Dear Researchers
I have done a Ni-20TiO2-20Al2O3 coating on the CA6NM turbine steel by the Flame spray method. The microhardness obtained after coating was observed lower than the literature. I want to know that, what is the effect of morphology and size of the raw feedstock powder on the microhardness obtained in the coating? The morphology of the 60%Ni was spherical, whereas; 15% of titania and alumina were having an irregular morphology. I have also added 5% nanostructured alumina/titania powder having a spongy morphology. plz, help.
dear Vibhu Sharma ;
The coating hardness depends on the grain size and porosity of the coating, whereas, coating particles of small grain sizes are more difficult to be dislocated because of the lower accumulation of dislocation pressure at the grain boundaries than that of larger grain sizes. Moreover, the coating of fewer surface pores indicates to the fewer voids and crevices inside the bulk of coating which make the coating harder and more adhesion with the substrate surface. for more details can read;
Chapter Thermal Sprayed Coatings Used Against Corrosion and Corrosive Wear
Article Grain size effect on the hardness of nanocrystal measured by...
Hi Vibhu,
hard phases in your coating are Al2O3 and TiO2 and the coarser these particles are, the higher the hardness is measured. Higher hardness in coarser layers does not mean that they are better, but if your coating is only rated on the basis of hardness, you have to choose the coarsest particles possible.
The explanation for this is very simple:
In the Vikers hardness measurement, the diamond indenter can meet different phases. If the indenter hits a large Al2O3 or TiO2 crystal, a very high hardness is measured. In contrast, the indenter penetration into a bunch of small particles shows a low hardness. In this sense, your nanoparticles are counterproductive.
Regards
Vadim
Dear all, regular shapes mainly symetric whithout sharp edges impart excellent stress transmission compared to irregular ones. Also the the interface binding strength has also its pivot role. My Regards
Dear Vibhu Sharma,
More than particle shape, particle size plays an important role during particles in-flight inside the flame.
Since flame spraying is a highly oxydating atmosphere, finer particles develop a higher content of metal oxydes. But, heating/fusion of the coarser ones is limitated. This may decrease the lamellae binding.
Besides, the co- spraying of different materials leads to an heterogeneous dispersion, causing different values, particularly to microhardness.
Regards
Ricardo
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