High Pressure Tribometer Research Articles

Pressurized environments directly influence friction and wear of dry steel contacts—Investigations in a novel high fluid pressure tribometer

To reduce the usage of classical lubricants in deep drawing, a new tribological system based on volatile lubricants was developed. Therefore, a volatile medium is injected under high pressure into the interstice between drawing tool and sheet metal. Depending on temperature and pressure, the temporary lubricant may exist in its gaseous or liquid phase. In this study, a novel high fluid pressure tribometer was designed to investigate the friction and wear of dry steel contacts under comparable conditions like in dry deep drawing. Therefore, a new ball-on-disc tribometer was designed and integrated into a high-pressure vessel. To specifically investigate the effects of different environments (technical air, liquid and gaseous carbon dioxide, nitrogen, argon) at atmospheric and high pressure (0.1 MPa, 6 MPa) on tribology, the specimens and all components were operating unlubricated. During the experiments, the friction was measured continuously. Results show that the highest friction occurs in air and the lowest in carbon dioxide environment. Subsequent to the experiments, the wear of the specimens was assessed along with changes in surface chemistry related to tribochemical reactions. Therefore, the tribology of the dry sliding contacts is correlated to changes of the surface chemistry. Also differences as well as similarities regarding the different fluid environments are shown. As the results show, the differences between the media used are most pronounced at elevated pressure. Concluding, this work gives clear indications on the suitability of volatile lubricants in dry friction or rather gas lubrication, especially for dry deep drawing.

Polymeric Coatings With Superior Tribological Performance for Lightweight Materials in HFO-1234yf Refrigerant

As global warming concerns continue to rise, research has shifted toward developing more climate-friendly refrigerants for air-conditioning and refrigeration compressors, with HFO-1234yf being a prominent candidate. In line with the need for lightweight engineering in electric vehicles, advanced composite coatings are necessary to improve the surface properties of parts and system components made from light metals such as magnesium and aluminum, which typically have poor tribological performance. This study investigates the tribological performance of advanced polymeric coatings (aromatic thermosetting copolyester [ATSP]-, polytetrafluoroethylene [PTFE]-, and polyether ether ketone [PEEK]-based) on lightweight aluminum alloy substrates in the presence of HFO-1234yf refrigerant and polyalkylene glycol (PAG) lubricant under starved lubrication conditions. Using a high-pressure tribometer, scuffing pressure, friction, and wear experiments were conducted, with all three coatings exhibiting excellent tribological performance compared to the bare substrate. Notably, PEEK and ATSP coatings demonstrated superior performance with high load-to-failure, low coefficient of friction, and negligible wear. Results also showed ATSP to have superior durability against a broad range of stress levels, while PEEK coating partially wears out at higher stress levels. These findings provide valuable insights for developing robust coatings on lightweight materials for high-stress applications.

Experimental study on the tribological characteristic of vane–roller interface of HC290 rotary compressor with mineral oil

As a natural refrigerant, HC290 is considered as an excellent long-term alternative refrigerant applied in room air-conditioners (RAC). In this paper, the experimental study on tribological characteristics of the vane–roller friction pair in HC290 rotary compressors was carried out and friction coefficients and wear depths were measured under the pure HC290 atmosphere, pure mineral oil and HC290/mineral oil mixture environment, respectively. In order to obtain the experimental conditions as close as possible to the actual operating conditions of a HC290 rotary compressor, the mass-produced vanes and rollers were adopted, and all tests were performed with a high-pressure tribometer under various sliding speeds, loads and temperatures which were set to simulate the different actual operating conditions of rotary compressors. Furthermore, the scuffing tests were performed and the Scanning Electron Microscopic has been used to investigate morphological changes occurred after test. The experiment results with the HC290/mineral oil mixture showed that better anti-scuffing ability but a higher friction coefficient of vane–roller interface were acquired compared to HFC410A/polyolester. The results obtained here may provide some guides for further investigations on tribological characteristics of the vane–roller interface of HC290 rotary compressors.

