Unlubricated Conditions Research Articles

Effect of Tightening Speed on Thread and Under-Head Coefficient of Friction

Abstract Applying torque to a fastener creates friction as well as clamp-load. Friction and clamp-load are inversely proportional: as friction increases, the amount of clamp-load generated decreases. The speed at which a fastener is tightened has a pronounced affect on the magnitude of friction, and thereby clamp-load generated in a metal joint. This paper examines the relationship between the tightening speed with friction and clamp-load. The applied torque, clamp-load, and under-head torque were measured as the fastener was tightened within a torque/tension load cell. The corresponding thread torque, under-head and thread coefficients of friction values were calculated from equations given in both the DIN946 and ISO16047 standards. One bolt, one washer, and one nut (all of similar hardness) were used for all trials of this study. Two different anti-seize lubricants were used (one composed of molybdenum-disulfide and graphite and the other of mineral oil, nickel, and graphite). Lubricant was applied to both the thread and under-head regions of the bolt prior to torquing. Further trials were executed with the bolt, washer, and nut in the dry, or plain, condition (all of similar hardness). The results of this investigation indicated that the thread and under-head coefficients of friction decreased as speed increased. The programmed speeds ranged from 25 to 700 r/min, with a torque target of 28 N·m. Since the friction coefficients decreased, the amount of clamp-load generated at target torque increased with increasing speed. The average difference in clamp-load from minimum to maximum speed was determined to be 6.3 kN for both lubricants and 1.2 kN for the unlubricated condition.

Friction and wear behaviour of β-silicon nitride–steel couples under unlubricated conditions

The friction and wear behaviour of colloidally processed and pressureless sintered β-silicon nitride (Si3N4) ceramics against steel DIN Ck45K under unlubricated condition were investigated using a pin on disk tribometer. β-silicon nitride (Si3N4) ceramics consolidated by slip casting from suspensions with different solid loading have been studied, aiming at increasing the use of β-Si3N4 as cutting tools in industrial applications. The morphology of the worn surfaces of β-silicon nitride ceramics was analysed by scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS). Under the conditions used, it was found that β-Si3N4 ceramics exhibited a low wear rate (10−6 mm3 N−1 m−1) and the frictional behaviour of β-Si3N4 ceramics–steel couples depended on a metallic layer transferred from the steel disk to the β-Si3N4 ceramic pin.

Wear properties of nickel coating film plated from emulsion with dense carbon dioxide

The microstructure and wear behavior of nickel coating film was investigated under dry conditions using a pin-on-plate type reciprocating diamond scratch tester. The nickel coating film was produced on brass substrate by an electroplating reaction that was carried out in an emulsion of dense carbon dioxide (CO 2) and an electroplating solution with nonionic surfactants. The microstructures of nickel coating film and wear characteristics were investigated by surface analysis using optical microscope, roughness tester and SEM. The wear tests were done under unlubricated condition at room temperature. The produced nickel coating films have a higher uniformity, a smaller grain size, and a significantly higher Vickers hardness (680 Hv) demonstrated very good friction and wear resistance properties. The results also showed that this nickel coating film plated from emulsion with dense CO 2 has better wear stability than that of nickel coating film produced with conventional electroplating system.

5228 無潤滑条件下におけるWCサーメット溶射皮膜の耐久性 : 母材硬さと下地加工の影響(S66-1 サステイナブルトライボロジー(1),S66 サステイナブルトライボロジー)

Using a two-roller testing machine, the authors examined the surface durability of thermally sprayed WC-Cr-Ni cermet coating under unlubricated condition. By means of the high energy type flame spraying (Hi-HVOF) method, the coating of about 110μm in thickness was formed onto the axially ground, blasted and circumferentially ground roller specimens made of a thermally refined carbon steel or a carburized and hardened steel. The WC cermet coated roller finished to a mirror-like condition with a roughness of 0.1μmRz was mated with the carburized and hardened steel roller without coating ground to a roughness of 0.1μm〜8μmRz, and a maximum Hertzian stress of P_H=1.2GPa was applied in line contact. In the case of the thermally refined steel substrate, the life to flaking became short independent of the substrate surface finish. On the other hand, the carburized hardened steel substrate roller showed a higher durability. The effect of substrate material on the durability of thermally sprayed coating was discussed by the elastic-plastic analysis of the subsurface layer.

