Growth Characteristics and Kinetics of Niobium Carbide Coating Obtained on AISI 52100 by Thermal-reactive Diffusion Technique

Niobium carbide coating was produced by thermal-reactive diffusion technique on AISI 52100 steel in salt bath at 1 123 K, 1 173 K, and 1 223 K for 1, 2, 4, and 6 hours. The salt consisted of borax, sodium fl uoride, boron carbide, and niobium pentoxide. The presence of NbC phase on the steel surface was confi rmed by X-ray diffraction analysis. Microscopic observation showed that niobium carbide coating formed on the substrate was smooth and compact. There was a distinct and fl at interface between the coating and substrate. The micro-hardness of niobium carbide coating was 2892±145HV. The thickness of coating ranged from 1.6 μm to 14μm. The forming kinetics of niobium carbide coating was revealed. Moreover, a contour diagram derived from experimental data was graphed for correct selection of process parameters. Some mathematical equations were built for predicting the coating thickness with predetermined processing temperature and time. The results showed that these mathematical equations are very practical as well as the kinetics equation.

Plasma electrolytic oxidation of AZ31 and AZ91 magnesium alloys:Comparison of coatings formation mechanism

The growth kinetics of PEO coatings on AZ31 and AZ91 magnesium alloys were studied and correlated with their structure,compositions(phase and elemental)and corrosion resistance.It was established that the coatings have a two-(outer and anodic)or three-layer structure(outer,inner and anodic)depending on the treatment time.Briefly,at short treatment time only an anodic layer and outer layer exists.Growth of the outer PEO layer takes place due to the micro discharges,which occur in vertical pores and voids with spherical cross-section.If the time is increasing,and electrolyte inside of the pores is heating-up,etching of the Mg substrate and oxide film becomes more dominant and horizontal pores in the interface between coating and metal are formed.In the pores new anodic layer will form and at this time the formation of the third inner layer starts.The growth of the inner layer happens via the anodic film as a result of micro discharge ignition in the horizontal pores,accompanied by formation of plasma in numerous micro-voids of this layer.The coatings formed on AZ91 alloy are denser,than those on AZ31,which is related to the difference in the rates of inner layer growth and dissolving of oxides which are located at the bottom of the horizontal pores.Because of the lower Al content,the AZ31 substrate itself and the also the oxide films are less stable and tend to dissolve at a higher rate compared to AZ91.Thus,it was demonstrated that a good corrosion resistance of the coatings was only obtained on AZ91 and if the average thickness of the coating is around 50μm,correlating with the formation of a sufficiently dense inner laye-Knowing this mechanism,a new two-step treatment was suggested,combining the standard PEO treatment with a subsequent PEO process in an electrolyte supporting the inner film formation.The concept was successfully applied and a further improved corrosion resistance was obtained compared to the single stage PEO process.This improvement of corrosion resistance was related to the better sealing of porosity and formation of a denser inner layer.

Effects of imidacloprid and thiamethoxam as seed treatments on the early seedling characteristics and aphid-resistance of oilseed rape

Seed treatments with the neonicotinoid insecticides imidacloprid and thiamethoxam were evaluated to determine whether the chemicals at effective concentrations for aphid control would influence the germination and early growth of oilseed rape,Brassica napus.Treatment with imidacloprid or thiamethoxam did not affect the cumulative germination rate,but significantly inhibited establishment potential by suppressing root system development in the cotyledon stage.However,these alterations in seedling development in the thiamethoxam-treated seeds appeared not to be detrimental as leaves developed;in contrast,for the seedlings with imidacloprid as seed treatment agent,a significantly decreased shoot/root ratio was still evident at the late two-leaf stage.After two leaves developed,chlorophyll content per leaf in the thiamethoxam treatment was significantly higher than that of the control,while chlorophyll content per leaf in the imidacloprid treatment remained close to that in the control.Most other parameters,i.e.,height,leaf area,weight of stem,leaf or root,and other growth indexes,between the treatments and the control showed no significant difference.Additionally,it was found that storage time of the treated seeds had a significant effect on cumulative germination rate.Treatment 30 d before planting significantly reduced germination relative to that of the control.All of the plants treated with neonicotinoids were shown to have significant anti-aphid characteristics that persisted until the end of the trial.