High-rate deposition of thick (Cr,Al)ON coatings by high speed physical vapor deposition

Accepted Manuscript High-rate deposition of thick (Cr,Al)ON coatings by high speed physical vapor deposition

K. Bobzin, T. Brögelmann, C. Kalscheuer, T. Liang PII: DOI: Reference:

S0257-8972(17)30502-9 doi: 10.1016/j.surfcoat.2017.05.034 SCT 22353

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

3 February 2017 18 April 2017 11 May 2017

Please cite this article as: K. Bobzin, T. Brögelmann, C. Kalscheuer, T. Liang , High-rate deposition of thick (Cr,Al)ON coatings by high speed physical vapor deposition, Surface & Coatings Technology (2017), doi: 10.1016/j.surfcoat.2017.05.034

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ACCEPTED MANUSCRIPT High-rate Deposition of Thick (Cr,Al)ON Coatings by High Speed Physical Vapor Deposition

K. Bobzin a, T. Brögelmann a, C. Kalscheuer a, T. Liang a* a

Surface Engineering Institute, RWTH Aachen University,

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Kackertstr. 15, 52072 Aachen, Germany

*Corresponding author. Phone: +49(0)241 809-5346

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Fax number: +49(0)241 809-2941

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E-mail: [email protected]

Keywords: CrAlON, High Speed PVD, Hollow Cathode Discharge, Thick PVD coatings,

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Oxidation Protection, Inter-diffusion

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Abstract

Inspired by diffusion barriers, (Cr,Al)ON coatings are developed as oxidation protection for gamma titanium aluminide alloys (γ-TiAl), offering oxidation resistance for applications at

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elevated temperatures, such as turbines. In this study, (Cr,Al)N and (Cr,Al)ON coatings were

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deposited by means of high speed physical vapor deposition (HS-PVD) technology, basing on hollow cathode discharge (HCD) and gas flow sputtering. Using argon as plasma gas and transport medium, the HS-PVD combines the advantages of thermal spraying and physical vapor deposition, which provides very high deposition rates. The objectives of this study were to synthesize thick PVD (Cr,Al)ON coatings and to understand how various process parameters influence coating morphology, deposition rate and phase composition. Keeping other parameters constant, the argon gas flow was varied between QAr = 6,000 sccm and QAr = 12,000 sccm, different N2:O2 gas flow mixtures from 1:2 to 4:1 were introduced into Page 1 of 32

ACCEPTED MANUSCRIPT the coating chamber and the bias voltage was adjusted between UB = -25 V and UB = -150 V. A special target configuration was applied for synthesizing coatings with different Cr:Al ratios within one coating cycle. The atomic Cr:Al ratio was varied between 25/75 and 75/25. The results showed that thick s > 35 µm (Cr,Al)ON coatings can be synthesized very efficiently at a deposition rate ds/dt ≈ 20 µm/h. Strong influences of argon gas flow and bias

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voltage on the deposition rate were revealed. Furthermore, stable plasma processes were observed, even at high oxygen flows QO2 = 600 sccm, which would lead to target poisoning in

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conventional PVD processes. In this way, the HS-PVD technology reveals a high potential for

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the deposition of high temperature oxidation protective coatings and extends the application

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range of conventional PVD technology significantly.

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Introduction

Owing to the outstanding thermophysical properties involving low density of ρ = 3.8 g/cm3 4.2 g/cm3, high temperature strength and excellent creep resistance, as well as promising oxidation resistance up to T = 850°C, the gamma titanium aluminide (γ-TiAl) alloys have been attracting increasing interest in gas turbine industries and are investigated as potential

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replacement for nickel-based superalloys [1–4]. However, for the elevated temperature range

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between T = 800 °C and T = 1,000 °C, the insufficient oxidation resistance of γ-TiAl limits

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their application in advanced aircraft engines and gas turbines [5–7]. Efforts to improve the oxidation resistance γ-TiAl at elevated temperatures have been centered on alloy design [8–

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10], surface modification like halogen implantation [11–13] and oxidation resistant coatings [14–16]. Under the category of coatings, MCrAlY (M = Co, Ni) and Ni-, Pt based coatings

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proved to be resistant against oxidation even at T = 1,000 °C [14,17,18]. Although great progress has been achieved with these coatings, the suppression of inter-diffusion between

