PVD Coating Enhances Heat, Wear Resistance of Cutting Tools
Arc technology currently is the most widely used industrial physical vapor deposition (PVD) technology for depositing wear protective hard coatings. However, sputtering technology often is used in the area of basic research primarily because it lends itself better to the thin-film technology analytical methods used.
For example, electron spectroscopy analytical techniques frequently are used, which provide a depth resolution of only a few atomic layers (only a few nanometers) with a lateral resolution of a few micrometers. This type of analysis is hampered when examining arc-deposited coatings due to the presence typically of so-called droplets from the process on the surface of coating. However, today process parameters of the arc process can be changed to minimize these minute (micrometer range) metal droplets to such an extent that they will not impair the functional properties of the hard coatings applied to tools for the purpose of heat and wear resistance.
Droplets do not occur in PVD sputtered coatings, which is the reason why such coatings are easier to analyze. However, in industrial applications, the deposition of coatings that are easy to analyze is not important, but rather, coatings need to be produced that offer considerable functional advantages.
Arc process advantages
The most important advantage of the arc process compared with sputtering is the considerably higher energy density of the plasma during the deposition process. The high level of ionization (up to 100%) in the arc process ensures significantly higher hardness and density, as well as better adhesion of the heat and wear resistant coatings compared with coatings deposited using the sputtering process. These are important coating characteristics for improving the performance properties of cutting tools, for example.
The level of ionization in a typical sputtering processes is only about 10 to 15%, and even the most modern sputtering processes offer a level of ionization of only up to about 40%. Thus, sputtered coatings cannot achieve the properties of arc coatings due to this restrictive deposition feature.
The most important PVD coatings for chip removing tool applications are titanium-aluminum nitrides (TixAl1-x N). MetaPlas Ionon has succeeded in significantly increasing the high-temperature oxidation resistance of these coatings compared with other TiAlN coatings through the development of its MAXIT(r) AlTiN-Saturn coating. This is of considerable importance particularly with respect to high-speed, high-performance dry or minimum quantity lubricated cutting operations.
The coatings are deposited using the arc process and have a very high content of aluminum. During tool use, the high aluminum content results in the formation of a thin oxidation-protective alumina (Al2O3) film at the surface of the coated tool, which continuously renews itself during operation. In addition, the high density of the arc coating results in an increased resistance of the AlTiN-Saturn coatings to oxidation compared with conventional TiAlN coatings.
While the addition of aluminum offers important benefits, it is important not to further increase the aluminum content of the coatings (which are of a cubic crystal structure type) beyond a certain point because this would result in the formation of a hexagonal phase, which is unsuitable for chip-removing operations. The high energy of the arc process compared with that of the sputtering process offers the advantage of permitting higher aluminum contents without allowing this undesirable phase transition to occur.
The high aluminum content in the AlTiN-Saturn coatings provides additional benefits by imparting good electrical and thermal insulating properties. The latter thus serves the purpose of protecting the tools against excessive heating during use. For example, the AlTiN-Saturn coatings can withstand operating temperatures to 900 C (1650 F).
An additional important property of the AlTiN-Saturn coatings is their nanocrystalline structure (Fig. 1). Optimizing the process parameters of the arc process makes it possible to achieve a very high degree of hardness of the coatings together with a high degree of fracture toughness. With materials such as steel and cemented carbide, grain refinement offers a good method of increasing strength, which at the same time also increases the toughness of the material. This effect also is used to advantage in the AlTiN-Saturn coatings; the coatings are deposited in a nanocrystalline globular morphology in contrast to the otherwise common columnar, rough crystalline morphology of PVD coatings. This is made possible due to the high energy of the arc process without needing to add either expensive grain refining alloying elements or increasing the aluminum content up to the point where undesirable, but fine, crystalline hexagonal phases are formed. In addition, the dense nanocrystalline structure prevents inward diffusion of oxygen, thereby significantly increasing high-temperature oxidation resistance.
In addition to the high energy of the arc process, a plasma etching process run immediately before PVD deposition is responsible for the excellent adhesive properties of the AlTiN-Saturn coatings. This proprietary, patented MetaPlas Ionon process, known as arc enhanced glow discharge (AEGD), is extremely effective.
During dry cutting of cold-work tool steel 1.2379 (D2 high-carbon, high-chromium cold-work tool steel) and hot-work tool steel 1.2344 (H13 hot-work tool steel) using carbide ball nose end mills, wear at the cutting edges of tools coated with AlTiN-Saturn is far below the wear levels of competitive TiAlN coatings (Fig. 2). Further, typical applications in which AlTiN-Saturn coatings offer clear advantages are cutting of tempered steel, stainless steel, higher strength precipitation hardening ferritic-pearlitic steel, gray cast iron, Inconel (Ni-base alloy) and titanium alloys. Today, AlTiN-Saturn coatings are applied to all commonly used carbide and high-speed steel shaft tools as well as indexable inserts.