Unlubricated Conditions Research Articles

Thermal effects influencing stability and performance of coatings in automotive applications

Abstract Carbon based coatings are well established in the automotive industry to solve tribological problems in automotive applications. In systems under high load and/or high contact pressures, these coatings are used to reduce friction and wear. Other coatings like chromium nitride are useful to prevent scuffing. However, a real challenge in evaluating coatings for their suitability in automotive applications is the broad range of conditions which defines the load collective of the tribological system. In this work, we focused on the thermal effects that might influence the stability and performance of coatings in tribological applications. In a car engine, the temperature range might be much broader than the − 20 °C to 120 °C as given by the average oil temperature. In the tribological contact zone the temperature can be higher up to several hundred degrees Celsius, especially if it comes temporary dry running due to starved lubrication. These locally high temperatures can affect the properties of a coated surface, but the coated surface itself might also have an influence on the temperature in the contact zone. This is especially the case, if coatings with low thermal conductivities like carbon based materials are used. Therefore, we focused our here presented work on the investigation of the influence of the temperature on the wear performance, the friction coefficient and the thermal stability of a DLC coating in unlubricated conditions. The DLC coating was submitted to dry running reciprocating sliding wear tests in a broad temperature range and then the thermal stability of the coating has been analyzed by means of hardness measurements (nanoinentation) and a structural approach (Raman spectroscopy). In addition, we analyzed the effect of a DLC coating on the tribological properties of a lubricated contact on a two-disc tribometer. These efforts were completed by measurements of the thermal conductivity of the DLC coating.

Deposition, characterization, and performance of tribological coatings on spherical rolling elements

Abstract Increasing demands on the performance of intricate parts like bearings in biomedical and aerospace systems have created the necessity for tribological coatings that can enable systems to achieve enhanced performance in challenging environments. In this work, we have developed a process to deposit wear-resistant (Cr x N, WC/a-C:H, TiC/a-C) and solid lubricant (Ti–MoS 2 ) coatings onto spherical rolling elements. The tribological performance of coated specimens has been evaluated under boundary layer lubrication in rolling and sliding contact tribometers. Although all of the coatings are wear-resistant under sliding contact, only Ti–MoS 2 coated balls provide a significant improvement over uncoated balls in rolling contact fatigue experiments. Specifically, in rolling contact fatigue experiments performed using M50 steel rods, Ti–MoS 2 coated balls improved L 50 life by more than two times over uncoated balls. Ti–MoS 2 appears to be a promising candidate for rolling bearing applications in both lubricated and unlubricated conditions.

Experimental study on frictional characteristics of tungsten carbide versus carbon as mechanical seals under dry and eco-friendly lubrications

Lubrication reduces the friction between the interfaces of sliding surfaces in the mechanical seals that operate for extended period of time. Due to environmental issues caused by mineral oil-based lubricants, the use of organic based vegetable oils had increased worldwide due to the nontoxic and biodegradable characteristics. In this work an experimental study was carried out by employing a seal pair of tungsten carbide and resin impregnated carbon mechanical seals, its frictional behaviour was studied under the eco-friendly lubricant from the class of vegetable oils — soybean oil and canola oil with an eco-friendly solid lubricant i.e. boric acid powder. An experimental setup was designed and fabricated to study the frictional characteristics of the seal for varying normal load and constant speed. The friction characteristics was studied under unlubricated conditions, independent paraffin oil, soybean oil, canola oil lubricating modes and finally 1wt.%, 3wt.% and 5wt.% of boric acid powder mixed individually with soybean and canola oil. After all running-in test of all lubricating conditions, 5wt.% of boric acid powder mixed with soybean oil had contributed a hybrid tribofilm and resulted in the lowest friction coefficient value in the range of 0.06–0.07.

Tribological properties of twin wire arc spray coated aluminum cylinder liner

Abstract The effects of honing process on the friction and wear behaviors of Twin Wire Arc Spray (TWAS) coated aluminum cylinder liners were investigated using a pin-on-reciprocating type of a tribotester. Two types of coated cylinder liners were prepared for the tests: Type I — smooth honing (SH) with ~ 40° honing angle, and Type II — helical structure honing (HSH) with ~ 140° honing angle. The aluminum cylinder liners were coated with an Fe0.8C wire by the TWAS process. In un-lubricated condition, Type II specimen showed lower coefficient of friction (COF) compared to Type I specimen. This result was due to the fact that the groove of Type II was sufficiently large to trap the wear particles that may otherwise contribute to three body abrasive wear. In lubricated condition, Type I showed lower COF due to its lower roughness in comparison to Type II. The experimental results indicate that TWAS process can be effectively utilized for engine applications in conjunction with optimum honing process for the cylinder liner.

