Hard Coatings and Coating Properties
1. Hard coatings, formed by reactive Physical Vapor Deposition ( PVD) processes, are becoming widely used in the decorative coating and tool industries.
2. Hard, decorative PVD coatings are more resistant to wear and corrosion than are electroplated decorative coatings, such as gold and brass, which must use a polymer topcoat for protection.
3. Such decorative, hard coatings are being used on plumbing fixtures, sporting goods, metal dinnerware, eyeglass frames, door hardware, and other such applications where the coating is subjected to wear, abrasion, and corrosion during use and cleaning. Titanium nitride ( TiN) is used for a gold-colored coating and zirconium nitride ( ZrN) looks like brass. Titanium carbonitride ( TiCxNy) can have a color that varies from bronze to rose to violet to black, depending on the composition. One manufacturer of door hardware gives a lifetime guarantee on its PVD-coated fixtures.
How to get high quality hard coating film?
1. In order to get the most hard, tense, wear-and corrosion-resistant coating, the substrate temperature should be as high as possible.
2. And concurrent bombardment by energetic atomic-sized particles ( Ion-plating ) during the reactive deposition should be used.
3. When coating temperature-sensitive substrate such as plastics, the temperature must be kept low, and concurrent bombardment can be used to densify the film.
4. One technique for coating temperature-sensitive materials uses the deposition of many thin layers, separated by a cooling period. This is done by mounting the parts on a rotating fixture that is passed in front of the deposition source multiple times.
5. Hard PVD coatings are also used for coating machine tools such as drills, lathe-tool inserts, stamps and punches, and expensive forming tools ( such as injection molds for plastics).
6. The PVD hard coating is advantageous for coating forming tools, in that the process does not change the physical dimensions of the part significantly.
In many cases the TiN coatings can be stripped from the tool surface, for repair and rework, without attacking the substrate material.
Generally the machine tools can be heated or rather high temperatures during deposition. For example, in coating hardened steel drills, the substrate is heated to 450℃ or so before deposition is started. This pre-heating can be done by ion bombardment, which also sputter cleans the surface, or by using other heating sources in the deposition chamber.
7. Industrial tool coatings are typically 1 micron to 15 microns in thickness. In addition to being hard and dense, tool coatings should also have a high fracture toughness to inhibit fracture initiation and propagation, and possibly have some compressive stress to inhibite fracture propagation. The most common tool coatings are: TiN, TiCxNy and TiAlN2, while other coatings-such as zirconium nitride, hafnium nitride, titanium carbide, and chromium nitride-are less commonly used.