Nondestructive Evaluation of Diesel Spray Holes Using Piezoresistive Sensors

A slender piezoresistive sensor that can measure the extreme 3-D surface topography of microscale mechanical parts is described. Nondestructive quality testing of high-aspect-ratio microholes is insufficient since suitable sensors are not available. As a consequence, more than 50 million injector nozzles are shipped every year, but the spray holes, which determine through their geometry both efficiency and exhaust emission of diesel engines, are not fully tested. Recently, we described a piezoresistive cantilever as the only sensor so far for measuring form and roughness directly inside injector nozzle holes. Based on our initial design, sensors are now available from a pilot production using a silicon wafer scale process. They show much improved performance (vertical/lateral resolution, speed, noise, and light sensitivity), enabling us, for the first time, to perform a comprehensive study with the actual generations of state-of-the art injector nozzles as well as prototype holes.