Exhibited Friction Coefficients Research Articles

Pulsed laser deposition of titanium oxide coatings at room temperature—structural, mechanical and tribological properties

Titanium oxide (TiO x ) coatings exhibit extraordinary optical, electrical and tribological properties, but very less is known about coatings deposited at room temperature. In the present work this gap was closed by employing the room temperature pulsed laser deposition (PLD) technique. In order to simulate different deposition conditions, the substrate (AISI M2, molybdenum) surfaces were situated parallel (‘on-axis’ geometry) and perpendicular (‘off-axis’) to the pure titanium targets ablated by the Nd:YAG lasers with a wavelength of 1064 nm. The combination of a four-beam PLD evaporator with a suitable movement of the substrates resulted in a high-rate film growth on large surfaces. The coating structure investigated by X-ray diffraction consists of a mixture of monoclinic β-TiO 2 and amorphous TiO x phase. The ratio of the amount of the phases, the hardness and the elastic constants strongly depend on the O 2 gas pressure and the target–substrate arrangement applied for deposition. Owing to the very high adhesion and their very smooth surfaces, the TiO x coatings exhibit friction coefficients of approximately 0.55 and wear rates of lower than 10 −15 m 3 N −1 m −1, turning out that titanium oxide coatings deposited by means of PLD are very promising candidates for wear protection applications.

Influence of valence electron concentration over friction coefficient of B2-based approximants

According to the valence electron concentration e/a criterion for approximants, some B2 superstructures may be regarded as a special group of approximants. The present paper reports on an investigation of the friction behavior of this group of phases. The results are compared with data specific to quasicrystals. We show that samples containing the B2 structure and its superstructures exhibit friction coefficients that decrease with increasing e/a and reach minimum at e/a=1.86, i.e. where the quasicrystal is stable. Therefore, the B2-based approximants and quasicrystals belong to a group of phases whose surface properties are mainly determined by electronic structure rather than crystal structure.

Influence of valence electron concentration over the friction behaviors of quasicrystal and B2‐type approximants in Al–Cu–Fe ternary system

Quasicrystals and their approximants are Hume–Rothery compounds having similar valence electron concentration. According to the valence electron concentration criterion for approximants, some B2 superstructures can be regarded as a special group of approximants. The present paper reports on an investigation of dry friction behavior of this group of phases. The results are compared with the data from quasicrystalline and related crystalline phases with similar composition. Specifically, we show that samples containing the B2 structure and its superstructures exhibit friction coefficients that decrease with increasing e/a and reach a minimum at 1.86, the value of the quasicrystal. Therefore, quasicrystals and B2‐based approximants belong to one group of phases whose surface properties are mainly determined by electronic structure characteristics rather than crystal structures.

Silicon Nitride Materials with Low Friction Coefficients

The use of silicon nitride materials in wear applications such as ball bearings or seals is often limited by the relatively high friction coefficient under nonlubricating conditions. Very fine homogeneous materials were produced using fine-grained {beta}-Si{sub 3}N{sub 4} powders. The microstructures of the materials were observed and correlated with the wear behaviour. The dry friction coefficients and wear were measured for self-mated sliding couples. The materials produced have a grain size of 0.1 {mu}m and exhibit friction coefficients of up to 0.1, whereas conventional materials have friction coefficients in the range of 0.5 under the same conditions. Reduced wear rates were also observed. (orig.) 6 refs.

Properties of materials and structural parts made by metal injection moulding: H. Cohrt et al (Schunk Sintermetalltechnik GmbH, Giessen, Germany)

In our present study, α-AlB12 and SiB6 powder compact disk specimens were prepared by spark plasma sintering α-AlB12 and SiB6 powders, respectively, and their X-ray diffraction patterns, microstructures, Knoop hardnesses were examined. Also, the friction and wear properties of the α-AlB12 and SiB6 powder compact disk specimens were investigated when they were slid against Si3N4 ball specimens in water. Both the α-AlB12 and SiB6 powder compact disk specimens exhibited friction coefficients lower than 0.1 while the blended elemental Al-92.3 at% B and Si-85.7 at% B powder compact disk specimens exhibited friction coefficients between 0.1 and 0.2. The specific wear rates of the α-AlB12 and SiB6 powder compact disk specimens were lower than those of the blended elemental Al-92.3 at% B and Si-85.7 at% B powder compact disk specimens, respectively.

