Rev.Adv.Mater.Sci.80 16(2007) 80-87N. Panich, P. Wangyao, S. Hannongbua, P. Sricharoenchai and Y.



N. Panich1, P. Wangyao1, S. Hannongbua1, P. Sricharoenchai2 and Y. Sun3

1 Metallurgy and Materials Science Research Institute, Chulalongkorn University, Phrayathai Rd., Pathumwan, Bangkok, 10330, Thailand 2Dept. of Metallurgical Engineering, Faculty of Engineering, Chulalongkorn University, Phrayathai Rd., Pathumwan, Bangkok, 10330, Thailand 3 School of Engineering & Technology, De Montfort University, Leicester LE1 9BH, UK Received: July 14, 2007

Abstract. It has been a common practice that in the deposition of a coating onto a sub- strate, a thin titanium interlayer is used to improve the between the coating and the . But the structure and strength of the interlayer play an important role in determining the

adhesion strength of the coating. This work involves the deposition of titanium diboride (TiB2) based nanostructured coatings on high speed steel substrates. A titanium interlayer was applied by sputtering a titanium target in pure argon and in argon-nitrogen gas to affect nitrogen of the interlayer. Structural and properties analysis revealed that various contents of nitrogen gas in argon-nitrogen gas during deposition by magnetron sputtering

has significant effect on the adhesion between the TiB2 coating and the high speed steel substrate. There exists an optimum degree of nitrogen incorporation in the interlayer that results in the maxi- mum enhancement in adhesion. The beneficial effect of nitrogen gas mixing is discussed in terms of the modified structure and increased hardness of the interlayer, which provides stronger sup- port for mechanical loading.

1. INTRODUCTION high quality coatings with good mechanical prop- erties and coating adhesion suitable for industrial Titanium diboride, TiB , is a ceramic compound with 2 applications. a hexagonal and possessing high The major problem is that TiB coatings are very hardness, good chemical stability, and good ther- 2 brittle and usually have poor adhesion with - mal and electrical conductivity. There have been lic substrates, which is the main barrier for the in- increasing interests in fabrication of this material dustrial application of TiB coatings. Recently, sev- in and coating forms for many potential 2 eral attempts have been made to overcome these applications, for example to combat wear, protec- problems, for example by sputter cleaning the sub- tive coatings, corrosion and tribological engineer- strate [6], applying substrate bias during deposi- ing components and particularly in material pro- tion [7,8], after deposition [3,4,9], form- cessing tools and dies [1-5]. Even though the struc- ing multilayer and composite coatings [10-12] and tures and properties of sputter-deposited TiB coat- 2 introducing nitrogen during deposition [13,14]. It has ings have been studied by many investigators, the been a common practice that in the deposition of a commercialization of TiB coatings has been im- 2 ceramic coating onto a substrate, a thin titanium peded mainly due to the difficulties in producing interlayer is used as a “glue” to improve the adhe- Corresponding author: P. Wangyao, e-mail: [email protected]

© 2007 Advanced Study Center Co. Ltd. Effect of argon-nitrogen mixing gas during magnetron sputtering on titanium interlayer... 81

Table 1. Deposition conditions.

Samples Mode Substrate cleaning Deposition details Percentage of N2

No.1 Graded Ti-TiB2 150 W 30 min Ti 200 W 30 min, TiB2 200 W 180 min 0

No.2 Graded Ti-TiB2 150 W 30 min Ti 200 W 30 min, TiB2 200 W 180 min 10

No.3 Graded Ti-TiB2 150 W 30 min Ti 200 W 30 min, TiB2 200 W 180 min 20

No.4 Graded Ti-TiB2 150 W 30 min Ti 200 W 30 min, TiB2 200 W 180 min 33

No.5 Graded Ti-TiB2 150 W 30 min Ti 200 W 30 min, TiB2 200 W 180 min 50

sion between the coating and the substrate. Genu- ered in the direct current (DC) mode. The sub- inely, the use of a thin titanium interlayer has been strates were positioned near to the edge of the found to considerably improve the adhesion of ti- working table below the TiB2 target. The substrates tanium coatings produced by physical va- were stationary and the distance between the TiB2 por deposition [15]. The role of the interlayer is target and the substrate was fixed at 60 mm. All considered to be twofold. First, the titanium the experiments were conducted at a constant interlayer acts as an getter for the substrate working pressure of 0.65 Pa, Ar gas flow rate of 20 surface. This helps to decompose the native sccm, varied nitrogen gas flow rates (see Table 1), film on the substrate surface and promote the ad- a constant substrate temperature of 400 °C and a hesion between the coating and the substrate. constant RF sputtering power (for TiB2 target) of Second, it serves as a compliance layer to accom- 200 W. A thin (about 50 nm) pure Ti interlayer was modate the stress at the coating-substrate inter- deposited first in all cases, by sputtering the Ti tar- face. Thus, the structure and strength of the get for 30 min with a DC power of 200 W. Before interlayer play an important role in determining the deposition, the substrates were sputter-cleaned for adhesion strength of the coating. 30 min to remove oxide layers or contaminants at

