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Fig. 1—The properties of a specific cutting tool represent a combination of its substrate material, manufacturing method, geometry and coating (type and deposition process).
Fig. 2—As shown, the coating materials and process affect five wear mechanisms, as well as the ability to fabricate sharp insert edges.
Fig. 3—Here is a comparison of coating cross sections from three Sandvik Coromant insert grades. Three coating layers (TiN, alpha Al2O3 and TiCN) are clearly defined. The surface layer reduces friction; the middle layer prevents heat from penetrating into the substrate; and the lower layer enhances toughness by dispersing cracks.
A key premise of coating research today is the idea that the coating actually cuts the workpiece, not the insert body. Among various factors affecting performance and lifetimes of cutting tools, coatings are probably the most important. The coating's composition and adherence qualities are powerful promoters of metalworking productivity.
An important recent development applies to inserts for steel turning. New inserts can increase metal removal rates by 20 percent without sacrificing finish quality. They present a harder, more wear-resistant edge to the cutting zone, while supporting that edge with tough flanks that withstand heat and penetration by cracking.
To understand how coatings contribute to cutting-tool performance, let's examine all the factors involved in making inserts. Figure 1 presents an outline of substrate and coating characteristics and how the finished insert's geometry relates to its function. The substrate, coating and geometric form work together to enable a particular insert to meet its performance requirements.
Two Coating Methods
Physical vapor deposition (PVD) and chemical vapor deposition (CVD) are two processes used to apply coatings to inserts. PVD is a well-established technique primarily used for coating high-speed steel tools. To produce a sharp cutting edge, this process is commonly used with cemented carbide. The PVD process vaporizes the coating material and exposes the insert to this vapor at a temperature of approximately 900ºF. On the other hand, CVD coating is accomplished by chemical reactions between gases. For example, hydrogen, titanium chloride and methane react in a nitrogen atmosphere to create a TiCN coating.
In general, PVD coatings are thinner than coatings applied by CVD. The PVD process applies titanium nitride (TiN), titanium carbo-nitride (TiCN) and titanium aluminum nitride (TiAlN) coatings. CVD can apply TiN and TiCN, as well as other coatings, in layers up to 0.007-inch thickness. This process improves wear resistance, particularly with aluminum-oxide coatings. Multi-layer coatings of varying thickness may also be applied with CVD.
In CVD processes, inserts typically are heated to approximately 1,800ºF. Because this process is relatively easy to regulate, it permits controlled application of multiple layers. Orientation of each layer's crystalline structure may also be controlled during this process. Performance improvements have resulted from a controlled temperature-reduction process known as moderate-temperature chemical vapor deposition (MT-CVD). This process increases resistance to abrasion and plastic deformation.
Why Coat?
The main purpose of a tool coating is to improve productivity by allowing higher cutting speeds or feed rates. Although coatings sometimes extend tool lifetimes, this isn't usually their primary purpose. The principal rationale is to identify factors that might cause insert failure and to develop coating attributes that deal specifically with these factors.
Each aspect of a well-designed coating has a different function. The combinations of coating processes and coating materials may be regarded as recipes to protect inserts against failure. Figure 2 illustrates how different coatings affect insert performance and wear, with (-) indicating a negative effect, (+) a positive effect and (o) denoting no effect.
Other factors contributing to insert failure are not shown in Figure 2. For example, an insert's surface smoothness significantly affects chip formation when machining sticky materials. Chemical reactivity may also cause insert degradation at high temperatures.
Failure Analysis
Evaluating multi-layer coatings requires establishing a connection between coating failure and fracture of the insert body. In a typical scenario, if the outer coating slowly abrades and wears off, the tool surface loses its lubricity and generates excessive heat. In this case, if an inner coating cannot provide an adequate thermal barrier, heat penetrates the substrate and weakens it. In turn, overheated substrates may deform under cutting pressures. Such deformation frequently causes flaking of the coating and separation of the substrate from the coating, or separation between coating layers.
The coating, its component layers or the substrate may also crack when subjected to mechanical shock. To avoid this, they must be made from durable materials. If chemical bonding adheres the coating to its substrate, these bonds may fail at high temperatures due to accelerated chemical activity. For this reason, the bonds should be resistant to chemical attack.
New Steel-Turning Inserts
With these possibilities in mind, we can interpret the cross section of a coating used for Sandvik Coromant's (Fair Lawn, New Jersey) GC4000 grades of steel-turning inserts. In Figure 3, the thin, outer layer at the top is a multi-layer TiN coating applied by the CVD process. This shiny, gold material imparts lubricity to the surface and indicates wear as it slowly rubs off. With smearing metals, this lubricity provides resistance to built-up edge formation.
The next layer is fine-grained, columnar, alpha-phase aluminum oxide, a very hard material that provides a temperature barrier to protect the tool's inner body from excessive heat. This layer is applied via a high-temperature CVD process.
