Dieter Neuschütz

Thermal stability of Al–O–N PVD diffusion barriers

Abstract The use of Al–O–N films as a diffusion barrier for components with a high thermal load, e.g. in gas turbines, was investigated. Therefore, films with compositions along the quasibinary section Al 2 O 3 –AlN were deposited onto Ni base superalloy (CMSX4) substrates by means of magnetron sputtering ion plating (MSIP) at 373 K substrate temperature and characterized with regard to their composition and structure using X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray diffraction (XRD). The phase stability of the films was examined by annealing under inert atmosphere at temperatures up to 1473 K for 4 h and subsequent XRD analyses. To investigate a possible application of these films as a diffusion barrier between Ni base superalloys and MCrAlY, the latter in technical applications serving as corrosion protection for superalloys, a MCrAlY coating was deposited onto selected samples. Without a diffusion barrier and operating temperatures of and above 1373 K, noticeable interdiffusion could be observed. After annealing the CMSX4–Al–O–N–MCrAlY composites for 4 h the interfaces CMSX4–Al–O–N and Al–O–N–MCrAlY were investigated by XRD and energy-dispersive X-ray spectroscopy (EDX). The analyses showed that ternary Al–O–N films were grown in X-ray amorphous structure and remained in that state after annealing the CMSX4–Al–O–N–MCrAlY composites for 4 h at 1373 K. Al 2 O 3 was also deposited in X-ray amorphous structure, but converted into α-Al 2 O 3 after annealing. In the case of the Al 2 O 3 , interdiffusion between CMSX4 and MCrAlY was observed during annealing, especially evident regarding titanium. In contrast to this, ternary Al–O–N films showed a better performance as a diffusion barrier, e.g. no titanium was detectible in the Al–O–N or MCrAlY film. A possible explanation is that the ternary films remained in their amorphous structure, whereas Al 2 O 3 , which at the low substrate temperature supposedly was deposited nanocrystalline in the cubic γ-Al 2 O 3 structure, converted into the hexagonal α-modification during annealing. This transition is assumed to be responsible for the permeability for interdiffusing elements.

Deposition of alumina hard coatings by bipolar pulsed PECVD

Abstract Pulsed and mid-frequency plasma sources are gaining increased attention for the deposition of ceramic hard coatings by PVD as well as plasma enhanced chemical vapor deposition. Especially bipolar techniques allow the deposition of insulating layers under stable plasma conditions. In this paper, the deposition of alumina films on the molybdenum based superalloy TZM and on steel substrates from gaseous mixtures of AlCl 3 , O 2 , H 2 and Ar in a bipolar mid frequency glow discharge at temperatures from 500 up to 620 °C is reported. Special attention was paid to the correlation between deposition temperature and power density and film properties. The measurements revealed that the structure as well as the properties of the resulting coatings were significantly influenced by the plasma parameters during deposition. Increasing plasma power densities led to the deposition of films with higher crystallinity and a change in structure from amorphous Al 2 O 3 to γ- and finally α-Al 2 O 3 . Depending on the gas composition and plasma parameters, alumina films with relatively good hardness and adhesion were deposited.

Plasma-enhanced CVD of Ti,Al N films from chloridic precursors in a DC glow discharge

Abstract Metastable Ti 1− x Al x N films have been deposited from gaseous mixtures of TiCl 4 –AlCl 3 –N 2 –H 2 –Ar in a pulsed DC glow discharge at 510°C. When the discharge voltage was kept constant, the Al content x of the films increased linearly with the AlCl 3 /TiCl 4 ratio in the feed gas. Increasing the discharge voltage also increased the Al content. Up to compositions of Ti 0.09 Al 0.91 N the layers remained single-phase cubic with a strong {100} texture. Films with a higher Al content consisted of two phases and their cubic phase showed a weak {111} texture. The lattice parameter of the homogeneous cubic films decreased with increasing Al content in accordance with Vegards law. Films with a low Al content exhibited a columnar morphology, while the films with high Al contents had a fine-grained structure. Increasing the discharge voltage also caused the grain size to decrease. The microhardness of the single-phase coatings increased with increasing Al content up to 3947 HV 0.05 for x =0.83, while the two-phase layers showed hardness values of approximately 5000 HV 0.05. The metastable films began to decompose at temperatures between 750 and 800°C, depending on the Al content. The decomposition of the films with an Al/Ti ratio below 1 caused the lattice parameter of the cubic phase to increase and the microhardness to decrease. Films with high Al contents did not show any increase in the lattice parameter after annealing and their microhardness strongly increased. Investigation of the oxide layer formed on a Ti 0.21 Al 0.79 N film after annealing in air at 800°C showed that an amorphous alumina layer with a thickness of approximately 100 nm was formed on the surface, preventing further oxidation. The films with high Al content exhibited advantageous tribological properties with friction coefficients of 0.5. Thus, they seem to be especially well suited for an application on cutting and metal working tools.