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coatings and substrate is still a big challenge. Even if a dense Al2O3 top-layer is formed, an

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inter-diffusion of Ti and Al is still possible, leading to depletion of Al in the coating and Ti in the substrate. Thus, the coating with a low Al concentration is no longer protective and the

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mechanical properties of the substrate will be affected negatively, accompanied by the transformation of γ-TiAl into AlCo2Ti or AlNi2Ti intermetallic phases [15–17]. Furthermore,

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the economic aspect must be taken into account, since those advanced coatings require complex deposition processes and cause high material cost, which limit the industrial implementation. Therefore, the development of oxidation resistant coatings for γ-TiAl, which can prevent the inter-diffusion at temperatures of T > 850 °C remains a great challenge for researchers to overcome.

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ACCEPTED MANUSCRIPT Apart from the deposition of a multilayer system, for example MCrAlY/Al2O3 [19,20], a high temperature resistant diffusion barrier could be a technical and economical alternative. Nitride coatings of transition metals, such as CrN and TiN, find a wide technological use as wear resistant and oxidation protection coatings [21–24]. In order to further improve the hightemperature oxidation resistance of these binary nitrides, ternary coating systems such as

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(Cr,Al)N, (Ti,Al)N have been developed. It has been widely researched and proved that the addition of Al into CrN can shift the oxidation temperature of the film to higher values [25–

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27]. Various studies demonstrated that a complex oxide scale consisting of Cr2O3 and Al2O3

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formed on the coatings, in which the outer layer is Cr-enriched, while the inner layer is rich in

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Al. The mixed Cr2O3 and Al2O3 oxide scale can provide much higher oxidation resistance than a Cr2O3 scale alone [27,28]. Moreover, Hoffmann et al. indicated that a complex

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(Cr,Al)ON layer, which formed on the (Cr,Al)N coating during the heat treatment, could reduce the diffusion rate of Cr significantly [28]. Further researches showed that the

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incorporation of oxygen into the nitride lattice, which leads to the formation of oxynitride

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coatings, can prevent further oxidation at elevated temperatures [29–31]. Knotek et al. produced amorphous AlON and (Cr,Al)ON coatings by means of low pressure plasma

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spraying (LPPS) and magnetron sputtering (MS), respectively, which offered promising oxidation protection and diffusion barrier for oxygen even at temperatures T > 1,000 °C

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[29,30]. Hirai et al. reported that (Cr,Al)ON thin films with a thickness of s = 1 µm, synthesized by pulsed laser deposition (PLD), were oxidation resistant up to T = 900 °C and at the meanwhile, their B1 (NaCl) structure remained stable at T = 1,000 °C [32]. Most of the researched (Cr,Al)ON coatings were deposited by magnetron sputtering [33–38] and cathodic arc evaporation [39,40]. However, up to now there is only little information available on the deposition and characterization of (Cr,Al)ON coating systems beside the referred publications. Moreover, due to the deposition characteristics of conventional PVD techniques such as low deposition rate and arcing [41,42], thick (s > 10 µm) (Cr,Al)ON coatings, which Page 4 of 32

ACCEPTED MANUSCRIPT could be beneficial for oxidation protection and diffusion barrier at high temperatures, have not yet been explored. In this study, we report for the first time on the development and deposition of thick (Cr,Al)ON coatings using high speed physical vapor deposition (HS-PVD), which presents a promising technology for the reactive synthesis of coating systems like oxynitrides. Basing on

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hollow cathode discharge (HCD) and gas flow sputtering as well as using argon as plasma gas and transport medium, the HS-PVD provides several advantages such as the generation of

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dense and highly ionized plasmas as well as a stable plasma process without target poisoning

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and high deposition rates especially for reactive coatings. In order to determine a stable

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process point, the ternary coating system (Cr,Al)N was firstly deposited, which served as predevelopment and transition to quaternary (Cr,Al)ON coating system. The coatings were

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synthesized with varying process parameters such as argon flow QAr and bias voltage UB, so that their influences on the coating properties like morphology and deposition rate ds/dt could

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be investigated. Moreover, different N2:O2 gas flow mixtures and target configurations with

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various Cr and Al contents were applied, resulting in different chemical compositions of the

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coatings, which influenced the coating properties, too.

