Effects of post-annealing on microstructure and mechanical properties of plasma sprayed Ti-Si-C composite coatings with Al addition

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Abstract

In this study, Ti-Si-C composite coatings were fabricated by plasma spraying using Ti/SiC/C/Al powder mixtures, and then these coatings were annealed at 500–900 °C in an Ar atmosphere. The influences of post-annealing on the microstructure and mechanical properties were analyzed via X-ray diffraction (XRD), scanning electron microscope (SEM) and micro-hardness tester. Results showed that the annealing treatment hardly changed the phase structure of the composite coatings. Coatings before and after annealing were composed of Ti5Si3, Ti3SiC2 and TiC phases. Compared with coatings without annealing, the peak intensity of Ti3SiC2 increased, the porosities decreased and the microstructure became more uniform after annealing. The composite coatings showed an increased microhardness and fracture toughness, which reached the maximum value of 1232 HV0.2 and 3.19 MPa·m1/2 when annealed at 700 °C. The improvement of mechanical properties was mainly attributed to the more uniform microstructure and the toughening effect of closed micro-cracks.

Introduction

Transition metal-based ceramic coatings (i.e., TiC, ZrC, Ti5Si3, MoSi2, etc.) have been widely used as protective coatings in aerospace and machinery fields owing to their high hardness, high melting point, high temperature strength and thermal stability, good wear resistance and corrosion resistance [[1], [2], [3], [4]]. However, single-phase ceramic coatings have greater brittleness and lower toughness, which limits their applications. Ceramic composites such as ceramic-ceramic and ceramic-metal could combine the advantages of each constituent, thus attracted increasing attentions [[5], [6], [7], [8]]. In Liu's study [9], TiC was incorporated into Ti5Si3 to fabricate the TiC-Ti5Si3 nanocomposite coating, the fracture toughness of which reached 11.5 MPa·m1/2, much higher than that of Ti5Si3 (2.5 MPa·m1/2). The fracture toughness of the TiC-TiB2 composite coatings was approximately 6.97 MPa·m1/2, which is better than that of individual TiB2 (~3.5 MPa·m1/2) and TiC (3.8 MPa·m1/2) [10]. Hence, the design of composites would be an interest for ceramic coatings.

Plasma spraying is a kind of thermal spraying technology, using a plasma flame to heat feedstock to a molten or semi-molten state and rapidly solidifies on the substrate to form coatings. It can be used to deposit wide kind of coatings including metal alloys (i.e., NiAl, Mo, CoCr, etc.) [[11], [12], [13]] and ceramics (i.e., TiC, TiCN, Al2O3-TiO2, WC/Co, etc.) [[14], [15], [16], [17]]. Due to the ultra-high temperature of flame and rapid cooling rate, plasma spraying is very suitable to produce ceramic coatings with nano-grained and amorphous structures. In Zhang's study, the plasma sprayed TiCN coating exhibited a nanostructure with 90 nm equiaxed grains, reaching the hardness of 1674 HV [18]. Hong et al. [19] also employed atmospheric plasma spraying (APS) to fabricate TiC coatings, which had the hardness of 784 HV and friction coefficient of approximately 0.49 under high load. Sun et al. [20] synthesized Ti5Si3-TiC composite coatings via reactive plasma spraying. The microhardness of the composite coating was 1782 HV0.2, which is approximately four times higher than that of TC4 alloy. At the same time, the Ti5Si3 improved the isothermal oxidation resistance of the Ti5Si3-TiC composite coatings. Zhang et al. [21] using APS to fabricate TiCN-Mo composite coatings. Compared to those coatings without Mo, the TiCN-Mo composite coatings had increased fracture toughness and reduced wear rate, making the composite coatings have a better wear resistance. These studies prove that the multi-component composite design could efficiently improve the performance of the single-phase ceramic coatings. Thus, plasma spraying would be a promising approach to produce ceramic based composite coatings.

