Deposition Of Tin On Metal Substrate Biology Essay

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This chapter deals with fundamental studies of different techniques and processes used for nitriding and deposition of metals and metal incorporated polymers such as through glow discharge, magnetron sputtering, by using laser and RF etc.

Polymers are usually known for their high thermal stability, chemical inertness, low surface tension and excellent tribological performance, properties that are advantageous for many applications .To increase the industrial applications, the polymer surfaces are modified by different techniques like nitriding, carburizing etc or incorporated with different materials for example with metals. The metal incorporated polymers improve the properties of polymers e.g; it increases the adhesion of polymer surfaces. Plasma polymerized fluoropolymer films have many potential applications, including use as non-wettable surfaces, dielectrics, optical layers and wear resistant coatings.

Transition metal nitrides are attractive materials for industrial applications due to their remarkable physical and mechanical properties including high hardness, high melting point and thermodynamic stability. For example, Titanium and its alloy are very attractive materials because of their excellent combination of properties that give them the possibility to be used in many industrial applications. However, the materials based on titanium metals have some disadvantages and to overcome these disadvantages by the incorporation of third elements during ion irradiation process (different types of the treatment like thermal treatment) results in the formation of composite films. The properties of composite films are more attractive than the binary nitrides/carbides of titanium. Thus the use of composite films consisting of titanium are being used in industrial applications rather its binary nitrides. Different types of techniques are used for this purpose. However, plasma polymerization is one of the thermo-chemical treatment methods which is being used to improve the surface properties of binary nitrides/carbides of titanium metal and are being used in many industrial properties due to the outstanding features of the titanium base composite films. Thus one can achieve the desirable properties of the composite films. Pulsed DC sputtering is one of the most popular and effective method used for plasma polymerization of the materials to form composite films but as far as plasma co-deposited polymer films are concerned, deposition of such films by pulsed DC glow discharge is a new work in my knowledge. Literature survey for the study of deposition of metals, polymers and metal incorporated polymers using plasma and other methods are explained below.

DEPOSITION OF TiN ON METAL SUBSTRATE

TiN deposition on metal substrate is conducted to improve the surface properties of metals.These have various applications in contemporary microelectronics as diffusion barriers, in the automobile and glass industries as reflecting materials and in jewelry as gold-colored surface-finishing paints (Smith,1995).

Rawat et al [1] studied the deposition of titanium nitride thin films on AISI 304 stainless steel substrate.A3.3 KJ pulsed plasma focus device was used to deposit thin films of TiN at room temperature. Films were deposited with different numbers of focus shots, at different distances from the top of the anode, and at different angular positions with respect to the anode axis. Deposited films were characterized for their structure by XRD, surface morphology by SEM, elemental composition and distribution mapping by EDX and hardness using a nanoindenter. XRD pattern showed the growth of polycrystalline TiN thin films and the uniform distribution of film was confirmed by SEM. The variation in structure, morphology, thickness and hardness of the deposited films with the variation of film deposition parameters was explained, qualitatively, on the basis of ion-emission characteristics of the focus device. It was demonstrated that polycrystalline, smooth and hard thin films of TiN can be successfully deposited onto stainless steel-AISI 304 substrates at room temperature.

Kelly et al [2] reported a comparison of the properties of titanium based films produced by two different techniques. They deposited TiO2 and TiN coatings by continuous and pulsed DC reactive sputtering. The coatings were characterized in terms of their structures and properties using scanning electron microscopy, x-ray diffraction analysis, electron probe microanalysis, micro-hardness testing, scratch adhesion testing, wear testing and surface profilometry. The optical properties of TiO2 films were also investigated. In this case, the pulsed films showed increased refractive index and peak transmission values in comparison to the DC films, while the tribological properties of both coating types were superior when pulsed processing was used in comparison with continuous processing.

Limsuwan et al [3] deposited titanium nitride coatings on stainless steel 304.They studied that the microstructure of the deposited TiN films are related to their properties and deposition conditions. The transition from porous to compact films, the change in their microhardness, lattice parameters, gas pressure and energy of ion bombardment were also investigated. He concluded that the extended crystallographic anisotropy of inhomogeneous lattice deformation is a new phenomenon in which thin polycrystalline films differ from bulk stress-free materials.

