Thermal Spraying Applications

Keith Harrison, Sulzer-Metco UK Ltd.

Originally Published in "Coatings" July 2000
 

Every time you drive a car, every time you fly, every time you switch on a fluorescent light then you will probably be relying on a thermally sprayed coating. Thermally sprayed coatings are used right across the spectrum of engineering and manufacturing to enhance the surface of components made from low cost or light weight materials. This allows  the designer to create cost effective, high performance parts with a functional surface exactly where it is required.

Thermally sprayed coatings can be used to impart many surface characteristics such as:

  • Wear resistance
  • Heat resistance
  • Oxidation resistance
  • Corrosion resistance
  • Electrical resistance
  • Electrical conductance
  • Restoration of size


Aerospace

Aero-engine manufacturers are the biggest users of thermally sprayed coatings, both at the original equipment manufacturing stage and during subsequent repair and overhaul. Coatings are applied to combat surface degradation mechanisms such as wear, corrosion and oxidation. Thermal barrier coatings are applied to the combustors and nozzle guide vanes to protect them against extreme heat. Clearance control coatings are designed to allow the rotating blade tip to machine its own seal path thus making significant savings in fuel consumption. Coatings are also used to rebuild mis-machined or service worn components. Similar applications also apply to industrial gas turbines and compressors.


Aircraft

The application of thermally sprayed coatings to aircraft has not been as extensive as that in the aero-engine industry, nevertheless there are a number of important areas where coatings have been used. Landing-gear light alloy Oleo housings, which suffer from galvanic corrosion, are reclaimed using aluminium bronze coatings. The coatings are sealed to prevent corrosion re-occurring.

Composite aircraft panels are metallised with aluminium to protect against lightning strikes or to dissipate static electricity. Some panels have been coated with refractory ceramic to prevent damage from the heat of missile exhausts.

More recently the aft V-bay tunnel skin panels on the prototype Eurofighter EF 2000 have been protected against exhaust-plume damages by a CoNiCrAlY/ZrO2-Y2O3 coatings system.

The sliding surfaces of slat-rails and flap-tracks are reclaimed with HVOF coatings of tungsten carbide/cobalt. Intermediate coatings of similar hardness to the substrate are sometimes used to build up the dimension.

Helicopter rotor shaft worn main bearing journals are coated with an HVOF tungsten carbide/cobalt  material. This provides wear resistance and repair build-up without affecting the fatigue strength of the shaft.


Automotive

The automotive industry has also been a major used of thermally sprayed coatings for many years. The combustion wire and plasma spray processes are used in the high volume

 

 

 

  

Gas Seals coated on the outside diameter with HVOF Tungsten Carbide, Photo courtesy of John Crane Limited.


production of a number of products of different shapes and sizes in the automotive industry world-wide. These applications include the outside diameters of piston rings, the bearing surfaces of forks, synchronisation rings and cones, fuel injector nozzles, alternator covers and exhaust oxygen sensors.

These days the automotive industry is not only driven by the requirement to make vehicles that are safe and reliable; but also by the need for low fuel consumption, low emissions of pollutants, as well as low mechanical and corrosive wear.

Thermally sprayed coatings can help the designer achieve these objectives by providing specific surface properties for lightweight components. Lowering the  un-sprung weight of a vehicle improves the fuel efficiency, reduces emissions and enables the manufacturer to add safety and luxury features whilst still maintaining maximum weight targets.

Two areas where this is happening are:

  1. Lightweight engine blocks - cast iron liners and replaced with a thin plasma sprayed coating. Production machines utilising rotating plasma spray guns and capable of processing 800 blocks per day have been built.
  2. Lightweight brake discs - aluminium metal matrix composite discs are coated with a special ceramic to give the desired frictional characteristics.

The success of the process in the automotive industry is due to  the wide range of materials available for thermal spraying, the properties of these materials and the ease in providing   high-rate/low-cost deposition for the high volume of parts involved. Thermal spraying is a versatile process that can easily be integrated into a production line.

Other applications include:

  • Surgical implants, such as replacement hips, are coated with porous titanium or with synthetic bone to promote fixation in the body.
  • Rolls used in the paper, printing and steel industries are protected against wear and corrosion.
  • Thread guides and other components used in the textile industry are ceramic coated to combat the abrasive nature of synthetic fibres.
  • Chemical industry pumps, sleeves and shafts and other components are coated to prevent corrosion and wear.
  • Paper or polymeric capacitors are metallised with thermally sprayed tin/zinc to enable electrical connection to be made.
  • Structural foam moulded VDU and other electronic cases are sprayed with zinc to shield against stray electromagnetic or radio interference.

The list is endless with new applications for thermally sprayed coatings being developed every day.

For other articles presented at the TSSEA conferences or printed in "Coatings" see the publications page or contact TSSEA for further information.

© Copyright 2001 TSSEA