Tribological characteristics of aloe mucilage

Abstract

Increased concerns about environmental damage caused by many lubricants, has created a growing worldwide trend of promoting new environmentally friendly lubricants. The tribological characteristics of aloe mucilage as a kind of original biolubricant have been investigated in the present work. The experimental results indicate that the variation of the film thickness of aloe mucilage is not the same as that in traditional elastohydrodynamic lubrication, but conforms to the lubrication regime of thin film lubrication under the present experimental conditions. The coefficient of friction (COF) of the aloe mucilage among different tribological pairs is significantly decreased by the increase in velocity, while there is little variation when the normal load is increased. The COF of aloe mucilage between WC and DLC surfaces is very small with a value of 0˙04, and the wear resistance of the aloe mucilage between WC/DLC is better than that between WC/Si and WC/steel.     

The Microstructure and Wear Characteristics of Cu–Fe Matrix Friction Material with Addition of SiC

The Cu–Fe matrix continuous braking friction materials using SiC as abrasive were fabricated by powder metallurgy technique, and the effect of content and size of SiC were investigated. The tribological properties of friction materials sliding against AISI 1045 steel ring were carried out on a block-on-ring tester at different loads and sliding speeds. The strengthening effect of nano-SiC (55 nm) was superior to that of micro-SiC (70 μm) of the tribological properties for friction materials. The friction coefficients of friction materials increased with increasing nano-SiC content. However, the wear rates decreased with increasing nano-SiC content and then increased when the content of nano-SiC particle exceeded 10 wt%. The specimen contained 10% nano-SiC had the best tribological properties at different testing conditions.     

Tribological Performance of Lubricating Greases Based on Calcium Carbonate Polymorphs Under the Boundary Lubrication Condition

In this article, we synthesized the calcium sulphonate grease (CSG) based on the calcite using the bright stock (150BS) as the base oil. In order to investigate the tribological performance of lubricating grease containing different calcium carbonate polymorphs under boundary lubrication condition, a calcium sulphonate complex grease (CSCG) based on the vaterite was used as a reference. An oscillating reciprocating friction and wear tester set at a series of applied loads and frequencies was adopted to evaluate the tribological performance under boundary condition. Results showed that the lubricating grease that was composed of crystalline calcite as the partial thickener had excellent friction-reducing and antiwear (AW) properties, regardless of the applied loads and frequencies. The vaterite in CSCG easily experienced a polymorph transformation into calcite or aragonite characterized by Raman spectroscopy. This polymorph transformation was attributed to the highly local friction temperature and activated hydrogen from water or acids oxidated in the rubbing process at high load or frequency. The physical polymorph transformation corresponded to the fluctuations of the friction coefficients, then contributed to the severe wear. XPS analysis indicated that two calcium sulphonate lubricating greases occurred a tribochemical reaction and boundary tribofilms consisted of CaCO₃, CaO, iron oxide and FeSO₄ were formed on the rubbing surfaces. The tribofilm formed by the introduction of the CSG that mainly depended upon the thickeners of calcite structure contributed to an excellent AW protection. The possible boundary friction mechanism for greases based on various calcium carbonate polymorphs was also proposed. Effect of calcium carbonate polymorphs on the tribological performance was discussed.     

碳酸钙纳米粒子润滑油添加剂的制备及其摩擦学性能的研究

本文利用十二烷基硫酸钠/异戊醇/环已烷/水微乳液体系制备了碳酸钙纳米粒子,用透射电镜(TEM)、X—射线衍射仪(XRD)和动态光散射仪(DLS)测定其物理形态,并将其作为添加剂分散到500SN基础油中,用四球实验机考察了其摩擦学性能和用X—射线光电子能谱仪(XPS)对磨斑表面进行分析。实验结果表明:所制备的碳酸钙纳米粒子的粒径约为13nm、呈球形和六方型晶体结构,具有较好的单分散性;具有较好的摩擦学性能,少量的碳酸钙纳米粒子即可提高润滑油的抗磨减摩性能。其摩擦机理是在磨斑表面形成了含有碳酸钙和由其分解而成的氧化钙的保护膜,从而表现出较好的摩擦学性能。     

