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Wear and T.E.M. studies of P.E.C.V.D. diamond-like-carbon films
Materials and Manufacturing Processes
Abstract
Diamond-like-carbon (D.L.C.) films are characterized by a low friction coefficient, a high wear resistance and a high corrosion resistance. D.L.C. films can be obtained by P.V.D., C.V.D. and duplex techniques with solid or gaseous precursors. Depending on the processing, hydrogen concentration ranges between 0 and 60%
In this study, hydrogenated D.L.C. has been deposited with a radio frequency system (13.56 MHz) onto a steel substrate. Wear was studied by pin-on-disc testing at room temperature with an alumina counterpart. Interrupted tests were carried out to go into wear mechanisms. Debris were collected after various given sliding times for transmission electron microscopy (T.E.M.) in addition to scanning electron microscopy (S.E.M.) applied to the wear tracks.
Diamond-like carbon (DLC) films were prepared on Ti alloy substrate with an rf-plasma enhanced chemical vapor deposition method (rf PECVD), their nano-mechanical properties were evaluated by using a Nano Indenter system with attachments of Continues Stiffness Measurement (CSM) and lateral force measurement (LFM). The results show that the hardness of those films increases with film thickness, but their increment is small; during scratching, three processes containing fully elastic recovery, asynchronous recovery of films and substrate, and delaminating of films, successively occur as the increase of load; at the first, regime, no damage could be found on the surface of films, in the second regime, the trace like fish bone formed; as the thickness of films increases, the critical load increases, but the delamination of films is more severe, which may be attributed to the increase of internal stress in the films.
Diamond-like-carbon coatings (DLC or a-C:H films) were prepared on Ti alloy substrate by rfplasma enhanced chemical vapor deposition. The surface morphology, surface roughness, hardness, critical load and frictional properties of the films were analyzed and evaluated by atomic force microscopy, nano-indentation, scratch test, friction test and fretting wear test. It was found that with the increase of film thickness, the surface roughness sharply increases and then significantly decreases, and it finally comes to a constant. The hardness and critical load determined by scratch test increase with the increase of film thickness, while the friction coefficient decreases with the decrease of surface roughness and the increase of hardness at ambient environment in air. After sliding for 5000 cycles, no evident wear trace was found on the film surface and the friction coefficient of DLC coating against corundum determined by fretting wear test decreases with the increase of relative humidity.
The nanoscrach properties of diamond-like carbon (DLC) films prepared on Ti alloy substrate with rf PECVD were evaluated using a Nano Indenter XP system with the attachments of lateral force measurement (LFM) and their elastic-plastic deformation and mechanism of cracks formation were also analysed during scratching. It was concluded that three processes containing deformation of films, together deforming of films and substrate, and pulling-off of films successively occur with the increase of load during scratching, corresponding to nearly overlaping, separating and abruptly changing of the P-D curves. During the third regime, plastic deformation exceeded elastic part films were dominated from the substrate, which may be attributed to combining effect of cracks in the film and the entire pulling-off between the film and the substrate due to asynchronous recovery of the films and the substrate. Therefore, it is important to enhance bond strength and fracture toughness of films.
Diamond-like carbon coatings with different thickness have been prepared on Ti alloy substrate by rf-plasma enhanced chemical vapor deposition (rf PECVD), their surface morphology and mechanical properties were analyzed and evaluated by AFM, nanoindentation, scratch test, friction test and fretting wear test. The results show that: with the increasing of coating thickness, the surface roughness sharply increases, then significant decreases, at last tending to stable; the hardness and bond strength increase with thickness, but the friction coefficient decreases; and the friction coefficient of DLC coatings against corundum obtained by fretting test decreases with the increase of relative humidity to be below 0.1 in an aqueous condition. All of that are beneficial to their biomedical applications in body fluids.
Nano-scratch behaviors of diamond-like carbon (DLC) films prepared on Si substrate by rf PECVD were evaluated using a Nano Indenter® XP system with attachment of lateral force measurement (LFM). It was found that elastic recovery, elastio-plastically deforming, cracking under loading and delaminating under unloading of the film, occur successively with the increase of scratching load. Though the DLC film has not been delaminated as continuous loading until cracking of film (cracking resistance LCL), the film was delaminated from the substrate after unloading, due to asynchronous recovery of the film and substrate. It is evident that nano-scratching test not only displays the cracking resistance charactering the cohesion strength of the film during loading, but also determines the critical load relating to adhesion of the film and the substrate during unloading.
Deformation characteristics and interfacial adhesion property of silicon nitride/gallium arsenide film/substrate systems were investigated by use of nanoscratch. During scratching, three different types of deformation, elastic, elastoplastic and fracture, were discovered. The critical loads for film failure were obtained. The critical loads, together with the other scratch parameters, were used to determine the interfacial adhesion energies. The practical adhesion energies per unit area of the bilayers obtained were 2.72 and 3.73 Joules/m², respectively. The tensile stress developed just behind the contact area and at the interface was attributed to the film failure. It was also found that residual compressive stress strengthened, but tensile stress weakened the interfacial adhesion.
