Materials Chemistry
High-Temperature Oxidation Behavior of 10 vol% AlN/Al2O3 Composites
2023 Volume 64 Issue 12 Pages 2776-2781
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2023 Volume 64 Issue 12 Pages 2776-2781
Alumina composites dispersed with 10 vol% AlN particles are prepared by commercial α-Al2O3 and AlN powder. The pulsed electric current sintering is used to densify the bulk samples. The high-temperature oxidation experiments are conducted at temperatures ranging from 1200 to 1350°C for 6, 12 and 24 h in the air environment. The internally oxidized zone (IOZ) is developed from the sample surface to consist of the Al2O3 matrix with small closed pores. The growth of IOZ obeys the parabolic law, which means the rate-controlling process is mass transport in IOZ. The high-temperature oxidation behavior of 10 vol% AlN/Al2O3 is compared with the 10 vol% Ni/Al2O3 composites. The differences in the microstructure at IOZ and the diffusion of oxygen through the Al2O3 matrix grain boundaries dependence on the oxygen partial pressure are discussed for understanding the high-temperature oxidation behavior of both composites.
Structural ceramics are the most popular materials in high-temperature applications because they can withstand high-temperature environments. Among the ceramic materials, alumina is well-known as a high-performance engineering ceramic because of its properties such as high hardness, good thermal and wear resistance, high melting point and good chemical stability.1) However, like other ceramic materials, alumina could suddenly fail from crack growth and propagation from stress concentration at the crack tips.2) Recently, engineering materials have been focused on improving the long-term integrity and reliability of the operating components.3) Self-healing in structural materials is a proposed concept to extend the lifetime and reliability of parts or materials. In alumina-based composites, the strength recovery occurred by crack-gap filling from the oxidation product of the dispersion phase or reinforcing phase, for example, SiO2 as the oxidation product in the SiC whisker/Al2O34,5,7) and SiC dispersed/Al2O3.6,8–12) The self-healing in alumina composites is reported with other non-oxide phases, Abe and co-authors reported NiAl/Al2O3 self-healing composites.13) The strength recovery in those alumina base composites was achieved after oxidation at approximately 1300°C.
Similarly, our group is interested in self-healing alumina-based composites; our group first reports Ni/Al2O3 self-healing composites, the surface crack disappearance is succeeded by covering NiAl2O4 as the oxidation product after oxidation at 1200°C for 6 h.14) The strength recovery after oxidation is reported as well, and the strength after oxidation is recovered at equal or higher as-sintered samples.15,16) The self-healing of Ni/Al2O3 composites is responsible for the outward diffusion of cations. Outward diffusion of Ni2+ ions result in the formation of NiAl2O4 as an oxidation product.17) Simultaneously, inward diffusion of O2− is promoted to the growth of the internally oxidized zone (IOZ), which follows the parabolic law.18) Moreover, doping elements such as Si,19,20) Y,20) or Sr21) affected high-temperature oxidation behavior by improving the oxidation resistance of Ni/Al2O3 composites.
In our previous research, Ni2+ in Ni/Al2O3 composites may affect the inward diffusion of O2− via the Al2O3 matrix. The IOZ in Ni/Al2O3 consists of NiAl2O4 grains dispersed in the Al2O3 matrix. The solubility of Ni2+ in the Al2O3 lattices is most likely at a very low level, which does not influence the diffusion of Al2O3. But Ni2+ ions are doped at grain boundaries of theAl2O3 matrix. Doping Ni2+ in the Al2O3 grain boundaries may affect grain boundary diffusion of O2− in the polycrystalline Al2O3. Consequently, Al2O3-based composites dispersed with AlN particles (AlN/Al2O3) were prepared to clarify the influences of Ni2+ to inward diffusion of O2− via the grain boundaries in the Al2O3 matrix. In this research, high-temperature oxidation of 10 vol% AlN/Al2O3 is investigated via the growth of IOZ. The differences in microstructure at the IOZ of both composites are discussed for the growth of IOZ. The effects of Ni2+ with inward diffusion of O2− was discussed by comparing the calculated diffusivity of oxygen through the Al2O3 grain boundaries dependence on the oxygen partial pressure of both composites via the mathematical model. The high-temperature oxidation behavior of 10 vol% AlN/Al2O3 is investigated and compared with that of 10 vol% Ni/Al2O3 composites.