Microstructure and Wear Behavior of a Manganese Bronze Bearing Material under Unlubricated Conditions

In an attempt to collect information about the tribological performance of copper-based bearings, the friction and wear behaviors of C86300 manganese bronze were investigated. The characteristics of the base material were determined by structural and mechanical investigations. Then, dry sliding pin-on-disc wear tests were performed against an AISI 52100 steel counterface. After the wear tests, the worn surfaces of the pins and wear debris were studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy. In addition, light optical microscopy and microhardness measurements were performed for examination of the steel counterfaces and worn pin subsurface layers. With increasing normal load, the wear rate of commercial C86300 alloy (containing 0.6 wt% Si) decreased initially and then began to increase. After reaching a maximum wear rate at the load of about 60 N, the wear rate decreased again with a further increase in the normal load. However, the wear rate of this C86300 alloy mainly decreased with increasing sliding speed. Adhesive and abrasive wear were the dominant wear mechanisms under the designed conditions.

Tribofilms generated from bulk polyphosphate glasses at elevated temperatures

In this study polyphosphate lubricant was introduced into steel rolling process. The friction coefficient reduced from 0.463 to 0.238 by the presence of polyphosphate lubricant as compared with unlubricated condition, calculated from 2-high Hille 100 rolling mill tests. A tribofilm of ~100nm can be observed on the strip surface, greatly improving the surface finish. This tribofilm well adhered on the oxide layer/steel base by the interfacial reactions. Laser-scanning confocal microscope well confirmed the reactions of polyphosphate lubricant with the oxide scales occurring. Furthermore, a feasible mechanism the composition and structure of this lubricant during thermal reaction was proposed when exposed to different temperatures.

Effect of oxide layers and near surface transformations on friction and wear during tool steel and boron steel interaction at high temperatures

Recent years have seen a continuously growing interest in high temperature tribological research. A significant part of this is driven by the need for improved understanding and knowledge pertaining to friction and wear and their control in the context of hot forming of high strength steels. Friction and wear characteristics of a sliding system are highly dependent on the properties of the two interacting surfaces. At high temperatures, the surface and material properties become extremely important since these systems often operate under unlubricated conditions. High temperature tribological processes are highly complex as these involve changes in mechanical properties due to microstructural changes; thermal softening; surface chemical and morphological changes due to oxidation and diffusion; deterioration of the surface and bulk material as a result of adhesive/abrasive wear and thermal fatigue. Many of these changes occur on the surfaces and/or in the near surface region. The formation of surface oxide layers and near surface layers with a highly refined microstructure (nano-structured) has been reported to have a significant influence on the tribological behaviour. An improved understanding of these effects is a prerequisite in an attempt towards controlling friction and wear at high temperatures. The main aim of this work is to investigate the formation of oxide layers and near surface transformed layers during tool steel and boron steel interaction at elevated temperatures and their relation to the friction and wear response. The results from sliding wear tests showed that under favourable conditions of temperature and load, a reduction of wear by three orders of magnitude and reduced friction by 50% was obtained. This was attributed to the formation of a composite layer structure involving a refined workhardened layer and a protective oxide layer on top. In the case of three body abrasive wear of boron steel, a reduction in wear rate when temperature increased (100–200°C) has also been found. This reduction in three-body wear is due to the formation of a workhardened layer with a mechanically mixed layer of wear debris and fragmented silica particles on top. At higher temperatures (>500°C), the softer matrix due to recrystallisation and phase transformations was unable to maintain a lower wear rate despite the presence of embedded fragmented silica particles.

On Friction and Wear Behavior of WC-12Co Microwave Clad

In the present work, the friction and wear behavior of microwave-clad WC-12Co cermet was examined using a pin-on-disk tribometer as per ASTM G99. Microwave clads were tested against an EN-31 countersurface in unlubricated conditions. The WC-12Co clads were developed using an industrial microwave applicator at 2.45 GHz and 1.4 kW. The influence of varying normal load on the tribological characteristics of the microwave-induced clads have been investigated. Responses of the WC-12Co microwave clads and AISI 304 stainless steel substrate were monitored and the resulting wear was subsequently analyzed in terms of wear rate, pressure–velocity–time (p-v-t) characteristics, and friction coefficient. The worn surfaces of the WC-12Co microwave clad and AISI 304 substrate were studied using scanning electron microscope. Wear debris was analyzed using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The developed clads exhibit significant resistance to wear attributable to the microwave-induced dense microstructure and material properties. The wear rate and friction coefficient were reduced by 67 and 56%, respectively for WC-12Co microwave clad compared to that of the AISI 304 substrate.

