Oxidation response of a SiCf/SiC CMC with a HfB₂-based coating in an arc jet test

Figures & data

Table 1. Arc jet conditions: measured data.

Disk ID	Total pressure (bar)	Cold wall FC heat flux (MW/m^2)	Stag. pressure (Pa)
I	1.11	1.73	3030
II	1.10	1.36	2670
III	1.17	1.70	2870
IV	1.17	1.52	2730

Table 2. Arc jet conditions: derived data.

Disk ID	Specific total enthalpy at s.l. (MJ kg^−1)	Cold wall NC heat flux (MW m^−2)
I	16.7	0.79
II	14.0	0.69
III	16.8	0.78
IV	15,4	0.73

Table 3. Pyrometer details.

Pyrometer ID	Manufacturer	Model	Range (°C)	Wavelength (µm)
P800	IMPAC	ISQ5	800–2500	0.7–1.15
M1000	MIKRON	M77S	1000–3000	≤2

Figure 1. (a) Overview of the as processed SiC/SiC-HfB₂ composite. (b) Magnified image of the surface showing the HfB₂-SiC and SiC phases in the matrix.

Figure 2. Temperature versus time plot for the four tested samples. The solid lines are the P800 pyrometer and the dotted lines of the corresponding color are the M1000 pyrometer readings. The locations marked SC are where the P800 was switched form dual-colour to single-color mode. The inset images show the surface of the samples after arc jet exposure for each disk.

Figure 3. Calculated spectral emissivity at about 1 µm for the considered angle of view (60°) versus temperature measured for Disk I, Disk II and Disk IV. No measurements were taken on Disk III.

Figure 4. (a) Secondary electron image of the surface of Disk II after arc jet exposure. From the EDS maps for (b) Hf, (c) O and (d) Si, it is shown that there is a thin SiO₂-HfO₂ layer at the surface which is shown for Disk III in (e). The oxide scale is less than 10 µm and consists of SiO₂ (dark phase) and HfO₂ (light phase).

Figure 5. (a) Overview of the oxide scale formed on Disk IV after arc jet exposure. The oxide scale forms three distinct layers 1: porous HfO₂; 2: SiC-depleted HfB₂ and 3: HfB₂-SiC. (b) Surface of the oxide scale. (c) Magnified image of the three layers. SiO₂ is not found at the surface in (a), but can be found in some instances as shown in (d).

Figure 6. EDS map showing the removal of SiO₂ in the outer layer and depletion of SiC from layer 2. Oxygen signal is associated with HfO₂.