Sputter deposition of crystalline alumina coatings

In the last years a variety of plasma sources have been developed for film deposition by magnetron sputtering. In addition to RF and DC sputter sources, pulsed DC and low-frequency plasma sources are gaining increased attention in sputter technology. This interest is driven by the wish of depositing coatings with superior properties as compared to those deposited by conventional techniques. One prominent example of coatings that are significantly enhanced by the usage of pulsed sputter deposition is alumina. Although crystalline α-alumina can be deposited by thermal CVD at temperatures above 1000 °C for two decades, no process for the deposition of crystalline alumina at low temperatures is commercially available up to now. In this paper, the results of a detailed study of the plasma parameters during bipolar pulsed magnetron sputtering and their effect on the properties of alumina hard coatings is reported. Langmuir type voltage measurements at the substrate position, optical emission spectroscopy as well as mass spectroscopy were used to monitor the effect of target poisoning on the reactive deposition of alumina. Those principal observations were connected to easily available process parameters like discharge voltage and oxygen partial pressure. Based on these measurements, the deposition of crystalline γ-alumina with high hardness and good adhesion under technical conditions was achieved.

A combinatorial approach to the optimization of metastable multicomponent hard coatings

Abstract Due to the variety of process parameters during the physical vapor deposition of metastable hard coatings, a conventional approach to the development of new coatings is time consuming and expensive. Thus, the ever increasing global market pressures lead to the demand of a cost efficient tool for the development of new products in the advanced materials industry. One possible approach to a fast screening of potential materials for hard coatings could be the adaptation of combinatorial or composition spread technologies to the specific requirements of coatings industry. Combinatorial synthesis and screening is an established technique in pharmaceutical industry and recently combinatorial approaches have been made to the screening of superconductive, magnetoresistant and photoluminescent materials but up to now only very limited work has been done concerning the investigation of hard coatings by combinatorial methods. Despite this, combinatorial approaches can significantly reduce time and costs for the development of multicomponent hard coatings. In this paper, the deposition and characterization of two dimensionally laterally graded hard coatings is presented. The coatings have been deposited by reactive magnetron sputtering, using three metallic targets at a low angle to the substrate surface as well as a system of apertures. To illustrate the advantages of combinatorial approaches for the development of metastable hard coatings, Ti–Al–Hf–N and Ti–Al–Si–N composition spreads will be discussed with special emphasis on the relationship between structure and composition on one hand and oxidation resistance of the quaternary films on the other hand. These two quaternary systems have been chosen because of their technical importance and as model systems with complete solid solution between two of the constituent binaries (TiN and HfN) and with a complete miscibility gap (TiN and Si 3 N 4 ), respectively. The results illustrate that the combinatorial approach is a powerful and cost efficient tool for the development and optimization of new materials for hard coatings.

Composition, binding states, structure, and morphology of the corrosion layer of an oxidized Ti0.46Al0.54N film

Abstract A metastable, single-phase, polycrystalline Ti0.46Al0.54N film was deposited on a HSS substrate by reactive magnetron sputtering ion plating and oxidized in synthetic air at 800°C. The build-up of the oxidic overlayer formed during 1 h of oxidation was analyzed with the followign results: Scanning electron microscopy (SEM) micrographs taken at the inner profile of a crater sputtered through the oxide overlayer showed a glassy morphology of the near surface region and, between the near surface region and the nitridic film, a sublayer consisting of crystals embedded in a glassy matrix. Electron probe microanalysis (EPMA) crater edge linescan analysis revealed that the oxide overlayer consists of two sublayers, which differ significantly in the Al and Ti content. At the surface and in the near surface toplayer the composition corresponds to Al2O3, whereas with increasing depth a composition conforming with Al2O3 and TiO2 was detected. The binding states of the components Al, Ti, O, and N were determined with increasing depth by high resolution Auger electron spectroscopy (AES) multipoint analysis as the inner profile of the crater. Evaluation of the spectra revealed that oxygen is present in two binding states. In the toplayer, oxygen is bonded to Al in Al2O3, and in the underlying layer, to Al in Al2O3 and to Ti in TiO2−y. Transmission electron microscopy (TEM) and high energy electron diffraction (HEED) analyses revealed that the Al2O3 rich toplayer is present in an amorphous state. With structure analysis by thin film X-ray diffraction (XRD), two modifications of TiO2, namely anatase and rutile, were detected.