The starting powder for making 10 vol% AlN/Al2O3 is commercial α-Al2O3 powder (Taimei Chemicals, TM-DAR, d = 0.14 µm) with 10 vol% AlN powder (Tokuyama corporation, d = 1 µm). Two powders were mixed in ethanol. A magnetic stirrer was used for 30 min to make the homogeneous powder mixture. Then, the powder mixture was wet-ball milling by using alumina grinding media in ethanol for 24 h to reduce the powder agglomeration. After wet-ball milling, the powder mixture was dried at 60°C for 6 h using a hot plate.
The powder mixture was ground by an alumina mortar for deagglomeration. The sintering was conducted using pulsed electric current sintering at temperature 1400°C for 5 min in holding time at the vacuum environment under applied uniaxial pressure of 50 MPa. The disk shape sintered sample measured the density after sintering using Archimedes’ principle. All sintered samples can attain at least 99% of their theoretical density. After that, the bulk disk shape samples were cut into small semi-circular shapes using a diamond-refine saw; one side of the sample surface was polished into a mirror surface with diamond polishing liquid. The high-temperature oxidation was conducted at temperatures ranging from 1200 to 1350°C for 6 to 24 h in air. The samples were placed on zirconia balls in an alumina crucible, and the polished side of the sample was exposed to the atmosphere. As-oxidized samples were cut into the cross-section surface; the high-temperature oxidation was investigated via the growth of IOZ. The thickness IOZ was evaluated using a scanning electron microscope (SEM) to observe the cross-section surface after the oxidation experiments. The phase identification before and after oxidation experiments was specified by X-ray diffraction (XRD).
Figure 1 shows the microstructure of sintered 10 vol% AlN/Al2O3 from the fractured surface. The average Al2O3 grain size after sintering is approximately 1 µm. The XRD pattern from Fig. 2 indicates that only 2 phases of α-Al2O3 and AlN are identified. Regarding the AlN–Al2O3 phase diagram, no AlON phase appeared at the sintering temperature.22) The sample oxidized at 1200°C for 6 h shows only the α-Al2O3 phase in the XRD result. Figure 3 shows the cross-section of samples after oxidation at (a) 1200°C for 24 h, (b) 1300°C for 24 h and (c) 1350°C for 24 h. The SEM images in Fig. 3 show that the cross-section surface can be divided into three different layers. The outer surface as shown in a bright color is the surface of a sample consisting of Al2O3, which is the oxidation product of AlN, as follows:23)
| \begin{equation} \text{2 AlN} + \frac{3}{2}\text{O$_{2}$} \rightarrow \text{Al$_{2}$O$_{3}$} + \text{N$_{2}$} \end{equation} | (1) |
The next zone is the internally-oxidized zone (IOZ), which consists of Al2O3 matrix and Al2O3 oxidation product of AlN. Many pores have appeared at the IOZ. The last zone under the IOZ is the non-oxidized zone, which lies under the IOZ. The thickness of IOZ is measured from the sample’s surface until the interphase of IOZ and non-oxidized zone, shown as the dashed line in Fig. 3 (oxidation front). Figure 4 shows the thickness of IOZ plotted as a function of oxidation time at various temperatures. The results indicated that the thickness of IOZ was increased with increasing heat-treatment temperature and time. The growth of IOZ followed the parabolic law:
| \begin{equation} x^{2} = k_{p}t \end{equation} | (2) |
where x is the thickness of IOZ, kp is the parabolic rate constant and t is oxidation time. After that, the values of kp at different temperatures are plotted with the reciprocal temperature for high-temperature oxidation in Fig. 5. Based on the straight-line slope, the apparent activation energy (Q) for the growth of IOZ of 10 vol% AlN/Al2O3 composites equals 319 kJ mol−1.
