Emerging technologies are focusing on the deposition methods of coating the steel surface with a hard metal (or combinations of metals), which is harder than that of the steel. These coatings are generally known as ultra-hard coatings or surface-deposition techniques. There are many different methods of the application of the coatings that can be put down onto a metal substrate to improve its wear and corrosion characteristics.
Generally, the coatings can be subdivided into many different categories. These include the simple groupings of decorative coatings, hard coatings and ultra-hard coatings. These coatings can be further categorized into temperature ranges (low, medium and high) that will be determined by the selected coating type. This will be followed by the following categories in relation to the method of applications of the coatings to the substrate material:
- Chemical vapor deposition
- Plasma-assisted vapor deposition
- Physical vapor deposition
A further subdivision can be made as to the nature of the method of application of the deposited coating: pack cementation and reactive gases. With the advent, development and growth of plasma technology, one is now able to process under low-pressure conditions (vacuum) and accomplish both controllable and repeatable results.
Chemical Vapor Deposition (CVD)
CVD has developed into a mature and versatile application procedure utilized to deposit layers of almost any elemental metal onto a substrate material. The primary deposition metals can be found in the periodic table of elements. Other compounds such as metallic oxides, nitrides, carbides and intermetallics can be deposited in a relatively easy manner.
Because there are numerous metallic elements and derivatives of these elements that are used in CVD, there are equally as many numerous reactive chemistries. These can include: thermal decomposition, carburizing and nitriding (gaseous, salt or plasma). A CVD reaction is controlled by the following factors:
- Thermodynamic, mass transport and kinetic considerations
- Chemistry of the reaction
- Process parameters such as temperature, pressure and chemical activity
When considering the application of the surface-deposition method, which is necessary to exercise good control procedures of the selected method to produce repeated and consistent depositions, it should be remembered that they are depositions and not diffusion processes. This means that pre-cleaning of the part prior to the coating application is of paramount importance. The degree of cleanliness accomplished will determine the success of the deposited coating.