Choosing the ideal sandpaper/abrasive type depends on the material you use it on and the finish you desire. When comparing silicon carbide vs. aluminum oxide (alumina), they do have some differences, but both are popular options for metalwork and sanding wood surfaces because of their performance level, lower prices, and versatility.
If you weren’t an earth science buff in high school, you might not know the difference between these abrasive tools. Below we have compared the two options and go over their best applications to help you make an informed decision for your next project.
What Is an Abrasive Grain?
In basic terms, an abrasive grain is a sharp and hard material used to wear down another material when moving in contact with applied pressure. One of the most used abrasives products is sandpaper, which manufacturers often make with silicon carbide (SiC) or aluminum oxide (Al2O3). These are the two most common abrasives types. They account for nearly 90% of the total amount of domestically-produced abrasive grains.
Other common abrasive materials include zirconia alumina, cubic boron nitride, synthetic diamonds, ceramic alumina, garnet, emery, quartz, silica sand, and crocus. Many of these abrasive grains mentioned are used to manufacture other types of abrasive products like grinding wheels, flap discs, flap wheels, and sanding belts.
The particles that form sandpaper grit need a sharp shape and high hardness values to become abrasive. You will often find them designated with a Grit number that describes their grain size. The Grit number will detail the number of small openings per inch in the screen that the grain passes. The values range from No. 4 to No. 2500, with particle sizes varying from a few millimeters to under two micrometers.
To determine the type of abrasives on your product, bonded abrasives and sanding discs have a code on the label that tell you the make-up. The symbol for aluminum oxide is A, and the one for silicon carbide is C. You can find this symbol in the first position from the left, followed by the characteristics of the particular abrasive, such as grit size and sometimes other special characteristics. See the image below for an example.
Choosing the correct sandpaper/abrasive can make the sanding/grinding process less time-consuming by smoothing the surface faster. You can improve the surface finish by using specific abrasives, techniques, and pressures, depending on the material. The right abrasives will reduce friction and heat, letting you preserve the abrasive longer and avoid burning the material.
What Are Aluminum Oxide Abrasives Used For?
Aluminum oxide sandpaper is the most widely used grain because it works very well on most metals, wood, and painted surfaces. The material is durable and cheaper to produce than most of the alternatives as well.
Aluminum oxide abrasives do come in different colors. You can find brown, white, and pink aluminum oxides, with brown being the most common.
White and pink will wear down faster, but they produce a smoother finish. These two aluminum oxides compare most to silicon carbide. You can find white aluminum oxide in coarse through fine textures. It works best on wood and lacquers as it produces less heat. You can also use white grains between finishing coats on wood. Pink aluminum oxide comes in coarse through fine textures, and you can use it on softer woods.
Brown aluminum oxide lasts longer since its grains break down far slower than the pink and white varieties. You can find it in coarse through micro-grit textures, and it can sand high-hardness materials like metals, drywall, fiberglass, wood, and painted surfaces. While the surface finish does not look as smooth as pink or white, you can more affordably and quickly grind your object with brown aluminum oxide.
You can find friable, semi-friable, regular, and heavy-duty aluminum oxides. Instead of dulling down, friable abrasive grains break to re-sharpen when used, constantly exposing sharp edges. Coarser grits can remove metal stock by letting the grains re-sharpen this way, while the smaller grains can finish metal surfaces.
Brown aluminum oxide is semi-friable. The grains will break and re-sharpen, but they take more use to break. The benefit of this is a longer lasting abrasive.
If you are using aluminum oxide sanding belts, you will find open, closed, and flexible closed coats. An open coat works best on soft metals and woods because of its 60% - 65% grain coverage.
Closed coats cover 90% - 95% of the belt surface, making these coatings the most appropriate for hard non-ferrous and ferrous metals. Lastly, a flexible closed coat sanding belt applies to recessed and curved surfaces on harder woods and metals.
You can only use aluminum oxide sanding belts for dry operations. If you plan to use a metal grinding wheel, you can utilize aluminum oxide on materials with high tensile strengths, like stainless steel and high tensile bronze and aluminum alloys.
What Are Silicon Carbide Abrasives Used For?
Silicon carbide sandpaper is the hardest and sharpest common abrasive, but it lacks durability due to its brittleness. Also, the narrow particle size wears down faster. The grains are razor-sharp so that they can sand metal, marble, glass, stone, cork, medium-density fiberboard, and plastic with minimal pressure. However, they struggle to sand harder metals and wood.
Silicon carbide works well on rough surfaces and for polishing. It is more friable than aluminum oxide, and you can use it in wet sanding operations. You can polish parts in automotive applications, remove rust, refinish wood floors, deburr metal, smooth glass edges, and sand between finishing coats.
Most people use silicon carbide sandpaper in conjunction with aluminum oxide. People will perform a rough sanding with aluminum oxide abrasives and finish off the project with silicon carbide. That way, you can produce a smooth surface finish without wearing down your sandpaper.
You can find friable (green) and regular (black) silicon carbide, with the friable being purer and harder yet more brittle than the regular variety.
Black silicon carbide can grind non-ferrous metals, ceramics, and hard nonmetals, while the green option works best for polishing purposes. However, it is not recommended to grind steel with silicon carbide.
Silicon carbide sanding belts work well on harder materials like wood finishes, stone, metal, paint, and softer ones like rubber, glass, and plastic. The grains will microfracture with repeated use (friability), making them unsuitable for softwood as they will not sharpen the edge. These belts have a closed coat with optimal grain coverage, so you can apply grain to a hard material or finish the surface with sealers or lacquers.
The grinding wheels and sanding discs work well on materials like cast iron, aluminum, and cemented carbide. If your project involves a nonmetallic or low tensile strength material, you will want to use silicon carbide rather than aluminum oxide.
Aluminum Oxide or Silicon Carbide Best for Grinding Steel?
The best sandpaper for metal depends on the material composition of the metal, but aluminum oxide works well in general. While silicon carbide can handle many hard and soft metals, it does not perform well on steel or stainless steel. Aluminum oxide can wear down the steel surface efficiently, but it may not create the smoothest finish.
Ceramic alumina and zirconia alumina can do a better job than aluminum oxide at sanding steel. They produce less friction and have greater durability, but they cost much more. However, the budget-friendly aluminum oxide can handle softer steel alloys. In the battle of silicon carbide vs. aluminum oxide, neither can sand stainless steel as well as ceramic or zirc, but aluminum oxide is a cost effective way to do minor work.
Aluminum Oxide or Silicon Carbide for Grinding Aluminum?
Actually, both. The best abrasives for aluminum grinding are often made with a special formulation of both alumina and silicon carbide. An additional lubricant is added to many of these hybrid abrasive products. The goal is to grind aluminum without creating too much heat that would damage this soft metal. The combination of both abrasives with a lubricant protects the abrasive from loading and building up too much heat that would melt soft metals like aluminum.
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