Evaluation of wear-resistant properties of microstructurally different PCD/PCBN
7.1 Characterisation of the benchmark electroplated pads
Electroplated pads with abrasive grits made of diamond and CBN have been selected as benchmarks for the wear tests of PCD and PCBN arrays.
The procedure for selection of benchmark specimens is widely described in Section 3.7.1. It is important, when performing comparative tests, to evaluate the similarity and differences between the tested PLA abrasive elements and the benchmark pads; for this reason, the characterisation of the benchmark was necessary for the evaluation of protrusion height and abrasive grit density and further comparison with the ultra-hard PLA generated arrays. This section reports the results for the characterisation
142 of the electroplated pads; its aims at understanding the main topographical/microscopic differences with the PLA generated arrays to allow a scientifically informed comparison between their performances in terms of wear and cutting forces as main output from the test.
7.1.1 Selection of the benchmark electroplated pads before the test
As already explained in Section 3.7.2, in order to benchmark the performances of the novel orderly micro-abrasive arrays, two types of electroplated abrasive pads (10mm x 10mm x 0.5mm) have been selected for a comparative test: an electroplated diamond abrasive (grade D501) shown in Figure 7. 1a, and an electroplated CBN abrasive (grade B501) depicted in Figure 7. 1b. In both cases, the substrate material is tungsten carbide in thickness of 1 mm (Figure 7. 2) and the bonding type is a Nickel electroplating.
a) b)
Figure 7. 1: ESEM image of the benchmark specimens before the test: a) electroplated D501 diamond, b) electroplated B501 CBN.
Diamond single grits
CBN single grits Matrix
Matrix
143 a) b)
Figure 7. 2: ESEM image (side view) of electroplated specimens showing the random height of the abrasive grits: a) electroplated diamond specimen; b)
electroplated CBN pad.
The sizes of the abrasives, graded to the Federation of European Producers of Abrasives (FEPA) standards, have an average nominal size of m Figure 7. 3b) with circa the same size/volume of the micro-abrasive/cutting edges to provide a reasonable comparison for benchmarked tests.
a) b)
Figure 7. 3: Example of ESEM image of a single CBN grit in an electroplated abrasive pad: a) tilted and b) top views.
Figure 7. 4 is representative of the different densities in the case of the micro-edges per array (Figure 7. 4a) and in the case of the grits per electroplated area (Figure 7. 4b). The reason of selecting different densities resides in the choice of a laser generated array which could enhance the wear performances in grinding/cutting applications.
Furthermore, the principle of perfectly spatially located features (Figure 7.
CBN single grit Diamond single grits
WC WC
Single grit tilted view
Grit edge
Grit edge d= 500 µm
144 4a) is not applicable for the electroplated specimen available in the market, where randomly oriented grits are typically bonded to a substrate in a disorder manner (Figure 7. 4b).
a) b)
Figure 7. 4: ESEM micrographs showing the variation in edges/grit density per same surface area of 1 : a) 12 micro-edges in the case of the array,
b) 7 grits in the case of the electroplated specimen.
7.1.2 Observation and discussion
The results from the optical analyses on the selected benchmark specimens before testing (Section 7.1.1) are essential for understanding how to approach the comparison of the forces produced during the wear test. In this regard, to calculate and compare the contact loads for the PCD abrasive array and the electroplated diamond pads, two main considerations are necessary: (i) the surface density of the orderly micro-abrasive edges (for orderly arrays) and the micro-abrasive grits (for electroplated elements) are significantly different; approximatively the ratio of 12:7 per mm2 for the orderly micro-arrays and for this reason, the measured forces during the wear test are not directly comparable; (ii) there is a geometrical variability in the grits (e.g. height, shape, orientation/angles of the active cutting edges) of the electroplated pads when compared to the repeatable micro-abrasive edges of the arrays.
Figure 7. 1 shows cross section micrographs for the electroplated specimens, indicating that the protrusion height of the grits is variable both for the electroplated diamond specimen and for the CBN one. It can be commented that the variability in height of the grits as well as edge orientation could affect noticeably the results of the cutting test because
Micro cutting-edge
Single grit
145 the number of grit in contact with the workpiece (i.e. shaft see Section 3.7.2) would be variable at different length of cut, affecting the acquired force signals and therefore producing an overall signal of lower intensity in comparison with the laser generated micro-cutting arrays (where the homogeneity in height of cutting edges will increase the number of features in contact with the shaft for the same length of cut). Also the density of grits per surface area is an important factor to be considered; in fact, because of the drop in grit density by 42 % for the electroplated pads (Figure 7. 4), less grits (compared to micro-cutting edges) are in contact with the shaft at the beginning of the test therefore representing the main grits responsible of the grinding/cutting process. To avoid problems related to differences in topographical characteristics of the PLA generated arrays and the electroplated specimen, a technical approach has been proposed in Section 3.7.3 for the comparison of cutting forces and wear progression characteristics of the array and corresponding benchmarks.