A method for the tribological testing of thin,hard coatings

A new method has been developed for tribological testing of thin, hard antiwear coatings, using a ball‐on‐disc tribosystem, under conditions of dry sliding. In this, an Al₂O₃ ball is pressed against a coated steel disc. Wear debris is removed from the contact zone by a stream of dry argon in this novel method. This improves the stability of the tribological properties and the repeatability of the test results. All test conditions are precisely defined, in particular: the type of motion, air relative humidity, ambient temperature, sliding speed, load, tribosystem spatial configuration, substrate material, substrate hardness and roughness, and coating thickness. The method developed has been used to test various physical vapour deposition coatings (deposited by the vacuum arc method), i. e., single‐layer TiN, Ti(C,N), CrN, and Cr(C,N), and multilayer Cr(C,N)/CrN/Cr and Cr(C,N)/(CrN+Cr₂N)/CrN/Cr. It is shown that CrN coatings exhibit the best antiwear properties, and Ti(C,N) the worst. Friction coefficients for CrN and Cr(C,N) coatings are much lower than for the more commonly used TiN. Multilayer coatings have better antiwear properties than single‐layer ones.     

Investigation of mechanical and tribological properties of amorphous diamond-like carbon coatings

We investigated the mechanical and tribological properties of amorphous diamond-like carbon (DLC) coatings deposited on Si(100) by a pulsed bias deposition technique. Tribological studies were performed using a pin-on-disc (POD) apparatus under a normal load of 6.25 N and at 10% relative humidity, with a ruby pin as a slider. Hardness measurements were performed using a nanoindenter and apparent fracture toughness using indentation techniques. We studied the influence of residual stresses on apparent fracture toughness. The data revealed that the thickness, hardness and compressive stress of the coating play different roles in the apparent fracture toughness. Crack initiation is influenced by the thickness and hardness of the coating, whereas crack propagation is influenced by the compressive stress in the film. The apparent fracture toughness of DLC coatings increased with coating hardness.     

Micro-scale abrasive wear testing of PVD coatings on curved substrates

A method is presented which enables a micro-scale abrasion test to be used to measure the wear performance of a coating over a small region, typically of millimetre dimensions, on a curved surface. The method is also applicable to studies of the wear resistance of any bulk material with a surface having complex curvature. The technique is illustrated by measurement of the intrinsic abrasion resistance of thin PVD coatings of TiZrN, ZrNbN and TiNbN on both flat and cylindrical tool steel and flat stainless steel substrates. The ability to measure the wear resistance of both a coating and its substrate, independently of each other and by a single test, is confirmed by experiment.     

Tribological characterization of thin hard coatings by reciprocating sliding tests

Thin hard coatings on metal or ceramic surfaces offer a large spectrum of improvements of the friction and/or wear behaviour of tribosystems. The development of coatings and the tailoring of their properties require test methods providing information about their friction and wear behaviour. A new wear test standard (ASTM) is under development for the evaluation of friction and wear quantities for sliding motions using the reciprocating sliding mode. The applicability of this test method to coated specimens was checked by testing uncoated and coated steel specimens in contact with alumina balls, whereby lower loads were used than in the ASTM proposal for bulk materials. Additionally, the influence of the relative humidity of the surrounding air at room temperature on friction and wear results was examined.     

Mechanical and tribological properties of vapour deposited low friction coatings on Ni plated substrates

Soft steel and aluminium substrates with load-carrying layers of electroplated nickel were coated with commercially available low friction vapour deposited coatings. The mechanical and tribological properties of the coating and substrate composites were evaluated with special emphasis on the influence of the nickel layer. Two different thicknesses of the intermediate load-carrying nickel layer were tested. The samples were evaluated regarding friction and sliding wear, abrasive wear, hardness and elastic modulus, morphology and coating thickness and adhesion between substrate and coating. It was found that all the evaluated low friction coatings were possible to be successfully deposited on the intermediate nickel layer. A relatively thick intermediate nickel layer is a promising candidate for improvement of the load-carrying capacity.     

