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Deformation mechanisms especially grain boundary(GB)-related plastic behaviors are essential for understanding and designing advanced hard coatings. This work systematically studied mechanical behaviors and the underlying interfacial mechanisms in Ti-Mo-N coatings, which is predicted to have both high hardness and toughness according to first-principle calculation. Mo-doped TiN coatings were prepared by multi-arc plating technique as a function of N2 pressure. The as-deposited coatings demonstrate a bct-Ti2N + fcc-TiN dual-phase structure below 1.2 Pa, and single-fcc structure at above 1.2 Pa. Single-fcc phase Ti-Mo-N coatings show considerable higher hardness and load-bearing capacity compared to pure TiN coating due to alloying effect of Mo. Transmission electron microscope (TEM) study unveil that GB plays the key role in the deformation behaviors. The Mo-doped TiN coatings exhibit enhanced plastic deformation ability of GBs, including substantial plastic flow and twinning, which retard intergranular cracking that is geometry necessary during GB glide and rotation. While the appearance of the bct-Ti(Mo)2N phase leads to stress concentration at bct-fcc interface during deformation because of the large lattice misfit, considerably lowering load-bearing capacity. In addition, Ti-Mo-N coatings have lower friction coefficient (COF) and wear rate than TiN coating in reciprocal tribology tests, which is attributed to the higher hardness and toughness, as well as tribochemistry-induced solid lubricant effect.

Carbon nanotubes (CNTs) with superior properties were introduced for the first time into Fe-TiB2 cermet coating to strengthen its wear resistance. Initially, composite feedstock in which every powder was not only highly spherical and dense but also contained uniform CNTs and TiB2 were successfully prepared. Then, Fe-based metallic and Fe-TiB2 cermet coating with or without CNTs were deposited via high velocity air fuel (HVAF) to mainly investigate the effect of CNT addition on the microstructure and wear behavior of coatings. It shows that, with incorporation of 2.5 vol% CNTs, the coating microstructure became denser and the microhardness and fracture toughness of Fe-TiB2 cermet coating obtain increases of 32% and 44% (or 61%). As a result, the improved tribological properties, the lowest porosity and fewest wear crack contribute to its best wear resistance with specific wear rate of 2.47 × 10−6 mm3/N·m. Oxidative and mild abrasive wears rather than severe adhesive wear were the dominant wear mechanism of coating reinforced by CNTs.

In this paper, agglomerated and dense nanostructured NiCrCoAlY-TiB2 composite powders prepared by spray drying and plasma spheroidization were used for activated combustion high velocity air fuel (AC-HVAF) spray. The effect of sprayed powders on the microstructure, tribological properties and corrosion behavior of the AC-HVAF sprayed coatings was systematically investigated. The present results demonstrate that plasma spheroidized powders are beneficial for dense microstructure and can effectively enhance coating's wear and corrosion resistance in turn. The coating sprayed by plasma spheroidized powders exhibits good wear resistance with the coefficient of friction of 0.48 ± 0.01 and wear rate of 1.6 ± 0.2 × 10−5 mm3 N−1 m−1. The good wear resistance is mainly due to the uniformly distributed nanostructured TiB2, which can work as the hard-skeleton support. The electrochemical polarization tests and electrochemical impedance spectroscopy (EIS) analysis show that the good corrosion resistance can be attributed to the TiO2 and B2O3 oxide films formed on the surface and dense microstructure of as-prepared coatings.

MoSi2–CoNiCrAlY nanocomposite feedstock was prepared by high energy ball milling starting from a mixture of 50 wt.% MoSi2 and 50 wt.% CoNiCrAlY powders. The microstructure and thermal stability were studied by means of scanning electron microscope (SEM), X-ray diffraction (XRD), laser particle size analyzer and differential scanning calorimeter (DSC). The results show that only grain refinement occurs in MoSi2–CoNiCrAlY powders during the whole milling process. The milled powders have nanocrystalline structure composed of nano-sized hard MoSi2 phase uniformly dispersed in CoNiCrAlY binder phase matrix. After milling for 30 h, both the average particle size and grain size of the powders are almost stabilized. The prepared nano-composite feedstock exhibits spherical morphology, uniform particle size and good thermal stability, which is expected to be used directly for thermal spraying hard coating after sieving slightly.

A nanostructured Ni60–TiB 2 composite coating (Ni60 is a brand of Ni-based self-fluxing alloy with a hardness of HRC60) was sprayed on steel substrate by high velocity oxy-fuel (HVOF) process using high energy ball milled powders. Its sliding wear resistance at room-temperature was evaluated by ball-on-disc testing. For comparison, conventional Ni60–TiB 2 composite coating was prepared by HVOF using mechanically mixed Ni60 and TiB 2 powders and tested under the same conditions. The results show that the nanostructured composite coating has excellent mechanical properties and sliding wear resistance due to the microstructural homogenization and the well preserved nanostructure characteristic of the ball milled powders. Adhesive and abrasive wears are found to be responsible for the wear down mechanisms of the nanostructured Ni60–TiB 2 composite coating.