Improvement of the Oxidation and Wear Resistance of Pure Ti by Laser-Cladding Ti<Subscript>3</Subscript>Al Coating at Elevated Temperature

Ti₃Al coating was in situ synthesized successfully on pure Ti substrate by laser-cladding technology using aluminum powder as the precursor. The composition and microstructure of the prepared coating were analyzed by transmission electron microscopy, scanning electron microscopy (SEM), and X-ray diffraction technique. Thermal gravimetric analysis was used to evaluate the high-temperature oxidation resistance of the Ti₃Al coating. The friction and wear behavior was tested through sliding against Si₃N₄ ball at elevated temperature of 20, 100, 300, and 500°C. The morphologies of the worn surfaces and wear debris were also analyzed by SEM and three-dimensional non-contact surface mapping. The results show that the Ti₃Al coating with high microhardness, high-temperature oxidation resistance, and high temperature wear resistance. The pure Ti substrate is dominated by severe adhesion wear, abrasive wear, fracture, and severe plastic deformation at lower temperature, and severe adhesion wear, abrasive wear, plastic deformation, oxidation, and nitriding wear at higher temperature, whereas the Ti₃Al coating experiences only moderate abrasive and adhesive wear when sliding against the Si₃N₄ ceramic ball counterpart. In addition, the wear debris of the laser-cladding Ti₃Al coating sliding and Si₃N₄ friction pairs are much smaller than that of pure Ti substrate and Si₃N₄ friction pairs at elevated temperature.     

Tribological behaviors and mechanisms of Ti<Subscript>3</Subscript>AlC<Subscript>2</Subscript>

Tribological behaviors and the relevant mechanism of a highly pure polycrystalline bulk Ti₃AlC₂ sliding dryly against a low carbon steel disk were investigated. The tribological tests were carried out using a block-on-disk type high-speed friction tester, at the sliding speeds of 20–60 m/s under a normal pressure of 0.8 MPa. The results showed that the friction coefficient is as low as 0.1∼0.14 and the wear rate of Ti₃AlC₂ is only (2.3–2.5) × 10⁻⁶ mm³/Nm in the sliding speed range of 20–60 m/s. Such unusual friction and wear properties were confirmed to be dependant dominantly upon the presence of a frictional oxide film consisting of amorphous Ti, Al, and Fe oxides on the friction surfaces. The oxide film is in a fused state during the sliding friction at a fused temperature of 238–324 °C, so it takes a significant self-lubricating effect.     

Microstructure and Tribological Properties of a HfB<Subscript>2</Subscript>-Containing Ni-Based Composite Coating Produced on a Pure Ti Substrate by Laser Cladding

A HfB₂-containing Ni-based composite coating was fabricated on Ti substrates by laser cladding, and its microstructure and tribological properties were evaluated during sliding against an AISI-52100 steel ball at different normal loads and sliding speeds. The morphologies of the worn surfaces were analyzed by scanning electron microscopy (SEM) and three-dimensional non-contact surface mapping. The results show that wear resistance of the pure Ti substrate and NiCrBSi coating greatly increased after laser cladding of the HfB₂-containing composite coating due to the formation of hard phases in the composite coating. The pure Ti substrate sliding against the AISI-52100 counterpart ball at room temperature displayed predominantly adhesive wear, abrasive wear, and severe plastic deformation, while the HfB₂-containing composite coating showed only mild abrasive wear and adhesive wear under the same conditions.     

Dry and oil-lubricated sliding wear of Si<Subscript>3</Subscript>N<Subscript>4</Subscript> and Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/BN fibrous monoliths