花状与球状MoS2对PAO15润滑油摩擦学性能的影响

为改善低黏度PAO15润滑油的摩擦学性能,通过水热法制备球形与花状MoS_2颗粒,采用X射线衍射仪(XRD)与扫描电子显微镜(SEM)对所制备的MoS_2颗粒进行表征。制备球形与花状MoS_2改性的PAO15油,利用四球摩擦试验机对比研究2种形貌MoS_2在不同用量条件下对PAO15油摩擦学性能的影响。采用光学显微镜、表面轮廓仪、扫描电子显微镜(SEM)与能谱仪(EDS)对磨痕表面进行表征。结果表明:制备的球形与花状MoS_2晶型均较好地符合MoS_2的晶型,掺杂至PAO15油中均能够提升其摩擦学性能,使其摩擦因数降低;随着MoS_2颗粒添加量的增加,PAO15油摩擦学性能有所提升,在质量分数为1.0%时达到最优;花状MoS_2具有更大的比表面积,其对PAO15油抗磨损性能的提升优于球形MoS_2,形成的转移膜能够更好地起到隔离摩擦表面的作用。     

Effects of Zinc Borate and Fly Ash on the Mechanical and Tribological Characteristics of Brake Friction Materials

Friction materials based on several combinations of zinc borate (ZB) and fly ash (FA) were fabricated and characterized for their mechanical and tribological performance. The triboperformance of the friction materials was evaluated on a Chase friction testing machine according to the brake lining quality test procedure as per SAE J661. The composites were manufactured based on a nonasbestos organic-based friction material for an automotive brake system and contained typical ingredients for commercial brake friction materials. The composites had a fixed composition of 15 wt% resin, 15 wt% fibers, and 5 wt% friction additives. ZB and FA as fillers were added to the the raw materials mixture at a total fraction of 65 wt%. The results showed that ZB and FA contents have a significant influence on the mechanical and tribological properties of the friction composites. In particular, the composites containing 0–5 wt% ZB and 65–60 wt% FA showed better friction stability and improved fade resistance compared to those containing 10–35 wt% ZB and 55–30 wt% FA, whereas the composite with 5 wt% ZB and 60 wt% FA showed a maximum friction coefficient. On the other hand, the specific wear rate of the composites decreased with increasing ZB and decreasing FA contents. The morphologies of the worn surfaces as well as wear debris were analyzed by means of scanning electron microscopy.     

Role of Al2O3 in Semi-Metallic Friction Materials and its Effects on Friction and Wear Performance

In this work the semi-metallic friction composites with moderately increased content of Al₂O₃ (0, 3.4, 5.6, 9.0, and 14.6 vol%) were successfully fabricated by casting technology and their tribological properties were evaluated. The character of friction surfaces was confirmed by SEM. The sample with 5.6 vol% of Al₂O₃ offers the best tribological property among the prepared formulations, and is characterized by high and steady friction coefficient (about 0.45) and low wear. With the temperature increasing, the stibnite patches population on friction surface increases and individual steel fibers spread due to their plastic deformation to form larger primary contact plateaus. The area covered by contact primary plateaus decreases with the alumina content increasing.     

The Wear Resistance of HVOF Sprayed Composite Coatings

The aim of the study was to determine the interactions between standard antiwear zinc dialkyldithiophosphate (ZDTP)-type additives and composite coatings containing hard phases of Al₂O₃, SiC, and TiN in the nickel matrix. The analysis was conducted for selected ceramic materials with different structures and different tribological behavior of ionic, covalent, and metallic bonds. The composite coatings were deposited on C45 steel using the high-velocity oxygen-fuel (HVOF) process. This process efficiently uses high kinetic energy and controlled thermal output to produce dense, low porosity coatings with highly predictable chemistries that are homogeneous in structure. The coatings can operate under harsh service conditions, because they are characterized by higher durability and higher wear and corrosion resistance. It was necessary to determine the interactions between the ZDTP-type antiwear additives (zinc dialkyldithiophosphates) and the coatings. The tribological properties of nickel and nickel-based composite coatings were examined by means of a T-01 M tester functioning in the ball-on-disc configuration during technically dry friction and boundary lubricated friction with lubricants containing 1% ZDTP. The comparative analysis confirmed different tribochemical activity and, accordingly, different tribological effectiveness of the nickel and nickel-based composite coatings during friction.     

