Abstract
The effect of 1800°C annealing on the oxidation behaviour of silicon carbide ceramics derived from polycarbosilane (PCS) was investigated by thermogravimetric analysis when the annealing atmosphere was Ar. These results indicated that the silicon carbide ceramics before and after 1800°C annealing both started to oxidise near 600°C. However, the oxidation behaviours of the silicon carbide ceramics were remarkably different above 600°C due to the microstructure changes during 1800°C annealing. The silicon carbide phase and the free carbon phase were almost oxidised together in the silicon carbide ceramics derived from PCS at 1200°C, while the free carbon phase had been mostly oxidised before the silicon carbide phase began to oxidise in the 1800°C annealing silicon carbide ceramics. Moreover, after oxidation, there were many nanopores present in the skeletons of the silicon carbide ceramics with 1800°C annealing.
Introduction
Silicon carbide ceramics are considered as structural materials for high temperature applications because of their excellent thermomechanical properties.1 In addition to conventional processing, a potential fabrication route for silicon carbide ceramics is pyrolysing polycarbosilane (PCS) at ∼1000°C in an inert atmosphere.2 – 6
Structural materials for high temperature applications in oxygen containing environments also must embody excellent resistances to oxidation. Hence, the oxidation behaviour is one of the subjects for silicon carbide ceramics.7 About the oxidation of silicon carbide ceramics, it is known that the protective silicon dioxide layer will be formed during the oxidation at high temperatures and high oxygen partial pressures.8 However, the silicon carbide ceramics derived from PCS are almost amorphous and not stoichiometric (an amount of excess carbon, called as the free carbon).3,5 When they are annealed in high temperature circumstances, the evolutions of compositions and microstructures are remarkable.5,6 Therefore, the oxidation behaviours of silicon carbide ceramics will be changed.
In this paper, we reported the effect of 1800°C annealing on the oxidation behaviour of silicon carbide ceramics derived from PCS.
Experimental
Substrate material
PCS (National University of Defense Technology, China) with molecular weight 1742 and softening point 175°C was used as precursor to silicon carbide ceramics. First, it was pyrolysed at 1200°C for 1 h when high purity nitrogen was the protective atmosphere. The ceramic products were called raw samples. Second, the raw samples were annealed at 1800°C under Ar for 1 h (called as the treated samples). Finally, the samples of the silicon carbide ceramics were all oxidised in air.
Analytical methods
Elementary analyses of the ceramic products were made on a LECO CS600 for carbon and TCH600 for oxygen and hydrogen when the weight concentration of silicon was confirmed by the perchloric acid dehydration gravimetric method. Thermogravimetric analysis (Netzsch STA 449C) was performed under an atmosphere of dry air according to a heating/cooling rate of 10°C/min. X-ray diffraction measurements were performed using a Bruker diffractometer (Model D8 ADVANCED, Cu Kα radiation; λ = 1·5406 Å). The pore characteristics were determined by nitrogen adsorption/desorption at −196°C (Quantachrome Monosorb).
Scanning electron microscopic (SEM) analysis was performed by a JEOL JEM-2010 operating at an acceleration voltage of 200 kV, and transmission electron microscopic (TEM) analysis was performed by a Philips CM200 operating at an acceleration voltage of 200 kV.
Results and discussion
Figure 1 shows the non-isothermal oxidation behaviours of silicon carbide ceramics in air. As seen in Fig. 1, the raw sample decreased a little below 200°C when the treated sample did not change. The main oxidations of the raw sample and the treated sample both started near 600°C. However, the oxidation behaviours of the silicon carbide ceramics were remarkably different above 600°C. The biggest weight loss ratio of the raw sample was 2·24 wt-% at 730·5°C, while it was 9·17 wt-% at 848·5°C for the treated sample. Therefore, the oxidation temperature of the silicon carbide phase was delayed more than 100°C in the treated sample.
Thermogravimetric curves of non-isothermal oxidations of silicon carbide ceramics from 29 to 1320°C with heating rate 10°C min−1
Then, the isothermal oxidation behaviours of the silicon carbide ceramics were respectively investigated at 700 and 1000°C, as shown in Fig. 2. In Fig. 2a, the raw sample decreased 1·4 wt-% in 13 min at 700°C, and then slowly increased 3·1 wt-% during nearly 300 min, when the treated sample increased 8·2 wt-% in 25 min and almost kept still in succession. In Fig. 2b, the silicon carbide ceramics were oxidised quickly at 1000°C. The raw sample decreased 0·8 wt-% in 1 min and then increased ∼10 wt-% in 50 min before not changing in the longer time. However, the treated sample was oxidised all the time. First, it decreased 8 wt-% in 2 min and then kept increasing for the rest of the time of oxidation.
a 700°C; b 1000°C
Obviously, the differences of oxidation behaviours of the silicon carbide ceramics before and after 1800°C annealing were notable because the compositions and microstructures of the silicon carbide ceramics remarkably changed as follows.
