Plasma projection of ceramics powders

Thermal projection by plasma allow to build
ceramics coatings which thickness can reach several millimeters.

To ensure sufficient quality of coating, powder used must have a granulometry which doesn't stand above a limit size (superior to several tenth of micron). Above, ceramics particles, strongly resistant to heat, don't melt correctly and create inclusions inside the coating. These unmelted particles cause stresses inside the coating and decrease its mechanical properties.

In the same way, too fine size particle (lower than a few microns), in too significant quantity (higher than several percents), causes overheating of plasma jet. Spraying of these fine particles of ceramics at the gun exit can involve its obstruction. Moreover, ceramics weight vaporized during spraying and which doesn't reach the substrate increase coating building cost. Finally, ceramics can decompose at hight temperature and form indesirable soluble phases in molten particles.

Oxygen contained in ceramics powders has a direct impact on oxides quantity inside the coating. Unfortunately, powders with lowest oxygen composition are also the most expensive. It is nevertheless possible to eliminate most oxides created in the coating by choosing a powder having a certain proportion of carbon such as carbon/oxygen mass ratio remains lower than 1. Carbon forms thus with oxygen volatil components such as CO which prevents oxides formation inside the coating.

Nature and pressure of the atmosphere, inside the projection booth, can be modified in order to improve coating quality. A gas mixture free from oxygen (lower than a few percents) can decrease oxides content inside the coating. Low gas pressure increases ceramic particles speed in the plama jet and allow to build denser coatings. Nevertheless, if pressure is too low (lower than several hundred mbars), deposition effciency can strongly fall and become null. Too high speed of particles, prevents them from melting correctly during their run into plasma flame.

The plasma-producing gas mixture in general consists of argon and hydrogen. Mass ratio of hydrogen compared to argon can vary between 1/2 and 1/6. Hydrogen proportion determines temperature of plasma flame and thus ratio of particles correctly melted during their run. After lighting of plasma torch, voltage imposed to the electrical discharge (between 50V and 100V) influence also particle state when they arrive on the substrate. Too low temperature of plasma involves an increase of nonmolten particles number and thus of inclusions percentage inside the coating.

Ceramic coating constitutes a deposit having good heat and electric insulation. It has great oxidation and heat resistance and it can be used in extremely corrosive environments at hight temperatures, until several thousands of Celcius degrees. Ceramics is a hard materials, having good wear resistance and high mechanical resistance, even at high temperatures.