Silicone Surfactants as Potential Lubricating Substances and Bases

This article presents an assessment of the physicochemical and tribological properties of silicone polyether aqueous solutions. Raw materials used in this research were characterized by different oxyethylation and oxypropylation degrees: PEG/PPG-20/20 dimethicone, bis-PEG/PPG-20/20 dimethicone, and PEG/PPG-25/25 dimethicone. The physicochemical properties were determined by surface tension, wetting angle, and atomic force microscopy images. The tribological properties were characterized by the motion resistance changes and wear in a concentrated ball (steel)-on-disk (steel) contact. The results of physicochemical properties indicate that these compounds are surface active and characterized with good wettability of the steel surface. In addition, it was shown that under the conditions of friction they can form an adsorption layer, which lowers friction and reduces wear of friction couples. In summary, oxyethylated and oxypropylated silicone aqueous solutions, particularly at higher concentrations (60–80%), and pure compounds can be used as potential and effective lubricants.     

Effect of MWCNTs-COOH Reinforcement on Tribological Behaviors of PI/MWCNTs-COOH Nanocomposites Under Seawater Lubrication

The carboxyl-functionalized multiwalled carbon nanotube (MWCNT-COOH) was achieved by grafting carboxyl (COOH) groups onto surfaces of MWCNTs. Then polyimide (PI)-based nanocomposites reinforced with MWCNTs-COOH and MWCNTs were prepared by in situ polymerization and the tribological behaviors of PI/MWCNTs-COOH and PI/MWCNT nanocomposites were studied under dry friction and seawater lubrication. The results showed that the incorporation of MWCNTs-COOH and MWCNTs could greatly improve the wear resistance of PI because of the lubricating effect of MWCNTs-COOH and MWCNTs. In additon, the PI/MWCNTs-COOH exhibited better tribological performance than the PI/MWCNTs under dry friction due to functionalization of MWCNTs. In addition, PI/MWCNTs-COOH nanocomposites presented better tribological properties under seawater lubrication than other conditions because of the excellent lubricating effect of seawater, especially when the content of MWCNTs-COOH was 0.7 wt%. Furthermore, the effects of applied load and sliding speed on the tribological behaviors of PI/MWCNTs-COOH nanocomposites were studied under seawater lubrication. It was found that 0.7 wt% PI/MWCNTs-COOH nanocomposites had the best friction reduction and antiwear properties when the applied load and sliding speed were 3 N and 0.26 m/s, respectively, under seawater lubrication.     

Tribological Properties of Lubricating Films on the Al-Si Alloy Surface via Laser Surface Texturing

Dimpled textures were prepared by using a pulse solid laser on the surface of Al-Si alloy. The combination of laser surface texturing (LST) and MoS₂ solid lubricant as well as their tribological properties were investigated in this article. The obtained friction and wear data were critically analyzed to investigate how the parameters of texture influence the tribological performance of Al-Si alloy. Furthermore, morphological investigations of the transfer layers on the worn surfaces were performed and the wear mechanisms are discussed. The results show that the combination of LST and solid lubricant improves the tribological characteristics of Al-Si alloy. The friction coefficient of Al-Si alloy: steel friction pairs can be reduced to 0.15 under dry friction. The lubrication mechanism is attributed to a synergetic effect of providing solid lubricant and traps wear debris in the dimples. It was found that the optimum density of structure was 37% for excellent tribological properties.     

Tribological Characteristics of Rubber-Based Friction Materials

This article deals with the rubber-based friction materials (RBFMs) which can be used in brake system. The physico-mechanical and tribological properties of a series of fiber filled RBFMs containing steel wool and aramid pulp at different concentrations along with a fiber-free reference material were characterized. Rubber–glass transition induced at higher sliding velocities was identified based on the friction fade behavior of the RBFMs. The rubber–glass transition which is inherently originated by viscoelastic response of polymeric binder was found to be influential on the tribological properties of the RBFMs. It was revealed that steel wool increased coefficient of friction (COF) and improved friction recovery behavior at low volume percent (7.5 vol.%) but it aggravated the COF at high concentration of steel wool (15 vol.%) and severe sliding conditions because of harsh abrasive mechanism. Aramid pulp improved the fade behavior at high sliding velocities and increased COF due to formation of sticky contact patches. It was revealed that steel wool increased the wear rate while aramid pulp did not affect the wear rate significantly, contrary to the increase in the friction coefficient of RBFM. SEM analysis was proved to be useful in correlating the wear rates of composites to the topographical changes on the worn surfaces.     