Table 1 shows the characteristics of the silicon carbide ceramics before and after 1800°C annealing. From the data, it could be found that there were ∼11 wt-% free carbon in the silicon carbide ceramics, and the elementary H in the raw sample disappeared after 1800°C annealing.
Characteristics of silicon carbide ceramics before and after 1800°C annealing
*It is based on the hypotheses that oxygen exists as silicon dioxide and hydrogen exists as carbon–hydrogen group.
Therefore, the oxidation of the carbon–hydrogen groups resulted in weight loss of the raw sample below 200°C, as seen in Fig. 1. Moreover, according to the content of the free carbon phase in Table 1, it was discovered that there was a little free carbon phase oxidised in the raw sample before the silicon carbide phase, and the free carbon phase was oxidised together in Figs. 1 and 2. However, the free carbon phase had been mostly oxidised in the treated sample before the silicon carbide phase began to oxidise, as seen in Figs. 1 and 2.
Figure 3 shows the X-ray diffraction patterns of the SiC bulk ceramics before and after 1800°C heat treatment. As seen in Fig. 3, the raw sample is amorphous. After 1800°C heat treatment, the crystalline degree of the SiC phase in the SiC bulk ceramics significantly increased. The phenomena were further supported by TEM images in Fig. 4.
X-ray diffraction patterns of SiC bulk ceramics before and after 1800°C heat treatment
a raw sample and carbon mapping (inset); b treated sample and silicon mapping (inset)
Figure 4 shows the TEM images and elementary distributions of the silicon carbide ceramics before and after 1800°C annealing. In Fig. 4a, the raw sample was amorphous, and the distribution of the elementary carbon was even in the carbon mapping, because there was part of the excess carbon dissolving in the amorphous silicon carbide continuum.9 As a result, the silicon carbide phase and the free carbon phase in the raw sample were almost oxidised at the same time.
In Fig. 4b, the silicon carbide ceramics became nanocrystalline from amorphous after 1800°C annealing, and the distribution of the elementary silicon was uneven in the silicon mapping, because the silicon carbide nanocrystals (nearly 7 nm) were surrounded by the turbostratic free carbon phase.10 Consequently, the free carbon phase in the treated sample had been mostly oxidised before the silicon carbide phase began to oxidise, and the oxidation temperature of the silicon carbide phase was delayed more than 100°C.
Figure 5 shows the SEM images of surfaces of the raw and treated samples before and after oxidation at 700°C. There were also notable differences. As seen in Fig. 5a and b, the skeletons of the raw and treated samples were all dense before the oxidation. After the oxidation, the skeleton of the raw sample was still dense just with a silicon dioxide film on the surface in Fig. 5c, when the treated sample had many nanopores in the skeleton in Fig. 5d.
a raw and b treated samples without oxidation; c raw and d treated samples with oxidation
Finally, the adsorption/desorption isotherm plot for the nitrogen sorption (at −196°C) and pore size distribution of the treated silicon carbide ceramics after the oxidation at 700°C are presented in Fig. 6. The pore size of the treated sample after the oxidation was mainly in the range of 7-100 nm, and the average pore diameter was 22·1 nm, which indicated that the sample had nanoporous characteristics.
Nitrogen adsorption/desorption isotherm (at −196°C) and pore size distribution (inset) of treated sample after oxidation at 700°C
Conclusions
In summary, the effect of 1800°C annealing on the oxidation behaviours of the silicon carbide ceramics derived from PCS has been studied. The silicon carbide ceramics before and after 1800°C annealing both started to oxidise near 600°C. However, the oxidation behaviours of the silicon carbide ceramics were remarkably different above 600°C due to the microstructure changes during 1800°C annealing.
For the silicon carbide ceramics derived from PCS at 1200°C, the silicon carbide phase and the free carbon phase were almost oxidised together because part of the excess carbon dissolved in the amorphous silicon carbide continuum. However, the free carbon phase in the 1800°C annealing silicon carbide ceramics had been mostly oxidised before the silicon carbide phase began to oxidise, and the oxidation temperature of the silicon carbide phase was delayed more than 100°C because the silicon carbide nanocrystals were surrounded by the turbostratic free carbon phase. After the oxidation at 700°C, there were many nanopores present in the skeleton of the silicon carbide ceramics annealed at 1800°C, and the average diameter was 22·1 nm.
Footnotes
Acknowledgements
The authors would like to thank the National Natural Science Foundation of China (grant no. 90916002) for financial support.