Reactive plasma spraying was carried out with aCAPS system, able to spray under various chamber gases(air, Ar,N2, CH4, etc.) and chamber pressures(from 10 to 400 kPa). Commercial Ti powders were used as a starting material (99% Ti,50-80 um average nominal size) and nitrogen was selected as the chamber and carrier gas. Chamber pressure was varied from 20 to 90 kPa to achieve different degrees of titanium nitride phase formation. Mixtures of Ar/H, and Ar/N2 were used as plasma gases. Coatings with an average thickness of 300 um were deposited on sandblasted AISI304 steel substrates. Input electrical power to the plasma torch was maintained in the range 36.0-40.6kW. More details on plasma spray parameters are reported in Table 1. Coatings were identified as 2A,2N,9A and 9N, the first figure stands for the spraying pressure(2=20 kPa,9=90 kPa), the letter for the type of plasma gas used(A=Ar/H2,N=Ar/N2).
Table 1 Main plasma spraying parameters
In Fig.1 cross-sections of coatings deposited under20 kPa and 90 kPa are shown.Titanium nitrides formation was achieved as confirmed by X-ray diffraction patterns and back-scattering scanning electron micro-scope images. Nitrogen content was measured by the inert gas melting method. Vickers microhardness measurements were carried out on coating cross-sections with an indenting load of 300 g, according to ASTM E384.
Fig.1. Cross-section of RPS sprayed coatings under Ar/H2plasma gas:(a)20 kPa;(b)90 kPa.
Coated samples were all grinded to an average final thickness of 200 um and, prior to performing corrosion tests, all specimens were mechanically polished with silicon carbide emery papers, diamond paste(3 um) on cloths and then resin embedded. Coatings from each set of similar samples were also mechanically detached from their substrates and tested. AISI304 steel andcommercially pure titanium specimens were also tested in the same conditions for comparison.
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