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.     

In Situ Tribological Evaluation of Greases and Solid Lubricants in a Simulated Atomic Oxygen Environment

Four greases and nine solid lubricants have been applied lo 440C steel surfaces and subjected to in situ tribo-testing in a simulaled-atomic-oxygen (SAO) environment. The test apparatus, procedures and results are described. The discussion addresses the calibration of the SAO tribometer using Kapton H film, the effect of SAO on wear and friction characteristics of unlubricaied 440C steel, and the screening test results for all the lubricants wider SAO conditions. Endurance test results of selected greases and solid lubricants, such as a PEPE-type grease and MoS₂ coatings, are also presented. The results of this research should facilitate selection of effective solid lubricants and greases and hence lead to improved lifetimes for mechanisms that must, operate in the atomic-oxygen environment of low-Earth orbits.     

Tribological studies of unpolished laser surface textures under starved lubrication conditions for use in air-conditioning and refrigeration compressors

An experimental study was carried out to investigate the tribological performance of different laser surface texture patterns, with unpolished material bulges around the dimples, under realistic operating conditions of starved lubrication, for use in air-conditioning and refrigeration compressors. Compared to untextured gray cast iron surfaces, the texture patterns showed significant tribological improvements. Long durability tests also highlighted the long term usefulness of surface texturing. The dominant wear mode of the texture patterns was found to be mechanical polishing and the tribological behavior was found to be largely independent of the type of lubricant or refrigerant.     

The Effect of FeS Solid Lubricant on the Tribological Properties of Bearing Steel under Grease Lubrication

In order to improve the tribological properties of 52100 steel under grease lubrication, FeS solid lubricant was used in two ways. Low-temperature ion-sulfurization technology was utilized to prepare solid lubricant iron sulfide (FeS) films on the surface of 52100 steel, and FeS particles were mixed into the lithium grease as additive. The friction and wear properties were examined systematically on a “ball-on-disc” testing machine. The results showed that the tribological properties of bearing steel under grease lubrication can be improved either by using ion-sulfurization technique or by adding FeS microparticles into the grease. The tribological performance of sulfurized surface lubricated by grease is better than that of a plain surface lubricated by grease containing FeS microparticles at lower load and speed. The plain surface lubricated by the grease containing FeS micropaticles possesses better antiwear property under harsher conditions. The mechanism of the experimental results is discussed in detail.     

The Structural and Tribological Properties of MoS2-Ti Composite Solid Lubricants

MoS ₂ -Ti composite solid lubricant films were deposited on an AISI D2 tool steel and silicon wafer by CFUBMS (closed field unbalanced magnetron sputtering). The deposition process was performed for nine different test conditions at various levels of target current, working pressure, and substrate voltage using the Taguchi L ₉ (3 ⁴ ) experimental method. It was observed that the chemical composition of MoS ₂ -Ti composite films was significantly affected by sputtering parameters. It was determined that the microstructure of composite films is neither crystalline nor amorphous; in other words, it is quasi-amorphous, and (002) and (100) planes characteristic of MoS ₂ occurred. The friction coefficients of the films were determined over 1800 s and at a loading of 10 N by means of a pin-on-disk tribotester. The changes in friction coefficient were related to structural changes based on Ti addition and the different levels of deposition parameters.     

Evaluation of Anti-Wear Performance of PFPE-Soluble Additives under Sliding Contact in High Vacuum

The anti-wear performances of perfluoropolyether (PFPE)-soluble additives were evaluated under vacuum using a vacuum four-ball tribometer with 440C stainless steel balls as test specimens. PFPE derivatives having the hydroxyl, carboxyl and phosphate groups at the end of the Type D-PFPE molecules were studied. The addition of either PFPE-soluble carboxylic acid or PFPE-soluble phosphates to the PFPE base oil remarkably reduced steady wear rates in a vacuum environment, whereas the addition of PFPE-soluble alcohol did not. Contrary to the performance in vacuum, an appreciable increase in wear rate was observed in the air atmosphere with all the types of additives used. The effect of moisture is studied in explaining the high wear rates obtained with the additives in the air environment. The mechanism of boundary lubrication with PFPE-soluble additives is discussed.     

Infrared and Visual Study of the Mechanisms of Scuffing

The development of a reliable model for predicting scuffing requires an understanding of the mechanism of scuffing initiation. This study examines the process of scuffing directly within a contact and thus tests some of the existing, proposed mechanisms.

