Under the present experimental conditions, the possible reactions for the coated samples are as follows [15–22]:
2C(s) + O2(g)→2CO(g) (1)
C(s) + H2O(g)→CO(g) + H2(g) (2)
4Cr3Si + 13O2(g)→6Cr2O3 + 4SiO2 (3)
4Cr7C3 + 27O2(g)→14Cr2O3 + 12CO(g) (4)
2SiC + 3O2(g)→2SiO2 + 2CO(g) (5)
SiC + 3H2O(g)→SiO2 + 3H2(g) + CO(g) (6)
2Cr2O3 + 3O2(g) + 4H2O(g)→4CrO2(OH)2(g) (7)
SiO2 + 2H2O(g)→Si(OH)4(g) (8)
Above 500 ºC, carbon oxidized according reactions (1) and (2) showing weight loss . Cr3Si and Cr7C3 begin to be oxidized above 660 ºC to form Cr2O3 and thus lead to weight gains [16–18], according reactions (3) and (4), respectively. While SiC will be oxidized above 800 ºC according to reaction (5), or above 1127 ºC according the reaction (6) that results in weight gain . On the other hand, in water vapor containing oxidizing environments, above 1100 ºC, the formed Cr2O3 can volatilize by further reacting with water vapor to form gaseous Cr(OH)x [19–22] according to reaction (7).
For the wet oxidation at 1300 ºC of the SiC/SiC/SiC coated C/SiC, as shown in Eq. 2, the exponent was 0.4782, which was close to 0.5, typical of parabolic kinetics. Moreover, as shown in Fig. 4 (d), the silica surface was smooth without pores, humps or bubbles. Therefore, it can be ascertained that the weight loss of the SiC/SiC/SiC coated C/SiC during the wet oxidation at 1300 ºC follows parabolic kinetics. As a result, under the present experimental conditions, (i.e. gas velocity of 3.0 cm s− 1, 10 vol. % H2O, and at 1 atm), the volatilization of SiO2 according to the reaction (8) was negligible .
The weight change of the C/SiC composite during oxidation was governed by the overall outcome of the competing reactions, and the crystallography of the oxidized samples depended on the wet oxidation temperature.
As the temperature was increased from 700 ºC to 1000 ºC, the width of the microcracks in the coating decreased as temperature increased . Consequently, the diffusion of the oxidizing gases into the PyC interlayer through microcracks in the coating decreased with increasing oxidation temperature. As a result, the weight loss due to oxidation of carbon phases decreased with increasing oxidation temperature for both the coated samples. For a specific oxidation condition (i.e. temperature, oxidizing atmosphere and reactant), the oxidation rate was constant. Since the width of the largest microcracks in the top layer of the Cr3Si-Cr7C3/SiC/SiC coating was lower than that in the top layer of the SiC/SiC/SiC coating (as shown in Fig. 7), the inward diffusion of oxidizing gases through the former was much lower. Thus, the Cr3Si-Cr7C3/SiC/SiC coated composite samples showed significantly lower weight loss when compared to the SiC/SiC/SiC coated samples.
At 700 ºC, since the oxidation of both Cr3Si and Cr7C3 was very slow, only a small amount of Cr2O3 was produced, and no SiO2 was detected. Hence, the XRD spectrum showed Cr3Si, Cr7C3 and a small amount of Cr2O3 were detected and C, O, and Cr were detected by EDS. On the other hand, the oxidation of carbon phase was faster than the oxidation of Cr3Si and Cr7C3. Therefore, the Cr3Si-Cr7C3/SiC/SiC coated composite samples showed a nearly linear weight loss. At 900 ºC, the oxidation of Cr3Si and Cr7C3 became faster, which led to the increased formation of Cr2O3 and the growth of Cr2O3 whiskers and platelets . The oxide layer became thicker with increasing time of oxidation. Thus, the ratio of the intensity of the Cr2O3 peak to that of the SiC peak was higher for the sample oxidized at 900 ºC when compared to that oxidized at 700 ºC. Correspondingly, the elements detected by EDS were O and Cr. However, due to the thick oxide layer, the inward diffusion of oxidizing gases slowed down first and then leveled out. As a result, the Cr3Si-Cr7C3/SiC/SiC coated composite samples showed a nonlinear weight loss behavior, with a very rapid weight loss within the first 2 h followed by an apparently gradual weight loss thereafter. The very low weight loss indicating the consumption of carbon was limited to the sub-surface region. At 1300 ºC, enhanced oxidation of the coating led to the rapid closure of the narrow microcracks in the outer layer of the Cr3Si-Cr7C3/SiC/SiC coating due to thermal expansion of the formed SiO2 and Cr2O3 [25, 26]. Moreover, presence of SiO2 scale was favorable to inhibit the diffusion of both Cr and O, leading to oxidation process slowed and a relative compact oxide scale formed underneath the external oxide scale . Additionally, water vapor decreased the viscosity of silica [28, 29] and thus, the carbon phases were protected from oxidation. On the other hand, the formed Cr2O3 volatilized by further reacting with water vapor to form gaseous Cr(OH)x [19–22]. Despite the fact that the rapid formation and escape of gases made the oxide films locally bulged and damaged, the sublayer was a compact oxide layer and retained the Cr3Si-Cr7C3/SiC/SiC coating (shown in Fig. 8). Therefore, the Cr3Si-Cr7C3/SiC/SiC coated samples showed non-linear weight gains, with a very rapid weight gains within the first 2 h folled by an apparently gradual weight gains thereafter. And detected phases were Cr2O3 and SiO2. Thus, after the wet oxidation for 10 h, the composite Cr3Si-Cr7C3/SiC/SiC coated composite samples showed nearly the same failure behavior and flexural strength as the as-received samples.
For the SiC/SiC/SiC coated C/SiC, high temperature water vapor enhanced the non-uniform oxidation consumption of carbon phases, which led to the formation of pipeline-shaped channels between the carbon fiber and the SiC matrix . As a result, the strength of the fibers and the bonding between the fiber/interphase/matrix would be weakened. Thus the stress-displacement curves showed a gradual stress drop after the maximum load point. Moreover, the non-uniform consumption of carbon phase increased the porosity of the composites increased. It is well acknowledged that the elastic moduli of porous materials decrease with increasing porosity [30, 31]. Therefore, the oxidized composite showed a reduced elastic modulus demonstrated by the decrease in the slope of the initial linear part of the stress-displacement curve. Above 900 ºC, the microcracks in the SiC coating are gradually sealed both by thermal expansion and by the formed SiO2, due to surface oxidation. However, even though the oxidation of SiC resulted initially in the formation of a protective SiO2 layer and a consequent weight gain, the rapid oxidation of carbon resulted in rapid weight loss. Therefore, the weight loss of the SiC/SiC coated composite samples decreased with increasing oxidation temperatures, showing the same trend as the flexural strength.
The above results suggest that the Cr3Si-Cr7C3/SiC/SiC coating has enhanced protection against wet oxidation. And this improved oxidation resistance originates from the rapid formation of a protective layer of Cr2O3 by oxidation of Cr3Si and Cr7C3 below 1000 ºC,while the formation of more protective layer of SiO2 by oxidation of Cr3Si and SiC at higher temperatures. The work provides support for the preparation of oxidation resistant coatings, which in return are essential to improve the service performance of carbon fiber reinforced composites in high temperature oxidizing environments.