Harder, Faster

With the variety of inserts on the market today, it is hard to believe that 30 years ago uncoated carbide and HSS were virtually the only materials used for turning applications. Since then, cutting tool developers have introduced a broad range of products as they have tried to keep pace with the drastic changes that have occurred in the metal-removal industry and satisfy the needs of users in such industries as aerospace and automotive.

Now, with so many choices in cutting tools available, users may find it hard to determine what they actually need for their particular turning operations. Some basic knowledge of the trends cutting tool manufacturers have been following in their development of cutting tool materials may help users sort things out.

Before looking at these trends, users should understand why these trends are taking place. Different types of coatings and substrates have evolved over the years in response to developments in the industries that use cutting tools. These developments include:

• The increasing use of high-performance materials;
• The growing number of machine tools in use that are capable of high productivity; and
• The growing number of applications that require lighter, faster cuts to save costs and improve quality.

Material Gains 
New insert materials have been developed primarily to machine the high-performance workpiece materials industry has begun using in recent years. In the metalcutting field, “industry” typically refers to the aerospace and automotive markets. In aerospace, manufacturers have developed most of these improved materials to reduce weight. Since flying machines were first invented, weight has been a major concern. Materials that are light yet strong are needed just to allow the aircraft to leave the ground. For manufacturers of aircraft-engine components, heat resistance is equally important to handle the stress placed on these parts.

Materials manufacturers have added a variety of alloying elements to base metals to meet aerospace’s demands for lower weight, higher heat resistance, and greater strength. The composition of cast-aluminum alloys can include magnesium, copper, zinc, silicon, and manganese to increase strength. Titanium alloys can contain aluminum, zirconium, molybdenum, niobium, vanadium, and tin.

The aerospace industry could not meet its objectives without these alloying elements, but these additives also create materials that are difficult to machine. The elements react with cutting tools, wearing away cutting edges with their abrasiveness and giving tool manufacturers headaches as their engineers try to develop products that can handle these reactions.

Weight is a concern in the automotive industry as well. Ever since fuel prices rose dramatically 25 years ago, automakers have been searching for ways to build lighter cars and trucks that go farther on a gallon of gas. Fuel economy was a concern before the 1970s, but it didn’t become a priority until interruptions in the United States’ oil supply caused the federal government to issue its corporate average fuel economy (CAFE) standards.

CAFE’s mandates forced automakers to build more efficient vehicles. To do this the automakers considered building vehicles out of lighter materials such as aluminum or titanium. They found that, while these materials are lighter than the same volume of steel, they also are much more expensive. As a more economical alternative, automakers turned to higher strength steels in smaller cross sections to decrease weight. Heat treating is one method steelmakers use to create stronger steels. They also use micro-alloying, adding small percentages of normal steel elements while maintaining tighter control for accuracy. Micro-alloying improves the properties that heat treating imparts to the steel.

Automakers also are using an increasing amount of stainless steel for greater corrosion resistance. And they have begun to find ways to use aluminum economically. Even traditional materials are being improved. Castings now have fewer inclusions, are less porous, and are heat treated to tighter specifications. The hardened steels that are being used by automakers require even harder cutting tools to turn them. Machining aluminum requires inserts with tough micrograin substrates and high positive rakes to shear the metal.

Another force driving the development of new cutting tools is the introduction of machine tools with advanced capabilities (although some might argue that machine tool manufacturers are simply trying to keep pace with cutting tool innovations). Whatever the motivation, machine tool manufacturers are producing machines that are more rigid and accurate and, in some cases, feature higher horsepower motors. These machines are capable of high-speed turning, a process that generates temperatures hot enough to accelerate tool wear and promote chemical reactions between the tool and the workpiece material. For operators to take advantage of these machines’ high-speed capabilities, they must have chemically stable, heat-resistant cutting tools.

The trend toward near-net-shape parts also dictates certain cutting tool characteristics. These parts require only light finishing cuts that produce good surface finishes. Harder and more-positive-rake carbide inserts are being developed to handle the requirements of these applications. These inserts must be strong enough to resist breakage on their sharp, thin cutting edges. Hard coatings must be used to withstand the punishment of faster machining and resist edge buildup.

The Evolution of the Insert 
The metalcutting industry’s need to machine difficult materials at increased speeds explains why shops moved from HSS to coated carbides in the 1980s. Coated carbides currently dominate manufacturing because of their wear resistance, chemical stability, and higher hardness. Within the coated-carbide category, there are a variety of substrate and coating compositions. To select the right combination of substrate and coating for a particular application, an operator must be familiar with the effects each component has on the insert’s performance.

The type of coating is important, but the coating must be applied to the appropriate substrate for the insert to achieve the desired characteristics. Even though the substrate on coated inserts never comes into contact with the workpiece, its properties will have a significant impact on the way the tool and its coating perform.

For example, a wear-resistant substrate can help prevent deformation from the heat of a high-temperature application. To gain this wear resistance, a fine-grain, low-cobalt tungsten-carbide substrate should be used. A substrate with even greater high-temperature hardness—and thus more deformation resistance—can be produced by adding titanium carbide, tantalum carbide, or niobium carbide to the fine-grain, low-cobalt tungsten carbide. By enhancing the substrate’s crater resistance, these additions create a substrate suitable for machining steels, while tungsten carbide by itself is suitable for machining cast irons. A coarse-grain, high-cobalt substrate offers increased toughness. Carbide enriched with higher cobalt at the surface provides more edge strength and works well in interrupted cutting.