Jingsheng et al [4] applied energetic cluster impact (ECI) to grow titanium nitride films on silicon (100) substrate at room temperature. It was found from XRD result the structure of TiNx films was sensitive to experimental conditions such as nitrogen partial pressure, sputtering current and substrate bias voltage and that TiNx films were (220) preferred orientation within some range of experimental conditions.

Kim et al [5] prepared titanium nitride thin films by low frequency (60 Hz) plasma enhanced Chemical-Vapor deposition (LF-PECVD) using a mixture of TiCl4 and N2 a source materials and H2 as an atmosphere material ,and investigated the effect of temperature and input power on the characteristics of the TiN films. The SEM result showed that the deposition rate increased as the substrate temperature and input power increased. The crystalline TiN films had a strong crystallographically preferred orientation of (200) on the XRD patterns.

Carrasco et al [6] studied the relationship between residual stress and process parameters in TiN coating deposited by DC planner magnetron sputtering (DCPMS) on copper alloy substrates, under different conditions of applied current intensity, Ar/N2 flow ratio and bias voltage. The increment of bias voltage and/or current intensity used during the deposition process promoted the generation of high compressive residual stresses. The variation in lattice parameters was also observed with relation to the deposition rate and with the content and the incorporation of nitrogen in the TiN lattice. The use of bias voltage increased the compressive residual stress and the hardness of the film.

Meng et al [7] prepared Titanium nitride films on glass substrate by using DC reactive magnetron sputtering and characterize at different nitrogen partial pressure. The film showed the (111) preferred orientation, the (111) peak intensity decreased as the nitrogen partial pressure was increased. The films had columnar structure and grain size both along the sample surface and normal to the sample surface increased as the nitrogen partial pressure increased. The tensile stresses were evident in the film and investigated to

be decreased as nitrogen pressure was increased.

Mashal et al [8] coated nickel substrates with hard titanium nitride (TiN) by employing a two-step titanization-nitriding Powder Immersion Reaction-Assisted Coating (PIRAC). The PIRAC nitriding of the titanized Ni specimen at 850-9000C resulted in the formation of a thin (~1μm) TiN surface film, whereas the underlying three-layer Ti2Ni-TiNi-Ni3Ti was eventually transformed into a single Ni3Ti layer. The results obtained are discussed in terms of Ni-Ti and Ni-Ti-N phase diagram.

Fu et al [9] studied the optimum parameters of laser nitride titanium to produce a smooth surface with more hard phases formed and without many cracks and pores on coating. Microstructure and XRD analysis showed that the laser treated coating consisted on dendrite TiN and needle-like TiN, Ti2N and Cr2N phases. The maximum hardness can reach 1600 HV compared with the substrate hardness of about 220 HV. Laser nitriding of pure titanium significantly improved both sliding and fretting wear resistance.

Fu et al [10] deposited TiN layer on TiNi thin films to improve surface properties, and explored the deposition and effects of TiN protective layer on TiNi thin films to improve their surface properties while retaining the shape memory effects. Results indicate that the presence of an adherent and hard TiN layer on TiNi based SMA film formed a passivation layer, improved surface hardness and tribological properties, without sacrificing the phase transformation and shape memory effect of the TiNi film.

Kashani et al [11] observed the effect of CVD deposition variables such as temperature, H2, N2 and TiCl4 concentration on deposition rate, lattice parameter, grain size, surface morphology, surface roughness, hardness and adherent of TiN coatings on tungsten carbide substrate were investigated. The result indicates that the kinetics of chemical reaction on the surface was the controlling mechanism for the deposition process. By decreasing nitrogen and increasing hydrogen concentration the deposition rate increased. By raising the amount of hydrogen and lowering the amount of TiCl4 in the system, the film became more uniform, denser and harder. TiN lattice parameter decreased by increasing temperature. TiN grain size decreased by increase in temperature and nitrogen concentration. The surface morphology was found to be sensitive to the deposition parameters and the growth feature changed from grains to columnar structure to powdery and flaky-type deposit. The coating roughness, adherence and hardness were closely related to the microstructual properties of the surface influenced by deposition conditions.

