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Syntheses of nano-multilayered TiN/TiSiN and CrN/CrSiN hard coatings.

Abstract Nano-structured superhard coatings represent the state-of-the-art in the rapidly increasing worldwide market for protective coatings. In this study, the combination of nano-composite and nano-multilayered structures into the same coating was attempted. Nano-multilayered coatings of TiN/TiSiN and CrN/CrSiN were deposited on tool steel substrates by closed-magnetic-field unbalanced DC magnetron sputter ion plating. The coating structures were characterized using X-ray diffraction, atomic force microscopy, and scanning electron microscopy. Mechanical characterizations were performed including nanohardness measurement, progressively-increasing-load scratch test, and wear test. TiN/TiSiN coatings have a nano-hardness of 40.2 GPa, whereas CrN/CrSiN coatings have a hardness of 30.9 GPa. TiN/TiSiN coatings also showed a higher critical failure force and scratch fracture toughness as well as belter wear resistance and lower acoustic emission signal, indicating less total damage to the coatings.

Keywords Nano-multilayer, Nano-composite, Nitride, Hard coating, Mechanical property

Introduction

Superhard and anti-abrasive coatings with high hardness above 40 GPa are in progressively increasing market demand in various industrial sectors. Much work has been dedicated to the hardness enhancement for the protective coatings, and to improving their chemical and thermal stability under severe environments. (1-5) By selecting the right materials with highest bonding energy such as Ti-N, B-N, Si-C, and C-C, etc., the coatings can be very hard. However, this approach gives only limited space for further increasing the coating hardness and stability under aggressive working conditions. Therefore, much attention has been directed toward the optimization of the material structure on different length scales.

Two theories in the structural design on the nanometer scale were put forward for preparation of superhard coatings. One is the superlattice or nano-multilayered structure suggested by Koehler in 1970. (1) Coatings with a periodic nano-multilayered structure of A/B/A/B could have dramatically increased hardness, relative to the individual components of A and B. The mechanism is that the dislocation propagation is restricted by the interfaces if several pre-conditions can be fulfilled. These conditions include the thickness of the constituent layers being on the nanometer scale, a big gap between the shear modulus of the two compositions, and weak intermixing at the interfaces. Another one is the nano-composite structure proposed by Veprek and co-authors (2-5) in 1995 based on nitride materials, i.e., nc-MeN/[alpha]-[Si.sub.3][N.sub.4], where nc indicates a nano-crystal phase, MeN means metal nitride, and the [alpha] phase denotes an amorphous phase. They prepared superhard nc-TiN/[alpha]-[Si.sub.3][N.sub.4] coatings with hardness above 49 GPa using plasma-enhanced chemical vapor deposition. (4) The generic aspect of their theory is that the network of the amorphous phase separates the nano-crystallites; thereby high structural flexibility can be assured to accommodate coherent strain. Consequently, the dislocation propagation is remarkably impeded in the coatings. In order to produce these nano-composite coatings, the two components should be immiscible, preferably even at elevated temperatures.

Nitrides have been most commonly used as the compositions for hard protective coatings owing to their valuable mechanical and thermal stabilities. Super hardness above 40 GPa has been obtained in nano-multilayered coatings of TiN/NbN, TiN/VN, TiN/CrN, TiN/[CN.sub.x], and ZrN/[CN.sub.x]. (6-10) Nano-composite coatings of nc-TiN/[alpha]-[Si.sub.3][N.sub.4], nc-TiA1N/[alpha]-[Si.sub.3][N.sub.4], and nc-TiN/[alpha]-BN/[alpha]-[TiB.sub.3] have also shown a superhard characteristic. (3), (4), (11-13) While both nano-structures are efficient in the hardness enhancement, their combination may produce new properties.

In this study, the combination of these two nanostructures into the same coating was attempted. The model material composition of nc-MeN/[alpha]-[Si.sub.3][N.sub.4] for the nano-composite structure was used as a constituent layer in nano-multilayered TiN/TiSiN and CrN/CrSiN coatings.

Experimental

Sample preparation

The nano-multilayered TiN/TiSiN and CrN/CrSiN coatings were deposited using a Teer UDP450/4 unbalanced DC magnetron sputter ion plating system (14) with two Ti metal targets and two Si targets, and two Cr and two Si targets, respectively. The targets were powered by a DC voltage, while the substrate was biased with 60 V of pulsed DC. With the closedmagnetic field lines linking the magnetrons, the plasma is enhanced and has a homogeneous distribution in the chamber. Flat samples of 30 mm x 30 mm x 3.0 mm M42 tool steel (hardness ~8 GPa as measured) were fixed on a threefold rotation fixture with a target distance of approximately 160 mm. The thickness of the layers on the scale of nanometer can be adjusted easily by controlling the rotation speed. The total pressure was 3.5 x [10.sup.3] Torr and the substrate temperature was 350[degrees]C. Nitrogen flow rate was kept at about 35 sccm. The deposition procedure required 20 min of glow discharge ion cleaning, followed by the deposition of a Ti or Cr adhesion layer of about 0.2 [mu]m thick. The reactive deposition of TiN or CrN with a thickness of about 0.3 [mu]m followed. During subsequent coating growth, the deposition rate of other elements could be gradually increased. Finally, a nano-multilayered coating was deposited while each individual layer had a thickness of about 6 nm. The coatings have a columnar structure, which is similar with previous studies. (14), (15) The relative concentration of the elements in the coating was controlled by the sputtering power applied on each target. Atomic ratios of Ti:Si:N and Cr:Si:N were 45:13:42 and 50:13:37, respectively.