A lubricated steel ball is loaded and rotated against a sapphire flat and the load increased in stages until scuffing occurs. Two methods of observation are employed. In one, the temperature of the steel ball across the contact is mapped continuously using an infrared microscope. A novel, nodding mirror set-up enables temperature profiles to be taken many times a second. This work shows that scuffing does not occur at either a critical maximum or at a critical inlet temperature. In the second approach, a solid-slate TV camera and video recorder are used to monitor the contact visually up to and during scuffing. This work suggests that, for a range of different lubricants, the onset of scuffing is always immediately preceded by the buildup of fine wear debris in the contact inlet. This then causes starvation followed by extremely rapid scuffing in the rear of the contact.

From these findings, an alternative method of scuffing based upon the influence of wear debris on lubricant film thickness is proposed.     

A comparison of the wear of cross-linked polyethylene against itself under reciprocating and multi-directional motion with different lubricants

A two-piece, silane cross-linked polyethylene (XLPE) finger prosthesis has been developed. To further the knowledge of the wear of XLPE against itself, a number of ‘pin-on-plate’ wear tests were undertaken, under different conditions of lubrication. These were distilled water, bovine serum and dry conditions. A second group of tests were then carried out, in which multi-directional motion was applied to the test pins. All tests had XLPE pins loaded at 40 N rubbing against XLPE plates. All the XLPE came from the same batch.

In all tests, pin wear was much less than plate wear. Under reciprocation only, the least wear was found when bovine serum was the lubricant (k=0.6×10⁻⁶ mm³/N m) and the maximum wear was when distilled water was the lubricant (k=5.8×10⁻⁶ mm³/N m). When multi-directional motion was applied to the test pins, increased wear occurred under lubrication with bovine serum (k=3.4×10⁻⁶ mm³/N m). Surprisingly, wear decreased when distilled water was used (k=0.7×10⁻⁶ mm³/N m), yet wear factors remained similar in the ‘dry’ test (k=0.7×10⁻⁶ mm³/N m). The dry tests had remarkably low wear.     

MoSi2/45#钢在油润滑状态下的摩擦学性能研究

在M - 2 0 0型磨损试验机上进行了金属间化合物MoSi2 / 4 5 # 钢的摩擦磨损试验 ,考察了载荷和润滑状态对MoSi2 材料摩擦磨损性能的影响 ,采用扫描电子显微镜 (SEM)和微探针观察了其磨损形貌 ,并分析了其磨损机理。结果表明 :油润滑明显改善了MoSi2 材料的摩擦学性能。MoSi2 材料的磨损机理在低载荷 (5 0～ 80N)下主要表现为疲劳磨损和磨粒磨损 ,高载荷 (12 0N以上 )下以氧化磨损为主。载荷为 15 0N时 ,MoSi2 材料具有较好的综合摩擦磨损性能 ,摩擦系数和磨损率分别为 0 1和 0 0 0 9g·km-1。     

Evaluation of the Tribological Properties of Polyimide and Poly(amide-imide) Polymers in a Refrigerant Environment

The tribological properties of three grades of polyimide (PI) and one grade of poly(amide-imide) (PAI) polymers are evaluated in a tetrafluoroethane (HFC-134a) environment. Friction coefficient, wear, and interface surface morphology are evaluated at temperatures of 20° and 120°C, sliding velocities from 0.127 to 3.75 m/s, contact pressures from 1.72 to 13.8 MPa, and counterface surface roughness of 0.06 and 0.40 μ Ra. Tests were conducted under dry or boundary lubricated conditions. Limited testing indicate that the polymers under study do not chemically degrade in HFC-134a environment. For the polymer grades tested, the combination of graphite and FIFE as filler materials provide the best wear characteristics in this environment. As expected, the environmental temperature has a strong effect on both the friction coefficient and the wear. Under boundary lubricated conditions, the presence of a liquid lubricant provides one order of magnitude lower wear compared to the wear obtained in dry sliding conditions. Results from tests conducted in air, argon, and HFC-134a show that the tribological properties of the polymers tested do not seem to be significantly influenced by the environment.     

