Study On Microstructure And Performance Of Aluminized Coatings On Pure Titanium

Due to its good corrosion resistance and high specific strength, titanium and its alloyshave been widely applied in aerospace and petrochemical field. However, oxidation is a bigproblem that has to be solved when titanium and its alloys are applied at temperatures higherthan600°C. In practice, the oxidation resistance of metals is determined by the forming abilityof oxide films that formed on the metallic surface. As so, aluminum was first selected as thehigh temperature oxidation resistant coating because of its best oxophilic nature, which hadled to a stable Al2O3film under high temperature reaction and contribute to a good protectionon the metal matrix.The present work employed arc spraying method to produce aluminum coating on a pureTi substrate, and was aimed to improve the high temperature oxidation resistance ability of thepure titanium substrate. In order to extend the working life of aluminum coatings, NiCrAl andNi95Al5coatings were also sprayed as a diffusion barrier to prolong the aluminum coating’slifespan, separately. Continuous oxidation experiments were operated on the specimens ofaluminum single coating and with diffusion barrier at800°C,900°C and1000°C for64h and100h, respectively. Microstructures evolution were observed with optical microscope (OM),scanning electron microscopy (SEM) and X-Ray diffraction analyzer (XRD), and thediffusion rule of aluminum element as well as the formation mechanism of the diffusion layerswere also studied. In addition, the effects of annealing temperature, keeping time, coatingthickness as well as the sealing way on the thickness of diffusion layers were also investigated.Results showed that, aluminum coating produced by arc spraying can afford an effectiveprotection of the pure titanium substrate at temperatures higher than700°C, and the hightemperature oxidation resistance of the samples was significantly improved. It revealed thatthe inter-diffusion of aluminum and titanium elements had occurred and an aluminized layerwas formed mainly composed of TiAl3, which may provide enough aluminum atoms togenerate Al2O3with the combination with the oxygen atoms. Al2O3was formed not only toestablish a continuous protective film on the coating surface, but also partially enclose thecoating porosity, thereby slowing the rate of oxidation. As compared with that of singlealuminum coating, the specimens with NiCrAlY or Ni95Al5diffusion barrier layer had anextending working life, as the inter-diffusion process of aluminum and titanium elements were effectively suppressed. The aluminized layer depth was studied in relationship with the heatingtemperature, the keeping time, the coating thickness as well as the sealing way. As it increasedthe aluminum coating thickness, extended the keeping time or increased the heatingtemperature, the depth of the diffusion layer can be expanded, while the concentration ofaluminum in the diffusion layer were reduced accordingly. Under high temperature, any of thesealing treatments was conducive to the formation of the aluminum infiltration layer, whereinthe coated water glass will form a new coating on the aluminum coating, which had effectivelyimproved the oxidation resistance of the coating.