Monodispersed chromium oxide (Cr2O3) precursor microspheres have been synthesized by an ethanol solvothermal process without using a template or surfactant. Porous Cr2O3 microspheres consisting of nanoparticles have been obtained from a thermal treatment of the precursor microspheres in air. Compositions of the precursor microspheres and the porous microspheres have been characterized by XRD, IR, TG–DTA and XPS; structures of the porous microspheres have been investigated in detail by SEM, TEM, HRTEM and N2 adsorption–desorption; the structure analysis ascertains that the porous microspheres with diameters of 1.0–1.2 μm are constructed from nanoparticles with sizes of 40–70 nm. Aggregation of the nanoparticles results in the formation of pores with sizes of 20–80 nm in the microspheres. Thick film sensors, made from these porous Cr2O3 microspheres, exhibit excellent toluene sensing performance with a detection limit of 1 ppm at 170 °C.

Highly toluene sensing performance based on monodispersed Cr2O3 porous microspheres

In order to improve the corrosion and tribological properties of steel, chromium oxide coatings were prepared by a new combined process, namely, chromizing and plasma oxidizing treatments using double glow plasma technology under various oxygen flow rates. The composition and microstructure of the coatings were analyzed respectively by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results indicated that the oxygen flow rates had a great effect on the surface structure of the prepared Cr2O3 coatings, and dense and smooth Cr2O3 coatings were prepared at the oxygen flow rate of 10 sccm. The Cr2O3 coatings exhibited the better corrosion resistance which was in good agreement with the results obtained by the microstructure studied. Further mechanical properties test showed that the Cr2O3 coatings with high hardness and elastic modulus adhered well to the steel substrates and displayed excellent wear resistance and low coefficient of friction under dry sliding wear test conditions. The wear mechanism was mostly dominated by the “soft abrasion”.

Corrosion and tribological behaviors of chromium oxide coatings prepared by the glow-discharge plasma technique

Sliding wear characteristics of plasma-sprayed Al2O3 and Cr2O3 coatings against copper alloy under severe conditions

In this study, Cr2O3 coatings were deposited on CF8M and CA6NM turbine steels by high-velocity oxy-fuel (HVOF)-spray process and analyzed with regard to their performance under slurry erosion conditions. High Speed Erosion Test Rig was used for slurry erosion tests, and the effects of three parameters, namely, average particle size, speed (rpm), and slurry concentration on slurry erosion of these materials were investigated. SEM micrographs on the surface of samples, before and after slurry erosion tests, were taken to study the erosion mechanism. For the uncoated steels, CA6NM steel showed better erosion resistance in comparison with CF8M steel. The HVOF-sprayed Cr2O3-coated CF8M and CA6NM steels showed better slurry erosion resistance in comparison with their uncoated counterparts. It may be due to the higher hardness as a result of HVOF-sprayed Cr2O3 coating in comparison with the uncoated CF8M and CA6NM steels.

Slurry Erosive Wear Evaluation of HVOF-Spray Cr 2 O 3 Coating on Some Turbine Steels

Wear behaviour of nanostructured alumina–titania coatings deposited by atmospheric plasma spray

Tribological behavior of Cr2O3 coatings as bearing materials

In this study, joining of dissimilar aluminum alloys 5059-H111 and 7075-T651 was carried out using friction stir welding. The joint has been investigated in terms of microstructure, hardness and mechanical properties. Optical microscopy was used to characterize the microstructure of the weld area. Seven different zones of the microstructure in the joint can be distinguished. It has been observed that a fine grain structure is formed in the nugget zone as a result of recrystallization. Although there is a general hardness decrease throughout the weld region compared to both base metals, the lowest hardness values are seen at the heat affected and thermo mechanically affected zones of aluminum alloy 5059. The mechanical strength of the dissimilar joint was found to be varying between 56 % to 68 % those of the base metal ultimate tensile strength values. The results show that friction stir welding can be successfully applied for the joining of dissimilar aluminums alloys.

Murat I. Kusoglu

Papers

Tribological behavior of Cr2O3 coatings as bearing materials

Mechanical and Microstructural Behaviour of Dissimilar Aluminum Alloys Joined by Friction Stir Welding