Study of tribological performance of ECR-CVD diamond-like carbon coatings on steel substrates: Part 1. The effect of processing parameters and operating conditions

Diamond-like carbon (DLC) coatings were prepared on AISI 440C steel substrates at room temperature by electron cyclotron resonance chemical vapor deposition (ECR-CVD) process in C₂H₂/Ar plasma. Using the designed Ti/TiN/TiCN/TiC interfacial transition layers, relatively thick DLC coatings (1-2 μm) were successfully prepared on the steel substrates. The friction and wear performance of the DLC coatings was evaluated by ball-on-disk tribometry using a steel counterbody at various normal loads (1-10 N) and sliding speeds (2-15 cm/s). By optimizing the deposition parameters such as negative bias voltage, DLC coatings with hardness up to 30 GPa and friction coefficients lower than 0.15 against the 100Cr6 steel ball could be obtained. The friction coefficient was maintained for 100,000 cycles (∼2.2 km) of dry sliding in ambient environments. In addition, the specific wear rates of the coatings were found to be extremely low (∼10⁻⁸ mm³/Nm); at the same time, the ball wear rates were one order of magnitude lower. The influences of the processing parameters and the sliding conditions were determined, and the frictional behavior of the coatings was discussed. It has been found that higher normal loads or sliding speeds reduced the wear rates of the coatings. Therefore, it is feasible to prepare hard and highly adherent DLC coatings with low friction coefficient and low wear rate on engineering steel substrates by the ECR-CVD process. The excellent tribological performance of DLC coatings enables their industrial applications as wear-resistant solid lubricants on sliding parts.     

Friction and Wear Performance of Boron Doped,Undoped Microcrystalline and Fine Grained Composite Diamond Films

Chemical vapor deposition(CVD) diamond films have attracted more attentions due to their excellent mechanical properties. Whereas as-fabricated traditional diamond films in the previous studies don’t have enough adhesion or surface smoothness, which seriously impact their friction and wear performance, and thus limit their applications under extremely harsh conditions. A boron doped, undoped microcrystalline and fine grained composite diamond(BD-UM-FGCD) film is fabricated by a three-step method adopting hot filament CVD(HFCVD) method in the present study, presenting outstanding comprehensive performance, including the good adhesion between the substrate and the underlying boron doped diamond(BDD) layer, the extremely high hardness of the middle undoped microcrystalline diamond(UMCD) layer, as well as the low surface roughness and favorable polished convenience of the surface fine grained diamond(FGD) layer. The friction and wear behavior of this composite film sliding against low-carbon steel and silicon nitride balls are studied on a ball-on-plate rotational friction tester. Besides, its wear rate is further evaluated under a severer condition using an inner-hole polishing apparatus, with low-carbon steel wire as the counterpart. The test results show that the BD-UM-FGCD film performs very small friction coefficient and great friction behavior owing to its high surface smoothness, and meanwhile it also has excellent wear resistance because of the relatively high hardness of the surface FGD film and the extremely high hardness of the middle UMCD film. Moreover, under the industrial conditions for producing low-carbon steel wires, this composite film can sufficiently prolong the working lifetime of the drawing dies and improve their application effects. This research develops a novel composite diamond films owning great comprehensive properties, which have great potentials as protecting coatings on working surfaces of the wear-resistant and anti-frictional components.     

Tribological properties of selected PVD coatings when slid against ductile materials

In this paper, a physical vapour deposited (PVD) deposited TiB₂ coating is compared in dry sliding with commercial PVD titanium nitride (TiN), titanium aluminium nitride (TiAlN) and titanium carbonitirde (TiCN) as to frictional properties and tendency of counter material pick-up. The aim is to investigate if the superior behaviour of the TiB₂ coating experienced in severe sliding applications against aluminium alloys can be extended to other materials with a similarly poor tribological characteristics.A new tribological test for sliding contact has been used. The test configuration involves two crossed elongated cylindrical test specimens which are forced to slide axially against each other at a constant sliding speed and a gradually increasing normal load, while recording the friction. The evaluation is performed by correlating the friction history with the width, topography and composition of the sliding tracks as detected by optical and scanning electron microscopy.Coated cemented carbide (CC) test cylinders have been slid against cylinders of a Ti alloy (Ti–6Al–4V), an Al alloy (Al 7075) and Inconel 718. It was shown that the TiB₂ surface displayed superior friction and anti-sticking properties, when tested against the aluminium alloy. Against the Ti and Inconel alloys no major difference between the coatings could be found. Instead, it is concluded that the friction coefficient is determined by the plastic properties of the counter material since a complete transfer layer instantly builds up on the coating.It proved possible to estimate the friction force from the width of the sliding tracks, the Vickers hardness of the counter material and simple plastic considerations. This estimation also verifies the unexpectedly low friction of all coatings against the Ti alloy.     

