ICMCTF2014 Session B4-3: Properties and Characterization of Hard Coatings and Surfaces

There is in the thin film and coating community a continuously growing interest in metastable phases and microstrucures, which are synthesised through low temperature deposition and are proposed for use in various fields of applications. A commonly used technique to analyse these coatings with regards to chemical composition and bonding is X-ray photoelectron spectroscopy (XPS) coupled with sputter etching using an ion gun to attain information below the outermost surface, which most commonly is oxidised. The use of sputter-etching may however affect the analysed material, which becomes more important to take into account as also metastable materials are studied.

We here present a study on AlN-based coatings, alloyed with Si, Ge or Sn, thus producing more or less metastable materials. These materials are hard multifunctional coatings with tuneable optical properties [1,2]. Through the use of in-situ UHV transfer of samples from the deposition chamber to the analysis instrument we can compare undamaged (and non-oxidised) surfaces with those resulting from sputter-etching to depth typically used in analysis of samples exposed to atmosphere. Our results show that standard XPS techniques can be used to analyse also metastable materials, but the utmost care must be taken, as extensive ion-beam induced damages are observed for some materials when ion Ar⁺ ion energies of 1 keV or above are used to sputter-etch the samples prior to analysis. For the here studied materials, sensitivity to ion beam damage (for Ar⁺ energies of 200 eV to 4k eV) is found to increase for the different alloying elments Si, to Ge to Sn; as well as with alloying content. Damages mainly manifest themselves as preferential nitrogen sputtering and subsequent metallisation of the alloying element. The observed trend agrees with the general nitrogen affinity of these elements.

These results are also relevant for other hard coating materials that include less stable nitride or carbide forming elements. Together with a previously published study on carbide materials [3], this underlines the importance of evaluating the possible presence of ion-beam induced damages, introduced during analysis when working with complex and (at least partly) metastable coating materials.

2. E. Lewin, et al., Journal of Materials Chemistry 22, 16761 (2012)

3. E. Lewin et al., Surface and Coatings Technology 204, 455 (2009)

Post-deposition blasting treatments are widely used to introduce compressive residual stresses into CVD hard coatings, which usually exhibit tensile stresses after deposition on cemented carbide substrates. Within this work, α-Al₂O₃ coatings grown by thermally activated CVD on TiCN base-layers were dry-blasted using a globular as well as an edged blasting medium. The as-deposited and blasted samples were characterized using X-ray nanodiffraction in transmission geometry. Since the point focus nanodiffraction [1] did not provide sufficient diffraction, a pencil X-ray beam with a size of 10 μm x 200 nm was implemented. The beam was aligned parallel to the interface and the cross-sections of the samples were scanned in order to analyze depth gradients of strains, phases and microstructure with a resolution of 100 ‑ 200 nm.

The results document that the maximum compressive stresses of ~4 GPa are much higher for the samples blasted with the edged medium compared to those blasted with the globular material, which showed maximum compressive stresses of ~2 GPa. The stress gradient obtained with the edged medium is steeper, while the affected zone of the sample blasted with the globular material reaches deeper into the coating. The results obtained from the synchrotron experiments are supplemented by laboratory XRD experiments. In addition, the observed stress gradients could be corroborated by the particle impact using a contact mechanics approach.

[1] J. Keckes, M. Bartosik, R. Daniel, C. Mitterer, G. Maier, W. Ecker, J. Vila-Comamala, C. David, S. Schoeder, M. Burghammer, X-ray nanodiffraction reveals strain and microstructure evolution in nanocrystalline thin films, Scripta Mater. 67 748 (2012).