A high-temperature ball-on-flat tribometer was used to investigate dry and oil-lubricated friction and wear of sintered Si₃N₄ and Si₃N₄/hexagonal boron nitride (H-BN) fibrous monoliths. The friction coefficients of base Si₃N₄ flats sliding against Si₃N₄ balls were in the range of 0.6–0.8 for dry and 0.03–0.15 for lubricated sliding, and the average wear rates of Si₃N₄ were 10^–5 mm³ N^–1 m^–1 for dry sliding and 10^–10–10^–8 mm³ N ^–1m^–1 for lubricated sliding. The friction coefficients of Si₃N₄ balls against composite fibrous monoliths were 0.7 for dry sliding and 0.01–0.08 for lubricated sliding. The average specific wear rates of the pairs were of the same order as those measured for the conventional Si₃N₄ pairs. However, the fibrous monoliths, in combination with sprayed dry boron nitride, resulted in reduction in the lubricated friction coefficients of the test pairs and significant reduction in their wear rates. The most striking result of this study was that the coefficients of friction of the Si₃N₄/H-BN fibrous monolith test pair were 70–80 lower than those of either roughened or polished Si₃N₄ when tests were performed under oil-lubricated sliding conditions over long distances (up to 5000 m). The results indicated that Si₃N₄/H-BN fibrous monoliths have good wear resistance and can be used to reduce friction under lubricated sliding conditions.     

Friction and wear properties of Fe7Mo6-based alloy in ethyl alcohol

In this study, the friction and wear properties of Fe₇Mo₆-based alloy, Fe and Mo disk specimens sliding against ASTM 52100 steel and Cu and SiC ball (or pin) specimens in ethyl alcohol were evaluated using an Optimol SRV oscillating friction and wear tester. The Fe₇Mo₆-based alloy disk specimens exhibited more stable friction coefficients than the Fe and Mo disk specimens when slid against the ASTM 52100 steel ball specimen. On the other hand, the Fe/SiC tribo-pairs exhibited the lowest average friction coefficients (0.14-0.17).     

Tribological Performance of Fe<Subscript>67</Subscript>B<Subscript>33</Subscript> Alloy Prepared by a Self-Propagating High-Temperature Synthesis Technique

A bulk Fe₆₇B₃₃ alloy was prepared by a self-propagating high-temperature synthesis technique that is convenient, low in cost, and capable of being scaled up for tailoring the bulk materials. The Fe₆₇B₃₃ alloy is composed of dendrites with the t-Fe₂B phase and eutectic matrix with the α-Fe and t-Fe₂B phases. The content of the dendrite t-Fe₂B is above 80 vol.%. The compressive fractured strength and Vickers microhardness are 3400 MPa and 12.4 GPa, respectively. The tribological performance of the Fe₆₇B₃₃ alloy is investigated under dry sliding and water lubricant against Si₃N₄ ceramic ball. The wear rates of the Fe₆₇B₃₃ alloy are of the magnitude of 10⁻⁵ to 10⁻⁴ mm³/m under water lubricant. It is lower than that of the Fe₆₇B₃₃ alloy under dry sliding (10⁻⁴ mm³/m). But both the friction coefficients are almost identical. Oxide layers form in both environments via different tribochemical mechanisms, which led to significant differences in wear behavior.     

Tribological Properties of a Nano-Eutectic Fe<Subscript>1.87</Subscript>C<Subscript>0.13</Subscript> Alloy Under Water Environment

Tribological properties of a nano-eutectic Fe_1.87C_0.13 alloy were investigated under distilled-water lubrication against AISI52100 steel ball for various applied loads and sliding speeds. For comparison, the tribological behavior of annealed coarse-grained Fe_1.87C_0.13 alloy was also examined under the same testing conditions. Worn surfaces of both alloys were analyzed by using a scanning electron microscope (SEM). The wear rate of nano-eutectic Fe_1.87C_0.13 alloy was on the order of 10⁻⁵ mm³/m. The wear rate of nano-eutectic Fe_1.87C_0.13 alloy was higher than that of annealed Fe_1.87C_0.13 alloy at lower load, but lower under higher load. The friction coefficients of the two alloys were similar and exhibited a slight increase with increasing sliding speed, but a small decrease with increasing applied load. The wear mechanism of the nano-eutectic Fe_1.87C_0.13 alloy was transformed from plowing and corrosion wear to slight fatigue cracking with increasing applied load, whereas that of the annealed coarse-grained Fe_1.87C_0.13 alloy was transformed from plowing and corrosion wear to severe fatigue flaking.     