Advanced polymeric coatings for tilting pad bearings with application in the oil and gas industry

Under extreme operating working conditions, the oil film in hydrodynamic bearings may get destroyed, resulting in mixed and boundary lubrication conditions; subsequently, the bearing surfaces might seize and cause catastrophic failure, rendering the machine non-operable. To address these extreme working conditions, three advanced coatings are proposed in this paper: polytetrafluoroethylene (PTFE) based, polyetheretherketone (PEEK) based and aromatic thermosetting polyester (ATSP) based coatings. A specialized high pressure tribometer, with a pin-on-disc configuration, combined with boundary/starved lubrication, was utilized to simulate the tilting pad bearing׳s severe working conditions encountered inside an Electrical Submersible Pump used in the oil and gas industry. Two sets of experiments, scuffing (step load) and constant load wear experiments were carried out. The coatings exhibited excellent performance compared to bare substrate materials. Scuffing experiments showed that all three coatings exhibited improved scuffing performance and wear experiments showed that the coatings exhibit relatively low coefficient of friction and low wear rate. Among coatings investigated, ATSP coating exhibited the best wear resistance.

TRIBOCAMADA FORMADA EM REVESTIMENTOS MULTIFUNCIONAIS: INFLUÊNCIA DO AMBIENTE

Friction and wear control can be achieved primarily by considering the nature of the counter faces, together with the environmental conditions. In most cases, a transfer film is found on the sliding surfaces. Environment plays a crucial role on the kinetics of formation and on the composition of the transfer film, and thus strongly influences friction levels and wear rates. In this paper, the effect of the actual environment (refrigerant) present in hermetic compressors on the tribological behaviour of a Si rich multifunctional DLC coating deposited on 1020 steel is analyzed. Unlubricated reciprocating pin‐on‐ disk tests are performed using a High Pressure Tribometer under different atmospheres (Air, CO2 and R600a). Samples tested in R600a environment present the lowest friction coefficient and the lowest wear rate for both body and counter‐body.

Effect of the actual environment present in hermetic compressors on the tribological behaviour of a Si-rich multifunctional DLC coating

Friction and wear control can be achieved primarily by considering the nature of the counter faces, together with the environmental conditions. In most cases, a transfer film is found on the sliding surfaces. The environment plays a crucial role on the kinetics of formation and on the composition of the transfer film, and thus strongly influences friction level and wears rates. In this paper, the effect of the actual environment (refrigerant) present in hermetic compressors on the tribological behaviour of a Si-rich multifunctional DLC coating deposited on 1020 steel and tested against 52100 steel pins was analyzed. Unlubricated reciprocating pin-on-disk tests were performed using a high pressure tribometer (HPT) under different atmospheres (air, CO 2 and R600a). The tribological behaviour (friction coefficient and wear rate results) was analyzed using laser interferometry, scanning electron microscopy and micro-Raman spectroscopy. Samples tested in R600a environment presented the lowest friction coefficient and the lowest wear rate for both body and counter-body.

Tribological Study Comparing PAG and POE Lubricants Used in Air-Conditioning Compressors under the Presence of CO2

Polyalkylene glycol (PAG) and polyolester (POE) synthetic lubricants are good candidates for air-conditioning systems that work with alternative refrigerants such as carbon dioxide (CO2). Both synthetic lubricants are widely used in air-conditioning compressors and have been optimized for use with hydrofluorocarbon (HFC) refrigerants. However, it is still not clear which lubricant is more suitable for use in compressors operating with CO2 as a refrigerant. This study compares the performance of PAG and POE lubricants of the same viscosity (ISO VG 68) used in air-conditioning compressors. The materials used were Al390-T6 disks and hardened steel SAE 52100 pins. The tests were performed using a high pressure tribometer (pin-on-disk configuration) in the presence of CO2. The results showed that scuffing and wear resistance of Al390-T6 tested with PAG were superior compared to the samples tested with the POE lubricant. Chemical analysis using X-ray photoelectron spectroscopy showed that PAG tends to promote the formation of carbonate layers on the surface, leading to improvement in the tribological performance of the interface.

Tribology of WC/C coatings for use in oil-less piston-type compressors

Compressors are being required to operate at increasingly severe conditions in order to increase efficiency, and with the transition from CFC to HFC and natural refrigerants, protective ferrous chloride films have been eliminated. To recover lost wear resistance and transition to oil-less operation, greater protection is sought through the application of hard coatings with enhanced tribological properties. Controlled reciprocating pin-on-disk experiments imitating the wrist pin-connecting rod interface were performed using a High Pressure Tribometer under unlubricated conditions. Specimens used for these experiments included WC/C coated cylindrical 52100 steel wrist pins and uncoated cast iron disks. The performance of WC/C with temperature variation as well as running-in characteristics were evaluated using surface profilometry and scanning electron microscopy. It was found that steady-state friction coefficients decreased with increasing temperature while friction coefficients increased during the running-in period. Also, wear decreased from testing in room temperature up to 80 °C and beyond that, increased due to unstable running-in. Based on the research presented in this work, hard coatings have the potential to replace oil in future compressors.

