Pool boiling of R-123/MO and R-134a/POE lubricant mixtures on pored surfaces

In a flooded refrigerant evaporator, where enhanced tubes are frequently used, lubrication oil inevitably circulates with the refrigerant. However, the literature shows that systematic studies on this subject are lacking. In this study, the effects of oil on the pool boiling of pored surfaces having a range of pore diameter (0.1 to 0.3 mm) and pitch (0.75 to 3.0 mm) were investigated using R-134a/polyester oil (POE) and R-123/mineral oil (MO) mixtures. The saturation temperature was 26.7 °C, and the oil concentration was varied up to 10 %. The results showed that the oil reduced the heat transfer coefficient. This was true for all the pored surface as well as the smooth surface. Overall, the samples having a ‘small open area’ yielded a significant degradation at a low heat flux, whereas the samples having a ‘large open area’ yielded a noticeable degradation at a high heat flux. Meanwhile, the heat transfer degradation was larger for R-134a/POE than R-123/MO, and the reason was attributed to the oil effect on the surface tension, which was stronger for R-134a/POE. The flow visualization results showed that, with the increase of oil concentration, the bubble departure diameter decreased. Similarly, the bubble generation frequency and nucleation site density decreased. These changes of the bubble dynamic parameters certainly were responsible for the heat transfer degradation. A model was developed extending that of Pastuszko et al. [27] to predict the heat transfer coefficients as well as bubble dynamic parameters. The model predicted 92 % of the heat transfer coefficients within ± 40 %.

     

Advantages of CO2 compared to R410a refrigerant of tribologically tested Aluminum 390-T6 surfaces

Carbon dioxide (CO₂) is currently being investigated as a viable alternative refrigerant due to its environmental advantages. Much research is still needed regarding thermodynamic, tribological, and design issues, yet CO₂ is a promising environmentally friendly refrigerant. This study investigates the physical and chemical changes of aluminum alloy disks that occur with increased severity tribotesting in the presence of either R410a or CO₂ refrigerants while submerged in POE lubricant. Visibly and through surface profilometry, this study clearly shows that R410a environment results in increased disk wear compared to CO₂ environment. In addition, Auger Electron Spectroscopy analysis was conducted showing that the oxygen concentration tends to increase with testing duration for both R410a and, to a greater extent, CO₂ tested samples. When Auger data were compared for the R410a submerged in POE, CO₂ submerged in PAG, and CO₂ submerged in POE, the CO₂ tested samples had significantly higher oxygen concentrations. This indicates that CO₂ refrigerant is promoting a strong oxygenated layer, which reduces wear.     

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.     

Tribological Characterization of Aromatic Thermosetting Copolyester–PTFE Blends in Air Conditioning Compressor Environment

The tribological behavior of a wide range of compositions using blends of aromatic thermosetting polyester (ATSP) with polytetrafluoroethylene (PTFE) has been investigated. PTFE was chosen as the blending material because of its low coefficient of friction and good performance at high temperatures and resistance to chemicals. ATSP blends were used to specifically combat some of the shortcomings of PTFE like its extremely low wear resistance and poor mechanical properties, and special processing requirements due to its high melt viscosity. Controlled tribological experiments simulating an air conditioning compressor operating with R134a refrigerant under realistic operating conditions were carried out with different ATSP/PTFE compositions, as well as four different state-of-the-art commercially available composites containing carbon fibers, graphite and PTFE. It was found that the newly synthesized composites exhibited superb tribological characteristics as far as low friction and low wear were concerned. The wear performance of PTFE was greatly improved, while it was shown that greater amounts of ATSP used in the blend lead to lower wear and the amount of ATSP did not significantly alter the friction coefficient. Material transfer and development of a weak film on the disk surface was observed, especially for the blends with higher PTFE content.     

