Diamonds for the rough

May 1, 1999
Manufacturing computer hard drives involves fine grinding with abrasive materials as small as pollen or bacteria and less coarse than cigarette smoke.

Manufacturing computer hard drives involves fine grinding with abrasive materials as small as pollen or bacteria and less coarse than cigarette smoke.

Texturing with diamond micron powder increases computer hard drive storage.

The El-Zone analyzes micron diamond samples to provide accurate particle distribution.

Micron diamond powder finishes and textures materials ranging from hard stone and gems to ceramics and glass.

Trace Element analyzers check the purity of diamond micron powder to one part/billion.

Your PC is more powerful because of a grinding process. Electronics manufacturers are fine-grinding computer hard drives to increase storage capacity from what was once a 30-Mbyte standard to what is now as much as 10 Gbytes. Specifically, these companies polish and texture hard disc and read/write head surfaces using diamond micron powder. Not only does the process boost drive storage, but it also lets the heads get closer to the disc surface for better signal pickup.

GE Superabrasives manufactures the diamonds and micronizes the powder used in these fine-grinding applications. The Worthington, Ohio, firm reports that disc drive texturing has spurred growth in micron diamond powder markets over the past five years. And, as companies strive to further increase storage capacities through tighter disc surface patterns, the average size of diamond particles used has decreased from about two microns to just 0.3 micron. This size imparts a surface roughness of 10 angstroms or less.

In disc manufacturing, the media part is stamped from aluminum. Diamond turning and back grinding flattens the parts, and, once machined, the 3.5-in. discs get a 20-micron-thick nickel plating. After that, polishing with an aluminum oxide slurry takes the surface finish to 5 angstroms with a random texture pattern.

The 5-angstroms disc surface is too smooth, so manufacturers will texture to 10 angstroms, imparting a circumferential pattern to the disk. This somewhat rougher surface helps align the magnetic media on the disc and lets the read/write head efficiently zip over the disc as it spins.

Standard read/write heads are made of alumina titanium carbide, a hard ceramic composite. Manufacturers get about 10,000 heads from a standard 4 3 4-in. wafer of this material. Using diamond micron powder, they polish and slice these wafers into bars, each representing approximately 30 to 40 heads. These rough heads are then lapped with diamond micron powder in a slurry, and diamond blades slice in grooves and rails. Finally, another lapping operation contours the heads for a degree of aerodynamics, so that they easily glide over magnetic storage media.

Controlling every particle
No matter what type of diamond micron powder is used, certain aspects of the particles can impede the powder's performance. These are size, shape, and surface of the particles.

A premium diamond micron powder will have a fairly consistent particle size for optimum material removal rates in free-abrasive lapping applications. When sizes differ, larger particles will remove most of the material. And, contact points are reduced, producing a non-uniform finish. In addition, there is an increased chance of gouging.

According to GE, even one over-size particle can scratch a surface. But, the company also explains that ultra-fine particles are also a problem, often indicating that a powder was not properly graded. Particle size analysis is accomplished using sophisticated techniques such as: electrical sensing zone, sedimentation, or microscopy techniques.

Electrical sensing zone methods are the same ones used to count blood cells. Particles pass through an orifice in an electrolytic medium, causing a voltage pulse. A device then detects, counts, and determines particle size by the size of the voltage pulse.

In sedimentation, particle size is determined by the speed at which particles settle while under a gravitational or centrifugal force. For measuring, particles are usually suspended in water.

A 2D profile of particles provides a size measurement when using microscopy techniques. Scanning electron microscopes are usually needed for particles measuring one micron or less.

Diamond particle shape, like size, is difficult to quantify, says Timothy Dumm, leader of market development for micron products at GE. He says that particle shape must be a consistently sharp and blocky form, but what is sharp and blocky to one person might not be to the next.

To get around this, he explains, GE uses quantitative shape measurement techniques. These are generally image-analysis systems involving a video camera. With the powders on a microscope slide, a video camera digitizes the particle images. Another instrument then defines the shape, assigning X/Y coordinate points that have mathematical algorithms associated with them. That way, users are plotting particle shape qualities in basically quantitative terms, says Dumm.

As particle size decreases, the specific surface area increases exponentially. Surface characteristics of the powder become important when particle size is below 10 microns. This is because the powder is more sensitive to surface effects such as moisture, static charge, or even suspension stability particles in a slurry. Particle surface area is measured using nitrogen adsorption techniques.

Texturing of hard drive discs is done with a slurry — in this case diamond micron powder dispersed in liquid — which disperses, transports, and distributes diamond particles evenly across the workpiece. This medium also lubricates, cools, and removes swarf.

Types of common diamond slurries include water-based, oilbased, or water/oil formulations. For an ideal slurry, GE recommends completely dispersing or separating diamond particles. One way is by adjusting the electrochemistry of the suspension, which negatively charges the particle surfaces so that they repel each other, preventing agglomerations from forming.

In addition, GE suggests that users consider the entire system when designing a slurry, which includes the diamond, workpiece, liquid medium, and tooling. Factors like wetting behavior, particle embedding, rinseability, and surface treatment will all come into play.

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