Fractured surface after sintering of 10 vol% AlN/Al2O3 composites.
XRD pattern of 10 vol% AlN/Al2O3 before and after oxidation at 1200°C for 6 h.
SEM images of cross-section surface of 10 vol% AlN/Al2O3 after oxidation for 24 h at (a) 1200°C, (b) 1300°C and (c) 1350°C.
Thickness of IOZ as a function of oxidation time at various oxidation temperatures.
Temperature dependence of parabolic rate constant of 10 vol% AlN/Al2O3 with the values of 10 vol% Ni/Al2O3 composites.24)
The microstructure of 10 vol% AlN/Al2O3 after high-temperature oxidation is observed by SEM. Figure 6(a), (b) and (c) shows the fractured surface of 10 vol% AlN/Al2O3, microstructure at IOZ and the non-oxidized zone after oxidation at 1350°C for 12 h, respectively. The average grain size of Al2O3 in IOZ is approximately 1.1 µm, while the non-oxidized zone is approximately 1.7 µm.
Fractured surface after oxidation at 1350°C for 12 h with their microstructure at IOZ and non-oxidized zone respectively, (a)–(c) for 10 vol% AlN/Al2O3 and (d)–(f) for 10 vol% Ni/Al2O3 composites.
As shown in Fig. 3, many closed pores appear at the IOZ. The formation of these pores is evidence of outward Al3+ diffusion during oxidation. The kp values of 10 vol% AlN/Al2O3 are compared with the 10 vol% Ni/Al2O3 (dash line)24) at the same oxidation temperatures in Fig. 5, the values of kp of 10 vol% AlN/Al2O3 are slightly higher than in 10 vol% Ni/Al2O3 at all oxidation temperatures. The slight difference in the kp values is discussed here. From the kinetic aspect, at high temperatures, the development of the Al2O3 oxidation product of AlN is contributed by the outward diffusion of cations through the grain boundary of the Al2O3 matrix. On the other hand, the growth of IOZ is promoted by the inward diffusion of oxygen ions via the grain boundaries of the Al2O3 matrix.17) Oxygen ions were diffused from high oxygen potential at the sample surface ($P_{\text{O}_{2}}^{1}$) to low oxygen potential at the oxidation front ($P_{\text{O}_{2}}^{2}$). Figure 7 shows the schematic illustration of mass transport in AlN/Al2O3 and the SEM image of surface morphology after high-temperature oxidation. The Al2O3 oxidation product of AlN formed a network-like structure at Al2O3 matrix grain boundaries on the sample surface. This network structure is strong evidence of Al3+ outward diffusion, similar to the Ni2+ outward diffusion in Ni/Al2O3 composites.15) The growth of IOZ follows the parabolic law; it is indicated that the inward diffusion of oxygen ions through Al2O3 matrix grain boundaries controls the reaction rate.25) From the transport of oxygen ions during high-temperature oxidation, the Al2O3 grain boundaries at the IOZ are indicated as a diffusion path for oxygen ions. The microstructure at the IOZ of both composites should be considered for the growth of IOZ. Figure 6 shows the fractured surface of 10 vol% AlN/Al2O3 composites (a, b and c) and 10 vol% Ni/Al2O3 composites (d, e and f) after oxidation at 1350°C for 12 h. The average grain size of 10 vol% Ni/Al2O3 composites after oxidation at 1350°C for 12 h shows the difference in grain size with the 10 vol% AlN/Al2O3 composites, Fig. 6(d), (e) and (f) show the fractured surface of 10 vol% Ni/Al2O3, microstructure at IOZ and the non-oxidized zone after oxidation at 1350°C for 12 h, respectively. The SEM images show that the grain size at the IOZ is larger than in the non-oxidized zone. The average particle size at the IOZ is 1.7 µm and the non-oxidized zone is 0.9 µm. Figure 8 compares grain size at IOZ and the non-oxidized zone of both composites after oxidation for 12 h at various oxidation temperatures. The graph shows that the grain size of the two composites increased with increasing oxidation temperature at both the IOZ and non-oxidized zone. Considering the grain size of both composites at IOZ, 10 vol% Ni/Al2O3 has a bigger grain size than 10 vol% AlN/Al2O3 because there is an enlargement of grain size during the high-temperature oxidation stage. The grain growth after high-temperature oxidation in Ni/Al2O3 composites is reported by Pham et al.; Ni particles acted as a grain growth inhibitor for the Al2O3 matrix, outward diffusion of cations in IOZ induced decrease in Ni concentration during the high-temperature oxidation stage.26) After that, grain growth occurs at the IOZ. Nevertheless, 10 vol% AlN/Al2O3 shows an opposites phenomenon by grain size at the IOZ is smaller than in the non-oxidized zone, the smaller Al2O3 grain size at the IOZ of 10 vol% AlN/Al2O3 may occur from the AlN particles being oxidized and forming small grains of Al2O3 oxidation product of AlN within the IOZ. The finer IOZ grain size in 10 vol% AlN/Al2O3 composites accelerated the growth of IOZ by showing little higher kp values.