Dry-sliding Tribological Properties of Nano-Eutectic Fe<Subscript>83</Subscript>B<Subscript>17</Subscript> Alloy

This paper reports the tribological performance of the nano-eutectic Fe₈₃B₁₇ alloy under dry sliding against Si₃N₄ ceramic ball in ambient environment with varying applied loads and sliding speeds. Worn surfaces of the nano-eutectic Fe₈₃B₁₇ alloy were examined with a scanning electron microscope (SEM) and an X-ray energy dispersive spectroscope (EDS). The wear debris of the samples were also analyzed by X-ray diffractometer (XRD). The wear rate of the nano-eutectic Fe₈₃B₁₇ alloy was of the magnitude of 10⁻⁴ mm³/m, which was lower than that of the coarse grained Fe₈₃B₁₇ alloy. The friction coefficient of the nano-eutectic Fe₈₃B₁₇ alloy was almost the same as that of the coarse grained Fe₈₃B₁₇ alloy. The Fe₂SiO₄ oxide layer was formed on the worn surface of the nano-eutectic Fe₈₃B₁₇ alloy. However, on the worn surface of the coarse grained Fe₈₃B₁₇ alloy was found only a little Fe₂SiO₄. These results demonstrated that the nanostructure improved the wear resistance of the Fe₈₃B₁₇ alloy, but did not significantly affect the friction coefficient. The wear mechanism of the nano-eutectic Fe₈₃B₁₇ alloy was delamination abrasion mainly.     

Tribological duality of Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript>

We report here on the friction behavior of fine- and coarse-grained Ti₃SiC₂ against steel and Si₃N₄ balls. Two successive friction regimes have been identified for both grain sizes and both counterparts. First, Type I regime is characterized by a relatively low (0.1–0.15) friction coefficient, and very little wear. Sliding occurs between a tribofilm on the ball and the Ti₃SiC₂ plane when against steel. Then, a Type II regime often follows, with increased friction coefficients (0.4–0.5) and significant wear. Compacted wear debris seems to act as a third body resulting in abrasion of the ball, even in the case of Si₃N₄. The transition between the two regimes occurs at different times, depending on various factors such as grain size, type of pin, and normal load applied. Some experiments under vacuum showed that the atmosphere plays also a major role. The reason for this evolution is not fully clear at that time, but its understanding is of major technological importance given the unusual good properties of this material.     

Ionic Liquid Lubrication Effects on Ceramics in a Water Environment

Ionic liquids were studied to determine their effectiveness as boundary lubricant additives for water. The chemical and tribochemical reactions that govern their behavior were probed to understand lubrication mechanisms. Under water lubricated conditions, silicon nitride ceramics are characterized by a running-in period of high friction, during which time the surface is modified causing a dramatic decrease in friction and wear. Two mechanisms have been proposed to explain the friction and wear behavior. Si₃N₄ sliding against itself may result in tribochemical reactions that form a hydrated silicon oxide layer on the surface of the sliding contact. This film has been suggested to mediate friction and wear. Others have suggested that tribo-dissolution of SiO₂ results in an ultra smooth surface and after a running-in period of high wear, the lubrication mode becomes hydrodynamic. The goal of this study was to examine the effects that ionic liquids have on the friction and wear properties of Si₃N₄, in particular their effects on the running-in period. Tribological properties were evaluated using pin-on-disk and reciprocating tribometers. The tribological conditions of the tests were selected to produce mixed/hydrodynamic lubrication. The relative lubrication mode between mixed and hydrodynamic was controlled by the initial surface roughness. Solutions containing 2 wt% ionic liquids were produced for testing purposes. Chemical analysis of the sliding surfaces was accomplished with X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The test specimens were 1 in diameter Si₃N₄ disks sliding against 1/4 in Si₃N₄ balls. The addition of ionic liquids to water resulted in dramatically reduced running-in periods for silicon nitride from thousands to the hundreds of cycles. Proposed mechanisms include the formation of BF_x and PF_x films on the surface and creation of an electric double layer of ionic liquid.     

