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Recent results of tribological properties of carbon nitride (CNx) coatings are reviewed. CNx coatings of 100 nm thickness were formed on Si-wafer and Si3N4 disks by the ion beam mixing method. Friction and wear tests were carried out against Si3N4 balls in the environments of vacuum, Ar, N2, CO2, O2 or air by a ball-on-disk tribo-tester in the load range of 80–750 mN and in the velocity range of 4–400 mm/s.It was found that friction coefficient μ is high (μ=0.2–0.4) in air and O2, and low (μ=0.01–0.1) in N2, CO2 and vacuum. The lowest friction coefficient (μ<0.01) was obtained in N2. It was also found that N2 gas blown to the sliding surfaces in air effectively reduced the friction coefficient down to μ&ap;0.017. Wear rate of CNx coatings varied in the range 10−9–10−5 mm3/N m depending on the environment.The wear mechanisms of CNx in the nanometer scale were studied by abrasive sliding of an AFM diamond pin in air. It was confirmed that the major wear mechanism of CNx in abrasive friction was low-cycle fatigue which generated thin flaky wear particles of nanometre size. [Copyright &y& Elsevier]

Wear particle generation in carbon nitride coatings by a spherical diamond counterpart in repeated sliding contacts has been studied with an emphasis on the effect of relative humidity varying from 0.4 to 80%.An environmental scanning electron microscope (E-SEM), in which a pin-on-disk type apparatus was installed, has in situ provided direct evidence that when and where the wear particle generation do occur. The in situ examination of non-conductive carbon nitride coatings are therefore available free from surface charging with controllable relative humidity and a sliding speed of 50 mm/s varying normal load from 10 to 250 mN.Based on the in situ examination, the shape transition maps for generated wear particles have been obtained for carbon nitride coatings in various relative humidity. The results show that the critical number of friction cycles (Nc) for detecting a cluster of wear particles is observed to be generally increasing with an increase in relative humidity.It also appears that the increase in relative humidity results in a decrease in peak-to-valley (P-V) values of contact tracks during the first friction cycle, which is essentially induced by a combined plastic deformation behavior of carbon nitride coated silicon surface. This further implies that the effective surface property such as hardness has been possibly changed by introducing water vapor into the contact interface between the carbon nitride coating and the spherical diamond counterpart, if assuming the contact pressure has a constant value. [Copyright &y& Elsevier]

The modes of wear of ceramics can be briefly described as mild wear and severe wear.For mild wear, tribochemical reaction forms a relatively soft and thin surface layer which is detached from the surface as thin flakes or dissolves in water. In addition to tribochemical wear, mild wear takes place by mechanically forming fine wear debris. They are smaller than the grain size by one-tenth or less and have the original chemical composition. Wear rate remains below 10−6 mm3/(N m) and the maximum surface roughness is in the order of one-tenth of the grain size.For severe wear, wear debris is formed by mechanical cracking of grains or delamination of tribo-films on the scale of the grain size. Wear rate is higher than 10−6 mm3/(N m) and the maximum surface roughness is in the order of the grain size.The mild and severe wear modes can be distinguished by the ceramic wear map in terms of the mechanical severity of contact Sc,m and the thermal severity of contact Sc,t. [Copyright &y& Elsevier]

With a pin-on-disk type apparatus installed in the chamber of an environmental scanning electron microscope (E-SEM), the motivation of this paper is to show the validity of the in situ examination to study the generation of wear particles of non-conductive carbon nitride coatings on silicon substrates under controllable relative humidity.Two typical shapes of generated wear particles can be observed in the light of in situ examination of contacting surfaces in the “wear particle generation” regime following the “no wear particles” regime. The observed two typical shapes are “feather-like wear particles” and “plate-like wear particles”, respectively. A shape transition map of wear particle generation as a function of normal loads and friction cycles is then obtained. The critical friction cycles Nc for detecting a cluster of feather-like wear particles is therefore determined from “no wear particles” to “feather-like wear particles” by the shape transition map at different applied normal loads, and is also used as the maximum value for the “no wear particles” regime for comparison.In light of the above approach to wear particle generation, the validity of using in situ examination makes further study of the effects of other important factors, such as the nitrogen incorporation conditions, coating thickness and the environments, on wear particle generation in carbon nitride coatings possible. [Copyright &y& Elsevier]