Tribology of Unfilled and Filled Polymeric Surfaces in Refrigerant Environment for Compressor Applications

The current study examines the tribological performance of polymer/metal contacts in the presence of refrigerant versus ambient air under conditions simulating refrigeration compressors. Ten different polymers were employed as potential compressor bearing materials; four unfilled polymers and six blended polymers. All polymers were tested against cast iron samples of RMS roughness 0.3--0.5 μm and representative testing was performed against aluminum alloy disks. Experiments conducted in R-134A refrigerant exhibited slightly favorable friction and wear characteristics compared to experiments conducted in ambient air. Moreover, all blended polymers have superior tribological characteristics to unfilled polymers and metals. PEEK and polyimide in both unfilled and blended forms exhibit minimal wear and do not adversely affect the metal disks. This study also shows that although coherent, uniform films were not produced under compressor-like conditions, as evidenced by scanning electron microscopy and energy dispersive X-ray spectroscopy, the tested polymers still have favorable tribological properties and are good candidates for tribological contacts in refrigeration compressors.     

Effect of R600a on tribological behavior of sintered steel under starved lubrication

This paper describes the impact of the isobutane refrigerant on the wear performance of tribopairs at hermetic compressor bearings. The tribological behavior of 100Cr6 steel pin is investigated under starved lubrication condition in air and R600a environments when running against sintered steel which was treated with and without steam. EDS and SEM are carried out on pin and plate samples after wear tests. The results indicate that wear durability is lower for the tests with R600a than those with air. The adverse effect of R600a on wear rate is linked to the change in the viscosity and foaming characteristics of the oil in the presence of R600a as well as the lack of oxides.     

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.     

Friction and wear properties of Ni–Cr–W–Al–Ti–MoS2 at elevated temperatures and self-consumption phenomena

The composites of Ni–Cr–W–Al–Ti–MoS₂ with different adding amount of molybdenum disulfide (6–20 wt.%) were prepared by powder metallurgy (P/M) method. Their mechanical properties and tribological properties from room temperature to 600 °C were tested by a pin-on-disk tribometer. The effects of amounts of molybdenum disulfide, temperature, load, and speed on the friction and wear properties of composite were discussed. Besides, the tribological properties against different counterface materials, such as alumina, silicon nitride and nickel-iron-sulfide alloys were also investigated. Results indicated that the molybdenum disulfide was decomposed during the hot-press process and the eutectic sulfides of chromium were formed. The hardness and anti-bending strength can be improved by adding 6 wt.% molybdenum disulfide due to reinforcement of molybdenum. The friction coefficients and wear rates of composites decrease with the increase of adding amount of molybdenum disulfide until a critical value of 12 wt.%. The composite with 12% MoS₂ shows the optimum friction and wear properties over the temperature range of RT 600 °C. The friction coefficients of composite with 12% MoS₂ decrease with the increase of temperature, load, and sliding speed, while the wear rates increase with the increasing temperature and are insensitive to the sliding speed and load. The friction coefficients of less than 0.20 at 600 °C and mean wear rates of 10⁻⁵ mm³/N m are obtained when rubbing against alumina due to the lubrication of sulfide films and glaze layer formed on the friction surface at high temperature, while a relatively low wear rate of around 10⁻⁶ mm³/N m presents when rubbing against nickel-iron-sulfide alloys. At high temperature, wear rates of composite containing sulfide are inversely proportional to friction coefficients approximately.     

Effect of grain size and hardness on fretting wear behavior of Inconel 600 alloys

The effects of grain size and bulk hardness on fretting wear behaviors were investigated by solution annealing and subsequently fretting wear test in Inconel 600 alloys. The results indicated that, with increase of solution temperature, the grain size increased while the hardness decreased. The average friction coefficients were the almost same, independent of grain size and hardness; while the wear volume decreased with increase of grain size, but the hardness played little role. The smaller grain was conductive to formation of tribological transformed structure (TTS) layer, and produced shorter delamiantion cracks in the TTS layer than larger one.     

Tribological Performance of Diamond and Diamondlike Carbon Films at Elevated Temperatures

In this study, the authors investigated the tribological performance of diamond and diamondlike carbon (DLC) films as a function of temperature. Both films were deposited on silicon carbide (SiC) by microwave plasma chemical vapor deposition and ion-beam deposition processes. Tribological tests were performed on a reciprocating wear machine in open air (20 to 30% relative humidity) and under a 10 N load using SiC pins. For the test conditions explored, the steady-state friction coefficients of test pairs without a diamond or DLC film were 0.7 to 0.9 and the average wear rates of pins were 10^?5 to 10^?7 mm³/N·m, depending on ambient temperature. DLC films reduced the steady-slate friction coefficients of the test pairs by factors of three to five and the wear rates of pins by two to three orders of magnitude. Low friction coefficients were also obtained with the diamond films, but wear rates of the counterface pins were high due to the very abrasive nature of these films. The wear of SiC disks coated with either diamond or DLC films was virtually unmeasurable while the wear of uncoated disks was substantial. Test results showed that the DLC films could afford low friction up to about 300° C. At higher temperatures, the DLC films graphitized and were removed from the surface. The diamond films could withstand much higher tempera-lures, but their tribological behavior degraded. Raman spectroscopy and scanning electron microscopy were used to elucidate the friction and wear mechanisms of both films at high temperatures.     