Tribological Properties of Nanocomposite CrC<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/a-C:H Thin Films

Recently we showed that coatings, prepared by unbalanced magnetron sputtering from a metallic Cr target in an Ar/CH₄ discharge are composed of nanocrystalline CrC _x embedded in an a-C:H matrix. This work investigates the structural correlation of such nanocomposite CrC _x /a-C:H coatings to their tribological properties. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to characterize the phase composition and the chemical bonding in the films deposited at different experimental conditions. The coating microstructure was investigated on selected samples by high-resolution transmission electron microscopy. For CrC _x -dominated coatings deposited at CH₄ partial to total pressure ratios (p_CH4/p_t) < 0.42, only minor changes regarding the friction coefficients and the abrasive wear rates were observed although microstructural changes towards a higher degree of crystallinity were proven by transmission electron microscopy and substantiated with XPS results. For a-C:H dominated coatings deposited at p_CH4/p_t > 0.42, the friction coefficients and abrasive wear rates were shown to decrease with increasing a-C:H phase content and its more sp²-like bonding configuration. It can be concluded that the microstructural changes in terms of CrC _x crystallite coarsening and bonding configuration of the a-C:H matrix phase are responsible for the observed changes of the friction coefficients and wear rates.     

Friction and wear of Y-TZP and Y-TZP-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramics in high-speed sliding on steel

The processes of wear of tetragonal zirconia-based ceramics partially stabilized with yttrium oxide (Y-TZP) are studied during dry high-speed sliding in pin-on-disk pairs at sliding velocities up to 47 m/s. It is shown that these wear tests produce tribolayers of intricate composition on the surface of the Y-TZP and Y-TZP-Al₂O₃ ceramic materials, inducing nonmonotonous wear-rate changes. The normal wear changes into disastrous highly intensive wear at the first stage when the sliding accelerates (from 0.1 to 4 m/s). At the second stage (in the velocity range from 6 to 47 m/s) the wear rate drops almost to its initial value, typical of slow sliding velocities (0.1 m/s). In this velocity range, the friction of the Y-TZP and Y-TZP-Al₂O₃ ceramics is almost wearless.     

Tribological behaviour of MW PACVD diamond coatings in vibrating contacts

Six diamond coatings were produced by microwave plasma-assisted chemical vapour deposition (MW PACVD) on polished SiC substrates. Process gas pressure and process gas composition were varied systematically, resulting in different coating morphologies, roughnesses and grain sizes. Friction and wear were investigated for unlubricated vibrating contact conditions in air at room temperature. Tribological tests on diamond coatings were performed in a ball-on-disc configuration, with steel balls (100Cr6) and alumina balls acting as counterbodies. For comparison of the tribological performance of the diamond coatings, standard tests were performed with a stroke of 200 μn, a load of 10 N, and a frequency of 20 Hz. The effect of relative humidity (RH) was investigated by testing in dry (3% RH), in normal (50% RH), and in moist air (100% RH), respectively To investigate running-in effects and wear propagation, the test duration (10⁴, 10⁵, and 1.2·10⁶ sliding cycles) was varied. For steel/diamond pairings, high wear at the steel ball and high friction were measured, influenced by roughness and grain size of the diamond coating and by relative humidity. For Al₂O₃ against diamond, a pronounced running-in of friction and wear was found. High friction and high wear at the beginning of each test are followed by a stationary phase with extremely low friction coefficients (⩽ 0.05) and with wear rates below the limit of resolution. This running-in depends on grain size and roughness of the coating, on relative humidity, as well as on the operational parameters. Surface analysis using SEM, EDX, LRS and AFM was conducted, to investigate tribologically induced surface changes.     

