HIGH VELOCITY SPRAY HVOF (High Velocity Oxy-Fuel)
This method is the state-of-art-technology in the heat spray sphere where oxygen and kerosene are the heat energy source. Oxygen and kerosene are axially let into combustion chamber, where they are atomised and ignition by a spark from the sparking plug. Combustion products are then accelerated in convergent divergent jet to supersonic values (Mach 1-2). The pressure in the combustion chamber is monitored to secure stabile burning conditions and the process reproducibility. The burner is cooled by water, which flows in the interspaces of the burner.
Combustion gasses are accelerated in the jet to supersonic values of output speed. Powder material is by carrying gas - nitrogen radially feeded from opposite sides to diffuser where they are smelted and markedly accelerated through the jet in the coated part direction. Thermodynamic process proceeded in the combustion gases flow running with supersonic speed gives rise to expansion and compression waves so called shock diamonds, which can be visible in the flame coming out of the burner.
High velocity of the powder particles is acting in impact an uniform spreading and particle anchorage to the background material and this generates high density and adhesion of HVOF spray coatings. The relative low flame temperature about 2600°C limits this technology for ceramic coating. On the opposite eliminates oxidation, phase changes and burn up of some elements of the sprayed material during the spray application. Unique properties of this technology is providing pressure stress coating in contrast to all other methods of heat spray. Pressure stress in the coating increases significantly adhesion of the spray application to the background material ( cohesion ca 70MPa) and is also favourable from the point of view of fatigue characteristics of coated particles.
The flame heat predestined this technology to spray of metals and its alloys with low and medium melting temperature. Typical materials for HVOF spray are cermets (ceramic-metal), most often tungsten carbides and chrome based. Wear and tear resistant and corrosion resistant coatings in different environments are typical applications.
Basic parameters of spray:
| Particles velocity [m.s-1] |
Adhesion [MPa] |
Oxygen content [%] |
Porosity [%] |
Spraying output [kg.h-1] |
Typical spraying thickness [mm] |
|---|---|---|---|---|---|
| 600 ÷ 1000 | < 70 | 1 ÷ 2 | 1 ÷ 2 | 3 ÷ 6 | 0,2 ÷ 2 |
- Added material - Powder
- Kerosene inlet
- Oxygen inlet
- Spray
- Background
- Sparkling plug
| Advantages | Disadvantages |
|---|---|
| High spray density | Noise level to 130 dB |
| High cohesion | Ceramic coating not applicable |
| Low porosity and oxygen content | Inner surfaces spray limitation |
| Low temperature of the background up to 150°C | |
| Pressure stress in spray coat | |
| Wide variety of powders |
Our company is using the equipment JP-5000 of American company Praxair TAFA with the system HP/HVOF (High Pressure High Velocity Oxy-Fuel) www.praxair.com with the powder feeder HVOF System Powder Feeder 1264.
Positioner with NC equipment enables with high accuracy coating of cylindrical and spherical surfaces with the diameter to 1100 mm and length 3500 mm.
As adding materials we use powder only from the company PRAXAIR, Inc. USA on the basis:
- Tungsten carbide
- Chrome carbide
- Nickel (Alloy 625)
- Cobalt (TRIBALLOY 400, TRIBALLOY 800)
- Iron (SS 316)
- and their mixtures