Tribological and structural characterization of a physical vapour deposited TiC/Ti(C,N)/TiN multilayer

A physical vapour deposited TiC/Ti(C,N)/TiN multilayer was investigated and compared with a PVD TiN monolayer coating in a ball-on-disc test. Wear and friction against a corundum ball were measured as a function of time and sliding velocity. In these experiments, the coefficient of friction remained constant at 0.2 as long as the ball was sliding on TiC or intermidiate Ti(C,N) layers. When the TiN layer was reached, the coefficient of friction became unstable and rose to an average value of 1–1.5, which is characteristic for a TiN/Al₂O₃ contact. Wear rates for the multilayer were found to be three to four times smaller as compared to the reference TiN. The multilayer morphology of the TiC/Ti(C,N)/TiN was revealed in a low-angle cross-section resulting from a prolonged ball-on-disc test. In that way, it was shown that the multilayer consisted of nine separate sublayers.     

Role of the Transfer Film on the Friction and Wear of Metal Carbide Reinforced Amorphous Carbon Coatings During Run-in

The relationship between friction, wear, and transfer films of three metal carbide-reinforced amorphous carbon coatings (TiC/a:C, TiC/a:C–H, and WC/a:C–H), sometimes referred to as metal-doped diamond-like carbon coatings, has been investigated. Tribological tests were performed in an in situ tribometer with sapphire or steel hemispheres run against coated flats in dry or ambient air. The sliding contact interface was observed and recorded by optical microscopy during reciprocating sliding tests. The friction and wear behavior during run-in depended on the number of sliding cycles to form a stationary transfer film on the hemisphere. Stationary transfer films formed rapidly (within ten cycles) and the friction coefficient fell to 0.2 (ambient air) or 0.1 (dry air), except with sapphire against WC/a:C–H in dry air; with the latter, a stationary transfer film required nearly 100 cycles to form, during which the friction remained high and the wear rate was from 10 to 100 times higher than the other two coatings. For all coatings, three velocity accommodation modes (VAM) were observed from run-in to steady-state sliding and were correlated with the friction and wear behavior. The delayed adherence of the transfer film to sapphire from WC/a:C–H coatings in dry air is discussed in terms of equilibrium thermochemistry. Friction and wear behavior during run-in, therefore, depended on transfer film adherence to the hemisphere and the VAM between transfer films and the coating.     

Patterned PVD TiN spot coatings on M2 steel: Tribological behaviors under different sliding speeds

Experiments were performed to investigate systematically the influence of sliding speeds on tribological behaviours of in-lined (IN), staggered (ST) spot-islandic and fully coated (FC) physical vapour deposition (PVD) TiN coatings on M2 steel discs sliding with ASSAB 17 tool steel pins. Results revealed that: (i) the friction coefficients of the individual mating couples generally decreased with the sliding speed and the order in increasing magnitude at each specific sliding speed was FC, IN, and ST pair, respectively and (ii) the wear loss was inversely related to the sliding speed, and the wear loss of both the pin and disc of FC mating pair was the largest with ST the second and IN the third. Relevant mechanisms for the friction and the wear loss are proposed and discussed in this paper.     

Influence of surface texture on boundary lubricated sliding contacts

The friction and wear behaviour of boundary lubricated sliding surfaces is influenced by the surface texture. By introducing controlled depressions and undulations in an otherwise flat surface, the tribological properties can be improved. Lubricant can then be supplied even inside the contact by the small reservoirs, resulting in a reduced friction and a prolonged lifetime of the tribological contact.In the present paper, well-defined surface textures were produced by lithography and anisotropic etching of silicon wafers. The wafers were subsequently PVD coated with thin wear resistant TiN or DLC coatings, retaining the substrate texture. The size and shape of the depressions were varied and evaluated in reciprocating sliding under dry and boundary lubricated conditions.     