Ti3SiC2 has excellent properties of both metals and ceramics due to its unique crystal structure and bonding characteristics [[22], [23]]. It has high melting point (~3000 °C), lower density (4.52 g/cm3), high fracture toughness, easy machinability, high strength, good oxidation resistance, good electrical and thermal conductivity [[24], [25], [26]], which makes it promising for high temperature protective coatings. However, Ti3SiC2 is relatively soft (~4GPa) and has a low wear resistance [27]. It is known that TiC and Ti5Si3 are impurity phases always accompanying the formation of Ti3SiC2 and usually coexist. TiC has high melting point (3140 °C), modulus and hardness (28GPa) [[28], [29]]. Ti5Si3 has high hardness (9.7GPa), excellent oxidation resistance and corrosion resistance [[30], [31]]. And their thermal expansion coefficient matches Ti3SiC2 (TiC: 7.4 × 10−6 °C−1, Ti5Si3: 9.7 × 10−7 °C−1, Ti3SiC2: 9.2 × 10−6 °C−1), thus they are suitable reinforcing phases of Ti3SiC2 [[32], [33]]. H. Fakih et al. [34] employed chemical vapor deposition (CVD) to prepare (Ti3SiC2/SiC)n multilayered coatings. However, the as-deposited Ti-Si-C film was thin (~20 μm) owing to the low deposition efficiency of CVD technology. With the increasing demand of thick Ti-Si-C coatings for severe work conditions, Pasumarthi et al. [35] explored the possibility of synthesizing thick Ti3SiC2 coatings by plasma spraying. However, due to the short resistance time of the original powder in plasma plume, the nucleation and growth of Ti3SiC2 phase were greatly constrained, resulting in the formation of only a small amount of Ti3SiC2. Studies on the synthesis of high-purity Ti3SiC2 bulk materials have shown that the addition of Al can suppress the evaporation of Si at high temperature and promote the diffusion of Ti and Si atoms, which made the Ti3SiC2 can be synthesized at lower temperature [[36], [37], [38]]. Our previous study has also shown that the addition of Al could increase the content of Ti3SiC2 phase in the Ti-Si-C composite coatings [39]. It inspired us to control the content of Ti3SiC2 phase in plasma sprayed Ti-Si-C coatings by optimizing feedstock or post treatment.

Moreover, annealing treatment was reported to improve the comprehensive performance of plasma sprayed coatings apparently [[40], [41]]. Zhang et al. [42] studied the effects of annealing temperature on the tensile bond strength and porosities of the plasma sprayed Al2O3-13%wt%TiO2 coatings. The results revealed that appropriate annealing temperature could reduce the porosities and improve the bond strength. The study of Dong et al. [43] showed that the porosities of the annealed coatings reduced, both the hardness and toughness improved compared with the coating without annealing. At the same time, the wear resistance increased by more than 25%. Chen et al. [44] also found that the annealing improved the hardness and fracture toughness of coatings fabricated by plasma spraying. Therefore, we expected to improve the properties of plasma sprayed coatings through post-annealing. In this work, we aimed to synthesize TiC-Ti5Si3-Ti3SiC2 composite coatings by plasma spraying Ti/SiC/C/Al powder and the coatings were annealed at different temperature to study the effects of post-annealing on the microstructure and mechanical properties of the composite coatings.

Section snippets

Materials

Commercial Ti powder (99.9% in purity, 30-45 μm), SiC powder (99% in purity, 30-45 μm), graphite powder (5 μm) and Al powder (99% in purity, 25-35 μm) were used as initial materials. The 45 medium carbon steel with size of 10 mm × 10 mm × 10 mm was used as matrix materials. The Ti, SiC, graphite and Al powder with a molar ratio of 3:1:1:1.5 were mixed with dispersing agent (PVP, the addition amount is 1% of the original powder), binder (a mixture of deionized water and CMC-Na with a mass ratio

Phase analysis

Fig. 1 shows the SEM morphologies of the starting material powders, agglomerated powder and the XRD results of the feedstock. As shown in Fig. 1e, irregular Ti and SiC powder were cladded by graphite and Al powder to form agglomerated powder. The agglomerated powder showed a near-spherical shape with average size of 40 μm, which is suitable for plasma spraying. Only diffraction peaks of Ti, SiC, graphite and Al were detected in the agglomerated powder, indicating that no reaction occurred

Conclusion

In this study, post-annealing treatments were conducted on the Ti-Si-C composite coatings by using plasma spraying Ti/Si/C/Al powder mixtures. The microstructure evolution and mechanical properties of as-annealed coatings were investigated. The composite coatings before and after annealing were mainly composed of Ti5Si3, Ti3SiC2 and TiC phases. Annealing treatment has not changed the phase structure of the composite coatings. The content of Ti5Si3 decreased while that of Ti3SiC2 increased,

CRediT authorship contribution statement

Liping Zhao: Data curation, Writing – original draft. Fanyong Zhang: Conceptualization, Methodology, Writing – original draft. Liangquan Wang: Data curation. Shu Yan: Methodology, Investigation. Jining He: Writing – review & editing. Fuxing Yin: Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge Natural Science Foundation of Hebei Province (Grant No. E2020202110) and National Natural Science Foundation of China (Grant No. 51701062) for the financial support of this research work.

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