Vacuum plasma spraying of coatings

Vacuum plasma spraying (VPS) is the injection of metal or ceramic powder (10-50microns) into a hot gas plasma (~10,000K) that melts and projects the molten droplets at high velocity onto a substrate to form a coating. After evacuation, the VPS spray chamber is filled with inert gas and maintained at low pressure (~100mbar) during spraying to allow coatings of reactive materials to be produced without oxidation. The high droplet impact velocities and low oxidation associated with VPS allow ~100% dense coatings to be manufactured. Vacuum plasma spraying is preferred to atmospheric plasma spraying (APS) for oxidation sensitive materials such as Ti alloys, and/or where improved adhesion and density is required.

The vacuum plasma spraying unit at Oxford features 5-axis robot manipulation of the plasma gun, an indexed linked turntable, a twin hopper feed system for co-spraying or graded coatings, infrared pyrometry for process temperature control and a range of customised features.


Coatings for plasma facing components (PFCs) in fusion devices
This project concerns the use of VPS to produce thick tungsten (W) coatings with through thickness variations in composition that are optimised for service in first wall applications in fusion reactors. Tasks include evaluation of possible materials combinations for plasma facing components from the perspective of all major contraints due to plasma surface interactions, physical properties for heat removal, radiation damage, etc.

Read more about this work at: doi:10.1016/j.msea.2007.05.120

Evaluation PFC materials are being manufactured in-house using processing strategies optimised for thermal exposure and CTE mismatch stress management, togethr with sufficient adhesion, minimum porosity and low surface roughness, etc. Current efforts are focused on W-based graded coatings on low activation steel substrates, and the use of surfi-sculpt substrates to enhance adhesion and to provide further options for graded structures (Funded by EPSRC and UKAEA)

A stainless steel substrate that has been sculptured using an e-beam at TWI, Cambridge, UK and then vacuum plasma sprayed with W. Note the excellent pentration of the surface sculpture, the high W density, and relatively smooth surface.

Processing of Advanced Materials