxidation [31]. It has been found that the formation of SiO2 in the TiN–Si3N4 coating plays a crucial role in hindering the diffusion of oxygen. The amorphous Si3N4 phase may also be introduced to the TiBN coating during the PACVD process to create a next generation TiBN coating with superhardness and enhanced resistance to oxidation. Another alternative is to add aluminium and silicon simultaneously to Ti–B–N to form Al–Si- and Brich oxide layers on the coating surface so as to prevent oxidation from taking place further [32].
Fig. 10. (a) rf-GDOES depth profile on the cross section of a thin PACVD TiBN coating (about 1.5 µm) after thermal treatment at 500 °C for 4 h, showing a thick titanium oxide layer (about 1.0 µm) and (b) microstructure of the titanium oxide layer on the cross section.
thickness of the oxidised layer (TiO2) increased from about 100 nm to 1.0 μm. (2) In the oxidised layer, boron vanished completely, because of the evaporation of low-melting point boron oxide formed. Nitrogen gas was another product of the oxidation reaction and escaped from the coating. Only TiO2 constituted the oxidised layer together with many microcracks and pores, exhibiting a pulverised morphology. (3) The pulverised TiO2 layer in the TiBN coating was difficult to act as an effective barrier to the diffusion of oxygen to hinder the progressive oxidation when temperature exceeded 450 °C. Gas protection during the preheating of TiBN-coated dies for aluminium extrusion and the development of a new-generation TiBN coating with enhanced oxidation resistance are recommended in order to secure the successful and extensive applications of TiBN coatings to extrusion dies.
5. Conclusions References (1) The nanocrystallite PACVD TiBN ternary coating with a gradient in the boron concentration over a range of 3–7 at.% exhibited good resistance to oxidation up to 400 °C. The critical temperature where rapid oxidation occurred was 450 °C. As temperature was further increased, oxidation was accelerated. At 500 °C, with the soaking time increasing from 2 to 16 h, the
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Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Oxidation behaviour of PACVD TiBN coating at elevated temperatures Y. He a, J. Zhou a,⁎, T. Walstock b, J. Duszczyk a a b
Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands Adex B.V., Tjalkkade 2, 5928 PZ Venlo, The Netherlands
a r t i c l e
i n f o
Article history: Received 12 May 2009 Accepted in revised form 27 August 2009 Available online 4 September 2009 Keywords: PACVD TiBN Oxidation Thermal treatment
a b s t r a c t The elevated-temperature oxidation behaviour of a TiBN coating on a plasma-nitrided hot-work tool steel (DIN 1.2367) by means of plasma-assisted chemical vapour deposition (PACVD) was investigated under the condition where a coated die would be preheated prior to being mounted on the press for aluminium extrusion. The TiBN coating was found to possess good resistance to oxidation up to 400 °C. Rapid oxidation started to occur at 450 °C. Radio frequency glow discharge optical emission spectroscopy (rf-GDOES) indicated that the oxidised layer was thickened from 100 nm to 1.0 μm, as the soaking time at 500 °C was prolonged from 2 to 16 h, which was attributed to the high temperature that promoted the penetration of oxygen into the coating. rf-GDOES also showed that boron initially in the coating vanished from the oxidised layer when the temperature was 450 °C or higher. X-ray diffractometry confirmed that the oxidised layer was composed mainly of TiO2. SEM revealed that the TiO2 layer was pulverised, leaving many microcracks and cavities, as a result of the losses of boron oxide and nitrogen. The rapid oxidation at above 450 °C was attributed to the pulverised TiO2 layer that was unable to hinder the diffusion of oxygen into the coating. It is therefore recommended to apply a protective gas during the preheating of the TiBN-coated die for aluminium extrusion. Alternatively, an advanced TiBN coating with enhanced resistance to oxidation must be developed, which will be conducive to its application for aluminium extrusion dies. © 2009 Elsevier B.V. All rights reserved.
