ICMCTF2006 Session TS1-1: Coatings for Automotive Applications

More than 100 years ago the development of internal combustion engine started. Amongst steam engines and others the spark ignated gasoline engine became the most used engine for passenger cars. In the beginning those cars were more or less motored coaches. Short time later they were not longer seen as replacement for coaches, but as a completely new method to travel. Therefore, very clever engineers like Wilhelm Maybach started the development of cars that were able to run faster, for longer distances, more comfortable and safer than before. This development has been continued by all the car manufacturers until now. Besides this development requirements regarding longer durability, affordability for a lot of people, fuel consumption and the enviroment have become more and more important. Especially the need for lower fuel consumption and increased durability forced the continuous development of Diesel engines for passenger cars that started in 1936.

Comparing the old engines with the up to date engines one can state that tremendous improvements have already been achieved with regards to all the above mentioned requirements. Engine manufacturers were able to do so by the help of new production processes and manufacturing technologies, new materials and advanced combustion processes.

However future emission legislation and future fuel consumption requirements oblige car manufacturers to improve their engines even more. Engine components therefore will be more loaeded. In some cases the contact stresses at the surfaces of sliding or rolling contact partners are too high to create sufficient wear resistance with conventional materials respectively with conventional heat treatments. Those applications need improved wear resistance, which can be given by the help of specially tailored coating systems. Depending on the wear characteristics, the stresses, the lubrication condition and the type of the contact the type of coating and the surface characteristics have to be choosen. In some applications those coatings have already proven their enormous potential to reduce different types of wear.

Therefore the Schaeffler KG created the coating system Triondur C⁺, with a coating thickness of just about 3 µm combining high wear resistance, low adhesion to metallic counterparts with high toughness and fatigue strength. To realize a high hardness with a strong support of the base material the coating temperature is reduced to 200°C. The transition from the metallic base material to the functional coating which must have low adhesive tendency to the metallic counterpart, is a chromium adhesion layer with a gradient a-C:H:W coating. The functional layer is an a-C:H-layer with a high hardness of about 2800 HV and very low chemical reactivity to the counterpart. Due to the multilayer architecture the coating system has a very high ratio of hardness and Young’s modulus with high chemical and physical wear resistance. Triondur coating systems are deposited by environmently-friendly PVD- / PACVD- processes, thus they make a contribution to a sustainable development in materials technology and whose application.

Due to stringent fuel consumption requirements car manufacturers do their best to increase the efficiency of the engines in every area. One possibility is to improve the mechanical efficiency by reducing the friction losses. Special coating systems have already shown their potential to reduce friction when they were used in sliding contacts of valve train components. Future developments will lead to coatings that will reduce friction losses between valve train components to an even higher amount.

Triondur CN is a tailor made coating system for tappets to reduce the friction in the valve train. The aim is reached by coating the cam contact surface by reactive magnetron sputtered CrN_x. The base material is carbonitrited and ground before coating down to roughness values Ra < 0,030 µm. The coating process is done by low temperatures below 250°C to stay under the annealing temperature to avoid distortion and secure the close tolerances of the thin cam contact surface. To have excellent tribological and mechanical properties the PVD-coating was tailored regarding the chemical composition, the microstructure and the morphology by creating a nanocrystalline metastable lattice structure of CrN and Cr₂N. The small grain sizes in the region of 10 nm and their different orientations lead to have a X-ray amorphous structure. In comparison to other coatings the nanocrystalline Triondur CN coating has a smaller content of defects with less increase in roughness and lower stress intensities because of the lower dislocation density. The grain borders work as effective crack barriers and because of the small grain sizes the stress intensities at the crack tip are too low to overcome the grain border. After the deposition of 2 µm coating the surface roughness is below Ra < 0,030 µm, without any subsequent treatment of the surface like polishing. This enables a robust surface regarding function and quality. The high wear resistance of the Triondur CN coated surface is reached by high, hardness values of about 2500 HVpl and an excellent oil wetability with an improved separation of the friction partners. According to the smooth surface topography with a high percentage contact area and flat surface angles the counterpart is smoothed in a soft way and therefore the combined surface roughness of the tribological system is reduced during the running in. The high wear resistance and the high surface roughness lead to a reduction of the mixed friction and protection of the excellent smooth surface during operation.

Triondur coatings enable high loaded modern valve train components and help to reduce fuel consumption with an environmental-friendly manufacturing process.

Today, the trends in the machine component industry are towards higher performance, improved reliability and tolerances, less lubricants and more environmental friendly products. Another strong driving force in automotive industry is decreased fuel consumption. Depleting fossil resources, economic competitiveness and especially environmental concerns has compelled to explore different possibilities to achieve these goals. And one of them is to improve the wear resistance and lower the friction of tribological systems by deposition of a low friction diamond like carbon (DLC) coating on critical components, often operating under boundary lubrication. Under boundary lubrication conditions, different oil additives are used to prevent metal to metal contact by boundary film formation and thus avoiding higher friction and wear of components. Because formation of boundary films involves reaction between additive molecules and the lubricated surface, additives are formulated for a specific application and specific material surface, in the vast majority for iron based surfaces. With the introduction of DLC coated surfaces the major concern is the compatibility of DLC coatings with formulated oils, originally designed for uncoated metallic surfaces.

In the present study, the tribological performance of hydrogenated amorphous carbon coating (a-C:H) and metal doped DLC coating (Me-C:H) under the boundary lubrication regime was investigated. The investigation employed ball-on-flat contact geometry in reciprocating sliding motion and six formulated oils (manual gearbox oil, automatic gearbox oil, hydraulic oil, compressor oil, and normal and high performance motor oil), with pure poly-alpha-olefin (PAO) oil used as a reference. In addition, DLC coatings behaviour in diesel and gasoline fuel was evaluated. Compared with the uncoated steel surface, metal doped DLC coating showed improved tribological performance under boundary lubrication even when using formulated oils.

Plasma nitriding (ion nitriding) was the first plasma assisted process for surface engineering in industry. Based on the pulsed dc-technology from the early 80ies in the last century a breakthrough could be achieved for the machine and automotive industry.

A big potential is opened for the combinations of the plasma nitriding with new plasma assisted coating technologies like PVD- and PA CVD-processes.

The competitiveness to other nitriding processes like salt bath and gas nitriding will be presented and discussed.

Examples out of the European automotive industry will be presented like: 1) tooling for sheet metal forming and aluminium forging, 2) gear components, especially for sintered gears, 3) crank shafts and cam shafts and 4)valves, piston rings, tappets etc. made of high alloyed steels.

The market potential for plasma nitriding in the automotive industry will be presented and discussed.