Illustrates mass transport in AlN/Al2O3 high-temperature oxidation and SEM surface morphology after high-temperature oxidation.
Grain size at the IOZ and non-oxidized zone of 10 vol% AlN/Al2O3 and 10 vol% Ni/Al2O3 after oxidation at various temperatures.
The parabolic rate constant is also affected by the diffusivity of oxygen ions through the grain boundary of the Al2O3 matrix. Hence, the effective diffusion coefficient is considered for the difference of the parabolic rate constant for the growth of IOZ. From the diffusion in polycrystalline materials, the effective diffusion coefficient is defined by the diffusion of ions in the material. In the case of a matrix with fine grain size, oxygen ions are almost diffused through the grain boundaries of the matrix (diffusivity through the grain boundary ≫ lattice diffusion). The effective diffusion coefficient of oxygen ions is dominated by the oxygen diffusivity through the grain boundary of the Al2O3 matrix as follows:27)
| \begin{equation} D_{\text{O}} = D_{gb}\frac{\delta}{d} \end{equation} | (3) |
where DO is the diffusivity of oxygen, Dgb is the diffusivity of oxygen ions through the grain boundaries of the Al2O3 matrix, δ is the Al2O3 grain boundary width and d is the average grain size of the Al2O3 matrix. From our recent work, the kinetic model was proposed for determining the growth of IOZ and diffusivity of oxygen ion through the grain boundaries of Al2O3 matrix in Ni/Al2O3 composite with different Ni concentrations as follows:24)
| \begin{equation} x^{2} = V_{\text{Al${_{2}}$O${_{3}}$}}\frac{D_{gb}\delta C_{\text{O}}}{dfv_{\text{Ni}}}\mathit{ln}\left(\frac{P_{\text{O${_{2}}$}}^{1}}{P_{\text{O${_{2}}$}}^{2}} \right)t = k_{p}t \end{equation} | (4) |
where $V_{\text{Al}_{2}\text{O}_{3}}$ is the molar volume of Al2O3, fvNi is Ni volume fraction, CO is the molar concentration of oxygen, $P_{\text{O}_{2}}^{1}$ is the partial pressure of oxygen at the sample surface and $P_{\text{O}_{2}}^{2}$ is the partial pressure of oxygen at the interphase of IOZ and non-oxidized zone (oxidation front). Moreover, because oxygen partial pressure at the oxidation front in AlN/Al2O3 equilibrium is quite low at approximately 10−20 atm at 1300°C, the diffusivity of oxygen ion as a function of oxygen partial pressure is applied to the equation. Kitaoka et al. studied the kinetics of oxygen permeation through the Al2O3 wafer via the oxygen vacancies following this defect reaction.28)
| \begin{equation} \text{1/2 O$_{2}$} + \text{V$_{\ddot{\text{O}}}$} + 2\acute{\text{e}} \to \text{O$_{\text{O}}^{\text{x}}$} \end{equation} | (5) |
Then the equilibrium constant for eq. (5) could be defined as follows;
| \begin{equation} k = \cfrac{[\text{O$_{\text{O}}^{\text{x}}$}]}{\biggl(\cfrac{P_{\text{O${_{2}}$}}}{P^{\circ}} \biggr)^{(1/2)}[\text{V$_{\ddot{\text{O}}}$}][\acute{\text{e}}]^{2}} \end{equation} | (6) |