Experimental details

2.1

Specimen preparation

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The investigated coatings were deposited on cemented carbide WC-Co substrates for crosssectional SEM analyses as well as on application-oriented substrates of γ-TiAl alloy Ti-48Al2Cr-2Nb. The specimen were grinded using 180, 400, 800 and 1200 grit SiC abrasive papers and then polished with a 3 µm water based diamond suspension to a roughness of Ra < 0.02 µm. After cleaning with isopropyl in an ultrasonic bath and drying, the specimen were loaded into the coating chamber that was subsequently evacuated to a base pressure of approximately p = 2 Pa. Page 5 of 32

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2.2

Coating process

The schematic principle of the HS-PVD technology is shown in Figure 1. The specimen were firstly heated up to T = 200 °C and etched in an argon atmosphere with a direct current (dc)

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bias supply UB = -600 V for t = 1 h in order to clean the surface and remove contaminations. In a second step, the power supply of the sputter unit was turned on, leading to the formation

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of a dense plasma so that total of 60 targets consisting of Cr and Al (purity: Cr 99.95 %,

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Al 99.5 %) were sputtered. The sputtered particles consisting of Cr and Al were then

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transported by the argon flow to the substrate. This deposition process was kept for t = 15 min to build an interlayer of CrAl for the purpose of a better adhesion between coating and

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substrate. Subsequently, nitrogen and oxygen gas flows were introduced through the gas nozzles into the vacuum chamber, so that interactions between reactive gases and the particles

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took place, forming a (Cr,Al)ON top layer. The deposition of the top layer took t = 2 h.

The target arrangement for the HS-PVD system offers the possibility to deposit coatings with

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different chemical compositions within one coating batch using flexible target configurations. For the deposition of (Cr,Al)N and (Cr,Al)ON coating systems, a special target configuration

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was developed and applied, as shown in Figure 2. A total of 30 Cr and 30 Al targets were mounted on both target plates. The targets left and right were set up symmetrically and diagonally. The Cr:Al ratios in the coatings were varied as a function of the substrate position. The samples coated at P1 position were supposed to have the lowest ratio of Cr:Al whereas the highest ratio should be found in the coatings at P3 position.

Since (Cr,Al)N and (Cr,Al)ON coating systems have not been deposited by HS-PVD before, a systematic parameter variation was conducted in order to investigate how different parameters Page 6 of 32

ACCEPTED MANUSCRIPT such as argon gas flow QAr, N2:O2 gas flow ratios and bias voltage UB influence coating morphology, deposition rate and phase composition. Therefore, this study also serves as a contribution to the basic research of gas flow sputtering. Table 1 gives an overview of the variation of process parameters for the coating deposition.

UC = 600 V,

cathode

current

density

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Following parameters were kept constant during the deposition process: cathode voltage IC = 0.007 A/cm2,

cathode

power

density

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PC = 7.14 W/cm2, bias voltage for etching UBe = -600 V, substrate-target distance d = 85 mm, no substrate rotation. For (Cr,Al)N coatings, the N2 flow QN2 was kept at 10 %

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of the Ar flow QAr, accordingly to previous investigations. Furthermore, owing to the special construction of the HS-PVD unit, in which reactive gases are directly injected into the coating

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chamber and have no possibility to reach the target area because of the argon flow, target poisoning is suppressed efficiently. Even at a high oxygen flow QO2 = 600 sccm, no

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appearances of target poisoning were observed. All of the investigated coatings could be

Analytical methods

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deposited in a stable plasma process.

Coating morphology and thickness were determined by means of cross-sectional micrographs

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using scanning electron microscope (SEM) of type DSM982 Gemini, Carl Zeiss GmbH, Jena, Germany. Compositional analysis of the coatings was carried out by glow discharge optical emission spectroscopy (GDOES) with JY 5000 RF, HORIBA Jobin Yvon GmbH, Bensheim, Germany. The chemical compositions of the coatings are presented in the form of (CrxAly)OuNv. Moreover, crystallographic phase analysis of the coatings was performed via X-ray diffractometry (XRD) using grazing incidence (GI) X-ray diffractometer XRD 3003, GE Energy Germany GmbH, Ratingen, Germany. The measurements were carried out using Cu-Kα (wavelength: λ = 0.1540598 nm) radiation at U = 40 kV and I = 40 mA with a Page 7 of 32

