Effect of particle size on anti-oxidation property of mullite coating prepared by pulse arc discharge deposition

Abstract

Mullite coatings with different particle sizes were prepared on carbon/carbon (C/C)–SiC composites surface by pulse arc discharge deposition (PADD). Phase compositions and microstructures of the as-prepared mullite coatings were characterized by XRD and SEM. The influence of particle size on the microstructures and oxidation resistance of the as-coated samples were investigated. Results indicate that the dense and homogeneous mullite coating was achieved when the size of mullite powder is about 1 μm, which could effectively protect C/C composites from oxidation at 1773 K in air for 156 h with a mass loss of 1.98%.

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1. Introduction

Carbon/carbon (C/C) composites have been widely regarded as one of the most promising structural materials for high temperature applications, owing to its excellent properties like low density, high specific strength, high thermal conductivity and high resistance to thermal shock especially at elevated temperature.^1–3 Nevertheless, the applications of C/C composites are generally limited to inert atmospheres and vacuum environment due to their oxidation character.^4–6 Applying ceramic oxidation-resistant coating to C/C composites is considered as an efficient method to address this issue.^7,8

Mullite has great potential to protect C/C composites for its high melting point, good stability, lower oxygen permeability at high temperature and good erosion resistance. In addition, the good match of thermal expansion coefficient and good physical and chemical compatibility between mullite (4.4–5.6 × 10⁻⁶ per °C) and SiC (4.3–5.4 × 10⁻⁶ per °C) are another advantage for the adoption of mullite as outer coating material.^9–12 Such as mullite–gadolinium,¹³ mullite¹⁴ and C–AlPO₄–mullite.¹⁵ The particle size is an important factor in the oxidation of carbon/ceramic composites.¹⁶ As far as the authors know, few literature has been published about researches on the effect of particle size on the oxidation of ceramic coatings.

In the present work, a mullite/SiC coating was prepared on C/C composites by pulse arc discharge deposition. At first, the SiC as a bonding layer was prepared on the C/C composites by pack cementation. Then the mullite powders were deposited on the SiC–C/C matrixs. The influence of mullite particle size on the phase composition, micro-structure and high-temperature oxidation property of the as-prepared coating were mainly investigated.

2. Experimental

2.1 Coating preparation

Cubic samples (10 × 10 × 10 mm³) used as substrates were cut from bulk 2-D C/C composites with a density of nearly 1.7 g cm⁻³, which were hand-abraded with 300 grit SiC paper, cleaned ultrasonically with ethanol (the ultrasonic power was kept at 200 W) for 0.5 h. The above steps were repeated for three times. Subsequently, the samples were dried at 373 K for 2 h as the deposition specimens. The SiC coating was then prepared by pack cementation. Details regarding the preparation of the SiC coated samples (SiC–C/C) were reported in ref. 17.

In the pulse arc discharge deposition process, three particle size of mullite powders with the mass of 5.1 g (prepared by the above process) were dispersed in 170 mL isopropanol with an ultrasonic bath for 30 min (the ultrasonic power was kept at 200 W) with a later magnetic stirring for 24 h. Next, 0.45 g crystalline analytical reagent iodine (made in San Pu chemical factory in Xi'an, China) as charge agent was dissolved in the above suspension with an ultrasonic bath for 30 min (the ultrasonic power was kept at 200 W) followed by constant magnetic stirring for 24 h. Finally, the above suspension was transferred into a hydrothermal autoclave. The anode of the autoclaves was a graphite substrate (20 × 10 × 3 mm³) and the SiC–C/C substrate was fixed to the cathode of the autoclave. After being sealed, the autoclave was put into a furnace. The processing parameters of preparing specimens were shown in Table 1. Choose a different deposition time is to get the same thickness of the coatings. The specimens marked A, B and C by different particle sizes of mullite with the corresponding average particle size 0.85 μm, 5.11 μm, 9.28 μm respectively. The particle size distribution was analyzed by laser particle size analyzer. Fig. 1 shows SEM micrographs of three mullite powders and the corresponding particle size distribution. It reveals that the three mullite powders had uniform distribution of particle size approximately obeying the normal and the morphologies of the powders were irregular.

Table 1 The processing parameters of the as-deposited samples

Mullite powders no.	Particle size/μm	Deposition time/min	Pulse duty ratio	Deposition temperature/K	Deposition voltage/V	Sample mark
1#	0.85	20	50%	373	400	A
2#	5.11	25	50%	373	400	B
3#	9.28	30	50%	373	400	C

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Fig. 1 SEM micrographs of three mullite powders (a) 1#; (c) 2#; (e) 3# and the corresponding particle size distribution (b) 1#; (d) 2#; (f) 3#.