1. Introduction TiBN ternary coatings are of great industrial value, considering their prominent mechanical and tribological properties, such as superhardness, low intrinsic stress and high wear resistance [1–3]. These coatings have been successfully applied to cutting tools, die-casting moulds, forging dies, aluminium extrusion dies, etc. Marked increases in lifespan and productivity have been achieved [4–7]. TiBN coatings developed so far have different compositions and structures in connection with a variety of processes employed, such as magnetron sputtering [8,9], physical vapour deposition (PVD), chemical vapour deposition (CVD) and plasma-assisted chemical vapour deposition (PACVD) [10–12]. For any of TiBN coatings, one of the important criteria for industrial application remains the same, i.e. thermal stability and oxidation resistance. For coated cutting tools and die-casting moulds, for example, the working temperature may reach 600–800 °C [4]. For coated aluminium extrusion dies, the preheating temperature is typically at about 500 °C, soaking time may be as long as 4–5 h, and in most cases no protective gas is applied [13,14]. Therefore, for these applications, the thermal stability and oxidation resistance of TiBN coatings will have a profound influence on the performance of the coated tools, moulds and dies during service.
⁎ Corresponding author. Tel.: +31 15 2785357; fax: +31 15 2786730. E-mail address: [email protected] (J. Zhou). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.08.039
In recent years, many investigations on the thermal stability of TiBN coatings have been carried out. It is known that TiBN coatings are thermally stable at a post-deposition annealing temperature higher than 900 °C and post-annealing hardness higher than that in the asdeposited condition may be achieved [15–17]. However, few research efforts have been made to evaluate the oxidation resistance of TiBN coatings at elevated temperatures. Lu et al. studied two TiBN coatings with a difference in chemical composition and found significant differences in oxidation behaviour over a temperature range of 600 to 1000 °C [8]. Héau et al. noticed that the mass gain of a TiNx(B)y coating started at 627 °C. Over a temperature range of 727 to 977 °C, the coating was completely oxidised to TiO2 and the mass remained almost unchanged [9]. There is a lack of understanding of the oxidation resistance of TiBN coatings at the typical temperatures to which they are exposed prior to or during service, typically at 400– 550 °C where, for example, a coated die for aluminium extrusion is preheated in an air furnace. The objective of the present research was to understand the oxidation behaviour of a PACVD TiBN coating with compositional gradients deposited on a hot-work tool steel during thermal treatments at 400–550 °C in the ambient surrounding that resembled the preheating condition of the coated die for aluminium extrusion. It was hoped that the understanding of the oxidation behaviour and oxidation mechanisms would be of use for making recommendations as to the need of applying protective atmosphere during the preheating of the coated die.
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2. Experimental details A TiBN ternary coating with compositional gradients was deposited on a DIN 1.2367 (X40CrMoV53) hot-work tool steel substrate using a PACVD-coating system with a bipolar DC voltage-controlled pulse generator. Prior to coating deposition, the steel substrate was subjected to a tempering heat treatment to achieve a hardness value of 50 HRC. The substrate was then plasma-nitrided in the PACVD equipment to create a nitrided diffusion layer with a thickness of about 120 μm without a compound layer. Plasma nitriding was performed in a mixture of H2, Ar and N2 under a total pressure of 300–600 Pa. Substrate temperature was kept at 400–530 °C so as to retain the hardness of the as-tempered tool steel. During coating deposition, a very thin TiN layer was first deposited for the purpose of enhancing adhesion to the substrate before TiBN coating deposition started. During subsequent coating, the BCl3 partial pressure was increased in order to increase the boron concentration in the coating toward the coating surface where high hardness and wear resistance were desired. Process parameters such as gas flow, wall heat, voltage, pulse-on and pulse-off times, and chamber pressure were closely controlled with a programmable logic controller (PLC). H2, Ar, N2, BCl3 and TiCl4 vapour were used as process gases for coating deposition. Pressure was kept at 70–150 Pa and substrate temperature at 530 °C in order to avoid exceeding the tempering temperature of the hot-work tool steel. The TiBN coating thickness was controlled at approximately 3 μm. To simulate the preheating condition of PACVD coated aluminium extrusion dies physically, TiBN-coated samples with a cubic shape and sizes of 8 × 8 × 8 mm were subjected to thermal treatments at 400–
