Development of Lightweight DLC Coated Valve Lifter Made from Beta Titanium Alloy for Motorcycles

An ultra lightweight valve lifter made from titanium alloy was developed for high performance motorcycles. A beta titanium alloy that was fulfilling not only higher strength but also cold forgeability was selected among several types of titanium alloys. A solution treatment and aging combined with the cold forging made a refinement of the grain size of beta phase. As a result, a tensile strength and an elongation of the alloy were reached to 1170MPa and 10.3% respectively. During the solution treatment, oxygen diffusion (OD) treatment was also performed on the beta titanium alloy. The surface hardness of OD layer was increased up to as high as 600 HV. Subsequently, a diamond like carbon (DLC) layer having a hardness value of around 2500HV was formed on the titanium substrate in order to provide a superior tribological property. In between the DLC layer and the substrate, a sputtered Ti layer was formed as an intermediate adhesive layer. The intermediate layer resulted in preventing from any damage for example flaking or pitting of the DLC layer and realized a good bonding strength. The high durability and superior tribological properties were proved by endurance test with using an actual motorcycle engine. The developed valve lifter achieved more than 30% of weight reduction compared to a conventional steel valve lifter.