The influence of surface roughness on friction and wear of machine element coatings

The use of low friction coatings like amorphous carbon or metal-doped carbon coatings on machine elements is constantly increasing. Most often, a surface treatment, e.g. grinding and polishing or honing, is required for optimal performance of the coated machine element. This can be time consuming and costly.In this study, the effect of surface roughness on friction and sliding wear of two different coatings, one tungsten containing and one chromium containing coating, were examined using a ball-on-disc test. Ball bearing steel plates were grinded to different surface roughnesses and coated with the two different coatings.The friction was found to depend on surface roughness where the rougher surfaces gave higher friction coefficients. The wear rate for the a-C:W coating was found to be independent of the roughness, whereas the roughness had a strong influence on the wear rate for the a-C:Cr coating. This could partly be explained by a difference in wear mechanism, where fatigue wear was observed for the a-C:Cr coating but not for the a-C:W coating.     

Triboengineering characteristics of unicomponent PVD coatings

In order to explore the opportunities of using vacuum coatings in tribounits, the triboengineering characteristics of unicomponent PVD coatings were studied based on titanium, zirconium, chrome, tungsten and aluminum in sliding friction interfaced with ShKh15 steel GOST 801-78. The study has revealed coatings with the optimal combination of properties, such as the friction coefficient, the wear resistance, the time of running-in, and the wear ability with respect to the counterbody.     

The effect of air humidity on tribological behaviours of W–C:H coatings with different tungsten contents sliding against bearing steel

Friction and wear behaviors of W–C:H coatings with different tungsten contents sliding against bearing steel balls at different air humidities were investigated. The worn out surfaces of steel balls and coatings were analyzed with the aid of scanning electron microscopy (SEM) and Raman spectroscopy. A tribolayer composed of a graphite-like material mixed with tungsten and iron oxides was observed on the friction surfaces of the steel balls. The chemical and phase compositions of the tribolayer, which depend both on the tungsten content in coatings and air humidity, determine the tribological properties of the W–C:H coating in a frictional contact with bearing steel. At average air humidity (50%), those coatings that contain less than 10 at% of tungsten in a frictional contact with steel exhibit favorable tribological properties. The friction coefficient of frictional contacts under test reaches a low value (f～0.01) at a low air humidity and increases with humidity of up to ca. 0.2. The best tribological properties in a wide range of air humidity (5–90%) have been found for W–C:H coatings with the tungsten content between 2 and 5 at%.     

Wear resistance and anti-sticking properties of duplex treated forming tool steel

The aim of this work was to investigate the potential of using hard physical vapour deposition (PVD) coatings on forming tools, as well as to determine the influence of plasma nitriding on the load-carrying capacity and wear resistance of coated tool surfaces. A load-scanning test rig was used for evaluation, where duplex treated cold work tool steel samples were loaded against soft austenitic stainless steel and hardened ball bearing steel, respectively. Four different coatings (TiN, TiB₂, TaC and DLC) and two substrate treatments (hardening and plasma nitriding in two different gas mixtures) were included.Plasma nitriding alone significantly improved the friction, wear, and anti-sticking properties of the tool steel. PVD coating, and especially PVD coating of nitrided tool steel further improved the performance. Therefore, from the point of view of tool life as well as work peace surface quality, the DLC coating with its excellent anti-sticking properties and sufficiently good wear resistance represent the best solution for forming tool applications of austenitic stainless steel.     

Friction and Wear Properties of Micro Textured DLC Coated Surfaces in Boundary Lubricated Sliding

In the present study, the friction and wear properties of boundary lubricated textured surfaces were investigated. The capability to feed lubricant into the interface of a sliding contact and to isolate wear particles was related to the shape, size and orientation of the texture patterns. Well-defined surface textures of square depressions or parallel grooves of different widths and distributions were produced by lithography and anisotropic etching of silicon wafers. Subsequently the wafers were PVD coated with thin, wear resistant DLC coatings, retaining the substrate texture. The surfaces were evaluated in reciprocating sliding against a ball-bearing-steel ball under starved or amply lubricated boundary lubrication conditions.     

