Keiji Nakayama

Triboemission from various materials in atmosphere

Abstract The triboemission characteristics of both negatively and positively charged particles from various materials such as metals, ceramics and glass were studied using an apparatus specially developed for measuring the triboemission of charged particles, including triboluminescence characteristics, even in atmosphere. The results obtained during scratching of the materials with diamond styluses having various vertical angles and tip radii in the atmosphere showed intense dependence of the emission characteristics of the charged particles and photons on materials, showing increasing emission intensity with increasing electrical resistance of the materials in the order conductor, semiconductor, insulator. Negatively charged particles were much more Intensively emitted from conductive and semiconductive materials. In non-conductive materials, both charged particles were emitted, with a slightly higher intensity of negatively charged particles.

Plasma generation in a gap around a sliding contact

It has been hypothesized that plasma is generated at a sliding contact. However, it has not yet been found. Here, we report on a discovery of plasma generated in the microscopic gap around a sliding contact, having an elliptical shape with a horseshoe pattern and with a size beyond a hundred micrometers. It emits mostly invisible ultraviolet (UV) photons and, to a lesser extent, infrared (IR) photons. It must be a main source of the curious tribophysical and chemical phenomena. The origin, characteristics and the relation to these curious phenomena are discussed.

Experimental investigation of the superfinishing process

Abstract The triboemission of charged particles and photons during scratching of ceramic materials of A12O3, ZrO2 and Si3N4 with a diamond stylus has been measured in saturated gas atmospheres of various hydrocarbons (methane, ethane, propane,n-butane andiso-butane), as a models for mineral lubricating oils, under a pressure of 1 × 105Pa. The triboemission of charged particles and photons was detected during wear of ceramics in the hydrocarbon gas atmospheres. The emission intensities of the negatively and positively charged particles and photons decreased as the number of carbon atoms in the hydrocarbon molecules increased, while the friction coefficient was independent of the number of carbon atoms. The dependence characteristics of the triboemission phenomena on the hydrocarbon series are discussed using a model of the gas discharge and mobility of ions in the contact zone.

Triboemission of Charged Particles and Photons from Wearing Ceramic Surfaces in Various Gases

Triboemission of charged particles and photons from Si3N4, Al2O3, ZrO2, and glass was investigated under He, Ar, N2, O2, dry air, and wet air atmospheres using a specially-developed triboemission measuring system for charged particles and photons. The results demonstrated that charged particles and photons were emitted simultaneously from wearing contact. Negatively charged particles were detected more intensively than positively charged particles from all ceramic materials and glass in every gas atmosphere tested. The charge intensity ratios of negative and positive particles were almost equivalent to mobility ratios of negative ions to positive ions. The emission intensities of both negative and positive particles decreased linearly with higher Vickers hardness, depended on gas, and linearly increased with an increase in the ionization coefficient. It was concluded that the triboemission of charged particles and photons from insulating ceramics originated from gas discharge through dielectric breakdown ...

Triboemission, tribochemical reaction, and friction and wear in ceramics under various n-butane gas pressures

Abstract The triboemission behaviour of negatively and positively charged particles and photons generated by scratching a Si 3 N 4 disc with a conical diamond stylus was studied under various gas pressures of n-butane as a model compound of typical paraffinic lubricating oil. The triboemission behaviour of photon emission in a frictional system of an Al 2 O 3 ball sliding on Al 2 O 3 and Si 3 N 4 discs was also measured under various n-butane gas pressures. Further, the friction and wear in the frictional system of an Al 2 O 3 ball sliding on an Si 3 N 4 disc were studied under various n-butane gas pressures. The triboemission intensities of three kinds of energetic particles, negatively and positively charged particles and photons, become a maximum at a particular n-butane gas pressure. When the n-butane gas pressure is such that triboemission is a maximum, the formation of friction polymer-like high molecular weight products is at a maximum, and friction and wear of the ball decreases to a minimum. It has been concluded that the friction polymer-like high molecular weight products are formed through tribochemical reactions triggered by triboemission phenomena and help reduce the friction and wear of the solids.

TRIBOEMISSION OF CHARGED PARTICLES FROM VARIOUS SOLIDS UNDER BOUNDARY LUBRICATION CONDITIONS

Abstract Triboemission under boundary lubrication conditions from various solids has successfully been measured. The dependence of triboemission intensity on time, normal force and sliding velocity was studied for negatively and positively charged particles emitted while scratching various metals, ceramics and polymers with a diamond stylus under boundary lubrication with saturated hydrocarbon liquids. Negatively charged particles were detected more intensely than positively charged particles for all solids tested except PTFE. The emission intensity increased with normal force and sliding velocity, and was high for insulating solids such as ceramics and polymers, while low for metals. The emission intensities under boundary lubrication were lower than under dry friction in room air, although the friction coefficients were comparable. The results are discussed in relation to the emission characteristics under dry sliding in room air.

