Fatigue tests of WC/Co(Material A)and WC/Hastelloy(Material H)thermally sprayed coatings fabricated by HVOFwere carried out in Na2SO4 solution(pH6.5)in order to investigate methods of improving corrosion fatigue strength.Corrosion fatigue behaviors of thermally sprayed coatings loaded with a small cyclic stress were carefully investigated inorder to discuss the effectiveness of thermally sprayed coatings near the threshold stress σu . When a fatigue test was carriedout in a solution, σu of Material A was same as that of the substrate, and σu of Material H was 45% larger than the substrate.The cross sections of the coatings were observed by SEM after the fatigue test, with σmax = σu and a stress ratio of R = 0.8fatigue. The crack in the substrate/coating interface, and the inter-formational delamination in the coating were observed inboth of the material coatings after the fatigue test at N = 1×106. An interface delamination was generated, and no cracks orcorrosion pits were generated from the interface to the substrate. This was observed on both materials over N = 2.0×107.The coating delaminations were also generated in the cases of loading cyclic stresses under σu , but the delamination stayed at the substrate/coating interface, and no crack propagation was discovered after reassessment.
Cold spraying is receiving attention for producing high quality metallic coatings. In this paper, the influence of particle velocity on the adhesion strength of cold-sprayed copper coating is carefully investigated without other factor's effect. In cold spray, residual stress is known as one of factor which affects adhesion strength. Residual stress changes with changing coating thickness. Therefore, if thickness kept same value in each gas conditions, effect of particle velocity could be evaluated without effect of residual stress. Heat treatment is done to evaluate other factor than particle velocity. Considering those, the following result is produced. 1) Adhesion strength decrease with increasing coating thickness. 2)Residual stress is big factor decreasing adhesion strength. 3) Residual stress decrease with increasing process gas temperature. 4) Adhesion strength linearly increases with increasing particle velocity.
The authors did thermal cycle test about an Al2O3 sprayed 1%Cr-0.5%Mo steel and a 18%Cr-8%Ni steel, and compared the failure type of thermal sprayed coating in both steels. The results obtained are follows. (1) The thermal cycle fatigue life of the thermal sprayed 1%Cr-0.5%Mo steel is worse than that of the thermal sprayed 18%Cr-8%Ni steel. (2) In the case of the thermal sprayed 18%Cr-8%Ni steel that is difficult to oxidize at high temperature, while thermal cycle, a tensile stress occurs in a sprayed coating and a compressive stress occurs in a substrate. So the thermal strain occurs in the interface between the sprayed coating and the substrate. The sprayed coating delaminates when the thermal cycle reaches the limit value. (3) In the case of the thermal sprayed 1%Cr-0.5%Mo steel that is easy to oxidize at high temperature, while thermal cycle, dense oxide film occurs in the interface between the sprayed coating and the substrate, and the adhesive strength in the interface becomes small. So the sprayed coating delaminates under the lower thermal strain.
In suspension spraying, the vaporization of ethanol solvent causes the agglomeration of fine particles, which leads to form molten alloy droplets composed of hundreds of fine mixed particles during the flight. Intermetallic compound of NiAl was coated by reactive suspension plasma spraying with fine nickel and aluminum mixed powder. Highly concentrated suspension of ethanol solvent is preferred to form NiAl coatings without retained elemental powders, however, the coatings contain appreciable amount of Al2O3 due to engulfed air into the plume. The shroud was installed at the gun tip to prevent the oxidation, so that the coatings are composed of mainly NiAl with carbide. The carbide is formed based on the sooting reaction of ethanol pyrolysis, instead of its burning. Mixed solvent of ethanol and water was supplied to control the carbide formation, since the decrease in carbide is expected through the water-gas reaction. The soot formation is suppressed by ethanol and water mixed solvent, so nearly pure NiAl coatings are formed without containing residual nickel and aluminum, as well as oxides and carbides.