Hot Properties

Author John Earnhardt
February 01, 1998 - 11:00am

Recent trends in manufacturing have turned up the heat at the cutting edge. Manufacturers are pushing their tools to cut faster. They are also trying to limit or eliminate the use of coolant at the cutting interface, and working with materials that are more challenging to machine. The high temperatures generated in high-speed machining, dry machining, and applications involving difficult-to-machine materials can accelerate tool wear. In turn, the poor tool life can result in more frequent tool changes, increased tool inventory, longer machine downtime, and lower throughput.

Some coatings can extend tool life by providing a protective layer that insulates the cutting edge from extreme temperatures. But not all coatings can provide this protection. Tools coated with titanium nitride (TiN) or titanium carbonitride (TiCN) typically don't last longer than uncoated tools in high-temperature cutting operations. Both of these physical-vapor-deposition (PVD) coatings have relatively low oxidation temperatures. When exposed to high heat, they transform into brittle oxides that do not resist the wear caused by cutting forces.

Titanium aluminum nitride (TiAlN) has proven more effective than TiN or TiCN in protecting tools from heat, because it has a higher oxidation temperature (1450° F). TiAlN can withstand higher temperatures without breaking down and degrading the tool substrate. Furthermore, when TiAlN's oxidation threshold is exceeded, its outer surface transforms into aluminum oxide, which has excellent hot hardness, low thermal conductivity, and high chemical stability. As a result, most of the heat generated by the cutting operation flows into the chips rather than the cutting tool. Furthermore, TiAlN provides the increased lubricity typical of PVD coatings.

Harder Is Better
Every coating manufacturer currently offers an aluminum-base TiN coating. Multi-Arc Inc., Rockaway, NJ, introduced the first TiAlN coating in North America in 1991. It was a graded, titanium-rich, mono-layer coating with a hardness of 2600 HV (micro-Vickers). Since then, the company has developed a TiAlN coating with increased aluminum content and a hardness of 4500 HV. Although the original coating and the harder coating have similar oxidation and surface-roughness properties, the harder coating has higher wear resistance and better thermal and chemical stability as the aluminum oxidizes. The harder TiAlN coating also has a lower friction coefficient that facilitates chip evacuation and maintains low, stable cutting forces. Technological advancements have allowed both TiAlN coatings to be deposited with adhesion levels comparable to those for TiN and TiCN exceeding 70 Newtons in normal-force scratch testing.

Without understanding the advantages of TiAlN over TiN and TiCN in high-temperature applications, a shop may be hesitant to pay the higher price for the premium coating. Table 1 demonstrates the merits of TiAlN by comparing its properties with those of TiN and TiCN. TiN's hardness is typically around 3000 HV, with an oxidation temperature near 950° F. TiCN has a higher hardness, approximately 4000 HV, but it suffers from a low oxidation temperature of 750° F. This makes the use of TiCN difficult without other means to manage the heat produced during high-speed machining, dry machining, and machining difficult materials.

TiN and TiCN may perform just as well as TiAlN in cutting processes that do not generate excessive heat. Therefore, a shop may not be justified in paying extra for the TiAlN coating for these applications. However, the properties of TiAlN make it the only choice for high-temperature operations. TiAlN's thermal and chemical stability allows tools to be run at higher speeds and feeds than TiN- or TiCN-coated tools, and it enables certain processes to be run dry. TiAlN's properties also make it suitable for machining a broad range of materials, including heat-resistant alloys, nickel- and cobalt-base alloys, titanium, stainless steels, tool and die steels, cast iron (gray, nodular, and austempered), heat-treated steels, and die molds.

A Variety of Uses
Many OEM tool manufacturers and tool users have achieved excellent results with TiAlN-coated tools. The coating has reportedly enabled tools to last two to five times longer than uncoated, TiN-coated, and TiCN-coated tools. Here are a few examples of TiAlN's performance in a variety of high-temperature cutting operations.

In the machining of titanium alloys, many users have reaped the benefits of TiAlN coatings by increasing production levels in endmilling and turning applications. This has been achieved mainly by increasing the cutting speed to between 400 and 500 sfm, which is significantly higher than the conventional range of 100 to 250 sfm for uncoated, TiN-coated, or TiCN-coated tools. This higher speed, combined with longer tool life, has helped to reduce overall production costs.

Endmilling titanium. One company that has achieved this level of performance is Pratt & Whitney. The manufacturer's East Hartford, CT, plant makes hollow fan blades for aircraft engines used to power the Boeing 777-300. The titanium 6A1-4V blade has a dovetail form that is milled at the ends. After experimenting with several styles of endmills and different substrate materials, Pratt & Whitney selected a serrated-tooth, uncoated carbide endmill. The tools were run at a speed of 300 rpm and feed rates of 2.5 ipm for the roughing pass and 3.0 ipm for the finishing pass. At these cutting parameters, average endmill life was five to 10 parts.

