Al2O3 multilayers

Accepted Manuscript The influence of varied modulation ratios on crystallization and mechanical properties of nanoscale TiB2/Al2O3 multilayers

X.D. He, L. Dong, J. Wu, D.J. Li PII: DOI: Reference:

S0257-8972(18)30715-1 doi:10.1016/j.surfcoat.2018.07.017 SCT 23581

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

9 March 2018 29 June 2018 4 July 2018

Please cite this article as: X.D. He, L. Dong, J. Wu, D.J. Li , The influence of varied modulation ratios on crystallization and mechanical properties of nanoscale TiB2/Al2O3 multilayers. Sct (2018), doi:10.1016/j.surfcoat.2018.07.017

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ACCEPTED MANUSCRIPT The influence of varied modulation ratios on crystallization and mechanical properties of nanoscale TiB2/Al2O3 multilayers X.D. He 1,2, L. Dong 1,2,*, J. Wu 1,2, D.J. Li 1,2, *

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College of Physics and Materials Science, Tianjin Normal University, Tianjin 300387, China 2 Tianjin International Joint Research Centre of Surface Technology for Energy Storage Materials, Tianjin 300387, China

* Corresponding authors: Tel.: +86-22-23766519; Fax: +86-22-23766519. 393#, Mingli Building, Bin Shui Xi Road, Xiqing District, Tianjin, China E-mail address: [email protected] (Lei Dong) [email protected] (Dejun Li)

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ACCEPTED MANUSCRIPT Abstract TiB2/Al2O3 multilayers with constant modulation periods and various modulation ratios ranging from 1:1 to 11:1 were synthesized on silicon wafer substrates by magnetron sputtering system. The amorphous structures of the TiB2/Al2O3 multilayers

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were characterized. The effect of modulation ratio on the microstructure and

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mechanical properties of the multilayers was investigated. The results revealed that

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crystallization of amorphous sublayer, performance of hardness, elastic modulus and residual stress of the multilayers largely depended on variations of modulation ratios.

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The hardness and elastic modulus of TiB2/Al2O3 multilayers were up to the maximum

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value (30.6 GPa and 460.1 GPa) at modulation ratio of 7:1. Moreover, the practical adhesion had been also remarkably improved. Further analysis suggested that

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crystallization of amorphous sublayer (individual layer) was the primary cause for

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mechanical performance improvement of the multilayers. This work proved that the modulation ratio was a key role in controlling the growth orientation of crystals of

TiB2/Al2O3

multilayers,

Hardness,

Magnetron

sputtering,

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Keywords:

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nanoscale TiB2/Al2O3 multilayers at room temperature.

Modulation ratio

1. Introduction

Hard coatings have been widely used to enhance the performance and lifetime of cutting tools in industrial applications. In particular, the coatings are usually subjected to high temperature up to 1000 ℃ during cutting, thus it is extremely important to improve the thermal stability of the coatings. For more than a decade, multilayer

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ACCEPTED MANUSCRIPT coatings made of two different kinds of materials stacks have been developed due to their extraordinary mechanical and chemical properties as compared to the individual layer coatings [1-4]. Lately, new developed deposition techniques and treatments have made it easier to produce coatings that perform well at high temperatures by synthesizing superlattice structure. But the superlattice structure of multilayers can

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only occur when the constituents are isostructural with similar lattice spacing,

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chemical bonding and atomic radius. Thus, searching for the suitable materials for

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preparation of these multilayers is becoming increasingly imperative [5-7]. TiB2 ceramic hard coating is potential candidate because it has widespread

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application in cutting tools or machine parts due to its high hardness, wear resistance

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and chemical inertness[3, 5, 8, 9]. There have been increasing interests in fabrication of this material in thin films or coatings for many applications, particularly in the

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material processing tools which needs to reduce wear and corrosion [10-12]. In

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addition to high hardness, low friction and good adhesion, tools working under extreme conditions must possess high-temperature oxidation resistance. Nevertheless,

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individual TiB2 coatings are also insufficient to survive such a high temperature due