Reciprocating Sliding Friction and Wear Property of Fe<Subscript>3</Subscript>Si Based Alloys Containing Cu in Water Lubrication

Fe₃Si, Fe₃Si alloys containing Cu were fabricated by arc melting followed by hot-pressing. The friction and wear behaviors of Fe₃Si based alloys with and without Cu addition against Si₃N₄ ball in water-lubrication were investigated. The friction coefficient and the wear rates of Fe₃Si based alloys decreased as the load increased. The wear rate of Fe₃Si was higher than that of AISI 304. The addition of Cu can significantly improve the friction and wear properties of Fe₃Si based alloys and substantially reduce the wear rates of Si₃N₄ ball. The wear rate of Fe₃Si–10%Cu was 2.56 × 10⁻⁶ mm³ N⁻¹ m⁻¹ at load of 20 N and decreased to 1.64 × 10⁻⁶ mm³ N⁻¹ m⁻¹ at load of 90 N. The wear rate of Si₃N₄ ball against Fe₃Si–10%Cu was 1.41 × 10⁻⁶ mm³ N⁻¹ m⁻¹, while the wear rate of Si₃N₄ ball against AISI 304 was 5.20 × 10⁻⁶ mm³ N⁻¹ m⁻¹ at load of 90 N. The wear mechanism was dominated by micro-ploughing. The combination of mechanical action (i.e., shear, smear and transference of Cu) and tribochemical reaction of Si₃N₄ with water was responsible for the improved tribological behavior of Fe₃Si alloys containing Cu under high loads.     

Wear Resistance of the La<Subscript>62</Subscript>Cu<Subscript>12</Subscript>Ni<Subscript>12</Subscript>Al<Subscript>14</Subscript> Bulk Metallic Glass Under Dry Friction Conditions

The wear properties of a La₆₂Cu₁₂Ni₁₂Al₁₄ bulk metallic glass (BMG) using sliding wear system under the various normal loads and the annealing conditions have been investigated. Although the La₆₂Cu₁₂Ni₁₂Al₁₄ BMG is brittle during the tensile testing, it exhibits ductile behaviors during the sliding wear process. The SEM and the EDS analyses of the wear tracks and the debris after the sliding wear processes indicate that the wear mechanism is a combination of abrasion, adhesion, and oxidation. It is found that the wear resistance is significantly affected by the normal loads. With the increases in the wear load, the wear loss and the friction coefficient decrease. In addition, it is found that the wear properties are significantly affected by the annealing conditions. Compared with the annealed BMG alloys, the as-cast BMG alloy with a low hardness exhibits good wear resistance, which is attributed to the better ductility during the wear testing.     

The effects of texture height and thickness of amorphous carbon nitride coating of a hard disk slider on the friction and wear of the slider against a disk

In order to minimize the stiction force caused by contact of the extremely smooth surfaces of head sliders and disks in hard disk drives, texture is usually applied on the disk surface. For future contact/near-contact recording, the stiction-induced high friction between slider and disk will become a problem. Texture on the slider/disk interface will still be an expected method to reduce friction. Recently, it was suggested to texture the slider surface. A protective coating is usually required on the textured slider surface to reduce wear of the texture. The results showed that texture on the slider surface was effective in reducing the friction between head sliders and disks. On the other hand, the texture and coating on the slider surface increase the spacing between the read/write element and the magnetic layer of the disk. The necessary and effective texture height and coating thickness are still not clear. In the present research, island-type textures with different heights (3–18 mn) were formed on slider surfaces by ion-beam etching. Amorphous carbon nitride (a-CN_x) coatings of different thicknesses (0–50 nm) were coated on the textured slider surfaces as a protective overcoat. The friction and wear properties of these sliders were evaluated by constant-speed drag tests against hard disks coated with diamond-like carbon (DLC). The results show that 2 nm texture on a slider surface is sufficient for low (0.3–0.5) and stable friction of the slider against the disk in a drag test, and coatings thicker than 5 nm show similar wear resistances of the texture on slider surfaces.     

