ICMCTF2012 Session B6-1: Coating Design and Architectures

The ever increasing complexity of modern coatings triggers the need of sophisticated technologies for rapid and commercially advantageous development methods. One possibility to significantly increase the speed of materials development is the use of combinatorial approaches.

In this paper, the applicability of such combinatorial methods in developing advanced materials is illustrated presenting various examples for the deposition and characterization of one- and two-dimensionally laterally graded coatings, which were deposited by means of magnetron sputtering, arc ion plating and plasma-enhanced chemical vapor deposition.

To illustrate the advantages of this approach for the development of advanced materials, the multi-component metastable hard coatings (Ti,Al)N, (Ti,Al,Hf)N, (Ti,Al,Cr)N and (Ti,Al,Si)N were investigated with respect to the relations between structure and composition on one hand and physical properties like hardness, erosion resistance, cutting performance and oxidation behavior of these coatings on the other.

The compositional and structural evolution of Ti_1-xAl_xN thin films has been studied as a function of the total working gas pressure (p_T), the N₂-to-total pressure ratio (p_N2/p_T), the substrate position, as well as the energy and ion-to-metal flux ratio of the ion bombardment during reactive sputtering of powder-metallurgically prepared Ti_0.5Al_0.5 targets of varying grain size (50, 100, 150, and 200 µm). Based on this variation we propose that the different poisoning state of the Ti and Al particles of the Ti_0.5Al_0.5 targets in addition to scattering and angular losses of the sputter flux cause a significant modification in the Al/(Ti+Al) ratio, x, of the synthesized Ti_1-xAl_xN thin films with 0.5 < x < 0.67. The dependence of the chemical composition on the N₂-partial pressure shows a strong correlation with the grain size of the powder-metallurgically prepared Ti_0.5Al_0.5 target.

Additionally, the compositional variation induces a corresponding structural modification between single-phase cubic, mixed cubic-hexagonal and single-phase hexagonal. Our results show, that in particular, the N₂-to-total pressure ratio in combination with the sputtering power density of the Ti_0.5Al_0.5 compound targets has a pronounced effect on the Al/Ti ratio and the structure development of the coatings prepared. The maximum Al content for single-phase cubic Ti_1-xAl_xN strongly depends on the deposition conditions and was obtained with x = 0.66 when applying a deposition temperature of 500 °C, p_T = 0.4 Pa, and p_N2/p_T = 17%. Furthermore we show that also the grain size of the Ti-Al targets has a specific influence, especially on the chemical composition (and local element distribution) of the films prepared, but also on the obtained morphology and evolving nanostructure. By modifying the target grain size, while keeping all other deposition conditions unchanged, Ti_1-xAl_xN thin films can be developed which already show a pronounced formation of Al-rich and Ti-rich cubic domains in the as-deposited.

The results obtained highlight the need for optimized target materials in order to design specific application tailored thin film structures and architectures.

In this work the (Ti,Al)N coatings were produced by cathodic arc evaporation using powder metallurgical targets. The magnetic field configuration on the targets surface was varied in order to influence the electron trajectory to the anode surface. The plasma characteristic was studied by optical emission spectroscopy (OES) as well as by Langmuir probe. The chemical composition of the coatings was investigated by means of energy dispersive X-ray spectroscopy (EDX). The evolution of the growth morphology and crystallographic structure as a function of plasma characteristic was studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The elastic modulus of the coatings was measured by nanoindentation method.

The variation of the magnetic field configuration on the cathode surface was found to be crucial for the plasma conditions near the substrate. The coating analysis shows strong dependence of the coating structure and properties on the plasma characteristic. The correlation between coating growth conditions and the cutting performance of the coatings is discussed.

For the synthesis of new coatings different coating architectures depending on the type of application are considered. This includes the development of nanocomposites, multi- or nanolayers as well as graded coatings. In the present work different types of coating architectures as well materials are presented. In this regard the production process and application of gear components serves as an example.

Gear components like gear shafts can be produced by cold forging. PM (powder-metallurgical) high speed steels are increasingly employed as tool materials for this applications due to their high abrasion resistance and toughness. Nevertheless, the machining of these materials is very challenging due to vanadium and iron carbides which lead to high abrasive wear on the cutting edges as well as the high toughness of these materials which leads to adhesive wear. To meet these challenges nitride nanocomposite coatings seem to be promising. In the present work the development of a (Ti,Cr,Al,Si)N coating is presented. The developed coating was tested in milling of S790PM, 62HRC ( 1.3345, M3 class 2) in comparison to two different commercial coatings: (Ti,Al)N deposited via MSIP and (Ti,Cr,Al,Si)N deposited via Arc Ion Plating. It was shown that, depending on the cutting parameters, the tool life is increased by 20 - 30% by the new developed coating.

After production of the mould the forging process is examined in a second step. Cold forging of steel leads to high forming loads and high adhesive and abrasive wear caused by friction between tool and workpiece material. To reduce friction the workpiece material is bonderized by a zinc phosphate conversion coating before forming operation. This coating step is very time-consuming, expensive and environmentally harmful. In this regard replacing the tribological function of bonder coating by PVD-coated tools and lubricants is interesting. The development of a (Ti,Zr)N/CrN nanolayer with a friction reducing CrN top layer is presented. In fist application tests the production of 500 parts without zinc phosphate conversion coating was possible.