After deposition, the autoclave was taken out of the furnace and cooled down naturally to room temperature. Then, the samples were dried at 333 K in air for 4 h. Finally, the homogeneous mullite coating coated SiC–C/C composites were obtained.

2.2 Pulse arc discharge deposition system

Fig. 2 shows a schematic diagram of pulse arc discharge deposition system. The anode of the autoclave is a graphite substrate (20 mm × 10 mm × 3 mm) and the SiC–C/C substrate was fixed on the cathode of the autoclave. The suspension is used to deposit coating for deposition processes. The apparatus of PADD is same with hydrothermal electrophoretic deposition device except the power. Pulse power supply takes the place of DC power supply in hydrothermal electrophoretic deposition process, and periodic arc discharges take place in an asymmetrical electric field.

Fig. 2 The device diagram of pulse arc discharge deposition system: 1-dryer; 2-hydrothermal autoclave; 3-suspension; 4-cathode (sample); 5-anode (graphite substrate).

2.3 Characterization

The as-coated specimens were heated at 1773 K in air in an electrical furnace to investigate the isothermal and thermal cycle oxidation behavior. During the tests, the weight of the samples was measured at room temperature by electronic balance with a sensitivity of ±0.1 mg. The mass loss was calculated by eqn (1)–(3). The end mass loss is from average value of the group of five samples after oxidation at high temperature for a certain time. Where m₀ is the original mass of the coated C/C composites, g; m_t is the mass of the coated C/C composites after oxidation at high temperature for a certain time, g; t is the oxidation time, h and S is the surface area of the specimen, cm². ΔW% is the percentage of weight loss, ΔW is weight loss per unit area, g m⁻² and V_oxidation is the weight loss rate, g m⁻² h⁻¹.

The morphology, the crystalline structure, and the element composition of the multi-layer coatings were analyzed by a scanning electron microscope (SEM, JSM-6390A) with energy-dispersive spectroscopy (EDS) and a X-ray diffraction (XRD, Rigaku D/max-3C).

3. Results and discussion

3.1 XRD analysis and surface morphologies of the coating

Fig. 3 presents the surface XRD patterns of mullite coatings with different particle size. The outer coating is composed of mullite (JCPDS Card no. 88-2049) phase, which agrees with the original powders phase composition. In addition, the peak intensity of the coatings increases with the decrease of average particle size. This may be due to the fact that the smaller particle size is beneficial to the discharge and sintering from the cathode to anode, leading to the improvement in crystallization of the mullite coatings.

Fig. 3 Surface XRD patterns of mullite coatings on SiC–C/C composites prepared at different particle sizes: (A) 0.85 μm; (B) 5.11 μm; (C) 9.28 μm.

Fig. 4 displays the surface SEM images of the mullite coating with different particle sizes. Obviously, the coating surface of all the samples are composed of mullite crystallites with some microholes, but no cracks in the coating surfaces are observed. The surface of the coatings exhibits different morphology with different particle sizes. Loose coating is obtained when the average particle size of mullite is 9.28 μm (Fig. 4(c)), and the microcracks are detected in the coating surface. Inhomogeneous and porous mullite coating is prepared at 5.11 μm (Fig. 4(b)). When the particle size reaches 0.85 μm, very compact and homogeneous mullite coating is achieved (Fig. 4(a)). The as-prepared mullite coating porous structure consists of numbers of random-oriented and distribution-uniform mullite particles (top-right corner in Fig. 4(a)–(c)). Table 2 shows the relative density of various specimens. Clearly, the relative density of the specimens decreases from 90.2% to 78.6% with the increase of particle size. This may be attributed to the truth that the smaller particle size is profitable to the large deposition mass per unit time, recrystallization and growth of mullite crystalline in the arc discharge sintering process.

Fig. 4 Surface SEM images of the mullite coating prepared at different particle sizes: (a) 0.85 μm; (b) 5.11 μm; (c) 9.28 μm; inset: high-magnification SEM images of the part area.

Table 2 The relative density of the deposited coatings prepared at different particle sizes

Sample mark	Particle size/μm	Relative density/%
A	0.85	90.2
B	5.11	85.0
C	9.28	78.6

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Cross-section SEM images of the mullite coating with different particle sizes are shown in Fig. 5. An obvious two-layer structure without microholes or penetrative cracks is achieved, which may be due to the good match in thermal expansion coefficient between the SiC bonding layer and the mullite outer layer. Additionally, the mullite coating prepared at low temperature with the arc discharge sintering process. As the particle size decrease, the improvement in density of the mullite coating is apparent. The enhancement in coating compact with the decrease of particle size is observed. Besides, homogeneous and crack-free mullite coating is gained when average particle size is 0.85 μm, while the microcracks in the outer coating are easily found when the particle size is 9.28 μm, which may result from the bigger particle size leading to the generation of thermal stress.

