The rise of the superabrasives

By IM Staff
Published: Thursday, 06 September 2018

Automotive manufacturers are demanding better accuracy and longer life from abrasive tools, and this has created an opportunity for a new family of high-performance synthetic abrasives, as Industrial Minerals correspondent Rose Pengelly discovers.

Demand for better performance, efficiency and accuracy in manufacturing is driving continuous improvement in the abrasives industry.

Mineral-based abrasives are used for grinding and smoothing applications, and have been an industrial tool for centuries, with metalworking generally topping the list of key markets.

Conventional mineral abrasives include silicon carbide, aluminium oxide, silica, garnet and quartz – minerals which occur naturally in the earth’s crust.

But with the evolution of the way metals are used, and now that they are being replaced with lighter, stronger, more flexible materials in everything from cars to industrial machinery, new approaches are starting to emerge to optimize the ways in which component parts are shaped and sharpened.

The automotive sector is one of the fastest-growing application areas for abrasives.

"If you look at the number of cars being produced – in the United States, but also globally – over the past few years, the numbers have been pretty robust," Jeff Wherry, managing director of the country’s Unified Abrasives Manufacturers’ Association (UAMA), told Industrial Minerals.

In fact, US vehicle production fell by around 8% year-on-year in 2017 to 11.2 million units, according to statistics published by the International Organization of Motor Vehicle Manufacturers (OICA). Of this total, just over 3 million were passenger cars and 8.15 million were commercial vehicles.

Globally, however, vehicle production grew by around 2.4% last year, to 97.3 million units (73.5 million passenger cars and 23.9 million commercial vehicles), OICA data showed. This was driven by output increases in China, France, Japan and Turkey, among others.

"Abrasives are used in almost every part of manufacturing cars, including in the electronics and seating, so when car production performs strongly, abrasives demand responds accordingly," Wherry said.

The trend toward lighter weight components to improve fuel efficiency has meant that plastics and composites are beginning to be used more widely in the automotive market as alternatives to metal parts, according to Wherry.

This trend has been widely regarded as a threat to abrasives demand, because the kinds of tough, mineral-based materials that are used for metalworking are not needed to finish plastic parts.

But Wherry believes that this threat is exaggerated. "What replacements [in the automotive sector] that need to be made with plastics have already been accomplished," he said. "The abrasives manufacturers I speak to say that business has never been so good."

The global abrasives market was 11.8 million tons (10.7 million tonnes) in 2016, according to analysis by US-based market research consultancy Grand View Research (GVR).

In terms of volume, GVR has forecast that the market will expand at a compound annual growth rate (CAGR) of 4.5% until 2025, when it will be worth $59.3 billion.

The metal fabrication sector is expected to be the fastest growing market for abrasives, with a CAGR of 5.2%, driven by the automotive and aerospace industries.

Polycrystalline cubic boron nitride (PCBN) is a superabrasive used for machining applications. 
Element Six 

Harder, better, faster, stronger

One of the chief goals for automotive manufacturers is to reduce their production costs, to bolster margins and help them to remain competitive in an increasingly crowded market.

Abrasive grinding wheels are necessary tools in the cutting and finishing of parts for cars, which are made from steel, aluminium and other durable materials, such as carbon fiber.

These parts are often overlaid with wear-resistant coatings, including ceramic coatings. These enhance their resistance to corrosion, but make them more difficult to shape and finish during the manufacturing process. Consequently, grinding wheels can quickly wear out and have to be replaced frequently.

One attempt to reduce the replacement rate has been the development of so-called "superabrasives" – extremely durable materials made from industrial minerals and metals treated at extremely high temperatures.

Conventional abrasives are typically natural grains that are fused or blended with other abrasive grains to make a grinding wheel with the right characteristics for the intended application.

The hardness of these conventional grains is usually in the range of 2,100-2,500 on the Knoop scale, a micro-hardness test developed by the US’ National Bureau of Standards (now NIST) in 1939.