Beneath this is a layer of TiCN applied via a proprietary MT-CVD process to substantially improve the insert's performance. The columnar orientation and optimal grain size of this coating's crystals result from careful adjustment of process parameters during application. The crystals' end-wise orientation presents a harder surface where abrasive wear is concentrated. In addition, descending stress lines between crystals dissipate mechanical stress into the substrate material. If heat or mechanical shock creates micro-cracks that penetrate the surface layers, they will be dissipated downward along lines between the crystals. When this remaining force reaches the substrate, it is absorbed without propagating larger cracks. This gives the insert toughness and strength.
Engineered Substrate
Controlled variation of substrate materials also enhances coating performance. The substrate must be able to withstand stresses encountered in high-production, steel-turning operations. To address this need, a gradient sintering process varies the substrate's tungsten carbide/cobalt composition along the tool's periphery. This enriches the tool flank with cobalt for toughness to ensure better edge integrity during high-speed cutting. The nose area is enriched with tungsten carbide to impart hardness, resist wear and minimize plastic deformation.
This new family of steel-turning inserts permits speeds at least 20 percent higher than those possible previously, and many current applications exploit this advantage. The principal benefits appear in cases where the insert's greater resistance to plastic deformation permits dry machining and application of wiper-insert geometry. General applications include wet or dry roughing, semi-finishing and finishing.
Understanding Wear Types
As a result of load factors exerted on the cutting edge during machining, a few basic wear mechanisms dominate metalcutting:
- abrasion wear
- diffusion wear
- oxidation wear
- fatigue wear (static or dynamic)
- adhesion wear
The tool material's ability to resist the loads determines how it will be affected by the wear mechanisms of metalcutting. Abrasion wear is caused mainly, but not entirely, by the hard particles of the workpiece material. This is similar to a grinding operation where the hard particles come between the surface of the workpiece and tool. The mechanical load on the insert leads to wear on the flat face of the cutting-edge flank.
To a large extent, the cutting edge's ability to resist abrasive wear is connected to its hardness. A tool material densely packed with the hardest of particles will stand up well to abrasive wear, but it may not be equipped to cope with other load factors during machining.
Diffusion wear is more affected by the chemical load during the cutting process. The chemical properties of the tool material and the affinity of the tool material to the workpiece determine the development of the diffusion-wear mechanism. Hardness of the tool material doesn't affect the process very much. The metallurgical relationship between the materials determines the extent of the wear mechanism. Some cutting tool materials are inert against many workpiece materials, while others have high affinities.
Tungsten carbide and steel have affinity toward each other that creates a diffusion-wear mechanism. This wear forms a crater on the insert's chip face. Because this mechanism is very temperature dependent, it's more pronounced at higher cutting speeds. Atomic interchange occurs with a two-way transfer of ferrite from the steel into the tool, as well as carbon diffusing into the chip.
In the presence of air, high temperatures produce oxidation wear in most metals. Tungsten and cobalt form porous oxide films that the chip rubs off more easily. But when stronger and harder oxides are produced (such as aluminum oxide), certain cutting-tool materials may be more susceptible to oxidation wear. This is especially true with regard to the interface portion of the cutting edge where the chip width finishes (at the depth of cut). At this point, air gains access to the cutting process, and oxidation can create notches in the edge. This form of wear, however, is relatively uncommon in today's machining.
Fatigue wear is often a thermo-mechanical phenomenon. Fluctuations in temperature and loading forces can crack or break cutting edges. Intermittent cutting action leads to temperature cycling that creates shocks at the point where the cutting edge engages the workpiece. Some tool materials are more sensitive than others to the fatigue mechanism. Pure mechanical failure can result from cutting forces that exceed the cutting edge's mechanical strength. This can be caused by hard or strong workpiece materials, high feed rates, or when the tool material isn't hard enough. Usually, however, plastic deformation is the principal form of fatigue wear.
Adhesion wear (also known as attrition wear) occurs mainly at low machining temperatures on the tool's chip face. This can affect long-chipping and short-chipping workpiece materials including steel, aluminum and cast iron. This mechanism often forms a built-up edge (BUE) between the chip and the cutting edge. In this process, successive layers from the chip are welded and hardened, becoming part of the edge.
As machining continues, the BUE may shear off by itself or cause the tool's cutting edge to break away (either in small pieces or by fracturing). Some cutting tool materials and workpieces (for example, very ductile steel) are more susceptible to this pressure-welding than others. At higher cutting temperatures, however, the conditions that produce this phenomenon usually do not exist. An adhesion-wear mechanism represents a combination of a specific temperature range, affinity between the tool and workpiece materials, as well as the load from cutting forces. When machining deformation-hardening materials (for example, austenitic stainless steel) this wear mechanism produces rapid local wear at the maximum limit of cutting depth. This is the most common type of notch wear, and it depends on the affinity between the tool and workpiece materials.
Source: Modern Metal Cutting from Sandvik Coromant, available through Hanser Gardner Publications (Cincinnati, Ohio).