In a third step the application of the produced gear components is considered. Gear components in tribological contact are subjected to high mechanical loads. At the Surface Engineering Institute low-temperature component coatings like the carbon based graded Me-DLC (metal containing diamond like carbon) coating zirconium carbide (ZrC_g) and the hard coating chromium aluminum nitride ((Cr,Al)N) were developed. The coatings exhibit hardness values up to 21 GPa and excellent friction behaviour as proven by tribological tests.

In this work, we prepared thin films of Nb-Si-N using two separate magnetrons, where one target was Si and the other Nb. The atmosphere was a variable mixture of argon and nitrogen. The film composition and structure was modified by varying the power of the two magnetrons independently. For the Si target, the rf source was varied between 20-300 W and for the Nb target, the power was fix at 300 W.

Ellipsometric spectroscopy was used to obtain the optical resistivity of the films as the silicon content was increased. The pseudo-dielectric function changed from metallic-like to semiconductor-like character at certain silicon content. The ellipsometric spectra were modeled using a Drude-Lorentz dispersion function for the film and including a small roughness layer, Brugeman effective medium approximation (50% voids and 50% film) From the Drude terms it was possible to estimate the optical resistivity, which showed three different regimes as the Si at% increased. The first regime, at very low Si contents (> 5 at%), corresponds mainly to NbN containing silicon within the crystalline structure, either as a substitution or interstitial atomic defect. The second regime corresponds to Silicon contents where the solubility limit has been reached and the silicon is largely segregated into the grain boundaries forming a layer of amorphous silicon nitride which covers the NbN crystal. Finally, at the largest Si content (13 at%), there is a large distortion of the NbN crystalline structure, leading to a quasiamorphous Nb-N-Si phase.

These results were confirmed by scanning tunneling microscopy images and X-ray diffraction that showed a decrement in the NbN grain size as a function of the Si content and variations in the film strain. Moreover, by analyzing the X-ray photoelectron signals from the Si, Nb and N atoms and the ellipsometric results, we proposed a model to determine the silicon nitride coverage of the NbN crystal, obtaining as results that the maximum hardness of the films was obtained when the coverage is about 1 monoatomic layer. The geometric model proposed is a combination between the model of Sandu et al. [1] for Nb-Si-N and Veprek et al. [2] for Ti-Si-N.

[1] C.S. Sandu et al. Surface and Coatings Technology 201 (2006) 2897–2903

[2] S. Veprek et al. Surface and Coatings Technology 133-134(2000)152-159

The design process for components made of carbon fibre reinforced composites (FRP) demands an increased level of interest for tribological questions, especially concerning load application zones. The tribological behaviour of FRP, particularly in contact with steel, is to be estimated as relatively weak. As a promising improvement the application of modern coatings is considered.

In this context the results of frictional and wear investigations of unidirectional carbon fibre reinforced epoxy composites (CF-EP) in contact with a coated and uncoated steel material are introduced. The used coating is based on a graduated build-up and extremely hard Diamor^® layer consisting of amorphous, diamond-like carbon which was applied on the steel surface by means of a commercial Laser-Arco^® process. It has been proven that this coating is extremely abrasion-resistant. Furthermore, the frictional coefficient of the material combination CF-EP/ coated steel is significantly lower in comparison with the combination CF-EP/ hardened and polished steel without coating. Also a remarkable outcome is the decrease of the specific wear rate over more than two magnitudes for the basic material (CF-EP). Based on these results the opening up of the material combination CF-EP/ coated steel to sliding application fields, so far reserved to specially developed maintenance-free slide bearings or slideways, can be attained. Therefore the achieved results are of great interest to applications with tribologically stressed contact zones between FRP components and metal structures.

Transition metal diborides MeB₂ are ceramics with high hardness, high melting points, and high temperature stability. These characteristics originate from their hexagonal and layered crystal structure, space group 191, where the transition metal atoms constitute the A layers (0,0,0) and the boron atoms occupy the trigonal prism interstitials (⅓, ⅔, ½) and (⅔, ⅓, ½) present in the structure. This arrangement enables both strong Me-B bonds given the electron transfer from the metal atom to the boron atoms and Me- Me overlap to yield metal-like properties as exemplified by a good electrical conductivity seen for the transition metal diborides. Furthermore, the boron atoms will form a covalently bonded honeycombed structured sheet, in which the electron injection from the metal results in graphite- likes properties; the sheet sometimes being refereed to as “borophene”. The above described property envelope suggests many potential applications for transition metal diborides as thin films ranging from hard protective coatings to high temperature resistant conductive layers.

For the transition metal diboride ZrB₂, we have studied growth at different conditions of target effect, substrate temperature, substrate bias, base pressure etc., using sputtering from a compound target in an industrial scale high vacuum system, CemeCon, CC 800^®/9 ML as well as growth in a laboratory scale ultra high vacuum system.

Our results from x-ray diffraction recorded from films deposited on Si(100) substrates show that 0001 oriented films can be deposited in both type of systems without external heating of the substrate and at growth rates of ~ 3nm per second. Such films are close to stoichiometric, B to Zr ratio of 2 to 2.1, and total level of contaminants less than 2%. Transmission electron microscopy and scanning electron microscopy images display a columnar growth mode. Nanoindentation performed on the films show that they are hard ~20-25GPa and with an elastic recovery of 96%. Four point probe measurements on films deposited on 1000 Å SiO₂/ Si(100) substrates yield resistivity values in the region of 150 to 180µΩ cm

Increased substrate temperatures affects the preferred 0001 oriented growth mode by allowing the nucleation of grains with other orientations as 101̅1 and 101̅0, and at temperatures above ~500 ^oC the deposited films are 101̅1 oriented.