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.     

纳米NiFe_2O_4与OLFs冷冻机油对往复式冰箱压缩机性能的影响

采用MINICER 993.0490分散机将纳米铁酸镍(n-NiFe_2O_4)和纳米洋葱状富勒烯(onion-like fullerenes,OLFs)添加到KFR22中制备出纳米冷冻机油。采用SRV(II)摩擦磨损试验机测试了该冷冻机油在冰箱压缩机工况下往复运动的润滑性能。依据SH/T0762-2005和GB/T5773-2004规定的测试方法和条件,对采用此纳米冷冻机油冰箱用往复压缩机性能进行了对比检测。结果表明:采用此纳米冷冻机油后压缩机的平均性能系数COP由1.5提高至1.58,增加5.3%。     

Structure effect of refrigeration polyolester oils for use in HFC-134a compressors

A study of polyolester oils (POE) for compressors using an ozone-friendly refrigerant is presented here. The effect of the chemical structure of POE was studied in relation to three lubricant properties: miscibility with HFC-134a (CF₃-CH₂F), lubricity (steel-on-steel and aluminium-on-steel contacts), and viscosity. The paper shows the strong structure effect of POE on miscibility and lubricity. Based on the data obtained, some model lubricants were blended, and the performance of these lubricants was assessed on refrigeration test rigs.     

Structural and tribological characterization of tungsten nitride coatings at elevated temperature

Transition metal nitrides exhibit excellent mechanical properties (hardness and Young's modulus), high melting point, good chemical stability and high electrical conductivity. However, tungsten nitrides still stand aside of the main attention. In our previous study, tungsten nitride coatings with different nitrogen content showed excellent wear resistance at room temperature. Nevertheless, many engineering applications require good tribological properties at elevated temperature. Thus, the present study is focused on the tribological behaviour (friction coefficient and wear rate) of tungsten nitride coatings at temperature up to 600 °C.

The structure, hardness, friction and wear of tungsten nitride coatings with nitrogen content in the range 30–58 at.% prepared by dc reactive magnetron sputtering were investigated. The tribological tests were performed on a pin-on-disc tribometer in terrestrial atmosphere with Al₂O₃ balls as sliding partner. The coating wear rate was negligible up to 200 °C exhibiting a decreasing tendency; however, the wear dramatically increased at higher temperatures. The coating peeled off after the test at 600 °C, which is connected with the oxidation of the coating.     

Lubricity of environmentally friendly HFO-1234yf refrigerant

Global warming concerns have triggered the interest in alternative refrigerants for air-conditioning and refrigeration compressors. Newly developed refrigerant HFO-1234yf is an attractive and environmentally friendly candidate to replace existing hydrofluorocarbon refrigerants, such as R-134a. Herein, the tribological compatibility of gray cast iron material with this new refrigerant was investigated under aggressive unlubricated conditions. HFO-1234yf exhibits excellent performance compared to R-134a due to the formation of a fluorine-containing protective tribolayer on the topmost surface as X-ray photoelectron spectroscopy (XPS) analysis, inside and outside the wear tracks, revealed. The morphology of the layer was also studied using scanning electron microscopy (SEM) studies.     

