Improvement on Fatigue Brittle Fracture Property of Al-Si Sintered Alloy by the DLC : The cure of the mesoscopic crack by DLC coating

Abstract

The improvement of the fatigue brittle fracture characteristics on DLC coated Al-Si sintered alloy was examined. Especially, healed effect of the damage by DLC coated was investigated in detail. The fatigue strength of DLC coated Al-Si sintering alloy has improved about 10%, and also in a fatigue brittle fracture region, the fatigue life was improved about 3 times compared to that of virgin material. From fracture surface analysis, the crack generating and crack progress mechanism in DLC coated differed from virgin material clearly. The crack of extraordinary length was observed on the surface. This crack was propagated along the surface of material in DLC coated. The meso-cracks of the length of about 1∿5μm were generated, and distributed in virgin material. The main crack was propagated in the direction of the inside of material. The causes of the healed effect by DLC coating were as follows; (1) The defect of the delamination between Si particles and Al matrix were covered with DLC. (2) The compressive residual stress existing in DLC coated suppressed generating of delamination between Si particles and Al matrix. (3) Since the crack was initiated by the delamination on the interface between Si particles and Al matrix in DLC coated, it can be regarded as an internal crack.. Therefore, a stress intensity factor becomes small as compared with virgin material.