Fig. 5 Cross-section SEM images of the mullite coatings prepared at different particle sizes: (a) 0.85 μm; (b) 5.11 μm; (c) 9.28 μm.

Fig. 6 displays the EDS element line scan analysis of the cross-section of the mullite/SiC multi-layer coating. It shows the concentration distributions of C, O, Al and Si elements along the coating cross direction. The element line scan analysis demonstrates that the multi-layer coating could be divided into three parts, designated as D, E and F. Part D is the C/C composites matrix infiltrated by Si, attributed to the pack cementation process. Part E is the SiC bonding layer, and part F is the mullite coating that agrees well with the experimental designation, XRD and SEM analyses.

Fig. 6 Cross-section EDS element line scan analyses of mullite/SiC multi-layer coating prepared at the particle sizes of 0.85 μm.

3.2 Oxidation test of the coated C/C composites

The isothermal oxidation curves of the SiC and mullite coating coated SiC–C/C composites with different particle size in air at 1773 K are shown in Fig. 7(a). After oxidation in air for 20 h at 1773 K, the mass loss of SiC–C/C composites is about 7.98%, which manifests that the single SiC coating cannot effectively protect C/C composites from oxidation for a long time. By contrast with it, the mullite coated SiC–C/C composites exhibit much better oxidation protective. The sample C can protect C/C composites just for 76 h due to the generation of microcracks in the outer coating according to the Fig. 5(c) and 4(c). The rate of mass loss decreases with the increase of homogeneity and density of mullite coating. Thus, the improvement of oxidation protective property is observed as particle size decreases to 0.85 μm. For the specimen A, the mass loss is only 1.98% after 156 h oxidation at 1773 K. The prime stage of isothermal oxidation curves (0–20 h) of the mullite–SiC–C/C samples with different particle sizes in air at 1773 K are shown in Fig. 7(b). Obviously, after 0.5 h oxidation test the samples exhibits different oxidation resistance. For the specimen A, the weight gain reaches 0.9%, while for the specimen C it is 1.21%. The weight gain of the specimens increases with the increases of particle size. This may be caused by bigger particle size with lower relative density (Table 2), which provides more channels for the oxygen diffused to the interface between the mullite and SiC. The formation of SiO₂ glass result in the sample gaining weight (eqn (4) and (5)).

2SiC(s) + 3O₂(g) → 2SiO₂(s) + 2CO(g) (4)

SiC(s) + 2O₂(g) → SiO₂(s) + CO₂(g) (5)

Fig. 7 (a) Isothermal oxidation curves of the SiC–C/C sample and the mullite–SiC–C/C samples prepared at different particle sizes in air at 1773 K; (b) isothermal oxidation curves of the mullite–SiC–C/C samples prepared at different particle sizes in air at 1773 K from 0–20 h.

Fig. 8(a) shows the weight loss rate curves of the coated samples during oxidation test. All the samples gain weight quickly at the primal stage of oxidation test (0–0.5 h), then the weight loss rate of the samples are almost constant. It can be found that the weight loss rate of the sample A is almost constant (4.49 × 10⁻⁴ kg m⁻² h⁻¹) after 22 h oxidation test. While the sample B and the sample C require 26 h and 35 h respectively, with the corresponding weight loss rate being 7.0 × 10⁻⁴ kg m⁻² h⁻¹ and 7.97 × 10⁻⁴ kg m⁻² h⁻¹, respectively (Fig. 8(b)).

Fig. 8 (a) Isothermal oxidation curves of the SiC–C/C sample and the mullite–SiC–C/C samples prepared at different particle sizes in air at 1773 K (the relationship between weight loss rate and oxidation time); (b) high-magnification image of the red area.

The oxidation behavior schematic of the samples with different particle sizes (a) sample A (b) sample B and (c) sample C were shown in Fig. 9. The sample A with a minor particle size will have fewer gaps (Fig. 8(a)), while the sample C has more gaps (Fig. 9(c)). The relative densities of the samples increase with the decreasing particle size (Table 2). The gaps will provide more channels for the oxygen diffused to the interface of SiC and mullite. The formation of SiO₂ glass will gradually seal the defects of the SiC inner layer and mullite outer layer. The sample A with higher relative density, the SiO₂ glass is easier to seal the defects. The sample will need lesser time to form a stable anti-oxidation coating (Fig. 8(b)).

Fig. 9 The oxidation behavior schematic of the samples prepared at different particle sizes: (a) 0.85 μm; (b) 5.11 μm; (c) 9.28 μm.

