At first glance, the science of sanding and grain technology may not sound like the most interesting of topics, but with the level of sanding needed to be done today for oil wax finishes, dark stains, and trying to please very picky clients, understanding the differences between abrasive products and the science behind producing a fine scratch pattern can help you achieve that furniture quality finish that is sought after.
The topics we will cover here are the different types of abrasive grains, the properties of abrasive grains that determine performance, and the unique properties of each type of grain to maximize performance for specific applications.
Hardness and Toughness
Four main types of grains are used for abrasives in the floor sanding and woodworking market: silicon carbide, aluminum oxide (alumina), zirconia alumina, and ceramic alumina, which are all man-made grains. Ceramic alumina and zirconia alumina are engineered grains that, unlike silicon carbide and aluminum oxide, can be manufactured more easily with specific properties and can be tailored to different applications. Each grain type has other properties that make it suitable for different applications.
Hardness and toughness are the two main properties of abrasive grains that determine life and performance. Hardness refers to the grain’s ability to scratch various materials. Toughness, or friability, refers to how easily the grain fractures or breaks down under pressure. The most-common hardness scale is the Mohs scale, which measures the hardness of various minerals with a rating of 1-10, with talc at 1 and diamond at 10. On the Mohs scale, silicon carbide sits at 9.5, aluminum oxide at 9, zirconia alumina at 8.5, and ceramic alumina at 9.4. However, just because silicon carbide is one of the hardest abrasive grains doesn’t mean it’s the best. Toughness and friability need to be factored in. The chart below shows the relationship of hardness and toughness for each type of grain.
Grains need to fracture under use to maximize the performance of the product. Fracturing allows the grain to constantly expose sharp edges, maintaining a consistent cut rate and extending product life. Abrasive grains fracture in two different ways. They are either macro-fracturing grains, which break apart in big pieces, or micro-fracturing grains, which break apart in small pieces. This is the key property that will determine product life. Silicon carbide and aluminum oxide are macro-fracturing grains. They can fracture a small number of times. Zirconia alumina can fracture hundreds of times, and ceramic alumina can fracture thousands of times, so it sometimes referred to as a “nano” fracturing grain.
Microstructure and Macrostructure
There are two terms used when describing attributes of different grains: microstructure and macro-structure. Microstructure refers to the grain’s crystal structure, which affects its toughness and how it fractures. The four types of grains can be split into two groups. Silicon carbide and aluminum oxide are what is referred to as “monocrystalline” grains. When they are fused during the manufacturing process, it creates one large crystal. When they fracture, it is usually along planes of structural weakness, which tend to be large pieces, giving them their macro-fracturing properties. In the case of ceramic alumina and zirconia alumina, each grain is composed of micron or sub-micron individual crystals. The manufacturing process for each provides for very precise control of the microstructure of the grain, which allows the toughness and fracturing properties to be tailored for different applications. Because these engineered grains are comprised of tiny crystals with a very refined microstructure, the fracturing is controlled, and they micro-fracture. Grains fracture in two ways: cleavage, and conchoidal fracturing. With cleavage, surfaces fracture along flat planes producing intersecting sharp edges. Conchoidal fracturing produces irregular curved sharp edges, similar to broken glass.
Macrostructure refers to the shape of the grain. Grains come in many shapes; they can be blocky, long, and pointy, or more symmetrical in shape. The manufacturing processes for silicon carbide and aluminum oxide give limited control over the end shape of the grain. Silicon carbide will vary slightly in shape, but is mostly long and pointy, and aluminum oxide tends to be blocky in shape. Because ceramic alumina and zirconia alumina are engineered grains, there is much more control over the shape of the grain produced, with ceramic alumina allowing for the most control over the shapes available.
Effects of Grain Structure on Performance and Scratch Pattern:
Both the shape of a grain and fracturing properties have a direct effect on the overall surface finish and performance in different applications. These properties make each grain better-suited for certain applications.
Silicon carbide is a hard, very sharp grain that can vary in shape and size, being mostly long and pointy. It works well sanding raw wood or removing old coatings, but because it is very friable and macro-fracturing, it wears down very quickly, giving it a short life. Its shape and sharpness allow it to leave a fine scratch pattern, but inconsistencies in shape and its high friability can cause errant scratches. Sometimes longer pieces of grain will dig more deeply into the surface, leaving a deep scratch. This is most noticeable when screening or using a multi-disc sander. Also, because of its friability, the grain tends to shed a lot during initial use, which can cause problems, especially on screens where fractured pieces of grain will get stuck under the screen and leave coarse scratches. This is visible when you first start a screen on the floor and can see the initial shedding of grain. Silicon carbide tends to fracture conchoidally.
Aluminum oxide, or alumina as it’s known, is a blocky-shaped grain with sharp edges. Because of its shape and high friability, it is not a good choice on its own for machine sanding because it wears down too quickly. It sometimes is blended with other grains, such as zirconia alumina and ceramic alumina, to create lower-cost products. Its blocky shape and high friability make it better-suited for use on low-pressure applications like orbital discs, hand sheets, and between-coats products because it leaves a fine consistent scratch pattern. Unlike silicon carbide, there are not any longer pieces of grain that can leave deep scratches, which is critical when abrading finishes. Aluminum oxide tends to fracture conchoidally.
Zirconia alumina is a long spindly shaped grain with sharp edges. It is a micro-fracturing grain, which gives it a longer life than silicon carbide, so it is better-suited for sanding tougher coatings and harder woods where silicon carbide would wear down too quickly. Because it is an engineered grain, it is more consistent in shape and size than silicon carbide. Zirconia tends to fracture along crystallographic planes producing flat intersecting edges that are very sharp.
Ceramic alumina is an engineered grain that actually is comprised of sub-micron crystals of aluminum oxide. The manufacturing process allows for multiple types of grains to be designed for different applications. Ceramic grains are the sharpest and longest-lasting grains, making them best-suited for sanding exotic woods, hard coatings like aluminum oxide finishes, and sports floors. Ceramic grains tend to be more consistent in shape and size than other grains and have a lower aspect ratio, meaning they are triangular as opposed to long and spindly. Ceramic grains fracture conchoidally and tend to fracture intergranularly, producing very small curved, extremely sharp edges.
I hope this deep dive into abrasives was informative and will help guide your future decisions on choosing the right product for the right application. It’s worth noting that this is just the tip of the iceberg regarding the science of abrasives.
Greg Mihaich is application engineer for Norton/Saint-Gobain Abrasives, and has been with the company since 2007. In this role, he works in the professional floor sanding and DIY/contractor markets on new product development, testing, and training. He can be reached at firstname.lastname@example.org. To learn more about the abrasives and grain manufacturing processes, visit nortonabrasives.com.