2/1/2001 | 6 MINUTE READ

Coating Improves Performance

Facebook Share Icon LinkedIn Share Icon Twitter Share Icon Share by EMail icon Print Icon

A coating so hard that aluminium can replace steel?


Facebook Share Icon LinkedIn Share Icon Twitter Share Icon Share by EMail icon Print Icon

A coating so hard that aluminium can replace steel? A coating so wear resistant that it extends the life of a component 60-fold? A coating so heat resistant that it can withstand temperatures up to 2000°C? A coating so corrosion resistant that it can endure more than 2,000 hours of salt spray without showing any signs of damage?

If such a coating exists, then there must surely be a downside, right? It must be toxic. It must demand uncompromising clean room conditions. Or a great deal of work must be carried out on the items before being treated.

There is apparently a coating that can do all of these things. And, no, those downside issues are not encountered.

The astonishing thing is that while this coating process is only now beginning to make an impact, it has been around for more than 100 years. And while it has been investigated by academics over the years, it only started to be commercialised in the 1990s. Because of the development of energy-efficient modes of the electrochemical process, along with environmentally friendly, long-lasting electrolytes, the technology has recently found industrial applications.

While research has been carried out in both Germany and the U.S., intriguingly, the main thrust has been in Russia, principally Moscow State University, although the research quickly came to a halt after the collapse of the Soviet Union, when funds dried up.

As the lead scientist responsible for the Keronite process at Moscow State, Dr. Alexander Shatrov was left in a limbo at this time and so set about trying to find a backer so that he could continue his work. He left the country to set up a company—Isle Coat Ltd.—in an offshore island to the U.K. Dr. Pavel Shashkov heard about the plight of his former university colleague and offered him an opportunity to continue his work in England. A company named Keronite Ltd. was subsequently created to market the technology.

The Keronite layer is a complex oxide ceramic consisting of hard crystalline phases dislocated in a matrix of softer phases of oxide. It is this that gives Keronite a combination of high hardness and wear resistance, shock resistance and vibro strength, and yet high flexibility. The fatigue strength is also three times greater than that of anodizing.

The Keronite coating involves a high-energy plasma discharge around a component immersed in an electrolyte that provides for surface oxidation resulting in a superhard ceramic layer on light alloys such as aluminum, beryllium, magnesium, and titanium. The process resembles anodizing because it uses an electrical power supply and a bath, but is significantly different as it produces harder and thicker layers. While items need to be de-greased prior to being coated, there is no need for surface etching as the electric micro discharge in an electrolyte sufficiently cleans the surface anyway. After the treatment, the part needs washing in warm water for several minutes.

The low concentration alkali electrolyte does not contain any toxic or aggressive chemicals and is no more hazardous than the water in a domestic washing machine. In particular, it is an effective substitute when applied to magnesium as it replaces the toxic chromating process.

"Compared to anodizing, electrolysing is far more environmentally friendly and contains 99% distilled water," says Shatrov. Its main features include extreme hardness—up to 2000 HV depending on the alloy; wear resistance; heat resistance—the coating can withstand short exposures of up to 2000°C; a good uniform coverage even in small cavities that are normally difficult to coat; high dielectric strength—it can withstand voltages in excess of 1,000 V DC. According to Shatrov, salt spray tests lasting more than 2,000 hours have been carried out on 6082 alloy, which can be even further improved by polymer sealing to over 5,000 hours. Once polished and infused with a polymer, the coating has a friction coefficient of less than 0.15 against steel and it has high adhesion properties. Since the coating is formed through a reaction that involves the substrate, adhesion constitutes up to 80% of the substrate metal strength.

In illustrating its wear properties, Robert Altham, chief executive of Keronite Ltd., explains that a steel engine pulley had a 3-mm reduction after 400 hours of accelerated testing; there was no sign of wear on an aluminium pulley that had been treated. This application is currently being evaluated by a manufacturer of earth-moving equipment.

What all this means in practice is that Keronite can be applied to a part working under high loads at a high temperature—and as the finished surface can be impregnated with lubricant materials—with virtually no lubricant.

Both Altham and Shatrov argue that the Keronite process is superior to hard anodizing and plasma spray ceramics. Compared to hard anodizing, Keronite offers higher microhardness and wear resistance; a wider range of coated alloys, it is environmentally friendly, it does not require preliminary preparation of the surface before coating, and the process can be carried out at room temperature.

"On some alloys, hard anodizing produces similar results," says Shatrov, "but the corrosion resistance of Keronite is 60 times better. So perhaps we've got 100 or 200 hours corrosion resistance of one alloy, but we get 7,000 hours on another alloy. We need to know which is the best one to use for a particular application. It is a similar situation for fatigue. If you're making something out of aluminum, you would probably anodize it, which reduces the fatigue strength by 50%. The Keronite process will reduce the fatigue strength by 15%, meaning items can be designed to be smaller."

"Keronite has very good adhesion properties," says Shatrov, "and compares well to other processes, particularly plasma-sprayed ceramics. A good example is the thermal barrier on the crown of a piston. If a portion comes off a piston that has been ceramic coated with plasma spray, it will immediately ruin the engine. A Keronite coating cannot unbond because it's a transformation of the surface and not something sprayed onto it so it has an atomic bond, not an adhesive one."

Magnesium is often the metal of choice, but there are so many negative aspects to it that it usually is relegated to structural components. However, argues Shatrov, the Keronite process opens up new opportunities. "The hardness is exceptional compared with most coatings and on most aluminum alloys it's extremely good," says Shatrov, "but on magnesium you can get hardness up to 2000 HV on the coating. This means that magnesium can now be used as a functional component whereas before it's only been used as a structural component."

As a Keronite coating significantly improves the wear characteristics of strong alloys like WE 54, it widens the range of applications in an engine including the pistons, the loaded element in pumps, fuel and pneumatic drives, valve trains, hydraulic systems, guide rods, and sliding bearings, amongst others.

"The possibility of having a magnesium piston that isn't going to wear out is here," says Altham, "and the implications are really substantial. As you are reducing the weight by 23% compared to aluminium this means that the con rod can be re-designed as it doesn't have to be quite so strong as the inertia is less. This means that the crankshaft can be re-designed from scratch as it doesn't need to be so large. "There are also an enormous number of possible applications in the gearbox," says Altham. "We can harden aluminum parts to make them even more wear-resistant way beyond that of steel. However, in terms of gear sets, aluminum is unlikely to have the bulk strength for it but titanium does, so there's a real possibility of having titanium gear sets because of the Keronite coating."

An advantage that aluminium has over steel, says Altham, is the manufacturing cost. "In some case studies we have undertaken, the manufacturing cost has been one third through making it in aluminum rather than steel. Additionally, the life of the product has increased four-fold, which means that there has been a 12-fold improvement in the cost/life relationship of the product."