Graduation Year

2011

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Mechanical Engineering

Major Professor

Ashok Kumar

Keywords

Adhesion, CVD, Delamination, Surface, Wear

Abstract

Several studies have been propose to improve the adhesion of diamond films on cemented carbide tool materials, however a systematic study in identifying the role of the factors that affect the final diamond adhesion and the resulting machining performance of the tool under real manufacturing conditions is still unexplored. CVD diamond film's extraordinary qualities bring little benefit if the film fails to adhere sufficiently to the substrate. Inadequate adhesion undermines tool performance and longevity, causing unpredictable behavior under load and possibly leading to unexpected failure of the tool in the production line. This dissertation investigates the effects of different surface pretreatments on the adhesion and performance of CVD diamond coated WC-Co turning inserts for the dry machining of high silicon aluminum alloys.

Different interfacial characteristics between the diamond coatings and the modified WC-Co substrate were obtained by the use of two different chemical etchings and a CrN/Cr interlayer, with the aim to produce an adherent diamond coating by increasing the interlocking effect of the diamond film, and halting the catalytic effect of the cobalt present on the cemented carbide tool. A systematic study is analyzed in terms of the initial cutting tool surface modifications, the deposition and characterization of microcrystalline diamond coatings deposited by HFCVD synthesis, the estimation of the resulting diamond adhesion by Rockwell indentations and Raman spectroscopy, and finally, the evaluation of the dry machining performance of the diamond coated tools on A390 aluminum alloys. Scanning electron microscopy (SEM), metallographic analysis, and x-ray diffraction techniques were used to characterize the morphology, cobalt content, and nature of the substrate surfaces before and after each pretreatment; optical interferometry was utilized to characterize the surface roughness. After successfully diamond depositions, the films were characterized again using SEM, Raman spectroscopy, XRD, Electron Probe Microscopy Analysis (EPMA), and optical interferometry.

The experiments show that chemical etching methods exceed the effect of the CrN/Cr interlayer in increasing the diamond coating adhesion under dry cutting operations. This dissertation provided new insights about optimizing the surface characteristics of cemented carbides to produce adherent diamond coatings in the dry cutting manufacturing chain of high silicon aluminum alloys. An alternative method to measure the practical adhesion on commercial diamond coatings that can be correlated with the machining performance is proposed to be used when other methods (scratch, nanoindentation, microindentation, pin-on-disk, etc.) are not viable.

This research is the first comprehensive and systematic work that links the surface/subsurface integrity of cutting tools with their ability to produce an adherent diamond coating capable to dry machine high silicon aluminum alloys.

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