Investigation On The Reactive Plasma Sprayed Nanocrystalline Titanium Nitride Coating

The Nano-metred coating has been a key branch in the field of surface engineering. And the reactively formed nano-metred coatings called more interest in these years, due to their further improved properties.Nano-meter TiN coating was fabricated via reactive plasma spraying (RPS), using the self-designed and made gas-tunnel type reactive plasma-spraying gun. The spraying parameters, the mechanical and tribological properties of the RPS TiN coating were investigated, and the formation mechanism of the nano-scaled TiN coating was discussed. XRD, SEM, TEM and HRTEM were applied to analyze the phase composition and microstructure, as well as crystal defects in the RPS TiN coating.It was found that the deposition efficiency and the cross-sectional micro-hardness of theRPS TiN coating could be maximized to 57.2μm / s and Hv100-2027.9 respectively, undercondition: P=28⁴0kW, N2/Ar= 1:1-3:1, the N2 gas flow rate in the reactive chamber assigned 20L/min, spraying distance being 20-30mm.Experimental results show that the as-deposited coating mainly contained TiN phase, also a small quantities of oxides of titanium were detected. The RPS TiN coating presented a typical wavy multi-layered structure, and the adhesive state among the layers is in good condition, without distinct interfaces. TEM analysis indicated that the reactive plasma sprayed TiN was composed of nano-scaled crystal grains ranging from 50⁷0nm; and the high resolution TEM study illustrates the existence of the amorphous matter, and that the nano-scaled crystal grains are enclosed by the amorphous matter. And the nano-crystallites in the as-sprayed TiN coating are separated by the small-angle (about 7) grain boundaries. Defects such as extendeddislocations, pile-ups of dislocation and grain boundaries (twin boundaries) were also investigated.An obvious indentation size effect (ISE) was surveyed, and the fracture toughness of theTiN coating was calculated using the micrographic indentation technology. The result is thatKIC = 4.51MPa m^1/2, which is quite superior to that of the thermal sprayed Al2O3 and ZrO2coatings.The wear-resistance of the RPS TiN coating is significantly higher than that of the quenched M2 high-speed steel. The wear resistance of the quenched M2 high-speed steel was improved about 2.5 times by the RPS TiN coating under high load. The antifriction performance of the TiN coating is also very excellent due to the self-lubricating effect of TiN, the friction efficient of which is about 5.3% that of the quenched M2 steel.Essentially, the process of reactive plasma spraying TiN coating is combination of the plasma-assisted chemical vapor deposition (PACVD), and combustion synthesis of Ti in nitrogen gas. The nitrogen atoms involving in the chemical reaction originated from the working gas and nitrogen gas in the reactive chamber, and the latter is chief.