With a degree in chemical engineering and after two years teaching experience, David Harvey joined TWI in 1986. He is now a Head of Section in the Arc Welding Department with responsibility for co-ordinating TWI's surface engineering activities.
Since joining TWI, David has looked at a number of difficult areas relating to MIG/MAG welding, ranging from materials problems such as porosity in aluminium alloys to process problems such as contact tip performance, welding wire condition and arc stability. More recently, he has been involved in application of magnetic arc oscillation to orbital TIG, and development of a backface penetration control system.
Breakdown and loss of operating efficiency in manufacturing plant often originate from what is loosely termed 'wear' in component surfaces. David Harvey outlines different types of wear and examines several surfacing processes used to tackle its effects.
The cost to industry of wear is high and recognition of this lies behind continuing development of the technology known as surface engineering. TWI has a long association with surface engineering and has recognised its important role in prolonging the life of components. Processes and applications for surface engineering can be found in many sectors and TWI has developed and expanded its activities to meet industry's needs.
Surface engineering is the co-ordinated design of surface and substrate to achieve economically results of which neither alone is capable. The composition of a material necessary to provide adequate strength in a component is usually different from those which provide wear resistance, so a composite product consisting of a structural material, with specially protected wearing surfaces, is the ideal compromise.
Surfacing technology covers a range of processes, whose common goal is to enhance not only wear resistance but quite possibly the corrosion, thermal, electromagnetic or optical performance of a particular surface.
Mechanisms of wear
The term wear is commonly used to mean any type of attack which may cause deterioration of metallic surfaces in use. Sometimes all surfaces of a part are exposed to wear, but more frequently, only a proportion of the surface is required to display high resistance to a specific form of attack. Wear is the result of relative motion of a surface with respect to another body and embraces metal to metal, metal to other solids, and metal to fluid contact.
Abrasion, adhesion and contact fatigue are generally regarded as the three basic wear mechanisms but other processes such as corrosion and erosion can intensify damage. Before selecting materials for surface coatings and choosing the means of their application, it is essential to appreciate the differences between wear mechanisms.
Abrasive wear
Abrasion arises from penetration of a surface by another body or surface - and usually involves a cutting or ploughing action. It may involve particles rubbing a surface, hard particles trapped between surfaces or a roughened surface moving over and penetrating into an opposing surface.
It is a common misconception that wear rate is inversely related to hardness, but results on abrasive wear tend not to confirm this direct proportionality. For pure metals there may be a direct relationship between hardness and wear rate, but with materials of a more complex structure, e.g. most engineering alloys, this is not the case.
In steels, wear resistance is related to hardness through the carbon content and by the structure of the matrix. The presence of secondary phases, particularly carbides, but also nitrides and borides, is widely used to provide resistance to abrasion.
In selecting an abrasion resistant material, it is essential to ensure there is strength to resist stresses from the wear mechanism and toughness to withstand impact. For surfaces subject to abrasion only, the ideal structure tends to be a hard matrix, e.g. martensite, with evenly distributed hard particles such as carbides.
If there is a large amount of impact loading then it may be necessary to revert to a tougher matrix at the expense of some wear resistance. This requirement for both strength and toughness often demands a compromise, but where there is any question, it is normal practice to choose extra toughness - a higher wear rate reduces life but less dramatically than a component fracture.
Adhesive wear
Although apparently smooth, the surfaces of granular crystalline materials such as metals are rough on a microscopic scale, comprising a series of peaks and troughs. Adhesive wear occurs when two such surfaces are brought together, and contact is made only at opposing asperities. Since asperities represent no more than a tiny fraction of the nominal contact area, even a small load can cause very high local pressure and temperature.
Adhesive wear is generally divided into two categories - mild and severe. Mild wear occurs in the presence of an oxidised surface which keeps clean metals apart. The wear process involves removal of fine oxide particles from the surface layer. Protective oxide films are mainly generated during rubbing or sliding, so conditions favouring oxide formation are most likely to confine wear to the mild regime. However, once the surface load becomes sufficiently high, fine cracks begin to grow in the oxide layer which can initiate plastic flow in the substrate and the onset of severe wear.
Adhesive wear can therefore be prevented by minimising direct contact between metallic surfaces. Surface films, whether of oxides or absorbed atomic layers, and lubrication may be used to reduce the rate of adhesive wear.
