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TiN-C60 - Thesis Presentation (Yan Valsky)
- 1. A Thesis submitted toward the degree of Master of Science in Tel Aviv University by Yan Valsky Supervisors: Engineering Faculty - Dr. Vladimir Zhitomirsky Engineering Faculty - Prof. Reuven L. Boxman Exact Sciences Faculty - Prof. Gil Markovich Novel Wear Resistant TiN-C 60 Coatings Deposited Using Combination of Filtered Vacuum Arc and Effusion Cell Techniques Aug 21, 2010 TAU - EDPL Tel-Aviv University Raymond and Beverly Sackler Faculty of Exact Sciences School of Chemistry, Materials Science Program
- 4. TiN (titanium nitride) coating Aug 21, 2010 TAU - EDPL
- 17. Experimental apparatus for deposition TiN-C 60 Aug 21, 2010 TAU - EDPL Effusion Cell
- 18. Cont’ Aug 21, 2010 TAU - EDPL Ti cathode
Editor's Notes
- This work was done in the electrical discharge and plasma laboratory. The main focus of the laboratory's research is a particular form of electrical discharge known as the vacuum arc, and the application of the plasma produced by the vacuum arc to form thin films and metallurgical coating
- The outline of my presentation includes some introduction to titanium nitride coating applications, it’s properties and main methods to improve its mechanical properties, I will present relevant information about fullerenes, and deposition processes which I used ; I will present the project goals as they were set at the beginning of the experimental work, I will present experimental apparatus and explain the deposition procedure, I will show you the results of that novel research following by discussion. Finally, I will summarize the main conclusions, and point out some open questions and future recommendation. Then I would be glad to hear your questions.
- This statement shows us the significant of wear phenomena effect in terms of money.
- Main functions of small thickness hard coating in tribological applications are: decreasing adhesive wear between metallic surfaces, offer protection against abrasive wear and act as diffusion barrier. TiN is widely used and widely studied coating for wear reduction: 3080 results for TiN, only 325 TiNAlN and 212 for TiCN on the “science direct” search. The most important field of application for TiN today are: wear reduction of tools, wear and friction reduction of machine parts, it also used as decorative because of its goldish yellow color.
- TiN color variations are determined by the film stoichiometry and lattice distortions. TiN crystallizes with the B1 (NaCl) crystal structure. It is generally found that arc deposited TiN has a strongly proffered (111) orientation and usually single δ -TiNₓ phase. It has combination of extreme hardness, high melting point and low coefficient of friction. Mechanical properties of the films can be controlled by varying the substrate temperature, substrate bias and the partial pressure of nitrogen and the film thickness.
- Oxidation resistance of TiN are improved when other metals are partially substituted for the titanium atoms. Considering the particular characteristic of the different hard materials, it is possible to construct coatings where the inner layer provides good adherence to the substrate, where one or more intermediate layers are responsible for hardness and strength and where an outer layers are reduces friction, adhesion, and reactivity. Multilayer superlattice structures produced by sequentially depositing, up to 1000 alternating layers, each layer is a few nm thicknesses. Superhard nanocomposites materials may be composed of both nanocrystalline and amorphous phases. The specific design of such coatings by completely enveloping nanocrystals with an amorphous phase introduces new phase boundaries and might be a tool to optimize both hardness and toughness in a coating. Nanocomposite coatings consisting of a hard matrix with embedded IFs could provide a way to independently control properties such as friction. It was suggested that the IF material is slowly released during wear, creating a self-lubricating tribo-film which reduces friction.
- Two-dimensional solids made of stacked molecular sheets. Each molecular sheet consists of a molybdenum layer sandwiched between two sulfur layers. The MoS 2 could serve as superior solid lubricants in the form of additives to lubrication fluids, greases and for self-lubricating coatings
- Fullerene molecule C 60 also known as Buckyball When sublimed, C 60 is found to produce yellow-gold colored film
- C 60 fullerenes may be identified and characterized using Raman spectroscopy, as well, X-ray photoelectron spectroscopy (XPS) and Time-of-Flight Secondary ion mass spectrometry (TOF-SIMS)
- VAD belongs to a family of Physical Vapor Deposition techniques The cathode vacuum arc is a high-current, low-voltage electrical discharge which produces plasma consisting of vaporized and ionized electrode material. A vacuum arc can arise when the surfaces of metal electrodes in a good vacuum begin to emit electrons either through heating (thermionic emission) or via an electric field that is sufficient to cause field emission. Once initiated, a vacuum arc can persist since the free particles gain kinetic energy from the electric field, heating the metal surfaces through high speed particle collisions. This process can create an incandescent cathode spot which emits more particles, thereby sustaining the arc in the cathode spot mode. In the cathode spot vacuum arc, the conducting medium is a highly ionized plasma, generated from highly luminous regions rapidly moving over the cathode surface, known as “ cathode spots ” (CS) . The cathode spots appear and move on the cathode surface as highly luminous regions of 1-100 mm size. The surface temperature in the spot (3000-4000 K). The spot appears to move on the cathode surface with velocities from several m/s and up to ~150 m/s Consequently, there is intensive evaporation from the cathode spots directed approximately normal to the cathode surface. Almost all vapor is ionized. The deposition of TiN films by arc evaporation is achieved by reactively evaporation of a pure Ti cathode in the presence of nitrogen.
