Nanocrystalline Diamond Coating around Sphere Analysis

Nanocrystalline Diamond Coating near Sphere AnalysisMechanical Properties and consent of Nanocrystalline Diamond cultivation around SphereHongyun ChenNanocrystalline base fruitcake field covers were deposited on spheres apply for ball baby buggy. The nanocrystalline destinations with a grain size of 50nm were confirmed by the open morphology and composition analysis. The strainingness of the conclusion is 20-40GPa tried by nanoindentation, which is higher than that of double-u carbide and te nitride substrates. The stopping point around the sphere discover from the Micro CT images is uniform with a onerousness of 12m.keywords unwavering lubricating refinement, nanocrystalline baseball baseball field, mechanical propertiesIntroductionMechanical parts be often utilize under extreme environment such as high temperature, prodigious load, radioactive and high vacuum, and so on. A wear-resistant, lubricating coating can cling to the mechanical parts and ensure the ir reliability under these extreme conditions1, 2. The advantages of adamant coating with high toughness, high elastic modulus, outstanding wear resistance, outset friction coefficient and good chemical stability make it to be an judge solid lubricating coating3, 4.The protective coating, e.g. baseball diamond like carbon (DLC) coating deposited on metals and some other materials can protect the interface of the metals from crack, solely likewise reduce the frictional wear of the opposing dig up due to the excellent tribological properties such as extremely low friction and wear resistance. rib et al5. deposited (DLC) coating with a thickness of 2m on silicon and carbonitride using pulsed-DC discharge and studied the tribological demeanor of DLC coating. Their solvents showed that the increase in surface indention reduced the friction coefficient, and wear rate of the carbonitride as the interlayer reduced three orders comp ard to that of silicon. Xie et al6. grew DLC coa ting with 600nm thickness on silicon wafer using micro-cook plasma chemical vapor deposition (MPCVD). It seemed that surface roughness, adhesion and junk accumulation collectively abnormal 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 3m on a sintered carbide (6%Co) ball of 15mm in diameter with a special support system in a hot string 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 use d for ball bearing to improve the wear-resistance. The mechanical properties and uniformity of the coating were evaluated by Micro CT and nano indenter.ExperimentalBy rotating the substrate holder, uniform diamond coatings around world(a) 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 amid 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 1-10m diamond powders by ultrasonic method, and rinsed in alcohol and dried forward to deposition.The nanocrystalline diamond coating was deposited for 20-60h at following parameters total gas contract was 4KPa, microwave power was 1400W, the s ubstrate 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 scan electron microscopy (SEM, JSM-7500F, Hitachi, Japan) to measure the crystalline grain size. An atomic force microscopy (AFM, MFP-3D, bema Research, USA) was applied for quantitative the surface roughness determination on a 2020m 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 (CT100, SCANCO, Switzerland).Results and discussion3.1. surface morphologyThe 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 samp les were treated under the selfsame(prenominal) 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 withstand cauliflower structure with a grain size of about 50 nm.The surface roughness is very important for solid lubricating application where a smooth coating surface can decrease the frictional wear. display panel 1 shows the transform in roughness due to the thickness change of coating on tungsten carbide using AFM method. The roughness of the coating followed the drive in thickness, which was increasing with the rise of the thickness of coating. The roughness of the coating with 5m thickness was under 150nm. Both RMS roughness and the average (Ra) roughness were between 100nm and 210nm lower than the florescence-valley (P-V) roughness. The latter had higher roughness look ons in the order of whizz micron, which accounted for the cauliflower stru cture 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 word e.g. chemical mechanical polishing.3.2. Uniformity and thicknessThe small sphere makes it hard to measure the thickness and the uniformity of the diamond coating. SEM image of the hybridisation subdivision is usually used to show the thickness and uniformity of the coating. However, solo one intersecting surface is ascertained, which cant represent the whole sphere. micrometer gauge 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(p) 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 maintain 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 14m among which 12m is dominant.3.3. CompositionCVD diamond coatings with different thickness were characterized by Raman spectroscopy as shown in Fig.4, a and b were the coating with 5m thickness, and c and d were the coating with 12m 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 prove that the composition of coating was in relation to diamond. Both of the two p eaks throw frequency shift caused by the compressive essay10. This accounted for the mismatch of the thermal expansion coefficient between diamond and substrate. Especially, the value of the tungsten carbide(4.36106/C, 20C) is big than 1.1810-6/C (20C) of diamondresulting in the far more upshift of the tungsten carbide shown in Fig.4a. The value of silicon nitride (2.8106/C, 20C), which is close to that of diamond, produced little residual compressive sample. 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 adult 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 CH chains proposed by R. Pfeiffer11 and his colleague. Their study considered this acetylene CH chains existed in the boundaries of nanocrystal diamond. Those bands around 1140cm1 and 1490cm1 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 flexibleness to fit curved surface of sphere because of the acetylene CH 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 propertiesThe 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 fain 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 threesome 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 sh ould 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 semblance with that of silicon nitride substrate. It suggests that the mechanical properties of both the tungsten carbide and silicon nitride are amend for its ball bearing application.ConclusionFor the purpose of protecting the spheres used for ball bearing, the diamond coatings were successfully deposited on the spheres. The coating is about 5-12m 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 plenteous due to its cauliflower structure and needs further polish. The hardness teste d 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 CH chains and graphite, which are responsible for the improvement of mechanical properties and fitness around sphere.