Proper coating selection for improved galling performance of forming tool steel

The aim of the present work was to investigate and compare different hard coatings as to the tendency for work material adhesion and galling properties when applied on forming tool steel and sliding against different work materials. The surface coatings included were PVD deposited TiN, TiB₂, VN, TaC and DLC coatings. They were all applied to cold work tool steel. Tribological evaluation was carried out in a load-scanning test rig, with the normal load being gradually increased during each test from 100 to 1300 N (1-3.5 GPa). The coated steel was tested against austenitic stainless steel and alloys of aluminium and titanium.This investigation clearly indicates that work material adhesion and galling performance of coated forming tool steel greatly depends on the type of work material. In the case of stainless steel, carbon-based coatings provide the best protection against the work material transfer, while forming of aluminium and titanium alloys, requires nitride type coatings, such as TiN.     

Tribological Properties of DLC Coatings in Helium

The aim of this research work was to investigate tribological properties of low-friction DLC coatings when operating in helium atmosphere. Two commercial DLC coatings (a-C:H and Me-C:H) were included in the investigation and compared to reference PTFE-based coatings, normally used on components operating in helium. Coatings were deposited on hardened 100Cr6 bearing steel discs and tested against uncoated steel balls in low-load pin-on-disc contact configuration. Investigation was focused on the effect of substrate roughness (R _a ?=?0.05?C0.2???m) and contact conditions, including contact pressure (150?C350?MPa) and sliding speed (0.2?C0.4?m/s) on the coefficient of friction of DLC coatings operating in helium. Results of this investigation show that for low-load sliding contact DLC coatings provide low friction in helium atmosphere, similar to soft PTFE-based coatings. At the same time DLC coatings investigated were found to substantially reduce wear of the coated surface. However, while the wear of the coated part has been more or less eliminated, application of DLC coating prolongs running-in and increases wear of the steel counter-part. Furthermore, also in helium atmosphere tribolgical behaviour of DLC coatings showed dependence on the coating type and contact conditions.     

A new test method for measuring the galling resistance between metal powders and die tool materials in powder compaction

The friction characteristics and galling resistance between metal powder and die tool material in metal powder compaction is of outmost importance since they will influence the porosity and surface quality of the green body and consequently the porosity, tolerances and surface quality of the final sintered product. In the present study, a new test method for evaluating the tribological performance of die tool materials aimed for powder compaction is presented. The test method is based on controlled scratch testing using a commercial scratch tester but instead of the commonly used Rockwell C diamond stylus a sample holder with a small green body of compacted powder particles is drawn over the surface in a well controlled multi pass linear reciprocating sliding contact. The capability of the test method was evaluated for different types of tool materials including two PVD coatings in contact with different types of metal powders to determine the friction characteristics and the adhesion and material transfer tendency at the sliding interface. Post-test examination of the tool surfaces using FEG-SEM and EDS were performed in order to evaluate the mechanisms controlling the friction behavior and the material transfer tendency. The results show that the proposed test is a simple and fast method to obtain relevant data regarding the friction and galling characteristics of die tool materials in metal powder compaction. The mechanisms prevailing at the green body/die tool material interface, e.g. cold welding, can easily be monitored by the friction and acoustic emission signals. Of the die tool materials investigated the low friction PVD a-C:Cr coating displayed the lowest friction and highest galling resistance.     

Tribofilm formation from TiC and nanocomposite TiAlC coatings,studied with focused ion beam and transmission electron microscopy

This work demonstrate how two different carbide coatings respond very differently to tribological stress and their very different ability to provide low friction tribofilms in dry sliding against steel. Both coatings, TiC and TiAlC, were deposited by DC-magnetron sputtering, but while the TiC is a thermodynamically stable coating, the TiAlC is made metastable with the addition of Al, and therefore releases carbon upon tribological testing. Thus, the TiAlC coating is shown to be self-lubricating on the atomic scale which makes very low friction achievable. The primary interest in this study is the differences in the tribofilms formed on the steel balls that have been sliding against the two coatings. Cross-section samples for transmission electron microscopy were extracted from the ball tribofilms using a focused ion beam instrument. X-ray photoelectron spectroscopy and Raman analysis were employed to provide information on the chemical and structural characteristics of the tribofilms. It was shown that tribofilms on steel balls largely inherit the structure and composition that evolve in the coating wear tracks, that the tribofilm microstructure greatly affects the friction level. It was also shown that tribofilm delamination, occurring with tribofilm growth, was initiated in weak ribbon like regions inside the tribofilm.     