Electron spectroscopy of single‐phase (Al,B)N films

In the present paper the metastable solid solubility between BN and AlN in the wurtzite structure of AlN has been investigated. Ternary Al-B-N films as well as the binaries AlN and BN have been deposited by reactive magnetron sputtering on Si(111) wafers. The composition, binding states of the components, electronic structure, crystallographic structure and texture of the films have been analysed by means of XPS, x-ray-induced Auger electron spectroscopy (XAES), electron energy-loss spectroscopy (EELS), x-ray diffraction (XRD) and reflection high-energy electron diffraction (RHEED). Up to a BN content of 33 mol.%, all films were deposited strongly textured in the single-phase sp 3 -bonded wurtzite structure of AlN. At higher boron nitride contents up to 68 mol.%, the films were still sp 3 -bonded but the binding states differed from those of the more AlN rich films. Only a pure BN film exhibited sp 2 bondings, corresponding to the stable hexagonal BN modification in graphite-like structure.

Preparation of a new tetragonal copper oxynitride phase by reactive magnetron sputtering

Cu–O–N layers were deposited on Si-〈100〉 wafers at a temperature of 90°C in a reactive magnetron sputtering ion plating system (R-MSIP). For this, a Cu-target was sputtered by a nitrogen/oxygen plasma, and the influence of the oxygen partial pressure on composition, structure, and texture of the Cu–O–N layers was investigated. The analyses of the films with EPMA, XRD, and HEED yielded the following results: with an appropriate setting of the oxygen partial pressure, the oxygen content of the Cu–O–N layers could be controlled between 2.4 and about 50 at. %. Structure analyses have shown changes in the crystal structure of the films with increasing oxygen and decreasing nitrogen content from the simple cubic (sc) structure of Cu3N, followed by a two phase region, where the sc structures of Cu3N and Cu2O appear, to a single phase film with the sc structure of Cu2O. With an oxygen content of 43.6 at. % a new Cu–O–N phase with the tetragonal structure of paramelaconite (≈Cu122+Cu4+O14) and the composition Cu27O22N was grown. The film with an oxygen content of about 50 at. % consists of monoclinic CuO.

Influence of composition and structure on the mechanical properties of BCN coatings deposited by thermal CVD

BCN films were deposited by isothermal chemical vapour deposition from gaseous mixtures of trimethylborazine, toluene and ammonia. The films were analysed with respect to chemical state, composition, morphology and microstructure on the one side oxidation behaviour and hardness on the other side. X-ray spectroscopy (WDX), Raman and infrared spectroscopy, differential thermal analysis, X-ray diffraction and transmission electron spectroscopy were employed for film characterization. A microhardness of maximum 20 GPa was achieved, affected by carbon content, by the way of its incorporation into the hexagonal turbostratic lattice as well as by the crystallite size and its texture.

Characterization of ternary Al-B-N films

Abstract Boron nitride under equilibrium conditions crystallizes in the sp2 bonded graphite like structure and therefore shows no solubility with aluminum nitride or the other group III nitrides. An incorporation of BN into the wurtzite lattice of AlN could extend the applications of (Al,Ga,In)N films due to a higher variability concerning band gap and lattice parameters. To investigate the regions of metastable solid solubility between AlN and BN, ternary films with compositions along the quasibinary section AlN-BN have been deposited on Si(111) wafers at substrate temperatures of 100 and 600°C by means of reactive magnetron sputtering. The structure and texture of the films have been analyzed by reflection high energy electron diffraction (RHEED) after the transfer of the films in a lock chamber under high vacuum into the analysis chamber. In the same device, the composition and binding states were measured by photoelectron spectroscopy (XPS, XAES). The lattice parameters of the films have been determined using X-ray diffraction with a grazing incidence attachment (GIXRD). The results show that it has been possible to deposit by reactive magnetron sputtering metastable single-phase (Al,B)N films in the sp3 bonded wurtzite structure of AlN with BN contents of at least 33 mol.%. The films are strongly textured with different [h0l] textures for deposition at 100°C substrate temperature and [001] textures at 600°C. The crystallinity, especially of the BN rich films, increases with higher temperatures.