3-2-14. Some of the coatings used for cemented carbide tools that have been successful are titanium carbide (TiC), titanium nitride (TiN), aluminum oxide (Al2O3), and titanium carbonitride (TiCN):
▪Titanium Nitride (TiN), a gold-colored coating, is an excellent general purpose coating for protecting a wide variety of tools from wear. TiN coated tools are used for machining high alloy steels and low alloy steels at medium and high cutting speeds. Tool life is three to five times longer than uncoated HSS and carbide end mills.
▪Titanium Carbonitride (TiCN), a blue-gray colored coating, is a high performance coating for milling cutters used for machining stainless steel at low cutting speeds, machining alloy steels, and when increased speed and feed rates are desired.
▪Chromium Nitride (CrN) is a silver-gray colored coating that resists adhesive wear, corrosion, and oxidization. It is used for machining copper alloys, bronze, aluminum bronze, nickel silver titanium, and titanium alloys.
▪Chromium Carbide (CrC), a silver-gray colored coating, has high temperature oxidization-resistant properties used for aluminum and magnesium die-castings.
▪Titanium Aluminum Nitride (TiAIN) is a violet-gray colored multi-layer coating used for machining cast iron, stainless steel, nickel-base high temperature alloys and titanium alloys. This coating is used for high-speed dry and semi-dry machining operations.
▪Tungsten Carbide/Carbon (WC/C) is a black-gray colored coating of hard tungsten carbide particles in a soft amorphous carbon matrix. It is used for precision components with abrasive and adhesive wear, seizure problems (poor lubrication) and for dry machining applications.
Fig. 3-2-14 Coatings on inserts help to increase productivity. (Niagara Cutter, Inc.)
▪Polycrystalline Diamond (PCD), a layer of diamond fused to the cutting tool, is used for machining abrasive non-metallic, non-ferrous materials, graphite, plastics, green compacts, and composites.
For a list of properties and applications for thin wear-resistant coatings refer to Table 3-2-5.
Principles and Background Information
▪Chemical Vapor deposition (CVD) coatings adhere well, but high temperatures 1760-2425°F (800-1100°C) can damage substrates.
▪The physical vapor deposition uses a lower coating temperature 440-1100°F (200-500°C). PVD coatings prove more useful for milling, parting, grooving, and drilling, while CVD coatings perform better in turning.
▪The medium-temperature chemical vapor deposition 1760-1870°F (800-850°C) produces smoother, less-brittle coatings with lower residual stress.
▪The combined result of cobalt-enriched substrate and the medium-temperature chemical vapor deposition produces a very hard and tough cutting edge that wears well and is crater resistant.
▪Chemical vapor deposition (CVD) physical vapor deposition(PVD), and more recent medium-temperature chemical vapor deposition (MTCVD) constitute the primary processes for 80% of all coating tools.
▪Multi-layer coatings (three to five layers) are used to combine with thermally-resistant materials such as AL203 and/or abrasion-resistant layers, Fig. 3-2-15.
Table 3-2-6 Properties and applications of various thin-film wear-resistant coatings. (Balzers Tool Coating, Inc.)
Fig. 3-2-15 Multi-layer coatings are used to combine thermal-resistant materials with wear-resistant materials. (Carboloy, Inc.)
Multi-layering improves adhesion and allows a wider range of substrate/coating combinations.
The CVD coatings allow the combined advantages of various coatings and can be used into an optimum sequence to handle specific applications.
PART 4 CERAMIC CUTTING TOOLS
CERAMIC CUTTING TOOLS
The strength of ceramic cutting tools has nearly doubled, their uniformity and quality have been greatly improved, and they are now widely accepted by industry. Ceramic cutting tools are used successfully in the machining of hard ferrous materials and cast iron. As a result, lower costs, increased productivity, and better results are being gained. In some operations, ceramic tools can be operated at three to four times the speed of carbide tools.
Manufacture of Ceramic Tools
Most ceramic or cemented-oxide cutting tools are manufactured primarily from aluminum oxide.
1.Bauxite (a hydrated alumina form of aluminum oxide) is converted into a denser, crystalline form called alpha alumina.
2.Ceramic tool inserts are produced by either cold or hot pressing.
▪In cold pressing, the fine alumina powder is compressed into the required form and then sintered in a furnace at 2912 to 3092°F (1600 to 1700°C).
▪Hot pressing combines forming and sintering, with pressure and heat being applied simultaneously.
3.Certain amounts of titanium oxide or magnesium oxide are added for certain types of ceramics to aid in the sintering process and to retard growth.
4.After the inserts have been formed, they are finished with diamond-impregnated grinding wheels.
Types of Ceramic Grades
Ceramic cutting tools can be divided into two grades or families: alumina-base ceramics and silicon-base ceramics.
▪Alumina-base ceramics offer superior wear resistance and chemical wear stability, and are used for high velocity semi-finishing and finishing of ferrous and nonferrous materials.
•The addition of silicon carbide whisker reinforcements has improved the reliability of some alumina-based ceramics, especially when machining nickel-base alloys.
•Alumina-base ceramics should be considered primarily for semi-finishing and finishing operations.
▪Silicon nitride-base ceramics offer increased toughness and thermal shock resistance over alumina-base ceramics and therefore are considered high-velocity ceramics. They retain the toughness and thermal shock properties of conventional ceramics but offer superior abrasion resistance.
Characteristics of Reinforced Ceramic Inserts
When using reinforced ceramic inserts, high temperature is needed ahead of the cutting tool to soften or plasticize the workpiece material and assist its removal. The ideal cutting temperature in nickel alloys is in the area of 2200°F (998°C). This cutting temperature is beyond the upper limit for sintered carbide inserts. At this temperature carbide will soften, deform, and fail. Successful cutting with reinforced ceramic inserts require high surface speed along with balanced feed rates.