550 °C for 2–16 h in the ambient atmosphere in a chamber furnace, followed by air cooling. The samples were cleaned using ethanol in an ultrasonic bath, prior to microstructure examination and chemical analysis. The surface morphology and cross-section microstructure of the TiBN coating in the as-deposited state were determined by using a Jeol JSM 6500F scanning electron microscope (SEM) for comparison with those after the thermal treatments. The phases and crystallographic structures of the as-deposited and as-oxidised coating were examined by means of a Bragg–Brentano X-ray diffractometer (XRD) where the Co Kα line at 0.179026 nm was used as the source for diffraction. Chemical composition profiles on the cross section of the coating in both the asdeposited and as-oxidised states were quantitatively analysed using LECO's GDS-750A radio frequency glow discharge optical emission spectroscopy (rf-GDOES). The sputtering diameter on the sample was 4 mm. RF-mode was run at 700 V plus 14 W as the true power and Ar pressure was typically 9 Torr (120 Pa). The sputter rate was 4–5 µg/s. Spectral wavelengths were as follows: Ti: 365.350, N: 149.262, B: 182.641, O: 130.217, C: 165.701, Cl: 134.724, Cr: 2425.433, and Fe: 2371.994 nm. 3. Results 3.1. Colour change The PACVD TiBN coating in the as-deposited condition was silvergolden in colour (Fig. 1a). After thermal treatment at 400 °C for 4 h, the colour remained essentially unchanged. However, after thermal
Fig. 1. Colour changes of the TiBN coating surface from (a) the initial as-deposited state as a result of thermal treatments at (b) 450, (c) 500 and (d) 550 °C for 4 h.
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exposure at raised temperatures, i.e. 450, 500 and 550 °C, the colour of the coating surface altered, as shown in Fig. 1, indicating oxidation to varying extents. After thermal treatment at 450 °C for 4 h, for example, the coating surface became reddish in some areas and bluish in other areas, while the original silver-golden colour was retained on the rest of
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the surface (Fig. 1b). As the temperature was increased to 500 °C, the silver-golden colour disappeared completely and the light blue-gray colour together with the light red colour characterised the coating surface (Fig. 1c). At 550 °C, the light blue colour dominated the coating surface (Fig. 1d). No thermal cracking, flaking or blistering occurred on
Fig. 2. Chemical composition profiles on the cross section of the TiBN coating determined by rf-GDOES: (a) in the as-deposited state and after thermal treatments at (b) 400, (c) 450, (d) 500 and (e) 550 °C for 4 h.
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the coating during the thermal treatments at these temperatures. The progressive changes of the coating colour with temperature indicate not only the onset temperature of rapid oxidisation but also the changes of oxidised layer thickness and the formation of different crystals of titanium oxides. 3.2. Compositional changes The as-deposited PACVD TiBN coating was composed of three layers with compositional gradients, as shown in Fig. 2a. The average concentration of titanium was about 50 at.% over the whole coating depth of about 3 µm. The concentration of nitrogen over the coating depth ranges of 0–1.0 µm, 1.0–2.0 µm and 2.0–3.0 µm step-wisely increased from about 36 at.% to 40 at.%, and to 42 at.%, respectively. In parallel with the changes of the nitrogen concentration, the concentration of boron decreased from 7 at.% to 5 at.%, and to 3 at.% over these depth ranges. In accordance with these compositional variations, the TiBN coating was composed of the mixtures of TiN0.72B0.14, TiN0.80B0.10 and TiN0.84B0.06 from the surface inwards. The observed compositional gradients of the TiBN coating were related to the gradual increase of the BCl3 gas pressure and corresponding decrease of the N2 gas pressure during the coating deposition process. By varying these partial pressures, leading to compositional gradients, a superhard surface layer with a relatively high boron concentration could be created and in the meantime the adhesion of TiN with the plasmanitrided substrate assured. In addition, about 20 at.% oxygen was present over a depth range from null to about 70 nm in the asdeposited coating. With increasing coating depth, the concentration of oxygen decreased to a level of about 3.5 at.% over a depth range of 0.5– 3.0 µm. Furthermore, about 2.5 at.% trace chlorine was retained in the as-deposited coating. rf-GDOES revealed an oxidised layer with a thickness of about 70 nm at the TiBN coating surface after the thermal treatment at 400 °C for 4 h, as shown in Fig. 2b. The oxygen profile over this depth was similar to that in the as-deposited coating without the thermal treatment (compare Fig. 2a and b). It indicates that the oxidised layer did not grow much during this thermal treatment. In the oxidised layer, the titanium and nitrogen concentrations both decreased and the boron concentration even decreased nearly to zero. After thermal treatment at 450 °C for 4 h, however, further compositional changes occurred (Fig. 2c). A titanium oxide layer formed over a coating depth range from null to about 250 nm, in which the oxygen concentration increased up to about 77 at.