ACCEPTED MANUSCRIPT diffraction angle 2θ between 20° and 80° and an incoming angle of ω = 2°. Step width and step time were chosen as s = 0.05° and t = 10 s, respectively. 2.4

Annealing

To examine their oxidation and inter-diffusion behavior, the samples B1 deposited on γ-TiAl substrate at P1, P2 and P3 position as well as an uncoated γ-TiAl substrate were annealed at

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T = 950 °C for t = 4 h in air. After that, the phase structures were investigated by XRD. The cross-sections of the annealed samples were examined by SEM. Moreover, EDX linescans

3.1

Chemical composition

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Results and discussion

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were carried out to evaluate the inter-diffusion.

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The surface near chemical compositions of the deposited (Cr,Al)N and (Cr,Al)ON coatings as measured by GDOES are summarized in Figure 3 and Figure 4, respectively. Through the

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diagonal target configuration, the atomic Cr:Al ratios are varied between 8/92 - 69/31 for the

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(Cr,Al)N coatings and between 26/74 - 73/27 for the (Cr,Al)ON coatings, respectively. Owing to target configuration, the Cr:(Cr+Al) ratio increases from P1 to P3, in all of the coatings.

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Moreover, the N content in all of the (Cr,Al)ON coatings is lower than x = 4 at.-% so that they can be defined as N doped (Cr,Al)O coatings. Even a high nitrogen flow for the B4

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sample at QN2 = 1,200 sccm, which is four times as high as the oxygen flow QO2 = 300 sccm, does not increase the N content in the coating, compared to those deposited by a lower N2 flow. This phenomenon has also been observed in other works [43–45] and can be explained by the much higher reactivity of O2 with metallic species compared to N2 among the possible reactions that take place between metallic and non-metallic species in the deposition chamber, together with the fact that oxygen may replace nitrogen in a nitride to form an oxide [43,44]. This statement is supported by the Gibbs free energy (∆G) values regarding the formation of oxides and nitrides for Al and Cr at T = 298.15 K, p = 1 bar [44]: Page 8 of 32

ACCEPTED MANUSCRIPT 2Al + 3O  Al2O3 (-1,582 kJ·mol-1); 2Cr + 3O  Cr2O3 (-1,053 kJ·mol-1); Al + N  AlN (-287 kJ·mol-1); Cr + N  CrN (-93 kJ·mol-1)

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Furthermore, the samples A5-A7 deposited by bias voltage UB between -25 V and -150 V

Coating morphology

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3.2

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of the chemical composition from the bias voltage .

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show almost the same chemical compositions as the A1 sample, indicating the independence

With the aim of investigating coating morphology and thickness of the deposited coatings,

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SEM analyses were carried out. The cross-sectional SEM micrographs of the (Cr,Al)N

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coatings deposited at P2 position are illustrated in Figure 5 and Figure 6.

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Comparing the micrographs of coatings A1-A4, it is obvious, that the coating thickness decreases radically with increasing Ar flow QAr and thus with the working pressure. Such a

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linear relationship has been also observed in previous studies for other coating systems deposited by HS-PVD and can be explained by the reduced mean free path (MFP), which

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describes the average distance travelled rectilinearly by a sputtered particle in a fluid without colliding with any other particles. According to the following equation [46]: MFP =

k∙T

√2∙π∙p∙(r1 +r2 )2

(𝐸𝑞. 1)

In our case, the MFP of the sputtered particles depends only on the working pressure p, since the temperature T and the radius r1 and r2 of the target atoms and the atmospheric gas molecule are constant. A higher working pressure p, mainly caused by a higher argon gas flow QAr, results in a reduction of the MFP, leading to a decreasing deposition rate ȧ . Page 9 of 32

ACCEPTED MANUSCRIPT Moreover, all of the (Cr,Al)N coatings reveal a PVD characteristic columnar structure. The morphology becomes denser with decreasing argon gas flow QAr as a result of the lower energy of the sputtered atoms which is affected by deposition rate [46,47].

