Thermal spray technology controls wear

Thermal spray technology controls wear in metal components In most industries significant losses can be incurred due to the accelerated wear of components. To minimise the effects of mechanical wear and extend product life, Poeton offers thermal spray coating solutions to meet the most demanding wear applications. Wear is defined as the unwanted removal of material from a surface as a result of mechanical action.

Controlling wear in metal components means controlling a complex set of system and process variables, starting with a clear understanding of the component, its material history (alloy chemistry, processing, surface hardness, grain size), the type of wear the component will be subjected to, and the type of environment (temperature, gas).

For each type of wear, there is a corresponding specific wear mechanism.

Applying coatings using thermal spray is a well-established method of resurfacing metal parts, by simultaneously melting and transporting sprayed materials, usually metal or ceramics, onto their surfaces.

The spray material is propelled as fine molten droplets, which solidify and adhere to the part by primarily mechanical and, in some cases, metallurgical interaction.

Each layer creates a strong bond to the previous layer and the process continues until the required coating thickness is achieved.

Thermal spraying can be used to apply coatings to machine or structural parts to achieve a number of engineering objectives.

Typically these include repairing worn areas on parts damaged in service; restoring dimension to mismachined parts; and increasing a component's service life by optimising its surface properties.

The main advantages of thermal spraying include the range of chemically different materials that can be sprayed, a high coating deposition rate that allows thick coatings to be applied economically, and the portability of the spray equipment.

The first step in selecting a thermal spray coating for a specific application is to define the coating function.

Thermal spraying consists of five basic processes: wire flame spraying; powder flame spraying; arc wire spraying; plasma arc spraying; and high-velocity oxy-fuel (HVOF) spraying.

Of these, the HVOF process gives coating microstructure densities closest to that of wrought materials, but all the processes can also be applied in a controlled environment in order to enhance coating properties.

Erosion occurs when solid particles impinge or impact on a surface, resulting in eventual material removal.

The mechanism of wear is based on the angle of impact, the size of the particles and the physical characteristics of the debris.

Temperature and environment can also play a role in wear.

A typically example is stainless-steel steam turbine blades, which are exposed to temperatures of up to 540C.

As the cost to replace the blades is significant, researchers have looked to coat them with proprietary materials.

High-temperature erosion is also a problem in boiler tube applications in the pulp and paper industry, where tubes are subjected to high-temperature corrosion (sulphidation) and erosion.

In black-liquor recovery boilers, for example, the combination of char (solid particles rich in sulphur) and the hot gas environment results in metal wastage of the tubes.

To minimise the problem, nickel-based alloys, high in chromium, have been developed.

Key contributors to sliding wear are mechanical loading, types of load, chemical media, temperature, and the type and amount of lubricant.

Sliding wear is common in automotive and heavy-duty piston ring applications, synchroniser rings and transmission systems, automotive and large-bore cylinder bores for gas transmission applications, hydraulic rods for earthmoving equipment, and landing gears for mainframe aerospace applications that replace hard chrome plating.

Due to the environmental restrictions on hard-chrome plating it is being replaced with other materials and processes, of which HVOF technology is an excellent alternative.

Its main benefits are ease of operation, quick turnaround, wide material selection, and low cost.

HVOF coatings are approved by the US Navy to replace hard chrome for hydraulic rods, and major aircraft manufacturers have approved materials such as SM5847 for landing gear applications.

Abrasive wear occurs with the removal of material by particles moving across a surface.

The particles may be loose or part of another surface that is in contact with the surface being worn.

Typical abrasive wear applications are found in the agriculture and glass industries, where cobalt-tungsten carbide/self-fluxing alloys allied with either low- or high-velocity oxy-fuel gun processes, are used successfully on cutting blades and glass mould plungers.

Fusing the coating creates a metallurgical bond between the substrate and coating, which improves the impact resistance.

Important considerations for abrasive wear are the coating's matrix hardness and chemistry.

Hardness and toughness typically run counter to each other therefore, the alloy design has to be optimised for the application.

Generally, plasma and HVOF processes are used for less severe impact applications while tungsten carbide-cobalt (chromium) and nickel chromium-chromium carbide are the main carbides of choice.

Tungsten carbide-cobalt is used for low temperature wear applications (400-500C), cobalt chromium tungsten carbide coatings for corrosive low-temperature wear, and nickel chromium-chrome carbides for high-temperature wear (1000C).

Typically, carbide coatings of cobalt chrome have more than four to six times the abrasive-wear resistance of hard-chrome plating and are approximately 30% better than nickel chrome-chromium carbide.

Material hardness is not the only factor in determining abrasive wear resistance between material systems, however.

The majority of applications are based not on one mechanism, but many.

A good example of this is a sucker rod coupling used in oil field pumping machinery, which experiences abrasive, sliding, and corrosive wear.

Another example is a pump volute used in the petroleum industry, which encounters abrasion, erosion, and corrosion.

Before selecting the proper coating system, several factors must be considered and evaluated.

Technical considerations are based on an understanding of the wear mechanism and environmental factors that the engineering parts are exposed to.

Cost considerations include the materials used, gun and process employed, and the normal life of the components before they need to be repaired.