413 無潤滑下における炭化ケイ素セラミックスとWC系超硬合金のすべり摩耗特性(材料工学IVほか)

413 無潤滑下における炭化ケイ素セラミックスとWC系超硬合金のすべり摩耗特性(材料工学IVほか)

Acetal gear wear and performance prediction under unlubricated running condition

Tests have shown that the acetal gear wear rate will be increased dramatically when the load reaches a critical value for a specific geometry. The gear surface will wear slowly with a low specific wear rate if the gear is loaded below the critical condition. The sudden increase in wear rate may be due to the gear operating temperature reaching the material melting point under the critical load condition. Gear surface temperature has been investigated in detail by studying three components: ambient, bulk and flash temperatures. Through extensive experimental investigations and modelling on gear surface temperature variations, a general relation has been built up between gear surface temperature and gear load capacity. A new design method for acetal gear has been proposed based on the link between polymer gear wear rate and its surface temperature. The method has been related to test results under different operating ambient temperature and gear geometries. Good agreements have been achieved between the proposed method predictions and experimental test results. The maximum torque the acetal gears can transmit is 9 Nm for both the driver and driven with a 2 mm module, 30 teeth and 17 mm face width. Copyright © 2006 John Wiley & Sons, Ltd.

Damping characteristics of unreinforced, glass and carbon fiber reinforced nylon 6/6 spur gears

Polymer based gears replace metal gears in many light duty power and/or motion transmission applications due to their noiseless operation even under unlubricated conditions. Visco elastic behaviour of the polymer, which is mainly responsible for the sound absorption, is altered by the addition of short fibers. Addition of high modulus fibers to the base polymer matrix reduces the damping characteristics of the composite. Material damping affects the hysteresis heating as well as the ability to absorb vibration during service. Dynamic mechanical analysis carried out on unreinforced Nylon 6/6, 20% short glass and 20% carbon fiber reinforced Nylon 6/6 gear materials indicates the reduction of damping factor due to the incorporation of fibers. Injection molded spur gears made of unreinforced and reinforced materials were tested for durability in a power absorption type gear test rig. Surface temperature of the test gears and noise generated near the gear mesh region were continuously measured and monitored using a personal computer based data acquisition system. Test results indicate that the reinforced gears generate more gear mesh noise than unreinforced gears.

Tribological characterisation of siliconcarbonitride ceramics derived from preceramic polymers

Amorphous SiCN ceramics were prepared in a laboratory scale as disk shaped specimens with 10 mm diameter and 0.3 mm thickness. The friction and wear behaviour was characterised in gross slip fretting tests under unlubricated conditions at room temperature against steel (100Cr6) and ceramic (Al 2O 3). Tests with a ball-on-disk contact were performed in laboratory air with different content of water vapour. The results show clearly that the relative humidity has a significant effect on friction and wear behaviour. All tests in dry air lead to higher friction and higher wear rate than in normal air. Improved friction and wear behaviour was observed with increasing pyrolysis temperature up to 1100 °C of the SiCN specimens. This is attributed to increasingly better mechanical properties and higher stiffness of the amorphous network due to the evaporation of gaseous organic species and the formation of free graphite like carbon.

Fretting wear behavior of liquid nitrided structural steel, En24 and bearing steel, En31

Fretting wear failures are reported in many bearing-shaft assemblies. Fretting wear characteristics of structural steel, En24 in as-received, hardened–tempered and liquid nitrided condition, when fretted with hardened–tempered bearing steel, En31 is reported in this paper. Tests were conducted at different normal loads and at constant slip amplitude under unlubricated conditions. Coefficient of friction under fretting conditions and wear resistance were measured. Surface chemistry influences the fretting wear behavior more than the hardness of the material. The mono phase epsilon iron nitride structure of compound layer formed in liquid nitriding process offers improved fretting wear resistance against bearing steels. The fretting wear resistance also depends upon the normal load and the nature of contact, stick–slip or gross slip.

