Exhibited Friction Coefficients Research Articles

Superlubricity of Zwitterionic Bottlebrush Polymers in the Presence of Multivalent Ions.

In this study, we report lubrication properties of physisorbed zwitterionic bottlebrush polymers in the presence of multivalent ions using the surface force apparatus. Unlike polyelectrolyte brushes, the lubrication properties of which diminish drastically in the presence of multivalent ions at concentrations as low as 0.1 mM, zwitterionic bottlebrush polymers exhibit friction coefficients as low as ∼10-3 at such concentrations of multivalent ions up to intermediate normal loads. This lubrication ability persists until surface wear occurs at high normal loads. The surface wear is demonstrated to be triggered by the multivalent ions bridging the polymer chains and dehydrating the zwitterionic moieties. Finally, the analysis of the polymer film stability suggests that the partial desorption of polymers in the presence of the ions does not affect the lubrication performance. Therefore, even in the physisorbed state, zwitterionic brushes perform significantly better than covalently grafted polyelectrolyte brushes in the presence of multivalent ions.

Mechanical and tribological properties of nanostructured TiAlN/TaN coatings deposited by DC magnetron sputtering

In this research, nanoscale TiAlN/TaN multilayer coatings was deposited by DC magnetron sputtering, controlling the substrate rotation speed in order to obtain different periods and to evaluate their effect on mechanical and tribological properties. FE-SEM images showed multilayer architecture of coatings with larger bilayer period, TEM analysis were necessary to observe the architecture for coatings with periods below 25 nm. Mechanical properties exhibited a parallel behavior. Hardness, Young's modulus and recovery percentage increase progressively as the bilayer period of multilayer coatings decreases. Regarding to the residual stresses, a significant reduction of residual stress was observed for multilayer coatings compare to monolayer constituent monolayers, decreasing the residual stress from −8 GPa up to −1 GPa. By FIB, it was possible to study the deformation mechanisms in nanoindentation tests; inter-columnar shear was the main mechanisms observed. The increase in critical load of coatings was one of the most relevant results obtained in the investigation, it was possible to increase two times the critical load of multilayer coatings with ʌ = 10 nm, compare to TiAlN and TaN monolayer coatings. With Regard to tribological properties, it was clearly observe the influence of multilayer architecture and the bilayer period. TiAlN/TaN coatings with ʌ = 5 mn exhibited friction coefficients lower than 0.2 and wear rates 2 time lower than the bare substrate. The optical images suggest the presence of tribo-oxidation reactions and with EDS it was possible to confirm the oxidation of coatings.

In-situ tribochemical formation of self-lubricating diamond-like carbon films

Diamond-like carbon (DLC) films were tribochemically formed from ambient hydrocarbons on the surface of a highly stable nanocrystalline Pt-Au alloy. A sliding contact between an alumina sphere and Pt-Au coated steel exhibited friction coefficients as low as μ = 0.01 after dry sliding in environments containing trace (ppb) organics. Ex situ analysis indicated that the change in friction coefficient was due to the formation of amorphous carbon films, and Raman spectroscopy and elastic recoil analysis showed that these films consist of sp2/sp3 amorphous carbon with as much as 20% hydrogen. Transmission electron microscopy indicated these films had thicknesses exceeding 100 nm, and were enhanced by the incorporation of worn Pt-Au nanoparticles. The result was highly wear-resistant, low-friction DLC/Pt-Au nanocomposites. Atomistic simulations of hydrocarbons under shear between rigid Pt slabs using a reactive force field showed stress-induced changes in bonding through chain scission, a likely route towards the formation of these coatings. This novel demonstration of in situ tribochemical formation of self-lubricating films has significant impact potential in a wide range of engineering applications.

First Principles Study of Water-Based Self-Assembled Nanobearing Effect in CrN/TiN Multilayer Coatings

Recently, we have reported on low friction CrN/TiN coatings deposited using a hybrid sputtering technique. These multilayers exhibit friction coefficients μ below 0.1 when tested in atmosphere with a relative humidity ≈ 25 %, but μ ranges between 0.6–0.8 upon decreasing the humidity below 5 %. Here we use first principle calculations to study O and H adatom energetics on TiN and CrN (001) surfaces. The diffusional barrier of H on TiN(001) is about half of the value on CrN(001) surface, while both elements are stronger bonded on CrN. Based on these results we propose a mechanism for a water-based self-assembled nanobearing.