- The Knudsen-cell provides a tool for depositing small area thin films, with coverage ranging from sub-monolayer to continuous films. The deposition rate is extremely stable, being determined by the temperature of the enclosure which is accurately controlled using a thermocouple and temperature controller.
- The rate of molecular effusion can be found from the equation. where R e is the effusion rate, P v is the vapor pressure of C 60 molecules, A 0 is the hole area, N A is the Avogadro number, M is the molar mass, R is the Boltzman gas constant, and T e is the vapor temperature/effusion temperature.
- TiN and C 60 and TiN-C 60 coatings were examined and tested by various techniques as follows
- The effect of N 2 pressure and substrate temperature on TiN coating color deposited on the Si substrates is presented. It may be seen that as the nitrogen pressure increased from 1 to 4 mTorr, the color of the coating become darker, from bright yellow to bright brown. No influence of substrate temperature on coating color was observed. No coating delamination was observed in all nitrogen pressures and substrate temperatures which were experimented
- The TiN deposition rate was determined by measuring the coating thicknesses using the Alpha-step profilometer at three deposition times: 60, 120 and 300 s. Deposition parameters were P N2 =3 mTorr and T substrate =100 o C. It may be seen that the TiN (111) plane (at 36 o ) was present in all three coatings. For P N2 =2 mTorr, the (222) plane at 75.5 o was also observed, while For P N2 =3 and 4 mTorr the (200) plane at 42.5 o and the (220) plane at 62 o planes were observed. EDX elemental showed that all samples contain titanium and nitrogen.
- C 60 fullerenes were deposited at two distances from the EC aperture, at 3 and 8 cm. The substrate was perpendicular to the effusion cell (that is the only experiment that the substrate was perpendicular to the EC, the rest experiment the substrate was oriented 45 o ). As the distance between the EC aperture and the substrate sample increases, the deposition rate decrease. The C 60 molecular beam is broader and thus spread the fullerenes, resulting in fewer fullerenes which deposit on the sample.
- Upper figure shows the Raman spectrums of C 60 fullerenes, deposited on glass substrate, and C 60 fullerenes powder, as reference. Three characteristic peaks of C 60 fullerenes may be seen on both spectra at 1425 cm-1, 1469 cm-1 and 1573 cm-1. No signs of polymerization or destruction of the C 60 fullerenes were observed. XPS figures show the XPS spectra of the C 60 fullerenes powder. Two characteristic peaks of C 60 fullerenes may be seen: a C1s core peak at 283.5 eV and a “ satellite ” peak at about 289 eV of the C 60 fullerenes powder. The C1s peak rather symmetrical and thus no polymerization or destruction of C 60 fullerenes observed.
- This figure shows the deposition structures of TiN-C 60 coatings: first layer, adjacent to the Si substrate, was TiN, and then, knowing the deposition rates of TiN and C 60 separately, number of C 60 and TiN layers was sequentially deposited. Two TiN-C 60 coating architectures were studied: Multi-layer consisting of alternate TiN and C 60 layers without any intentional mixing. TiN layers in the thickness range of 50-500 nm were deposited using VAD for a fixed short period (up to few minutes). C 60 fullerenes layers in the thickness range of 5-40 nm were deposited using effusion cell evaporation for a fixed rather long period (up to few hours). Composite TiN-C 60 coatings with a continuous TiN matrix and embedded C 60 fullerenes. TiN layers in the thickness range of 8-17 nm were deposited using VAD for a fixed very short period (up to one minute). C 60 fullerenes were deposited in such a way that only 0.3-0.7 layer of fullerenes was constructed, and enable continues TiN matrix connection.
- Figures show the delaminated layers of the coating, no fullerenes were directly observed, possibly because of the technical limitations.
- Coating adhesion can be estimated by testing for cracking around indentation craters. TiN-C 60 coatings were indented with 10 – 300 g loads for 10 sec, for each load two tests were made. The crack around the indentation was graded using 0-6 grading scale. 0 indicating good adhesion with no cracks, to grade 6 indicating low adhesion with pile up and brittle cracks. Upper figure shows the dependence of adhesion on mol.% C 60 in the TiN-C 60 coating. Pure TiN coatings had an average (two tests) adhesion grade 1. For all layered TiN-C 60 coatings, the average grade was above 3.5, indicating poor adhesion. With the embedded TiN-C 60 coatings, the tendency was that the grade increased with the C 60 content. One exception was (TiN-C 60 -26) with mol.0.8% C 60 , which was graded 0.5, better than pure TiN. Critical load (L c ) is defined as the indentation load at which a concentric crack in the coating around the indentation was observed. L c was determined when the crack around the indentation was graded 4 or higher using the scale. Next figure shows the dependence of critical load on the mol.% C 60 in TiN-C 60 coatings. For pure TiN, the critical load was 100 g. For all of the layered TiN-C 60 coatings, the critical load was 10 g (which was the lowest load tested). The critical load in the embedded TiN-C 60 coatings generally tended to decrease as the % C 60 increased. However one coating with mol.0.8% C 60 (sample TiN-C 60 -26) had a critical load of 200 g, twice as high as pure TiN.