The slurry erosive wear of physically vapour deposited TiN and CrN coatings under controlled corrosion

The erosion-corrosion of mild steel (BS6323), in the presence and absence of physically vapour deposited (PVD) TiN and CrN coatings, was studied, in comparison with that of AISI 304 stainless steel, in an aqueous alkaline slurry solution containing alumina particles. The influence of applied potential and particle velocity on the total erosion-corrosion loss was examined, and the respective corrosion and erosion damage (both contributing to the overall weight loss) then assessed by means of microscopical investigation of the morphology of the damaged surface, and subsequently evaluated quantitatively. The superior erosion-corrosion resistance of both the coatings compared to that of the uncoated mild and stainless steels was shown to be due to their resistance to both wear and corrosion. According to the detailed corrosion mechanisms revealed and different responses to wear, schematic diagrams were proposed to outline the main features of the corrosion-erosion process and the individual roles of erosion and corrosion. Discrete differences, in terms of the respective erosion and corrosion processes, between the TiN and CrN coatings, and between the mild and stainless steels, were also investigated and discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Characterization of wear protective coatings

Arising from the necessity and the suitableness of wear protection by coatings, common coating techniques and treatment procedures are presented in a survey. For evaluation of the effectiveness of these anti-wear coatings a testing process was proposed consisting of three parts: (1) a test procedure to simulate the basic wear mechanisms; (2) investigations of the tribologically relevant material properties; (3) an integral wear test procedure. Taking thermal sprayed coatings and electrolytic depositions as examples, the characterization procedure is explained. The test systems designed at the Technical University Chemnitz-Zwickau for simulation of the basic mechanisms of adhesive and abrasive wear and long-term fatigue-wear are described with regard to the functional principle, the structure and the parameters to be ascertained.     

Comparative tribological behaviors of TiN, CrN and MoNCu nanocomposite coatings

The purpose of this study is to investigate comparative tribological behaviors of Cu-doped TiN, CrN, and MoN coatings under a wide range of dry sliding conditions. TiN and CrN coatings have been developed and used by industry in numerous tribological applications including, machining, manufacturing and transportation. In contrast, MoN has attracted very little attention as a tribological coating in the past, despite being much harder than both TiN and CrN. In this paper, we will mainly concentrate on the Cu-doped versions of these coatings whose tribological properties have not yet been fully explored. The results of this study have confirmed that the addition of Cu into TiN, CrN and MoN coatings has indeed modified the grain size and morphology, but had a beneficial effect only on the friction and wear behavior of MoN. The tribological behavior of CrN did not change much with the addition of Cu but that of TiN became worse after Cu additions. Raman spectroscopy technique was used to elucidate the structural and chemical natures of the oxide films forming on sliding surfaces of Cu-doped TiN, CrN and MoN films. The differences in the friction and wear behavior of Cu-doped TiN, CrN, and MoN is fully considered and a mechanistic explanation has been provided using the principles of a crystal chemical model that can relate the lubricity of complex oxides to their ionic potentials.     

Friction of brushlike self-assembled monomolecular coatings

Friction modes of self-assembled monomolecular coatings (SAM) based on dodecyclophosphoric acid ester and its derivatives on a silicon substrate are investigated. The antifriction properties of the films are shown to be due to their ability to decrease the interfacial energy of friction contact. Models are suggested for friction force computation for single and multiple molecularly smooth contacts. They are based on consideration of the thermodynamic cycle of formation and break-off of the friction junction. On the example of composite coatings having different surface energy, a possible means of regulation of the tribological parameters of the silicon-SAM-silicon system is discussed.     

Formation mechanisms of low-friction tribo-layers on arc-evaporated TiC1−xNx hard coatings

A detailed correlation of the tribological performance of arc-evaporated TiC_1−xN_x coatings with testing temperature, atmosphere as well as variation in load and sliding velocity is presented in this paper. The low-friction behavior in combination with its mechanical integrity are the reasons for the extensive industrial application of TiC_1−xN_x over the last decades. Still the tribo-mechanisms behind this performance are not yet completely understood. The present study adds further understanding, as the low-friction behavior degrades at elevated temperatures and dry or inert environments, which is related to the different constitution of the tribo-layer formed as investigated by Raman spectroscopy. Surprisingly, the wear rate of the coatings does not correlate with the coefficient of friction indicating the presence of different wear regimes.     

