High Temperature Tribometer Research Articles

Temperature dependence of tribological properties of MoS 2 and MoSe 2 coatings

Transition metal dichalcogenides are well known for their lubricating properties. MoS 2 is the most popular member of this family and it is widely used as a solid lubricant in vacuum and inert gases. However, the lubricating properties of MoS 2 are deteriorated in humid air. It is known that molybdenum diselenide (MoSe 2) has the same crystal structure and also exhibits suitable lubricating properties, but their dependence on air humidity has not been studied yet in details. This paper is aimed to the comparison of tribological properties of MoS 2 and MoSe 2 coatings measured in air of different humidity and at elevated temperatures. Both coatings were prepared by non-reactive DC magnetron sputtering and tested with ball-on-disc high temperature tribometer. The results of measurements of friction coefficient and wear rate vs. tribometer revolutions and the resulting dependencies of friction coefficient and wear rate on ambient air relative humidity are presented. These results show that the friction coefficient of MoSe 2 was not influenced by air humidity. Wear rate of MoSe 2 in dry air was substantially higher than that of MoS 2; in humid air, the MoS 2 wear rate increased rapidly while wear rate of MoSe 2 remained unchanged. The operating temperature of both coatings was limited to 350 °C.

Comparison of tribological behaviour of TiN, TiCN and CrN at elevated temperatures

PVD-coated instruments are often used at elevated temperatures. However, the friction and wear data of these coatings at temperatures exceeding room temperature are still lacking. In order to help remedy this lack, three coatings widely spread in industrial applications, TiN, TiCN and CrN, taken from standard production of customer's coating centre were comparatively measured up to 500 °C. The coatings were deposited on hardened steel substrates either by unbalanced magnetron sputtering or arc evaporation deposition. The measurements were done using a high-temperature tribometer (pin-on-disc, CSM Instruments). The evolution of friction coefficient with sliding distance (number of cycles) was measured at different temperatures and the wear rate of the ball and coating were evaluated. 100Cr6 balls and the Si 3N 4 ceramic balls were used as counterparts. The wear tracks were examined optically and by SEM. The surface oxidation at elevated temperatures and chemical composition inside and outside the wear track was also measured by EDX analysis.

Tribological behavior of Stellite 21 modified with yttrium

Cobalt-base alloys have found a wide variety of tribological applications particularly at elevated temperatures or in corrosive environments in many industries, such as aerospace, automotive, power and gas turbines. One of the standard Co-base alloys, Stellite 21, is used predominantly to resist the synergistic effects of corrosion and mechanical attack, especially at elevated temperatures and continuing efforts have been made to improve its properties. One approach is to add reactive elements, such as yttrium, in order to beneficially affect its oxidation behavior. Research was conducted to investigate the effects of Y additions on Stellite 21 on its microstructure, mechanical behavior, and high-temperature wear performance. These studies employed various experimental tools, such as micro-mechanical probe, XRD, SEM, EDS, and high-temperature tribometer. The effects of Y addition on the properties of the oxide film formed on Stellite 21 were also investigated using grazing XRD and nano-mechanical probing techniques. It has been demonstrated that Y additions benefited the wear behavior of Stellite 21, especially at elevated temperatures. The oxide scale developed at 600 °C also showed markedly enhanced mechanical properties when Y was alloyed to the alloy.

Dry friction and wear rates as under liquid lubrication of ceramic/carbon couples up to 450°C

In a high temperature tribometer, stationary carbon has been tested against different rotating ceramics (SiC, Si3N4, Al2O3, WC‐6Ni, MgO‐ZrO2, (Ti, Mo)(C, N)) and stainless steel (DIN 1.4876). The rotating discs were grinded, polished and/or lapped. For most material combinations, the wear morphology is known from available literature. A transfer film with a typical wear pattern was found on the rotating disc. The combination of antimony graphite EK3245 against MgO‐ZrO2 did not form carbonaceous transfer layer. Through advanced variation of the roughness up to Rpk=0.011 μm, the wear rate has been reduced to Kv ≈ 3.5×10−8 mm3/N m at a stable coefficient of friction in a “millirange” of μ∼0.008 for a sliding distance of 20.000 m.

