Microstructure Characterization of Cr(N,C) Coating Deposited with Pulsed Laser Deposition (PLD) Technique

J. Morgiel1, Ł. Major1, H. Takahashi2, T. Shibayama2, D. Foord3, J.M. Lackner4 Abstract

The Cr/CrN/Cr/Cr(C,N) coating on stainless steel substrate were obtained using Pulse Laser Deposition (PLD) The standard transmission observations (TEM) showed that the coating consist of four main layers of 225/341/75/416 nm, of which the first one is divided to three sub-layers. The second and fourth layer was built from columnar grains, while all others showed amorphous like contrast. The local chemical analysis performed using X-ray energy dispersive spectroscopy attachment (EDS) confirmed high content of nitrogen and carbon in TiN and Cr(N,C) respectively and pointed toward a raised level of oxygen in all layers. These measurements indicated also an existence of strong Cr gradient in buffer layer. The atomic resolution observations (HREM) helped both to precise the size of columnar grains and roughly estimate their defect density. The same type of observations performed on Cr buffer and intermediate layer proved that they contain Cr nano-crystallites. Additionally, the density of nano-crystallites was found changing in between buffer sub-layers.

Introduction

Titanium nitride coatings have been used in industry since 1970 both for corrosion and wear protection of materials [1]. The present development in hard coatings is aimed at improvement of their high temperature oxidation resistance by substituting titanium nitride with chromium nitride, while partial replacement of nitrogen with carbon should increase coating hardness and in consequence wear resistance [2].

Building coatings from layers of different materials, i.e. inserting into hard coating softer interlayer may allow strengthening their adherence to substrate, relaxation of internal stresses and limiting crack propagation. The introduced recently Pulsed Laser Deposition (PLD) technique allows easy deposition of sequence of different layers. The conditions at which the PLD is performed decides on coatings microstructure and therefore on their properties. That is why, the relation between the deposition condition and microstructure of developed coatings must be fully understood before their future applications.

The X-ray investigations performed on the coating design as Cr/CrN/Cr/Cr(N,C) showed peaks both from CrN – type phase and from austenite present in the substrate, but non from the Cr buffer or intermediate layers. Additionally, the texture of CrN layers was found very similar to that present in the stainless steel substrate [3]. The inheritance of substrate texture by columnar CrN layer would not be surprising, if not the presence of Cr buffer layer having amorphous like microstructure as showed preliminary TEM observations [3].

Therefore, the aim of the next stage was provide more detail information on coatings microstructure and especially on chromium buffer and intermediate layers using high resolution and analytical electron microscopy, i.e. HREM and AEM.

1 Institute of Metallurgy and Materials Science, Polish Academy of Sciences,

25 Reymonta st., 30-059 Kraków, Poland 2 Centre of Advanced Research of Energy Technology, Hokkaido University,

Kita-ku, Kita-13, Nishi-8, 060-8628 Sapporo, Japan 3 FEI Company,

P.O. Box 80066, 5600 KA Eindhoven, Netherlands 4 Laser Centre Leoben, Joanneum Research Forschungesellschaft mbH,

Loebner Strasse 94, A-8712 Niklasdorf, Austria

Experimental procedures The Cr/CrN/Cr/Cr(N,C) coating was deposited using N:YAG laser (1064 nm). The

deposition chamber was first evacuated down to 2x10-3Pa and next argon, nitride or nitride and methane atmosphere was introduced. The buffer layer was deposited at argon flow of 20cm3/2min., 25cm3/3min. and finally 30cm3/5min., the CrN layer at 30cm3/40min., Cr intermediate layer 30cm/5min. and the top Cr(N,C) layer at (15cm3 N2 + 15cm3 C2H2)/ 40 min..The 99.999 N2 and Ar and 99.9 C2H2 were used.

The thin foils for transmission electron microscopy were prepared using standard Focused Ion Beam (FIB) technique [4]. The microstructure observations were performed using JEM-2010F, Philips CM20 TWIN, Tecnai G2 F20 equipped with EDS attachments. Resulsts

The detail microstructure observations of Cr/CrN/Cr/Cr(C,N) coating in cross-section confirmed previous observations of columnar nature of CrN and Cr(N,C) layers as well as amorphous like character of Cr buffer and intermediate layers as confirmed by micro-diffraction technique (Fig.1). Additionally, they not only confirmed that the first Cr buffer layer is divided to three sub-layers, but also showed that the sub-layers are characterized by step-like changes of their amorphous like contrast (especially between second and third from the substrate). That type of contrast variation in case of amorphous structure could be caused either by abrupt thickness or composition changes. However, as the thin foils were cut using FIB technique, which allows to cut from materials small pieces of nearly parallel surfaces, no thickness steps perpendicular to the thin foil surface should be expected. Fig.1. Transmission electron microstructure of Cr/CrN/Cr/Cr(N,C) with micro-diffraction

obtain from layers as marked by arrows.