Surface Films and Soluble Degradation Products Formed in a PFPAE Fluid and Their Implications to the Wear of Steel

The soluble degradation products were generated in a linear perfluoropolyalkylether (PFPAE) fluid in boundary lubrication. Perfluoropolyalkylether carboxylic acid species were found in the residual fluids from the sliding tests by vibration spectroscopy. Surface-bound organic and inorganic reaction products were identified by vibration microspectroscopy with a grazing angle objective attachment and X-ray photoelectron spectroscopy (XPS), respectively. Inorganic surface films were found to be composed primarily of FeF₃. A monodentate perfluorocarboxylate surface species was found on the sliding surfaces in 50° and 100°C tests but not found in 150°C tests. The higher friction and wear in 150°C tests as compared to 50° and 100°C tests were attributed to the absence of the perfluorocarboxylate species over the sliding surfaces at high temperatures.     

应用于跨临界CO_2压缩循环的双转子滚动活塞式膨胀机的研究

由于二氧化碳制冷系统的制冷效率与人工合成制冷剂相比处于劣势,故采用跨临界循环,并利用膨胀机回收膨胀功以减少膨胀过程的能量损失,达到提高整个循环效率的目的。针对目前膨胀机存在的诸多问题,设计应用于跨临界CO2压缩循环的新型双转子滚动活塞膨胀机。该膨胀机的一级气缸始终与进气管连通,二级气缸始终与排气管相通,随着转子的转动在两气缸之间形成膨胀空间,因此,不需要专门的进气控制装置。对膨胀机的运行过程进行理论分析和设计计算,得出主要结构参数,并进行受力和力矩计算。分析结果表明,在膨胀过程中,一级转子的总力矩始终为正值,膨胀将结束时两转子的总力矩变为负值。在角度θ2之前,二级转子的驱动力矩大于一级转子的驱动力矩,在角度θ2之后,一级转子的驱动力矩更大。     

Study on the Interaction between Talc and Perfluoropolyethers under Tribological Contact

Particle contamination at the head–disk interface (HDI) is a major concern for the long-term reliability of hard disk drives. In the current study, it is shown that, surprisingly, soft and lubricious talc particles have a significant damaging effect on the friction performance of media lubricants, nanometer-thick perfluoropolyethers. Thermogravimetric analysis (TGA) and the adsorption studies indicate that two mechanisms, Lewis acid–catalyzed decomposition and surface adsorption, are responsible for the observed tribological degradation. The potential impacts of the findings here on the design of next-generation media lubricant have been discussed.     

Preliminary Study of Nano- and Microscale TiO2 Additives on Tribological Behavior of Chemically Modified Rapeseed Oil

This study compared the tribological evaluation of chemically modified rapeseed oil as a potential biodegradable automotive lubricant with and without nano- and microscale titanium dioxide (TiO₂) particles, focusing on the influence of TiO₂ particles to improve the friction reduction and antiwear characteristics of chemically modified rapeseed oil. TiO₂ nano- and microscale particles of anatase phase and rutile phase were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Further, the analysis of chemically modified rapeseed oil with and without TiO₂ additives was carried out to determine its tribological behavior using a pin-on-disc tribometer. The experimental results showed that the addition of TiO₂ nanoparticles exhibited good friction reduction and antiwear properties compared with the addition of microscale TiO₂ and without TiO₂ additives to chemically modified rapeseed oil. Nanoscale TiO₂ is suitable as an antiwear additive in chemically modified rapeseed oil.     

二烷基胺基单硫磷酸酯基三嗪衍生物的摩擦学研究

合成了一系列双烷基胺基三嗪衍生物,2,4-双-二烷基胺基-6-(O,O′-二正丁基二硫代磷酸酯)基-s-l,3,5-均三嗪.在四球摩擦磨损试验机上研究了它们作为菜籽油添加剂的摩擦学性能。实验结果表明,该类化合物具有良好的极压抗磨性能,能提高菜籽油的极压抗磨性能。用X射线光电子能谱（XPS）分析了钢球磨损表面典型元素的化学状态，结果表明，在摩擦过程中，钢球表面形成了一层含硫，磷光机膜和含氮的有机膜。     

Study of the Influence of Contact Geometry and Contact Pressure on Sliding Distance to Galling in the Slider-on-Flat-Surface Wear Tester

One of the major causes of tool failure in sheet metal forming is wear in the form of galling. Galling is gradual buildup of adhered sheet material on the tool and leads to unacceptable scratches on the sheet surface and to components that fail to meet tolerances. Because it is difficult to reproduce operational and interactional conditions in laboratory test equipments it is hard to test, model, and predict galling initiation.

