Thermal spray and weld overlay coatings restore the proper dimensions of worn and incorrectly machined components. The rebuilt areas can be machined to tight tolerances with the required surface finish.
The coatings used for mismachined applications typically have a composition similar that of the substrate. For dimensional restoration of worn parts, typically a material more wear resistant than the substrate is used in order to prevent further damage. Specialized thermal spray coatings and honeycombs are used for clearance control in rotating equipment.
Thin films and plasma nitriding are applied to finished components. Machining is not required after application, but postprocess polishing may be performed.
Although dynamic systems rotating at high speed are designed to ensure concentric rotation, over time slight eccentricities can occur. Additionally, the inertial properties of rotation can cause component growth. It is therefore often necessary to design a gap between the rotating and stationary components of the system.
For gas turbine engines, such gaps in the gas path represent a loss of efficiency. A space of 0.125 mm (0.005 in) between the engine compressor blades and the outer casing can increase fuel consumption by 0.5%. Clearance control technologies minimize gap size by utilizing a surface that will abrade preferentially when contact is made with a mating part.
In rotating equipment such as compressors, gas turbines, and turbochargers, thermal and mechanical operational effects cause dimensional changes between the rotor and stator. These dimensional changes open gaps between the blade tips and the casing in gas path systems and between the seal and housing in labyrinth seal systems.
In these applications, clearance control systems can be installed that consist of a sacrificial element and a cutting component. Thermal spray abradable coatings and honeycomb seals form effective sacrificial systems.
Embossing and engraving
Industries that process plastic foils or paper frequently need to produce surfaces with an attractive matte appearance. To create the desired surface texture, the foil or paper is pressed between a textured embossing roll and a press roll.
A thermal spray coating applied to a new or used embossing roll can produce a surface having a homogeneously random pattern. The size range of the surface pattern is largely controlled by the roughness of the coating, which can range from 10 – 300 µm Ra. For pattern embossing, thicker thermal spray coatings of copper are often used.
Within the printing industry, thermal spray coatings are a mainstay for anilox rolls. A dense, homogeneous coating of chrome oxide is applied to the roll, which is subsequently laser engraved with a fine pattern. These coated rolls, used for ink transfer during the printing process, are both wear resistant and corrosion resistant to modern water-based ink formulas.
The surface profile of corrugating rolls is maintained using wear-resistant thermal spray or thin film coatings. These rolls are typically over 4 m (13 ft) long.
Salvage and restoration
The thermal-sprayed coatings are commonly used to repair mismachined and worn components. The coating material can be either metallurgically similar to the substrate or of a different material that provides desirable surface characteristics, such as wear resistance. These coatings can be applied very thick and can be machined to dimension the required surface finish.
For dimensional restoration, thermal-sprayed coatings are typically faster and more reliable than conventional welding. Thermal-sprayed coatings are applied quickly and do not metallurgically alter the substrate material.
Near-net shapes for prototyping
Thermal-sprayed coatings can be used to produce highly complex, near-net metallic components. Use our ChamPro controlled atmosphere plasma spray to manufacture components of exotic alloys that are difficult to cast or forge.
Near-net shape technology can also be used for rapid prototyping. The coating is sprayed onto a male or female mandrel to the desired thickness. Upon release from the mandrel, final machining is performed. For prototypes, this alleviates the need for expensive investment molds and test pours.