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to the intense oxidation. A promising way to solve the problem is the introduction of oxide, because most of the oxides, such as Al2O3 and SiO2, have good oxidation resistance and excellent high-temperature stability. As an in-corporation material, Al2O3 was chosen for the present investigation. Aluminum oxide possesses high thermal stability, which makes it an ideal candidate as a component to be incorporated into the multilayer system. TiO2/Al2O3, TiN/Al2O3, NiAl/Al2O3, FeAlN/Al2O3, SiO2/Al2O3 and TiN/Al2O3 nanomultilayers with better mechanical properties have

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ACCEPTED MANUSCRIPT been successfully synthesized [13-17]. Unfortunately, the nitrides without the doping of oxides will have so poor strength that withstand the mechanical abuse of cutting. In order to produce the superhard multilayer, the effect of the precursor layer is generally concerned. The thickness of the introduced layer is about 1 nm in which the first

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principles are in agreement with the explanation of the growth and the superhard

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mechanism [13, 14, 18]. Moreover, it is difficult to prepare TiB2/Al2O3 multilayers

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with crystal orientation due to the formation of amorphous with the increasing thickness of individual layers, and it is generally affected by the growth period and

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specific modulation ratio of the multilayer.

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The various modulation ratios have significant effects on microstructure and properties of multilayers coatings. The previous similar work[18] of Pan et al. showed an optimal modulation ratio (tVN: tTiB2 = 1:7) of VN/TiB2 exhibits growth mechanism

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of multilayers. The modulation ratio is also an important factor in other system. The

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lower modulation ratio of TiAlN/CrN multilayers structure exhibited improved oxidation resistance[36]. TiAlN/ZrN multilayers with a modulation ratio (t

TiAlN:t ZrN)

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of 3:2 and a certain modulation period of 6.5 nm showed the most excellent hardness[37].The present work aims to obtain further improved performance of

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TiB2/Al2O3 multilayers by optimizing the modulation ratio. The multilayers had constant modulation periods, thickness of two different sublayers, avoiding error of hardness measurement caused by the different thickness of multilayers. Besides, the various modulation ratios were synthesized on alumina substrates by RF magnetron sputtering system and the effect on layer-structure, orientation, mechanical and wear properties of multilayers were studied. 2．Experimental details TiB2/Al2O3 multilayers with constant modulation periods at 11 nm and various

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ACCEPTED MANUSCRIPT modulation ratios ranging from 1:1 to 11:1 were synthesized on silicon wafer at a fixed substrate bias of -60V by magnetron sputtering (FJL560C12, Chinese Academy of Science). The TiB2 and Al2O3 (both 99.9% purity) target were respectively connected to RF source sputter guns, which were installed with the horizontal plane. The substrates were paralleled to the targets surface with a vertical distance of 70 mm

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to the target. The base pressure of experiment was lower than 3×10-4 Pa. Substrates

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were cleaned in an ultrasonic agitator in acetone and absolute alcohol at least 15 min

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and dried using compressed air after each cleaning cycle. Subsequently, the substrates were sputter-cleaned at -400 V for 15 min in chamber prior to Al2O3 layer deposition.

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The argon gas flows were controlled independently using mass-flow controllers. Deposition was performed at work pressures of 0.4 Pa and room temperature. In the

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process of deposition, the TiB2 and Al2O3 targets were all in RF mode with power of 100W. To obtain various modulation ratios, we controlled the alternate time of

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substrates exposed to Al2O3 and TiB2 source by computer-driven shutter. All

around 450~500 nm.

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multilayers were deposited 40 periods. Total thicknesses of the multilayers were

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A D/MAX 2500 diffractometer operated with Cu Karadiation at 1.54056Å was used for layered structure analysis in the range of 1° to 10°. The wide angle X-ray

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diffraction (XRD) was used for crystalline analysis in the range of 20° to 70°, in where the step size and dwell time of theta-2theta are 0.02ºand 7.76 s, respectively. The hardness and elastic modulus and of the multilayers were measured using Nanoindenter XP system with a continuous stiffness measurement (CSM) technique. The diamond tip of Nanoindenter XP system is BERKOVICH TB from Micro Star Technologies. The indenters tip radius is less than 5 nm. In this measurement, the Poisson ratio was 0.25, and the maximum indentation depth was kept at 10% of the coating thickness (~500 nm) to minimize substrate effects. This Nanoindenter XP system was also used to perform nanoscratch test. In this test, the maximum load was 5