As shown in Figure 6 a)-d), the morphology and coating thickness were strongly influenced

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by substrate bias voltage. With increasing bias voltage, the deposition rate rises initially to a maximum of ds/dt = 25.1 µm/h at UB = -75 V and then decreases to ds/dt = 4.8 µm/h at

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UB = -150 V. An increase in bias voltage contributes firstly to attracting the positively

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charged molecule and clusters of the sputtered material in the plasma with enhancement of the

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probabilities for the sputtered material to arrive the substrate, leading to high deposition rates. However, as the bias voltage further increases, the ion bombardment at high levels flakes off

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the adatoms rather than drawing the metallic ions. Consequently decreases the deposition rate drastically. This phenomenon is known as “resputter effect” and has been reported

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numerously for the deposition of CrN and (Cr,Al)N coatings with diverse PVD technologies

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[47–51].

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With the aim of producing thick coatings by means of HS-PVD, argon flow QAr = 6,000 sccm and bias voltage UB = -75 V were chosen as suitable parameters and applied for the deposition

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of (Cr,Al)ON coatings. Figure 7 summarizes the cross-sectional SEM micrographs for the (Cr,Al)ON coatings B1-B5 with different O2/(N2:O2) gas flow ratios ranging from 20 % to 66.7 %.

By incorporating oxygen into the ternary (Cr,Al)N coating system, all of the (Cr,Al)ON coatings as deposited reveal a dense structure according to the SEM images of the cross section fracture. The deposition rate stays stable at a high level of ds/dt ≈ 20 µm/h, almost independent from the reactive gas flows. This corresponds very well with the chemical Page 10 of 32

ACCEPTED MANUSCRIPT compositions, since the O content in all of the (Cr,Al)ON coatings exhibits a clear dominance against N content and at the meanwhile, deviates hardly from each other.

3.3

Crystallographic phase analysis

Figure 8 depicts the XRD patterns of the (Cr,Al)ON coatings B1-B5, including three samples

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with different Cr:Al ratios (sample position P1, P2 and P3) for each coating series. The X-ray diffraction shows that all coatings deposited at position P1 are X-ray amorphous,

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independently from the reactive gas flows, which can be indicated by the absence of

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diffraction peaks corresponding to any crystalline phases. Considering the high Al-content in

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the coatings at P1 position as well as the deposition parameters, especially the coating temperature T = 200 °C, it can be assumed that the domination of amorphous alumina or

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chromium aluminum mixed oxide in the coatings or a decrease in the grain size with increasing aluminum content are responsible for the X-ray amorphous structure. With

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increasing Cr content, in case of the coatings B1 and B2 deposited at P2 position, two new

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peaks referring to fcc crystal structure can be clearly observed, which are in good agreement with the results in [36,44,52]. According to the peaks at 2θ = 44.8° ± 1° and 2θ = 65.8° ± 1°,

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with consideration of the peak shift induced by the internal stress, the presence of a cubic (Cr,Al)2O3 phase, which probably contains γ-Al2O3 (00-050-0741), CrO (01-078-0722), CrO3

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(01-072-0528), is identified. Additionally, no nitride was detected in the as-deposited coatings, which correlates with the low N content in the GDOES results and coincides with previous works on (Cr,Al)ON [44,53]. However, as the Cr content further increases (Figure 8 c), the peaks become broader and the intensity decreases, due to alternation of crystallinity or refinement of grain size, as illustrated in [40]. Associated with the formation of a mixture of chromium oxide of the +2, +3, +4 and +6 oxidation states as well as the decreasing amount of γ-Al2O3 with increasing Cr content, the coatings except B1 deposited at P3 position again exhibit an X-ray amorphous structure [54]. Generally, the coatings Page 11 of 32

ACCEPTED MANUSCRIPT deposited under high ratios of N2:O2 and O2:N2 gas flows, which is the case for the coatings B4 and B5, respectively, show X-ray amorphous XRD patterns at all of the three positions.

3.4

Annealing

After annealing at T = 950 °C for t = 4 h in air, the coated samples as well as an uncoated

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γ-TiAl substrate were measured with XRD and the patterns are presented in Figure 9. It can be seen that all of the samples show the presence of crystalline phases after annealing.