Sen et al [12] reported the growth kinetics of titanium nitride layer deposited on pre-nitride AISl 1020 steel samples by thermo-reactive diffusion (TRD) technique in a solid medium. Experimental results indicated that coating layer had a denticular , dense and porosity-free morphology. The longer the treatment time, the thicker the titanium nitride layer became .The Vickers micro hardness of nitride steel surface and titanium nitride layer were measured to be 520 and 1450 HV respectively.

DEPOSITION OF POLYMERS ON METAL SUBSTRATE

Polymers are used in many industrial applications for their properties such as high thermal stability, chemical inertness, heat resistance and hydrophobicity. By depositing polymers, the surface properties of metals can be altered for example, from conducting to non-conducting or partially conducting. The polymer coated metals also have a wide range of applications such as in kitchen utensils, aerospace and in electronics.

Womack et al [13] reported the deposition of polytetrafluoroethylene (PTFE) thin films (<1 micro meter) on single crystal silicon (100) by a Ti:Sapphire (IR,800nm). SEM,XRD, AFM,XPS and IR spectroscopy were used to characterize the structure, composition, and properties of PTFE films. Results showed that the femtosecond pulsed laser ablates the target cleanly and precisely compared to the traditional excimer laser. The deposition rate was higher and the film quality (particulate density, stochiometry, and smoothness) was superior to the excimer laser-deposited films. The films exhibited crystalline structures with a chemical composition same as the bulk target. So femtosecond laser was found to be a superior processing tool for micromachining and thin film deposition of PTFE.

Li et al [14] studied the production of PTFE thin films on silicon substrate by PLD using either pressed and sintered powder pellets or polished bulk pellets as target material. Based on IR spectroscopy, XRD, and visible inspection, it was concluded that all films have chemical composition similar to that of the source material. The effect of substrate temperature Ts on the morphology and crystallinity of the films was studied. At sufficiently high Ts, films are formed via melting and crystallization of laser transferred PTFE grains on the heated substrate. These films are continuous with a smooth surface and high transparency for non-polymerized visible light. These films have well adherence and pass the Scotch tape test. With the polished PTFE targets, laser irradiation results in rapid depolymerization and ejection of low molecular weight fragments that repolymerizes on the substrate. The films obtained were quite rough, show many particulates, and are opaque for visible light. In the Scotch tape test, the films show cohesion failure.

Bodas et al [15] suggested that polymer materials can be coated/deposited by various techniques like sputtering (magnetron, ion beam, RF or dc), plasma polymerization, etc. and can be used in coatings, paint industries, etc. They explored the possibility of depositing PTFE , known as Teflon, polymer thin films by RF plasma sputtering in presence of argon plasma on mirror polished silicon (100) at three different plasma powers of 100, 150, and 299 W. The time was kept constant at 60 min. The structural integrity of the deposited film was studied by FTIR, XPS and adhesion by contact angle. The sputtered film showed lower contact angle, hydrophilicity and good adhesion of the film with substrate. FTIR indicated presence of C-F, C-F2 bonding groups and XPS showed presence of CF3, CF2,C-F and C-CF moieties indicating deposition of PTFE films at higher power levels of plasma.

Stelmashuk et al [16] prepared fluorocarbon plasma polymer films of PTFE by RF sputtering; possess properties that depend on the argon gas pressure. The wettability of the films decreased with the increase in pressure of argon working gas. The films deposited at 70 Pa were found to be superhydrophobic plasma polymers with a static contact angle 146 degree for water. Sputtered fluorocarbon plasma polymer films were characterized by AFM, XPS and IR spectroscopy. His work showed that the surface composition and chemical structure of the films vary with altering the argon gas pressure.

