Combined influences of mechanical properties and surface roughness on the tribological properties of amorphous carbon coatings

Carbon films (∼2 μm thick) with a range of mechanical properties and underlying substrate roughnesses were evaluated for delamination and wear under conditions of combined impact and sliding contact. One-side-coated and both-sides-coated titanium alloy pin and disk wear couples were assessed using a custom-made impact/sliding pin-on-disk apparatus in a colloid-based blood volume expansion medium. The normal stress distribution along the film/substrate interface upon impact loading by a smooth titanium alloy pin was modelled analytically for each coated system. For disk surface roughness centre-line average (R_a) values below 0.05 μm, the area of disk film delamination was shown to correlate with the presence of tensile stress at the film/substrate interface. The interfacial tensile stress was shown to occur when there was a film/substrate elastic modulus mismatch, either within or outside the perimeter of contact, depending on the direction of the film modulus deviation. We propose that the tensile stress promotes delamination by lifting the film locally from the substrate. With increased disk R_a, the area of disk coating loss tended to decrease, because the surface roughness improved the mechanical bonding of the composite system.     

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.     

Mechanical and unlubricated tribological properties of titanium-containing diamond-like carbon coatings

Titanium-containing diamond-like carbon (Ti-DLC) coatings were deposited on steel with a close-field unbalanced magnetron sputtering in a mixed argon/acetylene atmosphere. The morphology and structure of Ti-DLC coatings were investigated by scanning electron microscopy, transmission electron microscopy, atomic force microscopy and Raman spectroscopy. Nanoindentation, nanoscratch and unlubricated wear tests were carried out to evaluate the hardness, adhesive and tribological properties of Ti-DLC coatings. Electron microscopic observations demonstrated the presence of titanium-rich nanoscale regions surrounded by amorphous carbon structures in Ti-DLC coating. The Ti-DLC coatings exhibit friction coefficients of 0.12–0.25 and wear rates of 1.82 × 10^?9 to 4.29 × 10^?8 mm³/Nm, depending on the counterfaces, sliding speed and temperature. The Ti-DLC/alumina tribo-pair shows a lower friction coefficient than the Ti-DLC/steel tribo-pair under the identical wear conditions. Increasing the test temperature from room temperature to 200 °C reduces the coefficient of friction and, however, clearly increases the wear rate of Ti-DLC coatings. Different wear mechanisms, such as surface polishing, delamination and tribo-chemical reactions, were found in the tribo-contact areas, depending on different wear conditions.     

Dependence of mechanical and tribological properties of diamond-like carbon coatings on laser deposition conditions and alloying by metals

The paper deals with the study of the effect of the intensity of the pulse laser irradiation of a graphite target (the fluence) on the rate of deposition, density, and the mechanical and tribological properties of diamond-like carbon coatings formed on a steel substrate at room temperature. The diminishing of the fluence from 50 to 5 MW/mm² yields a substantial (more than by an order of magnitude) increase in the coating deposition rate. However, these coatings have the lowered mechanical and tribological properties. The possibility of enhancing the coating characteristics by additional alloying with such metals as W and Ti in concentrations of up to ??10 at % is studied. The type of the metal and its distribution in the coating depth affect greatly the coating surfaces. The most stable combination of good antiwear and antifriction characteristics of the friction pair is reached after bulk alloying with tungsten.     

Wear of metal-containing diamond-like carbon coatings

Four commercial metal-containing DLC coatings were tested with a ball-on-disk tribomete to examine their modes of wear. Although all were sputter-deposited, the coatings differed in their compositions, thicknesses, and surface finish. The tests showed certain common wear modes. In each case the films wore away at constant rates until they were worn through. In this sense, interface adhesion was not an important issue. Since the nominal contact areas increased significantly during the course of the test, while the wear rates were constant, the wear rate was not controlled by the nominal average contact stress. Our data are consistent with the model of Greenwood and Williamson.     

Nanometre scale mechanical properties of extremely thin diamond-like carbon films

Abstract

Extremely thin diamond-like carbon (DLC) films are deposited by the filtered cathodic vacuum arc (FCVA) and plasma chemical vapour deposition (p-CVD) methods. The target thicknesses of the extremely thin protective DLC films deposited on a Si (100) surface by FCVA and p-CVD are 0·1, 0·4, 0·8, 1·0, 2·0, 5·0 and 100·0 nm. Nanoindentation hardness and nanowear resistance are evaluated by atomic force microscopy (AFM). The nanoindentation hardnesses of 100 nm thick DLC films deposited by FCVA and p-CVD are 57 and 25 GPa respectively. The nanowear test by AFM clarifies the mechanical properties of extremely thin DLC films. The wear depths of 1 and 2 nm thick FCVA-DLC films are extremely shallow. The wear depths of the 1·0 and 2·0 nm thick p-CVD-DLC films exceed the film thicknesses after five sliding cycles. These results reveal differences in the wear resistance of extremely thin DLC films and the superior mechanical properties of FCVA-DLC thin films.     

