Measurements of Coating Thickness of Diamond-coated Tools by White-light Interferometry

In this study, the coating thickness of diamondcoated tools was evaluated and investigated by non-contact measurements. Commercial cobaltcemented tungsten carbide (WC-Co) cutting inserts (edge radii from 5 to 80 microns) were used as substrates for varying CVD-diamond depositions. A white-light interferometer (WLI) was used to acquire the surface profile data around cutting edges. MATLAB algorithms were generated to further analyze the tool geometry using the data cloud obtained from the WLI. The characteristics of a cutting edge (the edge radius, the rake/flank surfaces, and the wedge angle) were then obtained by curve fitting. This methodology was applied to both uncoated and coated tools for the geometry characterizations. To evaluate the coating thickness, the fitted uncoated and coated profiles (from the same tool) were overlapped with the edge rounding centers kept at the same location. The coated profile was then rotated to have the rake face parallel to the uncoated one (as a reference). The normal distance between the two tool profiles (uncoated vs. coated) was used for coating thickness estimates. The combination of WLI and the developed algorithm is capable of efficient coating thickness measurements. Tools with large edge radii had high uncertainties in coating thickness. INTRODUCTION Chemical vapor deposition (CVD)-grown diamond films have been increasingly explored for cutting tool applications as an alternative to costly polycrystalline diamond (PCD) tools [7]. Coating attributes such as the coating thickness significantly affect the coating-substrate interface stresses resulted from the deposition process. Therefore, accurate estimates of coating thicknesses, especially around the tool tip is important for quality control and process modeling. Presently, multiple options exist for coating thickness measurements. Many accurate methods, such as differential interferometry [5], stylus profilometry [10], scanning electron microscopy [11], involve scratching/cratering [1] or other destructive methods. Acoustic thickness measurements are non-destructive; however, the resolution may not be sufficient for the thicknesses used for diamond-coated tools [13]. Raman spectroscopy is a promising prospect, but presently, it is only used for ultrathin coatings on the order of nanometers [14]. Optical pyrometry has also shown favorable results, but it can only offer an average coating thickness over a large surface [2]. WLI has been applied to coating thicknesses measurements before; however, most cases have only employed a single measurement after the deposition process. This approach is limited by the need to precisely know the refractive index of the tool materials (both coating and substrate) in order to calculate the difference in distances traveled by light reflected from the coating and substrate surfaces [3]. Also, the method has only been applied to ultra-thin coatings [8] and has proven largely ineffective on opaque coatings [6]. The objective of this study was to investigate the coating thickness of diamond-coated cutting tools by WLI. WLI data was processed using generated algorithms which objectively evaluate the tool geometry. These algorithms were applied to individual tools before and after the diamond coating process. The results from each tool were compared and used to calculate diamond coating thickness. METHODOLOGY Experimental Setup A white light interferometer, NT1100 from Veeco Metrology, was used to collect surface data around cutting edges using the vertical scanning interferometry mode. An objective lens of 50X and a 0.5X field of view lens were used. Commercial WC-Co cutting inserts of square shape were used (SPG422): 12.7 mm width and 3.1 mm thickness. The nominal corner radius was 0.8 mm and the wedge angle was 79°. Four separate tool sets (coded A through D) of various edge radii (A: 18 μm, B: 39 μm, C: 80 μm and D: 4 μm), each including five individual samples was tested. Straight cutting edges were measured, 4 edges for each tool sample. A single scan mode was used. In addition, the tools were subsequently diamond coated with 3 different thicknesses, and then further measured by the WLI. Edge Measurements An example of a cutting edge surface acquired by the WLI is shown in Figure 1. The instrument software (Vision) was also used for initial data processing including a low pass filter and data restoration to interpolate and replace any missing data. Once finished, the data file can be processed using MATLAB. FIGURE 1. A cutting edge image from