Effect of Silver Content on Tribological Property and Thermal Stability of HVOF-Sprayed Nickel-Based Solid Lubrication Coating

NiMoAl–Cr3C2–Ag composite coatings with different mass fraction of silver were deposited by high velocity oxy-fuel (HVOF) spraying, and their tribological properties from 20 to 800 °C under unlubricated conditions were evaluated by CSM high-temperature tribometer. Scanning electron microscopy, X-ray diffraction and Raman spectroscopy were used to characterize the coating and corresponding wear tracks to determine the lubrication mechanisms. Meanwhile, thermal stability of as-sprayed composite coatings were investigated by heat treatment at 800 °C for 200 h. The HVOF-sprayed composite coating showed an extremely dense structure, high bonding strength, excellent friction reduction and wear resistance property. The addition of silver effectively decreased friction coefficient of the coating and reduced wear rate below 400 °C. Characterization of NiMoAl–Cr3C2–Ag coating revealed that silver provided lubrication below 400 °C and Ag2MoO4 that was formed through tribo-chemistry reactions acted a as a high-temperature lubricant. However, when the content of silver increased to 15 %, more evident enrichment of silver on the coating’s surface at 800 °C was observed, which ultimately increased the oxidation degree of the composite coating.

The influence of phase gradient within the micro arc oxidation (MAO) coatings on mechanical and tribological behaviors

The alumina coatings of 100μm thickness were deposited on 6061 T6 Al-alloy through micro arc oxidation (MAO) technique. The phase composition across the coating thickness was evaluated using step-wise grinding followed by X-ray diffraction analysis. The microhardness and elastic modulus were measured through micro and nano indentation techniques respectively on the mounted and polished coating cross-sections as a function of distance from substrate–coating interface. The coatings represent typical graded composite of α-Al2O3 and γ-Al2O3 phases and accompanied by the corresponding hardness and modulus gradients across the coating thickness. Tribological performance of MAO coatings was evaluated through pin-on-disc wear test under dry (un-lubricated) conditions. The results obtained suggest that the transition from γ-Al2O3 rich surface sub-layers to α-Al2O3 rich inner sub-layers is gradual. On the basis of the above experimental results, the critical interrelationships between the coating phase composition, hardness distribution, modulus distribution and the accompanied sliding wear loss have been established. It was found that the simple rule of mixture explains (ROM) the hardness and modulus distributions as a function of phase gradient across the coating thickness while the inverse rule of mixture (I-ROM) is applicable in the case of sliding wear–phase gradient relationship. Further, the relationships established were utilized to evaluate the mechanical and tribological properties of pure α-Al2O3 and γ-Al2O3 phases. The properties of gamma alumina such as hardness, elastic modulus and sliding wear resistance have been reported for the first time in the present work. Further, the worn surface examination indicates that the formation of mechanically mixed layer (MML) which subsequently undergoes abrasive wear and micro-crack induced chipping as the predominant material removal mechanism during dry sliding wear tests.

The effect of surface roughness on the transfer of polymer films under unlubricated testing conditions

Blended Polyetheretherketone (PEEK) and polyimide-based polymers are excellent lighter tribomaterials that make possible the replacement of sliding metallic components in machinery. In this work a quantitative analysis of the effect of surface roughness on the transfer of polymer films was investigated using a specialized tribometer, under air-conditioning and refrigeration specific operating conditions. Specifically, polyimide (Vespel SP-21), polyimide (Vespel SP-211), PEEK with carbon fibers, and Aromatic Thermosetting Polyester (ATSP) polymer pins were tested against gray cast iron disks under unlubricated unidirectional conditions. It was found that a continuous transfer layer of polymer on the surface of the cast iron disks with mirror finish (low surface roughness) was produced in all the combinations of tested blended polymers. In the case of the polymer pins tested against cast iron disks with high surface roughness, a discontinuous transfer layer was observed.