Adhesion tendency of Ti-6Al-4V titanium alloys to three different types of coatings namely TiN, ZrN, and TiAlN was investigated after one pass sliding of a coated ball on a Ti-6Al-4V disc at 3N load, and sliding distance of 0.01m. The same testing conditions were performed for uncoated AISI 52100 steel balls for comparison purpose. After each test the sliding surfaces of the coated balls and the titanium discs were investigated using analytical microscopy techniques e.g. scanning and transmission electron microscopy (SEM, TEM), and energy dispersive spectroscopy (EDS). Microstructural analysis revealed that titanium adhered to the contact surfaces at the early stages, which, attributed as the main reason for an increase in the friction force. The microstructural investigations from cross sections of the transferred material to the ball surfaces obtained by focused ion beam milling showed a solid interface between the transferred Ti-6Al-4V and the coatings/uncoated steel ball. Transmission electron microscopy analysis and corresponding electron diffraction patterns revealed an embedded nanocrystalline structure for transferred Ti-6Al-4V close to the interface with the steel ball. Electron energy loss spectroscopy (EELS) mapping also displayed titanium oxides that were distributed in between the layers of transferred Ti-6Al-4V. These observations were used to determine the mechanisms of adhesion of Ti-6Al-4V to steel and nitride coatings.

In this contribution, the structure of a 13 Å thick SiN_x layer, epitaxially stabilized on TiN(001), is determined by atomically resolved scanning transmission electron microscopy ((S)TEM), annular bright field (ABF)-(S)TEM and spatially resolved electron energy loss spectroscopy (EELS) of the nitrogen (N-K) near edge fine structure (ELNES) in combination with full potential calculations and image simulations.

1 S. Veprek, M.G.J. Veprek-Heijman, P. Karvankova, J. Prochazka, Thin Solid Films 476, 1 (2005)

2 L. Hultman, et.al. Phys. Rev. B 75, 155437 (2007)

3 J. Schiøtz, F.D. Di Tolla, K.W. Jacobsen, Nature, 391, 561 (1998)

4 R.P. Feynman, Caltech Eng. and Sci., 23:5, 22 (1960)

5 H. Söderberg, M. Odén, J. Molina-Aldareguia, and L. Hultman, J. Appl.Phys 97, 114327 (2005)

6 M. Kong,W. Zhao, L. Wei and G. Li, J. Phys. D: Appl. Phys. 40 2858 (2007)

7 B. Alling, E. I. Isaev, A. Flink, L. Hultman, and I. A. Abrikosov Phys. Rev. B 78, 132103 (2008)

8 S.Hao, B.Delley, S.Veprek, and C.Stampfl, Phys. Rev. Lett. 97, 086102 (2006)

The Ni(P) films are superior con-tenders for corrosion and wear resistant coatings. Deposition of Ni(P) via electroless plating might possess a P content ranging from 2 at.% and 20 at.%. Electroless Ni(P) deposit is crystalline at low P contents (< 9.5 at.%), while at high P levels (> 9.5 at.%), the deposit is amorphous. It has been shown that the P content in the electroless Ni greatly influences the interfacial reactions with solders and the solder joint characteristic must also be closely related to the P content. Thus, the P content effect on the solderability of the Ni(P) film must be investigated as far as microelectronic packaging reliability is concerned.

In the present study, the chemical reaction and mechanical properties of the Sn-3Ag-0.5Cu/Au/Ni(P)/Cu joint system were investigated using a field-emission electron probe microanalyzer (FE-EPMA), field-emission scanning electron microscope (FE-SEM) equipped with an electron backscatter diffraction (EBSD) analysis system, and high-speed ball shear (HSBS) test machine. The P contents (x) in the Ni(P) films were 6 wt.% (10.8 at.%), 8 wt.% (14.1 at.%), and 10 wt.% (17.4 at.%), respectively, and all of them possessed a typically amorphous Ni(P) structure, as reflected its high P content in the Ni matrix.