Here the authors examine how changes from elliptical to line contact geometry influenced galling initiation under dry sliding by using a slider-on-flat surface (SOFS) wear tester. A micro clean tool steel was tested against ferritic low-strength and martensitic high-strength steel sheets.

The sliding distance to galling initiation was extracted from friction data and verified by scanning electron microscopy (SEM) observations. The presence of adhesive wear on worn tools after completed tests was used as a criterion. Experimental results showed that the elliptical contact causes galling quicker than the line contact.

Applicability of experimental results depends on the relevance of test conditions, so contact pressures calculated for the described tests were compared to calculated contact pressures in a semi-industrial U-bending test and to literature data relevant to industrial applications. Good agreement between values observed for SOFS and for most selected industrial applications was found, which assume that contact pressures typical for most common industrial applications can be successfully simulated by selection of tool geometry and normal load in the SOFS tester.     

超高压压缩机填料盘微动疲劳裂纹机理的研究

分析了填料盘裂纹的宏观特征和应力状态,并对填料盘磨损产生的原因进行了理论分析,通过计算得知两填料盘之间的位移幅值只有几十微米。依据微动疲劳产生的机理,确定填料盘的裂纹是在微动磨损下产生,并在压缩机交变压力的作用下不断扩展而造成填料盘失效的。     

Synergetic Effect of Lubricant Additive and Reinforcement Additive on the Tribological Behaviors of PEEK-Based Composites under Seawater Lubrication

Polyetheretherketone (PEEK)-based composites reinforced with lubricant additive (polytetrafluoroethylene, PTFE) and reinforcement additives including carbon fiber (CF), glass fiber (GF), and bronze powder were prepared using a hot-press molding technique. The synergetic effects of different additives on the tribological behaviors of PEEK-based composites sliding against 316 steel under seawater lubrication were investigated systematically using a ring-on-block test rig. The results showed that lubricant additive PTFE can decrease the friction coefficient and consequently improved the wear resistance of PEEK under seawater lubrication, especially when the volume fraction of PTFE was about 20%. It was also found that the incorporation of CF can further improve the wear resistance of PEEK blended with 20% PTFE, especially under high load and high sliding speed. This suggested that a synergistic effect on improving the wear resistance of PEEK existed between PTFE and CF, which originated from good lubrication of PTFE, good reinforcement of CF, and good interfacial combination between CF and PEEK-20%PTFE. However, two other reinforcement additives of GF and bronze powder had an antagonistic effect but not a synergetic effect with PTFE; that is, the incorporation of the two additives greatly deteriorated the wear resistance of PEEK blended with 20% PTFE.     

Effect of transfer film structure, composition and bonding on the tribological behavior of polyphenylene sulfide filled with nano particles of TiO2, ZnO, CuO and SiC

The tribological behavior of polyphenylene sulfide (PPS) filled with inorganic nano particles was studied. The fillers investigated were TiO₂, ZnO, CuO and SiC whose sizes varied from 30 to 50 nm. The polymer composites were compression molded with varying proportions of these fillers. Wear and friction tests were performed in a pin-on-disk configuration at a sliding speed of 1.0 m/s, nominal pressure of 0.65 MPa, and counterface roughness of 0.10 μm Ra. The polymer composite pins slid against hardened tool steel counterfaces. The transfer films of the composite materials formed on the counterfaces during sliding were studied by optical microscopy and X-ray photoelectron spectroscopy (XPS) and the adhesion between the transfer film and counterface was measured in terms of the peel strength. It was found that the wear rate of PPS decreased when TiO₂ and CuO were used as the fillers but increased with ZnO and SiC fillers. The optimum wear resistance was obtained with 2 vol.% CuO or TiO₂. These filled composites had the coefficients of friction lower than that of the unfilled PPS. The wear behavior of the composites is explained in terms of the topography of transfer film and adhesion of transfer film to the counterface as observed from peel strength studies. There is a good correlation observed between the transfer film-counterface bond strength and wear resistance.     

超临界CO2流体萃取流程中CO2尾气回收压缩机的设计及应用

某公司超临界CO2流体萃取流程中CO2尾气回收压缩机抽气时间比预期时间超出4倍以上,为解决这一问题,通过对超临界流体萃取流程和CO2的特性分析,提出了萃取流程中CO2尾气回收压缩机的设计方法及回收流程方案的选择,并对压缩机的使用与保养做了阐述。