Mechanical Properties and Uniformity of Nanocrystalline Diamond coating around Sphere

Nanocrystalline diamond coatings were deposited on spheres used for ball bearing. The nanocrystalline coatings with a grain size of 50nm were confirmed by the surface morphology and composition analysis. The hardness of the coating is 20-40GPa tested by nanoindentation, which is higher than that of tungsten carbide and silicon nitride substrates. The coating around the sphere observed from the Micro CT images is uniform with a thickness of 12μm.
keywords: solid lubricating coating, nanocrystalline diamond, mechanical properties

Introduction

Mechanical parts are often used under extreme environment such as high temperature, large load, radioactive and high vacuum, and so on. A wear-resistant, lubricating coating can protect the mechanical parts and ensure their reliability under these extreme conditions1, 2. The advantages of diamond coating with high hardness, high elastic modulus, outstanding wear resistance, low friction coefficient and good chemical stability make it to be an expected solid lubricating coating3, 4.
The protective coating, e.g. diamond like carbon (DLC) coating deposited on metals and some other materials can protect the interface of the metals from crack, but also reduce the frictional wear of the opposing surface due to the excellent tribological properties such as extremely low friction and wear resistance. Costa et al5. deposited (DLC) coating with a thickness of 2μm on silicon and carbonitride using pulsed-DC discharge and studied the tribological behavior of DLC coating. Their results showed that the increase in surface roughness reduced the friction coefficient, and wear rate of the carbonitride as the interlayer decreased three orders compared to that of silicon. Xie et al6. grew DLC coating with 600nm thickness on silicon wafer using microwave plasma chemical vapor deposition (MPCVD). It seemed that surface roughness, adhesion and debris accumulation collectively affected the frictional behavior while the tribological behavior of DLC coating mainly depended on the coating and its adhesion to the substrate. Gruen et al7. successfully deposited the nanocrystalline diamond coating with average grain size of 5-13nm on silicon at 750℃ by MPCVD. After that, there were much investigation of the nanocrystalline diamond coating, but the nanocrystalline diamond coating grown on the spheres are very few. B Lunn et al8. from Hull University deposited micro diamond coating with thickness of 3μm on a sintered carbide (6%Co) ball of 15mm in diameter with a special support system in a hot filament chemical vapor deposition (HFCVD) chamber.
The present work focused on that nanocrystalline diamond coatings were deposited on the sintered carbide spheres and silicon nitride spheres used for ball bearing to improve the wear-resistance. The mechanical properties and uniformity of the coating were evaluated by Micro CT and nano indenter.

Experimental

By rotating the substrate holder, uniform diamond coatings around spherical substrates with 1-3mm diameter were deposited by a lab-made MPCVD reactor. Tungsten carbide (WC-6 wt.% Co) spheres and silicon nitride spheres were pitched up as the substrates. The cobalt as the adhesive of tungsten carbide would convert the diamond into graphite, resulting in decrease in adhesion between coating and substrate. So firstly diluted nitric acid was used for processing the tungsten carbide spheres in order to selectively remove the cobalt of the surface9. Then, the spheres were scratched using 1-10μm diamond powders by ultrasonic method, and rinsed in alcohol and dried prior to deposition.
The nanocrystalline diamond coating was deposited for 20-60h at following parameters: total gas pressure was 4KPa, microwave power was 1400W, the substrate temperature was 870℃, 2.2% methane diluted in hydrogen. The Raman spectroscopy (LabRAM HR, HORIBA Jobin Yvon S.A.S, France) with a laser as light sources (wavelength 532nm) was used to analyze the quality of diamond coating on different substrates. The surface morphology was investigated by scanning electron microscopy (SEM, JSM-7500F, Hitachi, Japan) to measure the crystalline grain size. An atomic force microscopy (AFM, MFP-3D, Asylum Research, USA) was applied for quantitative the surface roughness determination on a 20×20μm scanned area. The mechanical properties were measured by MTS nano indenter (G200, MTS, USA) at an approach velocity of 5nm/s. The thickness and the uniformity of the diamond coating were investigated by Micro CT (μCT100, SCANCO, Switzerland).

Results and discussion

3.1. surface morphology
The surface morphologies of coatings deposited on different substrates can be seen in Fig.1a and c respectively, and b and d are the high magnification of images. Both the samples were treated under the same conditions. It is evident that there is no big difference between coating deposited on tungsten carbide sphere and silicon nitride sphere. The obtained coatings on both substrates have cauliflower structure with a grain size of about 50 nm.