Ultra High Pressure Tribometer for Testing CO2 Refrigerant at Chamber Pressures up to 2000 psi to Simulate Compressor Conditions

The growing interest for using the natural refrigerant carbon dioxide (CO2) in refrigeration and air-conditioning applications instead of HFC refrigerants, due to environmental concerns, has led to the development of an ultra high pressure tribometer (UHPT) specifically tailored for testing in CO2 environment. The existing research on tribology related to CO2 environment has focused on investigations at relatively low chamber pressures due to equipment restrictions. The UHPT is a unique tribometer that has been custom designed and manufactured to allow testing under CO2 refrigerant at environmental pressures comparable to those found in compressors. A special housing, which surrounds the tribological surfaces subject to testing, is capable of withstanding chamber pressures up to 13.8 MPa (2000 psi) and can be temperature controlled from 0°C to 100°C via a thermal control system. A multi-axis strain gauge force transducer measures the applied load, frictional forces, and moments during friction testing, and computer control permits different loading profiles. Using this machine, experiments were performed at a range of pressures between 1.4 MPa (200 psi) and 6.9 MPa (1000 psi) of CO2 refrigerant. The results suggest a slightly better tribological performance at higher pressures compared to lower pressures.

Tribological investigation of cast iron air-conditioning compressor surfaces in CO2 refrigerant

Tribological investigations of air-conditioning compressors have been a topic of great interest in recent years and gray cast iron has been a commonly used material by various compressor manufacturers. The scope of this paper is to determine the role of oxygen and in particular carbon dioxide refrigerant (R744) in cast iron samples tribologically tested using an Ultra High Pressure Tribometer that is suitable for tribological testing of compressor contact interfaces that operate with carbon dioxide refrigerant. A series of experiments was performed in environments of air, nitrogen (N2), oxygen (O2) and carbon dioxide (CO2). While it was found that the presence of oxygen is beneficial, CO2 has a more positive effect on the surfaces than in the case of pure O2 suggesting that the use of CO2 promotes a different wear mechanism. Also, it was found that CO2 has better tribological performance over a range of pressures between 100 psi (0.69 MPa) to 600 psi (4.1 MPa), compared to lower pressures. Furthermore, CO2 was compared with tetrafluorethane (R134a), a common hydrofluorocarbon refrigerant and found to have superior tribological performance. Two surface chemical analysis techniques were utilized to examine the surfaces after tribological testing. Auger electron spectroscopy (AES) was used to track changes in the elemental composition while X-ray photoelectron spectroscopy (XPS) was utilized to detect the different chemical states resulting from compound formation on the tribologically tested surfaces. It was found that CO2 leads to better tribological performance of the interface due to the formation of carbonates on the surface, which reduce friction and prevent wear.

Scuffing of cast iron and Al390-T6 materials used in compressor applications

Scuffing can lead to catastrophic failure in engineering components, but the surface mechanism that leads to scuffing is not well understood. In this experimental study, the surface and subsurface changes on gray cast iron and Al390-T6 were investigated under starved lubrication (a mixture of R410A refrigerant combined with a polyolester lubricant) and pure sliding. Controlled tribological experiments were conducted using a high-pressure tribometer to simulate the contact conditions found in typical air conditioning compressors. Tribological tests were conducted on several sets of tribo-pairs consisting of gray cast iron disks and gray cast iron pins, for periods of 25, 50, and 75% of the time-to-scuff in order to investigate the progression of surface and subsurface changes leading to scuffing. The evolution of the chemical composition in the topmost surface layer of the disks was investigated as the disk/pin tribo-pairs progressed towards scuffing. Important changes in chemical composition were observed on the disks that had scuffed. Specifically, a marked decrease in manganese compounds was measured, which indicates a significant loss of the protective coating that was applied to the disks. Vickers microhardness tests were also performed on the disks at all stages, and showed a reduction in the means of hardness measurements as the disk neared the scuffed stage, in agreement with other studies. Similar scuffing experiments were also performed with tribo-pairs of Al390-T6/52100 steel; surface and subsurface changes were also characterized using chemical analysis and cross-section scanning electron microscope (SEM). In this case, depletion of silicon, which is embedded in the aluminum matrix to increase the hardness of the alloy compounds, was observed at scuffing. This observed reduction in hardness not only agrees with the cast iron hardness study, but also was consistent with the chemical analysis of the cast iron where a chemical component associated with hardening is depleted at the instance of scuffing.