Effects of different metals on adhesive wear behaviour of alloy surface produced by TIG process

Abstract

Low carbon steel surfaces were alloyed with composite powders using the tungsten inert gas welding method. After the alloying process, the effects of cladding surface on the microstructural characteristics and adhesive wear of the alloyed samples were examined. The sliding wear behavior of samples was investigated in a block on ring apparatus under the loads of 20, 40, 60 and 80 N respectively. In the experimental investigation, a low carbon steel surface was alloyed with austenitic stainless steel powder and austenitic stainless steel powder mixed with 4·5% Co, Mo and Ti particles respectively. Following surface alloying, conventional characterisation techniques, such as optical microscopy, scanning electron microscopy, energy dispersive spectrograph and X-ray diffraction, were used to study the microstructure of the alloyed zone. Examination of the microstructure revealed the presence of M₂₃C₆ carbides, solid melt phases and intermetallic phases, such as Ni₃Ti, depending on the alloying element in the composite. As the amount of the reinforcing material increased, the saturation rates for the samples decreased, while their hardness increased. The adhesive abrasion tests conducted revealed that temperature input plays a significant role on the microstructure characteristics, which positively affected the adhesive abrasion values of the samples. Consequently, the tungsten inert gas welding method was successfully used for the surface alloying of low carbon steels.     

Wear of water lubricated silicon nitride in ball on disc test

Abstract

The ball on disc test configuration is preferred to the flat ended pin on disc because the ball is self-aligned and measurement of wear on the ball is of higher accuracy, compared to the pin. Silicon nitride, sliding on itself in water, was tested with the ball on disc tribometer. Misalignment of the test ball from its proper position behind the disc axis of rotation leads to friction measurement errors, which were analysed. The disc wears non-uniformly, the wear track depth and width vary in longitudinal direction by a factor of 2–3. The uneven wear of the disc is explained by the combined effects of sliding surface anisotropy and disc material non-homogeneity on the one hand and by the friction force and the normal load periodic variation on the other hand. During the running-in process at particular sliding velocity amplitude modulated friction force was observed and an explanation by the mechanical vibration 'beating' phenomenon was suggested. Predictive model of the running-in process is presented, which describes the evolution of the ball wear scar area, the contact pressure and the wear rate. The model predictions are consistent with the experimental data.     

Effects of thickness and surface roughness on mechanical properties of aluminum sheets

The effect of thickness on the mechanical properties of Al 6K21-T4 sheet specimens under uniaxial tension was investigated. In order to reduce the thickness of the specimens without changing the microstructure and grain size, chemical etching was carried out, resulting in Al sheets ranging from 0.40 mm to 1.58 mm in thickness. Additionally, the effect of surface roughness was determined by finite element (FE) calculations performed using FE code MARC 2007. Tensile specimens of varying surface roughness were modeled and simulated. An analysis of the combined effects of the thickness and surface roughness revealed that the yield and tensile strengths decreased when the number of grains over the thickness was decreased. The ductility also decreased when reducing the thickness. An FE simulation showed that both the surface roughness and thickness affected the flow-curve shape. Moreover, the effect of the surface roughness tended to increase when decreasing the sheet thickness of specimens having the same roughness.     

负偏压对含氢类金刚石薄膜性能的影响

为了解决类金刚石(DLC)薄膜与金属基材间的界面结合强度问题,本研究采用直流等离子体增强化学气相沉积(DC-PECVD)技术,以等时长、不同偏压条件在45钢基材上沉积复合DLC薄膜.采用扫描电镜、原子力显微镜观察薄膜形貌;采用拉曼光谱仪分析薄膜成分;采用涂层附着力自动划痕仪测定膜基结合强度.结果表明:制备偏压从-600...     

Application of diamond-like carbon coatings to elastomers frictional surfaces

Nitrile-butyl rubber-like materials were coated with amorphous hydrogenated diamond-like carbon (DLC) coatings in order to modify their surface and tribological properties. Measurements of water contact angle were performed by the sessile drop method and showed that the coated samples are more hydrophobic with water contact angles up to 116°. The surface free energy of the elastomers was calculated by the acid-base regression method considering polar and dispersive contributions and the results were correlated with changes in the surface chemistry measured by X-ray photoelectron spectroscopy. It has been found that the lower presence of oxygen functional groups on the elastomer surfaces led to lower surface free energies, even though the polar contribution was not predominant. We also found that the DLC coatings led to a significant decrease of the surface free energy (up to 16%) and that there is a good correlation between the surface free energy values and the corresponding water contact angle values. The coefficient of friction was also measured and presented a significant decrease after coating with DLC.     