%, while the concentrations of titanium and nitrogen decreased to about 20 at.% and 2 at.%, respectively. Furthermore, boron vanished completely at a depth of 250 nm in the titanium oxide layer. Over the remaining coating depth ranging from 250 nm to the interface between the coating and substrate, the concentrations of all the elements remained at the same levels as in the as-deposited state and boron still displayed a gradient distribution. As the temperature was increased to 500 °C and even to 550 °C while the soaking time remained to be 4 h, the depth of the oxidised layer in the coating reached about 350 and 750 nm, respectively, as shown in Fig. 2d and e. In the oxidised layers, the concentration of oxygen increased to about 65–70 at.%, while the concentrations of titanium and nitrogen fell into the ranges of 15– 30 at.% and 5–15 at.%, respectively. As in the case of thermal treatment at 450 °C, boron completely escaped from the oxidised layer after thermal treatments at 500 and 550 °C. The trace element of chlorine also disappeared from the oxidised layer at these elevated temperatures. Fig. 3 presents the development of the oxidised layer thickness in the TiBN coating during thermal treatments in a temperature range of 400 to 550 °C with soaking time increasing from 2 to 16 h. rf-GDOES clearly showed good resistance of the TiBN coating to oxidation up to 400 °C. Higher temperature and long soaking time caused severer oxidation in the coating. The thickness of the oxidised layer at 450 and 500 °C for 2 h were 70 and 100 nm, respectively, being similar to that at 400 °C for 4 h, however, after 16 h the thickness became about 500 nm and 1.0 µm,
Fig. 3. Depth of the oxidised layer in the TiBN coating as a function of soaking time at 400, 450, 500 and 550 °C.
respectively. Moreover, the thickness of the oxidised layer formed at 550 °C for 2 h only was about 500 nm, and after 8 h it reached 1.2 µm, being nearly one half of the thickness of the coating (3 µm). Apparently, temperature played a more important role than soaking time in promoting the penetration of oxygen into the coating and allowing boron to escape. 3.3. Surface morphology and microstructural changes The morphology of the as-deposited TiBN coating surface exhibited a cauliflower-like pattern instead of the end of epitaxial growth of crystallites, as revealed by SEM and shown in Fig. 4a. SEM on the cross section (Fig. 4b) showed that the TiBN ternary coating did not consist of columnar crystals, as in the case of TiN or TiB2 binary coating. The columnless structure was assumed to consist of nanocrystallites according to the findings of other researchers in TiBN coatings [18–20]. Mayhofer et al. give a clear explanation about the mechanism of the change from columnar crystals to nanocrystallites when TiN coating is doped with a small amount of boron [17]. After thermal treatment at 450 °C for 4 h, the cauliflower-like surface morphology and nanocrystallite structural characteristics on the cross section remained essentially the same as those in the as-deposited state, as shown in Fig. 5a and b. However, when the soaking time at 450 °C was prolonged to 16 h, the cauliflower-like pattern on the coating surface was retained and many microcracks, as indicated by arrows in Fig. 6a, appeared as a result of oxidisation, while on the cross section (Fig. 6b) the coating still had almost the same morphology as in the as-deposited condition except a very thin oxidised layer under the surface. When the temperature was raised to 500 and even to 550 °C, the surface of the TiBN coating appeared to be pulverised, as shown in Figs. 7 and 8, while the cauliflower pattern was preserved. On the cross section, SEM revealed that the thickness of the oxidised layer under the surface increased and, moreover, the microstructure of this layer changed from dense nanocrystallites to being porous, as shown in Figs. 7b and 8b. Below the oxidised layer, the microstructure of the coating remained the same as in the as-deposited state, i.e. being composed of nanocrystallites. 3.4. Constituent changes Fig. 9 compares the XRD patterns of the coating in the as-deposited state and after thermal treatments at 450 and 500 °C for 4 h. In the asdeposited state, the peaks of TiN (200), (111), (220) and (311) in the TiBN coating appeared at the 2-theta angles of 50, 43, 73 and 89°, respectively. The peaks of TiB were hardly visible, probably because
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Fig. 4. SEM micrographs of the TiBN coating in the as-deposited state: (a) top view showing the cauliflower pattern and (b) cross-section view showing the nanocrystallite microstructure.
