The crystalline structure of TiO2 coating is of significant importance for controlling its property and performance, such as photocatalytic activity. The aim of this study is to examine the phase formation mechanisms during high velocity oxy-fuel (HVOF) spraying of TiO2 coating. TiO2 coatings were deposited under different spray conditions using both anatase powder and rutile powder as feedstocks. The results showed that the anatase content in the coatings was increased with the increase of fuel gas flow when using the rutile powder as feedstock. A TiO2 coating of anatase content up to 35% can be obtained by well-melted particles. The high content of anatase phase possibly resulted from rapid solidification and cooling process of the particles. The anatase content in the TiO2 coating deposited with an anatase powder in partially melted state reached 55–65%. The coating deposited by well-melted anatase powder contained the same anatase content as that by rutile powder. A model was proposed to explain the phase formation within the coatings deposited through HVOF spraying.
Authors had reported already that the spreading of BAg alloy on mild and stainless steels could be improved by adding small amounts of NiCl2 or FeCl3 to KCl-LiCl cut. salt flux. In this study, then, when a 18Cr-8Ni stainless steel plate was pre-coated with the mixed salt fluxes and was heated, the corrosion products formed on the plate surface were analized by SEM and X-ray diffractometer. On the other hand, a spreading test of BAg-5 on the same treated plate was done, using only KCl-LiCl eut. salt flux.From these experiments, the effect of used fluxes on the spreading of BAg-5 was concluded as follows;1) The addition of NiCl2 to KCI-LiCl eut. salt promotes the dissolution of chromium and iron from the stainless steel and the resultant corroded surface becomes spongy intermetallic compound, FeNi3, furthermore, the particles layers of nickel were precipitated on the compound owing to electrochemical reaction.2) The addition of FeCl3 to the eut. salt also promotes the corrosion of stainless steel and the surface becomes the spongy compound, FeNi3.3) The spread area of BAg-5 on the particles layer of nickel was considerably larger than that on the compound, FeNi3. This result showed that the wettability of intermetallic compound, FeNi3, is not superior to the each wettability of both constituent elements, Fe and Ni.4) The spread area of BAg-5 on the particles layer of nickel was larger than that on a nickel plate, because the former was larger than the latter in a real surface area to be wet. Furthermore, since the spread area depended on the total surface area of particles distributed on the most outside layer, the amount of NiCl2 necessary to form mono-particle layer was considered to be about 10wt%.5) "De-chromiumnisztion" phenomenon on the surface of stainless steel may be one of the important effect of flux used.
Fracture strength of WC-12Co thermal sprayed coating is investigated experimentally and analytically. In the experiments, one pair of butt cylindrical specimen with coating is subjected to combined tension with torsion stresses. Fracture loci were obtained for three kinds of thickness of the coating in σ-τstress plane. Stress distributions at crack tip singular point on fractured surfaces are analyzed by Finite-Element-Method and approximated by the expression σ＝KR-λ where R means normalized thickness coordinate. It is found that the normal stress distributions are common to all cases of testing stress conditions and so fracture condition of the brittle coating is represented as K≧Kcr in the normal stress distribution even under mixed deformation mode I and III. A critical shear stress distribution for separation could also be obtained
In this study, fracture strength of WC-12Co thermal sprayed coating is investigated experimentally and analytically. In the experiments, one pair of butt cylindrical specimen with coating is subjected to combined tension with torsion stresses. Fracture loci were obtained for three kinds of thickness of the coating in σ-τ stress plane. Stress distributions at crack tip singular point on fractured surfaces are analyzed by Finite-Element-Method and is approximated by the expressionσ=KR-1 where R means normalized thickness coordinate. It is found that the normal stress distributions are common to all cases of testing stress conditions and so fracture condition of a brittle coating is represented as K≥Kcr in the normal stress distribution even under mixed deformation mode I with III. Delamination critical shear stress distribution could also be obtained.
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