Effect of vacuum annealing on tribological behavior of nanosized diamond-like carbon coatings produced by pulse vacuum-arc method

Results of tribotests of nanosized carbon coatings formed on silicon surfaces by the pulse vacuum-arc method at various orientations of substrates with regard to a carbon plasma flow are presented; the tribotests were carried out for as-deposited coatings and coatings annealed in vacuum at 600 and 800°C. It has been shown that, among as-deposited coatings, the carbon coating deposited on a substrate perpendicular to the axis of the plasma flow has the highest wear resistance. An increase in the wear resistance of diamond-like carbon coatings after annealing at 600°C has been found.     

类金刚石薄膜的光学性能的研究

利用脉冲真空电弧镀的方法,在硅基底上沉积类金刚石薄膜,研究薄膜的光学性能、光学常数和离子能量关系。结果表明:不同的离子能量可以得到不同折射率的薄膜,无氢类金刚石薄膜的折射率在2.5～2.7之间变化;通过改变工艺条件来制备不同折射率的薄膜,和不同折射率的基底材料相互匹配;折射率和光学能隙随离子能量具有相反的变化趋势,和理论预测的趋势相一致;对于硅、锗等红外材料,要求的薄膜应具有1.8～2.1左右的折射率,因此提出一种基于物理汽相沉积和化学汽相沉积两种相互结合的方法,来降低薄膜的折射率,以达到和硅、锗等材料的折射率匹配。     

Preparation,characterization and investigation of molecular films coated on diamond-like carbon substrate

For the successful application of boundary lubrication, detailed investigations about the influence of preparation process on molecular films are needed. In this paper, a specially designed device was used for the film preparation. The scanning electron microscope (SEM) combined with atomic force microscope (AFM) was employed to characterize the surface morphology and nanotribological behavior of molecular films. After the liquid phase deposition, molecular films are randomly and densely distributed over Ti-doped diamond-like carbon (Ti-DLC) substrates. Through rigorous surface treatments, island-like molecular films were finally achieved on substrate surfaces. The surface friction of molecular films is obviously lower than that of Ti-DLC surfaces. Then, pin-on-disk tribotests were performed to study the macrofriction behavior of molecular films under different preparation parameters. Based on the orthogonal experiment, the effect of five preparation parameters (solution weight percent, smearing force and processing time of three smearing steps) on initial friction coefficient of molecular films was investigated. The results indicated that the order of significance levels is as follows: processing time of the second smearing step > solution weight percent > processing time of step 1 > processing time of step 3 > smearing force. For the purpose of friction reduction, the appropriate level ranges are 0.75% (Solution), 2.5 N–15 N (Force), 1 min–10 min (Step 1), 1 min–2 min (Step 2) and 1 min, 2 min, 5 min and 15 min (Step 3). The initial friction coefficient under the optimized conditions is around 0.112, and the equilibrium friction coefficient is around 0.162, which is lower than that of unlubricated Ti-DLC substrates.     

Effect of surface roughness and surface texture on scuffing

This paper discusses the results of disk tests designed to examine the effect of surface roughness and surface texture on scuffing and related behavior. AISI 9310 steel disk of varying surface roughness and surface texture, along with a MIL-L-7808G lubricant, were used. It is shown that, under otherwise comparable situations, (a) an increase in the composite surface roughness increases the coefficient of friction at scuffing, decreases the scuff failure load, and decreases the critical temperature, (b) the cross-ground disks give a lower coefficient of friction at scuffing and a higher scuff failure load than the circumferentially-ground disks, but nearly the same critical temperature, and (c) an increase in the sliding velocity or sum velocity, at a constant sliding-to-sum velocity ratio, decreases the coefficient of friction at scuffing, decreases the scuff failure load, and decreases the critical temperature.The superior performance of the cross-ground disks compared with the circumferentially-ground disks is attributed to the effect of surface texture on microelastohydrodynamic action. The variations of the oil film thickness ratio at scuffing, the coefficient of friction at scuffing, and the critical temperature with respect to surface roughness, surface texture, and operating variables have been shown to correlate well with a dimensionless parameter $\text{ξ}{}_{\mathrm{?}}$.     