Characterization techniques

A Digital Instruments Dimension 3100 atomic force microscopy (AFM) in tapping mode was used to characterize the surface topography of the coatings. Scanning electron microscopy (SEM XL30 ESEM) was used for the thickness measurement of the coatings and the microstructural observation in the secondary electron and back-scattered electron modes. High-angle [theta]-2[theta] X-ray diffraction (XRD) spectra of the samples were measured on a Siemens Kristalloflex 810 with fixed slits and a step size of 0.02[degrees] for the incident angle ([theta]) using Cu [K.sub.[alpha]].

Mechanical tests were carried out using a CETR Universal Micro-Tribometer (mod. UMT-2). The nano-indentation tests were conducted for nano-hardness and reduced elastic modulus measurements. The maximum loads were selected through optimization by hardness measurements under a range of applied loads. (16)

Linear scratch tests with progressively increasing load were conducted for evaluation of the coating adhesion and scratch toughness properties. The tests were carried out on both specimens using diamond Rockwell indenter with the tip radius of 200 [mu]m. The scratch length was 10 mm while the dragging speed was 1 mm/s. The samples were tested in the load range from 1 N to 60 N. The test was repeated several times on each sample to verify the data consistency and repeatability. The indenter in a holder was mounted on the force sensor with a proper suspension. The tested sample was mounted on the table of the lower drive. An acoustic emission (AE) sensor was attached to the indenter holder to monitor the high-frequency signal generated during scratching, and to indicate the intensity of the material fracture. The initial normal force of 1 N was applied to the indenter and stabilized for few seconds. The scratch on the sample was produced by dragging the indenter along the sample surface by the horizontal linear X-slider. Simultaneously with the beginning of the indenter dragging, the normal load was increased linearly according to the preprogrammed test script, corresponding to the indenter motion. During the test, the normal load ([F.sub.z]), the friction force ([F.sub.x]), and the AE signal were continuously monitored and saved in the data files.

The linear reciprocating wear tests were used for the evaluation of the coatings friction and wear durability properties using 1.6 mm diameter sapphire balls at a constant normal load of 2 N over a distance of 5 mm on the specimens, with 3 Hz frequency of oscillation for 1 h (ASTM G133). The tested sample was mounted on the table of the lower drive and was reciprocating in contact with the stationary ball. The ball in the ball holder was mounted on the force sensor using a proper suspension. The sensor was attached to the carriage, which is a part of electro-mechanical loading mechanism. The X-slider connected to the carriage can be used for horizontal positioning of the ball on the sample mounted on the lower drive. The AE sensor was attached to the bail holder. The friction force ([F.sub.x]) and down force ([F.sub.z]) were simultaneously recorded during the tests. The width and depth of the wear tracks were measured after the test. The specific wear rate (WR) was calculated by using the following equation:

WR = 5 * A/[F.sub.z] * 3 * 2 * 0.005 * 3600 [mm.sup.3] * [N.sup.-1] * [m.sup.-1]

where A is the cross-sectional area of the wear scar in [mm.sup.2]. Wear depth and width were measured at five different locations on the wear scar and the average values were reported. From the measured values of wear depth and width, mean cross-sectional area was calculated, and finally specific WR was calculated.

Results and discussion

Structural characterizations

Figures 1a and 1b show the AFM images of the surface morphology of the TiN/TiSiN and CrN/CrSiN coatings, respectively. The surface roughness values are 54.3 and 46.8 nm for the TiN/TiSiN and CrN/CrSiN coatings, respectively. The typical size of the columnar structure of both coatings is about 300 nm.

[FIGURE 1 OMITTED]

Figure 2 shows the [theta]-2[theta] XRD patterns of the TiN/TiSiN and CrN/CrSiN coatings with the Bragg peaks being labeled with the corresponding phases and lattice planes. It can be seen that the Bragg peaks of TiN and CrN are very broad. According to JCPDF files of 038-1420 for TiN and 76-2494 for CrN, both coatings are under out-of-plane tension. Thus, it can be inferred that there is an in-plane compressive residual stress. Residual stress in magnetron sputter-deposited coating is often observed because of the highly energetic deposition species during the deposition. (17) It is suggested that the residual stress together with the small grain size should be responsible for the broadening of the Bragg peaks.

[FIGURE 2 OMITTED]

Figure 3a gives the SEM cross-sectional image of the TiN/TiSiN coating in the secondary electron mode. The coating has a total thickness of 4 [mu]m. The coating has a very homogeneous thickness and is free of structural defects like voids and cracks. It can be seen that the coating adheres to the substrate very well. Figure 3b illustrates the nano-multilayered structure of the coating and adhesion layers. Actually, a Ti and a TiN layer were deposited as the adhesion layers. However, the adhesion layers cannot be seen due to the weak contrast between the constituent layers even in the back-scattered electron mode.

