| In the past decades, TiN-based films deposited by physical vapor deposition(PVD) or chemical vapor deposition (CVD) method have found widespreadapplications in wear resistant situations such as cutting tools or machine partsbecause of their high hardness, wear resistance, chemical inertness, and hightemperature stability. Si3N4 is a material with high hardness, thermal and chemicalresistance and passivity towards a number of materials. It is found that multilayerscan combine the properties of the constituent materials and have improved propertieswhen compared to the individual single layer films. Multilayers with optimizedinterface areas seem to be most promising with respect to an optimumhardness-to-toughness ratio.It was found that combining layers with different mechanical properties such asdepositing Ti/TiN multilayers had enhanced the coating performance. There aresome discrepancies in both the maximum hardness and the critical period thicknessreported in these Ti/TiN multilayers studies. Some reports exhibited that the hardnessof Ti/TiN was enhanced significantly compared with that of the monolayer TiN, andthe maximum hardness was obtained when the period was in the range from 5 to 10nm. Daia et al. found that the maximum hardness, 1.6 times higher than the valueobtained by the rule of mixture, was obtained for Λ = 2.5 nm and the data of H(hardness) and period (Λ) gave a better fit with the Hall-Petch relationship. Otherwork did not confirm this report. Farhat et al. found that the maximum hardness wasobtained for Λ = 10 nm, far below the hardness of a thick TiN monolayer. Similarresults were also reported by Dück et al. Li et al. studied the effect of layerthickness ratio on the hardness and showed that the hardness enhancement is less forthin Ti layer than that for thick Ti layer because the Ti layer is too thin to form asharp interface due to diffusion atoms mixing. The difference among these researchreports possibly resulted from the different deposition parameters, both layer materialmicrostructure and interface structure. For tailoring certain properties, it is essentialto know the exact relationship between processing parameters and the correspondingmicrostructure of different single layers.With extensive data obtained for some systems, research interest is inunderstanding the mechanism of the hardness enhancement. Although it is not yetcompletely understood on the atomic scale, it seems that a large number of interfacesor laminated structures play important roles in the improvement of tribologicalproperties. To have a future understanding of the hardness enhancement for theTi/TiN films, it is of interest to systemically study the relationship between thedeposition parameters and the microstructure, interface structure of the multilayerfilms. Therefore, in this study, the influence of deposition parameters such aspressure, bias and temperature on the hardness, interfacial structure andmicrostructure of the multilayers is studied. So far no results have been reportedabout the thermal stability and diffusion behavior of the PVD Ti/TiN nano-structuredmultilayers.In this work, the focus of our study is investigation of Ti/TiN nanolayeredcoatings. The influence of the deposition parameters such as pressure, bias andtemperature on the interfacial structure and microstructure of the multilayers isstudied. The thermal stability of the multilayers is also investigated by means ofannealing for 2 h at different temperatures and at 600℃ for 2 to 4 h in vacuum. Anda very thin Si3N4 interlayer was introduced into the Ti/TiN multilayers trying toimprove its thermal stability.This thesis consists of two parts:1. The effect of deposition parameters(1) Deposition pressureThe sharper XRR spectra of 0.8 Pa and 1.6 Pa for Ti/TiN indicates that the layerstructure becomes more apparent and the cumulative layer thickness fluctuations, theinterfacial width and interfacial roughness less. With increasing pressure themodulation periods and the interface widths of the multilayer decrease and the grainsize increases. As the deposition pressure increases from 0.5 Pa to 1.6 Pa, the TiN(111), Ti2N (103) and Ti(002) peak intensity decreases, after 1.6 Pa Ti2N (103) andTi(002) peaks disappear and a new phase TiN (200) appears. The hardness of Ti andTiN single-layer film measured by Nanoindenter using CSM method is 10.5 GPa and18.4 Gpa, respectively. The hardness of Ti/TiN multilayer should be 14.5 GPaaccording to the rule of mixture, and the hardness enhancement for multilayer isperhaps due to the modulus difference mechanism proposed by Koehler. The mostapparent and sharp XRR peaks of Ti/TiN multilayers at 1.6 Pa mean a good interfacestructure and result in higher hardness.