Evelyn Adams looks at how a new generation of advanced coatings are helping cutting tools take on more exotic materials.
Coatings for cutting tools have come a long way since thin titanium carbide (TiC) layers were first introduced in the 1970s. Back then, the idea to include TiC coatings increased metal cutting performance by around 50 per cent seemingly overnight.
Not long after, tooling companies began using titanium nitride (TiN) coatings, and in 1973, aluminium oxide became popular. âSince then, many new coating innovations have taken machining performance to new levels,â explained Mia PÃ¥lsson, senior manager of turning tools at AB Sandvik Coromant. âFor instance, the release of the first really thick alumina coatings in 1980 lifted wear resistance and cutting data way beyond the level of existing coatings.â
Today, most coating-materials used on tooling inserts are classified as ceramic. An inner layer of titanium carbo-nitride (TiCN) is often used for wear resistance, and an outer layer of aluminium oxide (Al2O3) is used for heat protection. PÃ¥lsson says an important milestone in cutting tool coatings has been the introduction of physical vapour deposition (PVD) coatings, which was boosted when multi-layers and coatings with titanium aluminium nitride (TiAlN) came into the picture.
But while all this innovation has improved speed, strength and the life of cutting tools, in the past few years several companies have decided to take a closer look at their coatings. So much so, that changes to coatings are now being made on the nanoscale. Itâs caused an industry shift from exploiting chemistry to exploiting size. For instance, Seco Tools recently launched the latest version of its Duratomic coating, which it claims is the worldâs first atomically modified tool coating.
The technology toughens up cutting tools by arranging aluminium and oxygen atoms in a unique way to increase its resistance. As a result, the company says cutting tool inserts have a higher heat and wear resistance as well as chemical inertness.
âIn order to think big, we first had to think small, very smallâ, explained Roger GranstrÃ¶m, product manager, referring to the atomic manipulation. âPhysical vapour deposition (PVD) and chemical vapour deposition (CVD) coatings have performed very well for many years and will continue to do so in many applications, but our focus was to develop a brand new process.â
Duratomic began as an aluminium oxide formula. To include the added strength, researchers at Seco Tools controlled what is known as the ânucleation layerâ between an underlying titanium carbide (TiC) layer and the aluminium oxide to create a unique texture, or crystallographic orientation. In some orientations, the crystals can resist more wear. By exploiting this fact, the coating became 15 per cent tougher and 10 per cent harder than conventional aluminium oxide.
The Duratomic technology has now been incorporated in the companyâs TP2500, TP1500, TP0500 and TP3500 grades. âAs part of the next-generation Duratomic technology, the new TP grades feature used-edge detection so machinists can quickly and accurately identify used edges with the naked eye,â said John McGhee, product manager of turning, threading and advanced materials at Seco Tools UK.
Known as Edge Intelligence technology, it creates a chrome-coloured exterior that helps customer detect whether an insertâs edge is worn or not and when it need to be replaced. âWe determined this as a necessary benefit for todayâs manufacturers as recent surveys revealed that over 10 per cent of the edges on discarded inserts were unused â which represents a significant and costly waste,â added McGhee.
McGhee says that just as important is the machine toolâs substrate which must provide proper support for the coating to prevent damage. âThe basic insert â also known as the substrate â provides most of the required toughness for the cutting tool,â he said. âThe trick is to correctly combine the most suitable coating and substrate.â For instance, the Duratomic technology on TP2500 works with a cobalt-enriched substrate to increase toughness in certain applications by around 400 per cent.
A few years after Duratomic was released, Sandvik Coromant introduced its Inveio coating technology, which is based on something known as âuni-directional material crystal orientationâ. In a similar way to Duratomic, the coating is made by controlling coating crystals at an atomic level so that they are lined up, tightly packed and facing the same way. This, the company claims, creates a stronger, uniform structure.
âThis equates to remarkably long and predictable tool life, even at high cutting data,â said PÃ¥lsson. âAs a result, there are fewer tool changes, so downtime is reduced and component output is boosted.â These nanostructured coated cutting tools are increasingly being used to cut exotic materials in applications for industries such as aerospace, which require extreme precision when cutting materials such as composites, titanium, superalloy, aluminum and stainless steel. This can include anything from the composites as used on the Boeing 787 to the zirconium oxide ceramic used in hip replacements.
âLooking at the future for coatings, a strong trend is predicted in advanced nanocoating,â said PÃ¥lsson. âNew material systems being used for coatings will be enabled by the latest developments in nanotechnology and process steering. Over recent years, many have claimed that coating development has peaked and there’s little more to discover, butâ¦this fact is known to be wrong. The new products released in the coming years will prove that new levels of performance are within reach.â
Box: Common coating materials
Most coating-materials used on todayâs inserts are classified as ceramic. These are most common along with their main properties:
â¢ Titanium carbo-nitride – TiCN â has very good abrasive wear resistance and good adherence to the carbide substrate.
â¢ Aluminum oxide (alumina) – Al2O3 â provides very good outer thermal and chemical protection for the insert substrate.
â¢ Titanium nitride – TiN âmainly used for the golden colour which provides clear wear detection.
â¢ Zirconium oxide – ZrO2 â gives very good thermal and chemical protection for the substrate.