the amount of boron was very small (only about 7 at.%) and the intensities of TiB were too weak for reliable identification. It is important to mention that the intensity peaks of TiN in this TiBN ternary coating were all broadened in comparison with those of the same compound in TiN binary coating. This is believed to be associated with the presence of boron that is known for having a grain refining effect. Assuming the broadening to be due to crystallite size only, the mean size of the crystallites in the Ti(B)N coating was
calculated to be about 7.5 nm, using Scherrer's formula. The lattice parameter of TiN increased from a = 4.242 Å in the TiN binary coating to 4.264 Å in the TiBN ternary coating, a trend being in agreement with ab initio obtained values for NaCl structure TiBxN1 − x where B substitutes for N [21]. Obviously, the change of the lattice parameter is associated with the incorporation of boron in the TiN compound, but other factors such as defects or residual stresses may affect the lattice parameter. As the temperature was increased from 450 to 500 °C, the
Fig. 5. SEM micrographs of the TiBN coating after thermal treatment at 450 °C for 4 h: (a) top view showing the retained cauliflower pattern and (b) cross-section view showing a very thin oxidised layer under the surface.
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Fig. 6. SEM micrographs of the TiBN coating after thermal treatment at 450 °C for 16 h: (a) top view showing microcracks and (b) cross-section view showing a thin oxidised layer under the surface.
peak widths of TiN remained unchanged, but the peak intensities were weakened. This indicates that no crystallite growth or phase transformation occurred in the unoxidised TiBN coating part during the thermal treatment. A very weak intensity peak of titanium oxide in the TiBN coating in the as-deposited state appeared at a 2-theta angle of 32°. After thermal treatment at 450 °C for 4 h, however, noticeable peaks of TiO2 (rutile) appeared at the 2-theta angles of 44 and 64°. When the temperature was increased up to 500 °C, TiO2 peaks emerged at 32, 42, 48 and 64° over a 2-theta angle range from 30 to 130°. 4. Discussion The as-deposited TiBN ternary coating studied consists of three layers with a gradient of the boron concentration over a total depth of about 3 μm, as shown in Fig. 2a. The concentrations of boron in these three layers are all lower than 10 at.%. According to the Ti–B–N ternary phase diagram [1], the compositions of these three layers in the TiBN coating are within the triphase field of TiN–TiB2–Ti2N close to the TiN– TiB2 quasi-biphase line. The cauliflower-like morphology on the surface and the amorphous-like structure on the cross section (Fig. 4) indicate the absence of the columnar crystal structure as in TiN binary coating [17]. The broadened peaks of TiN in the as-deposited TiBN coating, clearly visible in the XRD pattern, are the result of refined grains and/or
higher lattice strains, associated with the involvement of boron. With reference to the previous research on TiBN coatings [18–20], it is likely that dense nanocrystallites are present in the TiBN coating. During thermal treatments at 400–550 °C, the nano-sized crystallites in the unoxidised TiBN coating part survived without remarkable growth of the nanocrystallites, as confirmed by the SEM micrographs presented in Figs. 4–8 and XRD patterns (Fig. 9). It indicates that the TiBN coating indeed possesses good thermal stability over this temperature range for a long time, which is in agreement with the findings from the research on other TiBN coatings with high concentrations of boron [15–17,21] that the crystallite size and hardness of the TiBN coatings remained stable up to an annealing temperature of 1000 °C [16,22]. However, for the TiBN coating, high thermal stability does not necessarily correspond to high oxidation resistance. In the as-deposited TiBN coating studied, 20 at.% of oxygen was detected over a depth range from null to 70 nm (Fig. 2a). The large amount of oxygen in the very thin layer at the TiBN coating surface is consistent with the weak peak of TiO2 in the as-deposited TiBN coating at a 2-theta angle of 32° in the XRD pattern (Fig. 9). The presence of such a large amount of oxygen in the asdeposited TiBN coating is attributed to its reaction with oxygen during storage after PACVD. If the oxygen had been introduced due to the leakage of the PACVD equipment used or due to residual oxygen in the reactant gases, the concentration of oxygen in the coating would have
Fig. 7. SEM micrographs of the TiBN coating after thermal treatment at 500 °C for 4 h: (a) top view showing a pulverised surface and (b) cross-section view showing an oxidised layer with a thickness of 200 nm under the surface.