Friction reduction and antiwear properties of the complex of lanthanum and a phosphate

A complex of lanthanum and di-(2-ethyl hexyl) phosphate (DEHP) was prepared by an extractive method. The friction and wear behaviour of this complex (called LaDEHP) as an oil additive was evaluated using a four-ball wear tester. For comparison, the friction reduction and antiwear properties of DEHP as an oil additive were also investigated. The chemical composition of the boundary film formed after wear tests was examined using X-ray photoelectron spectrometry (XPS). Four-ball tests showed that the prepared LaDEHP was effective in reducing friction and wear as well as in increasing load-carrying capacity. XPS analyses indicate the formation of iron phosphate and lanthanum oxide in the boundary films.     

The influence of tribochemical reactions of antiwear additives on heterogeneous surface layers in boundary lubrication

The article discusses methods for the deposition of antiwear, composite coatings with a metallic matrix. The Ni–P–Al₂O₃ composite layers were superimposed on carbon steel elements using the electroless plating technique. A comparative analysis of adsorptive interactions was carried out for model systems with antiwear characteristics, i.e., for the Ni–P matrix, and the composite material containing micro-dispersive corundum, i.e., Ni–P–Al₂O₃. The paper presents the results of tribological tests and analyses the effects of tribochemical interactions in the macroscale (using in a pin-on-disc system device) and in the micro/nanoscale—performed with the use of Atomic Force Microscopy (AFM).     

The Role of Microstructure in Ultralow Wear Fluoropolymer Composites

Past studies have shown that the inclusion of fillers in a polytetrafluoroethylene (PTFE) matrix can improve wear resistance by nearly four orders of magnitude. These discoveries have prompted several tribological experiments over the past decade that have highlighted the importance of particle size, tribofilm formation, filler percentage, and environment. To evaluate the effect that microstructure plays on a composite’s tribological performance, PTFE-filled polyamide-imide (PAI) composites were made and tested. To investigate the role of microstructure on the tribological performance of fluoropolymer composites, 12 composite formulations of PTFE and PAI over a range of 0 to 100 vol% PAI were tested. PTFE–PAI composite samples were slid against a stainless steel countersample using a linear reciprocating tribometer under a nominal 6.35?MPa contact pressure at 50.8?mm/s sliding speed. Of the samples tested, the 25 vol% PAI showed a remarkable mean steady-state wear rate of k?=?3?×?10^?9 mm³/Nm over an extreme distance of 360?km. A serial imaging investigation revealed that a mechanical interlocking of the two polymers occurred during the sintering process, which possibly contributed to the ultralow wear rates observed in this polymer–polymer composite.     

Tribological characterisation of a‐C:H coatings at room temperature: Effect of counterbody material

Metal‐free amorphous carbon (a‐C:H) coatings with 15% hydrogen were deposited on bearing steel surfaces. The friction and wear performance of these specimens was characterised in oscillating sliding tests with a ball‐on‐flat geometry. Balls of four ceramic and four metallic materials were investigated in tests at room temperature. Special attention was paid to the effect of moisture by testing in dry, normal, and moist air. The effect of water vapour on the friction and wear of the a‐C:H coatings was quite different for the different counterbody materials. The wear was in all cases very low, with a coefficient of wear below 10⁻⁷ mm³/N m for most cases. The coefficient of friction was also very low, between 0.04 and 0.12 for most of the tests. The smallest wear and friction coefficients were found for oxide ceramics, while during tests against SiC and Si₃N₄ the coating was worn through during the test. The effects of counterbody material and the humidity of the surrounding air are discussed in terms of friction and wear mechanisms.     

Damage of Stainless Steel Components by Olive Paste

Failure of equipment for processing olives interrupted oil production after only 3 weeks in service. Macroscopic and microscopic analyses were used in the present investigation to analyze damaged surfaces. Observations and data suggest that failure was induced via tribocorrosion with a predominance of mechanical damage. This damage is attributed to abrasive wear combined with corrosion of AISI 304L stainless steel in olive paste (seed particles and pulp) mixed with tap water. Microscopic observations revealed fracture and localized plastic deformation in the damaged area along with a tribologically transformed structure and work-hardened surfaces. The tribological behavior of AISI 304L was determined using a pin-on-disc tribometer, and these results were compared to damage on the olive processing equipment. The steel was sensitive to tribo-oxidation, mostly due to abrasion by seeds and steel wear particles, and somewhat due to corrosion reactions with the environment.     