Experimental Evaluation of Friction and Wear Properties of Solid Lubricant Coatings on SUS440C Steel in Liquid Nitrogen

The friction and wear properties of the prevailing different solid lubricant coatings (Ion-plated Au, Ion-plated Ag and RF-sputtered PTFE on SUS440C stainless steel) used in the bearings of high-speed cryogenic-turbo-pumps of liquid rocket engines were experimentally evaluated in liquid nitrogen immersed conditions. Also the above experiments were carried out with two newly proposed solid lubricant coatings of sputter-ion-plated MoSTi and a new ion-plated Pb on SUS440C stainless steel. The friction coefficient and wear rates of the coatings of ion-plated Au, ion-plated Ag, RF-sputtered PTFE, the new ion-plated Pb and MoS₂Ti-SIP (with coating thickness of 0.7±0.1 μm) on SUS440C steel against SUS440C stainless steel ball in liquid nitrogen were compared. Worn surfaces were examined microscopically with a microscope and a profilometer for understanding the mechanisms of friction and wear and transfer film lubrication in liquid nitrogen. It is found that the newly proposed solid lubricant coatings are showing promising results for their use in liquid nitrogen immersed conditions. The sputter-ion-plated MoSTi coating on SUS 440C steel shows a minimum value of friction coefficient (μ=0.015) and wear rate (w_c=0.56 × 10⁻⁶ mm³/N m ) in liquid nitrogen.     

Study of tribological performance of ECR-CVD diamond-like carbon coatings on steel substrates: Part 2. The analysis of wear mechanism

This paper describes the tribological performance of diamond-like carbon (DLC) coatings deposited on AISI 440C steel substrates by electron cyclotron resonance chemical vapor deposition (ECR-CVD) process. A variety of analytic techniques were used to characterize the coatings, such as Raman spectroscopy, atomic force microscopy (AFM) and nano-indentation. The sliding wear and friction experiments were carried out by the conventional ball-on-disk tribometry against 100Cr6 steel counterbody at various normal loads (1-10 N) and sliding speeds (2-15 cm/s). All the wear tests were conducted under dry sliding condition in ambient air for a total rotation cycle of 1 × 10⁵ (sliding distance ∼2.2 km). Surfaces of the coatings and the steel balls were examined before and after the sliding wear tests. The DLC coatings that had been tested all showed relatively low values of friction coefficient, in the range of 0.1-0.2 at a steady-state stage, and low specific wear rates (on the order of 10⁻⁸ mm³/Nm). It was found that higher normal loads or sliding speeds reduced the wear rates of the coatings. Plastic deformation became more evident on the coating surface during the sliding wear test at higher contact stresses. The friction-induced transformation of the coating surface into a graphite-like phase was revealed by micro-Raman analysis, and the flash temperature of the contact asperities was estimated. It was suggested that the structural transformation taking place within the wear tracks was mainly due to the formation of compact wear debris layer rather than the frictional heating effect. On the other hand, an adherent transfer layer (tribolayer) was formed on the counterface, which was closely related to the steady-state friction during sliding and the wear mechanisms. Fundamental knowledge combined with the present tribological study led to the conclusion that adhesive wear along with abrasion was probably the dominant wear mechanism for the DLC/steel sliding systems. Additionally, fatigue processes might also be involved in the wear of the coatings.     

Influence of Surface Roughness on the Transfer Film Formation and Frictional Behavior of TiC/a-C Nanocomposite Coatings

Influence of surface roughness on the friction of TiC/a-C nanocomposite coatings while sliding against bearing steel balls in humid air was examined by detailed analyses of the wear surfaces and the wear scar on the ball counterparts by atomic force microscopy, optical, and confocal microscopy. It was observed that the surface roughness of the coatings essentially determines the wear behavior of the ball counterpart, which consequently influences the transfer film formation. A rough coating causes abrasive wear of the steel ball during the running-in period, which impedes the formation of a stable transfer film and leads to higher values of coefficient of friction (CoF). Moreover, the CoF does not show a decreasing trend after the running-in period, although the roughness of the coating was greatly reduced. Replacing the worn ball with a new one after the running-in period yields lower CoF values similar to that observed for a smooth coating. In both of the cases, no wear of the steel ball occurs and a stable transfer film forms and effectively covers the contact area. The influence of the wear debris on the formation of the transfer film is also discussed.