Oxidation behavior of Si₃N₄-TiN composites at 1400 ^oC

Abstract

In this paper, the oxidation behavior of silicon nitride with different contents of TiN was evaluated at 1400 ^oC for 64 hours in air. The oxidized samples were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. Weight gain measurements have shown that the oxidation followed a multiple-law model with linear, parabolic, and logarithmic contributions. The samples presented high weight gain at the beginning of the process followed by the formation of an amorphous silica surface layer containing Y₂Ti₂O₇ and rutile crystals. Cracks and holes were detected on the oxide layer. The oxidation resistance of the composites was strongly influenced by the initial content of TiN.

Keywords:
silicon nitride; titanium nitride; oxidation; microstructure

1. Introduction

Silicon nitride ceramics are widely used in structural applications owing to their low thermal expansion, high strength, hardness, and corrosion resistance¹,². However, the fragile behavior of this material, which can reduce the lifetime of the components, has led researchers to investigate alternatives for obtaining silicon nitride ceramics with high strength and fracture toughness. One of them is related to the development of silicon nitride composites containing a second reinforced phase, such as fibers, whiskers or particles to induce fracture toughness mechanisms³-⁶.

Although many different composites have been studied, those with a conductive phase are more attractive since they can improve the mechanical properties and machinability of the sintered components by electrical discharge machining (EDM). In this context, TiN as a second phase in silicon nitride ceramics has been highlighted in the last years, mainly because of its high electrical conductivity, hardness and strength. Moreover, the thermal expansion coefficient of TiN higher than that of Si₃N₄ tends to induce the formation of tensile and compressive stress around the TiN particles which improves the fracture toughness of the composite⁵,⁷-⁹.

Si₃N₄-TiN composites are employed in many high-temperature applications, such as heat exchangers, crucibles for molten metal, turbine and automotive engine components, and cutting tools¹⁰-¹³. On the other hand, Si₃N₄-TiN composites exhibit lower oxidation than pure silicon nitride ceramics. The process just starts at around 600 ^oC, due to the oxidation of TiN¹⁴, which is governed by a second-order reaction. As the temperature increases, many diffusional mechanisms take place simultaneously, becoming the oxidation more complex. Deschaux-Beaume et al.¹⁵,¹⁶ studying the oxidation of Si₃N₄-TiN composites between 1000–1200 ^oC observed that the process occurred in three steps. The first step was characterized by the oxidation of Si₃N₄ and TiN into SiO₂ and TiO₂ phases, respectively. In the second step, the oxygen diffusion through TiO₂ and SiO₂ already formed was related to controlling the TiN oxidation. In the third step, they associated oxidation of the TiN and Si₃N₄ to the oxygen diffusion through the silica layer. Moreover, Bracisiewic et al.¹⁷ found oxide scales and a multilayer microstructure under the oxide towards the bulk due to the oxidation of hot pressed Si₃N₄–35 vol.% TiN in air at 800, 1000 and 1200 ^oC for 100 h. In turn, considering oxidation of sintered Si₃N₄/TiN nanocomposites at 850 and 1150 °C for 50 h, Zou et al.⁵ concluded that the oxygen diffusion at 1150 °C toward the matrix accelerated the weight gain of the materials as a consequence of the pore and cracks formed between the oxidizing layer and matrix phase.

Another factor with a great role in the oxidation of Si₃N₄-TiN composites is the intergranular phase present in the grain boundary. For Feldhoff et al.¹⁸ this process is strongly dependent on the glass transition temperature of the intergranular glass since this phase affects the species dissolution and diffusional transport. According to Bracisiewics et al.¹⁷ and Bellosi et al.¹⁹, sintering aid cations in the intergranular phase tend to diffuse from the bulk to the reaction interface, forming glassy silicates and other oxidation products. Moreover, the crystallization of SiO₂ in cristobalite becomes the oxidation process even more complex.

In this paper, Si₃N₄-TiN composites with 5 to 30 wt.% TiN were exposed at 1400 ^oC for 64 hours in air to evaluate the oxidation behavior and the morphology of the formed oxide layer. The tests temperature was selected considering the limited number of oxidation studies of Si₃N₄-TiN composites in such condition and their potential to be used in applications at temperatures as high as 1400 C, as in turbine blade¹² and space technology²⁰.

