FAILURE OF DIAMOND-LIKE CARBON (DLC) COATINGS IN AUTOMOBILE ENGINES – A REVIEW

Diamond-like carbon (DLC) coatings have become very attractive for various industrial applications, such as cutting tools, automotive engines, biomedical implants, micro-electromechanical devices (MEMS). Due to their surface energies and ability to interact with lubricants to form surface protective films, good adhesion with substrate, increased wear resistance, improved electrical conductivity, decreased internal compressive stresses during deposition and thermal stability there are used in automobile components. In the automobile industry, DLC coatings are usually applied on combustion engine components such as piston, tappet, camshaft, piston rings and gudgeon pin, valve stem and head and rocker arm. DLC coatings helps in reducing friction and wear of the moving parts. However, there are challenges facing the use of DLC coated components during service, which are; internal compressive stresses, low adhesion and low thermal stability leading to failures such as rolling contact fatigue, micro-pitting, delamination, oxidation and scuffing. Hardness and internal compressive stress increase with increasing sp3 content (sp3/sp2) ratio in DLCs. Internal compressive stress for DLC coatings > 1GPa in tribological applications is not good, due to the elastic strain energy that drives fractures along the coating/substrate interface, leading to delamination through blistering. The addition of non-metals (Si, N, F or O) or metals (W, Cr, Ta, Ti, Mo or Cu) can improve thermal stability of DLC up to about 500 oC. Above, 500 oC transformation of sp3 to sp2 begins to occur leading to graphitization. The addition of metals increases the interfacial fracture toughness and moderates the internal stress by creating two (2) interface; substrate/adhesion layer interface and adhesion layer/functional coating interface. This present paper will discuss the various failures that occur on DLC coatings such as internal compressive stresses, low adhesion and low thermal stability of non-metal and metal doped DLC coatings, regarding their applications in automobile engines. The effect of annealing conditions, tribological properties of non-metal and metal doped DLC, effect of sp3/sp2 ratio, and possible ways of reducing these failures on DLC coatings be discussed also.