The cyclic oxidation kinetics of uncoated TiBw/Ti64 substrate and TMCs coated with CoCrAlY-TiB2composite coating at various temperatures are demonstrated inFig. 1. The substrate suffered from a rapid oxidation stage at thefirst 10 h. Then the weight gain value obviously decreased but still followed a linear oxidation law. As a result, the substrate material exhibited an unstable oxidation phenomenon throughout the test, and the mass gain per unit area after oxidation for 100 h at 700 °C, 800 °C, and 900 °C reached about 10.5 mg·cm−2, 14.9 mg·cm−2, and 24.5 mg·cm−2(Fig. 1(a–c)). However, the TMCs coated with CoCrAlY-TiB2 experienced a stable oxidation after the initial relatively rapid mass gain stage, following the parabolic rate rule upon the whole procedure. After the cyclic oxidation for 100 h, the corresponding weight gain of the coated specimens was only 2.5 mg·cm−2, 4.0 mg·cm−2, and 5.4 mg·cm−2. Compared with the uncoated substrate, the gained weight was remarkably reduced by 66.7%, 73.1%, and 75.2%.
Moreover,Fig. 1(a1–c1) also show the correspondingfirst derivative curves of the mass-gain value to quantitatively describe the mass-gain rate over oxidation time. These curves also proved that the TMCs substrate presented higher mass-gain rate compared with the coated specimen. For instance, the initial rate of substrate at 700 °C was about 0.4 mg·cm−2h−1(Fig. 1(a1)), which was two times faster than the coated one (around 0.2 mg·cm−2h−1). Subsequently, the mass-gain rate decreased and the substrate exhibited a relatively constant rate of around 0.08 mg·cm−2h−1after 20 h, while the rate of CoCrAlY-TiB2 coated sample was stable and close to only 0.01 mg·cm−2h−1. Therefore, the mass-gain rate of the CoCrAlY-TiB2coated sample was obviously lower than the TiBw/Ti64 substrate, implying the oxidation resistance can be remarkably enhanced by introducing the CoCrAlY-TiB2coating. In addition, the mass-gain rate of the substrate at high temperatures also experienced a moderate increasing trend after the inflection points“A”(70 h at 800 °C inFig. 1(b1)) and“B”(60 h at 900 °C inFig.1(c1)). When the TMCs substrate was cyclically oxidized at elevated temperature, the oxide scale spalled and detrimentally leading to the exposed fresh surface which was easily oxidized and then caused the inflection points of the mass-gain rate curve. Nevertheless, the rate of the CoCrAlY-TiB2coated sample exhibited a comparatively stable trend after the initial 20 h. Consequently, the CoCrAlY-TiB2 coating presented stable and excellent anti-oxidation performance contributing to expanding the applicationfields of TMCs.
Fig. 1.The cycle oxidation kinetic curves of CoCrAlY-TiB2coated sample and uncoated TMCs substrate under different 700 °C, 800 °C and 900 °C: (a) (b) (c) mass- gain value; (a1) (b1) (c1) mass-gain rate.
Subsequently, the XRD patterns of CoCrAlY-TiB2 composite coating during the cyclic oxidation at different temperatures are shown in Fig. 2. A common phenomenon can be seen is that the change in phase constitution mainly occurred at thefirst 20 h, and then the transformed phases presented no change during the subsequent test. The main phase constitution change at the initial stage was that the original Al0.47Co0.53, AlCo, Al80Cr20, C o3Ti, CrSi2, CrB2phases were unstable in air atmosphere at elevated temperature, which reacted with oxygen and then gradually transformed into Co, Al2O3, C r2O3. Indicating that CoCrAlY-TiB2coating exhibited an enhanced oxidation resistance by forming Al2O3and Cr2O3oxides. It is worth noting that the diffraction peak of residual AlCo and Al0.47Co0.53solid solution phases (2θ= 44.7°) near the metallic Co peak was detected on the oxidized surface at 700 and 800 °C inFig. 2(a) and (b), but disappeared after oxidizing at 900 °C (Fig. 9(c)). In reality, when oxidized at 700 °C, only a portion of Al element transformed into Al2O3, and the rest of Al remained as the original solid solution element state. Therefore, the diffraction peak of residual AlCo or Al0.47Co0.53was detected inFig. 2(a) and a certain amount of metallic Co also emerged during this stage. With the increase of oxidation temperature and duration, the quantity of residual AlCo or Al0.47Co0.53phase on the oxidized surface gradually decreased and the corresponding diffraction peak intensity was reduced in Fig.2(b) and even disappeared at 900 °C inFig. 2(c) in association with the increased content of metallic Co. Moreover, there were also some differences in phase constitution change at various temperature. For instance, the diffraction peak of AlCCr2phase was observed when testing at 700 and 800 °C, which did not appear at 900 °C. Similarly, the Al0.67Cr0.08Ti0.25phase could only be detected at 800 °C. The possible reason accounting for this difference might be the instability of the corresponding phases in high-temperature environments.
Fig.2.XRD patterns of the CoCrAlY-TiB2composite coatings during oxidation test at (a) 700 °C, (b) 800 °C and (c) 900 °C.
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