where $\text{V$_{\ddot{\text{O}}}$}$ means oxygen vacancy, é an electron, k the equilibrium constant for the defect reaction $P_{\text{O}_{2}}$ oxygen partial pressure, P° the pressure of the standard state and [i] a concentration of species i. Give $[\acute{\text{e}}] = [\text{V$_{\ddot{\text{O}}}$}]/2$ then substitute to eq. (6) will give;
| \begin{equation} k = \frac{1}{4[\text{V$_{\ddot{\text{O}}}$}]^{3}}\left(\frac{P_{\text{O${_{2}}$}}}{P^{\circ}} \right)^{(-1/2)} \end{equation} | (7) |
From eq. (7), the concentration of oxygen vacancies could be defined as follows;
| \begin{equation} [\text{V$_{\ddot{\text{O}}}$}] = \left(\frac{1}{4k} \right)^{(1/3)}\left(\frac{P_{\text{O${_{2}}$}}}{P^{\circ}} \right)^{(-1/6)} \end{equation} | (8) |
The flux balance for oxygen ions and its vacancies could be established as follows;
| \begin{equation} D_{\text{O}}C_{\text{O}} = D_{\text{V${_{\ddot{\text{O}}}}$}}C_{\text{V${_{\ddot{\text{O}}}}$}} \end{equation} | (9) |
In ceramic materials, the concentration of vacancies is relatively low, then $D_{\text{V}_{\ddot{\text{O}}}}$ and CO are defined as constants. The diffusion coefficient of oxygen under fixed oxygen partial pressure is dependent with $[\text{V}_{\ddot{\text{O}}}] \propto P_{\text{O}_{2}}^{( - 1/6)}$, the diffusion coefficient as a function with oxygen partial pressure could be obtained as follow;29)
| \begin{equation} D_{\text{O}} = D_{\text{V${_{\ddot{\text{O}}}}$}}\frac{C_{\text{V${_{\ddot{\text{O}}}}$}}}{C_{\text{O}}} = D_{\text{V${_{\ddot{\text{O}}}}$}}[\text{V$_{\ddot{\text{O}}}$}] = D_{\text{O}}^{\circ}\left(\frac{P_{\text{O${_{2}}$}}}{P^{\circ}} \right)^{(-1/6)} \end{equation} | (10) |
where Di is the diffusivity of species i, Ci is the molar concentration of species i and DO and $D_{\text{O}}^{\circ}$ are the diffusivity of oxygen and the diffusivity of oxygen ions at $P_{\text{O}_{2}}$ equal 1 atm, respectively. From eq. (1), 3 mol of oxygen ions are diffused in the Al2O3 matrix to react with the AlN particle for forming 1 mol of Al2O3 oxidation product combining the relationship from eq. (1), the effective diffusion coefficient in eq. (3) our previous work model in eq. (4) and the diffusivity of oxygen as a function with oxygen partial pressure in eq. (10), the equation to obtain the coefficient of oxygen grain boundary diffusion in Al2O3 matrix is modified into
| \begin{equation} \delta D_{gb}^{\circ} = k_{p}\frac{fv_{\text{AlN}}d}{2V_{\text{Al${_{2}}$O${_{3}}$}}C_{\text{O}}}\cfrac{1}{\Biggl[\biggl(\cfrac{P_{\text{O${_{2}}$}}^{2}}{P^{\circ}} \biggr)^{(-1/6)} - \biggl(\cfrac{P_{\text{O${_{2}}$}}^{1}}{P^{\circ}} \biggr)^{(-1/6)} \Biggr]} \end{equation} | (11) |
where $D_{gb}^{\circ} $ is the diffusivity of oxygen ions through the grain boundaries of the Al2O3 matrix at $P_{\text{O}_{2}} = 1$ atm, fvAlN is the AlN volume fraction. Assuming the grain boundary width of Al2O3 equals 0.5 nm (δ = 0.5 nm),30) the values of $D_{gb}^{\circ} $ were calculated by using the kp values from the oxidation experiments. The $D_{gb}^{\circ} $ values of 10 vol% AlN/Al2O3 are calculated and compared with the values from 10 vol% Ni/Al2O3, as shown in Fig. 9. The calculated $D_{gb}^{\circ} $ values of 10 vol% Ni/Al2O3 