Cross sectioning is the first and most important step in the sample preparation process. Getting the best results involves obtaining a smooth surface finish and minimum chipping and material deformation without sacrificing cutting speed. Today, most laboratories work with a wide variety of materials, and frequently each material requires a different sectioning method and sample preparation approach. Selecting the right equipment, consumables and parameters for your specific material/application will significantly affect your sectioning operation, save you time and money, as well as set the stage for the rest of your specimen-preparation process.
Sectioning/wafering saw manufacturers typically recommend a different blade for each material; abrasive blades for materials such as ductile metals and sintered carbides or composites containing predominantly hard phases; sintered (metal-bonded) diamond wafering blades for cutting brittle materials such as ceramics or minerals; and CBN (cubic boron nitride) and resin-bonded diamond wafering blades for cutting metallic materials.
The development of new-generation diamond wafering blades and performance measurement and optimization methods has changed the way sectioning and specimen-preparation process is handled. This article discusses new developments in diamond wafering blade manufacturing, technologies, performance optimization techniques and several misconceptions regarding their use. Improvements in diamond wafering blade manufacturing technology have expanded the use of diamond into many other applications and materials. A recent technology in manufacturing diamond wafering blades has made use of diamond wafering blades more economically feasible on broader variety of materials and applications.
How diamond wafering blades work
A diamond wafering blade is simply a cutting tool that has exposed diamond particles captured in a metal matrix, each with a small cutting edge. During the sectioning operation, the surface speed may reach 30 m/sec using a high speed sectioning saw. The cutting action is performed by accumulation of small chips scratched out by the numerous diamond particles imbedded in the bond. Diamond particle size and blade thickness affect the cut, so blade selection together with feed rate, cutting speed and depth of cut determine sectioning success. Some factors to consider when selecting a diamond wafering/sectioning blade for an application include diamond grit (mesh size), cutting speed, CBN (cubic boron nitride) vs. synthetic diamond and diamond concentration.
Diamond mesh size plays a major role in surface finish quality, smoothness, level of chipping and microstructural damage. Finer mesh size (e.g., 220 and 320 grit) produce a very smooth surface finish with minimal amount of chipping on edges. Coarser diamond particles (e.g., 80 and 100 grit) frequently are used for fast cutting/material removal on harder materials such as silicon carbide, zirconia, Al2O3 and stainless steels, where a fine surface finish is not required. Diamond mesh size (grit size) should provide maximum removal rate at minimal acceptable finish. Often the desired finish cannot be achieved in a single step/operation, and lapping or polishing may be necessary.
Sectioning can be performed at low or high speeds. Diamonds may break at very high speeds and fall out at very slow speeds. Blade life usually increases at slower cutting speeds, but higher labor costs, utilities costs, depreciation of equipment and other overhead expenses can offset the savings in higher blade life. Currently, most sectioning is done at low speeds to minimize sample damage and preserve true material microstructure. However, sectioning materials at very low surface speeds creates a large impact force between diamond and material being machined. Even though the diamond may not break, the risk that the diamond will be pulled out of the blade and cause premature failure of the blade increases. Therefore sectioning every material at very low speeds by default is not always the optimum solution.
Today, many users believe that CBN blades with special formulations of CBN diamonds should be used to cut metals (e.g., irons, cobalt base alloys and nickel base superalloys), which will significantly reduce the sectioning time, while sintered (metal-bonded) diamond wafering blades should be used to cut brittle materials such as ceramics and minerals. CBN does not gum up as much as diamond when sectioning metals. However, CBN does not provide any real advantage in sectioning using ultrathin wafering blades. Studies show that new-generation metal-bonded blades with synthetic diamond significantly outperform CBN blades in cutting metals. Synthetic diamond is harder, more resistant to abrasion and more thermoconductive than CBN. Special coatings applied to synthetic diamond have tremendously improved synthetic diamond chemical reaction to metallic materials. These blades can cut harder materials and absorb more heat without transferring it onto the sample being sectioned.
The proportion and distribution of diamond abrasive particles, also known as concentration, has an effect on overall cutting performance and price of precision diamond blades. A concentration of 100 = 4.4 ct/cm layer volume (mesh size + bond), which means that the diamond proportion is 25% by volume of diamond layer, assuming that diamond density is 3.52 g/cm3 and 1 ct = 0.2g. Nominal diamond concentration in precision diamond blades range from 0.5 to 6 ct/cm3, which means diamond concentrations are available from 8 to 135.
Higher diamond concentration is recommended and usually used for cutting softer and more abrasive materials. However, the trade off is significantly slower cutting speed. Low diamond concentration is recommended and widely used for cutting ultrahard and brittle materials.
Diamond concentration and cutting performance
Today, most sectioning saw manufacturers and laboratory technicians recommend using low-concentration diamond wafering for sectioning ceramics, glasses, silicon, carbides, sapphire and other related semiconductor and optical materials, and using high-concentration diamond wafering blades on metals such as stainless steel, aluminum and titanium. A technological breakthrough in manufacturing diamond wafering blades allows the manufacturer to orient diamonds inside the metal matrix so every diamond is better able to participate in cutting action. This is changing these recommendations and setting new benchmarks on how diamond wafering blade performance is measured.
Studies and extensive testing show that diamond concentration in wafering blades manufactured using this new technology plays a minor role in determining overall blade performance. Every diamond is in the right place and at the right time, working where you need it most, resulting in maximum use of diamond and bond.
With conventional technology, where diamonds are placed inside the metal matrix with no control over diamond distribution, there is inconsistent diamond tool performance. Only about 40% of the diamonds are able to participate in the cutting action. The rest fall out, become dull, or disintegrate before they have a chance of being used. Improper spacing of diamond or CBN particles typically leads to premature failure of abrasive surfaces or structure. Excess diamond particles increase the cost of manufacturing diamond tools due to high cost diamond and CBN powder, yet they have no effect in increasing performance.
Conventional metal-bonded diamond blades without properly oriented diamonds quickly become dull and out of round. Further cutting requires continual blade dressing to expose new diamonds, and such dressing puts pressure on the material causing the tool to overheat and lose its tension. Users can ultimately find it necessary to use excessive force and pressure just to cut a small amount of material.
Most blades wear faster on the edge or in front than in the middle. Higher diamond concentrations are preferred in these locations to prevent uneven wear and, thus, premature blade failure. Making diamond and CBN particle distribution uniform and in a predetermined pattern tailored to individual customer applications distributes the workload to each particle evenly, resulting in faster sectioning and longer blade life. This new technology makes diamond an excellent, more feasible solution for a large majority of hard material and high metallic content sectioning applications.
In the past, preserving the true microstructure of the material was a strong argument against using diamond wafering blades due to possible contamination by the bonding material. The new generation metal-bond diamond blade resolves many of these concerns due to even diamond distribution and strong diamond retention and allowing the manufacturer to use a smaller number of diamond particles. Fewer and smaller diamond particles actually participate in the cutting action, yet cutting speed and surface finish is improved and optimized. This advanced formulated open bond design ensures minimal chipping, fast cutting, constant speed of cutting, minimal cutting noise and most importantly, minimum loss of precious material.
The controlled distribution of diamonds in the new generation metal bond often reduces the need for high diamond concentration, such as 100 con used in conventional wafering blades. IH