(2) BiasThe XRR spectra of Ti/TiN indicate that the layer structure is not apparent atfloating voltage. As the substrate bias increases, the layer structure becomes moreapparent. This is probably due to increasing energy coming into the growing film andimproving the interfacial smoothness. The floating voltage does not providesufficient mobility to surface species during deposition, resulting in rough interfaces.The XRD at floating voltage shows TiN (111), Ti2N (103) and TiN (200) preferredorientation and those at other different substrate biases show only TiN (111) andTi2N (103) preferred orientation. The Ti (002) peak is apparent at bias –100 V andperhaps is covered by Ti2N (103) at other bias. The substrate bias plays an importantrole in the formation of the microstructure for Ti/TiN multilayer. With the increase inabsolute bias voltage the modulation periods of the multilayer decrease but meanroughness increases first, and then decreases and has a maximum value 0.706 nm at–50 V. The interface width decreases as the absolute bias voltage increases. Thehardness of Ti/TiN multilayer without bias voltage is the lowest. This is perhaps dueto its poor interface and poor effect of modulus difference hardening.(3) Substrate temperatureThe XRR spectra for Ti/TiN indicate that the layer structure is poor at roomtemperature . As the substrate temperature increases, the layer structure becomesmore apparent, which is probably due to decreasing layer intermixing and interfacialroughness. The broad XRR peaks for RT to 300 ℃ samples perhaps mean theapparent cumulative layer thickness fluctuations in multilayers. The XRD spectrareveal that the TiN (111), Ti2N (103) and Ti (002) peak intensity decreases as thesubstrate temperature increases. At 300 ℃, the Ti2N (103) peak becomes very weakand Ti (002) peak disappears. At 400 ℃, besides the disappearance of Ti2N (103)and Ti (002) peaks, a weak TiN (200) peak appears. As the substrate temperatureincreases, the interface width remains almost unchanged but the modulation period Λdecreases first and then increase from 300℃ to 400℃. The hardness and modulusincrease as the substrate temperature increases. The increased hardness of multilayercoatings with substrate temperature is believed to be due to reduced porosity of thecoatings at higher deposition temperature. And moderately higher substratetemperature is also known to reduce the incorporation of argon atoms in theinterstitial sites in sputtered coatings.2. Thermal stability(1) The periodic layer structure of Ti/TiN multilayer deteriorates apparently at 600℃.The amount and strength of XRR peaks becomes few and weak apparently. Thismeans that the interdifussion and grain coarsening occurred in the multilayer. Thiseffect becomes more significant at 800℃.(2) The hardness of Ti/TiN multilayers increases from 18.5 GPa to 28.5 GPa asannealing temperature increases to 600℃, and then decreases to 27.4 GPa whileannealing temperature increases further to 700℃. It is interesting to note that thehardness decreased drastically (18.8 GPa ) for the films annealed at 800 ℃. Thisdecrease in the hardness at higher annealing temperature may be attributed todisappearance of the periodic structure. Low temperature sputter-deposition is knownto produce highly imperfect crystalline layers. Once the crystalline nature of both theas-deposited layers was established the changes that occurred in the diffraction peakareas were attributed to diffusion processes occurring in the course of the heattreatment. Thus, thin enough initial Ti layers will disappear after a short-time anneal.The decreased width of the Ti layers is due to its progressive transformation into TiN,as a result of the influx of nitrogen atoms from the adjacent nitride layers. Meanwhile,the crystallinity of the TiN component is improved as evidenced by the narrower TiNdiffraction peaks, and the porosity and imperfects in the coatings reduced due to thehigher temperature annealing. The apparent hardness enhancement for the annealedsamples may result from these factors in annealing process.(3) Ti/Si3N4/TiN /Si3N4 multilayer coatings in which the S3N4 interlayer thickness is0.8 nm and Ti and TiN layer are 5 nm, respectively, was deposited at roomtemperature and after annealed at 800 ℃ for 2 h. There was not obviously change forXRR of the annealed multilayer coatings compared to the as-deposited sample.Therefore, the Ti/TiN multilayer with 0.8 nm thick Si3N4 interlayer had apparentperiodic layer structure after annealed at 800 ℃ for 2 h. The inter-diffusion betweenS3N4 and TiN or Ti was very poor due to the immiscibility and a very sharp interface.Hence, the S3N4 interlayer was helpful for the improvement of the thermal stabilityand stable periodic structure. The film was grown in a mixed TiN (111)/(200) texturefor Ti/Si3N4/TiN /Si3N4 and TiN (111) texture for Ti/TiN multiplayer. Adding Si3N4interlayer to Ti/TiN multilayer decreases the strain energy obviously and results inthe formation of a mixed TiN (111)/(200) texture. |
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