The hardest known superabrasives are made from synthetic diamonds, which have a Knoop hardness around 7,000 and are made from graphite.

Cubic boron nitride (CBN) is the second-hardest material known after synthetic diamond. It has a Knoop hardness of 4,700, and is synthesized from hexagonal boron nitride under conditions similar to those used to produce synthetic diamond from graphite.

The process which raw materials undergo to produce superabrasives is known as vitrification. The word comes from the Latin vitreum, meaning "glass", and it refers to the transformation of a substance into a glass- or ceramic-like material – technically, a non-crystalline amorphous solid.

This is usually achieved by heating materials until they liquidize before cooling them rapidly, so that there is no time for crystals to form. This results in a vitrified solid.

Vitrified diamond and vitrified CBN grinding wheels are regarded as being among the most efficient grinding superabrasive products available.

Diamond abrasives are typically used for non-metallic and non-ferrous materials, while CBN is mostly used to grind metals. Diamond abrasives are not usually suitable for grinding steel because the carbon in the diamond reacts with the carbon content of the steel, wearing down the abrasive tool too quickly.

More and more manufacturers are looking into the use of superabrasives to optimize their grinding processes, according to David Goetz, corporate applications engineer at Norton Abrasives, a subsidiary of French industrial conglomerate Saint-Gobain, based in Ontario, Canada.

Superabrasives are considerably more expensive than conventional abrasives, but the payback in performance and longevity is greater, so manufacturers will use them if the value of the end-product is high enough to justify the input cost.

Superabrasives are more thermally stable and durable than conventional abrasives, so they give a longer life for grinding wheels, particularly in demanding applications. They also typically offer a much higher G-ratio (the ratio of the volume of material ground to the volume of wheel wear) than other materials.

The G-ratios of conventional abrasives are usually in the 1-20 range, ceramics in the 10-200 range, while superabrasives are in the 500-10,000+ range, depending on the application.

Because higher G-ratios typically equate to shorter grind cycles and more jobs per hour, grinding with superabrasives can increase productivity. Superabrasives do, however, require higher machine stiffness compared with conventional abrasives, because they are less forgiving.

Synthetic diamonds are made from heating graphite to extremely
high temperatures to form the hardest material known.
Instytut Fizyki Uniwersytet Kazimierza Wielkiego, via Wikimedia Commons 

Diamonds grind forever

One of the companies spearheading the development of diamond superabrasives is Element Six, a private company which is part of the De Beers diamond business empire.

Founded in 1946 and with its headquarters in Luxembourg, the company sells super-hard engineering tools all over the world, with manufacturing sites in Ireland, Germany, South Africa and China.

In 2013, and at a cost of £20 million ($25.9 million), Element Six opened the world’s largest and most sophisticated research and development facility for synthetic diamond supermaterials at the Harwell Campus in Oxford, UK. The unit employs more than 100 expert scientists, engineers and technicians.

"Synthetic diamonds are harder than traditional abrasive materials," Hubert Koegel, global sales director for automotive and aerospace at Element Six, told Industrial Minerals.

The extreme properties of synthetic diamonds, including the highest known thermal conductivity and electromagnetic transmission, make them ideal for a variety of advanced industrial applications, such as mining and tunneling, road planing (stripping off a road surface before replacing it), and various uses in the automotive industry, he added.

Element Six makes a range of abrasives for the automotive sector, including diamond and CBN powders for grinding and finishing metal surfaces and glass; polycrystalline diamond (PCD) materials for cutting aluminium or carbon fiber-reinforced polymers (CFRP); polycrystalline cubic boron nitride (PCBN) used in finish machining for steel in drivetrain gears and shafts; and tungsten carbide burr blanks used for metal cutting and the removal of rough edges.

"Automotive customers require faster, more precise machining, with materials that enable a longer tool life and reliable wear," Koegel said.