ACCEPTED MANUSCRIPT up to 80 mN in order to measure the fracture resistances by observation of scratch width, residual depth and pile up height after scratching. The friction coefficient was obtained by MFT-4000 multi-functional tester in ambient air at room temperature. The sliding Si3N4 ball, 2 mm in diameter, held in a pen-like ball holder was moved a circle with a radius of 6 mm on the surface of the coatings. In this test, the load was 2 N, the

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frictional rotate speed was 150 r /min, and the test time was 1 min. The residual stress (σ) of the multilayers was calculated by the Stoney formula[35]：𝜎 = 6𝑡

𝐸𝑠 t2𝑠 𝑐

(1−𝜈𝑠 )𝑅

, in

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which Es, ts and vs are elastic modulus (131 GPa), thickness(0.0005 m) and Poisson's

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ratio(0.28) of the substrate respectively; tc is the multilayer thickness; R is the radius of the curvature of the multilayer coated on substrate, which was determined by the

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multilayer curvature using a surface profilometer (Ambios XP-2, America). The high resolution transmission electron microscopy (HRTEM, FEI, Tecnai G2 F20) was used

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to the characterization tests of modulation structure. In addition, the multilayers cross-sectional structure was also observed with the scanning electron microscopy

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3. Results and discussion

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(SEM; TDCLS-8010, Hitachi Japan).

Clear X-ray reflectivity (XRR) curves are clearly observed for TiB2/Al2O3

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multilayers with various modulation ratios (tTiB2:tAl2O3), indicating that these samples have chemical modulation structure and almost identical period structure (Λ), as

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shown in Fig.1. The XRR patterns also give information on the Λ of the multilayers. Measurements were taken in a θ-2θ geometry from θ = 0.1° to θ =8.0° at 40 kV/30 mA tube power to maintain linearity in the detector response. The thickness of the Λ can be determined using the Bragg relationship (nλ=2d sin θ) for peaks occurring at θ >∼2°. For smaller incident angles, the thicknesses must be determined using a form of Bragg’s law appropriately modified to account for refractive effects which become significant for θ < ∼2°. Thus, the Λ is calculated by the modified Bragg’s equation[17], 6

ACCEPTED MANUSCRIPT s i n2 n （

n 2 ）  2 2Λ

(1)

Where n is the ordinal number of reflection peak, λ=1.54056 Å is the X-ray wavelength, and δ is the correct value which is related to the average reflective index. According to the equation, the Λ of the multilayers of two different modulation ratios

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are calculated as 10.8 nm and 11.1nm, which are nearly consistent with the established modulation period (11nm). Nevertheless, the most pronounced difference

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between the curves is the variation in the number of observable Bragg reflections. The

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attenuation of the Bragg-like peaks in the reflectivity curve provides a measure of

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interfacial quality encompassing both roughness and inter-diffusion effects at the interface. The multilayer has the sharpest interfaces at tTiB2:tAl2O3=1:1 in Fig.1. Among

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samples with different tTiB2:tAl2O3, however, multilayers with thinner Al2O3 or thicker TiB2 constituent layers exhibit fewer orders of reflection, indicating the formation of

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the rough interfaces in the multilayers. The poorer interface definition can be

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attributed to the interfacial diffusion occuring between the two layers[19-21]. In order to prove the consistency of the periodic structure between the observed

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and the calculated values, cross-sectional SEM image of a TiB2/Al2O3 multilayer with tTiB2:tAl2O3 of 7:1 is given in Fig. 2. The dark and light layers correspond to the TiB2

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and Al2O3 layers, respectively. The interfaces between TiB2 and Al2O3 are fairly clear and straight, indicating that the coating has formed distinct modulated structure. This observed thickness of about 11.5 nm is very close to the calculated value from XRR result and our design value. Fig. 2 shows that the multilayers possess a well-defined layered structure with flat interfaces parallel to the surface. The highly magnified cross-sectional HRTEM images (Fig. 3) demonstrate the structure characteristic comparison of (TiB2/Al2O3) multilayers at the modulation ratio of 5:1 (Fig. 3 a) and 7:1 (Fig. 3 b). The both samples exhibit a series of 7