2. Experimental Materials

α-Si₃N₄ powders (UBE, SN-E10), TiN powders (H.C. Starck), Al₂O₃ (CT 3000SG, Almatis) and Y₂O₃ (Sigma Aldrich) were used as raw materials. As shown in Table 1, four compositions of powders with 90Si₃N₄-5Y₂O₃-5Al₂O₃ (in wt.%), coded as SNYA, and TiN were mixed by ball milling using Al₂O₃ balls for 24 h in isopropanol and then dried using a rotary evaporator. The powders were uniaxially pressed at 50 MPa into cylindrical pellets (12×20 mm) followed by cold isostatic pressing at 250 MPa. The green bodies were then placed in a graphite crucible in a powder bed and sintered using a graphite resistance furnace (Thermal Technology) at 1750 °C under a nitrogen atmosphere.

Table 1
Studied compositions.

Designation	Composition
SNTN-5	SNYA+5 wt.% TiN
SNTN-10	SNYA+10 wt.% TiN
SNTN-20	SNYA+20 wt.% TiN
SNTN-30	SNYA+30 wt.% TiN

For the oxidation tests, the sintered samples were rectified in a diamond wheel and cleaned using acetone in an ultrasonic bath. After that, the specimens were placed in an alumina crucible with minimal contact area and inserted in a tubular furnace under air atmosphere at 1400 °C for 64 h. Every weight was measured after 1, 2, 4, 8, 16, 32 and 64 h of exposure while the microstructural characterization of the samples surface was carried out by scanning electron microscopy (SEM, Philips XL-30), energy dispersive X-ray spectroscopy (EDS, JEOL JSM6701F). One specimen of each composition mounted in epoxy resin was cut to observe the cross-section and the oxide layer thickness by SEM/EDS which was estimated using ImageJ²¹ as image analysis software. The oxidized surfaces also were analyzed by X-ray diffraction (XRD) using a Bruker D8 Advance X-ray diffractometer (Cu Kα radiation source operated at 40 kV and 40µÅ) in a range from 5 to 90^o (2θ) with step of 0.02^o (2θ) and 5s/pass. To do that, the pellets were previously fixed onto sample holders with modeling clay and then slightly pressed to adjust their height. To estimate the amount of crystalline phases, we used the RIR method²², while the amount of amorphous phase was estimated through the peak areas of the amorphous and crystalline phases. The software used for data acquisition and evaluation was Bruker including the PDF-2 database²³.

3. Results

Figure 1 shows the oxidation kinetic of Si₃N₄–TiN composites at 1400 °C. At the beginning of the process, the samples presented an increased weight gain which was more significantly for those ceramics with higher content of TiN. The influence of TiN on the oxidation behavior is also observed during the entire process since the overall weight gain was greater for SNTN-30, SNTN-20, SNTN-10 and SNTN-5 samples, respectively. This behavior can be attributed to the greater Ti affinity for oxygen than Si, which favors the oxidation of TiN, according to the reaction¹⁷:

Figure 1
Plots of weight gain versus exposure time of the Si₃N₄–TiN at 1400 °C in air.

$$TiN+O_2\to Ti{O}_2+\mathrm{½}{N}_2$$ (1)

In addition, the shape of the curves in Figure 1 is similar. They were fitted by the multiple-law model proposed by Nickel²⁴ (Equation 2) since the oxidation kinetics could not be described by a simple linear or parabolic law.

$\frac{\Delta m}{A}=k_{l}t+k_p\sqrt{t}+k_{log}\text{log}\left(t\right)$ (2)

Where $\frac{\Delta m}{A}$ corresponds to the mass change as a function of time (t) and k_l, k_p and k_log are linear, parabolic and logarithmic rate constants, respectively.

Table 2 summarizes the kinetic parameters (k_l, k_p and k_log) and fit goodness values (R²) for the fitted curves. While the parabolic term is related to the oxide formation on the samples’ surfaces, the linear parameter is negative due to the evaporation of species from the oxide scale, and the logarithmic term can be attributed to crystallization processes²⁴,²⁵. Although each rate constant has a physical meaning, there is interaction between the phenomena which also interfered with the constants values of each studied composition.

Table 2
Linear, parabolic and logarithmic contributions, and R² (fit goodness) values obtained from fitted oxidation curves.

