3.1 Microstructure and phase of the bi-layer EBCs
Surface XRD patterns and SEM images of bi-layer YbDS/SiC coating are shown in Fig. 2. There are only the diffraction peaks of SiC and YbDS phases on the external of SiC inner coating and YbDS outer coating, respectively. There are no diffraction peaks of the SiC phases detected on the external of YbDS outer coating, which shows that the thickness of YbDS outer coatings is uniform without obvious defects. Meanwhile, no macroscopic cracks are detected on the external of SiC inner coating and YbDS outer coating in circular specimens, respectively, which shows that a dense and uniform coating can be prepared on the external of special shaped specimens by the CVD method and the sol-gel united with air spray method. SEM images show that the SiC coating prepared by CVD method is very compact and with the typical “pebble-like” morphology. After high-temperature sintering, no visible cracks are detected on the external of YbDS outer coating owing to the good match in CTE between SiC and YbDS ceramics. However, there are some micro-holes in YbDS outer coating due to the volatilization of solvents during the process of coating preparation.
Fig. 3 shows the cross-section SEM image of bi-layer YbDS/SiC coating covered Cf/SiC composites. As can be seen from Fig. 3(a), the bi-layer YbDS/SiC coating is uniformly, and compactly deposited on the external of Cf/SiC matrix, which can effectively protect the contact between the matrix and the external oxidation environment. Meanwhile, the surface of YbDS outer coating prepared by sol-gel united with air spray method is very smooth and the thickness distribution is uniform. Fig. 3(b) shows that the average thickness of bi-layer YbDS/SiC coating is about 70 μm, and the average thickness of SiC inner coating, YbDS outer coating are about 30 μm, and 40 μm, respectively. Moreover, the interface between SiC inner coating and Cf/SiC substrate, YbDS outer coating and SiC inner coating are closely bonded, and no penetrating cracks are detected in YbDS/SiC coating.
Fig. 4 shows the map elements distribution (Yb, Si, O) in the cross-profile of bi-layer YbDS/SiC coating covered Cf/SiC composites. As can be seen from Fig. 4(a), the structure distribution of each layer in the bi-layer YbDS/SiC coating is very obvious. Figs. 4(b) to (d)) shows the obvious Yb, Si, and O elements in YbDS outer coating. Fig. 4(c) shows the obvious Si element in SiC inner coating and Cf/SiC composites. All the element distribution diagrams (Figs. 4(a) to (d)) show that the chemical stability between each coating and Cf/SiC composites is good, and no element diffusion of Yb, Si, O is detected.
3.2 Gas erosion resistance of the bi-layer EBCs
Fig. 5 shows the surface macrophotograph of bi-layer YbDS/SiC coating covered Cf/SiC composites during thermal cycle process between 1773 K and room temperature in the burner rig tests. Fig. 5(a) shows that no visible cracks are detected on the external of bi-layer YbDS/SiC coating covered Cf/SiC composites in the initial state. Meanwhile, there are still no visible cracks detected on the external of bi-layer YbDS/SiC coating covered Cf/SiC composites after 12 thermal cycles (180 min) at 1773 K (Fig. 5(b)). This is mainly attributed to the close CTE of the SiC inner coating and the YbDS outer coating. However, after 24 thermal cycles (360 min), the visible cracks began to appear on the local and edge of the bi-layer coating (Fig. 5(c)). With the increase of thermal cycles between 1773 K and room temperature in the burner rig, the number and width of visible cracks on the external of the coating covered Cf/SiC matrix are gradually increased (Figs. 5(d) to (f)). After 36 thermal cycles (540 min) at 1773 K, there are a lot of visible cracks not only on the surface but also on the arc edge of the bi-layer YbDS/SiC coating covered specimen (Fig. 5(f)). At the same time, no obvious coating shedding phenomenon was found due to the better bonding strength between YbDS outer coating and SiC inner coating.
