Choosing The Right Diamonds

July 20, 2000
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Advanced engineered ceramics, such as alumina, silicon nitride, silicon carbide, zirconia and others, are showing many advantages over fabricated metallic components in a growing number of applications, and their use has grown rapidly in the global industrial market. In 1997 alone, the amount of ceramic used in advanced applications was valued at $6.5 billion, with most of the growth coming in automotive turbine engine technology, power generation systems and aerospace applications. The lower weight-to-strength ratio of ceramics provides lower rotating inertia, which improves engine efficiency and prolongs engine life.

The aerospace industry leads in ceramic adoption, since weight reduction for space vehicles can be valued as much as $20,000 per pound. In commercial aircraft, these cost savings are approximately $200 per pound, while in automotive applications, the value of weight reduction is reduced to about $2 per pound.

However, weight reductions are only one aspect of the potential benefits of ceramics usage. Ceramic components can operate at higher temperatures, making them less prone to heat related failures. The increased wear and corrosion resistance make ceramics ideal for bearings and seals, especially when used in wet or corrosive environments, such as salt water, acids, caustics and fuels. In addition, there exists an even larger market for ceramic materials in the electronics and medical industry.

In fact, any industry producing parts that require wear, abrasion, and corrosion resistance, low weight-to-strength ratios, high temperature stability, as well as unique mechanical, thermal, and electrical properties, benefits from employing ceramics. Table 1 lists some of the more common components made from ceramic materials.

Barriers in Grinding Ceramics

Despite the overall growth in ceramics, several factors hinder their widespread adoption into more applications. One of the largest barriers is the high cost of grinding, which can easily exceed 50% of the finished part value.

The primary method used in finishing these components is grinding with diamond wheels. Ceramic materials vary greatly in their grindability based on their compositions and physical properties. It is extremely difficult to grind ceramics to close dimensional tolerances in the absence of external and internal flaws due to the toughness and heat sensitivity of these materials, yet it is important to minimize micro-cracks and fractures that negatively effect the integrity and strength of the finished ceramic part.

Many properties of the grinding wheel influence the outcome of the manufacturing process and directly affect the total cost of processing ceramics. The diamond type and its coating, the abrasive concentration and mesh size, bond type and hardness, all influence the productivity of the process as well as the integrity of the workpiece.

While it is outside the scope of this article to look at each of these factors in detail, we can gain a great deal of understanding by looking closely at the diamond and its coating.

Diamond Types and Friability

There are several types of diamond, each tailor-made in a high-temperature, high-pressure process to produce a wide range of physical properties. The differentiation of diamond centers on crystal shape and strength, which is determined by its toughness or friability (see Figure 1).

Tough diamonds have a tendency to present relatively flat faces to the workpiece, causing the diamond to rub and produce harmful friction-generated heat to the part. This increase in cutting forces is detrimental to the ceramic material, resulting in external and internal fracturing.

Friable diamond, on the other hand, micro-fractures, constantly regenerating sharp cutting edges. This increased fracturing of the abrasive grain results in lower normal forces placed on each crystal. Lower forces, in turn, reduce the tendency of the crystal to be pulled out of the bond. Maintaining the number of crystals in the grinding wheel keeps the chip thickness and abrasive concentration constant, which is critical in minimizing wheel wear rates and limiting damage to the ceramic workpiece. Friable diamond is therefore the most appropriate crystal to grind ceramic materials.

Metallic Coatings

Resin-bond wheels are the most common for ceramic grinding, as they provide a grinding action gentle enough to limit damage to the ground ceramic components. While diamond friability is important, crystal retention in resin bonds depends upon the proper selection of a coating on the diamond in order to strengthen the diamond-to-bond interface.

A nickel (Ni) coating is typically applied to diamond in resin-bond wheel systems. The coating strengthens the crystal in the bond matrix by providing microscopic roughness and enlarging the surface area of the crystal. The coating is measured by percent weight (wt.%) and can range from 30% to 60% of the total coated crystal weight.

