The Development Of Coated Grades Of Carbide Tools

By Mia Paulsson and Christer Richt, Sandvik Coromant

Coated carbide indexable inserts are ideal tools for many machining applications. They have an incomparable capability to remove metal across a broad range of applications.

When it comes to cutting tools, coated indexable inserts are responsible for the major share of metal removed throughout machine shops today.

These are the tools that, since 1970, have continually lifted manufacturing productivity and, consequently, material prosperity.

And, these tools have a potential that is far from depleted. The more that coated inserts are developed, the more possibilities appear for them.

This article discusses how this part of tool material science is developing, and the best ways to approach its applications.

New insert grades are introduced continually, and contribute considerably to the improvement of productivity in machine shops.

Specially coated inserts have elevated performance since their introduction, and continue to do so. And, they show further potential for development.

About 80 percent of the inserts used in machining today are coated cemented carbide grades. These tool inserts earned and maintain that growing market share because of their broad application for removing large amounts of material while they sustain a long tool-life.

The high material removal rates and long life for these tools are achieved through an incomparable balance of wear resistance and toughness. Relatively thin insert coatings -- 0.00004 in. to 0.0007 in. -- protect inserts from the heat, corrosion and abrasion that shorten their lives.

The tool-material science of coated inserts is very much a high-tech area today.

The compositions of insert grades and the processes used to produce them have evolved after being extensively researched and tested, and include technologies that re-arrange the atoms of the material from which tools are made to technologies that led to the development of microrounding cutting edges.

The basic insert – the substrate – provides most of the required toughness for the cutting tool, while the coating adds wear protection and increased hardness. Most coatings are ceramic by nature, and if an insert was made solely of the coating material, it would be too brittle for general use.

The advantage is developed by correctly combining the most suitable coating and substrate.

Uncoated insert grades also have an important place, but mostly in niche areas, such as the machining of super alloys and materials that smear when they are cut, such as titanium alloys. Some of these materials will kill an unsuitable insert in a very short time, so they require the right type of substrate composition.

Aluminum can be very soft and can smear, so it requires an extremely sharp, uncoated cutting edge. Aluminum alloys also can be very abrasive, requiring a diamond coated insert

The primary reason that cemented carbide is such a good cutting tool material is its versatility.

The CVD-coating process dominates for indexable inserts providing excellent heat, chemical and abrasive wear barriers for the insert. New developments in relieving stress in the coating have improved performance as have gradient technology, where the insert substrate is provided with hardness and toughness where it is most needed. This photo shows a steel turning grade with a thick, alumina coating on top of a titanium carbo-nitride layer, with good adherence to a gradient-substrate.

Cemented carbide is hard and tough at the same time, and it is very forgiving as a material. Other materials used for cutting tools are hard or tough, but not both.

Compared to ceramics, cemented carbide is not anywhere near as sensitive to mechanical variations, and carbide is a tool material that can be used in most circumstances while making good use of very hard ceramic coatings. That gives these inserts the best of both worlds.

When it comes to the actual insert substrate, a lot of development has taken place to match the grade with the coating. The properties of cemented carbide are established mainly through:

• Varying the amount and size of hard particles. • Varying the amount as well as the proportions of different types of hard particles. • Or, alloying the insert-substrat binder in different ways.

Cemented carbide

By definition, cemented carbide is made up of hard particles in a binder, and slight differences to the type and size of hard particles lead to considerable changes in insert performance.

Other ways of designing the cemented carbide insert substrate is to differentiate parts of the substrate – giving hardness where it is most needed and toughness where it is most beneficial. This is called gradient technology and is used today on insert substrates for many of the latest coated grades, often to provide what may be compared to a car airbag effect to cushion the effects of machining.

Typically, a hard and heat resistant part of the substrate – that part that stands up to the effects of high cutting speeds – is combined with a tough, cobalt-enriched, outer substrate zone to secure durability in the high cutting-edge line durability. The insert substrate then has both higher hardness and higher toughness where it is most needed providing a balance that can cope better with the effects of speed and mechanical shocks.

Coating materials

Most coating-materials used on today×s inserts are classified as ceramic. These are most common along with their main properties:

• Titanium carbo-nitride - TiCN – has very good abrasive wear resistance and good adherence to carbide substrate.
• Aluminum oxide (alumina) - Al2O3 – provides very good outer thermal and chemical protection for the insert substrate.
• Titanium nitride - TiN –mainly used for the golden color which provides clear wear detection.
• Zirconium oxide - ZrO2 – gives very good thermal and chemical protection for the substrate.

By using more than one layer of different coating materials it is possible to combine the benefits that each provides. Many insert grades have three layers of coatings to ensure good adherence between the insert substrate and coatings. Also, the laminating effect of coatings provides added strength.