Contact fatigue
Contact fatigue occurs in surfaces experiencing rolling contact, such as gears and roller bearings. Surfaces are exposed to fatigue conditions during rolling, and if the endurance limit is exceeded fatigue failure eventually occurs. Failure usually happens suddenly, after an incubation period during which there is little obvious loss of performance. Direct contact of surfaces is not necessary, as stresses can be transmitted through interposed material such as a lubricant film.
Generally, the biggest contribution to contact fatigue wear is load - even a small reduction greatly increases the life of a part. Lubrication only helps to reduce contact fatigue indirectly by keeping surfaces dirt-free and smooth, and by reducing adhesive wear, corrosion and local stress concentrations.
Materials
Most metals, alloys, cermets and ceramics can be applied as coatings either individually or as mixtures, but their characteristics often limit the processes which can be used for their application. For example, ceramics cannot be applied by welding processes, and carbides require a metal or an alloy matrix. The material/process relationship dictates when they can be used together and the properties which can be expected from the coating, such as density and adhesion to the substrate. Thickness of the coating material is normally consistent with the amount of wear permitted before the component is no longer fit for use, and takes into account an allowance for finishing the deposit to a specified dimension.
Surfacing processes
Welding
Using properly operated procedures, welding provides a metallurgical bond - the highest bond strength between deposit and substrate of the surfacing processes. Furthermore, welded deposits can be laid down in greater thicknesses than most other processes, typically between 2-10mm. Most welding processes can be used for applying surface coatings. Suited to on-site operation, they can be carried out either manually or mechanically.
Weld surfacing involves application of considerable heat to the substrate. Influence over the thermal cycle is essential for controlling the microstructure, level of dilution of the coating by the substrate, and stresses in the coating/substrate as a result of thermal expansion and contraction. A typical thermal cycle may require preheat, interpass control and a post-weld heat treatment to optimise the properties of the welded coating.
A wide range of metals and alloys can be used for welded coatings, i.e. irons, steels and copper, nickel and cobalt based alloys. Other higher melting point and non-conducting materials such as ceramics are not suited to welding processes.
Thermal spraying
Although some thermal spraying processes involve a degree of high temperature diffusion between the coating and substrate, the majority of coatings are
mechanically bonded to the substrate. In this respect, substrate to coating bond strength, and interlayer bond strengths are not as high as in welded coatings. Furthermore, the coatings exhibit some porosity and oxidation. Thermally sprayed coatings are thin when compared with welded coatings, although there are examples where thick sprayed coatings can be deposited.
Thermal spraying offers two distinct advantages over weld surfacing. Firstly, it can apply non-weldable coating materials such as ceramics, and secondly it can apply coatings to materials unsuited to weld surfacing because of composition or susceptibility to distortion. Generally, there is little or no distortion, dilution or metallurgical impact on the substrate.
Other processes
Electrodeposition is a well established process for applying a range of metallic coatings,. generally for engineering applications. Two processes are available - vat plating and electroless plating. Although essentially for applying metallic coatings, non-metallic particles such as carbides can be co-deposited to produce metal matrix composite (MMC) coatings. The principal features of electrodeposition are low temperature of operation, low distortion, access to internal surfaces and accurate control of deposit thickness. Deposition rates are generally in the range 20-100 µm/hr, but some process variants can achieve up to 400 µm/hr. Coatings are thin - generally less than 100 µm.
Turning to vapour deposition, the process is divided into physical vapour deposition (PVD) and chemical vapour deposition (CVD). PVD encompasses a number of variants - vacuum evaporation, ion implantation, ion plating and sputtering - generally used to deposit very thin coatings in the range 0.1 - 10 µm. CVD techniques involve production of a volatile carrier gas, transport of the gas without decomposition to the deposition site and a chemical reaction to produce the surface coating. Higher deposition rates and thicker coatings are achieved by CVD, but the substrate must be able to withstand higher operating temperatures - typically 600-1200°C. Whilst access is limited to line of sight for the most widely used PVD processes (vacuum evaporation and ion implantation), CVD has excellent throwing power and will coat all exposed surfaces uniformly.
Summary
Although the choices of materials and processes for surfacing are many, often there are constraints because of materials compatibility, substrate size, configuration and access. It is essential to address several fundamental points. There is a need to understand the wear mechanism, as this is the key to selecting correct materials. Equally important is the choice of application process for the material and this depends on the required thickness and integrity of the coating, material composition and adhesion between the substrate and the coating.