The SEM photographs in Figure 2 illustrate how the diamond texture changes as nickel coating is applied. A spiked nickel coating enhances to a greater degree the irregularity and surface area of the crystal, which, in turn, further strengthens the interface between the diamond and the resin matrix.

Using metal coatings on diamond also increases the heat capacity of the crystal and allows for a uniform dispersion of the heat into the bond, keeping the bond matrix undamaged.

Other metallic coatings, such as copper and silver are more thermally conductive than nickel. They have the potential to further reduce heat-related damage to both the bond and the crystal during applications with high material removal rates, particularly in higher temperature polyimide resin bond systems. Silver is also an excellent lubricant and can improve grinding performance by reducing friction and thus reducing grinding forces.

Spiked Nickel-Coated Diamond

Researchers at General Electric Superabrasives Application Development Center in Worthington, Ohio, have conducted experiments to determine the most suitable nickel-coated diamond in surface grinding hot pressed silicon nitride (Si3N4). In these grinding tests, a nickel-coated diamond, a spiked nickel-coated diamond and an uncoated diamond were evaluated in medium hardness resin-bond grinding wheels. Grinding wheel specifications and test conditions are shown in Table 2.

The test results revealed large variations in wear rates of the different grinding wheels. Testing with the wheel containing the uncoated diamond was terminated after only 0.060 in3 (0.98 cm3) of Si3N4 had been removed. At that point, 0.003 in. (76 m) radial wheel wear had occurred. The wheel containing the nickel-coated diamond was able to grind 13 times more material when the testing was stopped. The radial wheel wear was 60% higher compared to the wheel containing the uncoated diamond. However, the optimum performance was seen with the wheel containing the spiked nickel-coated diamond. A total of 2.160 in3 (35.40 cm3) of material was ground, with only 0.0006 in. (15 m) of radial wheel wear taking place. The chart in Figure 3 plots radial wheel wear against volume of material ground for the three grinding wheels.

Immediate failure was seen with the grinding wheel containing the uncoated diamond due to crystal pullout. The superior performance of the spiked nickel-coated diamond is a result of an increased number of crystals retained in the bond. SEM photographs in Figure 4 illustrate the improved mechanical retention of the spiked nickel-coated diamond compared to the nickel-coated crystal. The spiked nickel-coated crystal is completely encapsulated in the nickel coating and firmly anchored in the bond. With the nickel-coated crystal, the advantage of the friable diamond is never realized because of the failure between the bond/coating interface.

Improved Edge Quality

Maximizing the number of crystals in the bond of the grinding wheel keeps the theoretical chip thickness and normal force of each individual grain at low levels. This reduces the wear rate of the grinding wheel and promotes long wheel life. Maintaining minute chip thicknesses and low grinding forces is also responsible for improvements seen in the edge quality of the silicon nitride workpieces. SEM photographs in Figure 5 show a significant reduction in edge fracturing of the ceramic component that was ground with the wheel containing the spiked nickel coated diamond. The edge quality and surface finish variations seen in Figure 5 strongly suggest that differences exist in the structural integrity of the specimens.

Achieving Profitability

Many parameters influence the total cost and ultimate profitability of a ceramic grinding process. The diamond type, coating, concentration, mesh size, bond type and hardness all have a major influence, not only on the productivity, but on the surface integrity, finish and total quality of the finished component. Grinding experiments revealed that crystal retention is the single most important attribute among these factors.

Since each application varies, manufacturers should consult with their suppliers to determine the best diamond type for their needs. Selection of the most appropriate diamond type, combined with enhanced coating characteristics such as a spiked nickel coating, can produce significant cost savings in grinding ceramics and bring real value to the ceramic grinding industry.

For More Information

For more information about ceramic grinding, contact Terry M. Kane, GE Superabrasives, 6325 Huntley Road, P.O. Box 568, Worthington, OH 43085; (614) 438-2851; fax (614) 438-2829; e-mail terry.kane@gep.ge.com.

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