Yang Alternatives to hard chromium plating on piston rods Degree Project of 30 credit points
As a main part of hydraulic cylinder, the piston rod is used in tough surrounding and corrosive condition; consequently a high quality protecting layer is essential. Currently EHC, electroplating hard chromium (hard chrome) is a widespread method for piston rod due to powerful performance and low cost. Unfortunately the plating process contains hexavalent chromium (Cr) which is toxic and carcinogenic; perhaps it will be prohibited in the nearing future. Therefore it is crucial to seeking alternatives to hard chrome for piston rod manufacturer. Through extensive searching for alternatives to hard chrome, the competitors like HVOF, electroplating tungsten plating and stainless steel linear were studied in the present work. Since microhardness and corrosion resistance are top priorities, after comparison perhaps Cr3C2-NiCr20% HVOF is the most promising technique. Distinct pretreatments Cr3C2-NiCr coatings were compared with hard chrome in pendulum impact test, hardness test and acetic acid salt spray (AASS) test. The results indicate that hard chrome show higher toughness more than 40% compare with HVOF, the microhardness of HVOF is higher than hard chrome, mean value 1784 Vs. 1244 HV and both coatings have excellent bonding strength. After 40 hours in AASS test, the hard chrome was rated No. 10, however the HVOF just No. 6.
Thin hard coatings fracture propagation during the impact test
One of the most significant applications of the impact test is to investigate thin hard coatings fatigue properties. Herein the test conditions and duration up to the film damage initiation, are considered in order to determine the critical stresses associated with the coating fatigue strength. Moreover, the subsequent film damage propagation is a significant mechanism as well, since it refers to the ability of the coating to withstand loads after its fatigue damage initiation. In order to describe the film fracture propagation the failed area ratio was introduced and an algorithm to determine this magnitude developed, based on the analysis of imprint scanning electron microscopy graphs. The application of the coating failed area ratio will be demonstrated in various impact test film cases, also with superficial thin layers. The top layers do not affect, in general, the failure initiation of the basic coating. However, according to the obtained results, the superficial films influence the basic coating failure propagation rate during the impact test.
Surface damage of SiC(W)/SiC by abrasion and indentation
Abstract Scanning electron microscopy has provided direct evidence that microcracking occurs during the room-temperature abrasion and indentation of the SiC(W)/SiC composite. Based on a comparative study of five kinds of surface morphology, i.e. abraded surface, abraded and etched surface, fractured surface, polished surface and indented surface, examination of 5% SiC whisker-reinforced SiC specimens showed that the occurrence of microcracking is a consequence of grain boundary cracking resulting from the nature of this normally brittle material under pointed diamond indenters and irregularly shaped SiC abrasive particles. However, the feature of transgranular fracture and plastic deformation can also be observed on the fractured surface and worn surface respectively. A preliminary link between the microdamage to the abraded surface and the radial and lateral cracks on the indented surface is discussed.