Wear Mechanisms in Functionally Graded Aluminium Matrix Composites: Effect of the Presence of an Aqueous Solution

Functionally graded aluminium matrix composites reinforced with SiC particles are attractive materials for a broad range of engineering applications whenever a superior combination of surface and bulk mechanical properties is required. In general, these materials are developed for the production of high wear resistant components. Also, often this kind of mechanical part operates in the presence of aggressive environments, such as marine atmospheres. In this work, aluminium composites with functionally graded properties, obtained by centrifugal cast, are characterised by reciprocating pin-on-plate sliding wear tests against nodular cast iron. Three different volume fractions of SiC reinforcing particles in each functionally graded material were considered. Sliding experiments were performed with and without the presence of a lubricant (3% NaCl aqueous solution). In the case of the lubricated tests, electrochemical parameters (corrosion potential) were monitored during sliding. Friction values were in the order of 0.42 for unlubricated conditions, but varied between 0.22 and 0.37 when the aqueous solution was present. For all test conditions, relatively high wear rates (over 1×10-6 gm-1) were obtained, particularly for the cast iron pin. The volume fraction of SiC particles exerted a net effect on the tribological response of the composites, although conditioned by the presence or absence of the aqueous solution. The worn surface morphology of the composites indicated that the presence of the aqueous solution modifies the protective action promoted by the combined effect of the presence of reinforcing particles as load bearing elements and the formation of adherent iron-rich tribolayers. The evolution of the corrosion potential during the sliding action is in accordance to the degradation mechanisms proposed for these systems.

Wear and micropitting of steel ball sliding against TiN coated steel plate in dry and lubricated conditions

Wear of the steel part sliding against titanium nitride both in lubricated and unlubricated conditions have been investigated. An unexpected micropitting like wear on the steel surface was found in lubricated conditions, after very short run time. Possible wear mechanisms—fatigue, chemical reaction or electrical discharge—are discussed. A special attention is given to electrical discharge mechanism. A remarkable result was obtained when the lubricant, white oil—a good insulator fluid—was replaced with glycerin, an electrical conductive fluid. With glycerin as lubricant no micropitting was observed on the steel surface even after long run time. In unlubricated condition intense, predominant oxidative wear of steel part was found.

Effect of CNT distribution on tribological behavior of alumina–CNT composites

Alumina–carbon nanotube (CNT) composites with a different volume fraction of CNTs were fabricated by two different processing methods to investigate the effect of the distribution of CNT in the ceramic matrix on the tribological behavior. Alumina–CNT composites with CNT content up to 12 wt.% were fabricated by tape casting, followed by lamination and hot pressing. For the comparison, composites with the same composition were prepared by hot pressing. The friction and wear behaviors of the CNT reinforced composites were investigated using a ball-on-reciprocating wear tester under an unlubricated condition at room temperature. The wear rate of hot-pressed samples decreased with increasing up to 4 wt.%, but increased with further addition of CNT. The wear rate of the tape casted composites decreased steadily with increasing CNT addition up to 12 wt.%. Density and mechanical properties depending on CNT addition were measured. SEM was employed to observe worn and unworn surface. All the results demonstrated that tape casting process significantly improved the uniform distribution of CNT in the alumina, resulting in enhanced wear resistance of composites.