Surface and sub-micron sub-surface evolution of Al390-T6 undergoing tribological testing under submerged lubrication conditions in the presence of CO2 refrigerant

Carbon dioxide (CO2) with its environmental benefits is considered a good replacement for commonly used synthetic refrigerants. In this study, the surface and sub-surface changes in simulated CO2 environment during the initial or transient stages of a sliding contacting interface were investigated. Pin-on-disk configurations involving Al390-T6 disks in contact with 52100 steel pins were used in controlled tribological experiments using a High Pressure Tribometer. In order to evaluate the effectiveness of CO2 refrigerant, comparative tribological experiments involving a conventional refrigerant and different commonly used lubricants were initially performed in a step-increasing load manner under submerged lubricated conditions. Subsequent detailed experiments for investigating the surface and sub-surface changes were performed in the presence of CO2 refrigerant and the best performing lubricant, polyalkyline glycol. Burnishing was observed on the surfaces during the transient (evolutionary) stage, which indicated asperity contacts due to the breaking of the elasto-hydrodynamic lubrication film. In order to quantify the surface and sub-micron sub-surface changes that occurred during this transient stage of tribological operation, several analytical tasks were performed, which involved the measurements of nanomechanical properties, chemical compositions of the topmost 200 nm surface layer, and surface roughness. Such studies of detailed evolutionary changes that occurred during the transient stage of a tribopair shed light on the complex interactions between surface and sub-surface changes that determine whether successful tribological conditions will eventually be achieved. Based on the analyses presented in this work, it is concluded that CO2 is a viable refrigerant from a tribology point of view.

Tribological Studies on Scuffing Due to the Influence of Carbon Dioxide Used as a Refrigerant in Compressors

Because hydrofluorocarbon (HFC) refrigerants in air-conditioning systems are known to have a negative effect on the environment, carbon dioxide (CO2) is a candidate as a replacement refrigerant. Research work related to CO2 as a refrigerant has been focused primarily on its thermodynamic performance, whereas work in the area of tribology related to carbon dioxide is absent. In this study, the effects of CO2 used as a refrigerant on the tribological behavior of surf aces in contact in such systems were investigated. Controlled experiments were performed at constant loads in environments of CO2 and the conventional HFC refrigerant, R134a, as well as under conditions of step-increasing loads in the presence of refrigerant (CO2 or R134a) and polyalkylene glycol lubricant. The experiments were performed on a high-pressure tribometer that is particularly suited for tribological testing of compressor contact interfaces. The tribological behavior of contacting surfaces in a CO2 environment was nearly identical to that in an R134a environment when tested under the same operating conditions.

Nanomechanical Properties of Aluminum 390-T6 Rough Surfaces Undergoing Tribological Testing

The nanoindentation technique was used to quantify nano-scale changes in material properties (effective elastic modulus and hardness) of Al390-T6 samples that have undergone tribological testing under a protocol in a high-pressure tribometer where the applied normal load was step-wise increased until failure by scuffing occurred. The test was highly repeatable, so additional tests were run to three intermediate fractions of the total-time-to-scuffing-failure, which provided data on the progressive wear of the surfaces preparatory to reaching the scuffed condition. The samples were engineering surfaces with significant surface roughness, nonhomogeneous surface microstructure and unknown, nonuniform surface layers. This study demonstrated that nanomechanical techniques can be extended to characterize the material properties of rough engineering surfaces. For the samples subjected to tribological testing, the material at the surface, and to approximately 60 nm below the surface, exhibited significantly higher hardness than the bulk material. Also, progressive wear of the surfaces resulted in a corresponding weakening of the near-surface material below the surface to a depth of 60 nm, while the hardness of material below the 60 nm depth remained relatively unchanged. The hardness data for the scuffed samples showed a large amount of scatter in the data, indicating that the surface is not homogeneous and that the protective surface layer is removed, at least at some points on the surface.

