Ultrasonic cavitation erosion of gas nitrided Ti-6Al-4V alloys.
Ultrasonic cavitation erosion experiments were performed on Ti-6Al-4V alloys samples in annealed, nitrided and nitrided and subsequently heat treated state. The protective oxide layer formed as a result of annealing and heat treatment after nitriding is eliminated after less than 30 min cavitation time, while the nitride layer lasts up to 90 min cavitation time. Once the protective layer is removed, the cavitation process develops by grain boundary erosion, leading to the expulsion of grains from the surface. The gas nitrided Ti-6Al-4V alloy, forming a TixN surface layer, proved to be a better solution to improve the cavitation erosion resistance, compared to the annealed and nitrided and heat treated state, respectively. The analysis of the mean depth of erosion rate at 165 min cavitation time showed an improvement of the cavitation erosion resistance of the nitrided samples of up to 77% higher compared to the one of the annealed samples.
Ultrasonic cavitation erosion of high-velocity oxygen-fuel (HVOF) sprayed near-nanostructured WC-10Co-4Cr coating in NaCl solution.
The high-velocity oxygen-fuel (HVOF) spraying process was used to prepare near-nanostructured WC-10Co-4Cr coating. The cavitation erosion behavior and mechanism of the coating in 3.5 wt.% NaCl solution were analyzed in detail. The results showed that the amorphous phase and WC grain were present in the coating. The cavitation erosion resistance of the coating was about 1.27 times that of the stainless steel 1Cr18Ni9Ti under the same testing conditions. The effects of erosion time on the microstructural evolution were discussed. It was revealed that cracks initiated at the edge of pre-existing pores and propagated along the carbide-binder interface, leading to the pull-out of carbide particle and the formation of pits and craters on the surface. The main failure mechanism of the coating was erosion of the binder phases, brittle detachment of hard phases and formation of pitting corrosion products.
Mechanism of high-temperature plasma nitriding of titanium
Abstract The paper presents the results of studies on plasma nitriding of titanium in nitrogen at 1030°C. For the nitriding process, flat specimens were placed on the cathode and, after the treatment had been carried out, they were analyzed for differences between the layers produced on their upper and lower surfaces. The nitrided layers have the same thickness on both sides of the specimens. Microhardness and corrosion resistance in 15% H2SO4 of the layers produced on the upper surfaces are higher than those of the layers formed on the lower surfaces of the specimens. The reason for these differences is the contamination of the lower layer by carbon. Carbon also causes an increase in the value of the lattice parameter of the TiN nitride. The action of nitrogen ions on the upper surface of the specimens entails sputtering of the surface and removes the contamination by carbon. For this reason, the nitrided layer produced on the upper surface contains less impurities and has better properties.