The diesel-engine industry is literally facing great pressure for technology advances to provide the level of combustion efficiency essential to meet environmental demands. Next-generation diesel-injection systems will use extremely high pressures to produce smaller fuel droplets—which will provide a greater surface area for combustion.
Typically, pressures in common-rail turbodiesel car engines are now in the 1600-1800 bar (23.2-26.1 ksi) range but are set to go considerably higher soon as the need to further reduce CO2 levels continues. For example, Bosch has announced that it is developing common-rail systems with piezo and solenoid actuators with pressures increased to as much as 2000 bar (29.0 ksi). But the raising of pressures will bring fresh technology challenges to ensure enduring system precision. These include a demand for tighter tolerances on injector components.
Currently, many injector components are coated with DLC (diamond-like carbon), but this can flake away under very high pressures. Specifically, DLC does not adhere satisfactorily using traditional physical vapor deposition (PVD) techniques, so it cannot be built up to any great thickness, said Professor Papken Hovsepian, head of the Nanotechnology Center for Physical Vapor Deposition Research (NTCPVD), part of the Materials and Engineering Research Institute at Sheffield Hallam University in the UK.
Although DLC has very good sliding wear, which makes it an attractive material for automotive applications, it is not sufficiently tolerant to shear forces. Seeking a solution to this problem, and other coating issues, is the Innovatial project, which is supported by the European Commission through the Sixth Framework Program for Research and Development. The project involves 24 high-profile European partners, all operating within the automotive and aerospace sectors, and is focused on developing innovative processes and materials to allow high-performance nanostructured coatings to be applied to components.
The NTCPVD has a key role in the project and, as a world leader in the sector, has taken the lead in a ground-breaking technology for applying several types of coating materials in a “superlattice”—building up, at the atomic level, dense coating layers.
The new technology is called HIPIMS (High Power Impulse Magnetron Sputtering) PVD, which may prove to be the most significant breakthrough in PVD in the last 30 years.
Within the HIPIMS chamber at
“Whilst HIPIMS has the potential to improve greatly the reliability and particularly the adhesion of DLC coatings, we are also keen to develop new coatings,” Hovsepian explained. “One of these is made by alternating layers of titanium aluminum carbon nitride and vanadium carbon nitride. This low-friction coating is showing promising results, as it adheres extremely well and is much harder than DLC.”
NTCPVD is also working on new coatings for valves and piston rings (which are subjected to very high heat levels) and has recently deposited alternating layers of chromium aluminum yttrium nitride and chromium nitride.
“Using HIPIMS, it is possible to lay a dense, superlattice coating, which is twice as hard as chromium nitride on its own and is extremely resilient to high-temperature oxidization,” Hovsepian added. “Furthermore, the coating shows excellent tribological behavior as the coefficient of friction is actually reducing when the coating is exposed to high temperatures.”
The Innovatial project and the NTCPVD’s activities promise to provide a substantial increase in the durability and performance of coated components within the automotive industry, notably those that work in harsh environments.“At Sheffield, we are equipped with world-class industrial and laboratory scale PVD systems and a large variety of advanced systems for plasma diagnostics,” added Hovsepian. “This allows both fundamental and applied research to be carried out, and we are collaborating with several companies in Europe, the U.S., and the Far East, as well as leading research organizations such as the Lawrence Berkeley National Laboratory, the Center for Microanalysis of Materials, University of Illinois, and the Fraunhofer Institute.”