Amorphous-carbon thin films used as protective coatings for magnetic hard disks were subjected to wear by full-sized alumina-titanium carbide sliders. The associated microstructural changes were analyzed by Raman spectroscopy and high-resolution transmission electron microscopy (TEM). Raman spectroscopy did not detect structural differences between worn and unworn regions. However, TEM micrographs directly showed structural differences between worn and unworn regions, and indicated increased graphitic content. For comparison, unworn carbon samples were annealed and analyzed. The annealed unworn carbon films showed structural changes similar to those of worn samples. This paper presents the experiments and the correlation between the temperature and microstructure of the films, and proposes a wear mechanism.
Friction and wear of TiN, TiCN and diamond-like carbon (DLC) films were studied experimentally using a ball-on-disk tribometer. The rider was a ball of 100C6 steel or polycrystalline alumina. Two steel substrates with different hardnesses were used (250 and 880 HV).When the steel ball is in sliding contact against the films, the results show that friction coefficient is not affected by the hardness of the substrate. For TiN and TiCN, adhesion takes place at the interface followed by metal transfer onto the films; transfer from the coating to the steel ball was detected in the case of the DLC.When the rider was an alumina ball, the experimental results show that the friction coefficient and wear of the coatings depend strongly on the hardness of the substrate.
The nano-scratch behaviour of diamond-like carbon films on a Ti alloy and Si substrate was evaluated. For both samples, three processes---fully elastic recovery, plastic deformation, and delamination and pulling-off of the films, occur successively with increasing load during scratching. The loads (LcL) corresponding to the peeling-off of the films during the up-loading were 75 and 70 mN for Ti alloy and Si. However, the films on Si were delaminated during unloading, and the relevant load (LcU) was only 45 mN. This probably originates from the distribution status of the plastic deformation both in the films and the substrates. Therefore, the nano-scratch test can be applied not only to obtain the cracking resistance (LcL) characterizing the cohesion strength of films during up-loading but also to determine the delamination resistance (LcU) related to the adhesion strength of the film-substrate during unloading.
The present study deals with the tribological behaviour and the wear mechanisms of 2-μm thick diamond-like carbon coatings. The examined coatings were deposited by plasma-enhanced chemical vapour deposition (PE-CVD) on three tool steel substrates, of different hardnesses. Tribological studies against alumina were performed using a pin-on-disc apparatus under various normal loads (2–20 N) and sliding speeds (0.1–0.3 m s−1), while the relative humidity of the environment was kept constant, equal to 25%. The influence of the testing parameters (normal load and sliding speed) and the mechanical properties of the substrate on the wear lifetime of the coatings was determined and the involving wear mechanisms were identified. It was found that both the sliding conditions and the hardness of the substrate affect strongly the wear rate and the wear lifetime, but have no significant influence on the value of the friction coefficient, which was found to be lower than 0.15 for all the testing parameters.
In this paper the structural investigation by TEM of debris generated in ball-on-disc tests is discussed. A 200 μm thick B4C-Cr2O3-TiO2 coating deposited by air plasma spraying on a steel disc was worn in a sliding wear (ball-on-disc) test, and a steel disc coated with a 2 μm thick (Ti, Nb)N layer deposited by PVD was tested under similar conditions. After a discussion of the preparation of TEM samples, the TEM analysis of the debris is discussed. Evidence is given for the fracture and pull-out of carbide particles, and for the oxidation of boron carbide and (Ti, Nb)N during the wear test.
The unlubricated wear of ductile cast iron sliding against an alumina counterbody under mild pin-on-disc testing conditions was studied. Wear plots showing the evolution of wear coefficient with normal load, sliding velocity and humidity have been established. The evolution of the wear coefficient as well as metallographic observations revealed a three stage oxidational wear mechanism. A decrease in humidity resulted in an increase of wear coefficient, mainly owing to abrasive debris behaviour. The removal of wear debris during testing helped in understanding its role in the wear process of ductile cast iron. This role appeared to be both detrimental under dry and beneficial under humid conditions respectively.
The bonding in hard, hydrogenated, amorphous carbon films (a-C:H) prepared by plasma decomposition of benzene was investigated by high-resolution electron-energy-loss spectroscopy. In all asgrown films we find that one-third of the carbon atoms are trigonally bonded and two-thirds of the carbon atoms are tetrahedrally bonded. For the trigonally bonded part, the dielectric properties reveal a transformation from polymeric state for samples prepared with low ion energy to a more graphitic state for films prepared with higher ion energy. Upon annealing, the number of carbon atoms in sp2 configuration increases. The graphitization of the films is observed in two steps at 200°C and at 400°C. A closing of the optical gap and a delocalization of the π electrons is observed between 400°C and 600°C.