ACCEPTED MANUSCRIPT multilayer films with good periodic structure composed of two sections which are the dark layers (TiB2) and light layers (Al2O3). The blurred interface of TiB2/Al2O3 multilayers also validates the inference about interfacial diffusion form X-ray reflectivity (XRR) curves above mentioned in Fig. 1. Similarly, the realistic modulation periods are 10.9 nm and 10.7 nm respectively, which is nearly consistent

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with the established modulation period (11 nm) and the previous calculated values.

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The diffraction rings are ambiguous in Fig.3 a. While Fig.3 b shows the obviously

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diffraction rings belong to TiB2 (100), (101) and (002). It can be concluded that (TiB2/Al2O3) multilayers at the modulation ratio of 7:1 display the more tendency of

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crystallization.

The Figure 4 displays XRD patterns of TiB2/Al2O3 multilayers with different

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tTiB2:tAl2O3 and Al2O3 and TiB2 monolayers, respectively. The Al2O3 and TiB2 monolayers both present amorphous structure. Compared with the monolayers, the

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multilayers at tTiB2:tAl2O3= 1:1 presents the textures of (001) at 27.59ºand (002) at

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56.99º. Only when it comes to 7:1, strong TiB2 (001) as well as weak TiB2 (002), (100), and (101) textures are observed. Then the TiB2 and Al2O3 layers transform to

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an amorphous structure with the continually increasing of modulation ratio. It reveals

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that cubic Al2O3 and nanocrystalline hexagonal TiB2 orientation growth are formed during deposition. This information is consistent with the result presented in Figure 3 b. The coherent multilayer growth model reported in Pan’s work[18] can also be applied in to this system. Based on the theory of lowest energy structures of first principles, the excessive increase of TiB2 monolayers will cause preferred orient growth and undermine the coherent growth[38]. The interface energy can be dominated by the growth condition of smaller thickness in the existing first principles calculation[39,40,18]. TiB2 and Al2O3 monolayers were alternately deposited with a 8

ACCEPTED MANUSCRIPT constant period of 11nm in this work. Al2O3 monolayer will undermine the randomly orientation growth of TiB2 monolayer. A suitable modulation ratio will limit the space making the TiB2 layers crystallize and grow epitaxial with the Al2O3 layers. Based on the theory of the lowest energy structures of first principles, TiB2 layers crystallize and grow epitaxial with the Al2O3 layers at a suitable modulation ratio

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(tTiB2:tAl2O3=7:1).

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Fig. 5 shows the variations of hardness and elastic modulus as different tTiB2:tAl2O3 in the modulation period of 11 nm, together with the hardness and elastic modulus of

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amorphous TiB2 and Al2O3 individual layers. With increasing modulation ratios, the hardness of the multilayers increases first before it decreases again. When tTiB2:tAl2O3

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of 7:1, the maximum hardness of 30.6 GPa is obtained. The change trend of elastic modulus is similar to one of hardness. Actual hardness and elastic modulus of our

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multilayers at tTiB2:tAl2O3=7:1 are the higher 3.45 GPa and 104.35 GPa than calculated values obtained from rule-of-mixtures hardness of monolithic TiB2 and Al2O3

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individual layers.

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The two modulation layers have formed coherent interfaces according to the XRD patterns mentioned above. The Hall-Petch strengthening, stress effects in the

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interfaces and dislocation slip impeded by the interface are always used to explain the hardness improvement of the multilayers [22, 23]. The interfaces act as barriers to the

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motion of dislocations in multilayer when explaining the mechanisms of promoted hardness and modulus of multilayers [24, 25]. In order to move the dislocation across the interface, a significant additional stress has to be required, and therefore anomalous hardness enhancement can be found. That is why hardness and modulus of multilayers are superior to the expected result from the rule of mixtures [5, 26, 27]. Residual compressive stresses in films are also associated with the increases in the hardness of both monolithic and multilayer films. The residual compressive stress of the multilayer coatings determined by Stoney’s equation [28, 29] and influenced by 9

