Suspension and coating characterization of high velocity suspension flame sprayed (HVSFS) mixed titanium oxide-titanium carbide coatings

Three different suspensions of pure titanium carbide were thermally sprayed through high velocity suspension flame spraying (HVSFS) in order to deposit mixed titanium oxide titanium carbide coatings. One suspension was prepared by using milled feedstock powder (TC1), while two suspensions were prepared using non-milled carbide powder dispersed in pure deionized water (TC2) and deionized water with additives (TC3), respectively. As reference, a pure titanium oxide HVSFS coating was investigated, which was deposited from a commercial aqueous suspension W 740 X. Each suspension was set to a solid content of 20 wt% and showed shear thinning behavior. Through the use of pure titanium carbide suspensions, it was possible to achieve a two-phase titanium oxide titanium carbide coating with significant oxidation of the carbide phase. In the composite coatings, the retained titanium carbide particles with their higher Young's modulus act as reinforcement of the titanium oxide matrix (anatase and rutile phases) and therefore raised the microhardness up to 1014 HV0.05 compared with 527 HV0.05 of the pure titanium oxide coating, composed of anatase and rutile phases. The lowest porosity and the highest hardness values among the mixed coatings were achieved by the use of suspension TC1, milled powder with a median particle size of 2.66 mu m. Each composite coating showed high surface roughness with R-a values over 15 gm. The addition of an organic additive to the suspension and the modification of its pH value by ammonia water resulted in coatings with homogeneously distributed titanium carbide particles. By comparison, the pure titanium carbide coating exhibited the lowest porosity beneath 1% and a significantly lower surface roughness than the mixed oxide carbide coatings.