Comparison of the friction and wear properties of titanium and oxidised titanium in dry sliding against sintered high speed steel HS18-0-1 and against C45 carbon steel

The paper presents the results of conformal pin-on-disc tribological tests concerning the hard oxidised zone, α-Ti(O), created at the surface of a titanium specimens sliding against the sintered and hardened up to 850 HV30, Grade T1 high speed steel (HS18-0-1) counterspecimens. Comparison with the results obtained for the tests performed against hardened up to 620 HV30 C45 carbon steel counterspecimens has been done. Testing was carried out at room temperature and at unlubricated conditions. After tests the structures and compositions of the worn surfaces and wear debris were a subject of LOM, SEM observations and X-ray microanalyses. In comparison with the baseline—the Ti/T1 HSS couple, the wear resistance of the α-Ti(O)/T1 HSS couple appeared to be poorer with the factor of 1.4–2.3. Strong adhesion between mating bodies and abrasive action of the hard α-Ti(O) wear particles were found responsible for high intensity of wear of the oxidised titanium. In opposite to that, the wear resistance of the α-Ti(O)/C45 carbon steel couple appeared to be better with the factor of 28–68. Fine powder of iron oxides between mating bodies has been found playing protective role.

Tribological properties of Ti-aluminide reinforced Al-based in situ metal matrix composite

For structural application of moving components, the tribological properties (friction and wear) are considered to be one of the major factors controlling the performance. In recent years, lightweight metal matrix composites (MMC) have received wider attention for their technological application, such as automotive parts etc. This paper reports the tribological behavior of Al based composites reinforced with in situ Ti x Al y and Al 2O 3 particles. The wear experiments were performed on a newly designed fretting tribometer to evaluate the role of intermetallic particulates on the wear performance of in situ composites against bearing steel under the ambient conditions of temperature (22–25 °C) and humidity (50–55% RH). Based on the topographical observation of the worn surfaces the plausible wear mechanisms are discussed. An important result is that Al-based composites with 20 vol% reinforcement exhibit an extremely low coefficient of friction of 0.2 under unlubricated conditions. Also, around five times lower wear volume is measured with 20 vol% composites when compared to unreinforced Al.

Electrotribological behavior of Cu−Ag−Zr contact wire against copper-base strip

Cu−Ag−Zr alloy has an excellent combination of mechanical strength and electrical conductivity, and is a promising contact wire material for high-speed electrified railways. An investigation of the electritrobological behavior of Cu−Ag−Zr wire is presented here. Wear tests are conducted under laboratory conditions with a specified sliding wear tester that simulated train motion under an electrical current applied across the sliding interface. The Cu−Ag−Zr alloy wire is slid against a copper-based powder metallurgy strip used in railway systems under unlubricated conditions. Worn surfaces of the Cu−Ag−Zr alloy wire are analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectrum (EDS). Within the studied range of electrical current, normal pressure, and sliding speed, the wear rate increases with increasing electrical current and sliding distance. Adhesive wear, abrasive wear, and electrical erosion are the dominant mechanisms during the electrical sliding processes. Compared with a Cu−Ag contact wire under the same test conditions, the Cu−Ag−Zr alloy wire has much better wear resistance.

Friction and wear properties of titanium and oxidised titanium in dry sliding against hardened C45 steel

The aim of the paper is to present the results of conformal pin-on-disc tribological tests concerning the hard oxidised zone created in superficial layer of a titanium specimen sliding against the hardened to a hardness of 690 HV carbon C45 steel counterspecimen. Tribological testing was carried out at room temperature and at unlubricated conditions. After tests the structures and chemical compositions of the wear debris and worn surfaces were a subject of SEM and X-ray investigations. In comparison with the baseline the technical quality titanium/C45 steel couple, the wear and lifetime performance of the oxidised titanium/steel C45couple was markedly superior. Dominant wear debris discovered to be chips of metallic titanium with local isles of rutile (TiO 2) or to be a fine powder of iron oxides, respectively, to the mating pair. It was confirmed that strong rise of the titanium surface hardness after oxidising increases the couple wear resistance within the factor of ∼160.