ACCEPTED MANUSCRIPT modulation ratios are shown in Fig. 6. It is well known from the work of Holmberg, Khan and others that large residual stresses can be generated during the coating growth process [30,31]. High residual stress (σ) is the main reason for coating delamination and plastic deformation. Therefore, the reduced residual stress in coatings is a key factor for these coatings to explore more applications [32]. The

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compressive stresses generally share the same trend with the hardness and elastic

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modulus. The compressive stresses increase with the increase of modulation ratios

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(tTiB2:tAl2O3). When the tTiB2:tAl2O3=7:1, the compressive stress reach to the highest value (-2.7 GPa) too. Fig.6 also indicates that all multilayers have lower residual

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stress than the rule-of-mixture value of monolithic coatings. Such residual stresses can greatly increase the measured hardness, although the origin of the effect in films is

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still obscure. As for this experimental work, an appropriate explanation is that periodically introduction of Al2O3 layers into TiB2 layers can help to relax the stress

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built in TiB2 layers.

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Fig.7 shows the results of scratch test reflecting the fracture resistance of TiB2/Al2O3 multilayers with different modulation ratios (tTiB2:tAl2O3). The scratch scan

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profiles of all multilayers indicate an abrupt increase point in scratch depth. The normal load corresponding to an abrupt increase point in the scratch scan profile is the

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critical fracture load of coating (Lc). The Lc can characterize the adhesion strength of the coating or the coating’s fracture resistance. The coating’s fracture resistance may be also interrelated with inherent residual stress, hardness and plastic recovery of the coatings with multilayered structures. The Lc of monolithic TiB2 and Al2O3 coating are 40mN and 16mN, respectively. TiB2/Al2O3 multilayers at tTiB2:tAl2O3 of 1:1 exhibits smooth scratch-scan and post-scan surface profiles. Its Lc valve approaches to 26 mN. With modulation ratios increasing to 7:1, the Lc value is up to 48 mN,

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ACCEPTED MANUSCRIPT displaying the higher indentation fracture resistance. The friction coefficients of monolithic and multilayered coatings with various tTiB2:tAl2O3 are displayed in Fig. 8. It embodies that the friction coefficient of TiB2/Al2O3 multilayer films at tTiB2:tAl2O3 of 7:1 is the lowest among the multilayers. Friction coefficient is qualitatively correlated with the changes in surface chemistry at

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the sliding interface. The wear behavior in this tribological situation seems to be

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influenced by the grain orientation of multilayer multilayers. Meanwhile, the surface

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with smaller nanocrystalline grains gives a contribution to the decrease of friction coefficient [33, 34].

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4. Conclusions

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A series of TiB2/Al2O3 multilayers and monolayer TiB2 and Al2O3 coatings are synthesized by a magnetron sputtering system. The influence of modulation ratios on

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structure and mechanical properties of TiB2/Al2O3 multilayers is investigated in order

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to deepen the understanding of multilayer growth. TiB2/Al2O3 multilayers are crystallized with orientations in the c-Al2O3 (222) and h-TiB2 (100). The Al2O3

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monolayer with thicker thickness would block the grain growth of TiB2 to form nanocrystals in the multilayers. The mechanical properties of TiB2/Al2O3 multilayers

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are directly bound up with the Al2O3 monolayer thickness. The maximum hardness and elastic modulus about 30.6 GPa and 460.1 GPa of the multilayers appears on the modulation ratio of 7:1 and modulation period of 11 nm. The TiB2/Al2O3 multilayers with tTiB2:tAl2O3=7:1 has displayed better practical adhesion strength and fracture resistance. The results indicate that the coherent growth in TiB2/Al2O3 multilayers is the primary causes for crystallization of amorphous sublayer by changing modulation ratios. Therefore, the TiB2/Al2O3 multilayers could be an effective and suitable candidate for the hard coating system such as coated cutting tools. 11

ACCEPTED MANUSCRIPT Acknowledgements This work was supported by National Natural Science Foundation of China (51772209 and 51472180).