Huber et al [17] studied the deposition of high quality PTFE films on metallic microstructures and metal backplates for electroacoustic applications by KrF excimer-laser ablation of sintered powder PTFE targets. The films were found to be highly crystalline, consisting of large spherulites with diameter up to 1mm.XPS of the films revealed the chemical similarity of press-sinter target pulsed-laser-deposited films with bulk PTFE. Negatively charged PTFE films on stainless steel back-plates exhibit an exceptional charge stability with practically no decrease of the surface potential up to 225 degree C in open-circuit thermally stimulated discharge.

Gaur et al [18] presented the 40kHZ magnetron plasma polymerization of hexamethyldisiloxane (HMDSO) results, with respect to flow rate of monomer, power input, and XPS analysis of the film .The presence of magnetron caused electrons to move in spirals instead of straight lines,which increased the frequency of collisions and hence fragmentation, resulting in dense polymer film deposits at low pressures. The deposition rate at constant power,increased/increases at first and then decreases with further increase in flow rate. It was found that the film had Si, O, C as main elements but target sputtering was present which added Fe, Cr, Ni and colour to the film.

Guo et al [19] studied that the rapid formation of PTFE-like polymer nanocrystals was achieved by a novel synthesis method oriented plasma polymerization (OPP) at atmospheric pressure. The entire process was completed within a short period of time ranging between a few seconds to several minutes through dielectric barrier discharge (DBD) at atmospheric pressure. It was found that the rapid formation of nanorods and nanotubes are dependant on the plasma conditions. Discharge time, power and ratio of the monomer to carrier gas strongly influence the structure and morphology of fluorocarbon films. As a result, the physical morphology and structure could be controlled through the conditions of oriented plasma polymerization to some extent. The surface morphology of the films were investigated through SEM, TEM, and XRD.

Bodas and Gangal et al [20] described the deposition of PTFE films on a silicon substrate by RF sputtering. These films were evaluated as masking material for anisotropic etching of silicon in aqueous KOH solution. Sputtered PTFE films were characterized by FTIR, XPS and contact angle. FTIR and XPS data showed the presence of PTFE-like film on silicon substrate. From the contact angle value, interfacial tension was calculated, which gave a value of 0.7 dyne per cm which confirmed good adhesion of the film and substrate. The etch rate was calculated from the masking time data of the films. An etch rate of 4 angstrom per minute proved the better masking characteristics of PTFE. And it was demonstrated that PTFE can be a good substitute for the conventional masking materials, namely SiO2 and Si3N4.

Kitoh et al [21] deposited a sputtered ultra thin polyimide film by RF sputtering from a polyimide resin target. The polyimide deposition rate increased along with gas pressure. The molecular structure of the sputtered polyimide film was different from that of the target. The imides ring decomposed, the carbon component increased, and the nitrogen and oxygen components decreased remarkably in comparison with the target. The sputtering process of polyimide was basically a plasma chemical reaction process involving decomposition, recombination, and carbonization. The tribological properties of sputtered polyimide were investigated. The friction coefficient of sputtered polyimide measuring 20-30 nm in thickness was low. The film also exhibited high abrasion resistance. Sputtered polyimide film therefore acted as an effective solid lubricant.

Msckie et al [22] studied pulsed plasma polymerization. He used this process to produce aromatic thin films from inductively coupled RF plasmas with benzene,1,2,4-trifluorobenzene,and hexafluorobenzene as monomers. The effects of aromatic monomer fluorination and duty cycle variation on the resulting films' properties was examined. The surface and bulk properties of the films were were determined using FTIR, XPS, SEM, static secondary ion mass spectrometry (SIMS) and differential scanning calorimetry (DSC).Analysis using these techniques showed a strong dependence of film chemistry and deposition rates on the plasma duty cycle. On the basis of the SIMS,XPS, and DSC data, films deposited under the optimal pulsed conditions were similar to polystyrene in structure but were more complex ,somewhat cross-linked networks. These studies showed pulsed plasma polymerization affords high control over film chemistry and properties.