Fig. 6 shows the weight change curves of bi-layer YbDS/SiC coating covered Cf/SiC composites with the thermal cycle process between 1773 K and room temperature in the burner rig. See in Fig. 6, the oxidation behavior of the YbDS/SiC coating covered Cf/SiC specimen in high-speed and high-temperature gas environment can be divided into two stages denoted as A and B. A stage is a stable oxidation stage (0-300 min), the weight change of the YbDS/SiC coating covered specimens is steadily and slowly increased with the oxidation time. After 20 thermal cycles (300 min) at 1773 K, the weight change of the covered specimen is only 5.93 × 10-3 g·cm-2, which shows a good oxidation resistance of the bi-layer YbDS/SiC coating for Cf/SiC specimen in A stage. However, the weight change of the covered specimen is rapidly increased in B stage (300-540 min). As can be seen from the macrophotograph of Fig. 5, the reason for the rapid increase of weight change is the formation of visible cracks on the external of YbDS/SiC coating. After 36 thermal cycles (540min) at 1773 K, the weight change of the covered specimen is 37.67 × 10-3 g·cm-2, which indicates that the weight change of the covered specimen has entered the rapid oxidation stage.
Fig. 7 shows the surface SEM images of bi-layer YbDS/SiC coating covered Cf/SiC composites with different thermal cycles between 1773 K and room temperature in the burner rig. After 24 thermal cycles (360 min) at 1773 K, a large number of micro-cracks are found on the external of YbDS/SiC coating except for a few visible cracks, this micro-cracks provide a channel for oxygen diffusion to the Cf/SiC matrix, and resulting in partial oxidation of the carbon fibers and matrix in Cf/SiC composites. The above phenomenon is the reason for the stable oxidation of covered specimen in A stage (Fig. 6). However, as the number of thermal cycles increases to 36 (540 min), there are a lot of cracks with a wider size on the external of YbDS/SiC coating (Fig. 7(b)). Meanwhile, a large number of cracks can be seen on the external of YbDS/SiC coating at the edge of the circular specimen. The oxygen content diffused into the Cf/SiC substrate by YbDS/SiC coating is greatly increased due to the increase of the number and width of cracks, and resulting in the rapid increase of the weight change of the specimen in B stage (Fig. 6). The EDS analysis on the external of YbDS coating in Fig. 7(b) shows that the atomic percentage ratio of each element in YbDS coating is approximately in accord with that of 2:2:7, respectively (Fig. 7(d)). This indicates that the YbDS coating has very good chemical stability under a high-speed and high-temperature gas environment.
Fig. 8 shows the cross-section SEM images of bi-layer YbDS/SiC coating covered Cf/SiC composites after 36 thermal cycles (540 min) between 1773 K and room temperature in the burner rig. Compare with the bi-layer YbDS/SiC coating covered Cf/SiC specimen before thermal cycles (Fig. 3(a)), the surface of YbDS outer coating becomes rough and the thickness distribution of YbDS outer coating becomes uneven after 36 thermal cycles in high-speed and high-temperature gas environment (Fig. 8(a)), which caused by the continuous erosion of YbDS outer coating by high-speed gas. Fig. 8(b) shows that the crack on the external of YbDS/SiC coating can be divided into penetrating cracks and non-penetrating cracks. The penetrating cracks in bi-layer YbDS/SiC coating are caused by the difference of CTE between integral YbDS/SiC coating and Cf/SiC composites. Meanwhile, the non-penetrating cracks are caused by the difference of CTE between YbDS outer coating (~4.1 × 10-6/K) and SiC inner coating (4.3-5.4 × 10-6/K). Fig. 8c shows the penetrating crack in YbDS/SiC coating, the oxygen diffuses into Cf/SiC matrix through penetrating cracks, and resulting in rapid oxidation of carbon fibers and matrix near the penetrating cracks. As can be seen from Figs. 8(b) and (c), there are a large number of pores left by the oxidation of carbon fibers. Fig. 8(d) shows the interface between YbDS outer coating and SiC inner coating after 36 thermal cycles (540 min), the interface is still tightly bonded and has a very obvious SiO2 film due to the oxidation of SiC coating surface in 1773 K.
In order to find out the reason why the surface of bi-layer YbDS/SiC coating becomes rough after thermal cycles in high-speed and high-temperature gas environment. Fig. 9 shows the surface 3D morphology of bi-layer YbDS/SiC coating after 36 thermal cycles (540 min) to better observe the three-dimensional structure on the external of the coating. There is a very obvious annular corrosion pit area on the external of YbDS outer coating, and two cracks are intersected in the center of the corrosion pit area. According to the distribution curve of the depth in the coating section, an obvious arc-shaped corrosion pit curve is gradually formed after high-speed and high-temperature gas corrosion. The above results show that in high temperature and high-speed gas environment, it is easy to form annular corrosion pits on the external of YbDS outer coating in the region of higher gas velocity. Meanwhile, the center of the corrosion pit is easier to be the origin area of the crack due to the greater impact force of the gas.