REMAKING THE GRADE

Atomic level changes in coating and substrate development have caused revolutionary results in the grade performance of cutting tool inserts. Here’s how one unique CVD coating methodology has changed the expectations of insert grade performance in a variety of applications.

Coated carbide inserts have long dominated the majority of metal removal operations in job shops due to their wear resistance, chemical stability and high hardness. With the ability to remove large amounts of material across a range of applications while retaining a long tool life, about 80 percent of inserts used in machining today are coated carbide grades. Coatings improve wear resistance, increase tool life, and can broaden the functionality of a particular grade as well as allowing for higher machining speeds.

The coating is important, but the coating must also be applied to the appropriate substrate for the insert to achieve the desired characteristic. This multilayer coating technology means that each layer has its own function, be it toughness, wear-resistance, lubricity, etc. So, although the substrate is protected by the coating until there is significant tool wear, its properties have a significant impact on the way the tool and the coating perform. Within this realm, there is a variety of substrate and coating compositions.

Numerous approaches have been taken to improve the performance of coated products – fine adjustments to the chemistry in the coating furnaces, more careful structural control of the individual coating layer, and even polishing the edges after coating. All of these are designed to improve quality, productivity or reduce cost.

Historically, advancements in coatings technology have been incremental. A rare revolutionary change did come about with the unique Duratomic^® methodology from Seco Tools (Troy, MI) that involves a chemical-vapor-deposition (CVD) applied coating. This process starts with aluminum oxide (Al₂O₃), which uses two different forms for coatings: kappa and alpha. The difference between these two forms is the crystal structure, or the way the atoms are arranged within the material.

To illustrate the impact of the crystal structure on material property, it is noteworthy that toilets and rubies are both made out of aluminum oxide. The difference between the two items is in the way the atoms are arranged.

When developing cutting tool (coating) materials, the alpha form of aluminum oxide is preferred for use in most applications. However, because the kappa form is much easier to deposit, producers have typically deposited the kappa form and then heat-treated it in the coating furnace to transform it into alpha. This heating process results in an unavoidable contraction in volume that forms cracks in the coating.

Through significant research and development, metallurgists learned how to deposit the original alpha form to avoid the cracking issue and produce a more homogenous and tougher structure. Eliminating cracks also creates a more effective thermal barrier that reduces the amount of heat actually getting to the substrate. Through this technique, engineers learned how to control the alumina crystal growth. By modifying the coating deposition techniques, the individual crystals that make up the coating are “tilted” to bring a more favorable crystallographic direction into the cut.

To use an analogy, think about how diamonds can be cleaved relatively easily in certain crystallographic directions, producing the facets that reflect and refract light and make gems so attractive.  However, in other crystallographic directions diamond is very resistant to any form of breakage. Essentially, this structural alteration creates a coating that offers improved mechanical and thermal properties in combination with better wear resistance and toughness – beyond the capabilities of all previous Al₂O₃ on the market.

Where conventional aluminum-oxide coatings have a hardness of about 27.5 (GPa), Duratomic hardness is closer to 30.5 (GPa), a nearly 11 percent increase that translates directly into greater abrasion resistance and, therefore, tool life. The coating also runs cooler, down a full 60 deg C (or over 9 percent) in a typical application, enough to substantially reduce the tendency for the insert to crater.

Compared with conventionally produced Al₂O₃, the Duratomic coating shows less crater wear, less deformation of the cutting edge and longer tool life. When compared with titanium carbonitride (TiCN) coatings produced by MTCVD, or conventional Al₂O₃ coatings, the Duratomic-developed grades consistently show increased flank wear resistance and improved toughness.

The optimized Duratomic coating structure is composed of two functional parts: the inner TiCN base layer that is responsible for adhesion and the basic cutting edge strength, and the top layer of Al₂O₃ that acts as an effective thermal barrier to permit higher cutting speeds. This combination of substrates and coatings and the fine-tuning within the Duratomic range has delivered insert grades that can run faster, longer and suited to a wide range of applications, making them first-choice grades for a variety of turning, milling and holemaking applications.

The turning grade TP2500 for general machining of steels was the first grade to incorporate this technology. Beneath the Duratomic coating on TP2500 was a brand new substrate tailored to be an ideal partner for this coating, which also featured a specialized cobalt enriched zone. Cobalt enrichment refers to the process where cobalt is taken from the core of this substrate and moved to the surface, thereby further increasing the toughness and chippage of the cutting edge.

After testing TP2500 across a broad range of steel turning applications, impressive gains of 100 percent productivity in conjunction with 400 percent increase in tool life improvement were achieved in certain applications.

Since the introduction of TP2500, numerous Duratomic turning, milling, drilling and reaming grades have been developed that double existing tool life across a broad range of applications. For example, the turning grade TP0500 builds upon the existing foundation of Duratomic knowledge. But, rather than suited to a wide application range, it is designed for great wear resistance in a specific area – high temperature, high speed steel machining  the hardest, most wear-resistant grades must be applied in order to remove a lot of metal quickly.

Applications that can make significant gains using TP0500 include those requiring high metal removal rates, such as the machining of large components for the power generation segment or for high-speed, high productivity applications found in the highly competitive automotive sector. TP0500 allows users to stock one reliable grade for these applications, simplifying inventory and reducing tool change-over.

It seems the prophecy of Roger Granström, a product manager for Seco Tools, revealed itself to be true: “We believe Duratomic will totally change performance expectations of future grade developments.”

Seco Tools Inc., 2805 Bellingham Drive, Troy, MI 48083, (248) 528-5440, Fax: (248) 528-5603, www.secotools.com.