Katoh et al [23] deposited thin films of Teflon-polymers such as PTFE (polytetrafluoroethylene)., FEP (polytetrafluoroethylene-co-hexafluoro-pro-Pylene). and PFA (polytetrafluoroethylene-co-perluoroalkoxy vinyl ether). on Si (100) substrates by synchrotron radiation, the critical wavelength of 1.5 nm. Etching of their corresponding starting materials in vacuo. Analysis of the deposited films were carried out with X-ray photoelectron spectroscopy (XPS), Fourier transfer infrared (FTIR) spectroscopy as well as X-ray diffraction (XRD). Their surface morphologies were observed under scanning electron microscopy (SEM).For the deposition of the Teflon-polymer films, the processing with the synchrotron radiation photo-etching was compared with that using laser ablation and significant differences between them were discussed. It was concluded that the process developed, can deposit high quality thin films of the Teflon-polymers without reducing the deposition rate or introducing any gas, demonstrating its application potential.

SURFACE MODIFICATION OF POLYMERS

Polymer surfaces are also modified to improve their surface properties like increasing their surface energies and wettability. Many different techniques are used for this purpose, deposition of metal thin films is one of the useful technique for surface modification of polymers having numerous applications.

Liu et al [24] investigated that dense aluminium oxide coatings can be deposited on PTFE substrates in a closed-field unbalanced magnetron sputtering (CFUMS) system by a reactive sputtering technique. The coating composition and properties varied by changing the deposition parameters, such as oxygen concentration in the working gas, plasma power and deposition time. The resulting coatings were analyzed by microanalyser SEM, and a ball-on-disc wear test. Depending on the preparation conditions, the coatings were shown to have different compositions and different mechanical properties. The results revealed that ductile, dense coatings could be obtained in a low oxygen concentration plasma, while a high oxygen concentration in the plasma resulted in a granular, porous structure to the coatings.The coatings with a medium Al/O ratio exhibited good wear and impact resistance; high Al/O ratio coatings adhere to the substrate well but have low wear resistance; and low ratio coatings possesss poor impact resistance, while cracking and debonding from the PTFE substrates were observed durig a ball-on-disc wear test.

CO-DEPOSITION OF METAL AND METAL

Metals are co-deposited to get and study the heterogeneous thin films and also to obtain the specific industrial requirements such as high hardness, corrosion and wear resistance etc.

Schultes et al [25] described the attempt to embed Ag nanoclusters emitted from a gas aggregation cluster source into a growing matrix of alumina originating from sputter sources for the study of heterogeneous thin films. The characteristics of the cluster source were first resumed, with their mean masses ranging from approx.1000 to 100,000 atoms per cluster. The expelled and soft landed clusters were extensively examined by TEM verifying their crystalline form. The use of RF source for the embed material destroys and annihilates the Ag clusters even at very low sputter power. If a reactive DC processes was performed, it did not destroy the crystalline structure of the clusters. Almost all of the crystalline Ag was oxidized to various Ag oxides as revealed by XRD, but remained as crystalline clusters. A subsequent heat treatment reduced the Ag oxides back to metallic Ag clusters.

Pelleg et al [26] investigated the reactions between Ti and Si in co- sputtered (Ti + Si) blanket films with and without a TiN overlayer. The formation of TiSi2 was studied by XRD, TEM and Auger spectroscopy. In all cases,a metal rich silicide identified as TiSi3, was the first phase to form from the amorphous (Ti +Si) regardless if a TiN overlayer was or was not present. At temperatures of 923 K and below it only Ti5Si3 was observed in all specimens.C54 TiSi2 formation was enhanced in specimens having a TiN capping and its formation occurred at lower temperatures and at a faster rate at some appropriate temperature than in specimens without TiN.The effect of tensile stress induced by the TiN layer was suggested as the reason of C54 TiSi2 formation.

Wuhrer et al [27] investigated (Ti,Al)N coatings by reactive magnetron co-sputtering technique with separate titanium aluminum targets at 30o magnetron configuration. It was found that increasing the aluminum magnetron discharge power caused the deposition rate and the aluminum content to increase, and the grain size and surface roughness of the coating to decrease substantially. Tighter packing of the grain columns occurred and the microstructure changed from a porous densified structure, resulting in a continuous increase of the coating hardness. It was found that the microstructure and hardness enhancement of the coating was associated with an increases formation of the TiAlN and AlN phases and a densified, fine grain structure at higher magnetron discharge powers.

