Microscopy study of amorphous/nanocrystalline coatings thermally sprayed

Abstract

A commercial NiCrBSi and FeCrPC amorphous–nanocrystalline coatings were fabricated on a carbon steel substrate by plasma thermal spraying in a controlled atmosphere vessel. A cold helium gas stream can be blown directly onto freshly deposited layers to form amorphous coating or suppressed to allow formation of crystalline coating. Microstructural studies are investigated by means of X-ray diffraction (XRD), optical microscopy, scanning (SEM) and transmission (TEM) electron microscopy. Hand fracture surfaces in the fully amorphous coatings specimens exhibited a vein like morphology typical of many metallic glasses. When coatings are aged at 300 °C for 15 min, the vein like morphology is still present. If the coating is heated to 500 °C for 15 min, the vein pattern, characteristic of amorphous materials, disappears completely and nanocrystalline structure appeared. TEM was made to give more detailed information on the nanocrystallized structure. Fine nanocrystalline particles were grown on the amorphous matrix in the aged coatings having grain size ranged between 50 and 100 nm. From XRD, full amorphous coatings seem to be obtained but meticulous TEM examination has revealed the presence of crystallites of approximately 100 nm within size in the amorphous matrix. The thermal plasma technology is well adapted to the manufacture amorphous coatings on a substrate of any shape. However, the numerous operating parameters that govern the deposition of a coating render difficult the preparation of fully amorphous coatings. X-rays diffraction is not sufficient to assert that the crystallization is totally suppressed. Careful observation by the TEM microscope is necessary.