Diamond-like carbon, or DLC, films, prepared by the rf plasma decomposition of acetylene, have been deposited at substrate temperatures of 100 to 250 °C, with the substrate at a negative bias of 80 V dc. The DLC films have been annealed in vaccum at temperatures up to 600 °C for 3–4 h. The optical properties of the as-deposited and annealed films have been characterized by ellipsometry and Fourier transform infrared spectroscopy. The wear resistance of the thin DLC films and their friction coefficients have been characterized by a specially designed tribotester. The stresses in the films have also been determined. No significant differences were found between the index of refraction and IR absorption spectra of the as-deposited films, or films annealed at up to 390 °C. The DLC films begin losing hydrogen after annealing above 390 °C and only sp2 bond carbon is observed after annealing at 590 °C. The wear behavior of the as-deposited films was identical for all deposition temperatures. DLC films deposited at 250 °C were more stable and could withstand higher annealing temperature than films deposited at lower temperatures, and remained wear resistant after anneling at 390 °C.
Amorphous carbon films were deposited onto Si, SiC, and Ti alloys by implanting low‐energy C+ ions, laser ablation, or double ion beam sputtering (DIBS). Their soft x‐ray emission spectra were recorded by means of an electron probe microanalyzer (EPMA). Comparison of the energy position of peaks with characteristic spectra of various standards has shown that the short range order in films formed by 2 keV C+ implantation or laser ablation was about the same as in diamond. On the contrary, DIBS films and implantation films formed at lower energies were rather graphite‐like. The former were constituted of true diamond‐like carbon, since other analysis indicated that they were free of H and O contamination and that a carbide of the substrate species did not grow. It is concluded that EPMA is a valuable method for characterizing the bonding in carbon films.
The mechanical properties of the diamondlike coatings deposited with mass‐separated C+, CH+ 3 , CH+ 4 , and C 2 H+ 2 ion beams have been compared. The hardness, abrasive wear resistance, and adhesion of the coatings prepared with the C+ ion beam were superior to those of the coatings prepared with other ions. The most serious drawback of the films prepared with hydrocarbon beams was their brittleness and weak adhesion.
Hydrogenated amorphous carbon (a-C:H) films with “diamond-like” properties have been deposited in dual-mode microwave-radio frequency discharges in CH4, CH4 + Ar or CH4 + H2 mixtures. Properties, such as the microhardness, density, friction coefficient, stress and adhesion, have been studied and correlated with the films' microstructure. It is shown that the mechanical and tribological properties strongly depend on the amount of unbonded hydrogen in the films. They have been shown to belong to one of the following three microstructural categories. (a) Films deposited under high ion energy bombardment (more than 200 eV). They possess high fracture strength and are resistant to severe conditions of humidity, load and sliding speed. This film type is a hard, brittle hydrocarbon, with a high compressive stress (about 8–12 GPa) and high content of unbonded hydrogen (about 80% of the total hydrogen content cH. (b) Films deposited under high ion flux and intermediate ion energy (about 100 eV). These are characterized by moderate fracture strength and they resist moderately severe sliding at a load of 0.5 GPa and a speed of 50 mm s-1. This film type is a hard hydrocarbon with a low compressive stress (about 1–3 GPa) and high content of bonded hydrogen (about 40%–60% of cH). (c) The final category results from a high ion flux and low ion energy (20 eV or less). The films are soft, polymer-like and possess low fracture resistance.
The wear behaviour of sintered high-speed steel-type particulate composites depends significantly on microstructural parameters, such as those of the metallic matrix and primary carbides, but also on some additional parameters related to the powder metallurgy processing. Pin-on-disc test results, coupled with scanning electron microscopy (SEM) observation of the wear tracks and microstructure image analysis, indicated the prominent role of the added ceramic particles. More precisely, the size, the mechanical resistance and the cohesion with the metallic matrix of TiC, uncoated Al2O3, and TiN-coated Al2O3 particles, largely determine the wear behaviour of such materials. In addition, the roles of the residual porosity and of the presence of copper as a solid lubricant are pronounced in this investigation.
Diamond-like carbon (DLC) films are characterized by very low friction coefficients, high wear resistance and high corrosion resistance. Depending upon the testing environment, the coefficient of friction can be as low as 0.01. As-deposited films are wear resistant in vacuum as well as in atmospheric ambient. However, the tribological properties of DLC are strongly affected by the deposition method. This paper reviews the friction and wear properties of DLC, and of similar materials derived from DLC, as a function of the preparation method and testing environment. Mechanisms proposed to explain the tribological properties are presented and discussed.
The wear of ceramics is often affected by tribochemical reactions with the surrounding environment. This paper examines the development of aluminium hydroxide interfacial layers in the wear of alumina at intermediate wear rates and the effect of these layers on wear and friction. Since the humidity of the surrounding atmosphere promotes the formation of these layers, a dramatic effect of humidity on the wear rate is brought about by the changes in layer formation. In supporting experiments, it was found that alpha-alumina reacts directly to form boehmite (Al(OH)) under moderate pressures and high temperatures.