Diamond-coated carbide tools on the rise

  • 26-Mar-2009 01:18 EDT
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Getting holes right is a priority for Lockheed Martin in assembly of the F-35. Lockheed Martin

Carbon-fiber-reinforced polymer (CFRP) composites are built of layered carbon fibers, arranged in specific patterns for flexibility and strength. They behave very differently from the traditional materials such as steel and aluminum, the hardness of which is generally consistent and predictable from the point of a tool’s entry to its exit.

By contrast, composites vary widely in the amount of fiber content, composition, and ply direction. Manufacturers are exploring custom-designed tools from companies such as Amamco Tool of Greer, SC, to penetrate quickly and perform efficiently within the matrix of the composite material.

Lockheed Martin recently selected an Amamco coated tungsten-carbide drill to replace a polycrystalline diamond (PCD) drill. A production process change led Lockheed to consider alternatives to the PCD drill, a tool that has been popular in the industry because of its long life cycles. Prior experience with an Amamco coated compression router helped in its ultimate selection of the diamond-coated drill.

The defense company got help in its considerations by reaching out to a third-party testing organization, the National Center for Defense Manufacturing and Machining (NCDMM) of Latrobe, PA. NCDMM was asked to study the entire machining process and developed a proven best-practice procedure.

NCDMM testing revealed that the combination of precise cutting edge geometries, only possible with a tungsten-carbide tool, along with a coating application that greatly extended its resistance to wear, was by far the best solution for the project and process. The tool finally selected allows operators to increase feed rates by over 12 times and substantially reduce span time. The tool and the process change netted a 450% increase in the number of holes drilled per tool.

During tests, which allowed Amamco to perfect geometries that cannot be duplicated by PCD tools, a protective coating was applied to the carbide that protects critical cutting geometries and covers the entire tool’s surface. The coating, applied by a chemical vapor deposition (CVD) process, is a multilayered diamond material provided by Diamond Tool Coating of North Tonawanda, NY. The tool produced more than1200 holes compared to the 275 produced by the original PCD drill on the same task. Test results showed a 75% reduction of span time and huge increases in feed rates, which contributed to the significantly faster production achieved. In all, Lockheed achieved a 97% reduction in tool cost. Along with the process change savings and tool cost reductions, significant efficiency improvements are expected for Lockheed as it builds 2500-plus F-35 aircraft.

Tungsten carbides, first developed in Germany in the 1920s, produced such advanced steel-milling grades they revolutionized most industries by the 1930s, when carbides containing both titanium and tantalum were introduced. A variety of hard metals were then further developed in response to the demands of the emerging steel, mining, and manufacturing industries.

A composite material itself, carbide, in combination with the binder cobalt, can be formulated for specific hardness and bending strength ratios. Now, by including chromium, nickel, and other metals to carbide formulas, developers can custom-produce tool base materials according to very specific user requirements and preferences for hardness, toughness, and wear, as well as the ability to accommodate precise cutting geometries. Most recently, innovators have increased cutting edge sharpness substantially by using smaller grain size, even to microsize grains, and by controlling grain growth during the sintering process.

Although it outlasts any other metal tool, carbide’s one limitation has been life span, as compared to PCD-tipped tools. However, coated carbide tools now reduce friction along the entire tool surface, lowering cutting edge temperatures and greatly extending tool life, to the equivalent or exceeding that of PCD. CVD diamond coating also enables the carbide tool to smoothly and quickly penetrate composite material without causing delamination, which is the separation of layers within the composite material.

As a result of the NCDMM testing for Lockheed, Amamco recommends the CVD process, which applies layers of 100% real diamond crystals to the surface of its solid carbide tools. The result is an ultrahard coating 8 to 12 µm thick that significantly increases the life of tools in abrasive CFRP applications. Diamond grown in nature can take hundreds of years to develop. But diamond grown in a hot-filament chemical vapor deposition process requires only 20 to 60 h, depending on the size of the tool and desired coating thickness.

The process involves the introduction of methane gas into a vacuum chamber that is super-heated by an array of hot filaments. The process temperature of 1500ºF precludes anything but solid carbide tools from being coated. An extensive pretreatment process requires 6% fine grade carbides to ensure good adhesion.

“Creating the conditions to grow diamond crystals is nearly as difficult as getting the diamond to adhere to the surface of tools,” said Doug Mueller, General Manager of Diamond Tool Coating. “In the absence of any pretreatment, the diamond crystals simply flake off. But roughening the surface of tools with a chemical pretreatment creates pores for the diamond to anchor into.”

Once the diamond begins to adhere to a tool, a continuous layer will coat the surface. Initially, only a single layer of large PCD crystals could be produced. But recent advances have made it possible to grow nanocrystalline diamond to produce a smoother part finish. Diamond Tool took this a step further, engineering a multilayer diamond coating made of alternating poly- and nanocrystalline applications. This innovation, DiaTiger multilayer diamond, increases the wear resistance and fracture toughness of the coating by a factor of 10, making it suitable for advanced composite applications and giving the carbide tool a wear advantage.

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