Friction and wear of self-mated SiC and Si3N4 sliding in water

The friction and wear of self-mated SiC and Si₃N₄ with different initial roughness sliding in water were investigated with pin-on-disk apparatus at normal load of 5 N and sliding speed of 120 mm/s in ambient condition. It was found that, for self-mated Si₃N₄, the wear mechanism for surface smoothening to obtain low friction was tribochemical wear, but for self-mated SiC, it changed from mechanical wear into tribochemical wear with increasing sliding cycles. After running-in in water, self-mated Si₃N₄ exhibited lower steady-state friction coefficient than self-mated SiC did. For these two ceramics, initial and steady-state friction coefficients were hardly dependent on initial roughness. Initial roughness mainly affected the running-in period. The larger the initial roughness, the longer the running-in period, but the running-in period was much shorter for self-mated Si₃N₄ at each initial roughness than that for self-mated SiC.     

Aqueous Lubrication of SiC and Si<Subscript>3</Subscript>N<Subscript>4</Subscript> Ceramics Aided by a Brush-like Copolymer Additive,Poly(<Emphasis Type="SmallCaps">l</Emphasis>-lysine)-<Emphasis Type="Italic">graft</Emphasis>-poly(ethylene glycol)

We have examined the adsorption properties of poly(l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG)—a brush-like polymer—on Si₃N₄ and SiC surfaces and determined its impact on the aqueous lubrication of Si₃N₄ and SiC at various speeds and applied loads. The addition of PLL-g-PEG in aqueous solution reduces the interfacial friction forces significantly for self-mated sliding contacts of these two ceramics, as compared to lubrication with water or buffer solution alone. For SiC, the improved lubricating performance by addition of PLL-g-PEG was apparent for all tested speeds (from 1.4 to 185 mm/s under 2 N load). For Si₃N₄, the effect was more apparent in the slow-speed regime (≤20 mm/s under 2 N load) than in the high-speed regime (>100 mm/s), where extremely low coefficients of friction (μ ≤ 0.006) are readily achieved by aqueous buffer solution alone. It was further observed that the optimal lubricating effect with Si₃N₄ is achieved when the tribopairs are first run-in in polymer-free aqueous buffer to render the sliding surfaces smooth, after which the PLL-g-PEG copolymer is added to the buffer solution.     

Ceramic particulate composites in the system SiC–TiC–TiB2 sliding against SiC and Al2O3 under water

The tribological behaviour of SiC, SiC–TiC and SiC–TiC–TiB₂ was determined in oscillating sliding against SiC and α-Al₂O₃ in water at room temperature. The tribo-systems with the composite materials containing TiC and TiB₂ differ significantly from the systems with the single phase SiC: The wear is reduced and the friction is increased. The wear reduction up to a factor of 10 is mainly due to the formation of an oxide film containing titanium oxides which is soft, stable in water and well adhering to the bulk material. This oxide film is transferred to the alumina ball but not to the silicon carbide ball.     

Parametric study of the dry sliding wear behaviour of AA6082-T6/SiC and AA6082-T6/B<Subscript>4</Subscript>C composites using RSM

In this paper a parametric study of the wear behaviour of Aluminum matrix composites has been carried out. AA6082-T6/SiC and AA6082-T6/B₄C composites were fabricated using stir casting technique. The percentage of reinforcement was taken as 5, 10, 15 and 20 wt.% for both SiC and B₄C particulates. Dry sliding wear tests were conducted using pin-on-disc apparatus at room temperature and process optimization was done using Response surface methodology (RSM). Weight percentage (wt.%) of reinforcement, sliding speed, load and sliding distance were the four process parameters considered to analyse these composites wear behaviour. Analysis of variance (ANOVA) showed that sliding distance exerted the highest contribution (60.24 %) to AA6082-T6/SiC wear, followed by sliding speed (14.28 %), load (11.88 %) and reinforcement content (4.31 %). The same trend was found in AA6082-T6/B₄C composites with slightly different contribution values, namely sliding distance (63.28 %), sliding speed (14.02 %), load (10.10 %) and reinforcement content (4.05 %). RSM analysis revealed that increases in the reinforcement content and sliding speed reduce the wear rate in both composites. On the other hand, increases in load and sliding distance led to higher AA6082-T6/SiC and AA6082-T6/B₄C composites wear. The two predictive models were validated by conducting confirmation tests and certified that the developed wear predictive models are accurate and can be used as predictive tools for wear apllications.