Formation and Characterization of Corrosion-Resistant Amorphous Coatings by Thermal Spraying*

Since it was first discovered that amorphous metals could be formed by rapid quenching from the liquid phase, these so-called metallic glasses have attracted an increasing amount of attention due to their unique properties. Forming amorphous metals by quenching from the liquid phase requires cooling rates on the order of 10⁵-10⁶ K/s. In order to achieve such cooling rates, one of the dimensions of the samples must be sufficiently small. For instance, a very thin ribbon enables amorphous alloys to be produced by the well known melt-spinning process. An alternative: approach is the use of thermal spraying to produce amorphous coatings. The first commercial product exploiting this approach was developed in 1984 and the subject has received increasing attention since this first development.

The purpose of the present work is to explore the conditions necessary for the formation of combustion flamesprayed amorphous coatings from NiMoFeB and NiCrFeSiB alloys. A technique based on differential thermal analysis has been developed to characterize and quantify the fraction of amorphous phase present in the partially amorphous coatings. It has been shown for the current two alloys that the amorphous content of coatings can be varied through control of the maximum substrate temperature, the initial powder condition and the powder size. These results will be compared to existing knowledge regarding the formation of amorphous alloys and, specifically, thermally-sprayed coatings.