Based on the superior properties, such as high specific strength and stiffness together with outstanding chemical corrosion resistance, titanium matrix composites (TMCs) are considered as promising candidates for structural components in automobile, aviation, and aerospace engineering. However, the discontinuously reinforced titanium matrix composites (DRTMCs) with a homogeneous reinforcer distribution often inevitably suffered from ambient temperature embrittlement challenge, limiting the scope of potential application in multiplefields. To combat this problem, it has been documented in literature that the DRTMCs with a special quasi-continuous network architecture fabricated through a cost-effective powder metallurgy (PM) method exhibited appreciable ductility and excellent strength, which overcame the bottleneck problem of DRTMCs, leading to better strength-ductility synergy. Unfortunately, even though the chemically stable ceramic reinforcements were beneficial for the improvement of oxidation resistance, the presence of large volume fraction of titanium matrix still exhibited strong propensity for oxidization especially in the atmosphere environment beyond 550 °C. With regard to this, further enhancing the oxidation resistance of network-strengthened TMCs and thereby promoting their service temperature will require the introduction of suitable anti-oxidation coatings.
Coating material systems like aluminide, Ti-Al-X (X = Cr, Nb, N), silicide ceramics, and MCrAlY (M = Ni, Co, or NiCo) were previously reported to possess better oxidation resistance. The most promising candidate amongst them is the MCrAlY alloy showing desirable ductility, high strength, outstanding anti-oxidation performance, and hot corrosion resistance at elevated temperatures. In particular, the slowly formed dense Al2O3film and the coating microstructures suppressed elemental diffusion, contributing to excellent oxidation resistance. MCrAlY also has capability to produce a continuous and adherent thermally grown oxide (TGO), as well as good cracking/spallation resistance. As a result, this sort of coating system demonstrated the potential to be employed as bond coatings in thermal
barrier coating (TBC) application. The commonly used processing methods for MCrAlY coating include: air plasma spray (APS) , vacuum plasma spray (VPS), high velocity oxygen fuel deposition (HVOF), magnetron sputtering, electron beam physical vapor deposition, and laser deposition. Among them, the air plasma spray technique is considered as a suitable method to produce the coatings, because of its appreciable tunability and desirable cost-effectiveness. For instance, Vermaak et al. produced theγ’strengthened MCrAlY coating by plasma spraying, which presented superior adherence than the NiAl-based coatings. Chenet al. studied the effect of alloying elements (Ta, Mo) on the microstructures and properties of NiCoCrAlY coating by APS technology, revealing that Ta addition improved the oxidation resistance, and Mo addition enhanced the adhesion between coating and substrate. Furthermore, it was reported that post treatments could also modify the MCrAlY coating. For example, Cai et al. found that the high-current pulsed electron beam irradiation refined the grains of as-sprayed CoCrAlY coating, effectively improving the oxidation resistance. Deng et al. reported that the cathode plasma electrolysis treatment also enhanced the anti-oxidation performance of thermal sprayed NiCoCrAlY bond coat.
In summary, the CoCrAlY-TiB2composite coating was synthesized on the TMCs surface by APS technique to enhance the oxidation resistance. The microstructures were characterized and the cyclic oxidation behavior was elucidated in detail. The mainfindings are listed as follows:
(1) The optimized APS power was 30.25 kW, and the corresponding CoCrAlY-1TiB2(wt.%) coating was composed of TaC, SiC, Co3Ti, CrSi2, CrB2, Al-richβ-(Co, Cr)Al andγ-(Co, Cr) solid solution phases;
(2) The bonding strength between the coating and substrate experienced an increasefirst and then decrease trend with the increase of APS power, and maximally reached 27.63 MPa. Besides, the TiB2 addition was beneficial to improving the bonding strength when the power was higher than 27.50 kW, and the maximum enhancement was about 22.6% under 35.75 kW;
(3) The anti-oxidation performance was significantly enhanced by introducing CoCrAlY-TiB2composite coatings. Compared with the uncoated substrate, the weight gain of the coated specimens during cyclic oxidation test reduced by 66.7%, 73.1%, and 75.2% at 700, 800, and 900 °C, respectively. Such enhancement was attributed to the enhanced bonding strength and the formation of uniform and dense Al2O3and Cr2O3oxides which hindered internal oxidation.
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