Looking to improve tool life and reduce machining time, Pratt & Whitney tested two TiAlN coatings with different hardnesses. The coating with the lower hardness enabled the endmill to cut 22 parts while running at the same speed and feed as the uncoated tool. For the endmills with the harder TiAlN coating, Pratt & Whitney increased the speed 33% to 400 rpm and increased the feed rates to 3.0 ipm for the roughing cut and 3.5 ipm for the finishing cut. Even though these higher cutting parameters produced more heat, the tool with the harder coating cut 28 parts, 30% more than the previous tool and almost three times more than the uncoated tool. Pratt & Whitney hopes to achieve even better performance with its TiAlN-coated tools. Using endmills coated with the harder TiAlN, the company is looking to increase the speed of the finishing cut to 600 rpm and the feed rates to between 4.5 and 5.0 ipm.

Facemilling titanium. In another application involving an aerospace titanium alloy, a pump body was being facemilled with standard 15°-rake, TiCN-coated inserts. TiAlN-coated inserts were used on a 2"-dia. milling cutter with a 45° lead angle and five teeth. Cutting conditions were 200 sfm, 3.0 ipm, and 0.100" depth of cut, with coolant. Full-diameter cuts were taken up to a 45° shoulder. The TiAlN-coated inserts lasted 3 1/2 times longer than the TiCN-coated inserts being supplied by a tool vendor.

Shops have used TiAlN-coated tools to cut other materials in addition to titanium and in applications other than milling. The switch to TiAlN resulted in higher productivity for these manufacturers as well.

Broaching Inconel. One aerospace manufacturer tested the high-hardness TiAlN coating in the broaching of Inconel 718. Typically, this material was cut with uncoated tools, with an average broach life of 10 to 12 parts. After TiAlN was applied to the surface of the tool, broach life was doubled, with an average of 23 parts per grind. In addition to minimizing the flank wear, the coating made the wear pattern more predictable. The results were less downtime, reduced tooling investment, and lower overall part cost. And the manufacturer could continue to regrind its TiAlN-coated tools, recoating them after sharpening. The predictable wear pattern of the TiAlN-coated tools meant that the manufacturer could schedule a tool's regrinding before the tool suffered significant damage in most instances.

Drilling aluminum. Although coatings are not typically used in the machining of aluminum, TiAlN was used in the drilling of 10% silicon aluminum to provide added abrasion resistance. In this application, TiAlN-coated solid-carbide drills produced more than three times the number of holes that uncoated drills did. Because the operation was on a transfer line, the user could not increase the cutting parameters on one machine without overloading machines down the line, but the improvement in tool life justified the cost of the TiAlN coating. In another application on the same transfer line, TiAlN-coated indexable drills running at more than 1700 sfm lasted more than twice as long as uncoated drills, reducing transfer-line downtime.

Dry hobbing steel. With coolant disposal becoming more complex and costly, an increasing number of shops are pursuing dry machining. An automotive-transmission manufacturer has been investigating the dry hobbing of gears made of medium-carbon steel. The TiAlN-coated carbide hobs allowed the company to machine parts dry at increased cutting parameters and still hob nearly as many parts per regrind throughout the regrindable life of the hob as it did with conventional methods using coolant.
With continued success in a variety of operations, TiAlN is expanding the realm in which cutting tools are applied in today's manufacturing environment. Higher productivity, reduced costs, and environmental liability are growing concerns in manufacturing. While addressing these issues, TiAlN has demonstrated the ability to run faster, longer, and cooler in repeated applications. Through a combination of properties, including thermal stability and a high hot hardness, TiAlN coatings open new opportunities for manufacturing.

About the Authors
John Earnhardt is general manager, northeast regional coating center, Multi-Arc Inc., Rockaway, NJ. Raymond May is general manager, Multi-Arc de Mexico, Queretaro, Mexico.

Related Glossary Terms

  • Vickers hardness number ( HV)

    Vickers hardness number ( HV)

    Number related to the applied load and surface area of the permanent impression made by a square-based pyramidal diamond indenter having included face angles of 136º. The Vickers hardness number is a ratio of the applied load in kgf, multiplied by 1.8544, and divided by the length of diagonal squared.

  • alloys


    Substances having metallic properties and being composed of two or more chemical elements of which at least one is a metal.

  • aluminum oxide

    aluminum oxide

    Aluminum oxide, also known as corundum, is used in grinding wheels. The chemical formula is Al2O3. Aluminum oxide is the base for ceramics, which are used in cutting tools for high-speed machining with light chip removal. Aluminum oxide is widely used as coating material applied to carbide substrates by chemical vapor deposition. Coated carbide inserts with Al2O3 layers withstand high cutting speeds, as well as abrasive and crater wear.

  • broach


    Tapered tool, with a series of teeth of increasing length, that is pushed or pulled into a workpiece, successively removing small amounts of metal to enlarge a hole, slot or other opening to final size.

  • broaching


    Operation in which a cutter progressively enlarges a slot or hole or shapes a workpiece exterior. Low teeth start the cut, intermediate teeth remove the majority of the material and high teeth finish the task. Broaching can be a one-step operation, as opposed to milling and slotting, which require repeated passes. Typically, however, broaching also involves multiple passes.