Effect of a combined machining/burnishing tool on the roughness and mechanical properties

Burnishing is a cold working surface treatment process in which plastic deformation of surface irregularities occurs by exerting pressure through a very hard and a very smooth roller or ball on a surface to generate a uniform and work‐hardened surface. This treatment occurs generally after the machining process. In this study, a new combined machining/burnishing tool is designed and is fabricated. This tool allows for generating simultaneously the machining (turning) and the burnishing of the cylindrical surface using a turning machine. First, turned surfaces at different conditions, sketches, finishing and half finishing were performed using only the cutting tool. The evolutions of a surface roughness parameter and the technological time relative to every test condition have been investigated. Second, using the combined machining/burnishing tool at coarse conditions, the evolutions of the surface roughness and the technological time have been also investigated. A comparison among the parameters obtained under different machining conditions and those obtained using the combined machining/burnishing tool has been carried out. Moreover, the analyses of the layers obtained on the combined machined/burnished surface have shown that the burnishing process induces compressive residual stresses on the subsurface treated specimens. Copyright © 2013 John Wiley & Sons, Ltd.     

Self-assembled monolayer on diamond-like carbon surface: formation and friction measurements

We have investigated self-assembled monolayers (SAMs) of heptadecafluoro-1,1,2,2-tetradecyltrietoxysilane (FTE) on diamond-like carbon (DLC) surfaces formed by a simple immersing process. SAM formation on DLC surfaces was verified by contact angle measurements, ellipsometry and X-ray photoelectron spectroscopy (XPS). Water and hexadecane contact angles increased gradually with immersing time and saturated at about 110 and 70 degrees, respectively. Ellipsometric measurements showed that the film thickness was 1.4 to 1.6 nm, which corresponded reasonably to the thickness of FTE monolayer. XPS data showed the presence of FTE molecules on the DLC surface. These results ensured the SAM formation of FTE molecules on the DLC surface.We further measured and compared the friction of unlubricated, SAM coated and 2 nm thick perfluoropolyether (PFPE) coated DLC surfaces using lateral force microscopy (LFM) as functions of the applied load and the sliding velocity. The SAM coated DLC surfaces showed lower friction than the unlubricated DLC surfaces and the friction coefficient decreased by about 15% compared to the unlubricated DLC surfaces. Scratch tests revealed that the critical load of the DLC film increased due to the SAM deposition. These results are attributed to the hydrophobic nature of the SAM coated surface. On the other hand, even though the water contact angle of the SAM coated surface was larger than the 2 nm thick PFPE coated surface, the friction of the SAM coated surface was larger than that of the PFPE coated surface. Also, the critical load of the SAM coated DLC surface in scratch test was lower than the PFPE coated surface. These results indicate that the hydrophobic nature of the surface is not the only factor which determines the friction characteristics in the nano-lubricating system, and it is attributed to the mobile characteristic of PFPE lubricant.     

The influence of surface defects on the mechanical and tribological properties of VN-based arc-evaporated coatings

The influence of surface defects, i.e., droplets and craters, on the mechanical and tribological properties of arc-evaporated V_xN coatings deposited on cemented carbide has been investigated in a scratching contact using a diamond stylus and a sliding contact using a stainless steel pin. Post-test characterisation using 3D optical surface profilometry and scanning electron microscopy was performed in order to investigate the mechanical and tribological response of the coatings. The results show that scratch induced coating cracking mainly is restricted to larger droplets showing a low interfacial bonding to the adjacent coating matrix. The influence of coating defects on the cohesive strength, i.e., the tendency to chipping of small coating fragments, was found to be relatively small. In contrast, the presence of defects may have a significant impact on the interfacial adhesive strength, increasing the tendency to spalling. In sliding contact, surface defects such as droplets and craters have a strong impact on the tribological behaviour of the coatings causing abrasive wear of the less hard counter material surface and material transfer to the coating, both mechanisms affecting the friction characteristics of sliding contact tribo systems.     

An investigation on the micro cutting performance of diamond-like carbon coatings using finite element method

In an effort to prolong the tool life and improve the tooling performance in micro cutting, it is attractive and promising to apply diamond-like carbon (DLC) coatings on micro tools. Comprehensive understandings of micro cutting performance under various coating circumstances are essential for choosing optimum coating conditions so as for potentially improving cutting tool designs. In the study, the cutting characteristics of a DLC-coated tool has been extensively evaluated and compared with those of an uncoated tool under constant and various uncut chip thickness (UCT) using 2D plane-strain finite element method (FEM). The thermo-mechanical modelling approach has been validated at different UCT in micro milling. Besides, the influence of coating friction coefficient, coating thickness as well as UCT on the cutting forces and tool temperatures has been determined and analysed through design of experiment. It is found that appropriate UCT in micro cutting is of the greatest importance for achieving desirable coating performance of micro tools.     

表面粗糙度对微动摩擦特性的影响

通过实验研究,探讨了表面粗糙度对微动摩擦特性影响的作用。表面粗糙高时接触表面之间的摩擦系数会有明显增加,在微动状态下表面粗糙度对表面磨损影响不大。