[FIGURE 3 OMITTED]

Figure 4a gives the SEM secondary electron image of the cross section of the CrN/CrSiN coating. The total thickness of the coating is 3.5 [mu]m. The coating has a very homogeneous thickness in the field of view. Figure 4b shows the adhesion layers of Cr and CrN between the nano-structured coating and the steel substrate. Thus, the structure of the coating is substrate/Cr/CrN/nano-multilayer. The coating has high-quality bonding to the substrate, and no defects can be found at the adhesion layers.

[FIGURE 4 OMITTED]

Mechanical characterizations

Nanohardness (H) and reduced elastic modulus ([E.sub.r]) of the TiN/TiSiN and CrN/CrSiN coatings are 40.2 and 279.9 GPa, and 30.9 and 258.6 GPa, respectively. The TiN/TiSiN showed higher values of H and [E.sub.r]. The typical loading-unloading curve is shown in Fig. 5.

[FIGURE 5 OMITTED]

Figure 6 shows an example of the progressively increasing load scratch test, illustrating the definition of the critical load for CrN/CrSiN coating. In the beginning of the scratch, at low normal load, the indenter tip was mostly sliding on the sample surface without causing significant coating damage. In the course of the test, when the normal load reached a certain critical level, the coating started fracturing and/or delaminating from the substrate, since at higher normal load and with the indenter having large tip radius, the depth of the mechanical stress distribution in the sample can exceed the coating thickness. This process was reflected in a sharp increase in both friction coefficient and AE signal. The value of the critical load depends on the properties of the tested coating, and can be used for the coating scratch toughness and adhesion evaluation.

[FIGURE 6 OMITTED]

Table 1 lists the mean value of the coefficient of friction (COF) and AE signal before and after the coating failure, and the critical load values for both samples. The CrN/CrSiN coating exhibited lower critical force and higher AE signal level after the coating failure than did the TiN/TiSiN coating. It could indicate that the CrN/CrSiN coating has lower scratch fracture toughness at higher load or lower adhesion to the substrate. At the same time, the friction coefficient values were almost identical for both samples.
Table 1: Macro-scratch tests with Rockwell Indenter, tip radius
200 [mu]m, distance 10 mm, velocity 1 mm/s, and load from 1 N to 60 N

Coatings   Mean     COF      Mean     AE       Critical load (N)
           Before   After    Before   After
           failure  failure  failure  failure

TiN/TiSiN     0.08     0.15     0.05     0.40               50.7

CrN/CrSiN     0.08     0.14     0.04     0.75               29.9


Table 2 lists the mean values of the COF and AE signal after the break-in period, and the WR for both samples. In this test, CrN/CrSiN had lower friction coefficient, whereas TiN/TiSiN demonstrated better wear resistance and lower AE signal, indicating a smaller degree of interaction between the contacting parts, and less total damage to the sample surface.
Table 2: Friction and wear tests with 1.6 mm sapphire ball, 2 N load,
3 Hz frequency, distance 5 mm, and 1 h duration

Coatings   Mean COF  Mean AE  WR ([mm.sup.3] [N.sup.-1] [m.sup.-1])

TiN/TiSiN      0.41      4.0                                   0.93
CrN/CrSiN      0.32      5.1                                   1.18


Conclusions

Nano-multilayered TiN/TiSiN and CrN/CrSiN hard coatings with dense structures were deposited on tool steel substrates by a magnetron sputtering method. The TiN/TiSiN and CrN/CrSiN coatings have a nano-hardness of 40.2 and 30.9 GPa, respectively. It is found that both coatings are under in-plane compressive residual stress. Comprehensive mechanical tests revealed that the TiN/TiSiN coatings have higher indentation hardness, higher critical load, and longer wear durability than the CrN/CrSiN coatings.

Acknowledgment The authors acknowledge the support of European Union Project RESTOOL.

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J.-L. Cao (*)

Institute for Advanced Materials and Technology, University of Science and Technology Beijing. Beijing 100083, China

e-mail: jlcao@mater.ustb.edu.cn

J.-L. Cao, K.-L. Choy

School of Mechanical, Materials, Manufacturing Engineering and Management, University of Nottingham, Nottingham NG7 2RD, UK

K.-L. Choy

e-mail: Kwang-Leong.Choy@nottingham.ac.uk

H.-L. Sun, H.-Q. Li, D. Teer

Teer Coatings Ltd., Droitwich,

Worcestershire WR9 9AS, UK

M.-D. Bao

Institute of Materials Engineering, Ningbo University of Technology, Ningbo 315016, China

DOI 10.1007/s11998-010-9275-0

[C] ACA and OCCA 2010
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Author:Cao, Jiang-Li; Choy, Kwang-Leong; Sun, Hai-Lin; Li, Hui-Qing; Teer, Dennis; Bao, Ming-Dong
Publication:JCT Research
Article Type:Report
Date:Mar 1, 2011
Words:3059
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