CrC·CaF2—A PVD stratified coating for friction and wear under heavy load at high temperature

This study is to enable a cobalt superalloy to work at 700°C in air under a normal load of 5500 N corresponding to a nominal pressure of 25 MPa. Preliminary experience shows that the cobalt superalloy does not have sufficient mechanical and tribological properties to support this load under sliding conditions. In fact, both the applied normal load and the induced tangential force rapidly damage the sliding surface. So it is necessary to modify its surface properties. The present study used a stratified coating with a hard layer made of chromium carbide (CrC) and a soft layer made of calcium fluoride (CaF2) for improving the alloy's tribological properties. All the coatings were deposited by PVD. The CrC layer has a thickness of 3 μm, and the CaF2 varies from 0 to 3 μm. In order to assess the tribological properties of the coatings, a reference couple consisting of a triboalloy PS212 to PS212 as well as three couples consisting of CrC to CrC, CrC·CaF2(0.6 μm) to CrC·CaF2(0.6 μm) and CrC·CaF2(3 μm) to CrC·CaF2(3 μm) have been tested at 500 and 700°C at ISMCM-CESTI using a specific high temperature tribometer FHTO (Frictiographe à Haute Température en atmosphère Oxydante). These tests have revealed that, by comparison to PS212, the stratified coatings have a slightly lower friction coefficient (0.28–0.32 instead of 0.35) and a significantly lower wear rate (30×10−6 mm3/Nm instead of 1000×10−6 mm3/Nm).

Mechanical and tribological properties of Ni-based alloy/CeF3/graphite high temperature self-lubricating composites

In this paper, Ni-based alloy/CeF3/graphite high temperature self-lubricating composites with different graphite addition amounts (0, 1, 3 and 5wt.%) were prepared using P/M (powder metallurgy) method. Findings indicated that the addition of graphite could increase the compressive strength and hardness of the composites, while poor results were obtained for the impact toughness at room temperature. The microstructures of the composites were studied using SEM with EDS and were correlated to their mechanical strengths with respect to the formation of carbides and graphite during the process. The effects of graphite addition amount on the friction and wear performances of the composites both at room temperature and 600°C in air in sliding against high speed steel (W18Cr4V) were investigated using a pin-on-disk high temperature tribometer. Results showed that the wear rates of the composites and their counterparts could be remarkably reduced at room temperature with graphite addition. As for 600°C, with no apparent improvement on wear resistance of the composites with graphite addition, the wear rate of high speed steel was higher than that with no graphite addition. Finally, the wear rate of high speed steel at 600°C was correlated to the hardness of the composites and the wear mechanism was discussed.

Tribological behavior of silicon nitride that is compositionally modified by high-temperature electrolysis

This paper describes the effect of grain boundary chemistry, which can be easily altered by the amount of introduced sintering additives or impurities, on the high-temperature wear behavior of silicon nitride. Two different kinds of silicon nitride were made to investigate the effect of minor additives on the high-temperature wear behavior. A small amount of either CaO or Fe 2O 3 was added to the silicon nitride. Specimens were cold pressed and then sintered by gas pressure sintering. An electric field was applied to the silicon nitride in a furnace at 1300 °C. Additive ions were expected to be drawn out of the intergranular glassy phase during the electrolysis treatment. The wear tests were conducted at temperatures up to 600°C with specimens before and after ion release. The wear tests were carried out on a high temperature tribometer with ball on plate geometry. Sliding velocity and applied load were 5 × 10 −3 m s −1 and 20 N. Examination by electron probe micro-analysis revealed that the concentration of the cations was influenced by the electrical treatment. A detrimental effect on the high temperature wear resistance due to addition of the minor additives was observed. The wear test results showed that the change in composition of the grain boundary phase with the electric treatment at 1300 °C resulted in a 50% decrease in wear rate at 600 °C compared with otherwise identical material. This study showed that the composition of the glass phase present greatly influences the high-temperature wear behavior of silicon nitride.

High temperature tribology of ceramics

The friction and wear behaviour of self-mated couples of MgOZrO 2, Al 2O 3 and two types of SiSiC were studied under dry sliding conditions in a special pin-on-disc high temperature tribometer. The temperature was varied between 25 and 1000°C, and the sliding speed from 0.03 m s −1 to 3 m s −1. The morphology of the worn surfaces was studied by means of SEM, and their phase distribution by X-ray diffraction and TEM analyses. The results show that the wear coefficients of all couples mostly increase with increasing temperature and sliding velocity. The wear of MgOZrO 2 is influenced by tribo-induced phase transformations while α-Al 2O 3 retains its original structure for all test conditions. For SiSiC delamination and fatigue of the interface Si/ß-SiC predominate. At higher temperatures and sliding velocities tribo-oxidation is effective. The friction coefficients lie between 0.5 and 1.0 under steady-state conditions but for short test durations lower values can occur. The couple SiSiC/SiSiC has low friction coefficients at low sliding velocities and temperatures, even if the steady-state region is reached.