The Role of Surface Engineering in the Automotive Industry

The Role of Surface Engineering in the Automotive Industry

The subject of surface engineering in the automotive industry has developed significantly in the last decade. A large driving force for the need for surface treatments has been energy consumption. 30 per cent of all energy consumed in the European Union derives from transportation activities, relying solely on fossil fuels. Due to this, and the push to reduce the emissions of polluting gases, car manufacturers must produce increasingly energy efficient and environmentally friendly vehicles. One way to do this is by reducing energy losses and wear rates of vehicle components, by either implementing new materials or applying improved surface treatments. In addition, surface treatments have also found decorative applications. [1, 2]

Increased demands from modern automotive systems have necessitated the use of these new technologies to give such improved properties as increased load handling, longer lifetimes and corrosion resistance to name but a few. Engines in particular have received much attention. The engine and its components alone contribute to 15 percent of the total frictional losses in the vehicle. Aluminium alloys have more recently been preferred to cast iron to reduce the overall weight of the car, to improve fuel efficiency and road performance. However, as engine technology continues to develop, and more power is demanded from the unit, so the severity of the operating conditions of the components increases. This has lead to a downturn in engine performance, mainly due to piston ring contact with cylinder sleeves. Figure 1 shows the distribution of frictional losses in the engine. Methods that have been developed for these will be discussed later. [1, 2, 3]

Figure 1 - The distribution of frictional losses in IC engines [a]

A wide range of surface treatments can be used in all aspects of the vehicle, including:

* Thermal and plasma spraying

* Physical vapour deposition (PVD)

* Plasma enhanced chemical vapour deposition (PECVD)

* Thermochemical heat treatments

* Electroless deposition

The applications of these processes can be seen in table 1. This also shows some resulting properties from the treatments.

Table 1 - Selected properties of the different treatments and typical applications [b]

The applications can also be seen in figure 2. This shows components of the car, each of which usually requires one salient property. Simply, these can be split into; corrosion protection; friction reduction; wear reduction; and decorative appearance, due to the factors previously discussed. For each requirement, some processes are more pertinent than others are. Sulzer Metco use their Ionit Ox process ("gas nitriding and/or plasma nitro carburising combined with plasma activation and oxidising" (Sulzer Metco, 2006 [7])) to give corrosion protection, e.g. for ball pivots. Friction reduction can arise from PVD or PECVD, e.g. for injection systems. Plasma spraying is implemented, e.g. in piston rings, to give wear reduction. PVD, but also electroless deposition, are used for decorative purposes, e.g. for exterior trim or interior furnishings. [1]

Figure 2 - Selected parts to be treated by surface technology [c]

The processes for more specific components shown in table 1 and figure 2 will be discussed.

Gears

For gear components, surface hardened steels provide high hardness and wear resistance, and usually have a low carbon content of 0.1-0.25%. Their lower carbon content means the process of carburisation is much more efficient. Nitriding is also an option for increased surface hardness but has disadvantages such as an increase in processing time. A typical gear wheel will require a Vickers' Hardness of 550HV, achievable with a carbon content at the surface of 0.7%wt., and a case depth of approximately 0.6mm. Case gaseous carburising can be used, whereby a carburising gas flows over the heated sample to allow carbon to diffuse into the surface. Further heat treatment is then necessary (austenitising), as both the core and case will have a coarse grain structure. Subsequently, the case can then be refined by heating and quenching to produce a martensitic structure. Martensite is a very hard phase, ideal for the high surface hardness required.

More recently, PVD processes are being implemented to improve not only surface hardness, but also surface roughness, friction coefficients and oil wetability. To give these properties, B4C, WC-C:H and CrN coatings are laid. [2]

Since around 2000, diamond-like carbon (DLC) coatings have been used to improve lubrication in gears. DLC coatings can be deposited by reactive PVD and PECVD processes, but only in small lot productions. This has contributed to the lack of application in powertrain production. The a-C:H:Me (metal containing amorphous hydrogenated carbon) and a-C:H (amorphous hydrogenated carbon) coatings put down by these processes had lead to significant performance improvements. This is shown in figure 3. Other applications in the coating of cams and cam followers in formula one motorcycle engines results in a massive 8 BHP gain compared to conventionally treated cams. [4, 5]

Load 1500 N/mm2 2000 N/mm2

Cycles 1.4 x 106 5.4 x 107

Pitting 20% 0%

Figure 3 - Reduction of pitting and load increase by a-C:H:Me (W-C:H) (shown right) coatings gears [d]

Cylinder Liners and Piston Rings

The advantages of using aluminium instead of cast iron for engines have already been explained. With the interaction of cast iron liners being so critical to energy consumption, the correct surface characteristics are crucial. To achieve this, coatings such as hard chrome platings, thermal sprays and plasma sprays are widely applied. PVD coatings are seen as the next step to achieve further improvements. This would involve using totally aluminium engine blocks with