"The automotive market is very significant for Element Six, and we expect it to remain that way due to the emergence of new technologies, such as hybrid and electric cars, as well as changes in trends of mobility," he added. "Urbanization and the increasing mobility craved by individuals around the world will create new opportunities for everyone in the automotive industry."

American and, increasingly, European drivers are choosing SUVs and trucks over
smaller car models, pushing up the demand for abrasives.
Mic, via Flickr 

Changing market demands

The shift from conventional abrasives to vitrified CBN technology in grinding automotive components has been driven mainly by the machine-tool builders which serve the vehicle manufacturing industry.

Machine tool builders have had to innovate to meet the demands of automotive producers for higher productivity, and have worked with abrasives manufacturers to develop new vitrified CBN products.

The new developments lower the cost per unit of automotive parts by making longer-lasting, faster-grinding abrasives with higher material removal rates.

Advances in vitrified bond technology, coupled with more choice in superabrasive grain types, mean that abrasive wheels can perform for longer at the same power level, or lower power, than traditional grinding wheels.

"This is a key benefit, because new advanced materials such as High-Velocity Oxygen Fuel ([HVOF] coatings are being introduced for today’s gas engine components," Norton’s Goetz said.

He explained that one of Norton’s principal objectives was to reduce the number of machining steps between the "blank" material and the final product.

Abrasives that stay sharper for longer and have a much lower energy-to-grind ratio also help to reduce the stress and heat introduced to an automotive part during the manufacturing process, which improves the structural integrity and safety of the finished vehicle.

New developments in superabrasives

Although vitrified CBN is still the most common superabrasive technology used today, there has been a trend toward using more plated and metal single layer (MSL) products – such as nickel-bond grinding wheels.

Superabrasive product selection is process-driven. The geometry of the part being worked, stock removal and finish requirements, as well as the material being ground, all help to determine the grain size and type, the grade, concentration and bond. Their matrices of performance benefits are used by manufacturers to justify to customers why they should shift to super or metallic abrasives from conventional abrasives.

So far, the shift to superabrasives has not generally been regarded as having a negative effect on prices for conventional abrasive minerals, where bulk pricing has been fairly steady for some time.

Strong demand for conventional abrasives minerals in the steelmaking industry has generally supported prices, while Chinese supply has been brought under control after a few years of undisciplined production and export.

UAMA’s Wherry said that although there is a definite trend toward the use of synthetic and superabrasives, the market for natural and conventional abrasives will remain strong, at least in the US.

The US abrasives sector is benefiting from the repatriation of the industry from Asian competitors, he added.

"There was a time when people were very keen to take advantage of China’s low-cost labor and lax environmental standards for producing abrasives products there and importing them into the US," he said. "But recently there has been some concern about the quality of Chinese material, and manufacturers have complained, so some companies are beginning to rethink their strategies."

This shift is backed up by anecdotal evidence from other sectors, such as aluminium, but is taking some time to materialize in abrasives.

According to the US Geological Survey (USGS), US abrasive mineral production in the first quarter of this year was flat compared with January-March 2017.

Imports of crude fused aluminium oxide totaled 35,400 tonnes during the period, 51% more than in the first quarter of 2017. Of this year’s first-quarter imports, 22,100 tonnes, or 64%, came from China, up from 14,500 tonnes (62%) in 2017.

For silicon carbide, all but 100 tonnes of the US’ 11,600 tonnes of imports came from China in the first quarter of last year. In the equivalent period this year, the US imported 26,700 tonnes of silicon carbide, 22,000 tonnes, or 82%, of which was from China.

Wherry felt strongly that change was under way and believed that the election of Republican Donald Trump as US president in 2016 "was the best news the US economy has seen in 30 years."

"Trump’s election was very good news for US abrasives manufacturers, certainly," Wherry said. "His focus has been on creating a level playing field in terms of tariffs, so US companies have a chance to compete fairly with imports."