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The surface roughness is very important for solid lubricating application where a smooth coating surface can decrease the frictional wear. Table 1 shows the change in roughness due to the thickness change of coating on tungsten carbide using AFM method. The roughness of the coating followed the trend in thickness, which was increasing with the rise of the thickness of coating. The roughness of the coating with 5μm thickness was under 150nm. Both RMS roughness and the average (Ra) roughness were between 100nm and 210nm lower than the peak-valley (P-V) roughness. The latter had higher roughness values in the order of one micron, which accounted for the cauliflower structure on the surface of the coating as shown in Fig.1. The rough surface does harm to the solid lubricating application. So the roughness will be decreased through post treatment e.g. chemical mechanical polishing.
3.2. Uniformity and thickness
The small sphere makes it hard to measure the thickness and the uniformity of the diamond coating. SEM image of the cross section is usually used to show the thickness and uniformity of the coating. However, only one intersecting surface is observed, which can’t represent the whole sphere. Micron CT can get a 3D image of the coating and directly give the whole feature of the coating. Because the metal absorbs the X-ray, the coating on silicon nitride which is inorganic material was measured.
Fig.2 is the CT image of the diamond coating around sphere. Fig.2a and b are the 2D and 3D images of the sphere and c is the 3D CT image of the shell whose silicon nitride substrate is removed through analysis software simulation. As the images shown, the coating is uniform and no obvious protuberance on the surface can be observed. The cross section of coating in Fig.4a indicated that the concentricity between substrate and coating was maintained to assure uniform coating thickness. No separation between the silicon nitride substrate and coating was observed, suggesting that the diamond coating attached the sphere tightly. Fig.3 shows the thickness distribution of the coating. The thickness of coating is between 10 to 14μm among which 12μm is dominant.
3.3. Composition
CVD diamond coatings with different thickness were characterized by Raman spectroscopy as shown in Fig.4, a and b were the coating with 5μm thickness, and c and d were the coating with 12μm thickness. The peak at 1332cm-1 is the characteristic of the diamond lattice which can be used to identify diamond. Two sharp peaks at 1337.87cm-1 and 1333.64cm-1 in Fig.4a and b proved that the composition of coating was in relation to diamond. Both of the two peaks have frequency shift caused by the compressive stress10. This accounted for the mismatch of the thermal expansion coefficient between diamond and substrate. Especially, the value of the tungsten carbide(4.36×10−6/°C, 20°C) is larger than 1.18×10-6/°C (20°C) of diamond，resulting in the far more upshift of the tungsten carbide shown in Fig.4a. The value of silicon nitride (2.8×10−6/°C, 20°C), which is close to that of diamond, produced less residual compressive stress. With the thickness of coating increasing, there was almost same frequency shift shown in Fig.4c and d. Compared with Fig.4a and b, the diamond peak of the thicker coating has a large upward shift that attributed to the increase in compressive stress with thickness increasing. The compressive stress is also related to other factors such as defects, composition of coating. The stress from defects and composition appeared to be dominant in thick coating.
The features at 1145cm-1 and 1490cm-1 are possibly related to acetylene C–H chains proposed by R. Pfeiffer11 and his colleague. Their study considered this acetylene C–H chains existed in the boundaries of nanocrystal diamond. Those bands around 1140cm−1 and 1490cm−1 were usually observed in nanocrystalline diamond coating. So Fig.4a and b confirmed the deposited coatings were nanocrystalline diamond, which is consistent with the result of the SEM. In addition, the coating got flexibility to fit curved surface of sphere because of the acetylene C–H chains in coating.
In Fig.4c and d, the peaks at 1580cm-1 is labeled as G peaks which are due to the sp2 sites. Compared with Fig.4a and b, the G peak of the graphite is obviously observed on Fig.4c and d. Although G peak at 1560cm-1 possibly overlapped the peak at 1490cm-1, it was obvious that the composition of the thicker coating was different from that of the thinner coating which affected by substrate to some extent. The band at 1146cm-1 is related to nanocrystalline diamond as discussed above.
3.4. Mechanical properties
The modulus and the hardness of diamond coatings were characterized by the nano indenter designed by the MTS Company. The sphere was too small to find an applicable flat surface to get an accurate result. The diamond coating deposited on silicon wafer was prepared with the same conditions as the control.
As known to all, the hardness and the modulus of the diamond coating prepared by CVD are normally lower than that of the natural diamond. The Fig.5a and c show the modulus and the hardness of the diamond coating deposited on sphere, while the Fig.5b and d exhibit the modulus and the hardness of the diamond coating on silicon wafer deposited in same conditions. The hardness of the coating on sphere was about 20GPa, only a half of that on silicon wafer, and the modulus was only one third of that on silicon wafer. The curved surface and cauliflower structure of the coating on sphere led to lower hardness and modulus measured. The true hardness and modulus of the coating should be higher than that of the measured. In terms of the measured value on silicon wafer, the hardness of coating on sphere was estimated to be 20-40GPa and the modulus was 200-600GPa. Therefore, the diamond coating was expected to improve the wear-resistance of tungsten carbide and silicon nitride substrates whose hardness are about 17GPa and 15.6-9.8GPa respectively11, 12. The modulus of coating also increased in comparison with that of silicon nitride substrate. It suggests that the mechanical properties of both the tungsten carbide and silicon nitride are improved for its ball bearing application.

Conclusion

For the purpose of protecting the spheres used for ball bearing, the diamond coatings were successfully deposited on the spheres. The coating is about 5-12μm in thickness depending on the deposition time and is uniform as the result of the Micro CT shown. The surface of coating is not smooth enough due to its cauliflower structure and needs further polish. The hardness tested by the nano indenter was 20-40GPa larger than that of tungsten carbide and silicon nitride. The Raman spectra reveal that the coating deposited on sphere is composed of diamond, acetylene C–H chains and graphite, which are responsible for the improvement of mechanical properties and fitness around sphere.