Advanced ceramics are the leading candidates for high-temperature engine applications — offering improved engine performance and reduced emissions. One type of ceramic, silicon nitrides, is being evaluated for use in valve train materials for diesel and natural gas engines.
Because they operate in high-stress, high-temperature, corrosive environments, these materials must be highly durable and reliable. Accurate and efficient surface and subsurface characterization methods are essential to identify damage caused by machining and to ensure the reliability of the valves.
A group of researchers at Argonne National Laboratory (ANL), led by J.G. Sun, have been investigating nondestructive evaluation (NDE) methods to detect surface and subsurface defects caused by abrasive machining processes. Such defects — which include microstructural discontinuities such as spalls, cracks, and voids — are typically within 200 µm of the material's surface and can significantly degrade the fracture strength and fatigue resistance of siliconnitride ceramics. Because these ceramics are partially translucent in light, a laser-scattering method based on the detection of optical scattering from the subsurface can be used to measure variations in subsurface microstructure — without contacting the material's surface.
The ANL researchers investigated the machining-induced damage in silicon-nitride ceramic valves intended for use in diesel and natural gas engines. The team developed a high-speed, automated, laser-scattering system for the NDE of these valves. The valves were subjected to a coarse and a finish machining process, then tested in a bench rig. The NDE system scans the entire valve surface and generates a two-dimensional scattering image that is used to identify the location, size, and relative severity of subsurface damage from the machining process and the rig tests. The results indicated that the machining damage in coarse-machined valves was significant. In particular, severe damage at the edge of the keeper groove caused premature failure of a valve during the rig test.
After finish machining, the team found that most of the initial damage in the valves was removed. The data were analyzed and compared with surface photomicrographs.
ANL is currently evaluating these valves in 1,000-hour engine durability testing.
