Takayuki Tokoroyama

Effect of nitrogen atoms included in CNx coatings on friction sliding against Si 3 N 4 ball in nitrogen gas

Carbon nitride (CNx) coatings are thought to have good tribological properties because of low friction and high wear resistance. This material shows a friction coefficient lower than 0.01 when slid against a Si3N4 ball in dry N2 gas. The mechanism of superlow friction was believed to be due to the change in the topmost layer of the CNx surface in the wear track to a low-shear-strength layer by forming a graphite-like structure. However, the effects of nitrogen atoms in the CNx coating and the ambient N2 gas on the transformation of the surface layers of CNx remain unclear. In this work, we investigated the relationship between the friction coefficient and the nitrogen concentration in the wear track on the CNx coating. We measured the intensity of C KLL and N KLL on the wear track of CNx by Auger electron spectroscopy (AES), and conducted Raman analysis. We compared the N KLL/C KLL ratios of wear tracks from sliding in ambient N2, Ar, and O2. Only in the case of the wear track from material slid in N2, did the N KLL/C KLL ratio decrease to almost zero and the friction coefficient became lower than 0.01. On the other hand, in the Ar environment, the N KLL/C KLL ratio in the wear track was comparable to the as-deposited CNx coating, the friction coefficient became 0.03, and the IG position of Raman shift increased at most 3 cm-1. The N KLL/C KLL intensity of the wear track after friction tests in Ar and then in N2 was approximately 0.2 times as low as the as-deposited CNx. The friction coefficient in N2 gas was 0.03 when incorporated nitrogen remained in the CNx surface. On the other hand, the friction coefficient was lower than 0.03 when incorporated nitrogen was desorbed.

Effect of tetramethylsilane flow on the deposition and tribological behaviors of silicon doped diamond-like carbon rubbed against poly(oxymethylene)

In this study, silicon doped diamond-like carbon (Si-DLC) was deposited on stainless steel (JIS SUS304) by using surface wave-excited plasma (SWP). The effects of tetramethylsilane (TMS) flow on the composition, topography, mechanical properties and tribological behavior were investigated. Pin-on-disc tribo-meter was used to investigate the tribological behavior of the Si-DLC coating rubbed against poly(oxymethylene) (POM). The results show that the deposition rate, roughness of Si-DLC increased and the hardness of Si-DLC decreased with the increase of TMS flow rate from 2 to 4 sccm; the roughness increase therein led to the increase of ploughing term of friction. The increase of adhesion term was also seen with the increase of TMS flow rate, being attributed to the decrease of hydrogen concentration in the coating. It was considered that more POM transferred onto the Si-DLC deposited at higher TMS flow rate due to larger heat generation by friction.

The effect of DLC wear particles on friction under lubrication

Almost sliding parts in machine elements are immersed in lubricant to prevent wear, to reduce friction coefficient and to eliminate wear particles from sliding interface between two faces. Recently, automobile industries are required to use less lubricant for environment friendly, high normal load and low speed at the contact or sliding part which generates less amount of oil film thickness between faces and it causes severe friction condition. In this situation, carbonaceous coating such as Diamond-Like Carbon (DLC) is one of the attractive candidates to generate low friction under boundary lubrication because of its low friction property at dry. However, wear particles coming from DLC is believed to be a kind of hard slurry that scratches surfaces as abrasive and it shortens lifetime of DLC. Generally, these wear particles are collected by filters that 0.1 μm pore size is the smallest in commercial, therefore, nm scale wear particles are eliminated from the discussion. So, it is unclear that nm scale particles have affection onto tribological properties. In this study, we carried out friction test between the side surface of DLC coated cylinder against DLC coated disk under lubricant at boundary lubrication condition. The friction test was conducted 2000 cycles, then lubricant was replaced to new lubricant, friction test was continued at same contact point after the sequence. We carried out these procedures to reach to 12000 cycles, then wear particles were collected from lubricant by using electric field to gather nm scale particles. The lubricant was kerosene at room temperature. The wear particles were gathered near plus voltage electrode, then these particles stayed onto glass plate, then the lubricant was dried out. Finally, we conducted scanning electron microscope (SEM) observation and laser microscope observation to the particles.

The effect of ultraviolet light irradiation on tribological properties of hydrogenated DLC

The carbonaceous coating such as Diamond-Like Carbon is one of the promising materials as well tribological properties, especially low friction coefficient. The friction coefficient of hydrogenated DLC was reported not lower than 0.1 when it slid in lubricants. However, the hydrogenated DLC is easy to coat to the three dimensional mechanical parts, so it is required to decrease the friction coefficient of hydrogenated DLC in lubricants. The low friction property is important for hydrogenated DLC, however, it is worse to get the low friction property by the running-in process because of the complex procedure, long period to get low friction. So, it is required to obtain the frictional property by a substitute process. From the view point of reformation of the topmost surface of hydrogenated DLC, we focused on ultraviolet light. It is assumed that ultraviolet light will change the DLC topmost surface to graphite-like layer because the light intensity exceeds C-C bonding energy, and then generated dangling bonds will combine. We hypothesized that ultraviolet light irradiation could make the DLC coating topmost surface graphitic. We prepared the DLC coating irradiated ultraviolet light of 254, 312, and 365 nm wave length for 30 min. The C-C bond is 348 kJ/mol and the other ultraviolet lights are 469, 382, and 327 kJ/mol. Then these specimens and as-deposited DLC coating were tested in air to compare frictional property. DLC coating was synthesized with PECVD as 1.8 µm thickness on Si(100) substrate. Friction tests were carried out by a ball-on-disk type frictional tester in air, under lubricant, sliding against Si3N4 ball and SUJ2 ball which diameters were 8 mm.

Micro Texturing of Polycarbonate Plate with Ion Beam Irradiation and its Friction Properties in Water

In order to fabricate micro textured polymer plate, some kinds of ions were irradiated to the substrate by using an Electron Cyclotron Resonance (ECR) ion shower apparatus. Surface morphology of specimens was observed by SEM, and measured by AFM. We observed a lot of micro polymer brushes on the substrate. The surface roughness of untreated specimen increased and reached to 248 nm by N2 ion irradiation. We have used Argon, Nitrogen and Oxygen ions. Oxygen ion shows more effective to etch than the other two ions. Also we measured the contact angle of water on the surface of specimens with optical microscope. The contact angle of untreated specimen decreased and reached to 8 degree by N and O ion irradiation. After the irradiation, we conducted sliding friction test between ion irradiated polycarbonate slider and flat polycarbonate specimen. It was observed that friction coefficient of ion irradiated polycarbonate slider showed lower friction than flat polycarbonate specimen under mixed lubrication condition in water.

Effect of the Shear Strength of Coating of CNx on Friction Sliding Against Si3N4 Pin in Nitrogen

It was reported that CNx coating showed low friction coefficient less than 0.01 when it slid against Si3 N4 ball in dry N2 . It is suggested that the super-low friction was caused by the structural change of CNx surface to graphite-like structure during sliding in N2 . In this study, to investigate the effect of N2 gas on shearing and adhesive strength of CNx, micro scratching tests were examined in N2 and air with a diamond tip (radius = 2.0 μm). Main results are the followings: (1) CNx surface after sliding in N2 against Si3 N4 ball showed lower scratching friction coefficient in N2 than that in air. (2) Adhesive strength Si was estimated as 1/10 times lower than that in air.Copyright