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Armor for Cutting Tools

Armor for Cutting Tools

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Selecting the right coating for tooling —based on the application— is one of the most important factors to ensure effective machining. The right choice of coating can cut tooling costs per job or per piece because it can lead to reduced friction; less heat development, less cold welding, reduced cutting forces; increased feed and speed rates; improved workpiece surface quality and reduced downtime. Choosing the right coating for an application can be a confusing and arduous task. Each coating that is available has advantages and disadvantages, and the wrong choice can cut tool life and can multiply problems.

What to look for in a coating

Tool geometry, material and diamond coating increase tool life: cross-section image of CVD coating on carbide tool’s cutting edge – courtesy of Diamond Tool Coating.

Hardness is critical in a coating because the harder the material or surface, the longer the tool will last. With a surface hardness near 9,000 HV, CVD diamond coatings that have been grown onto the tools have as much as 20 times better tool life compared to PVD coatings. This is the coating of choice for cutting nonferrous materials because of its higher hardness and its ability to run at two to three times the speed of uncoated tooling.

What to look for in a coating

Hardness is critical in a coating because the harder the material or surface, the longer the tool will last. With a surface hardness near 9,000 HV, CVD diamond coatings that have been grown onto the tools have as much as 20 times better tool life compared to PVD coatings. This is the coating of choice for cutting nonferrous materials because of its higher hardness and its ability to run at two to three times the speed of uncoated tooling.

Wear resistance is the ability of the coating to protect against abrasion. Although a material may not be hard, elements and processes added during production may aid in the breakdown of cutting edges or forming lobes.

Surface lubricity is important because a high coefficient of friction causes increased heat that shortens the life of a coating or causes the coating to fail. However, a lower coefficient of friction increases tool life. A slick surface — one that lacks coarseness or irregularities — lets chips slide off the face of the tool, and generates less heat. High surface lubricity also allows for increased speeds, when compared with tools that are not coated, and helps to prevent galling of the workpiece.

Oxidation temperature is the point at which the coating starts to break down. A high oxidation temperature rating improves success in high heat applications.

Anti-seizure capability keeps material from depositing onto the tool by preventing chemical reactivity between the tool and the cutting material. This capability reduces built up edge (BUE), a problem that is common when machining nonferrous materials such as aluminum or brass, leads to chipping of the tool or oversizing of the part. Once material starts adhering to the tool, it continues to attract more material.

Multilayers of PVD coatings reduce the spread of cracks. Photo courtesy of Emuge Corp.

Common Coatings

Titanium nitride (TIN) is a general purpose coating produced by the physical vapor deposition (PVD) process. This coating has a surface hardness of 81 HRc and a coefficient of friction of 0.4. It is thermally stable to 1,000f (550C), and is suited to cutting a variety of materials including iron-based materials, unalloyed and alloyed steels and hardened steels with high-speed steel (HSS) tooling.

Titanium carbonitride (TiCN) has more carbon than tin for higher hardness (90HRc) and better surface lubricity (coefficient of friction of 0.3). This coating also is good for HSS cutting tools and, with increased wear resistance, is good for machining more difficult-to-machine materials such as cast iron, aluminum alloys, tool steels, copper, inconel, titanium alloys and nonferrous metals.

Titanium aluminum nitride (TiALN) is a high performance coating with a surface hardness in the upper 80-HRc range. It holds its hardness at high temperatures due to a layer of aluminum oxide that forms between the tool and the cutting chip that transfers heat away from the tool and into the part or chip. Its superior oxidation resistance provides unparalleled performance in high-temperature machining. Because carbide tooling is run at higher speeds than HSS with little to no coolant, tialn is the preferred choice when coating carbide. With a coefficient of friction less than that of tin, tiain excels at machining abrasive and difficult-to-machine materials such as cast iron, aluminum alloys, tool steels, and nickel alloys. the coating’s high ductility makes it a good choice for interrupted-cutting operations.

Aluminum titanium nitride (AlTiN), with a higher aluminum content than tialn, has a higher surface hardness and also forms a layer of aluminum oxide for good tool life in high-heat applications and is a viable option for high-speed machining.

