什么叫hard coating?

fzhrxy · 发表于 2008-4-6 20:55

什么叫hard coating?

都市 · 发表于 2008-4-7 08:31

What are the hard coatings?

The tools are the key of every industrial production. The tool price usually presents a sizeable share in the product end-price, therefore it is important to enhance the tool lifetime and productivity. The first way is the proper tool material choice. Generally this includes various tool steels and hard metals, whereas in the last year the ceramic and composite materials (e.g. cermets) have been implemented.

Together with the development of the tool materials, the protective coatings have been developed as well. Their common feature is the protection of the tool by their high hardness and chemical inertness, however, they are relatively brittle and expensive so they are not appropriate for bulk tool material. The combination of bulk material and the coating ensures optimal tool properties. The ecologically questionable galvanic processes (e.g. hard chrome) have been used for over a century to protect the tools, and many other techniques are known today, such as electroless plating, gas and plasma nitiding etc.

The development of the hard protective coatings in the narrower sense started in the sixties with the chemical (CVD) and physical (PVD) vapor deposition techniques. There are many PVD variants in use today (magnetron sputtering, evaporation by laser, cathode arc, electron beam etc.). Their common feature is the vacuum environment, the substrate temperature is from room temperature up to about 500 °C, the coating thickness does not exceed a few micrometers.

The most common hard coatings are based on transition metal nitrides (TiN, CrN), but in the last decade there has been a vast increase in multicomponent coatings (TiAlN), multilayer coatings (TiN/TiAlN) as well as carbon-based coatings (DLC). The newest generation is represented by nanocomposite (TiN+DLC) and lubricating coatings (WC+C).

The most important feature of the hard protective coatings is to reduce wear and in this way to increase the tool lifetime. So it is important to know which mechanism has the highest contribution to the wear (abrasion, adhesion, corrosion, high temperature, material sticking etc.) in order to find the most suitable coating for the desired process. A combination of certain coating properties opens possibilities for new technological procedures, e.g. low coefficient of friction and resistance against high temperatures enables dry machining without use of cooling-lubricating liquids. Aside from the price reduction such a procedure is superior from the ecological standpoint.