MoSe_2纳米片的制备及其摩擦性能

通过固相反应方法合成了MoSe2纳米片,并采用X射线衍射仪、扫描电子显微镜、透射电子显微镜等对其进行了表征;通过摩擦试验研究了MoSe2纳米片作为150bn润滑油添加剂的摩擦性能。结果表明:MoSe2具有纳米片状结构,长为100～500 nm,厚为10～50 nm,添加MoSe2纳米片基础油的摩擦因数要比添加MoS2纳米片基础油的高,其中MoSe2添加5%(质量分数)的润滑油的摩擦性能最好。     

磁控溅射制备高熵碳化物(AlTiVCrNb)C涂层摩擦学性能

为改善高熵合金涂层的摩擦学性能,通过石墨与AlTiVCrNb高熵合金靶共溅射制备(AlTiVCrNb)C涂层,采用扫描电子显微镜(SEM)、X射线衍射仪(XRD)分析涂层的成分、表截面形貌和物相,采用纳米压痕仪、球盘式摩擦磨损试验机测试涂层的硬度、弹性模量和摩擦学性能,采用白光干涉三维形貌仪表征涂层的磨损情况。试验结果表明：随着涂层中碳含量增加,高熵组分从BCC/FCC双相向单一FCC结构转变,且涂层的微观组织结构也随之变化;由于碳化物的形成和固溶强化,涂层呈现良好的摩擦学性能;在涂层碳原子分数为20.83%时,涂层的摩擦性能和力学性能达到最优,此时摩擦因数最低,为0.35,涂层硬度与弹性模量最高,分别为17.84、182.72 GPa。研究表明,在磁控溅射工艺中石墨与AlTiVCrNb高熵合金共溅射,可以获得摩擦学性能良好的高熵碳化物(AlTiVCrNb)C涂层。     

Friction coefficient and wear rate of sputter-deposited thin films and plasma-sprayed thick coatings at high temperatures in room air

Silver, calcium fluoride (CaF_x with x = 1.85) and chromium-carbon (Cr₃C₂) thin films were deposited onto various tribological test specimens by sputtering. The friction properties of sputter-deposited Ag and CaF_x single layers as well as Ag/CaF_x multilayer films were determined by ball-on-disk tribological tests conducted in room air under various experimental conditions. The tribological properties (friction coefficient and wear rate) of sputter-deposited CaF_x films were also determined at 500°C by pin-on-disk tribological tests performed with pin specimens made of cobalt-based alloy (alacrite). Chromium-carbon films sputter-deposited onto alacrite disk and counterfaces were found to be of interest for reducing the formation of alacrite wear debris in the wear tracks; thus reduced friction coefficient and wear rate values were obtained. The friction behavior of sputter-deposited CaF_x/Cr₃C₂ thin bilayer structures and plasma-sprayed (PS) chromium carbide/Ag/BaF₂-CaF₂ eutectic composite coatings (PS-212 type coatings) was investigated by plane-on-plane tribological tests conducted in room air at 500°C and 700°C. The friction performance of solid lubricant thin bilayer films was compared with that of thick PS-212 type coatings similar to coatings developed by NASA.     

Tribological Investigation of MC PA6 Reinforced by Boron Nitride of Single Layer

Monomer casting polyamide 6 (MC PA6)/boron nitride of single layer (SBN) nanocomposites were successfully synthesized by in situ ring-opening polymerization. SBN was prepared by sonication assisted with solution beforehand. Studies on frictional and wear performance of the nanocomposites were carried out on a block-on-ring tester. The results showed that the nanocomposites had lower wear rates and friction coefficients in comparison with neat MC PA6. The incorporation of 0.25 wt% SBN into MC PA6 significantly reduced wear and friction under dry sliding; however, with further increasing the SBN loading, both wear rate and friction coefficient began to increase. The SEM micrographs of the worn surface revealed their friction and wear mechanisms. Pure MCPA6 was characterized by severe lamellar spalling with adhesive wear being the major wear form which caused high friction and wear. The wear modes of the nanocomposites became mainly abrasive wear with lower content of SBN and displayed fatigue wear with higher content of SBN. The morphology of a uniform transfer film on the counterpart ring and fine wear debris for the nanocomposites corresponded to the improved tribological performance.     