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Coatings B1 deposited at P1 and P2 positions depict very similar patterns, in which the peaks

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are assigned to a mixture of α-Al2O3 and Cr2O3 phases in corundum-type structure. The two oxides are isostructural and similar in the ionic radii of the metal ions (0.57 Å for Al3+, 0.64 Å

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for Cr3+). This favors the formation of solide solid solutions between these oxides, which can

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be referred as (Cr,Al)2O3 [55]. Compared with the uncoated γ-TiAl substrate after annealing, no evidence for Ti containing phases, such as TiO2 and Ti-Al compounds can be obtained in

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these coatings. Therefore, it can be concluded that the coatings B1 deposited at P1 and P2

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positions with high or medium Al content, respectively, act as oxidation protection and diffusion barrier up to T = 950 °C for the γ-TiAl underneath. However, as the Cr content

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further increases, in case of P3 position, the XRD pattern shows the existence of TiO2 and Al3Ti (for a better legibility, peaks of Al2O3 and Cr2O3 are not plotted), which can also be

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observed on the uncoated γ-TiAl substrate. This can be explained by the delamination of the (Cr,Al)ON coating due to volume change during the phase transition, so that the substrate is exposed to the atmosphere causing oxidation. Therefore, the Cr:Al ratio is supposed to trigger the phase transformation, as also observed in [56]. As explained in previous chapter, mixed chromium oxides of the +2, +3, and +4 and +6 oxidation with increasing Cr content will be formed in the as-deposited coatings. This leads to a large amount of oxides, which will transform into the stable chromia while annealing and thus is probably reasonable for the delamination of the coating. Page 12 of 32

ACCEPTED MANUSCRIPT Cross-section analysis of the polished annealed samples was carried out by means of SEM and the results are demonstrated in Figure 10. The coating deposited at P1 position shows no visible cracks after annealing at T = 950 °C, indicating a high thermal stability due to the high Al content. Moreover, a good adhesion between coating, interlayer and substrate is identifiable. EDX linescan was performed and confirmed the absence of Ti in the coating and

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O in the substrate, respectively. The coating deposited at P2 position reveals numerous cracks. Nevertheless, no Ti can be identified in the coating according to EDX linescan result. In this

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case, the CrAl interlayer turns into a TGO sub-layer, which can act as a diffusion barrier in

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view of the short annealing time t = 4 h and prevents the inter-diffusion. However, the coating

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deposited at P3 position shows a damaged columnar structure, along with cracks, pores and gaps between the columns, which offers a diffusion pathway for Ti and oxygen.

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Consequently, as shown in Figure 10, TiO2 and Al3Ti could be identified in the XRD pattern and thus the oxidation protection for γ-TiAl and the diffusion barrier function are not given.

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As a comparison, the cross-section of the uncoated γ-TiAl substrate after annealing is present

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in Figure 10 d). Obviously, the γ-TiAl undergoes massive oxidation without the protection of coatings. As it can be seen, the outer TiO2 layer lies over a thin layer enriched in alumina.

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Beneath these layers is a mixture of alumina and TiO2. Due to the continuous consumption of Al, a titanium-enriched area consisting of α2-Ti3Al phase is formed on the oxide/γ-TiAl

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interface, which is not visible in the XRD spectra because of its distance to the coating surface. The structure of the observed layers after annealing is in agreement with the literature [57]. Furthermore, it can be clearly seen, that the uncoated γ-TiAl was damaged by oxidation very quickly, at a high rate about 2.5 µm/h.

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Summary

(Cr,Al)N and (Cr,Al)ON coatings were deposited by HS-PVD at different argon gas flows QAr, bias voltages UB and N2:O2 gas flow mixtures, respectively. Owing to flexible target Page 13 of 32

ACCEPTED MANUSCRIPT configuration, coatings with different Cr:Al ratios were synthesized within one coating batch. During the deposition, a stable plasma process without target poisoning even at high oxygen flow QO2 = 600 sccm was observed. Generally, high deposition rates up to ds/dt = 25.1 µm/h could be achieved for (Cr,Al)N and (Cr,Al)ON coatings. It was found that a relatively low Ar flow QAr = 6.000 sccm, which led to low working pressure, contributed to a high deposition

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rate. Moreover, a bias voltage at UB = -75 was proved to be suitable for the coating deposition in terms of coating morphology and deposition rate. “Resputter effect” was observed at higher

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bias voltage. By incorporating oxygen into the ternary (Cr,Al)N coating system, all The as-

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deposited (Cr,Al)ON coatings revealed a dense structure according to SEM cross-section

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micrographs. GDOES measurements confirmed the extremely low N content x < 4 at.-% in the coatings, which is almost independent from the nitrogen gas flow. XRD measurements

