Research to respond to demands for improving usability of passenger vehicles has played important roles. Some aspects can be attributed to friction behavior of the steering and suspension components. In this study, we focus on an approach to improve handling, steering feel and ride-comfort of a vehicle by applying the appropriate friction behavior to tie-rod end ball joint. To control not only friction coefficient but also static-kinetic transient behavior, we investigate the potential use of diamond-like carbon (DLC) coatings. Different DLC coatings varied widely in hydrogen content, mechanical properties and micro-surface roughness are applied to the ball studs. Friction behavior corresponds to material characteristics and surface roughness of DLC. The “polymer-like” DLC with higher hydrogen tend to show higher kinetic friction and remarkable stick-slip behavior at the static-kinetic transient period, whereas the “graphite-like” DLC with lower hydrogen show lower kinetic friction and smooth transient behavior. From dynamic evaluation results of actual vehicle, the “graphite-like” DLC gives an appropriate friction behavior for the ball joint, leading to an improved steering feel, vehicle stability and ride-comfort. In contrast, the excessive friction causes a sticky steering feel and an increased harshness with high frequency vibration. We discuss the effectiveness in friction behavior of DLC-coated ball joints depending on the DLC coating properties, with either positive or negative effects found for vehicle handling and ride-comfort.
