In this paper, the development of surface oxide scale and the evolvement of spallation mechanism of Fe-21Cr-5.6Al super alloy were investigated at 1200°C and 1300°C. The oxidation kinetic curves were obtained by isothermally measuring the weight gain of alloy oxidized with various time durations. The morphologies of oxide scale and grain structures were observed by SEM/EDX, and the phase structure was analyzed by XRD. The results show that the oxidation processes follow the parabolic law and the oxidation rate is higher at 1300°C than 1200°C. Though the FeCrAl alloy shows capabilities against oxidation even at a high temperature of 1300°C, the oxidation behavior and mechanism are distinct from those at moderate temperatures (<1000°C). Different morphologies and phase structure were found in oxide scales generated at different temperatures within the same time duration. Typical buckling was observed in the super alloy when it was subjected to 1200°C. Equiaixed grains with multiple voids were found near the alloy surface. At 1300°C, a flat and thicker oxide layer was formed. The grains were stretched vertically against the alloy and presented as coarse and compact near the interface. The vertically stretching of grain was triggered by fast element transportation inside the alloy. The differences in grain morphologies among the different test temperatures demonstrated that although the super alloy followed parabolic law at both test temperatures, the oxidation processes were different due to the evolvement of grain morphologies and oxide scale structures caused by exposure to high temperature.

This study assesses the effect of carbide grain size on the wear performance of HVOF-sprayed WC-CoCr coatings. The coatings were produced from two powders, one of conventional grain size and one with submicron carbide. Both coatings were subjected to abrasive rubber wheel wear tests in wet and dry conditions with 220 nm titania particles and 368 μm sand particles, respectively. Detailed examination before and after wear tests shows that the wear mechanism depends on the relative size of the carbide and abrasive particles.

HVOF spraying of fine feedstock powder allows the deposition of cermet coatings with outstanding properties, but selecting and optimizing process parameters can be difficult. In this study, investigators employ a design of experiments (DOE) approach to identify the most relevant process parameters in the spraying of 2-8 μm Cr 3 C 2 -NiCr powders. In a screening step, all parameters were assessed in a 12-run Plackett-Burman experimental design and linear models were used to estimate their effect on coating properties and deposition efficiency. The five most influential parameters were then analyzed in a 16-run fractional factorial set of experiments in order to determine their effect, along with all possible two-way interactions, on coating quality.

A new plasma chemical process has been found that produces tungsten thin films. Using fine powders or precursors as feedstocks, the process vaporizes the feedstocks and then deposits nanometer size grains. The deposition kinetics of structures produced with this technique vary greatly from classical plasma spraying methods. Equiaxed and columnar grains (30-150 nm) are formed instead of splat structures, although the grains may continue to grow after spraying.

This work evaluates the microstructure and composition of zirconia films produced by thermal plasma chemical vapor deposition (TPCVD). The results show that TPCVD has the potential to produce durable ceramic films with columnar structure, even in open air. Paper includes a German-language abstract.

In this study, nanostructured TiO 2 deposits are produced by liquid flame spraying using a two-fluid atomizing nozzle. The coatings are systematically analyzed to determine the effect of spray parameters on grain size and phase structure. Using a butyl titanate ethanol feedstock and appropriate process conditions, TiO 2 deposits have been produced with a phase concentration of more than 80% anatase and a grain size of 30 nm. Paper includes a German-language abstract.

For many years, a new interest in nanomaterials, with grain sizes smaller than 100nm, has emerged. This is due to the enhanced properties of the resulting sintered materials or coatings compared to those with coarser-grained materials. This paper is devoted to the feasibility to produce nanomaterial coatings by a dc plasma spray process. Until now, only thick coatings (> 100µm) have been elaborated using this technique, by injecting, with a carrier gas, micrometric particles in the plasma flow. But, it is not possible to inject too small particles (<5µm) without perturbing drastically the plasma jet by the high carrier gas flow rate necessary to give them a high enough momentum. This work presents a new dc plasma spray process, designed to elaborate alumina nanocoatings. The most important step of the process is the control of the ceramic nanometric particle penetration in the plasma. Because of their small size, a liquid, which density made the momentum transfer more efficient, replaced the carrier gas with an injector creating calibrated droplets with controlled velocity and flow rate. To study the liquid-plasma interaction, the penetration of pure water in an Ar/H 2 plasma jet was investigated by means of emission spectroscopy. The modification of temperature field together with oxygen concentration was determined quantitatively. Emission spectra were treated with a new localization method, avoiding the use of Abel's inversion implying a cylindrical symmetry, destroyed by the liquid injection. Such measurements allowed optimizing the liquid penetration in the plasma jet. Alumina nanopowders were dispersed in a liquid to form a stable suspension, which was injected in the plasma. The layered particle morphology, collected on glass substrates at different distances downstream of the injection point, was then studied.

Ni powders prepared by mechanical milling under liquid nitrogen for 15 hr were sprayed using two stoichiometric ratios of the oxygen-fuel mixture in an effort to promote the formation of fine oxide phases. The oxide phases were introduced in an effort to improve mechanical properties and thermal stability of the coatings, via chemical reaction between oxygen and milled powders during flight and after impingement. The microstructure and properties of the milled powders and as-sprayed coatings were characterized by scanning electron microscopy, transmission electron microscopy and nanoindentation. The average grain size of the milled powders was 15.7 ± 5.1 run and ultrafine NiO and Ni 3 N particles with a size less than 5 run were distributed in the milled powders. These fine oxide and particles distributed in the powders were formed as a result of interaction between Ni, N from the milling slurry, and O from the surrounding environment under the energetic milling conditions. The coating microstructure was composed of nanocrystalline grains with an average grain size of 92.5 + 41.6 nm and extremely fine NiO particles of ~5 nm distributed homogeneously inside the grains. Ni 3 N phase was not found in the coating as it appears to have decomposed during HVOF thermal spraying. The coating sprayed with higher oxygen fraction in a hydrogen-oxygen mixture showed no significant increase in hardness and elastic modulus when compared to those of the coating sprayed with lower oxygen fraction in hydrogen-oxygen mixture. This was attributed to the small difference in the volume fraction of NiO particles between the coatings. These results indicate that new techniques of ultrafine dispersoid introduction in nanocrystalline coatings are potentially attractive as a means to improve the mechanical properties of the coating through reactive HVOF spraying.