Friction and Wear Properties of αAlB<sub>12</sub>-NiAl Cermet Prepared by Spark Plasma Sintering

In this study, αAlB12-20vol% NiAl cermet disk specimens were prepared by spark plasma sintering, and their microstructure, Knoop hardness, fracture toughness, and friction and wear properties were investigated. The αAlB12-20vol% NiAl disk specimens were obtained by spark plasma sintering blended αAlB12 and NiAl powder at 1573 K for 600 seconds. No reaction product phases were observed between the αAlB12 and NiAl phases. The αAlB12-20vol% NiAl disk specimens exhibited friction coefficients lower than 0.2 and specific wear rates as low as 1.3 × 10-6 mm3/Nm when sliding against Si3N4 ball specimens in water. O-rich phases were observed on the worn surfaces of the NiAl and αAlB12-20vol% NiAl disk specimens after sliding against Si3N4 ball specimens in water. The Knoop hardness of the disk specimens was as high as 10 GPa and the fracture toughness was as high as 7 MPa m1/2.

Friction and wear properties of spark-plasma-sintered α-AlB12 and SiB6 powder compacts in water

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-92.3at% B and Si-85.7at% 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-92.3at% B and Si-85.7at% B powder compact disk specimens, respectively.

Mechanical properties of pulsed laser deposited nanocrystalline SiC films

The mechanical properties of nanocrystalline SiC thin films grown on (100) Si at a substrate temperature of 1000°C under a CH4 atmosphere using the pulsed laser deposition (PLD) technique were investigated. Nanoindentation results showed that films exhibited hardness values around 36GPa and Young modulus values around 250GPa. Scratch tests found that films were adherent to the substrate, with critical load values similar to those recorded for other hard coatings deposited on significantly softer Si substrates. Wear tests performed at a temperature of 900°C showed that films exhibited friction coefficients and wear rates very similar to those measured at room temperature, due to the presence of C–C bonds as evidenced by X-ray photoelectron spectroscopy investigations. These results recommend such coatings for demanding high temperature applications such as nuclear fuel encapsulation.

Friction and Wear Properties of the Siliconized, Chromized and Borochromized Steel Substrates

In this study, the friction and wear properties of the siliconized, chromized and borochromized SS 400 steel/Si3N4 tribopairs were investigated in ethyl alcohol at 298 K. The surface of the siliconized plate specimens mainly consisted of Fe3Si, while the surfaces of the chromized and borochromized steel plate specimens were mainly composed of Fe-Cr alloy and Cr2B phases, respectively. The borochromized specimens exhibited the highest microvickers hardness of all the plate specimens. Also, it was found that both the chromized and borochromized steel specimens exhibited friction coefficients as low as 0.2 in ethyl alcohol, and that the friction coefficients of the siliconized steel specimens were slightly lower than those of the non-coated steel specimens.

Friction and wear properties of α-AlB12- and SiB6-based ceramics in water

In this study, α-AlB12- and SiB6-based ceramic disk specimens were prepared by spark plasma sintering, and their X-ray diffraction patterns, microstructure, microvickers hardness were examined. Also, the friction and wear properties of the α-AlB12- and SiB6-based ceramic disk specimens were investigated when they were slid against Si3N4, Al2O3 and SiC ball specimens in water. Additionally, the friction and wear properties of the Si3N4–Si3N4 and Al2O3–Al2O3 tribopairs were investigated in water as references. Both the α-AlB12- and SiB6-based ceramic disk specimens exhibited friction coefficients lower than 0.2 when they were slid against the Si3N4, Al2O3 and SiC ball specimens. On the other hand, the Si3N4–Si3N4 and Al2O3–Al2O3 tribopairs exhibited friction coefficients as low as 0.6. The α-AlB12- and SiB6-based ceramic disk specimens and their paired Si3N4, Al2O3 and SiC ball specimens exhibited much lower specific wear rates than the Si3N4–Si3N4 and Al2O3–Al2O3 tribopairs.

Friction and wear properties of αFeSi2–Si alloy, ReSi1.8 and MoSi2 in ethyl alcohol

In this study, Fe–Xat% Si alloy (X=70.5, 80.0 and 96.0), Re–64.3at% Si and Mo–66.7at% Si disk specimens were prepared by spark plasma sintering, and their friction and wear properties were investigated when they were slid against Si3N4 ball specimens in ethyl alcohol. The friction and wear properties of Si ingots were also examined. Fe–70.5at% Si, Fe–80.0at% Si, Fe–96.0at% Si and Re–64.3at% Si disk specimens exhibited friction coefficients as low as 0.15. It is considered that the low friction of the Fe–70.5at% Si, Fe–80.5at% Si and Fe–96.0at% Si disk specimens was due to the formation of low friction silicon alkoxide and polyoxysilane on the worn surfaces of the disk specimens and the paired ball specimens. Re–64.3at% Si disk specimens exhibited the highest microvickers hardness of all the disk specimens prepared in this study. In addition, the microvickers hardness of the Fe–Xat% Si (X=70.5, 80.0, 96.0 and 100) disk specimen increased with increasing the Si content. Moreover, it was difficult to obtain dense Fe–90.0at% Si disk specimens by sintering the annealed and crushed Fe–90.0at% Si powder. However, dense Fe–96.0at% Si disk specimens could be obtained by sintering the Fe–90.0at% Si powder at 1403K.

