The composite powders of the above two systems were prepared into coatings by plasma spraying. Fig. 1 shows the XRD patterns of the two systems coatings. According to Fig. 1a, the coating obtained by plasma spraying Ti-SiC composite powders were composed of TiC, Ti5Si3and SiC phases. According to Fig. 1b, the coating obtained by
plasma spraying Ti-SiC(ht) composite powders was mainly composed of TiC, Ti5Si3, and a small amount of SiC.It was found that TiC and Ti5Si3were formed by the reaction of Ti with SiC (reaction Eq. (3)) during plasma spraying. It can be obtained that Ti3SiC2was oxidized and decomposed into TiC and Ti5Si3according to Eq. (1) under the condition of plasmaspraying, and TiSi2was transformed into Ti5Si3phase through a series of intermediate phases (TiSi2→ TiSi→Ti5Si4→ Ti5Si3).
3Ti3SiC2+O2→4TiC +Ti5Si3+2CO(g) （1）
Zhou et al. also found that Ti3SiC2was difficult to exist in rapid non-equilibrium processes, such as reactive plasma spraying. The relative contents of TiC, Ti5Si3and SiC in the two composite coatings can be quantitatively analyzed by reference intensity ratio (RIR) of characteristic peaks. The calculation results show that the coating obtained by plasma spraying Ti-SiC composite powders contained TiC (35 wt%), Ti5Si3(42 wt%) and SiC (23 wt%). The coating obtained by plasma spraying Ti-SiC(ht) composite powders contained TiC (47 wt%), Ti5Si3(40 wt%) and SiC (13 wt%).
Fig. 1. XRD patterns of the two coatings prepared from two composite powders: (a) Ti-SiC and (b) Ti-SiC(ht), respectively.
The cross-sectional SEM micrographs of the two composite coatings were presented in Fig. 2. As can be seen from Fig. 2a, there were many pores in the coating obtained by plasma spraying Ti-SiC composite powders, so the density of the coating was poor. However, the coating obtained by plasma spraying Ti-SiC(ht) composite powders had smaller porosity. Comparing Fig. 2c and d, it was found that the plasma sprayed Ti-SiC(ht) composite powders had higher microstructure density and more uniform distribution. The spray-dried Ti-SiC composite powders were formed by binding irregular Ti particles and SiC particles together with a binder. The spray-dried Ti-SiC composite powders had low bonding strength and contains holes inside. Therefore, the spray dried composite powders were easy to be broken into small particles in the high temperature plasma jet, resulting in the low deposition efficiency of the powders and the poor densification of the coating. After heat treatment at 1200 °C, the residual moisture and binder in the composite powders completely disappeared, and solid phase diffusion reaction occurred between Ti and SiC. Therefore, Ti-SiC(ht) composite powders had higher density and cohesive strength, and the coating structure obtained by spraying Ti-SiC(ht) composite powders had higher density and more uniform distribution.
Combining Fig. 2 and the EDS analysis results (see Table 1) of the two composite coatings, it can be seen that in the coating obtained by plasma spraying Ti-SiC composite powders, the A region was mainly SiC and TiC phases, and the B region contained TiC and Ti5Si3phases.In the coating obtained by plasma spraying Ti-SiC(ht) composite powders, the C region was rich in SiC phase, the D region was rich in TiCphase, and the E region contained TiC and Ti5Si3phases. In summary, the phases composition of the two composite coatings were TiC, Ti5Si3 and SiC. However, the phase distribution of Ti-SiC(ht) composite coating was more uniform.
Fig. 2. Cross-sectional SEM micrographs of the two composite coatings: (a) Ti-SiC, (b) Ti-SiC(ht), (c) high magnification of (a) and (d) high magnification of (c).
Table 1
EDS results of the two coatings (the regions in Fig. 8c and d) (wt%).
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