Substrate dependence of the scratch resistance of CrN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> coatings on steel

Hard CrN_x coatings were sputter deposited on hot work tool steel (HWTS) and high speed steel (HSS) in an industrial PVD reactor. Coatings were deposited under various nitrogen flows. The thickness, density, hardness, elastic modulus, composition, and stress were determined for the coatings. The specimens were subjected to scratch testing. Two different failure mechanisms were investigated: chipping and complete coating removal. For all specimens, the coating-to-substrate adhesion was that good that adhesion did not limit the scratch resistance. Therefore, the minimum loads at which a given type of failure was initiated were not a measure for the coating-to-substrate adhesion. The scratch resistance was better for coatings on HSS than for coatings on HWTS. This is due to the higher hardness of the HSS. Substrate independent measures for the scratch resistance of the coating were obtained by considering critical track widths instead of critical loads. The hardening of the coating--substrate systems due to the coating was investigated. The uncoated substrates exhibited track width independent scratch hardness. For the coated specimens the scratch hardness increased with increasing track width until chipping of the coating occurred. Complete coating removal coincided with a decrease in hardness. Although the elastic properties, hardness, and thickness of all coatings were more or less equal, CrN_1.0 coatings outperformed CrN_0.6 coatings in scratch tests both on HSS and on HWTS.     

Investigation on the Fretting Wear Behavior and Mechanisms of Alloy 690TT during the Transformation from Mixed Regime to Partial Slip Regime Induced by Elevated Temperature

Fretting wear of Alloy 690TT can occur in the steam generator of a nuclear power plant, in which the interfacial conditions are changed as the temperature varies. In this study, the gradual transformation from the mixed fretting regime (MFR) to the partial slip regime (PSR) occurred with an increase in test temperature. Correspondingly, there was a competition in wear modes due to the formation of a nanostructured tribologically transformed structure (TTS), presence of delamination cracking, nucleation of fatigue cracks, and oxidation. Delamination within the TTS dominated first. As the area of TTS and the plastic deformation region was reduced, fatigue cracks began to initiate. Oxidation was accelerated by an increase in the test temperature, and that effect resulted in reduced wear volume of Alloy 690TT in the MFR.     

Improvement of antiseizure,antiwear, and anticorrosion properties of thin nitrided layers by impregnation with oil‐ or solvent‐based formulations

The results of some corrosion and tribological investigations are presented in this paper. Samples with thin (white layer, WL = 16.5 μm) nitrided layers on 45‐grade steel obtained in a controlled gas nitriding process (at 570°C, 6 h; WL = 16.5 μm; grey porous zone or grey layer, GL = 6.0 μm) and impregnated by multifunctional oil‐based formulations containing BS‐MOD corrosion inhibitor (CI) as well as tribological additives (TAs) were used. The TAs when present alone (without the CI) in oil have no corrosion‐inhibiting features and also do not affect the corrosion resistance of WLs impregnated with formulations containing both the CI and TAs; corrosion resistance is mainly connected with the presence of the CI. Synergistic effects observed between TAs and the CI as well as TA concentration vs. BS‐MOD in multifunctional additives are more apparent in seizure features and wear resistance of impregnated WLs. The influence of the TAs in combination with the CI depends on their type and concentration; it can be positive or negative. Hence, the choice of composition of CI and TA for multifunctional impregnating formulations is determined, above all else, by the relation between the CI and the grade of TA. The anti‐corrosion effect as well as the tribological function is dominated by the CI component (BS‐MOD).     

Investigation of the diamond machinability of newly developed hard coatings

Presently, coatings of electroless nickel are used for diamond turning molds for injection molding of optical lenses. We have investigated the diamond machinability of substoichiometric hard nitride coatings (TiN_x, TiAlN_x, and CrN_x). These coatings have a superior hardness compared to electroless nickel suggesting an improved wear resistance of molds with optical surface quality. In the case of CrN_x and TiAlN_x, high tool wear occurred, even after small cutting distances, and the surfaces showed a roughness larger than R_a = 0.5 μm. A considerably higher surface quality was obtained on TiN_x coatings. The best results (R_a = 15 nm) were achieved with a nitrogen content of x = 0.03. As a first application, a mold for a diffractive optical element was machined using this newly developed substoichiometric titanium nitride deposit.