Titanium diboride (TiiB2) coating is harder than tin and tialn coatings and has an extremely smooth surface, resulting in low surface friction, speedy chip flow, and high wear resistance. in addition, built-up edge is prevented because this coating has a very low affinity for aluminum. the coating is recommended for applications in silicon aluminum, titanium, magnesium and copper alloys.

Chromium nitride (CrN), also known as rainbow coating, has characteristics and capabilities that surpass other coatings. With high corrosion resistance, hardness (90 to 92 HRc), wear and abrasion resilience and heat transformation, crn is good for virtually all materials including high silicon aluminum, stainless steel, high nickel alloys, titanium and composite materials. With an extremely low coefficient of friction of 0.027, crn’s lubricity surpasses that of tin, ticn, tiain and Zrn coatings. the anti-seizure properties of this coating make it preferred in situations where built-up edge is common. crn coatings have a thickness of .00065 in., and help to ensure that machined products maintain tolerances.

Diamond coatings, produced by the chemical vapor deposition (CVD) process, offer the highest performance available for nonferrous materials. they are suitable for cutting graphite, metal matrix composites (MMC), high silicon aluminum and many other abrasive materials. Diamond coatings should not be used for machining steels because the heat generated causes chemical reactions that break down the bonds that hold this coating to the tool.

DIAMOND COATING CURES COMPOSITE MACHINING WOES

An advanced composite wing skin material for the f-35 Joint Strike fighter was a headache for components manufacturer Lockheed Martin aeronautics co. The tooling Lockheed was using had short life and produced excessive delamination on the carbon composite material. The National Center for Defense Manufacturing and Machining and its alliance Partners — AMAMCO Tool Co., Diamond Tool Coating, Kennametal Inc., McCullough Machine and MDT inc. — developed a remedial approach that was aimed at improving router geometry and material, and the tool’s coating.

Diatiger multi-layer diamond coating produced by Diamond Tool Coating LLC  was chosen as the coating for the router. This coating gains strong protection against abrasive wear, galling, adhesive wear and cutting edge damage from mechanical shock from its interlocking layers of uniquely structured diamond crystals. The Diatiger coating is recommended for difficult-to-machine non ferrous metals and composites.

The new tooling strategy produced dramatic improvements for the live of the tools that Lockheed Martin was using. The number of tools needed per wing skin was reduced from 24 to two — one to rough and one to finish; Cutting distance increased six-fold — from 9 linear feet at one-third the total material thickness to 57 linear feet at full material thickness. These improvements produced a savings of $80,000 per aircraft.

COATINGS FOR HARD MACHINING

To compete with low-wage global competition, many mold and die shops are turning to hard milling. Hard milling, combined with high speed machining (hSM) techniques, is the machining of high-tensile-strength materials that cannot be economically machined with hSS tools. The threshold of hard milling begins with materials that have a tensile strength greater than 1,800 mm, or 52hrc. Cutting materials typically are micrograin carbides, cermet or cubic boron nitride (cBn), and cutting usually is done dry with pressurized air.

To protect carbide tool substrates from the high cutting temperatures generated by hard machining, high-performance tialn coatings are preferred. Coating structures (layers) and compositions are application specific, as is cutting tool geometry. Tialn coatings can consist of a single, mono-layer, multiple layers of various coating formulations, such alternate layers of tin and tialn, or as very thin nano layers.

Mono-layer coatings enhance lubricity and multi-layer structures provide for greater heat resistance. a major advantage of multi-layer coating structures is that they inhibit the progression of a crack directly through the coating to the substrate. Should a crack occur in an outer layer, it would progress only to the interface with the next lower layer and then shift sideways, thus preserving the integrity of the lower layers.

Contributors to this article include:

CemeCon AG (www.cemecon.com)
Diamond Tool Coating (www.diamondtc.com)
Emuge Corp. (www.emuge.com)
Hardcoating Technologies Ltd. (www.hardcoatingtech.com)
National Center for Defense Manufacturing and Machining (www.ncdmm.org)

To see the original article on: American Machinist