Tribological Behavior of Amorphous Hydrogenated Carbon Films on Silicon

Unlike polycrystalline diamond films, amorphous hydrogenated carbon (AHC) films can be deposited at room temperature, are amorphous in atomic structure, and form very smooth surfaces. Amorphous hydrogenated carbon film consists of very small 10–20 Å sp2 bonded (graphitic) clusters captured in a largely sp3 coordinated, partially hydrogenated random network of covalently bonded carbon. Because of the extreme stiffness of the carbon-carbon bond, this hydrocarbon composite, less dense even than graphite, exhibits hardness rivaling that of the hardest ceramics. We report a systematic study of the tribological characteristics of AHC films deposited on silicon substrates by radio frequency plasma assisted chemical vapor deposition. The friction and wear behavior of these films in sliding contact with a steel ball without any lubrication was evaluated as a function of film deposition conditions, contact stress, sliding speed, sliding distance, and relative humidity. The friction coefficient and the wear of both the contacting surfaces were found to increase with relative humidity. At low relative humidity, (a) the films exhibited friction coefficients in the range of 0.05–0.16 under a contact stress ranging from 0.83 to 1.66 GPa and a sliding speed ranging from 0.03 to 1 m/s and (b) the wear rates of the film and the steel ball were significantly lower than that of other hard coating, such as TiN or TiC, evaluated under similar conditions. These results are very encouraging for some tribological applications of AHC films.

Friction and wear behavior of diamond films against steel and ceramics

Friction and wear tests were conducted at room temperature on diamond films deposited on silicon substrates by a microwave plasma-assisted chemical vapor deposition technique. Three different diamond film morphologies ( i.e. faceted, cauliflowered and smooth) were evaluated in sliding contact with steel, α-alumina, silicon nitride and magnesia-stabilized zirconia balls. In addition to the uncoated balls, a few tests were also conducted with silicon nitride balls coated with either a cauliflower-type or a smooth diamond film. All the material combinations, except for diamond films sliding against diamond films, exhibited friction coefficients in the range 0.38–0.90. The average friction coefficient for diamond films sliding against diamond films was 0.10. The faceted films exhibited higher friction coefficients and higher wear rates of the counterface materials than those for the smooth films. Examination of the worn surfaces of the diamond films and steel counterfaces revealed that the diamond films wore primarily by fracturing the tips of crystals. A secondary wear mechanism of delamination within the diamond films was also observed. The wear of the steel counterface occurred by abrasion.

Friction and wear properties of diamonds and diamond coatings

Abstract The recent development of chemical vapor deposition techniques for diamond growth enables bearings to be designed which exploit diamond's low friction and extreme resistance to wear. However, currently produced diamond coatings differ from natural diamond surfaces in that they are polycrystalline and faceted, and often contain appreciable amounts of non-diamond material ( i.e. graphitic or amorphous carbon). Roughness, in particular, influences the friction and wear properties; rough coatings severely abrade softer materials, and can even wear natural diamond sliders. Nevertheless, the best available coatings exhibit friction coefficients as low as those of natural diamond and are highly resistant to wear. This paper reviews the tribological properties of natural diamond, and compares them with those of chemical vapor deposited diamond coatings. Emphasis is placed on the roles played by roughness and material transfer in controlling frictional behavior.

Evaluation of Low Friction Materials for the Central Solenoid of the Compact Ignition Tokamak (CIT)

The status of the R&D program to evaluate low friction materials for use in the Compact Ignition Tokamak (CIT) is given. The goal is to provide material with a friction coefficient of 0.1 or lower at a 77 K interface between the central solenoid and the case of the toroidal field (TF) coil inner leg. The material must withstand 3000 machine pulses under a transverse compressive stress of about 207 MPa and be able to withstand relative surface motions of the order of 2 mm. A testing machine capable of providing 1.3 MN of compressive load and 0.44 MN of oscillating thrust in the direction of relative motion was prepared and is described in the paper together with the instrumentation and control system. Data can be taken at room temperature and at 77 K. Measured room temperature friction coefficients of the initial single stroke tests are plotted for twelve candidate materials as a function of compressive stress. All candidates exhibited friction coefficients below 0.1 for compressive stress above 172 MPa. Single stroke results on two candidates at 77 K showed friction coefficients higher than their room temperature values by a factor of about 3 at 44 MPa compression, with the coefficients tending toward their room temperature values as the compression was increased to 228 MPa. Of the seven candidates tested cyclically with a 5 mm stroke at 253 MPa and 77 K, only two types of Fiberslip materials met the requisite 3000 cycles. Friction coefficients for all materials are plotted as a function of the number of cycles.