The surface engineering industry has grown massively over the last few years. What was once the domain of the racing and other high-performance engines has now spread to the everyday powertrain and its components. A longer life, less wear, and increased durability are all mentioned as the results that these coatings offer. Needless to say, there is not just one coating, but a whole range. To make matters more interesting, though, there is now a pre-coating process that is transitioning from the aerospace industry to motorsport and from there into the general automotive industry.
Plasma nitriding has been used in the aerospace industry for some time. Its advantages include a hard, wear-resistant surface without brittleness, galling, or spalling, and the process envelops the entire surface, achieving a consistent hardness and case depth. This is especially noticeable on complex geometries where gas nitriding case depths can be non-uniform. Plasma nitriding also enables a higher surface hardness while maintaining the material’s core properties due to the lower processing temperatures of 350° to 570°C, whereas traditional nitriding operates at higher temperatures and can change the material’s core properties. It means that there are no phase transformations, which guarantees no distortion and a calculable growth of the parts.
On the downside, though, there are some operating considerations that have limited its general acceptance. For example, the treatment temperature depends upon the proper selection of a large number of operating parameters, including nitriding temperature and time, gas pressure and gas mixture composition, and the plasma voltage. The proper arrangement of the workpieces within the furnace also requires considerable operating experience. If the workpieces are not properly arranged, some surfaces are not nitrided or they may be overheated.
A few years ago, German company Eltro developed and patented the Eltropuls process that uses a pulsed nitrogen plasma to infuse nitrogen ions into the steel surface to improve surface characteristics. Completely reproducible, it allows the surface morphology of a part to be optimized, taking into account the material and application. It guarantees the best results for manufactured parts and production tools and can meet, the company claims, the most-demanding requirements of any nitriding process. A pulsed voltage is applied between the workpieces and the furnace wall. This voltage accelerates electrons to very high velocities, permitting them to ionize and activate normally inert gases like nitrogen so that the specified surface treatment can take place. The high electron energy in the plasma permits physical and chemical reactions to take place that would normally require much higher surface temperatures. The Eltropuls system allows the temperature of the workpieces and the surrounding inert gas to remain relatively low during the entire surface treatment so the workpieces can retain their original core properties. Process parameters are microprocessor controlled and optimized throughout the heat treatment cycle to produce the best metallurgical results. Parts can be heated in the shortest possible time, in vacuum, by radiation or in an inert atmosphere, with natural or forced convection. Even the most complicated programs can be easily reproduced. The characteristics of the nitrided surface can be controlled to produce a variety of results, influencing the final hardness profile. This includes the structure, the nitrogen content and thickness of the compound zone, the final concentration of the new elements added to the surface and the nitrogen concentration gradient in the diffusion zone.
“Nitriding is one of the heritage technologies that have been around for a while,” says Jeremy Cockrem, managing director of Eltro GB, the British arm of the business that focuses on the high-performance end of the business. “The process consists of infusing nitrogen into the steel which reacts with the alloy elements to produce a hard, wear-resistant layer. It also expands the surface of the material generating residual compressive stresses, which is why you get such a huge improvement in fatigue resistance.”
“Plasma is a fourth state of matter so you go from solid, liquid, gas to plasma so what we are doing is bringing ionic nitrogen directly to the surface by plasma ready to diffuse into the steel,” says Cockrem. “The mass transport is very efficient and can be precisely controlled to create whatever layer is required by using nitrogen and carbon. We can build the required surface layer with the right surface stresses and the correct hardness very easily with our equipment. We normally just use hydrogen and nitrogen but there are a few applications where we want to add a little bit of carbon to create a different layer for something like camshafts but it does depend on the base material and what the end requirement is. Half the trouble is choosing the right layer for the right application. It means we can produce the appropriate hardness whether it is for machine cutting tools, clutch and valve springs or gears. Any steel can be nitrided to improve the fatigue resistance but to improve the wear resistance and hardness we need alloy elements like chromium and sometimes aluminum in the steel. It means the appropriate steel must be chosen for the right application. We can also do titanium but that’s mainly for aerospace applications. We can optimize for fatigue resistance, wear resistance or corrosion resistance or a compromise of the whole lot depending what the customer wants. For example, Formula One engine manufacturers don’t care anything about corrode resistance on their crankshafts and just want the best wear resistance. For damper rods, though, we want corrode resistance as well.”