TWI Technology Briefing 588 - January 1997
A J Sturgeon
High velocity oxyfuel (HVOF) spraying is increasingly considered as an alternative thermal spraying process to air plasma and detonation flame spraying for the preparation of high quality coatings of tungsten carbide-cobalt (WC-Co). These coatings provide protection against severe wear such as abrasion, adhesion, fretting and particle erosion for a wide range of industrial applications. The achievement of good wear protection is highly dependent on deposition of a coating having the correct microstructure.
Background
There is very little published information on the microstructure of HVOF sprayed WC-Co coatings and on how this influences wear performance. This work has sought to characterise the microstructure of two WC-Co coatings with very different abrasive wear rates. Characterisation of the coating microstructure was undertaken using scanning electron microscopy, X-ray diffraction and chemical etching techniques.
Objectives
- To characterise the microstructure of WC-Co coatings prepared using the HVOF process.
- To determine the microstructure necessary to give best resistance to abrasive wear.
Main conclusions
- The coatings contain hexagonal WC as the primary crystalline phase distributed in an amorphous Co-rich matrix. Crystalline hexagonal α-W2C and cubic W arepresent, but only as minor phases. The coatings also show the presence of 'ribbon-like' amorphous Co-W-C phases.
- Resistance to abrasive wear is associated with low levels of 'ribbon-like' amorphous Co-W-C phases in the coating. The majority of the coating should consist of fine and uniformly distributed grains of WC in an amorphous Co-richmatrix.
Recommendations
To achieve best wear performance from a tungsten carbide coating prepared using the HVOF process, it is recommended that spraying conditions are used which minimise the formation of amorphous Co-W-C ribbon structures within the coating.