건축단열재 생산시스템 브레이커 마찰특성

Construction heat insulating material for construction is used in large amounts in industry. In the manufacturing process of this insulation material, a thermal insulation material is completed while a polymer in a liquid state passes through Hall breaker. At this time, the quality and form of a product are determined by a hole in the breaker according to the oil pressure of the fluid and the change of the flow velocity. The friction wear action with regard to partner movement between the two levels of quality of materials affects the performance and the lifetimes of machine parts. In this study of a friction test, SM45C, which is a material used to create brake holes, was used. PVC was used to create the specimen. Moreover, an experiment divided a lubricous state and an unlubricated condition. The resulting value over the load of a pin, the revolving speed of a disk, and the standby state of an experimental result disk could be acquired.

Microstructural Characterization and Tribological Behavior of HVOF Sprayed NiMoAl Coating from 20 to 800 °C

NiMoAl coating was deposited by high velocity oxy-fuel spraying from gas-atomized powders and its tribological properties from 20 to 800 °C under unlubricated conditions were evaluated. Scanning electron microscopy, x-ray diffraction, and Raman spectroscopy were used to characterize the coating and corresponding wear tracks to determine the lubrication mechanisms. The friction coefficient of NiMoAl coating that decreased gradually with the increase of temperature exhibited the highest value of 0.8 at 20 °C and the lowest value of 0.29 at 800 °C. Meanwhile, NiMoAl coating also possessed an excellent anti-wear property and the wear rate of the coating maintained at a relatively low value at all test temperatures. Characterizations of worn surfaces revealed that the coating suffered abrasive wear at the low temperature. When the temperature elevated to 600 and 800 °C, molybdenum oxide and nickel molybdate that were formed through tribo-chemistry reactions acted as lubricants at the high temperature. In addition, NiMoAl coating experienced no obvious oxidation or phase transition on the unrubbed surface during the friction test at 800 °C, indicating that the coating performed both thermal stability and lubrication function at the high temperature.

Influence of Subsurface Structure on the Linear Reciprocating Sliding Wear Behavior of Steels with Different Microstructures

The present work investigates the influence of subsurface microstructure on the linear reciprocating sliding wear behavior of a number of steels with ferrite-pearlitic, pearlitic, bainitic, and martensitic microstructures under dry unlubricated condition. The change in the underlying microstructure with depth from worn-out surface of steel sample intimately relates to the associated hardness variation and wear volume. The present paper is not about comparison of wear resistance of steels with different structures; rather it is on mutual influence of wear and substructure for individual microstructure. Inherent toughness of the matrix and ability of microstructural components to get deformed under the reciprocating action of the ball decide the wear resistance of the steels. Bainite has good amount of stability to plastic deformation. Ferrite shows severe banding due to wear action. Work hardening renders pearlite to be wear resistant. Temperature rise and associated tempering of martensite are observed during wear.

Tribological coatings with a chromium-based composite structure obtained by reactive deposition in vacuum

The peculiarities inherent to the chemical and phase compositions, microstructure, and tribological behavior of Cr-O, Cr-C-O, Cr-C-H and Cr-C-N-H coatings, obtained by plasma-assisted physical vapor deposition in a chemically active gas atmospheres (activated reactive deposition and magnetron sputtering methods), have been studied. It has been demonstrated that all coatings have a micro- and/or nanocomposite structure comprising nanoscale chromium-based domains, as well as chromium oxide, carbide and nitride phases with different stoichiometries. The tribological properties of the coatings investigated in a broad temperature range under unlubricated friction contact conditions, as well as their correlation with microstructures and reactive-deposition technology parameters, are discussed.

HVOF-Sprayed Adaptive Low Friction NiMoAl–Ag Coating for Tribological Application from 20 to 800 °C

An adaptive NiMoAl–Ag composite coating was deposited by high-velocity oxy fuel spraying, and its tribological properties from 20 to 800 °C under unlubricated conditions were evaluated using a CSM high-temperature tribometer. Scanning electron microscopy, X-ray diffraction and Raman spectroscopy were used to characterize the coating and corresponding wear tracks to determine the lubrication mechanisms. The results showed that the friction coefficient of the NiMoAl–Ag composite coating was around 0.3 from 20 to 600 °C and reached the lowest value of 0.09 at 800 °C. Meanwhile, wear rates of the coating were maintained on the order of 10−5 mm3/N m at the test temperatures except for 400 and 600 °C. Characterization of the NiMoAl–Ag coating revealed that silver provided lubrication below 400 °C. Ag2Mo2O7 and Ag2MoO4, which were formed through tribochemical reactions, acted as high-temperature lubricants above 400 °C. It was especially proposed that silver in a nearly molten state was effective in reducing the friction of the NiMoAl–Ag coating at 800 °C. Moreover, a comprehensive lubrication mechanism model of an NiMoAl–Ag composite coating at 800 °C was established to explain the extremely low friction coefficient and wear rate of the coating.

