HVOF versus Plasma coating

Since many years ago, liners have been provided with a ceramic coating (plasma spray process). A relatively unknown, but better alternative has been around for several years: a coating of tungsten carbide applied using the HVOF process.

Liners wear, and preventing this has always been a struggle. The challenge is to protect a liner at its running surface in such a way that it will give many years of service. The purpose of the application of a coating layer is to prolong the gland packing/lifetime of the liner.

Lagersmit’s HVOF coating is called HML and stands for High Velocity Oxygen Fuel. It is a unique thermal spray coating process in which the powder particles are transported to a work piece at high speed by means of a gas/oxygen flame. It forms a coating that attaches to the surface at a molecular level. The result is a very dense, homogeneous coating with very high bond strength.

The diagram demonstrates the HVOF process.

The gas/oxygen flame, with a temperature of about 3000 °C, ‘bombards’ the work piece with powder particles, such as tungsten carbide, which travel at a speed of more than 1,000 m/s. This is approximately four or five times the speed of particles in traditional thermal spraying. HVOF spraying allows particularly dense and homogeneous coatings to be created with high adhesion value, low internal stresses and high bond strength. Ideal for a liner.

HML coating versus ceramic coating

A ceramic layer is an outdated method of protecting a liner. The ceramic coating has a higher porosity value (2 – 5%). An HML coating has a very low porosity value (< 1%). The advantage of an HML coating compared with a ceramic coating is that the liner is more resistant to:

• The dissipation of heat generated by friction of the lip seals
• Hardening and ageing of the lip seals
• Sub-corrosion

The Hard Metal Layer ensures a dense and smooth surface with good running properties (low friction coefficient). Nevertheless, the layer should have a certain roughness (Ra 0.8) in order to prevent leakage. This reduces resistance of the lip seals to a minimum. The resistance is mainly caused by the medium, the pressure and peripheral speed. The coating seals against water, thus preventing sub-corrosion. This is a result of the special design parameters of an HML coating, namely very low porosity (less than 0.5%). The layer has to have a certain roughness (Ra 0.8). A liner with low and/or high roughness on the running surface also causes leakage and wear. In addition, the HML coating is resistant to abrasive wear (sand

The probability that a liner with HML coating is damaged is therefore many times smaller than that of a liner with ceramic coating.

Lifetime

Tests under ideal conditions indicate an impressive total lifetime of 20 years.

Opt for an HML coating

Does your liner need replacing? Or are you thinking about buying a complete new seal and liner? Opt for an HML coating on the liner, and increase its lifetime by many years. Lagersmit works with tungsten carbide Hard Metal Layers, applied using the HVOF spray process.