PVD-coatings are on the increase. There is a growing need for these thinner coatings, more suitable for positive, sharper tools. Smaller machine tools need tools that cut with lower power consumption and are more forgiving when it comes to instability, intermittent cuts and unfavorable tool entries and exits. This photo shows a thin PVD-coating on a positive indexable insert for an endmill.

An inner layer of TiCN often is used to achieve abrasive wear resistance, and an outer layer of Al2O3 is used to make sure of heat protection. The layers of coating material vary in thickness depending upon on the properties required for the application that the coated grade is intended for. The more heat the insert is exposed to, the thicker the alumina coating should be so as to form a satisfactory barrier.

If the workpiece material has more of an abrasive character, such as highly alloyed steel, the TiCN should be thicker to stand up to wear.

The coating materials are hard because they are ceramic materials, and as such they are more brittle. So, the thicker the coating, the more sensitive it is to mechanical variations.

The insert substrate and the coating properties always must be balanced to suit the requirements of the application for which the grade is intended. For example, an insert with a thick coating may be unsuitable for finishing operations because it requires more of a rounded edge while it may be exactly right for roughing cast-iron. A thin coating with high adherence to the substrate is suitable for stainless steel machining.

Coating processes Two principal coating processes are used for indexable inserts to provide cutting edges with fundamentally different properties for machining:

• Chemical vapor deposition (CVD) which uses a higher temperature and gives thicker coatings.
• Physical vapor deposition (PVD) uses a lower temperature and gives thinner coatings.

Each of these processes give rise to different tensions in the coating material. Chemical vapor deposition develops tensile stress in the substrate, while physical vapor deposition tends to develop compressive stress. These stresses provide different desirable characteristics for the insert.

The CVD-coated insert typically has a thicker coating, and has a high degree of wear resistance and coating adherence. The PVD-coated insert has a thinner coating, high toughness and is more suitable for sharper cutting edges.

Practically, the CVD insert is more suitable for higher cutting speeds, while the PVD insert is more suitable for lower cutting speeds.

The CVD insert can cope better with heat, longer insert engagement times and larger chip thicknesses.

The PVD insert copes well with instability and more demanding chip evacuation from the machining zone as well multiple as tool exits from the workpiece such as those encountered in milling.

A CVD insert typically is chosen for turning of steel and cast-iron while the PVD is typically a solution for endmilling in a machining centre with limited power. The PVD insert is well suited to the growing number of positive, sharper cutting edges required for intermittent cuts and also is used widely for solid carbide tools – endmills and drills especially. But these characteristics are broad for the insert-coating types, and overlap in both turning and milling.

The PVD process is the subject of intense development today because it offers great potential and can make use of more coating materials. The CVD and PVD processes generally should not be seen as competitors, rather as complimentary to each other. Both offer potentials for optimizing machining.

CVD is the dominant coating process, partly because it is the only process capable of satisfactorily depositing layers of Al2O3.

This process has been developed over the years to reduce the negative side-effects of high temperatures on the substrate.

Recent developments include CVD processes for depositing TiCN and Al2O3 in ways that lower stress levels and develop fewer tendencies for crack formation in the coated surface of the insert. After the heat of the CVD-coating processes a network of cooling cracks tend to form, resembling a dry river-bed. The resulting tensile stresses, due to the different coating materials involved in the several layers, can negatively reduce the toughness properties of the insert.

Coatings with low stresses Coatings with low inherent stresses have proven to have better properties for tackling the demands of machining. Stress-relief therefore has been developed, and now is achieved by new after-coating processes that transform the coating to provide a smooth, low-stress insert surface. To a certain extent, the cooling cracks from the CVD-process also are closed.

The PVD-coating process does not involve as much heat, so it does not need similar precautions. Stress in PVD coatings is reduced through a process that involves high-impact treatment that counters any tensile stresses with suitable levels of compressive stresses. This results in the edge-line of these sharper cutting edges having greater levels of toughness.

The use of low-stress CVD coatings on new generations of inserts is providing clear benefits. By minimizing material stresses in the coating it has been found that layers can be made thicker without sacrificing insert toughness. Thus, practically in machining, modern lowstress coatings have better resistance to insert-fracture, edge chipping, flaking of coatings as well as cutting edge integrity.

The result is that inserts with such coatings can be used at higher cutting speeds while lasting longer and having better predictability. The primary advantage of this development for machine shops is that it provides trouble-free machining at higher metal removal rates.

Modern insert coatings are characterized by this low-stress technology as well as by having several sub-layers of coating materials, creating a laminating effect. The coatings are optimized to act as heat and chemical barriers, to resist mechanical wear and to promote better adherence between coatings and substrate. The coating combinations and substrates for each insert grade-type today are individually designed to match machine shop requirements.