Effect of Processing Parameters on Properties of Plasma Sprayed CuAl Coating

Plasma sprayed aluminum–bronze (CuAl) alloy coating was fabricated on steel substrate, and the effects of processing parameters on the structure and tribological behaviors of the resultant CuAl coating were reported. Results show that although the aluminum–bronze coating deposited with high primary gas flow rate and low energy density (input power divided by primary gas flow rate) has the highest hardness and lowest defect level, it just possesses a medium wear resistance under sliding wear condition. The main wear mechanism of plasma sprayed CuAl coating is splats delamination resulted from surface fatigue cracks and fracture, which are not only related to coating's hardness and defect level. Furthermore, the most wear-resistant CuAl coating is the one deposited at higher input power with low primary gas flow rate. Such processing parameters can furthest restrict the precipitation of brittle β′ phase and consequently relieves the surface fatigue cracks and fracture under unlubricated wear condition.

Tribological comparison of different C-based coatings in lubricated and unlubricated conditions

The use of carbon-based coatings (hydrogenated and non-hydrogenated DLC, doped and alloyed-DLC) is of wide interest due to its applications in mechanical components submitted to friction and wear including sliding parts in automotive engines. A tribological comparative analysis using a reciprocating (SRV) tester in lubricated and unlubricated conditions with a 4-stroke motor oil has been carried out on six currently relevant state-of-the-art coatings (namely WC/a-C, TiBC/a-C and TiC/a-C:H nanocomposites, Ti-doped DLC, BCN film and a crystalline monolithic TiC film as reference). The quantification of the fraction of the sp2-bonded matrix has been done by fitting of C 1s XPS peak and the mechanical properties evaluated by nanoindentation. The comparative analysis has allowed us to identify the capabilities of each system depending on the testing conditions and the possible synergies as a function of the chemical composition and film nature. Under lubricated harsh conditions (max. contact pressure 1.7GPa) only coatings displaying hardness superior to 20GPa could stand the sliding motion without failure. At lower contact pressures, a significant fraction of sp2 carbon (≥75%) is advantageous for reducing wear in boundary lubrication. WC/a-C, BCN and Ti-DLC films showed the best tribological response in dry sliding conditions. This fundamental information would be of relevance for assisting engineers in selecting best partnership for lubrication systems.

Microstructure and tribological property of HVOF-sprayed adaptive NiMoAl–Cr3C2–Ag composite coating from 20 °C to 800 °C

Adaptive NiMoAl–Cr3C2–Ag composite coating was deposited by high velocity oxy-fuel (HVOF) spraying and its tribological properties from 20°C to 800°C under unlubricated conditions were evaluated. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy were used to characterize the coating and corresponding wear tracks to determine the lubrication mechanisms. The results showed that HVOF-sprayed composite coating exhibited an extremely dense structure, high bonding strength, and excellent friction reduction and wear resistance. When the applied load is 15N, friction coefficients of the composite coating were below 0.3 and wear rates maintained at the order of 10−6mm3/N·m except 400°C. Post-wear testing structural and chemical characterization revealed that silver with low shear strength provided lubrication below 400°C. Subsequently, the composite coating suffered serious adhesion wear at 400°C, nevertheless, the adhesive degree could be greatly alleviated at a lower applied load. At higher temperatures, silver molybdate that was formed through tribo-chemistry reactions acted as high temperature lubricants.

The use of niobium carbide (NbC) as cutting tools and for wear resistant tribosystems

The tribological profile of alumina (99.7%) mated against rotating disks made in binder-less niobium carbide (NbC) and cobalt-bonded NbC were determined under unidirectional sliding tests (0.1m/s to 8.0m/s; 22°C and 400°C) as well as in oscillation tests (f=20Hz, Δx=0.2mm, 2/50/98% rel. humidity, n=105/106cycles) under unlubricated (dry) conditions. In addition, the microstructure and mechanical properties of binderless NbC and NbC bonded with 8% cobalt were determined as well. The reason for testing hot-pressed NbC was to avoid side effects generated by sintering additives and/or second phases. The tribological data obtained were benchmarked with different ceramics, cermets and thermally sprayed coatings. NbC and cobalt-bonded NbC exhibited low wear rates under dry sliding associated with high load carrying capacity. The tribological profile established revealed a strong position of NbC bearing materials under tribological considerations and for closed tribo-systems against traditional references, such as WC, Cr3C2 and (Ti,Mo)(C,N).