The frictional behavior of carbon nitride coatings on silicon substrates repeatedly sliding against a spherical diamond is presented. The motivation is to clarify the coating thickness effect on friction with relation to applied normal load and repeated friction cycles. Repeated friction tests on silicon substrates with sputtered carbon nitride coatings of various thickness were conducted using a pin-on-disk type apparatus under a controlled environment. The results indicate that the maximum value of friction coefficient occurs around certain coating thickness with no relation to applied normal load. Considering the nano-scale sub-asperities on the tip of the diamond counter-face and the ratio of substrate to the coating hardness, the results from the experiments are favorably comparable to the results obtained with a contacting model proposed by Halling and Sherbiney . [Copyright &y& Elsevier]

Abstract: Ion beam assisted deposition (IBAD) techniques have been performed to synthesize ternary compound films composed of boron, carbon and nitrogen atoms (BC x N y ). BCN coatings were deposited on Si wafers and Si3N4 disks by evaporating B4C target, with simultaneous N ion bombardment. The composition and chemical bonding were analyzed by XPS and the nanohardness was measured by Nano Indenter. The friction and wear property of the BCN coatings against Al2O3, SiC, Si3N4 and SUS440C balls was investigated in air, N2 gas and water, respectively. The worn surfaces on the BCN coatings and the balls were observed by optical microscope. The results showed that, the atomic ratio in the BCN coatings was 49at.% B, 42at.% C and 9at.% N and there were several bonding states such as B–N, B–C and C–N with B–C–N hybridization in the BCN coatings. The nanohardness of the BCN coatings was 33GPa. As the BCN coatings slid against ceramic balls, the friction coefficients in air were largest, while those in water were smallest among three kinds of environment. But as the BCN coating slid against the SUS440C ball, the friction coefficients in N2 gas were slightly higher than those in air, whereas the friction coefficients in water also exhibited the lowest values. Among four kinds of tribo-pairs, the BCN/Al2O3 tribo-pair showed the largest friction coefficient of 1.13 in air, but the BCN/Si3N4 tribo-pair exhibited the smallest friction coefficient of 0.64 in N2 gas and 0.03 in water. The specific wear rates of the BCN coatings were largest in N2 gas, while lowest in water in all cases. The observation of worn surfaces on the BCN coatings and balls showed that the many roll-like materials were produced during sliding tests in air and N2 gas, while the smooth surfaces were formed in water. The experimental results indicated that the interface reaction between the BCN coatings and water or oxygen had many influences on the friction and wear property of the BCN coatings. [Copyright &y& Elsevier]

Abstract: A new experimental apparatus which can evaluate particle erosion at high temperature is proposed. This apparatus has the following characteristics:[•] Particles are accelerated by centrifugal force of a rotating nozzle. So, particle impingement velocity is easily adjusted by rotating speed of the nozzle. [•] Nine test pieces can be supplied for one test, and temperature of the test pieces is controlled independently. So, erosion data under various temperature conditions can be obtained in only one test. [•] A test piece is heated by a cartridge heater directly. So temperature of test pieces rise in short duration, and electric power required for erosion tests is lower than blast type high temperature erosion apparatuses. To confirm that the new apparatus can obtain similar erosion data to the data obtained by conventional apparatuses, some preliminary experiments have been performed. As a result, it was confirmed that the new centrifugal erosion apparatus can obtain similar erosion rates and velocity exponents to the data obtained by conventional erosion apparatuses. The erosion rates of steels containing various amount of Cr obtained by the new centrifugal erosion apparatus showed similar tendency to the data obtained by Levy and Man [A.V. Levy, Y.-F. Man, Erosion–corrosion mechanisms and rates in Fe–Cr steels, Wear 131 (1989) 51] using a blast type erosion apparatus. [Copyright &y& Elsevier]