This paper describes recent effort to synthesize Fe-based amorphous alloys coatings using Cold Gas Dynamic Spraying. Characterization of the gas atomized Fe-based (Fe-Cr-Mo-WC-Mn-Si-Zr-B) powder shows that fully amorphous powder is found when particle diameter is below 20 µm. The coatings produced were composed of the same microstructure as the one observed in the feedstock powder. The overall deformation suggests the occurrence of a localized deformation process at the particle/particle boundary and possible adiabatic deformation softening inside the powder particles during splat formation. The influence of the substrate material on the coating deposition process was also investigated. The synthesis of fully amorphous, porous free coatings using Cold Spray was demonstrated in this work.

This work describes recent progress in Cold Spray of 2618 (Al-Cu-Mg-Fe-Ni) aluminum alloy with Sc addition. Atomized 2618+Sc aluminum powder was sprayed onto aluminum substrates. 2618Al alloy is used for intermediate to high temperature applications in aerospace and automotive industry. The addition of Sc provides higher thermal stability to this alloy by the formation of fine Al 3 Sc particles. The Cold Spray process allows the fabrication of refined microstructure when compared to conventional manufacturing techniques such as casting. The microstructure of the powder and the resulting coating was analyzed using scanning electron microscopy. The mechanical behavior of the powder and the coating was studied using micro indentation measurements. The influence of different heat treatment conditions on the coating was also analyzed. This work shows that Al-Cu alloys with a refined grain structure microstructure can be successfully produced by the Cold Spray process.

WC-Co has been extensively investigated for use in wear resistant coatings for engineering applications. In principal, when the WC particle size decreases in the starting powder, the decomposition of WC increases, and therefore, significant amounts of W 2 C and W 3 C, and even metallic phases, are observed in nanocrystalline WC-Co coatings. The reported increase in hardness of nanostructured materials is generally attributed to the significant decrease in grain size or particle size. However, the presence of brittle, non-WC phases in nanostructured WC-Co coatings leads to sliding and abrasive wear by removal of large plates of the coating. Concurrently, the greater degree of decomposition suffered by the nanostructured powder during spraying leads to a reduction in the volume fraction of the wear-resistant primary WC phase. For the reasons presented above, the present efforts are directed towards the synthesis of a wear-resistant coating using a multimodal WC size distribution of particles in the starting powder. The multimodal distribution is characterized by small WC particle(~50 nm) and coarse WC particles (1.7µm). In addition, the distribution of Co also spanned an order of grain size, hence the name multimodal. The coatings were deposited using HVOF technology.

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.

Thermal sprayed nanoscale materials produce coatings with enhanced properties. Thermally spraying nanopowders, in non-agglomerated form, include several potential advantages. These advantages can not be tested because nanoparticles do not have the inertia required to cross streamlines in typical plasma spray flows. Hence, they are swept away without depositing on the substrate. Analysis of inertial deposition reveals that the non-dimensional Stokes number largely characterizes deposition efficiency. Under low pressure plasma spray conditions, with reduced stand-off distance, the Stokes number of nanopowders can be made to approach the Stokes numbers of typical thermal spray powders. This paper explores the conditions that make the inertial deposition efficiency of nanoparticles quantitatively similar to the typical inertial deposition efficiency of microparticles. Paper includes a German-language abstract.

Nanocrystalline Inconel 718 and Ni powders were prepared using two approaches: methanol and cryogenic attritor milling. High velocity oxy-fuel (HVOF) spraying of milled Inconel 718 powders was then utilized to produce Inconel 718 coatings with a nanocrystalline grain size. Isothermal heat treatments were carried out to study the thermal stability of the methanol milled and cryomilled Inconel 718 powders, as well as the HVOF Inconel 718 coatings. All nanocrystalline Inconel 718 powders and coatings studied herein exhibited significant thermal stability against grain growth as evidenced by a grain size around 100 nm following annealing at 1273 K for 60 min. In the case of the cryomilled nanocrystalline Ni powders, isothermal grain growth behavior was studied, from which the parameters required for the prediction of the microstructural evolution during a non-isothermal annealing were acquired. The theoretical simulation of grain growth behavior of nanocrystalline Ni during non-isothermal annealing conditions yields results that are in good correspondence with the experimental results.

Recent experimental investigations of reactive spray deposition of aluminum alloys have indicated that oxides could not be detected for atomization gas oxygen contents lower than 10%. In order to elucidate this behavior, an analysis of the oxidation kinetics during reactive spray deposition based on the Mott-Cabrera theory of oxidation is proposed herein. A linear growth law is obtained that indicates that the oxide growth rate decreases with decreasing temperature or oxygen pressure. Furthermore, the oxide growth rate is found to decrease faster at low oxygen pressure with decreasing temperature as well as at low temperature with decreasing oxygen pressure. Calculations of the width of oxide stringers as a function of oxygen content and superheat temperature based on this analysis are in good agreement with the experimental observations.

The present paper describes the synthesis of nanocrystalline 316-stainless steel coatings by high velocity oxy-fuel (HVOF) thermal spraying. The feedstock powders were synthesized by mechanical milling to produce flake-shaped agglomerates with an average grain size of less than 100 nm. The powders were introduced into the HVOF spray to successfully produce nanocrystalline coatings. X-ray diffraction analysis and transmission electron microscopy were used to determine the average grain size of the milled powders. Scanning electron microscopy and transmission electron microscopy were used to study the morphology of the nanometric particles and the microstructure of the as-sprayed coatings. The properties of various coating materials were characterized by microhardness measurements performed on the polished surface of the cross section.