Scuffing behavior of 390 aluminum against steel under starved lubrication conditions

The scuffing characteristics of 390-T6 aluminum–silicon alloy sliding against 52100 steel are examined under starved lubrication conditions. The evaluation is based on a shoe-on-disc geometry which simulates the plate/shoe contacts in an automotive air conditioning swashplate compressor. Four shoe geometries (flat shoe, crowned shoe, crowned shoe with a dimple at the center and crowned shoe with a groove) are tested against 390-T6 aluminum discs. All tests are conducted in a high-pressure tribometer (HPT) under R134a (tetrafluoroethane) environment with a base polyalkylene glycol (PAG) lubricant. The effects of degree of lubricant starvation, sliding velocity, geometry of contact, surface topography, and a tin coating on scuffing are evaluated. It is hypothesized that scuffing of 390-T6 Al alloy under starved lubrication conditions is due to macroscopic adhesions leading to plastic shearing of the bulk material.

Scuffing of Area Contacts Under Starved Lubrication Conditions

The scuffing behavior of 390-T6 and DHT3 aluminum alloys, Si-Pb brass and gray cast iron, sliding against 1018 carburized steel, is experimentally studied under starved lubrication conditions. The major emphasis of the study is on the 390-T6 aluminum alloy. All tests are conducted in a high pressure tribometer (HPT) under RI34a (tetrafluoroethane) environment with polyalkylene glycol (PAG) and polyolester (POE) lubricants to simulate failures of critical tribocontacts in refrigerant compressors. An area contact, pin-on-disc geometry, is used to examine the effects of degree of lubricant starvation, sliding velocity, materials, and lubricant/refrigerant mixtures on scuffing. The scuffing transitions characteristics of 390-T6 aluminum as a function of lubricant supply rate are also examined. The processes leading to scuffing and failure mechanisms are studied by examination of scuffed surfaces and subsurfaces. Based on the experimental observations, it is hypothesized that bulk material failure during scuffing is due to plastic shearing or smearing. Presented at the 55th Annual Meeting Nashville, Tennessee May 7–11, 2000

Simulative friction and wear study of retrofitted swash plate and rolling piston compressors

A specimen testing programme was conducted to approximately simulate critical contact in retrofitted swash plate and rolling piston compressors. Friction and wear data were obtained by means of a specially designed high-pressure tribometer. The substituted refrigerant-oil mixtures under study were tetrafluoroethane glycol (PAG) or R134a-polyester oil mixtures. The oil residue was either a mineral or an alkylbenzene oil, and the residual refrigenaant was dichlorofluoromethane (R12). The amounts of residual oil used were 0%, 20%, an% by volume. The residual amount of refrigerant was 1–5% by weight. For the mixtures and conditions under study, the presence of R12 and a residual amount of alkylbenzene or mineral oil did not have a negative effect on the lubricative properties of the R134a-oil mixture. In fact, for both the counterformal and area ontacts under study, the mixture with 50% residual oil tended to improve the tribological conditions of the contacts. The improvement was probably due to the difference in the tribological properties of the oils by themselves and not to the presence of R12.

Friction properties of highly elastic materials at high contact pressures

A newly designed high-pressure tribometer has been employed to investigate the pressure dependence of the friction force of SKN-40 crosslinked butadiene-nitrile rubber in contact with a steel surface on the pressure range to 1200 kgf/cm2 (20°C). Over the entire range of contact pressures the friction process is molecular-kinetic in nature and characterized by a linear dependence of the friction force on the logarithm of the sliding velocity. In the region of normal pressures up to 200 kgf/cm2, where the effect of pressure on the friction force reduces to the formation of the actual contact area, the friction constant (proportionality factor relating the friction force and the actual contact area) is practically independent of the pressure. At pressures above 200–300 kgf/cm2 the increase in the friction force at fixed actual contact area is attributable to the effect of pressure on the friction constant. The nature of this effect is related not with an increase in the chain-surface interaction energy (the activation energy does not increase), but with an increase in the forces of adhesion owing to the greater number of polymer chain-steel surface contacts on the actual contact area (increase in contact density).