The main aim of surface engineering is to improve the superficial properties of materials while the bulk properties—such as mechanical strength, impact resistance, density, etc.—are still maintained. The surface aspects of materials, which engineers consider in their design processes, are mostly categorized as: wear, fatigue and corrosion. Although wear and fatigue are considered to be mechanical damage, they are often coupled with corrosion.
When these types of damage are combined, the results can be complicated. For example, fatigue fracture in steel components is prevented when the applied cyclic stress is less than a specific value known as the "fatigue limit." Knowledge of this value is important for mechanical engineers who want to control fatigue. However, in the presence of a corrosive environment, the fatigue limit of the materials is greatly reduced, and thus determining the fatigue limit becomes more difficult. That is why it is very common to enhance the corrosion resistance of materials while also improving wear or fatigue resistance.
Methods of Increasing Durability
The methods that are used in surface engineering to control wear and fatigue fractures are divided into two types:
- Coating Application
This involves applying high-performance coatings to the surface of the material. In some cases, very hard coatings such as chromium, titanium carbides or titanium nitrides are applied to the surface. These coatings usually have good corrosion resistance and high wear resistance. However, unavoidable defects in the coatings, such as holidays, cracks and pinholes, can cause trouble in terms of corrosion by exposing the substrate to a corrosive environment. There are many different methods to apply wear and fatigue resistance coatings, such as electroplating, physical vapor deposition (PVD), chemical vapor deposition (CVD), pulsed laser deposition (PLD), etc.
- Surface Modification
In this method, both the structure and chemistry of the material surface are changed without applying an external coating; the surface of the metal is modified to satisfy surface requirements. Surface modification is usually conducted by diffusion of one or several elements into the surface of the metal. For example, in the carburizing process, carbon atoms penetrate the steel surface to produce a very high carbon content and high-hardness martensitic structure on the surface, while the core of the steel is a high-toughness and ductile metal. These types of processes are often known as "diffusion coatings."
Protection by Surface Modification
Because elemental diffusion is a function of time and temperature, these processes are performed at high temperatures. The elements that are chosen to diffuse into the steel parts are usually elements with a small atomic radius, such as carbon, nitrogen and boron. These kinds of elements have a higher diffusion rate in steel, and small amounts of them can increase the hardness of steel drastically.
As mentioned above, surface modification is predominantly used to improve surface features against mechanical damage. However, some of them have excellent corrosion resistance as well. Therefore, some of these coatings can be good candidates to protect against both mechanical damage (wear and fatigue) and corrosion simultaneously.
The Nitriding Process
Nitriding, one of the most important and common surface modification processes, can improve the surface corrosion resistance of steel parts along with increasing wear and fatigue resistance. In this process, which was invented in the early 1900s, atomic nitrogen penetrates the steel surface and reacts with substrate atoms (predominantly iron) to produce nitrides. The metallic nitrides are hard and enhance the wear resistance of the surface. This is a well-known process in the heat treatment of steel parts to improve their wear and fatigue resistance. The hardness of the nitrided layer reaches up to 65 Rockwell Hardness (RC) while the substrate hardness is usually around 45 RC. The corrosion resistance of the nitrided layer is excellent in many environments.
It is possible to induce both nitrogen and carbon atoms into the metallic surface simultaneously. This process is called nitrocarburizing or carbonitriding, depending on which element is predominant in penetrating.
Details of Nitriding
There are many different trade names—such as Arcor, Tenifer, Tufftride, Melonite, Tenoplus, QPQ, etc.—that have been patented. These vary according to the specific conditions of the nitriding process. The medium used as a source of atomic nitrogen can be plasma, gas or molten salt.
The nitrided case on steel is not a single layer, and actually consists of two zones:
- Compound zone, which is called a "white layer" and identified with the mixture of different phases of metallic nitrides.
- Diffusion zone, which is placed beneath the white layer and consists of iron containing high nitrogen in conjunction with metallic nitride particles.
Since the white layer is extremely hard but very brittle, it is advisable to eliminate this zone for wear and fatigue applications. However, the white layer has a high corrosion resistance. Therefore, in cases where corrosion is a problem, the white layer would be helpful. For example, when water-based lubricants are used in a wear application, the surface must have high corrosion resistance.
For the Arcor process, after nitrocarburizing in a molten salt, a black iron oxide layer (Fe3O4) is formed on the surface by immersing nitrided steel components in another molten salt for a couple of minutes. The process of formation of black oxide, which is known as the blackening process, can increase the corrosion resistance of the surface in atmospheric and neutral solutions. The black oxide layer is porous and this feature not only enhances the friction behavior of the surface by improving lubrication run-in, but can also provide a good substrate for rust-preventive coatings, because the open pores in the oxide layer can enhance the adhesion of coatings on the surface.
A Mitigation Method
Although nitriding is a well-known process in the heat treatment of steel parts to reduce superficial mechanical damage such as wear, galling, erosion, fatigue and fretting, this process could be useful when the corrosion in a system has to be controlled. In other words, corrosion engineers should consider nitriding as a method to mitigate corrosion, particularly when the corrosion is likely to be accompanied by mechanical damage.