Friction and Wear Characteristics of TiN Coated Vane for the Rotary Compressor in a R410A Refrigerant©

HFC is a potential alternative refrigerant for CFC, which depletes the ozone layer. The rotary compressor has been widely used for refrigeration and air-conditioning systems due to its compactness and high-speed operation. R410A, an HFC refrigerant, is used in a refrigerator compressor, but its frictional characteristic is not established. In this study, the influence of R410A refrigerant on the roller-vane surfaces was studied, and friction and wear characteristics of the TiN coated vane were investigated. The friction and wear data were obtained with a specially designed high-pressure wear tester. A testing environment charged with HFC refrigerant more closely simulates the operating conditions of a real rotary compressor. In pure oil without R410A, wear of the TiN coated vane was larger than that of the uncoated vane. But when the refrigerant was dissolved in oil, wear of the uncoated vane was larger than that of the TiN coated vane. This showed that a TiN coated vane is good relative to wear resistance in the refrigerant/lubricant mixed environment. As the rotating speed of the frictional motion increased, wear increased. But in the high-velocity region, wear decreased because the boundary lubrication is changed into the mixed lubrication in the lubrication region. As the pressure grew larger, wear volume and coefficient of friction became larger. This is because the amount of the refrigerants dissolved in oil increased and the viscosity of oil dropped as the pressure increased.     

汽车空调系统中用烃混合物取代HFC-134a的可行性分析和实验研究

随着全球气候变暖的影响，对现有空调系统制冷剂取代问题的研究也越来越深入。针对汽车空调，从理论和实际两个方面论证用烃混合物取代HFC-134a的可行性。并证明，丙炕（R290），丁烷（R600）和异丁烷（R600a）按照50%、40％、10％的比例混合是取代HFC-134a的最佳比例。     

A test method to investigate the tribological behaviour of friction materials under ultrasonic fretting conditions

To investigate and understand the tribological behaviour of high-frequency tribosystems such as ultrasonic motors, a specific test method is necessary. This work reports on the construction of a test machine to evaluate the friction and wear behaviour of friction materials under ultrasonic fretting conditions, as well as giving some representative experimental results. Hard/soft (steel/polymer) and hard/hard (steel/alumina, alumina/alumina) couples were studied with respect to their application as contact materials in ultrasonic motors. Investigation of friction behaviour at high frequencies showed that friction-induced vibrations lead to friction forces of much lower magnitude than predicted by quasistationary friction coefficients obtained for sliding friction. The wear behaviour is characterised by abrasive, adhesive, fatigue and oxidative mechanisms, depending on the mating materials. For polymeric friction materials, the influence of fibre reinforcement and the incorporation of PTFE as a solid lubricant were evaluated. The presence of PTFE resulted in a strong improvement of both friction and wear behaviour.     

Microstructural,Mechanical, and Tribological Properties of Rice Husk–Based Carbon: Effect of Carbonizing Temperature

In this study, we investigated the microstructural, mechanical, and tribological properties of rice husk (RH)-based carbon carbonized at various carbonizing temperatures under dry conditions. All samples exhibited amorphous carbon structures and the X-ray diffraction spectra of the samples carbonized at 1300 and 1400?°C indicated the presence of a polymorphic crystals of silica. The hardness increased with temperature due to the densification of the structure and the presence of the hard crystalline silica. At low normal loads, the mean friction coefficient of the material decreased as the carbonizing temperature was increased from 600 to 800?°C and slightly decreased as the carbonizing temperature was further increased from 800 to 1400?°C. At the highest load, all samples, except for that carbonized at 600?°C, exhibited extremely low friction coefficients (around 0.05). The wear rates of the all samples were smaller than 10^?5 mm³/N·m, indicating that RH carbon exhibits sufficient wear resistance. A Raman spectroscopic analysis of the worn surface of a steel ball revealed that the transfer layer at 600?°C had a less graphitic structure compared to the other carbonizing temperature. Based on these findings, we recommend an optimal carbonizing temperature for applications of sliding materials exposed to dry sliding contact.     

Stress-overstress-surface fatigue: new approach in the understanding of the tribology of plasma coated sintered steels

This paper deals with the tribological evaluation of plasma nitrocarburizing of commercially sintered low alloy steels. The samples were tested in dry sliding conditions against a cast iron pin. A model for particle detachment in the coatings is proposed, which takes into account the volume expansion in the coating process as a source of internal stresses in the compound layer. Two different routes were used for plasma nitrocarburizing—a ferritic (560°C) and an austenitic (600°C) treatment—and the tribological behaviour of the coatings was found to reflect the stress condition in the compound layer as a function of its thermal history. The experimental wear data fit well with the model, and the ferritic treatment was found to induce more stress in the coating, as predicted. Comparative results of tribological tests (wear and friction data) for as-received, coated and polished samples reinforce the ‘stress-overstress-fatigue’ sequence.