A modified stainless steel coating, named as M-SUS here, was prepared by the air plasma spray process (APS) and the high velocity flame spray process (HVOF) and compared with a conventional stainless steel (JIS: SUS316L). Anodic polarization tests using NaCl, HCl solutions, neutral salt spray test, and exposure test in actual tank for HCl storage were employed for the evaluation of corrosion resistance. Structure of the coatings was investigated by use of optical microscope, scanning electron microscope, electron probe micro-analyzer, and transmission electron microscope. It was found that the coating M-SUS exhibited a remarkably superior corrosion resistance by all tests mentioned above, compared with the conventional ones. Although both of the coatings compose of gamma-austenite (γ-Fe) and delta-ferrite (δ-Fe) phases, the coating M-SUS reveals much less oxide with chromium and more delta-phase enriched with molybdenum. Another exposure test using a mixed acid of 25%HNO3 and 75%HCl yielded that the δ-Fe was not etched out but the γ-Fe vanished, that is, the δ-Fe of M-SUS has a strong anti-corrosion property. It is considered that the superior corrosion resistance of coating M-SUS is attributed to the extensive formation of anti-corrosive δ-Fe and inhibition of chromium depletion resulting from oxide formation.

Oxidation is a serious issue for thermally sprayed alloy coatings, especially in atmospheric plasma spray (APS) process, by which it is believed to be impossible to obtain alloy coatings with low oxide content. Oxidation behavior of Ni20Cr alloy and NiCrSiBC alloy in APS were compared by evaluating collected in-flight particles and sprayed coatings in our previous study. The results clearly show that the oxide content in the in-flight particles and coating of NiCrSiBC alloy was significantly lower than that of Ni20Cr alloy. In the present study, further study of the surface oxide layer of NiCrSiBC coating was carried out by TEM, which showed 10 nm thick oxygen enriched layer. The effects of particle temperature and velocity on the oxidation of the NiCrSiBC alloy were investigated. It was found that increasing particle temperature and velocity did not affect the oxidation of inflight particle of the NiCrSiBC alloy but significantly increased the oxidation after particle impacting substrate. The results are discussed in conjunction with effectiveness of preferential oxidation of Si, B, C and simultaneous vaporization of the formed oxides on suppressing in-flight oxidation and influence of a large amount of sub-particles produced upon impacting substrate on oxidation characteristics when increasing plasma power.

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.

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.