substrate

buffer 3 buffer 2

buffer 1

intermediate

The maps showing distribution of coating forming elements obtain using Energy Dispersive X-ray Spectroscopy (EDS) attachment are showed in Fig.2. They allowed to separate Cr layers, i.e. buffer and intermediate from CrN and Cr(N,C). However, the apparent nitrogen concentration in CrN and Cr(N,C) was practically the same, while recorded carbon concentration level was the same all over. The oxygen distribution map corresponding to that of nitrogen results probably from surface chromium oxide layer Fig.2. EDS maps of Cr, N,O and Fe distribution respectively The EDS local chemical analysis acquired point by point along the line marked on chromium distribution map in Fig.2 is shown in Fig.3. The much longer acquisition time assign to each point, than those used for mapping, benefited in increasing the measurements sensitivity. The Cr line showed not only very clearly the presence of intermediate Cr layer but also gave strong indication of the chromium concentration gradient in the buffer layer. The presence of nitrogen in the coating was raised in ceramic layers as expected but surprisingly high in Cr intermediate and buffer layer. Additionally, oxygen was found to follow distribution of nitrogen. The carbon, being the most problematic in that type of analysis, was inasmuch found in the coating top most layer as expected. As the EDS line-scan is of qualitative nature and therefore interpretation of exact level of content of certain element like for example oxygen is impossible. However, the using FIB for sample preparation allowed to produce thin foils with relatively similar thickness making at least interpretation of relative content of the same element in different places more reliable than usual.

Fig.3. EDS line-scan along the line marked on chromium distribution map (Fig. 2a)

Cr N O Fe

0

500

1000

1500

2000

0 10 20 30 40 50 60 70 80 90 100layer depth [a.u.]

X - r

ay c

ount

s [a

.u.]

Cr

N

O

C

Ni

Fe

Fig.4. HREM image of columnar grain (broken line shows direction perpendicular to coating surface) in CrN layer with Fourier Transform (FT) diffraction pattern obtain from area marked with a square

The high resolution electron microscopy observation of CrN and Cr(N,C) layers confirmed that they are built from large columnar heavily faulted CrN –type grains (Fig.4). The maximum column thickness was no bigger than 15 nm, while height approached 50 nm. The distortion or of lattice image every few nanometers confirmed the high defect density predicted from standard transmission observations. The Fourier Transform obtain from part marked by square indicated the [100]CrN-type phase orientation. The observation of the Cr buffer layer performed using the same technique showed that even as all the sub-layers (as marked in Fig.1) contain nano-crystals, though their density was evidently different in-between them. The strongest lowering of nano-crystals density is observed between layer 2and 3 as shown in Fig.5. The lattice spacing measured from this images of 0,22 nm and 0,145 nm are corresponding to d011(Cr) = 0,24 nm and d002(Cr)=0,144 nm within the measurement error. The larger spaced lattice fringes crossed at 60o, i.e. like the [110]Cr.

5 nm

Fig.5. HREM image of nano-crystals at the interface between Cr buffer sub-layers 2 and 3

Conclusions

The application of standard, analytical and high resolution (TEM/AEM and HREM) techniques allowed to characterize both microstructure and local chemical composition of Cr/CrN/Cr/Cr(N,C) coating. The following conclusions were drown: 1. The coating consisted of four main layers of 225/341/75/416 nm, of which the first one

is divided to three sub-layers. The second and fourth layer was built from columnar grains, while all others showed amorphous like contrast at standard imaging conditions.

2. The local chemical analysis using EDS attachment not only confirmed high content of nitrogen and carbon in TiN and Cr(N,C) respectively, but also pointed toward a raised level of oxygen in all layers. Additionally, these measurements indicated an existence of strong Cr gradient in buffer layer.

3. The atomic resolution observations (HREM) helped both to precise the diameter of columnar grains at below 15 nm and asses their defect density. The same type of observations performed on Cr buffer and intermediate layers proved that they contain Cr nano-crystallites. Additionally, the density of nano-crystallites was found changing in between buffer sub-layers.

5 nm

Cr buffer sub-layer 2 Cr buffer sub-layer 3

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Engineering, Chapman and Hall, N.Y. 1991 2. S.J. Bull, D.S. Rickerby, Compositional, Microstructural and morphological effects on

mechanical and tribological properties of CrN films, Surface Coatings Technology, 43/44(1990)732

3. B. Major, Ł. Major, J.Morgiel, J.M. Lackner, W. Waldhauser, W. Mróz, T. Wierzchoń, Nanocrystalline nitride-based coatings produced by pulsed laser deposition, Proc. XVII Int. Conf. on “Advanced Materials and technology”, AMT 2004, Łódź, published in Inżynieria Materiałowa, 25(2004)623 (in English)

4. T. Ishitani, H. Tsuboi, T. Yaguchi, H. Koike, Transmission Electron Microscope Sample Preparation Using Focused Ion Beam, J. Electron Microscopy, 43(1994)322

Acknowledgements Partial support from European Union in the frame of Eureka project E!2841 is appreciated