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carried out at room temperature indicated the formation of X-ray amorphous (Cr,Al)ON coatings with either very high Cr content (Cr:Al = 75:25) or very high Al content

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(Cr:Al = 25:75). The domination of amorphous alumina, the mixture of chromium oxide of

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the +2, +3 and +4 and +6 oxidation states as well as the alternation of crystallinity or refinement of grain size were considered as possible reasons for the X-ray amorphous

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structure. Furthermore, (Cr,Al)ON coatings and uncoated substrate consisting of γ-TiAl were annealed at T = 950 °C for t = 4 h in air and analyzed by XRD and EDX linescan afterwards

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to investigate the oxidation protection. Compared with the uncoated γ-TiAl substrate on which numerous oxidation products and delamination could be found, the (Cr,Al)ON coating protected the γ-TiAl substrate effectively. EDX linescan confirmed the absence of Ti in the coating and O in the substrate, respectively. Therefore, it can be concluded, that thick, amorphous (Cr,Al)ON coatings, especially with high Al contents, can prevent the interdiffusion at high temperatures and provide a high oxidation protection for γ-TiAl.

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Acknowledgment

The authors gratefully acknowledge the financial support of the German Research Foundation, Deutsche Forschungsgemeinschaft (DFG), within the project BO1979/40-1

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“High Speed-PVD XAlON oxidation protection coatings for γ-TiAl alloys”.

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ACCEPTED MANUSCRIPT List of figures: Figure 1:

Principle of the HS-PVD deposition technology

Figure 2:

Target configuration for the deposition of (Cr,Al)N and (Cr,Al)ON coating systems

Figure 3:

Chemical composition and Cr:(Cr+Al) ratio of the (Cr,Al)N coatings deposited

Chemical composition, Cr:(Cr+Al) and O:(O+N) ratio of the (Cr,Al)ON coatings deposited at different positions

Cross-sectional SEM micrographs of the (Cr,Al)N coatings (A1-A4) deposited

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Figure 4:

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at different positions

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at P2 position with different Ar flow QAr

Cross-sectional SEM micrographs of the (Cr,Al)N coatings (A1, A5-A7)

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deposited at P2 position with different bias voltage UB Cross-sectional SEM micrographs of the (Cr,Al)ON coatings (B1-B5)

XRD patterns of the (Cr,Al)ON coatings B1-B5 deposited at P1, P2 and P3

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deposited at P2 position with different O2/(N2:O2) gas flow ratios

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XRD patterns of the (Cr,Al)ON coating B1 deposited at P1, P2 and P3 position

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Figure 9:

and the uncoated γ-TiAl substrate after annealing at T = 950 °C for t = 4 h SEM images of the cross-section of the (Cr,Al)ON coatings B1 deposited at a)

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P1, b) P2 and c) P3 position and d) uncoated γ-TiAl substrate after isothermal oxidation at T = 950 °C for t =4 h

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ACCEPTED MANUSCRIPT Tables: Table 1:

Parameters for the deposition of (Cr,Al)N and (Cr,Al)ON coatings O2 flow Bias voltage Temperature

Pressure

QAr

QN2

QO2

UB

T

p

[sccm]

[sccm]

[sccm]

[V]

[°C]

[Pa]

A1

6,000

600

-

-75

200

28.1

A2

8,000

800

-

-75

A3

10,000

1,000

-

-75

A4

12,000

1,200

-

-75

A5

6,000

600

-

-25

A6

6,000

600

-

A7

6,000

600

-

B1

6,000

300

B2

6,000

600

B3

6,000

900

B4

6,000

B5

6,000

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Ar flow N2 flow

36.8

200

44.8

200

53.2

200

28.1

-100

200

28.1

-150

200

28.1

300

-75

200

28.5

300

-75

200

30.0

300

-75

200

31.4

1,200

300

-75

200

32.2

300

600

-75

200

30.0

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Cr,Al)ON

(Cr,Al)N

Sample

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ACCEPTED MANUSCRIPT Highlights

 Deposition of thick (Cr,Al)ON coatings by means of HS-PVD technology  Stable plasma process without target poisoning even at high oxygen flow  High deposition rate ds/dt ≈ 20 µm/h for (Cr,Al)N and (Cr,Al)ON coatings

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 Cr:Al ratio affects microstructure of the coatings

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