  • coolant


    Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.

  • cutting speed

    cutting speed

    Tangential velocity on the surface of the tool or workpiece at the cutting interface. The formula for cutting speed (sfm) is tool diameter 5 0.26 5 spindle speed (rpm). The formula for feed per tooth (fpt) is table feed (ipm)/number of flutes/spindle speed (rpm). The formula for spindle speed (rpm) is cutting speed (sfm) 5 3.82/tool diameter. The formula for table feed (ipm) is feed per tooth (ftp) 5 number of tool flutes 5 spindle speed (rpm).

  • depth of cut

    depth of cut

    Distance between the bottom of the cut and the uncut surface of the workpiece, measured in a direction at right angles to the machined surface of the workpiece.

  • endmill


    Milling cutter held by its shank that cuts on its periphery and, if so configured, on its free end. Takes a variety of shapes (single- and double-end, roughing, ballnose and cup-end) and sizes (stub, medium, long and extra-long). Also comes with differing numbers of flutes.

  • endmilling


    Operation in which the cutter is mounted on the machine’s spindle rather than on an arbor. Commonly associated with facing operations on a milling machine.

  • facemilling


    Form of milling that produces a flat surface generally at right angles to the rotating axis of a cutter having teeth or inserts both on its periphery and on its end face.

  • feed


    Rate of change of position of the tool as a whole, relative to the workpiece while cutting.

  • flank wear

    flank wear

    Reduction in clearance on the tool’s flank caused by contact with the workpiece. Ultimately causes tool failure.

  • gang cutting ( milling)

    gang cutting ( milling)

    Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.

  • hardness


    Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.

  • inches per minute ( ipm)

    inches per minute ( ipm)

    Value that refers to how far the workpiece or cutter advances linearly in 1 minute, defined as: ipm = ipt 5 number of effective teeth 5 rpm. Also known as the table feed or machine feed.

  • lead angle

    lead angle

    Angle between the side-cutting edge and the projected side of the tool shank or holder, which leads the cutting tool into the workpiece.

  • lubricity


    Measure of the relative efficiency with which a cutting fluid or lubricant reduces friction between surfaces.

  • milling


    Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.

  • milling cutter

    milling cutter

    Loosely, any milling tool. Horizontal cutters take the form of plain milling cutters, plain spiral-tooth cutters, helical cutters, side-milling cutters, staggered-tooth side-milling cutters, facemilling cutters, angular cutters, double-angle cutters, convex and concave form-milling cutters, straddle-sprocket cutters, spur-gear cutters, corner-rounding cutters and slitting saws. Vertical cutters use shank-mounted cutting tools, including endmills, T-slot cutters, Woodruff keyseat cutters and dovetail cutters; these may also be used on horizontal mills. See milling.

  • physical vapor deposition ( PVD)

    physical vapor deposition ( PVD)

    Tool-coating process performed at low temperature (500° C), compared to chemical vapor deposition (1,000° C). Employs electric field to generate necessary heat for depositing coating on a tool’s surface. See CVD, chemical vapor deposition.

  • stainless steels

    stainless steels

    Stainless steels possess high strength, heat resistance, excellent workability and erosion resistance. Four general classes have been developed to cover a range of mechanical and physical properties for particular applications. The four classes are: the austenitic types of the chromium-nickel-manganese 200 series and the chromium-nickel 300 series; the martensitic types of the chromium, hardenable 400 series; the chromium, nonhardenable 400-series ferritic types; and the precipitation-hardening type of chromium-nickel alloys with additional elements that are hardenable by solution treating and aging.

  • titanium aluminum nitride ( TiAlN)

    titanium aluminum nitride ( TiAlN)

    Often used as a tool coating. AlTiN indicates the aluminum content is greater than the titanium. See coated tools.

  • titanium aluminum nitride ( TiAlN)2

    titanium aluminum nitride ( TiAlN)

    Often used as a tool coating. AlTiN indicates the aluminum content is greater than the titanium. See coated tools.

  • titanium carbonitride ( TiCN)

    titanium carbonitride ( TiCN)

    Often used as a tool coating. See coated tools.

  • titanium carbonitride ( TiCN)2

    titanium carbonitride ( TiCN)

    Often used as a tool coating. See coated tools.

  • titanium nitride ( TiN)

    titanium nitride ( TiN)

    Added to titanium-carbide tooling to permit machining of hard metals at high speeds. Also used as a tool coating. See coated tools.

  • titanium nitride ( TiN)2

    titanium nitride ( TiN)

    Added to titanium-carbide tooling to permit machining of hard metals at high speeds. Also used as a tool coating. See coated tools.

  • turning


    Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.

  • wear resistance

    wear resistance

    Ability of the tool to withstand stresses that cause it to wear during cutting; an attribute linked to alloy composition, base material, thermal conditions, type of tooling and operation and other variables.


General Manager

John Earnhardt is general manager, northeast regional coating center, Multi-Arc Inc., Rockaway, New Jersey.