Stock et al [28] successfully deposit Ti-Ni-N coatings by unbalanced magnetron sputtering with a pulsed DC discharge. The coatings show a sufficient Plastic universal 31 hardness between 7 to 21 GPA and a good adhesion to the steel substrates. The resulting roughness of the coating's surface depends on the finishing of the steel substrate prior to coating and is only slightly increased by the deposition process. Moulding inserts coated with Ti-Ni-N can successfully be used to produce PMMA lenses. During the process no noticeable wear occurs on the coated inserts. More than that, the contact to the hot polymer matrix causes the dome-shaped top of the Ti-Ni-N columns to flatten. This results in a reduction of the overall roughness but not the total thickness of the coating.

CO-DEPOSITION OF METAL AND POLYMER

Different types of metallic and polymeric coatings, used in automotive industry are widely applied to improve the tribological behaviour of steel mechanical components where wear resistance is needed as fundamental property. Anti- wear coatings show their protective action through two main characteristics: high surface hardness and self lubrication properties.

Wang et al [29] electrochemically prepared nickel/ultra dispersed PTFE composite films with different PTFE content from the Watt's nickel plating bath containing a cationic surfactant as the dispersing agent of PTFE particles. The microstructure of Ni-PTFE composite films was analyzed by means of XRD,SEM, and TEM, and their water repellencies were also measured. The results indicate that a flat and smooth Ni-PTFE composite film was obtained.The film contained uniform dispersion of PTFE due to the surfactant used and homogeneous distribution within the Ni matrix. The PTFE particle content in the composite film is dependent upon the particle concentration in the plating bath and the plating parameters. Ni-PTFE composite films with higher particle content show a superior water-repellency.

Liu et al [30] co-deposited polytetrafluoroethylene (PTFE)-rich surface layer and functionally graded titanium-titanium carbide-PTFE mixed sublayer onto stainless steel substrates using a radio frequency (RF) unbalanced magnetron sputter system with the purpose of improving the tribological performance of the substrate and endowing the surface with hydrophobicity. The results showed that decomposition of PTFE during RF plasma sputtering results mainly in the evolution of fluoropolymer species. High incident power resulted in low F/C ratio in the resulting films, and low incident power resulted in high F/C ratio films. The tribological performance of the coatings depend on the fluoropolymer content in the film bulk, as well as on the film growth process. The films with high fluoropolymer content possess good tribological performance especially at the initial stage of testing. During the co-deposition process, the segments were inlaid into the titanium matrix . It was speculated that some of the carbon atoms may react with titanium to form titanium carbide in the coatings. Multilayer structure attributed to the decrease in the stress developed between layers. All these factors attributed to the improvement of the tribological performance of the stainless steel

substrate.

Guo et al [31] studied about one of the wide range applications of co-deposited metal and polymer named as" Ni-PTFE LIGA MOLD". He investigated that most problems in polymer micromolding are caused by demolding,especially for hot embossing. The demolding forces are related to the side-wall roughness of the mold insert, the interfacial adhesion, and the thermal shrinkage stress between the mold insert and the polymer.The incorporation of PTFE particles into a Ni matrix can have the properties such as antiadhesiveness, low friction, good wear etc. To minimize the demolding forces and to obtain high-quality polymer replicas, a Ni-PTFE composite microelectroforming has been developed.

Zhao et al [32] studied the modification of surface energy of stainless steel 304 by electroless plating Ni-P and small amount of PTFE to minimize bacterial adhesion. The effects of PH temperature, PTFE concentration in solution on the deposition rate of electroless Ni-P-PTFE coatings were also investigated. The thickness and the compositions of the coatings were measured using a digital micrometer and an EDX respectively. Surface morphology was investigated by SEM. The results showed that the deposition rate of Ni-P-PTFE decreases with increasing PTFE, but increases with increasing temperature and p H of plating solution. The Ni-P-PTFE coating with low PTFE content performed better than the coating with high PTFE content.

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