Evaluation and optimisation of diamond-like carbon for tribological applications

Fuel economy and environmental protection have become increasingly important over the last few decades and this has led to the demand for higher efficiency within the automotive sector. Non-ferrous coatings with excellent and unique tribological properties, intended to increase efficiency by lowering friction and wear internally within the car engine are becoming increasingly more prevalent within this sector. Since current lubrication practices are geared towards ferrous materials, it is important to determine the effect that lubricants have on non-traditional coatings.
In this study, three Diamond-Like Carbon (DLC) coatings are assessed for their viability within this application, more specifically for the cam-follower interface. The coatings are an amorphous hydrogenated Diamond-Like Carbon and a silicon doped amorphous hydrogenated Diamond-Like Carbon, both produced at the University of Leeds, using a Hauzer Flexicoat 850 deposition system, and a tungsten doped Diamond-Like Carbon coating from Oerlikon Balzers, type Balanit C*. The coatings are characterised to quantify their hardness, thickness, elastic modulus and sp2 content before being tribologically tested in group III base oil and fully formulated oil containing the anti-wear additive zinc dialkyldithiophosphate on a pin-on-reciprocating plate tribometer. The three DLCs are tested against a cast iron counter-body and analysed for wear and the presence of a phosphate tribo-film. The nature of growth of the tribo-film is commonly known for ferrous contacts but much uncertainty still remains for DLC lubricated contacts.
The Hauzer Flexicoat 850 system houses within it two microwave sources which aid in the Plasma Enhanced Chemical Vapour Deposition (PECVD) of DLC. This new technology offers potentially improved coatings for tribological coatings and the ability to tailor the coating for a particular lubricant. In this study the microwave PECVD process is tested to determine its viability within a commercial setting, and to discover the effect of different processing parameters on the mechanical properties of the coatings. The parameters of particular interest are the bias voltage, gas ratio and power of the microwave sources. Preliminary tests were undertaken in order to determine the optimum position and other fundamental deposition properties before the full scale tests began.
This study has shown that a-C:H DLC is the most suitable coating of those tested here for implementation into this application. The Si-DLC and W-DLC were not compatible with the lubricants or the system as will be shown. The lubricant additive ZDDP is important in ferrous systems, and it is shown here to be useful within a DLC environment
The microwave DLC coatings have demonstrated good mechanical properties, in particular a high hardness. They have proven to be very promising, and with a substantial reduction in deposition time, demonstrate their potential commercial viability for tribological applications