Patrick Foster What's New in Metallography
Patrick Foster
LECO Corporation
The main focus at LECO corporation and particularly in the Application's Lab is in metallographic sample preparation. The LECO strategy has not been toward "full automation" but rather to preserve a balance between efficiency, improvement of results, and the "Human Factor", since it is recognized that ultimately there will always be a human evaluating what a metallographic laboratory facilitates. At one in the same time, the "Human Factors" which lead to less consistancy of results are being minimized.

The most significant advance in sample preparation is a 3-M LECO introduction of TRIZACT abrasives. Traditional sample preparation papers are SiC and have several inherent disadvantages. Due to great variation in particle height on the SiC paper, they tend to be very agressive on initial contact with the sample, and wear out rapidly (usually in 20 - 30 sec). Because of this a whole series (usually 120 180 240 400 and 600) of abrasive sizes must be used to bring any one sample to the polishing stage. TRIZACT abrasives are radically different. Particles are shaped (as discrete pyramids) on the backing and have an entirely uniform height and cutting characteristic. Grinding is a very uniform operation (it is neither "very agressive" on first contact, nor becomes ineffective after several minutes). Because of this, a sample can often be taken from "raw cut" down to "ready-to-polish" on one single TRIZACT faced cloth. Moreover, the cloth is usually good for about 30 minutes (not seconds) of grinding, so many, many (about 80) samples can be prepared on the same cloth. After the 30 minutes of grinding time, the backing cloth (a different color) shows itself as a color change, indicating the TRIZACT is now worn out. While costing about 5-6 times (per sheet) what SiC does, it can be readily shown that using TRIZACT ultimately reduces good sample preparation costs from about $0.46 / sample to $0.08 / sample accounting for time saved, as well as extended media life.
Using TRIZACT will require a learning curve, since cutting is so uniform. Actual grinding time is less (about 120 - 180 sec / sample), (compared to the time spent on 5 SiC papers) but seems longer since it is not split-up on five media with an initial aggressive surge of removal. Technicians require considerable experience with the media to get used to this concept. The main differences are: 1) the media will wear down evenly 2) there is less vibration and "feel" to the process 3) there is less stress (grabbing) of the sample. 4) the only lubricant required is water (only enough to flush debris). TRIZACT works well with most metals, alloys, and coatings EXCEPT aluminum. Aluminum will tear up a TRIZACT cloth almost immediatey.

Another improved approach to sample preparation is something LECO calls "Linear Sectioning" and is applied to its new line of Cut off and sectioning equipment. Most recent equipment has used a "plunge" approach, in which the cut-off wheel is plunged into the stock. With "linear sectioning" the stock is traversed in the cutting line of the wheel, and multiple passes are made (if necessary) to complete the cut. This "cutting line" motion is referred to as the X-axis. Z-axis (plunge) indexing is used to do multiple passes, and Y-Axis (perpendicular to the cutting line) indexing is used to make multiple parallel cuts (if required). This approach has been found to do a much better job on large irregular shapes, and combined with the availablility of variable blade speeds both minimizes deformation and speeds up the cutting process. Elimination of "burning" reduces work in subsequent grinding.
(REVIEWERS COMMENT: none of this is new -- when I began my metallography work 27 years ago, such equipment was in common usage, [ie. table saw, or radial arm saw style cutting which achieves the same result] however; nothing [particularly the operator] was protected from dissintegrating blades, flying chips, etc. The safety factors present in today's equipment were not in place.)
Since "enclosed" cutting is now available in this style, LECO has been able to go to much thinner blades (which while fragile) greatly improve microsectioning efficiency.

In the area of "automation" of Grinding and Polishing, LECO has choosen to produce a line of equipment which is "modular" and easily "upgradable". A basic wheel, is the core. A solid state speed control can be the first module. This module can be replaced by a multiple sample manipulating "head" with automatic control from a LCD control screen; and ultimately basic "wheel" units can be linked together (two wheels sharing one sample head) into a chain, for nearly total automation. At any level, the same "modules" are used and the original investment in the "wheel", or "head" modules is never lost.

LECO has introduced a "new generation" of microhardness testers, from "fully automatic" to a "semi-automatic" (which actually is faster than the "full automation" unit). The "semi-automatic" does indentions per a program schedule and downloads the images to a computer screen for "user assisted" measurement, and the final results to a spreadsheet for useful analysis and archiving.

Finally, microphotography, is rapidly moving in the direction of digital images. Such images are now beginning to rival those of traditional photography, and are much more convenient for storage, archival, and retrieval than the old file of polaroids.