Boride coatings obtained by pack cementation deposited on powder metallurgy and wrought Ti and Ti–6Al–4V

Abstract

Wear resistance of Ti alloys needs to be improved, and an effective way to achieve this is through surface treatment. Boronizing is a surface treatment in which boron diffuses into the surface of Ti leading to the formation of hard and wear-resistant Ti borides. Boronizing of wrought and/or cast Ti alloys by pack cementation has been studied, while similar coatings on Ti alloys produced by powder metallurgy (PM) have not been reported. Also critical process parameters for boronizing Ti alloys, such as pack cementation powder composition and the process temperature have not been systematically studied and analysed. The present work reports on the surface modification of PM Ti and PM Ti–6Al–4V by boronizing, and presents some important thermodynamic aspects of the process comparing it with similar coatings applied to wrought Ti–6Al–4V. The coatings were characterised using scanning electron microscopy and X-ray diffraction. For both Ti and Ti–6Al–4V alloys the use of amorphous B as a B element supplier in the boriding powder pack led to the formation of a uniform external boride layer, while the use of B4C as a B element supplier in the pack and under the same boronizing conditions, led to the formation of an external TiN layer and an internal layer containing B. The thermodynamic calculations performed proved successful in determining the appropriate conditions for boride coating deposition and estimating the phases likely to be formed. Finally the effect of surface roughness on the coating quality is discussed.