Wear and friction behaviour of alumina/mullite composite by sol–gel infiltration technique

Structural ceramics are those ceramic materials that possess sufficient mechanical properties for use as load bearing components. Ceramics based on alumina have been used in commercial applications for many years because of their availability and low cost. In this investigation we studied and explored the influence of mullite phase, speed and load values on friction and wear behaviour of alumina ceramic. Wear tests for alumina and composites contained mullite from 0 to 11 vol% were carried out with a ball-on-disc machine. Tribological tests were under 2.5, 5, 7.5 and 10 N load, at the speeds of 0.15, 0.3 and 0.6 m/s and unlubricated conditions in air. Wear rates were deduced from mass loss. The wear rate for alumina was in order of 6.76 × 10 −5–1.66 × 10 −4 mm 3/N m, while the wear rate of alumina/mullite composites was in the order of 4.6 × 10 −5–1.3 × 10 −4 mm 3/N m. The results indicate that wear rates and friction coefficient of alumina/mullite composites are lower than alumina ceramics.

Tribo-oxidation of cathodic arc ion-plated (V,Ti)N coatings sliding against a steel ball under both unlubricated and boundary-lubricated conditions

The friction and wear characteristics of (V,Ti)N coatings deposited by cathodic arc ion plating (CAIP) have been studied by using a Schwingungs Reibung und Verschleiss (SRV) tester with reciprocating motion against a SAE 52100 steel ball from room temperature to 300 °C. The (V,Ti)N coatings exhibit a friction coefficient of 0.57–1.28 under unlubricated condition and of 0.18–0.19 under boundary-lubricated condition, depending on wear parameters used. The effects of load, frequency and temperature on the friction and wear of the (V,Ti)N coatings have been discussed. The worn surfaces and wear debris generated under different wear conditions are examined by means of scanning electron microscopy and Raman spectroscopy to identify the tribochemical reaction and phase structures generated during the wear process. Metastable FeO-type and stable Fe 2O 3-type iron-based oxides are formed on worn surfaces under both boundary-lubricated and unlubricated conditions as well as some (V,Ti,Fe) 2O 5 and rutile TiO 2 oxides. The dissolved oxygen in the lubricant plays a very important role in oxidational wear of (V,Ti)N coatings under boundary-lubricated conditions.

Ultra Grain Refinement and High Strengthening of Al Based MMC by Accumulative Roll Bonding Process

Aluminum based metal matrix composite (MMC) was processed by accumulative roll bonding (ARB) for ultra grain refinement and high strengthening. The ARB process up to 4 cycles was performed for the composite with 5vol.%SiC at ambient temperature under unlubricated conditions. The ARB of unreinforced aluminum powder compact was also performed for comparison. The tensile strength of the composite increased with the number of ARB cycles, and reached a maximum of 375MPa at the 3rd cycle, which is 1.8 times higher than that of the initial material. An increment of the strength per cycle was much larger in the composite than that in the unreinforced 6061 aluminum powder compact. The elongation of the composite decreased gradually with the number of ARB cycles, became almost zero after 4 cycles. TEM observation revealed that the composites fabricated by 1 to 3 cycles showed a dislocation cell structure, but after 4 cycles it showed an ultra-fine grained structure with mean grain size below 500nm. The ultra-fine grains developed at lower cycles in the composite than in the unreinforced one.

Friction and wear behaviour of cemented carbides

In this paper the friction and sliding wear of WC–Co cemented carbides are studied. Friction and wear tests were carried out using six different WC–Co alloys (Co ranging from 6 to 20 wt.%) under unlubricated conditions against steel (0.45 wt.% C) disk. Tests were performed at sliding velocity of 2.2 m s −1 and normal load of 40 and 180 N. Sliding wear tests were carried out using moderated block-on-ring equipment. It is shown that wear resistance depends on the chemical composition and hardness of cemented carbides. The wear rate of cemented carbides decreases with an increase in hardness and decrease in binder content. Means, standard deviations (S.D.) and COV of wear data were calculated. The morphological analysis of the worn surface and wear debris were made by SEM. The wear mechanism has a complicated character and depends on the applied load and material composition. The wear of WC–Co cemented carbides under dry conditions is caused mainly by removal of the cobalt binder followed by fracture of intergranular boundaries and fragmentation of carbide grains.