THERMAL SPRAYING

 


Le principe de la projection thermique

 

Whatever it is plasma spraying, (conventional or HVOF) flame spraying or electric arc spraying, in all the cases the principle is the same, only temperatures and velocities of powder particles differs.

Flame spraying is well suitable for manufacturing low melting temperature materials (polymers and certain metals), electric arc spraying is use for manufacturing coatings with electrical conductive materials, and plasma spraying is well suitable for spraying refractory materials like oxides.

The thermal spraying process happen according the following steps:

> The selected material designed as to be the coating is injected in the melting source (flame, plasma or electric arc) under the form of powder or wire,

> The material is thus melted under the form of fine droplets which are propelled with the stream of the melting source to impact with a high velocity onto the substrate to be coated,

> Particles cool down very quickly by giving their calories to the previous deposited layers,

> Bond coat with the substrate is high if it has been properly prepared

 

Le procédé de projection thermique

 

 

 

Powder flame spraying
2700°C 50m/s

Wire or rod flame spraying
2700°C 200m/s

HVOF spraying 2700°C 550m/s

Electric arc spraying
4000°C 150m/s

Plasma spraying 20 000 K 450m/s
Detonation gun spraying
4000°C 750m/s

 

 

> POWDER FLAME SPRAYING

>WIRE OR ROD FLAME SPRAYING

>HVOF SPRAYING

> WIRE ARC SPRAYING

> PLASMA SPRAYING

> DETONATION GUN SPRAYING

> Preparation of substrate before spraying

> Methodology for spraying coatings

 

 

Spraying tool Flame Arc Plasma HVOF
Coating porosity ( %) 10 à 20 5 à 20 1 à 10 <0.5
Bond strength MPa 15 40 40 >60
Hardness WC-12Co 600 - 700   850 - 1150 >1200
Advantages    low cost low cost
high coating speed
versatile tool low porosity
Limits in most case high porosity, low bond strength, limited possibilities

sprayed material has to be electrically conductive, high porosity

expensive no possibility to spray oxides
Sprayed materials metal and certain ceramics metallic or cored wires all materials carbides métals cermets

 

 

Chart température / cinetic energy of various thermal spray processes

 

 

Micro-hardnesses of various coatings of WC-Co powders sprayed with
various spraying guns