Sample	kl (mg.cm-2.min-1)	kp (mg.cm-2.min-1/2)	klog (mg.cm-2)	R2
SNTN-05	-3.117x10-4	0.047	0.128	0.992
SNTN-10	-6.742x10-4	0.101	0.048	0.995
SNTN-20	-5.034x10-4	0.043	0.497	0.980
SNTN-30	-1.801x10-4	0.251	0.675	0.999

XRD patterns of the oxidized samples are shown in Figure 2. The presence of rutile (TiO₂) confirms that the reaction described in Equation 1 occurred on the surface of all samples. Cristobalite (SiO₂) and an amorphous phase with a broad band between 15º and 30º, characteristic of amorphous silica were also detected. These results are attributed to the Si₃N₄ oxidation which tends to form SiO₂ which reacts with yttrium and aluminum cations from the amorphous intergranular phase of the sintered composites. The formation of amorphous silica tends to create a protective coating on the samples’ surfaces, avoiding the diffusion of oxygen and reducing the oxidation rate of the composite. Moreover, the cristobalite is a consequence of the partial crystallization of the amorphous silica contributing to the logarithmic term in Equation 2.

Figure 2
XRD spectra of oxidized Si₃N₄-TiN samples after 64 h at 1400 °C.

The diffractograms in Figure 2 also show that the Y³⁺ ions, diffused from the intergranular phase of composites, reacted with TiO₂ formed during the oxidation process forming yttrium titanate (Y₂Ti₂O₇) during the tests. The presence of osbornite (TiN) and β-Si₃N₄ was also detected confirming the microstructure of the samples was preserved.

By XRD semi-quantitative analysis (Figure 3), we can observe that the prevalent phases on the surface of all samples are Y₂Ti₂O₇ and the amorphous phase. Moreover, the amount of Si₃N₄ and TiN on the oxidized surfaces tended to be decreased for samples with higher content of TiN in the initial composition, while the amount of TiO₂ was increased and that of cristobalite was found to be almost constant. These results are in good agreement with those of weight gain in Figure 1, demonstrating that the oxidation resistance of the composites is directly related to the reaction shown in Equation 1.

Figure 3
Evolution of the phases on the oxidized sample's surface with the content of TiN.

Figure 4 illustrates SEM-backscatter images of polished cross-sections of the oxidized samples with EDS maps for O, Si, and Ti. The Y and Al distributions could not be analyzed, as the low concentration of these elements in the samples did not make it possible to differentiate their peaks from the background. In general, the cross sections are formed by three areas, characterized by (i) the bulk, (ii) a sub-layer and (iii) an external layer. Although all analyzed elements are present in the entire area, the surface is formed by a grey phase containing silicon and oxygen, with Ti-rich crystals that appear in the SEM images as white structures. The sub-layer is extremely porous and formed basically by silicon and oxygen, while the bulk has titanium and silicon, as the main elements. Associating with XRD results, the external layer is probably the amorphous phase and/or cristobalite with Y₂Ti₂O₇ and/or TiO₂ crystals, as identified in Figure 2. Moreover, the sub-layer must be formed by cristobalite or amorphous phase, and the bulk is Si₃N₄ and TiN, suggesting that the inner region of the composite remained well kept up. Besides the porosity and number of Ti-rich crystals on the surfaces, the oxidized layer thickness tended to increase with the initial content of TiN on the samples, as shown in Table 3.

Figure 4
SEM images of cross-sectional oxidized samples and corresponding X-ray maps of oxygen, silicon, and titanium obtained by EDS.

Table 3
Oxide layer thickness on the samples oxidized at 1400^oC for 64 hours.

Sample	Thickness (µm)
SNTN-5	85.78 ± 10.70
SNTN-10	78.81 ± 16.62
SNTN-20	153.28 ± 24.30
SNTN-30	285.26 ± 31.44

These results are in line with those found by Bracisiewicz et al.¹⁷ in their study about the oxidation of Si₃N₄-TiN composites up to 1200 ^oC. In such work, the authors also identified a porous sub-layer rich in titania and silica, attributed to the titanium diffusion toward the surface. For them, with the porous sub-layer formation, the migration of titanium cations became limited which led to the oxidation process being controlled by oxygen diffusion through TiO₂ and porosity. In addition, oxidation of Si₃N₄ formed a silica glassy phase, which filled the porosity in the TiO₂ sub-layer formed. After that, oxidation of both TiN and Si₃N₄ phases is controlled by oxygen diffusion through the silica sub-layer¹⁶.

