In this paper, by R. Ikeda, H. Ogi, T. Ogawa and M. Takemoto, the mechanical properties (elastic stiffness and fracture strength) were measured of polycrystalline diamond film with different structure synthesised by chemical vapour deposition method. Advanced new methods for the evaluation were utilised. Elastic stiffness (C11, C12, C14) was measured by resonant-ultrasound spectroscopy. The stiffness changed by grain boundary (GB) structure and chemistry. The stiffness C12 of cubic diamond film consisting of fine grain of 10 - 60 nm was found to be quite large due to the graphite phase existing along the GB. A spherical indentation test assisted by highly sensitive microcrack detection method utilising acoustic emission (AE) and corrosion potential fluctuation (CPF) was developed for the fracture strength evaluation. AE and CPF were available for thick and thin film, respectively. The fracture strength, estimated from stress distribution calculation by finite element method (FEM), changed from 4.0 GPa to 6.4 GPa depending on film thickness and grain size.

Nowadays polycrystalline diamond film synthesised by chemical vapour deposition (CVDD) is a indispensable material in manufacturing tools such as machining tools, wear resistant parts, manufacturing dresser tools etc. For those applications, mechanical properties of CVDD are important factors that strongly affect their functions, and are remarkably changed by the microstructure of the film such as grain size, orientation and grain boundary (GB) structure. Therefore, it is necessary to evaluate mechanical properties of the CVDD. Conventional methods, however, are scarcely available for CVDD due to its high hardness of the diamond, high film stiffness, and being difficult to process. In this study, the authors developed advanced new methods which can evaluate mechanical properties (elastic stiffness and fracture strength) of polycrystalline diamonds and studied the relation between microstructure and the properties of the film. Elastic stiffness was measured by the resonant-ultrasound spectroscopy coupled with laser Doppler interferometry (RUS/LDI). This system, developed by Ogi et al. [1-7], allows us to determine the stiffness coefficients: Cij of the thin film by the inversion process. We measured the diagonal coefficients, C11 and C44, and the off-diagonal coefficient C12 of the film, and discussed the relationship between the C12 and grain boundary chemistry based on the micromechanics model [8,9]. Fracture strength was measured by indentation test with spherical indenter. Kagawa et al. [10,11] developed a static fracture test of hard coatings by modifying the Hertzian fracture test [12]. In this test, detection of the first ring crack is of most importance and we developed a hybrid monitoring system using improved acoustic emission (AE) analysis method, classifying fracture mode and corrosion potential fluctuation (CPF). Elastic stiffness evaluation RUS/LDI method Fig 1 (a) shows a setup of the RUS/LDI method. A rectangular-shaped specimen was held by three edge-shaped supports. Two supports with piezoelectric transducers of 10-MHz resonant frequency were utilised for vibration excitation and detection. The support A oscillates the specimen and the support B detects the resonance frequencies. By sweeping the frequency of the driving signal from 50 to 1500 kHz and acquiring the vibrational amplitude as a function of the frequency, we obtain a resonant spectrum.

Results and discussion

Fig 6 shows corrosion potential fluctuation and load history for the test of MCD-21 and MCD-70. AE generation timings were also plotted near the curve. Mode-I AE designated by corresponds to the ring crack generation while Mode-II AE the interfacial shear crack or slip noise between indenter and film surface. For the thin diamond film (MCD-21), the potential shifted to active side quickly at F = 82 N and recovered slowly after staying for about 50 seconds. Similar potential shift was also observed at F = 194 N. We observed multiple ring cracks on the film surface after the test. Two potential shifts observed during loading correspond to the ring crack generations. The potential recovery is considered to be due to the crack closure by further indenting. The potential shifted to active side largely at F = 75 N during unloading. This occurred due to penetration of the solution into the ring crack again when the indenter departs from the film surface. No Mode-I AE or ring crack AE was detected for the test of thin MCD film. For thin film, released energy with crack generation is very small and AE is hardly detected, while the CPF can be easily detected because the crack opens quickly. Contrary to the thin diamond film, one Mode-I AE but no potential shift was detected for the thick diamond film (MCD-70). For thick film, the solution can not reach the substrate surface due to small opening of the crack. As the result, the corrosion potential does not show any shift. However AE was detected easily because large energy is released. We next evaluated fracture strength of three diamond films utilising FEM. Ring cracks were detected at F = 81.9 N (MCD-21) and F = 90.0 N (MCD-70). Surface stress distributions at ring crack initiation were computed by FEM and shown in Fig 7.Ring crack diameters (αm), determined by a microscope, were shown with hatched bar in the figure. The radii (α) at the largest tensile stress (σa) agree well with the radii (αm) of observed ring cracks. As a result, fracture strength of 6.4 and 4.0 GPa were obtained for MCD-21 and MCD-70 respectively. The difference in strength between MCD-21 and MCD-70 originate in grain size of the film. MCD film grows columnar style along normal to the substrate surface indicating that the thicker film has larger grains at the surface than those near the interface. Fig 8 shows the surface SEM images of MCD-21 and MCD-70 as deposition condition (before polishing). MCD-21 consist of 3 ~ 7 μm-grains and MCD-70 of 10 ~ 18 μm-grains at the surface. The largest dormant defect most likely to causes fracture in brittle material. Size of defect depends on grain size of polycrystalline diamond film and crack propagates along the cleavage plane of (111) in diamond crystal. MCD-70 shows lower strength than MCD 21 due to large grain size.

Summary

Mechanical characteristics of polycrystalline CVD-diamond film were studied by developing new methods. Results are summarised below.

1) Resonant-ultrasound spectroscopy with the laser-Doppler interferometry determined the independent elastic constants of MCD and NCD films, and we found that both the diagonal and off-diagonal coefficients (C11, C44) are close to those of the bulk diamond for MCD film and NCD film poses quite large off-diagonal component (C12) originating in graphite phase along grain boundary.

2) Fracture strength of MCD diamond films with different thicknesses were measured to be 4.0 to 6.4 GPa by spherical indentation test with AE and CPF, assisted with FEM analysis. The difference in strength could be explained considering the difference in grain size at the film surface generated by grain growth behaviour of CVD-diamond. We studied method to detect the first ring crack which is important for determination of the fracture strength. It was found that AE was available for thick film and CPF was available for thin film.