Indirect monitoring of the dynamics at the contact interfaces of a pair of abrading titanium specimens

The novel method of using continuous tribo-electrification variations to monitor the dynamic tribological properties between metal films has been applied successfully [Y.P. Chang, A novel method of using continuous tribo-electrification variations for monitoring the tribological properties between the pure metal films, Wear, 262 (2007) 411–423]. The method was shown to produce clear and strong signals, superior to monitoring continuous friction coefficient variations. However, the above method was only shown to be suitable for the tested material pairs that were studied. In this paper, the method was improved and applied to monitoring the dynamic tribological properties between titanium oxide (TiO₂) films in the friction process. The experiment was conducted on a purposed-designed friction tester with a suitable measuring system. In order to investigate the tribological property of titanium (Ti) sliding against Ti with TiO₂ films in detail, the continuous variations of electrical contact resistance and friction coefficient were measured for monitoring the onset of film rupture between the TiO₂ films and the chemical reactions between the interfaces. Wear loss was measured by an accuracy balance and scan electron microscopy was used to observe the microstructures and material transfer. The experiments demonstrated that the novel method of using electrical contact resistance variations has great potential for monitoring the dynamic tribological properties and the chemical reactions of titanium specimens.     

Frictional Properties and Mechanisms of an Organic–Metal Brake Pair Braking Repeatedly in Magnetic Field

With the aim to investigate repeated braking of organic–metal brake pairs, tribological and scanning electron microscopy (SEM) experiments were performed to reveal the influence of a magnetic field on the tribological performance of brakes. A nonasbestos copper-based brake pad and gray cast iron brake disc were selected as the brake pair. The X-DM pad-on-disc friction tester was improved to set up a tribological tester under a magnetic field. The worn surfaces were observed by SEM to reveal the friction mechanisms. It was found that a magnetic field can ameliorate the dynamic friction and wear. In addition, the global mean friction coefficient increases and the wear resistance of brake materials improves. A magnetic field promotes surface oxidation and aggravates the surface heat emission condition. As a result, the mean temperature on the friction surface increases obviously. An appropriate magnetic field can improve the dynamic temperature rise and decrease the global temperature rise on the friction surface. It is considered that a magnetic field has important influences on tribological performance in repeated braking. Therefore, this research could provide theoretical references for studying the tribological performance in repeated braking and/or under a magnetic field.     

改性碳纤维增强纸基摩擦材料摩擦磨损性能

采用双-[γ-(三乙氧基硅基)丙基]四硫化物(Si69)和乙烯基三乙氧基硅烷(A151)对PAN基碳纤维(CF)进行表面改性处理,利用SEM、FTIR、EDX对改性前后的CF进行表征,测量接触角和表面能、力学性能和界面性能;通过湿法成形技术,制备不同改性CF增强聚酰亚胺纸基摩擦材料,并测试其孔隙率和摩擦学性能。结果表明:与未改性CF相比,Si69和A151能够有效地增加CF表面粗糙度,且新基团的引入使接触角变小,提高了CF表面活性,改善了纤维与树脂之间的结合力,使得A151-CF表面能增加了37.3%,Si69-CF表面能增加了109.4%,A151-CF/聚酰亚胺复合材料界面性能增加了19.1%,Si69-CF/聚酰亚胺复合材料界面性能提高了45.3%;相比未改性CF,Si69改性CF使纸基摩擦材料孔隙率下降了20.2%,A151改性CF使纸基摩擦材料孔隙率下降了8.8%;表面改性CF能够提高纸基摩擦材料的摩擦学性能,其中Si69改性CF增强纸基摩擦材料摩擦学性能优于A151改性CF增强纸基摩擦材料。     

Tribological investigation of CaF2 nanocrystals as grease additives

Calcium fluoride (CaF₂) nanocrystals with average grain size of 60 nm were synthesized via a precipitation method. The morphology and structure of nanocrystals were characterized by means of transmission electron microscopy (TEM) and X-ray powder diffraction (XRD). TEM and XRD showed that CaF₂ nanocrystals are cubic particles in submicron scale. The tribological properties of the prepared CaF₂ nanocrystals as an additive in lithium grease were evaluated with a four-ball tester. The results indicated that these nanocrystals exhibit excellent antiwear, friction reduction and extreme pressure (EP) properties. It was also found that the EP and antiwear capabilities of the grease are not proportional to the content of CaF₂ nanocrystals but there existed a certain value. The rubbed surface after friction test was investigated with X-ray photoelectron spectroscopy and scanning electron microscopy to understand the action mechanism. The results show that a boundary film mainly composed of CaF₂, CaO, iron oxide and some organic compounds was formed on the rubbed surface after friction test and the thickness of boundary film was about 12 nm. The disproportion of stoichiometric ratio of Ca and F in boundary lubrication film indicates that tribochemical reaction of CaF₂ nanocrystals occurred on the worn steel surface at severe tribological conditions.