SUS316L coatings were sprayed by a high-velocity air fuel (HVAF) system to reduce oxidation and thereby improve the corrosion behavior of stainless coatings. The effects of powder feed rate and particle size on the microstructure, oxide amount and adhesion strength of the coatings were investigated. The corrosion resistance of the coatings was evaluated by conducting salt spray tests. The oxide amount in the coatings sprayed by the HVAF process is below 7 % and adhesion strength is below 34 MPa. In comparison with those of coatings sprayed by a high velocity oxygen fuel (HVOF) system, the oxide amount and adhesion strength are decreased because the particles heated below the melting point of the alloy and insufficient softened in HVAF process. The coatings deposited are original porous, and they become denser through the impinging effect caused by the following sprayed particles. With the increase of powder feed rate and particle size, there is a tendency of reduction in oxides, and an obvious decrease in adhesion strength. Corrosion resistance of the unsealed coatings is insufficient, and this becomes notable with increasing powder feed rate and particle size. The sealed HVAF coating sprayed with the largest particles shows the best corrosion resistance.

The effect of pack boronizing on microstructure and hardness of WC-12wt%Co coating sprayed on a low carbon steel (SS400) was studied using two kinds of HVOF-sprayed WC-Co coatings consisting of a single phase of WC and several phases of WC, W 2 C and Co 3 W 3 C, respectively. Pack boronizing was applied at 1273K for 3.6ks under an argon flow atmosphere, using 5%B4C, 5%KBF4 and 90%SiC powders. Microstructures obtained were characterized by X-ray diffraction, SEM and EDX analyzer. After boronizing, WC and CoW 2 B 2 phases were detected in the both sprayed WC-Co coatings. This suggests that not only WC but also W 2 C and Co 3 W 3 C of WC-Co coatings decomposed by boronizing, resulting in the development of CoW 2 B 2 . However, many porosities with a size of more than 10µm were formed on the coating consisting of WC, resulting in a low hardness of HV600. On the contrary, the coating with W 2 C and Co 3 W 3 C has little porosity and a high hardness of HV2600.

Most of the work published to date on thermally sprayed titanium has been carried out in controlled atmospheres, yielding little information about the reaction of titanium with nitrogen and oxygen. The aim of this study is to investigate the influence of atomization gas on the formation of titanium nitrides and oxides during wire arc spraying. In the experiments, three types of gases (air, nitrogen, and argon) are used to deposit Ti on steel substrates and the microstructure and composition of the coatings, as well as the wire feedstock, are assessed by means of SEM and XRD analysis. The effect of spraying distance on crystal structure and nitrogen content is also investigated in the case of the argon-atomized coating. Paper includes a German-language abstract.

The microstructure of arc sprayed stainless steel 316L coatings appears mainly in bright white matrix, deteriorated layers (grey), and black pores under optical microscopy. The black pores and the chromium-depleted areas in the deteriorated layers are known as the factors for decreasing the ability of protecting substrate under corrosive environments. Results of experiments in this paper suggests, in the condition of this study besides the factors mentioned above, Fe-Cr oxides should be another factor of dominating the corrosion resistance in the coatings. It also describes that the quantity and the distributions of such oxides are great influence on the corrosion behaviors. In this study, two kinds of coatings were used, one with thick deteriorated layers and another with thin deteriorated layers, which were sprayed on mild steel substrate by air atomization and nitrogen atomization respectively. Salt spray test and salt-water dip test were carried out to investigate corrosion behavior in macro and micro view. An effect of sealing treatment on the performance of the coatings was also examined. Results of metallographic examination and image processing analysis are well supported by a detailed investigation of corrosion behaviors of individual phases.

Fatigue strength and fracture mechanism of a medium carbon steel with HVOF thermally sprayed WC-Co coatings were investigated under rotating bending. Two types of commercially available WC-Co powders were used, which have similar total chemical composition with different manufacturing processes. The fatigue strengths of the specimens with thick coatings were lower than that with thinner ones. Especially, it was found that the fatigue strengths of the thick coating specimens decreased more greatly than that of the grit blasted ones. Also, the morphology of the fatigue crack depended on the type of powders and the thickness of the coating. Futhermore, an embedded Al 2 O 3 grit and a crack generated during the grit blasting were found near the fatigue fracture origin of the coated substrate.

Two types of WC-12wt%Co powders, each manufactured by a different process, were thermally sprayed on a medium carbon steel by HVOF, and repeated load tests (rolling contact fatigue test and high cycles fatigue test) were carried out. The surface damages for the two types of coatings were investigated. It has been clear that the coating damages depend on the types of powders. It has been found that in rolling contact fatigue, there are the coatings, in which damage is characterized by delamination, and by a mixture of delamination and cracks. And it has also been found that in high cycles fatigue, there are the coatings, in which damage is characterized by net-like fatigue cracks, and by linear fatigue cracks.