Fabrication of plasma-sprayed TiC-Ti₅Si₃-Ti₃SiC₂ composite coatings from the annealed Ti/SiC powders

Highlights

• The sprayed composite coatings mainly consisted TiC, Ti₅Si₃, and MAX phase Ti₃SiC₂.

• The annealing temperature increased the content of TiC and MAX phase Ti₃SiC₂ in the coatings.

• The microhardness of the sprayed coatings were enhanced by 33% after the Ti-SiC powders were annealed.

• The mass loss of the sprayed coating decreased by 80% after the Ti-SiC powders were annealed.

Abstract

TiC-Ti₅Si₃-Ti₃SiC₂ composite coatings were plasma sprayed in-situ using the annealed Ti/SiC powders and raw graphite powder. The microstructure and properties of the sprayed coatings depending on the annealing temperatures of the Ti-SiC powders were investigated. For the annealed Ti-SiC powders, the interface structure of Ti/TiC/Ti₅Si₃/Ti₃SiC₂/SiC was formed between Ti and SiC. The sprayed coating from raw powders consisted of TiC, MAX phase Ti₃SiC₂, Ti₅Si₃, and residual Si. The sprayed coatings from the annealed Ti-SiC powders consisted of TiC, MAX phase Ti₃SiC₂, Ti₅Si₃, and residual SiC. With the increase of the annealing temperature of Ti-SiC powders, TiC and MAX phase Ti₃SiC₂ increased, and Ti₅Si₃ phase decreased. For the sprayed coatings from the annealed Ti-SiC powders at 1200 °C, their hardness and fracture toughness were enhanced maximally by 33% and 53%, respectively. Their wear resistance was also improved by decreasing the mass loss maximally by 80%.

Introduction

Titanium silicon carbide (Ti₃SiC₂), as a MAX phase, has received much attention because of the mixed salient properties of metals and ceramics [[1], [2]]. It has low density of 4.52 g/cm³, high elastic modulus (320 GPa), better fracture toughness (7 MPa/m²), remarkable damage-tolerant, excellent high-temperature stability (~1700 °C), and good machinability [[3], [4], [5], [6], [7], [8], [9]]. In addition, due to the layered structure similar to graphite, Ti₃SiC₂ can be also used as a lubricating phase [[10], [11]]. These properties make Ti₃SiC₂ useful in engineering applications.

Since Ti₃SiC₂ was first formed by sintering method [12], many synthesis techniques have been adopted to synthesize Ti₃SiC₂ based bulk structural materials, e.g. arc melting [13], hot-pressing (HP) [14], spark-plasma sintering (SPS) [15], pulse discharge sintering (PDS) [16], self-propagating high-temperature synthesis (SHS) [17], and hot-isostatic pressing (HIP) [18]. There are many starting materials employed to synthesize Ti₃SiC₂, such as Ti + Si + C [13,17,19], Ti + Si + TiC [[15], [16]], Ti + SiC+C [7,18,20], Ti + Si + C + SiC [21], Ti + SiC+TiC [22] and TiC+Si [23]. However, based on the above synthesis techniques and starting materials, high temperature (1450–1700 °C) is required during the synthesis process [24]. This makes producing Ti₃SiC₂ based bulk materials expensive and time-consuming.

Compared with producing bulk structural materials, fabricating surface coatings to improve the serve performance of components is more economic [[25], [26]]. In the recent years, Ti₃SiC₂ based coatings have been prepared successfully by many physical/chemical vapor deposition techniques (PVD and CVD) [[27], [28], [29], [30]]. For PVD and CVD, long processing time is also necessary to deposit a limited thickness coating, which is due to the low deposition rate. In contrast with PVD and CVD, plasma spraying could be used to fabricate thick protective coatings on the structure components owing to the high deposition rate, simple process, low cost and the availability of diverse coating materials [[31], [32], [33]]. However, so far, few studies on the fabrication of thick Ti₃SiC₂ based coatings using plasma spraying has been reported. The lack of commercial Ti₃SiC₂ powder is one possible reason. Pasumarthi et al. [24] prepared Ti₃SiC₂ based coatings (~200 μm) by plasma spraying using Ti, SiC and graphite, where plasma was used as the ignition source of the reaction of Ti-SiC-graphite to form Ti₃SiC₂ phase. They discussed the reaction mechanisms of Ti-SiC-graphite and the sprayed coatings consisted of Ti₃SiC₂, TiC, Ti₅Si₃, TiO and Ti phases. Recently, Li et al. [34] prepared TiC-Ti₅Si₃-Ti₃SiC₂ coatings (~200 μm) by atmosphere plasma spraying using Ti + SiC+graphite powders. They found that the coating had the microhardness of 1013 HV_0.2 and fracture toughness of 1.54 MPa/m². They also plasma sprayed TiC-Ti₅Si₃-Ti₃SiC₂ coatings using Ti + Si + graphite powders and obtained a high microhardness (1205 HV_0.2) and fracture toughness (2.39 MPa/m²) [35]. Jiao et al. [36] investigated the influence of metal Mo on the plasma sprayed coatings from Ti + Si + graphite powders. They found that the main phases of the coatings changed from TiC+Ti₅Si₃ + Ti₃SiC₂ to TiC+Ti₅Si₃ + Ti₃SiC₂ + Mo₅Si₃ after adding Mo, and the microhardness increased to 1400 HV_0.2, combining with the fracture toughness improving from 1.96 to 3.35 MPa/m². Zhang et al. [37] plasma sprayed TiC/Ti₅Si₃/Ti₃SiC₂ coatings using Ti + SiC+graphite+Al powders and found the coating had the highest microhardness of 1188 HV_0.2 and fracture toughness of 2.91 MPa/m². These studies indicate that Ti₃SiC₂ phase could be synthesized using Ti + SiC/Si + graphite powders in the plasma flame with high temperature. However, decarburization and decomposition of carbides and MAX phases are inevitable due to the high temperature during spraying [38]. This would weaken the serve performance of the sprayed coatings.

In view of the above problems, in order to inhibit the decarburization of carbides and the decomposition of MAX phases during spraying, many attempts, e.g. changing the spray conditions, reducing the flame temperature and using metal coated powders have been reported [[39], [40], [41], [42]]. In addition, annealing of the sprayed coatings has been also reported as an effective way to increase MAX phases content even if the decomposition of MAX phases occurred during spraying. Frodelius et al. [43] prepared Ti₂AlC coatings by HVOF using Ti₂AlC powder and investigated the phase transformations during annealing of coatings. They found that Ti₂AlC phase was decomposed and the sprayed coating consisted of Ti₂AlC, Ti₃AlC₂, TiC, TiAl, and oxides. After annealing above 700 °C, more Ti₂AlC were formed. Recently, Zhang et al. [44] sprayed Cr₂AlC based coatings using Cr, Al, and graphite powders, and studied the effect of post-heat treatment on Cr₂AlC phase content. They found that the increase of the annealing temperature could increase Cr₂AlC phase content. As reported [45], annealing the powders for spraying could also inhibit the decarburization of carbides effectively during the coating formation and then improved the properties. However, much less attention has been paid on the sprayed coatings from the annealing powders to form MAX phases (especially Ti₃SiC₂ phase).

In the present work, the mixed Ti-SiC powders were annealed at different temperatures. Then, TiC-Ti₅Si₃-Ti₃SiC₂ composite coatings were plasma sprayed using the annealed Ti-SiC powders and raw graphite powder. The evolution of the structure and properties of the coatings from the annealed Ti-SiC powders were studied. The phase formation mechanism was also discussed.