The properties of the substrate are enhanced by the type of coating that is used. These coatings can be applied by PVD, a 500° F process, or chemical vapor deposition (CVD), a 1,000° F process. Before a CVD coating can be applied, a hone of 0.001" to 0.006" must be put on the cutting edge. Also, the high temperature of the CVD process increases the chances that eta phase will form. This brittle interface between the coating and the substrate can cause insert breakage. PVD coatings allow a sharp edge to remain, but the thickness of a PVD coating is limited to less than 5µm, making it useful only in tougher applications. The thicker coatings are more wear-resistant, but they lack the toughness of a thinner coating.

The current trend among toolmakers is to coat tools with multiple layers of different coatings. With multilayer coatings, the insert benefits from the combination of each layer’s attributes to optimize metal removal. By adding multiple coating layers, toolmakers can create either an insert with properties targeted for one particular material or one type of application, or an insert with wide ranging properties to handle a variety of materials or applications. Multilayer coatings are the principal reason CVD is a more popular process than PVD. Until recently, CVD coatings were the only ones applied in multiple layers. Multilayer PVD coatings have been developed, however, and they are beginning to reach the market.

CVD/PVD combined coatings are available from a few cutting tool manufacturers, but these coatings are expensive and they are only being used on milling inserts. In theory, a thin PVD coating applied over a CVD coating compresses the CVD coating, which is in tension after it is applied. This prevents the tiny cracks that are believed to be in the CVD coating from growing larger when the insert is used. Some have questioned whether one PVD coating is thick enough to provide enough compression, however. For turning inserts, the CVD/PVD process has little application. In turning, the insert is in constant contact with the workpiece. This provides enough compression to maintain the integrity of the CVD coating without a PVD coating over it.

Since titanium-carbide (TiC) coatings were first introduced in the late 1960s, several other coatings have been developed. Each has properties that make it suitable for a particular range of applications. The most common carbide-tool coatings are listed below in the order in which they were first sold:

• TiC is a wear-resistant coating suitable for speeds below 600 sfm. Because TiC coatings by themselves are not very heat resistant, they are slowly being phased out as single-layer coatings. TiC is still being used in multilayer coatings, however.
• Titanium nitride (TiN) is a heat-resistant coating with a low coefficient of friction for intermediate speeds. TiN coatings are still very common, especially as PVD coatings.
• Titanium carbonitride (TiCN) is an abrasion- and wear-resistant coating. TiCN coatings were introduced originally as tri-phase coatings along with TiN, but now they are commonly used as single-layer coatings.
• Aluminum oxide (Al2O3) is a heat- and wear-resistant coating (black color) best suited for high-speed applications. Al2O3 coatings have been used only as CVD coatings and mainly in multilayer coatings.
• Titanium aluminum nitride (TiAlN) is the best heat- and wear-resistant physical-vapor-deposition (PVD) coating available. TiAlN is the newest coating to come around. It is being applied exclusively as a PVD coating.
• Multilayer coatings use a combination of these coatings. The most popular combinations on the market include TiN-TiCN-Al2O3, TiC-TiN, and TiN-TiCN-TiN.

Many other coatings are available, such as hafnium nitride or zirconium nitride, but these command very small percentages of the market. Coating companies are constantly developing new technologies for multiple coatings. One promising development is super-lattice coatings. These coatings are applied in numerous ultrathin layers, ideally no more than one or two atoms thick each. The tension that exists within these layers creates an extremely hard coating. These coatings are only in the research phase, however. To date, no production inserts are using this technique.

Coating developers also are researching new ways to apply standard coatings to increase tool life. One coating, for which a patent is pending, is built up from layers of TiN, TiCN, Al2O3, and then TiN again. As they are applied, the intermediate layers are altered with an infusion of nitrogen, causing them to interlock. This strengthens the Al2O3 layer, which extends tool life by increasing wear and crater resistance. The manufacturer has developed a wear-resistant grade, a general-purpose grade, and a very tough grade with this type of coating.

Beyond Carbide 
The trends that have driven tool manufacturers to develop new carbides and coatings also are leading them to try other materials. Because of their increased hardness, the newer materials can handle even higher speeds and lighter depths of cut in harder materials. Closest in cost and capabilities to carbide are cermet and ceramic tools. These tools are slowly increasing their share of the market. Advancing up the scale in terms of hardness are polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN), and diamond-coated carbide inserts.

Figure 1 shows how these materials compare in toughness and wear resistance/hardness. Tracing these tools from toughness to hardness not only shows the properties of these materials but also the evolution of cutting tools over the years. The chart illustrates the relationship between speed and wear resistance, which also can be expressed as the relationship between heat resistance and wear resistance. Tool steels, with their relatively low wear resistance, could only be run at slow speeds (150 sfm). But as the wear resistance increases, so does the speed at which the materials can cut.

Several new developments in cutting tool materials will arise in the 21st century as cutting tool manufacturers continue their search for better combinations of geometry, substrates, coatings, and edge treatments. All of this effort is to help shops be ready for whatever the next trend in manufacturing turns out to be.

About the Author 
Tim Malone is involved in marketing for Stellram, LaVergne, TN.