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[35] G.C.A.M. Janssen, M.M. Abdalla, F. van Keulen, B.R. Pujada, B. van Venrooy, Celebrating the 100th anniversary of the Stoney equation for film stress:

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developments from polycrystalline steel strips to single crystal silicon wafers, Thin

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Solid Films 517 (2009) 1858–1867.

[36] Y.X. Xu, L. Chen, F. Pei, Y. Du, Structure and thermal properties of TiAlN/CrN

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multilayered coatings with various modulation ratios, Surf. Coat. Tech. 304 (2016)

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[37] D.J. Li, M. Cao, X.Y. Deng, X. Sun, W.H. Chang, W.M. Lau, Multilayered coatings with alternate ZrN and TiAlN superlattices, Appl. Phys. Lett. 91 (2007) 715. [38] K. Lu, L. Lu, S. Suresh, Strengthening materials by engineering coherent internal boundaries at the nanoscale, Science, 324 (2009) 349-352. [39] N. Wang, L. Dong, C.K. Gao, D.J. Li, A study of structure, energy and electronic properties of TiB2 /c-BN interface by first principles calculations, Opt. Mater. 36 (2014) 1459-1462. [40] M. Wen, C.Q. Hu, C. Wang, T. An, Y.D. Su, Q.N. Meng, W.T. Zheng, Effects of 16

ACCEPTED MANUSCRIPT substrate bias on the preferred orientation, phase transition and mechanical properties for NbN films grown by direct current reactive magnetron sputtering, J. Appl. Phys.

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ACCEPTED MANUSCRIPT Figures Captions Fig. 1 XRR patterns of TiB2/Al2O3 multilayer at various modulation ratios of two typical samples. Fig. 2 Cross-section SEM images of TiB2/Al2O3 multilayer at the modulation ratio of 7:1.

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Fig. 3 Cross-sectional HRTEM images of the TiB2/Al2O3 multilayer coatings at Ʌ = 11

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Fig. 4 XRD patterns of the Al2O3, TiB2 monolayers and TiB2/Al2O3 multilayers at different modulation ratios.

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Fig. 5 Hardness and elastic modulus of TiB2/Al2O3 multilayers at different modulation ratios.

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Fig. 6 Residual stresses of TiB2/Al2O3 multilayers at different modulation ratios. Fig.7a Surface profiles of the scratch-scan on TiB2/Al2O3 multilayers at different

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modulation ratios.

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Fig.7b The surface morphologies of the scratched samples in Figure 7 a. Fig.8 Friction coefficients of TiB2, Al2O3 and TiB2/Al2O3 multilayer coatings with

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Figure 1: XRR patterns of TiB2/Al2O3 multilayer at various modulation ratios of two

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Figure 2: Cross-section SEM images of TiB2/Al2O3 multilayer at the modulation ratio of 7:1.

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Figure 3: Cross-sectional HRTEM images of the TiB2/Al2O3 multilayer coatings at the modulation ratio of 5:1 (a) and 7:1 (b).

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Figure 4: XRD patterns of the Al2O3, TiB2 monolayers and TiB2/Al2O3 multilayers at

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different modulation ratios.

Figure 5: Hardness and elastic modulus of TiB2/Al2O3 multilayers at different

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Figure 6: Residual stresses of TiB2/Al2O3 multilayers at different modulation ratios.

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Figure 7 a: Surface profiles of the scratch-scan on TiB2/Al2O3 multilayers at different modulation ratios. 22

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Figure 7 b: the surface morphologies of the scratched samples in Figure 7 a

Figure 8: Friction coefficients of TiB2, Al2O3 and TiB2/Al2O3 multilayer coatings with different modulation ratios.

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ACCEPTED MANUSCRIPT Highlights

Nanoscale TiB2/Al2O3 multilayers are prepared by magnetron sputtering.

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Mechanical properties of multilayers can be changed by modulation ratios.

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Modulation ratio can significantly affect the growth orientation of crystals.

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Insertion of Al2O3 sublayer into TiB2 can improve the crystallinity and properties.

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