Figure 5 shows a backscattered electron image of a detailed area from the external layer of SNTN-20 coded sample. The points where EDS analysis was performed (Table 4) are also indicated. From the different contrasts produced by the difference between the atomic numbers of elements, it is possible to observe the presence of three phases characterized by grey crystals (Points 1 to 3), white crystals (Points 4 to 6) and the matrix (Points 7 and 8). Combining diffractograms in Figure 2 with the EDS semi-quantitative results in Table 4, probably the grey crystals (Points 1 to 3) correspond to rutile, while the white crystals are Y₂Ti₂O₇, and the matrix is cristobalite and an amorphous phase containing Si, Al, Ti, and O. Aluminum cations in the amorphous phase come from the intergranular phase of the sintered composites. Before the oxidation tests, aluminum cations are present in the intergranular phase of Si₃N₄-TiN composites as a result of cooling of the liquid phase during the sintering process due to the reaction between the additives (Al₂O₃ and Y₂O₃) with silica (SiO₂) on the surface of the Si₃N₄ particles, as described in our previous work²⁶.

Figure 5
Backscattered electron (BSE) image of the oxidized SNTN-20 sample cross section showing the locations of the EDS analysis points of Table 4.

Table 4
Content of elements from Point 1 to Point 8 (at.%) of Figure 5.

	N-K	O-K	Na-K	Al-K	Si-K	Ti-K	Y-L
Point 1		48.1		0.1	0.2	51.6
Point 2		46.6		0.2	0.3	53.0
Point 3	4.9	44.8		0.2	0.6	49.5
Point 4		44.9		0.3	0.0	31.5	23.4
Point 5		27.2		0.2		45.3	27.3
Point 6		72.1		0.2		15.6	12.1
Point 7		76.4		2.4	19.9	1.3
Point 8		70.4	1.4	2.9	23.6	1.7

The surface morphology backscattered electron images of the samples after oxidation are shown in Figure 6. The oxidized surface of all samples is formed by a grey matrix besides grey and white crystals also observed in Figure 5. However, the greater the amount of TiN in the initial compositions, the greater the number of crystals on the layer. Since the XRD semi-quantitative analysis (Figure 3) shows that the amount of rutile increased with the initial content of TiN, we can associate the increased number of crystals to the greater formation of rutile. As well as this, it is possible to notice many holes due to bubbles produced by gaseous products (such as N₂ and SiO) released during the oxidation process. The volatilization of these species leads to mass loss justifying the negative linear terms in Table 2.

Figure 6
Backscattered electron (BSE) images of the external layer of oxidized samples at different magnifications.

Cracks are also present on the oxide layer. They were probably initiated during the cooling after oxidation, as a result of tensile stresses created by a volume contraction due to the transformation of the high-temperature cristobalite phase to the low-temperature cristobalite phase²⁷. Both holes and cracks contributed to decline the oxidation resistance since they favor the oxygen diffusion through the formed oxide layer to the bulk of the material²⁸.

4. Conclusion

The oxidation behavior of Si₃N₄-TiN composites was investigated at 1400 ^oC for 64 hours in air. The oxidation curves were well fitted by a multiple-law model with linear, parabolic, and logarithmic terms. After 64 hours of exposition in air, it was formed on all samples' surfaces, a protective layer characterized by amorphous silica containing cristobalite, Y₂Ti₂O₇, and rutile crystals. Holes and cracks were also presented, contributing to accelerating the oxidation of the samples since they can provide channels for oxygen diffusion.

As a result of the great Ti affinity for oxygen, both weight gain and oxidized layer thickness increased with the initial content of TiN. Hence, although TiN additions may improve the mechanical behavior of silicon nitride ceramics, their concentration must be carefully selected to avoid high reductions in oxidation resistance of the final sintered composites.

5. Acknowledgments

The authors thank the Laboratory of Microscopy and Microanalysis of IPEN. This research was funded by Fundação de Amparo à Pesquisa do Estado de São Paulo, grant number 2015/02265-7.

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Publication Dates

• Publication in this collection
  13 Nov 2023
• Date of issue
  2023

History

• Received
  10 Apr 2023
• Reviewed
  16 Aug 2023
• Accepted
  12 Sept 2023