Coating Improves Performance

Automotive Finishing, Summer, 2001 by William Kimberley

A coating so hard that aluminum 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 2000C? A coating so corrosion resistant that it can endure more than 2,000 hr 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 cleanroom 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 commercialized 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 research quickly came to a halt after the collapse of the Soviet Union when money dried up.

As the lead scientist responsible for the Keronite process at Moscow State, Alexander Shatrov was left in a limbo at this time and so set about trying to find a way so that he could continue his work. He left the country to set up Isle Goat Ltd., on an island offshore to the United Kingdom. Dr. Pavel Shashkov heard of the plight of his former university colleague and offered him an opportunity to 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. This is what gives Keronite its combination of high hardness and wear resistance, shock resistance and vibro strength, while maintaining high flexibility. The fatigue strength is also three times greater than that of anodizing.

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

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

"Compared to anodizing, electrolyzing is far more environmentally friendly and contains 99% distilled water," said Dr. Shatrov. Its main features include extreme hardness, up to 2,000 HV, depending on the alloy; wear resistance; heat resistance, the coating can withstand short exposures of up to 2000C; good, uniform coverage even in small cavities that are normally difficult to coat; and high dielectric strength, it can withstand voltages in excess of l,000v DC.

According to Dr. Shatrov, salt spray tests lasting more than 2,000 hr have been carried out on 6082 alloy. The salt spray resistance can be further improved to 5,000 hr using polymer sealer on the parts. Once polished and infused with a polymer, the coating has a coefficient of friction 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., explained that a steel engine pulley had a 3 mm reduction after 400 hr of accelerated testing. There was no sign of wear on an aluminum pulley that had been treated. An earth-moving equipment manufacturer is currently evaluating this application.

What all this means in practice is that Keronite can be applied to a part working under high loads at high temperatures. As a finished surface it can be impregnated with lubricant materials with virtually no lubricant.

Both Mr. Altham and Dr. 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.