Deposition of CrSiN/AlTiSiN nano-multilayer coatings by multi-arc ion plating using gas source silicon

Quinary CrAlTiSiN coatings have been prepared on cemented carbide and Si substrates using Cr and AlTi (67:33at.%) alloy targets in the mixture flow of N2 and SiH4 gas by multi-arc ion plating method. X-ray diffraction and transmission electron microscopy revealed that the multi-element coatings had a nc-CrSiN/nc-AlTiSiN multilayered structure with a thickness period of 10nm. The microhardness and wear of the coatings were found to be strongly influenced by SiH4 flow rate. The hardness increased monotonically with the increase of SiH4 flow rate and reached 37GPa at 23sccm. The coatings exhibit friction coefficients varying between 0.58 and 0.76 against Si3N4 ball.

Friction and Wear Properties of Fe<sub>7</sub>Mo<sub>6</sub>-Based Alloy under Various Sliding Conditions

In this study, the friction and wear properties of the Fe7Mo6-based alloy/Al2O3 tribopair were investigated at high temperatures in air and at 298 K in water. The Fe7Mo6-based alloy/Al2O3 tribopair exhibited friction coefficients as low as 0.5 at 298 K and 873 K in air. On the other hand, this tribopair exhibited higher friction coefficients at 573 K than at 298 K and 873 K. It was also found that the friction coefficients of this tribopair were approximately 0.5 in water. The Fe7Mo6-based alloy disk specimens and their paired Al2O3 ball specimens exhibited the highest specific wear rates at 873 K in air. Oxygen-rich phase was observed on the worn surfaces of all the disk specimens. Concerning the friction and wear tests at high temperatures in air, the amount of the oxygen-rich phase increased with increasing the testing temperature. In addition, a little larger amount of the oxygen-rich phase was observed on the worn surface formed at 298 K in water than that formed at 298 K in air.

Microstructure and tribological properties of gray cast iron specimens coated by aluminizing, boronizing, chromizing and siliconizing

ABSTRACTIn this study, aluminized, boronized, chromized and siliconized gray cast iron plate specimens were prepared, and their microstructures and tribological properties were investigated. The surfaces of the aluminized, boronized, chromized and siliconized specimens mainly consisted of FeAl, Fe2B, (Cr, Fe)23C6 and FeSi phases, respectively. Also, the surface of the boronized specimen exhibited the highest microvickers hardness of all the specimens. The aluminized, boronized and chromized specimens exhibited friction coefficients as low as the non-coated specimens when sliding against AISI 52100 steel ball specimens in poly-alpha-olefin. In addition, the boronized and chromized specimens exhibited much higher wear resistance than the non-coated specimens.

Microstructure and tribological properties of Fe-Mo alloy-coated steel specimens prepared by low-pressure plasma spraying

Fe7Mo6-based alloy powder was low-pressure plasma sprayed on SS400 steel substrates, and the microstructure, hardness and tribological properties of the prepared substrates were investigated. Metastable BCC-coated and FeMo-coated steel specimens were obtained by low-pressure plasma spraying. The BCC-type Fe-42at% Mo alloy was stable at temperatures below 955 K. In addition, the BCC-type Fe-42at% Mo alloy coating layer exhibited micro-Vickers hardness as high as the FeMo coating layer. The metastable BCC-coated and FeMo-coated steel disk specimens exhibited friction coefficients as low as Fe7Mo6-based alloy specimens prepared by spark plasma sintering. Moreover, the specific wear rates of the FeMo/ASTM 52100 steel tribopairs were almost the same as those of the Fe7Mo6-based alloy/ASTM 52100 steel tribopairs.

Effect of Tribofilm Formation on the Dry Sliding Friction and Wear Properties of Magnetron Sputtered TiAlCrYN Coatings

Nano-structured TiAlCrYN coatings, grown by unbalanced magnetron sputtering on various steel substrates, exhibited friction coefficients 0.6–0.8 and wear coefficients 10−16–10−15 m3 N−1 m−1 in dry sliding wear tests. This article reports comprehensive worn surface analyses using SEM, TEM, EDX, EELS and Raman spectroscopy. A ~80 nm thick tribofilm formed on the TiAlCrYN worn surface was found to have dense amorphous structure and homogeneous oxide composition of Cr0.39Al0.19Ti0.20Y0.01O0.21. Viscous flow of the amorphous tribofilm was dominant in causing the high friction coefficient observed. The coatings showed combined wear mechanisms of tribo-oxidation and nano-scale delamination.