indicated higher values than 10 vol% AlN/Al2O3, the apparent activation energies for oxygen grain boundary diffusion of 10 vol% AlN/Al2O3 and 10 vol% Ni/Al2O3 are estimated at 449 and 554 kJ mol−1, respectively. As reported by numerous research, the alteration of mass transport in polycrystalline Al2O3 occurred by segregation of doping elements at the Al2O3 grain boundaries, which changed the diffusivity of outward cations diffusion or inward oxygen ions diffusion.31–34) Considering the characteristic of matrix grain boundaries at the IOZ in both composites, the Al2O3 grain boundaries in 10 vol% Ni/Al2O3 are regarded to be doped with Ni2+. On the other hand, in 10 vol% AlN/Al2O3 composites, the grain boundaries of the Al2O3 matrix can be regarded as “un-doped”. The increase in the $D_{gb}^{\circ} $ values of 10 vol% Ni/Al2O3 tend to be influenced from the Ni2+-doped Al2O3 grain boundaries.35,36)
Coefficient of oxygen ion through alumina grain boundaries diffusion at $P_{O_{2}} = 1$ atm as a function of the reciprocal temperature of 10 vol% AlN/Al2O3 and 10 vol% Ni/Al2O3.24)
The growth of IOZ in 10 vol% Ni/Al2O3 was accelerated by the higher diffusivity of oxygen ions through the Al2O3 grain boundary ($D_{gb}^{\circ} $), the activation energy for the growth of IOZ indicated 399 kJ mol−1, as shown in Fig. 5. Regarding the point of view on high-temperature oxidation, the smaller grain size at IOZ in 10 vol% AlN/Al2O3 and higher $D_{gb}^{\circ} $ in 10 vol% Ni/Al2O3 promoted the growth of IOZ and made two composites that had a similar magnitude of kp values. The difference in the magnitude of apparent activation energy for the growth of IOZ at least two or three times indicates the difference in oxidation behavior. However, the apparent activation energy of 10 vol% AlN/Al2O3 and 10 vol% Ni/Al2O3 is shown to be similar in magnitude at approximately 400 kJ mol−1, the two composites implied similar high-temperature oxidation behavior.
The high-temperature oxidation behavior of 10 vol% AlN/Al2O3 is investigated at a temperature ranging from 1200 to 1350°C for 6, 12 and 24 h. The internally oxidized zone (IOZ) is developed from the sample surface to consist of the Al2O3 matrix with small closed pores. The growth of IOZ follows the parabolic law; the apparent activation energy for the growth of IOZ was approximately 319 kJ mol−1. The oxidation behavior of 10 vol% AlN/Al2O3 is compared to the 10 vol% Ni/Al2O3 in previous research. The finer grain size at the IOZ accelerated the growth of IOZ of 10 vol% AlN/Al2O3. The apparent activation energies for diffusivity of oxygen ions through alumina grain boundary in both composites show at about 449 and 554 kJ mol−1 for 10 vol% AlN/Al2O3 and 10 vol% Ni/Al2O3, respectively. Ni2+-doped Al2O3 grain boundaries in 10 vol% Ni/Al2O3 accelerated the oxygen grain boundaries diffusion ($D_{gb}^{\circ} $), which promoted the faster growth of IOZ as well. The comparable activation energy for the growth of IOZ led to similar high-temperature oxidation behavior of both composites.
