Mullite Diffusion Barriers for SiC-C/C Composites Produced by Pulsed Laser Deposition
- PDF / 2,330,910 Bytes
- 6 Pages / 391.5 x 607.5 pts Page_size
- 31 Downloads / 199 Views
advantages as the ablation of non-conductive and high melting targets, the preparation of films of complex composition and a freely choosable working atmosphere. Furthermore, the high energy particle beam is highly ionized [5]. SAMPLE PREPARATION
Polished SiC substrates (sintered SiC produced by BCE Special Ceramics GmbH, Germany) and commercial C/C-Si-SiC substrates (size: 20x20x2 mm 3; CFC 222 HT 3K, infiltrated with liquid silicon under pressure and coated with CVD-SiC, produced by Schunk Kohlenstofftechnik GmbH, Germany) were used.
High Vacuum Chamber Hg-Cd-Te Sensor
\
Deposition Rate 100 nms-1 Ptot< 5-10-5 mbar
Meaii Power 200 W Pulsee Energy 600 mJ Pulsie Length 170 ps 2 Powcer Density 5- 107 Wc-f Wav elength 10.6 prm Repeztition Frequency 100Hz Fig. 2 PLD-mullite layer on SiC
Fig. 1 Scheme of the PLD equipment Tab. 1
PLD: Transfer of composition
sample
treatment
composition [at.%]
SI
mullite target'
sintered powder
mullite layer 2
green layer
2.2
cc I 61.9 28.6
ci
52.1
11.3
34.4
cAAcS)
0.752 0.753
Prior to the mullite deposition the samples were preoxidized in air. The resulting SiO 2 layer of about 0.2 pm serves as a bond coat. Finally, 2.5 pm thick mullite coatings were deposited by PLD (Fig. 1 and 2). High energy impact CO 2 laser pulses (X = 10.6 pm, At = 170 ps, j = 3.107 W cm-2) led to melting and evaporation of the target material in a single step. The flux of the metal components was stoichiometric. As the work was carried out under reduced pressure (p,., = 10-5 mbar) the oxygen content of the PLD layer decreased [5]. For these targets (sintered mullite powder, Baikowski Chimie, France) the typical deposition rate was 100 nm s-'. Even with target rotation deposition rates up to 500 nm s-' can be reached only by using larger targets. The substrate temperature did not rise to more than 37.3 K.
I calculated from nominal powder composition WDX analysis, EPrjm = 6 keV
2
80
CHARACTERISATION OF THE PLD MULLITE LAYERS BY IR SPECTROSCOPY IR-reflection spectra between 650 and 1300 cm-' of the target material and of PLD coatings on SiC heated at 1673 K for 15 min are shown in Fig. 3. Both spectra show similar band shapes with a peak at about 720 cm-' and broad structures between 800 and 1000 cm-' and between 1050 and 1200 cm'. Therefore, the IR reflection spectra indicate that mullite is formed after only a short oxidation. The formation of mullite was also confirmed by XRD investigations. 0.3 SiO
0.15-
:", 2.
'1873 K
0.2 .t
0.10Mullite
0.1
S0.05-
0.0
......
...
.... .
.
1773K"..
..
73 K
0.00
700 * 800 ' 900 "10"00 Wavenumber [cm-']
1100 12*00
Wavenumber [cm-1]
Fig. 3
Reflectivity of the target in comparison with the reflectivity of a PLD coating on SiC annealed in air at 1673 K for 15 min
Fig. 4 Comparison of reflectivity spectra of PLD coatings on C/C-Si-SiC substrates annealed at different temperatures in air
After oxidation runs at elevated temperatures in air (see below) the reflection spectra show the formation of amorphous SiO 2, which is indicated by
Data Loading...