Gradient CrCN cathodic arc PVD coatings

This work reports on the growth and properties of Cathodic Arc Physical Vapor Deposition of low friction-coefficient CrCN coatings, as deposited on the basis of complementary carbon/nitrogen gradient concentrations through the film thickness. The coatings developed in this study exhibited friction coefficients ranging from 0.2 to 0.6, against 100Cr6 bearing steel, depending on the preparation conditions. Gradient CrCN films with COF of 0.35 and comparable mechanical strength to that of CrN were achieved. The adhesion rate of metallic elements such as Fe during the sliding process of CrCN coated tools on AISI316L steels were monitored using a pin-on-disc experimental set-up. The results demonstrated a low tendency of the gradient CrCN deposited films to pick up material from the AISI316L steels surfaces, in contrast to that found when using standard CrN cathodic arc PVD films.

Gradient CrCN cathodic arc PVD coatings

This work reports on the growth and properties of Cathodic Arc Physical Vapor Deposition of low friction-coefficient CrCN coatings, as deposited on the basis of complementary carbon/nitrogen gradient concentrations through the film thickness. The coatings developed in this study exhibited friction coefficients ranging from 0.2 to 0.6, against 100Cr6 bearing steel, depending on the preparation conditions. Gradient CrCN films with COF of 0.35 and comparable mechanical strength to that of CrN were achieved. The adhesion rate of metallic elements such as Fe during the sliding process of CrCN coated tools on AISI316L steels were monitored using a pin-on-disc experimental set-up. The results demonstrated a low tendency of the gradient CrCN deposited films to pick up material from the AISI316L steels surfaces, in contrast to that found when using standard CrN cathodic arc PVD films.

Mechanical and unlubricated tribological properties of titanium-containing diamond-like carbon coatings

Titanium-containing diamond-like carbon (Ti-DLC) coatings were deposited on steel with a close-field unbalanced magnetron sputtering in a mixed argon/acetylene atmosphere. The morphology and structure of Ti-DLC coatings were investigated by scanning electron microscopy, transmission electron microscopy, atomic force microscopy and Raman spectroscopy. Nanoindentation, nanoscratch and unlubricated wear tests were carried out to evaluate the hardness, adhesive and tribological properties of Ti-DLC coatings. Electron microscopic observations demonstrated the presence of titanium-rich nanoscale regions surrounded by amorphous carbon structures in Ti-DLC coating. The Ti-DLC coatings exhibit friction coefficients of 0.12–0.25 and wear rates of 1.82 × 10 −9 to 4.29 × 10 −8 mm 3/Nm, depending on the counterfaces, sliding speed and temperature. The Ti-DLC/alumina tribo-pair shows a lower friction coefficient than the Ti-DLC/steel tribo-pair under the identical wear conditions. Increasing the test temperature from room temperature to 200 °C reduces the coefficient of friction and, however, clearly increases the wear rate of Ti-DLC coatings. Different wear mechanisms, such as surface polishing, delamination and tribo-chemical reactions, were found in the tribo-contact areas, depending on different wear conditions.

Effects of high-temperature hydrogenation treatment on sliding friction and wear behavior of carbide-derived carbon films

In this study, we investigated the effects of a high-temperature hydrogenation treatment on the sliding friction and wear behavior of nanostructured carbide-derived carbon (CDC) films in dry nitrogen and humid air environments. These films are produced on the surfaces of silicon carbide substrates by reacting the carbide phase with chlorine or chlorine–hydrogen gas mixtures at 1000 to 1100 °C in a sealed tube furnace. The typical friction coefficients of CDC films in open air are in the range of 0.2 to 0.25, but in dry nitrogen, the friction coefficients are 0.15. In an effort to achieve lower friction on CDC films, we developed and used a special hydrogenation process that was proven to be very effective in lowering friction of CDC films produced on SiC substrates. Specifically, the films that were post-hydrogen-treated exhibited friction coefficients as low as 0.03 in dry nitrogen, while the friction coefficients in humid air were ~0.2. The wear of Si 3N 4 counterface balls was hard to measure after the tests, while shallow wear tracks had formed on CDC films on SiC disks. Detailed mechanical and structural characterizations of the CDC films and sliding contact surfaces were done using a series of analytical techniques and these findings were correlated with the friction and wear behaviors of as-produced and hydrogen-treated CDC films.

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.