The field of industrial applications of coatings is steadily increasing and more and more coatings are used in applications where their mechanical strength comes to a limit. Apart from their primary properties for the intended use (optical, electrical, chemical, biological) also mechanical properties have to be considered and tested to guarantee the lifetime of the components. The preferred mechanical measurement technique for thin coatings is nanoindentation. It is an established and precise technique with high local resolution and provides typically hardness and modulus of the uppermost layer. However there are more mechanical properties responsible for lifetime and wear behaviour like adhesion, internal stresses, tensile strength, Poisson’s ratio, fatigue behaviour, friction against a certain counterpart, roughness and defect density of all layers and the substrate in a multilayer. Therefore only a combination of several measurement techniques can deliver a more comprehensive view of the mechanical properties of coatings. Additionally several of these techniques can be used to investigate the mechanical behaviour of bulk materials in small volumes (grains, pillars, micro beams) which can be different from that of large samples.

The talk will give an overview of the state of the art of different mechanical characterization methods for coatings. It will mainly focus on nanoindentation in combination with the measurement of lateral force-displacement curves. Additionally other methods like ultrasonic surface waves, centrifuge test, scratch test or atomic force acoustic microscopy are mentioned and compared. Further it will be shown that it may be very helpful to combine tests with advanced stress calculations to extract more mechanical properties and to come from a more qualitative comparison of coatings to a quantitative comparison which allows an optimization of coated systems.

Epitaxial 3C-SiC films were deposited on Si substrates by the vapor-liquid-solid (VLS) tri-phase growth method. We show that such a technique was able to overcome some drawbacks encountered in the epitaxial growth of 3C-SiC films by sole physical or chemical vapor deposition methods. In the VLS method a metallic thin layer, which was evaporated on the Si substrate prior to the growth, was melted at high temperature as the flux and then, methane gas (carbon source) was diffused into the liquid layer to react with Si, leading to the epitaxial growth of 3C-SiC on the substrate surface. The VLS-grown films were characterized by a wide range of techniques including high-precision X-ray diffraction, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. We will report on the effects of the growth parameters, such as flux thickness, substrate temperature, methane flow rate, etc., on the microstructures of the grown films, as well as the growth mechanisms of the VLS process.

Keywords: SiC, thin film, epitaxy

The major drawback in electroless coating is high cost because the coating efficiency and nickel recovery efficiency are considerably low. It reflects in cost of the final product. Surfactant (Surface active agent) is an additive to reduce the surface tension into the electrolyte bath. There are 4 types of surfactant anionic, cationic, zwitterionic, Nonionic. However no researchers were concentrated in zwitterionic surfactant in electroless coating process. Zwitterionic (amphoteric) surfactant consists of two charges (positive and negative) in its head group called hydrophilic and negative charge in its tail group called hydrophobic. Influence of both positive and negative (hydrophilic effect) charged particles in head group shows effective changes in electroless coating process. The problem occurs during coating is that the H2 bubbles floats as the reaction starts, while floating the H2 bubbles carries the Ni particles along with it and as the H2 bubbles bursts the Ni particles become turbulent and coat other than the substrate. Role of surfactant is to reduce the surface tension energy in the EN bath and clears the interfacial bonding energy between Ni particles. Coating was carried out on mild steel specimens. Coating was done for two hours and volume of bath was fixed as 200ml. Nickel chloride, sodium hypophosphite, ammonium chloride and tri sodium citrate were used as source of nickel, reducing agent, complexing agent and stabilizer respectively. The coating parameter pH was maintained at 8-9.The effect of surfactants on the surface roughness, hardness and microstructure of electroless nickel – phosphorus (ENi-P) surface protective coating obtained from an alkaline bath and its mechanism is presented in this paper. In this study the influence of zwitterioinic surfactant 3-(n,n-dimethylmyristylammonio)propanesulfonate on the surface roughness, hardness and microstructure of electroless Ni–P coated samples has been investigated. The variation on surface morphology was examined using scanning electron microscope (SEM), surface roughness values were measured using a stylus instrument and surface hardness values were measured using a hardness tester. It was observed that t he surface roughness, surface hardness and surface morphology of Ni-P coating were clearly influenced by the addition of zwitterioinic surfactant. The complete experimental details, results obtained and their analysis are presented in this paper.

Keywords: electroless coating, zwitterionic surfactant, coating efficiency, surface roughness, surface hardness and surface morphology