- Upper figure shows the measured Vickers hardness (HV) for 10 g load vs. mol.% C 60 in TiN-C 60 coatings. It may be seen that TiN coating showed the highest hardness relative to the TiN-C 60 coatings tested. No significant difference in hardness was observed between the embedded and the layered coating. Two samples with ~500 HV were delaminated. The friction coefficient was calculated for various TiN-C 60 embedded coatings. Next figure shows the dependence of COF (20 g load) on mol.% C 60 in the TiN-C 60 coatings. Two pure TiN coatings had a COF of 0.18-0.21 after 3 and 12 min of scratch. After 3 min of wear, COF of most TiN-C 60 coatings was somewhere in that range or higher. One sample, TiN-C 60 -18 (mol.0.8% C 60 ) had a COF of 0.15 less than for pure TiN. After 12 min of wear, no samples had a lower COF then pure TiN.
- Wear tracks left on the coating surfaces by ball-on-disk tests were graded on a scale from 1 to 4. This test was only semi-quantitative since the results are very subjective. Figure shows the dependence of the wear track grade on the concentration of C 60 in 450±20 nm thick TiN-C 60 coatings. It may be seen that as the concentration of C 60 increased from 0 to mol.1.6% C 60 , the wear track grade increased from 1 to 4. Further increase in the C 60 concentration lowered the wear track grade to 2.
- Accordingly, two modes for incorporating C 60 into the TiN matrix were explored: layering and embedding
- Adhesion at micro indentation of layered coatings failed between the layers, resulting in visually observable delamination. The inter-layer adhesion failure was probably caused by stress forces in the TiN coating which were higher than the adhesive forces between the TiN layer and the C 60 layer. In contrast, embedded coatings adhered well most probably due to continuous TiN matrix connection All but one embedded sample showed lower (better) indentation adhesion than pure TiN coatings. Vickers microhardness testing showed that the coating sample was softer, ~950 HV, than TiN, 1300 HV. This could explain the indentation behavior – the indenter penetrated the TiN-C 60 coating without brittle fracture or coating detachment and thus the embedded C 60 increased the L C
- The reason for this behavior possibly lies with the tendency of the fullerene to clump and compress into a high shear strength layer which is harder to deform than pure TiN
- Deposition rate of C 60 molecules, by means of the effusion cell, may be calculated using the following assumptions: all evaporated C 60 are deposited on the substrate, the C 60 molecules deposits and grow through a layer by layer mechanism when each layer is 1 nm thick
- The erosion products are plasma of the cathode material, including positive ions, electrons and neutrals, as well as liquid droplets, known as macroparticles. The microdroplets, are molten droplets, usually of 0.1-10 μ m size. If the MPs are incorporated into a deposited coating, they can degrade its properties. The main idea in MP filtering is to block any direct path between the cathode and the substrate. However, the path is blocked both for the undesirable MP's and also for the metal plasma flux. MP-free deposition is more effective if an appropriate magnetic field is applied to bend the plasma beam towards the substrate, while the MP ’ s generally move along straight trajectories and are hence separated from the plasma flux. An apparatus combining an obstacle and a magnetic field to bend the plasma around the obstacle is referred to as Filtered VAD. Usually the magnetic-field strength in the duct is sufficient to “ magnetize ” the electrons, i.e. to produce a Lamor radius for the electrons which is smaller than the duct width, but not the ions. The ions are confined electrostatically by the field generated by the drift of the ions away from the electrons. Because of the mobility of the cathode spots, it is necessary to control the spot motion, so that they are on the “ front ” surface of the cathode facing towards the substrates, both so that the deposition device will be efficient and also to prevent destructive erosion from occurring on support structures. It is also desirable to control the spot motion in order to spread out the heat flux to the cathode surface and thus prevent local overheating and to erode the cathode relatively uniformly. Two magnetic field phenomena influence cathode spot motion: (a) primarily, the spot tends to move in the retrograde – JxB direction with a velocity approximately of 50-150 m/s and (b) secondarily, the spots tend to move in the direction of the opening of the acute angle between the magnetic field line and its projection on the cathode surface ( “ Acute angle rule ” ). An advantage of the arched magnetic field concept is that the cathode spot motion can be controlled over large area cathodes, which are often desired in order to obtain uniform deposition over large areas and to have long cathode life. A disadvantage is that the efficiency of the cathode material utilization is low, as evaporation occurs only from the narrow track and not from the entire cathode surface.