Thermal wave imaging of indented diamond coated WC
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Thermal wave imaging of indented diamond coated WC Albert Feldman National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 12 July 1996; accepted 6 January 1997)
Photothermal radiometry has been used to obtain thermal wave images in the vicinity of indentations in WC–6% Ni coated with chemical vapor deposited (CVD) diamond. Features in the magnitude and phase of the thermal signal profile are consistent with a one-dimensional thermal wave theory that assumes (i) an air gap extending well beyond the visibly observable indented region, and (ii) a thermal resistance interface between the diamond film and the substrate over the entire coated surface. The theory allows us to estimate the air gap thickness, which decreases as the distance from the indented region increases. Air gap variations of tens of nanometers appear to be easily detectable.
I. INTRODUCTION
Tungsten carbide cutting tools coated with chemical vapor deposited (CVD) diamond are becoming important commodities because the lifetime of a diamond-coated tool can be many times the lifetime of an uncoated tool.1 Unfortunately, delamination of the diamond coating due to poor adhesion will severely limit the usefulness of a coated tool.2 Indentation testing with a diamond indenter is commonly used to measure the adhesion of diamond coatings.3,4 However, in many instances, the full extent of delamination may not be readily observable by optical microscopy or electron microscopy. Thermal wave imaging is a technique that allows us to probe for structures beneath the surface of a coated specimen, such as a delamination.5 We have employed photothermal radiometry6 to obtain thermal wave images in the vicinity of indentations in WC –6% Ni coated with CVD diamond. The images reveal a structure beyond the immediate location of the indentation, suggesting a region of delamination. The spatial features of both the magnitude and phase of the thermal signal and their dependence on modulation frequency can be explained on the basis of a onedimensional thermal wave theory that assumes (i) an air gap between the diamond and the substrate extending well beyond the visibly observable indented region, and (ii) a thermal resistance interface between the diamond film and the substrate extending over the entire coated surface. The theory allows us to estimate the air gap thickness which decreases as the distance from the indented region increases. Air gap variations of the order of 10 nm in width are easily detectable. II. THE SPECIMEN
The specimen was a cylinder, 12.5 mm in diameter by 19 mm long, made of WC–6% Ni coated with CVD
diamond on one of the flat cylinder bases by a dc plasma torch method.7 Prior to the deposition, the substrate had been etched by a proprietary procedure in order to enhance adhesion of the diamond coating. The diamond film was nominally 20 mm thick. The coated surface was indented with a diamond brale indenter under a load of 450 N. The brale indenter has a conical shape with a
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