The results of the isothermal oxidation test in air at 1773 K (sample A) are shown in Fig. 10. After oxidation in air for 156 h, the weight loss of the coated C/C composites is only 7.02 × 10⁻² kg m⁻², whose corresponding weight loss rate is 4.5 × 10⁻⁴ kg m⁻² h⁻¹. According to the oxidation curve shown in Fig. 10, the oxidation behavior of the coated C/C composites could be divided into three stages marked as G, H and I. The corresponding oxidation kinetic equations are shown in eqn (6)–(8), among which t (h) is the oxidation time and ΔW (×10⁻² kg m⁻²) is weight loss per unit area.

ΔW = −3.92256 + 0.10537t + 0.00372t² (process A: 0 ≤ t ≤ 22 h) (6)

ΔW = −0.50192 + 0.0906t − 4.75477 × 10⁻⁴t² (process B: 22 ≤ t ≤ 92 h) (7)

ΔW = 1.812 + 0.00863t + 1.5625 × 10⁻⁴t² (process C: 92 ≤ t ≤ 156 h) (8)

Fig. 10 Isothermal oxidation curves of the mullite–SiC–C/C sample in air at 1773 K: (a) the relationship between weight loss and oxidation time; and (b) the relationship between weight loss rate and oxidation time.

At the primal stage of oxidation (0–22 h), the weight loss of the sample with time follows a parabolic law and the obvious weight loss of sample is detected (stage G). It is clear that the samples gain weight quickly to 1.09 × 10⁻² kg m⁻² during the initial 22 h oxidation. The sample gains weight due to the formation of SiO₂ glass. The SiO₂ glass as a buffering layer will gradually seal the defects of the SiC inner layer and mullite outer layer. From 22 to 92 h (stage H), the mullite outer layer can be gradually transformed into the glass layer (eqn (9) and (10)) and synchronously the formation of Al₂O₃ from the silicates glass occured. Consequently, the dense and smooth silicates glass can be completely formed (Fig. 10(a)), which may result in the best protection for the C/C matrix due to their low oxygen permeation constant and good self-cure ability (Fig. 10(b) and 11(a)). The average weight loss rate in this process keeps below 5.39 × 10⁻⁴ kg m⁻² h⁻¹, which manifests that the oxidation rate of the coated sample is controlled by the diffusion rate of oxygen along the silicates glass film. Above 92 h (process I), the weight loss increases linearly with time. This may be attributed to the volatilization of the glass film and lead to the decrease in coating thickness. As a result, oxygen can diffuse through the thin silicates glass and react with the C/C matrix as shown in eqn (11) and (12). These reactions may create microholes and microcracks (Fig. 11(c) and (d)) in the glass layer due to the escape of CO and CO₂. At the same time, micro-cracks will be generated due to the frequent thermal shocks from high temperature to room temperature in the isothermal oxidation tests. The as-generated micro-holes, micro-cracks and oxidation holes provide the channels for oxygen to attack the C/C matrix, which may lead to the decrease in oxidation resistance of the coated sample (Fig. 12(b)). The convincing reason for the formation of these holes needs further research.

SiO₂(s) + 3Al₂O₃·2SiO₂(s) → silicates glass(m) (9)

Silicates glass → Al₂O₃ + SiO₂ (10)

2C(s) + O₂(g) → 2CO(g) (11)

C(s) + O₂(g) → 2CO₂(g) (12)

Fig. 11 Surface SEM images of the mullite–SiC–C/C sample after oxidation at 1773 K in air for different hours: (a) 22 h; (b) 40 h; (c) 76 h; and (d) 156 h.

Fig. 12 Cross-section SEM images of the mullite–SiC–C/C sample after oxidation at 1773 K in air for different hours: (a) 40 h; (b) 156 h.

4. Conclusions

Based on this work, it can be concluded that a compact and homogeneous mullite coating for SiC–C/C composites was successfully prepared by a novel pulse arc discharge process at low temperature. The compactness and oxidation resistance of the as-prepared coating improve with the decrease of particle size from 9.28 μm to 0.85 μm. The mullite coating prepared at the particle size of 0.85 μm can effectively protect C/C composites from oxidation in air at 1773 K for 156 h with a weight loss of only 7.02 × 10⁻² kg m⁻², and a corresponding weight loss rate of 4.5 × 10⁻⁴ kg m⁻² h⁻¹. The failure of the multilayer coatings is attributed to the generation of micro-holes and micro-cracks. It may be caused by the escape of CO and CO₂ and the volatilization of silicate glass layer.

Acknowledgements

This work has been supported by the National Natural Science Foundation of China (51272146), the Fund of State Key Laboratory of Solidification Processing (NWPU), China (Grant No. SKLSP201301) and the Graduate Innovation Foundation of Shaanxi University of Science and Technology.

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