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Fig. 8. SEM micrographs of the TiBN coating after thermal treatment at 550 °C for 4 h: (a) top view showing a pulverised surface and (b) cross-section view showing an oxidised layer with a thickness of 600 nm and with cavities under the surface.
been at a constant level, for example, at 4 at.% over a depth range of 0.5– 3.5 µm in Fig. 2a. Furthermore, the possibility of the leakage of the rfGDOES equipment was excluded by analysis of other coatings, such as Cr7C3 + TiN coating, which showed a significantly smaller amount of oxygen across a depth of a hundred nanometers under the surface. At room temperature, the main phase TiN in the TiBN coating may react with oxygen in the ambient atmosphere, because of the following thermodynamically favourable reaction: TiN þ O2 ðgÞ→TiO2 þ 1=2N2 ðgÞ;
ΔG ¼ −576:95kJ=mol
ð1Þ
The change in the Gibbs free energy of the reaction ΔG was calculated from the thermochemical data [23]. From the kinetic point of view, the oxidising rate at room temperature must be very low and the diffusion of oxygen into the coating must be very slow, as the TiBN coating studied was kept for about one year after PACVD. Over a depth range of 0.5–3.0 µm in the as-deposited TiBN coating, about 3.5 at.%
oxygen might stem from residual oxygen in the deposition chamber. Such a small amount of oxygen was also found in other TiBN and TiN coatings [8,24,25]. XRD and rf-GDOES analyses confirmed the formation of TiO2 over a certain depth and disappearance of boron at an elevated temperature of 450 °C or higher. The loss of boron also occurred in other TiBN coatings at higher temperatures (above 600 °C) [8,15]. It is likely that the following reaction occurred as the temperature was increased to 450 °C in an oxidising atmosphere: TiBNðsÞ þ O2 ðgÞ→TiO2 ðsÞ þ N2 ðgÞ þ Bx Oy ðzÞ
ð2Þ
where boron oxide BxOy(z) may be of different types: B2O2(g), B2O3(l), B2O3(g), B2O3(s), B2O3(s, amorphous). These types of boron oxides are all possible products of the reaction, because the Gibbs free energy changes (ΔGθf ) are all very low at 450 °C, namely, −463.8, −1325.2, − 846.56, − 1351.0 and − 1326.0 kJ/mol, respectively, calculated
Fig. 9. XRD patterns of the TiBN coating in the as-deposited state and after thermal treatments at 450 and 500 °C for 4 h.
