In this investigation, a nanostructured NiCr coating was deposited on boiler steel by cold spraying and its oxidation behavior was evaluated under cyclic thermal conditions. Oxidation kinetics were established through weight change measurements and the oxidized samples were characterized using XRD and SEM analysis and X-ray mapping. The nanostructured coating was more than two times harder than its microstructured equivalent and in oxidation tests, reduced the weight gain of the boiler steel by 68%. The coating was found to have protective oxides in its oxide scale and was shown to be spallation-free.

The Cold spray method of material deposition is widely used for surface enhancement, to improve properties such as corrosion and wear. A detailed microstructural analysis of cold sprayed Ni based coatings (IN625 (Inconel 625 is a Trademark of Huntington alloys corporation) using the transmission electron microscope, revealed the occurrence of three distinct types of microstructures in the as sprayed condition, adjacent to each other. These include the occurrence of large (> 1 μm) grains having a high dislocation density, along with regions comprising shear bands (20-30 nm wide) and twins with a large aspect ratio (> 1000), along with locations having a very fine grained structure (20-30 nm). The crystallite size, using a Hall-Williamson plot measures an average 50 nm. The substrate deformation indicates a bilayer dynamic recrystallization, as a means to accommodate the strain. The microscopy studies will serve to correlate the bonding mechanism of cold sprayed IN625 on 4130 steel.

In this study, Ni-20Cr alloy powder was synthesized and deposited on T22 boiler steel by HVOF spraying. Coated and uncoated test samples were placed in a silicon carbide tube furnace and subjected to cyclic oxidation conditions in 900 °C air. Oxidation kinetics were established via weight change measurements and oxidation products were characterized based on XRD and SEM-EDS analysis. The results show that in addition to improving the oxidation resistance of the steel, the Ni-20Cr coatings also reduced spallation due to the formation of a protective Cr2O3 phase.

Thermal spray of Al was carried out on the modified 9Cr-1Mo steel to evaluate the steam oxidation resistance of the sprayed Al coating. Atmospheric plasma spray process (APS) was used to coat aluminum on sandblasted 9Cr-1Mo steel substrate. The coating thickness was around 40 µm. The coated specimens were steam oxidized in four different temperatures, ranging from 600 to 750°C. The results show that the scale growth occurred in the interface between coating and substrate subsequently it penetrated into the coating structure. Al diffused into the alloy substrate with high solubility. The diffusion increased with increase in the steam temperature and test duration. Diffused aluminum formed the high hardness intermetallic compound in the substrate near the coating/substrate interface. With increase in the test duration, the intermetallic compound moved towards the bulk and at prolonged aging, it became dissolved. This was identified from the decrease in the micro hardness values at coating/substrate interface at prolonged duration. The scale growth at the substrate surface of Al sprayed steel was much controlled compared to the uncoated specimens.

Thermally sprayed hardmetal coatings are often used to protect valves in steam turbines against wear. Normally the valve spindles are made of CrMo steel and their bearings are made of Stellite, a cobalt-base alloy. In this investigation, a block-on-ring wear test is used with rings and blocks made of the same materials as the spindles and bearings. The rings were coated with Cr3C2-NiCr by HVOF spraying. Wear tests showed increasing weight loss up to 400°C and a sharp decline at 600°C due to a thin surface oxide layer that begins growing at 450°C. The coatings are characterized based on SEM, EDX, and hardness measurements. Paper text in German.

The work reported herein deals with the interaction among the thermally sprayed coatings and steel substrate in acid aqueous solutions investigated by using the electrochemical process. Thermally sprayed coatings on steel substrate for such as the tribological applications except for the sacrificial anode property have been extended. The environment of these machinery components is often utilized accompanied with the aqueous solutions. The galvanic corrosion occurred in aqueous solutions among the metals or alloys have been well known. In the actual spray process, the formation of penetrated pores or crevice defects are not generally avoided. In the aqueous solution environment, the penetrated defects cause the galvanic corrosion between coating and steel substrate, and tend to be occurred the coating spalling. In this work, the electrochemical process was employed to determine the corrosion phenomena. The preferable combination of sprayed coatings and steel substrate are discussed and the examples of design of the intermediate layers are proposed.

Low cycle fatigue tests were performed at room temperature (RT) and at 673 K for l%Cr-0.5%Mo steel comparing the specimens coated with chromium carbide by gas spraying and the ordinary uncoated specimens, and the mechanism of fatigue crack formation was investigated. Following observations and conclusions were made: (1) When sprayed with ceramic, the fatigue life suffers reduction at either temperature, but at 673 K, the degradation was so much smaller than that at RT that the fatigue life was actually, though slightly, longer than that at RT. (2) The cracks are initiated in the ceramic layer very early in the whole fatigue life, the crack initiation lifetime becoming the longer, the smaller the strain range. (3) The fatigue failure process can be viewed as comprising following steps: first, early initiation of fatigue crack at the surface of the ceramic coating, rapid propagation through it to the substrate metal, and initiation of crack in the metal, the initial rate of propagation of such a crack being a number of times (perhaps as much as one full order of magnitude) faster than that in uncoated steel.