Diamond-coated carbide inserts - ready, set, go!

Modern Machine Shop, April, 1996 by Chris Koepfer

The indexable insert is a marvel of metalcutting efficiency. In all of their various shapes, sizes and formulations, these tool tips deliver cost-effective, high metalcutting performance in the gamut of application materials. Moreover, the search is on for better chipbreakers, geometries and coatings to push performance even farther.

Thin-film diamond-coated inserts are among such frontier-expanding tools. While the ability to apply a thin, chemically grown layer of pure diamond on a carbide substrate has been a reality for many years, commercialization of the process is relatively recent.

The potential for cutting non-ferrous materials with diamond is broad. Material candidates for these tools include aluminum - (especially its high-silicon alloys like A1390), brass, copper, bronze, fiber-reinforced composites, titanium, ceramics, graphite, hard carbon, and laminates. High-speed machining and long tool life are the economic incentives propelling development.

To understand this tooling technology and how it can be applied in general manufacturing, we talked to Crystallume (Santa Clara, California), a manufacturer of diamond-coated inserts. While there are some application parameters a shop needs to be aware of, "diamond-coated inserts should be considered a tooling alternative for any shop cutting non-ferrous materials." says Dave Kropfl, senior engineer at Crystallume.

Insert Anatomy

The base material, or substrate, for most thin-film coated-diamond inserts is tungsten carbide (grade C-2). It's basically the same substrate material used for many carbide inserts coated with titanium nitride or titanium carbide.

A carbide insert is made from two main ingredients, carbide and cobalt, which are mixed together in various ratios and sintered. Carbide manufacturers guard their ratios and sintering process details the way chefs protect their recipes.

The carbide used in an insert is granular, although individual grains are very small - many formulas call for grains in the single-digit micron range. By using different grain sizes specific cutting performance can be selected. For diamond coatings to adhere more firmly to the substrate, a larger grain carbide is usually specified so the surface has some roughness.

Cobalt is the "cement" that bonds the carbide grains. It gives the carbide its cutting characteristics of hardness and toughness.

The mixture of carbide and cobalt is pressed in a mold, giving it the shape of an insert. Chipbreakers and other performance enhancing bumps and valley are formed as part of the molding process. These pressings are then sintered. At high temperature, the cobalt flows around the carbide grains creating a strong and very hard bond.

CVD - Remarkable Science

The key to making practical thin-film diamond-coated insert is getting strong adhesion between the diamond film and the carbide substrate. The process of applying diamond film to the carbide is chemical vapor deposition (CVD). Making the diamond stick to the carbide substrate with sufficient tenacity for metalcutting has been the focus of development work by companies like Crystallume.

However, it's the creation of the diamond film by CVD that is so remarkable. Basically, two gases, hydrogen and methane, are used to create a pure diamond crystal, every bit as real as any mined in South Africa. Even industry veterans marvel at the idea of making a diamond crystal from relatively simple ingredients of hydrogen and methane.

Simplified, the CVD process works by dissociating and ionizing the hydrogen and methane gases at high temperatures and under moderate pressure. Generally, one of three deposition methods are used to create diamond coated carbide inserts (see box).

In a vacuum chamber, the dissociated and ionized gases pass over the carbide substrate which is heated to 700-1000 [degrees] C. A chemical reaction between the tungsten carbide and the gases causes pure diamond to grow on the surface of the substrate.

The rate of deposition varies among the CVD methods, but 1 micron per hour or higher over an area of 10 [cm.sup.2] is considered an acceptable production rate. The diamond film deposited is usually about 25 microns thick when finished. Film thickness is controllable, however, so thicker or thinner films can be deposited to suit an application.

Cobalt As Culprit

The cobalt binder used to hold together the carbide grains in an insert substrate creates a problem in achieving high adhesion between the diamond coating and the surface of the insert. Any cobalt on the surface causes the reacting gases used in the CVD process to form graphite instead of diamond. The graphite areas are weaker than the diamond - they do not adhere as well. Too much graphite weakens the entire film, creating the potential for insert failure.

To reduce the occurrence of cobalt, specially formulated carbide substrates (cobalt content 6 percent or less) are used for diamond-coated inserts. These formulations vary among manufacturers.

Ideally, good adhesion results in a cutting edge that wears through to the carbide substrate. If the diamond film adheres poorly, it will spall or flake off, resulting in premature failure of the insert.