Effect of Surrounding Gas Pressure on Triboemission of Charged Particles and Photons from Wearing Ceramic Surfaces

Triboemission of charged particles and photons during scratching of Al2O3 surfaces with a diamond stylus was investigated under various pressures of Ar, He and O2 gases. The results showed that the emission intensity of negatively and positively charged particles and photons peaked, dependent on the gas pressure at a pressure inherent to gas, while the friction coefficient was shown to be independent of gas type and gas pressure. The ratio of the charge intensity of the negative to positive particles was unaffected by gas pressure and the values were identical to those of the mobility ratios of the negative to positive, ions under any gas pressure. The three kinds of emission peaks for positive and negative particles and photons appeared at the same pressure inherent to gas. The peak values in charged particles decreased in the order O2 > Ar > He, while those of photon emission decreased in the inverse order of the charge intensity, i.e., He > Ar > O2. The gas pressure at which the emission curve peaked i...

Characteristics and Pattern of Plasma Generated at Sliding Contact

To verify the tribo-microplasma concept proposed by Nakayama, who suggested that a microplasma is generated in the gap of a sliding contact due to electrical discharge of the ambient gas in the electric field caused by tribo-charging, we challenged to observe directly the tribo-microplasma and to measure spectral characteristics of the emitted photons. In experiments to observe plasma image (with a diamond hemispherical pin sliding on a sapphire disk) it was found that the plasma was generated in the several micrometer gap of the sliding contact. The plasma had a shape of an ellipse with a tail, surrounding the contact and spread to the rear of the sliding contact. The plasma image observed through the UV transmittable filter (UV image) had a horseshoe pattern, while the IR image had a shape of a ring on the ellipse. The strongest UV emission was observed in the center of the horseshoe pattern outside the sliding contact, while the IR photon image showed that the most intense emission occurred at the sliding contact. The electrical discharge origin of the photon triboemission was proved by comparing spectra of tribophotons with spectra of photons emitted from plasma by electrical discharge in parallel electrodes in various gases. The results showed that the spectra of photons emitted from the sliding contact and those of gas-discharge completely coincided for all gases tested, i.e., dry air N 2 , O 2 , H 2 , He, CH 4 , C 2 H 4 and C 3 H 8 , except peaks originated from the excited atoms of the sliding surfaces. It was concluded that microplasma is produced by electrical breakdown of ambient gas at sliding contact. These results corroborate the tribomicroplasma concept.

On the possible role of triboplasma in friction and wear of diamond-like carbon films in hydrogen-containing environments

Hydrogen-free diamond-like carbon (DLC) films (both amorphous (a-C) and tetrahedral amorphous carbon (ta-C)) suffer high friction and severe wear losses when tested in inert and/or high vacuum environments. However, they provide anomalous superlow friction and wear coefficients in the presence of hydrogen gas, water vapour and alcohol molecules in the test environment. In this paper, we used such films in a systematic study to further confirm that hydrogen indeed plays an important role in their friction and wear behaviours. To study the effect of hydrogen, we conducted sliding tests in a hydrogen-containing test chamber and analysed the chemistry of their sliding contact surfaces using a time-of-flight secondary ion mass spectrometer. Clearly, the sliding contact regions of the carbon films became very rich in hydrogen after tribological tests in the hydrogen-containing chamber. In an attempt to understand the fundamental tribochemical mechanisms involved, we performed additional tests on these DLC films using a highly instrumented tribometer that permitted us the visualization of triboplasmas generating at or in the vicinity of the sliding surfaces. In this test system, we confirmed the formation of a triboplasma inside the contact area of the DLC films as evidenced by the characteristic UV radiation. Based on these observations, we believe that the formation of such triboplasmas within the contact zones of these DLC films may have triggered unique tribochemical reactions between hydrogen and carbon atoms on their sliding surfaces and thus resulted in very low friction and wear during tests in hydrogen-containing environments.

Triboemission of charged particles and resistivity of solids

The relation between the triboemission intensity of negatively and positively charged particles and the resistivity of solids was investigated for various solids including metals, ceramics and polymers. It has been found that the triboemission intensity increases steeply and the ratios of the charge intensity of the negatively to positively charged particles decreases gradually with an increase of the resistivity of the solids. These results show that the resistivity of solids governs the triboelectromagnetic phenomena and that the emission mechanism and the characteristics shift from those of conductors through semi-conductors to insulators with the increase of the resistivity of solids.