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according to the thermochemical data [23]. It was thought that boron in the TiBN coating might be prone to form B2O3 (l), B2O3 (s) and B2O3 (s, amorphous) due to their low values of ΔGfθ. The boron oxide of B2O3 (l), B2O3(s) and B2O3(s, amorphous) might be evaporated from the TiBN coating during thermal treatments at 450–550 °C, because of the lowmelting temperatures of both B2O3(s) and B2O3(s, amorphous) at 450– 460 °C. This explains the disappearance of boron in the oxidised layer, as found from the rf-GDOES analysis. The pulverised oxidised layer with microcracks and porosity might result from the disappearance of boron oxide. In addition, the nitrogen gas as a product of the reaction (2) and the entrapped trace element of chlorine escaped from the oxidised layer at elevated temperatures, which resulted in the low concentration of nitrogen and depletion of chlorine. The residual nitrogen in the oxidised layer might form the TiNxOy phase which was found by the X-ray photoelectron spectroscopy (XPS) of TiN and magnetron sputtering TiBN coatings when temperature exceeded 600 °C [8,26]. The rapid oxidation of the TiBN coating occurring at a temperature of 450 °C, as shown in Fig. 3, is partly in connection with the evaporation of boron oxide. Obviously, the oxidation rate must have been further accelerated at 500 and 550 °C, as the thickness of the oxidised layer rapidly increased. It may be farfetched to calculate the activation energy of oxidation according to the titanium oxide layer thickness, because it is clear that the thickness and microstructure of the TiBN coating have significant influences on the diffusion of oxygen and the thickness of the titanium oxide layer. This is ascertained by comparing the presently investigated PACVD TiBN coating (being 3 µm thick and containing 7.5 at.% boron), Fig. 2, with a much thicker titanium oxide layer (being 1 µm thick) formed in a thinner PACVD TiBN coating (being 1.5 µm thick and containing 7.5 at.% boron), Fig. 10, subjected to the same heat treatment at 500 °C for 4 h. It indicates that the diffusion of oxygen through the already grown titanium oxide layer in the PACVD TiBN coatings is not a rate-limiting process as in the case of TiN coating [27]. The pulverised titanium oxide in the oxidised TiBN coating was difficult to act as an effective barrier to the diffusion of oxygen, as can be seen in Figs. 7b and 8b. Over the temperature range of 400 to 600 °C, oxidation kinetics follows cubic rate law, depending on the dissolution and diffusion of oxygen and thus on temperature [28]. A pulverised TiO2 layer makes a large specific surface available for mass diffusion of oxygen such that oxygen can easily penetrate through the pulverised TiO2 layer and react with boron and titanium in the TiBN coating with a total depth of 3 µm. The results of the present research indicate that, in order to avoid further oxidation of the TiBN coating on the aluminium extrusion die, gas protection should be employed during preheating in a single-cell die furnace [14]. An alternative approach is to improve the resistance of the TiBN coating to oxidation. In recent years, the amorphous Si3N4 phase has been successfully introduced into TiN coatings [29,30], which significantly improves the resistance to oxidation [31]. It has been found that the formation of SiO2 in the TiN–Si3N4 coating plays a crucial role in hindering the diffusion of oxygen. The amorphous Si3N4 phase may also be introduced to the TiBN coating during the PACVD process to create a next generation TiBN coating with superhardness and enhanced resistance to oxidation. Another alternative is to add aluminium and silicon simultaneously to Ti–B–N to form Al–Si- and Brich oxide layers on the coating surface so as to prevent oxidation from taking place further [32].
Fig. 10. (a) rf-GDOES depth profile on the cross section of a thin PACVD TiBN coating (about 1.5 µm) after thermal treatment at 500 °C for 4 h, showing a thick titanium oxide layer (about 1.0 µm) and (b) microstructure of the titanium oxide layer on the cross section.
thickness of the oxidised layer (TiO2) increased from about 100 nm to 1.0 μm. (2) In the oxidised layer, boron vanished completely, because of the evaporation of low-melting point boron oxide formed. Nitrogen gas was another product of the oxidation reaction and escaped from the coating. Only TiO2 constituted the oxidised layer together with many microcracks and pores, exhibiting a pulverised morphology. (3) The pulverised TiO2 layer in the TiBN coating was difficult to act as an effective barrier to the diffusion of oxygen to hinder the progressive oxidation when temperature exceeded 450 °C. Gas protection during the preheating of TiBN-coated dies for aluminium extrusion and the development of a new-generation TiBN coating with enhanced oxidation resistance are recommended in order to secure the successful and extensive applications of TiBN coatings to extrusion dies.
5. Conclusions References (1) The nanocrystallite PACVD TiBN ternary coating with a gradient in the boron concentration over a range of 3–7 at.% exhibited good resistance to oxidation up to 400 °C. The critical temperature where rapid oxidation occurred was 450 °C. As temperature was further increased, oxidation was accelerated. At 500 °C, with the soaking time increasing from 2 to 16 h, the
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