This paper involves the deposition of TiB2 based the substrate surface. In addition, in order to avoid nanostructured coatings on high speed steel sub- the formation of compound during deposition strates. A titanium interlayer was applied by sput- of nitrogen-doped Ti interlayer, no substrate bias- tering a titanium target in pure argon atmosphere ing during deposition was applied for all deposi- and in argon-nitrogen gas mixtures to influence the tions. Table 1 summarizes the deposition details in interlayer. Structural and properties analysis re- this study. vealed that argon-nitrogen gas mixture is benefi- The composition of the resultant coat- cial in improving the adhesion between the TiB2 ings was examined by Rigagu X-ray diffractometer coating and the high speed steel substrate. with Cu-Kα radiation. Crystallographic phases were deduced by comparing the experimental diffraction 2. EXPERIMENTAL PROCEDURE patterns with the standard JCPDS data. The mor- phology of surfaces and fractured cross-sections High-speed steel (HSS) was chosen as substrates of the coatings were imaged using a field emission in this study. The commercial HSS, SECO WKE45 scanning microscope (FESEM), Jeol JSM (Sweden) in fully hardened and tempered condi- 6340F. The coating thickness was measured by tion was cut into 12×12×3 mm pieces. The making a ball-crater on the coating surface using specimen’s surface was manually ground and pol- the Calotest machine manufactured by CSEM. A ished. The HSS substrates were then ultrasonically ball of 25.4 mm diameter was used cleaned with and before charging for cratering with a speed of 500 rpm for 240 s. the deposition chamber. High-purity argon gas was The roughness of surfaces (Ra) was measured then introduced into the chamber after it was evacu- using an atomic force microscopy (AFM), Digital . -4 ated to below 5 10 Pa. The TiB2 target (75 mm Instrument and the average roughness (Ra) val- diameter and 5 mm thick) was powered in the ra- ues are listed in Table 2. dio frequency (RF) mode and the Ti target was pow- 82 N. Panich, P. Wangyao, S. Hannongbua, P. Sricharoenchai and Y. Sun

Table 2. Properties of resultant coatings.

Samples Percentage Coating Ra Hardness Reduced Critical load

of N2 Thickness (nm) (GPa) modulus Lc (mN) (micron) (GPa)

No.1 0 1.90 6.95 31.6 321.3 1,089.1 No.2 10 1.85 1.87 35.6 316.2 1,784.5 No.3 20 1.87 1.62 33.3 355.8 1,352.4 No.4 33 1.89 1.51 33.2 332.7 1,446.1 No.5 50 1.92 0.81 29.9 310.5 1,387.5

Fig. 1. XRD patterns of all samples.

Nanoindentation test was performed using the 3. RESULTS AND DISCUSSION NanoTestTM instrument (Micro Materials Limited, UK), with a Berkovich indenter. All experi- 3.1. Structural characterization of ments were performed at a constant loading and resultant coatings unloading rate of 0.05 mN/s and to a penetration Fig. 1 shows the X-ray diffraction patterns recorded depth of 50 nm. The unloading curves were used for all five samples listed in Table 1. Each pattern to derive the hardness and reduced modulus val- shows several broad reflection peaks correspond- ues by the analytical technique developed by Oliver ing to the hexagonal TiB2 structure. The dominant and Pharr [16]. The reported hardness and modu- peak is (001), which indicates that the basal plane lus values are the average of 10 measurements. of TiB2 is preferentially oriented parallel to the sub- The microscratch test was performed using the strate surface in all samples. The degree of such multipass microscratch mode available in the preferred orientation seems to increase with in- NanoTestTM device with a diamond indenter topped creasing content of nitrogen in argon-nitrogen gas with a conical with spherical end form of 25 µm in mixture, as evident from the increased relative in- radius. A new test method was employed in this tensity of the (001) peak. Such a (001) texture may work, as described by Xia et. al. [17]. account for the observed higher hardness of the

coatings, since TiB2 coatings with (001) orienta- Effect of argon-nitrogen mixing gas during magnetron sputtering on titanium interlayer... 83 tion is known to yield the highest hardness as com- pared with TiB2 coatings with other orientation [18]. The broadness of the reflection peaks indicates the nanocrystalline of the coating structure, as further confirmed by FESEM examination (Fig. 2). No reflection peaks corresponding to TiN phase were observed in the X-ray diffraction patterns, in- dicating that under the present argon-nitrogen gas mixture conditions did not to TiN compound formation, and nitrogen are dissolved in the α-Ti lattice to form solution. The morphologies of the as-deposited coating surfaces and fractured cross-sections were exam- ined under FESEM, as illustrated in Fig. 2 for sample 1. Similar morphological features were also observed for other coatings. In all cases, the coat- ing surface is populated with granular grains with typical size less than 30 nm. The virtual grains are formed from many nanoclusters and they have well- defined boundaries with the others. Some pores of less than 10 nm size could be observed at vir- tual grain boundaries. Regarding the fractured cross-section (Fig. 2b), it is obvious that a thin Ti interlayer could be observed in all samples. From Fig. 2b, it can also be seen that the resultant coat- ing exhibits a columnar structure, which is typical of at relatively low adatom ener- gies and limited mobility. Fig. 2. FESEM images showing the surface mor- phology and fractured cross-sectional morphology 3.2. Surface roughness of TiB2 coatings (sample 1). The surface morphology and roughness values were measured by AFM for the coatings deposited on substrates under the same conditions as listed in Table 1. In this study, silicon wafer was used as a substrate material for AFM measure- ments due to the beginning smoothness with an average roughness (Ra) of about 0.15-0.20 nm. The surfaces of the resultant coatings deposited on sili- face morphological features, characterized by the con substrates were found to be quite smooth and densely populated conical projections. But the num- ber and individual size of these conical projections homogenous, with Ra at the nano-scale, as listed in Table 2. It can be clearly seen from Table 2 that vary with the nitrogen content in argon-nitrogen gas argon-nitrogen gas mixture is very effective in re- mixture: with increasing nitrogen content, the num- ducing the surface roughness of the subsequent ber of projections increases and the size of indi- vidual projects decreases. These are in good TiB2 coating. For example, the Ra value of TiB2 coating surface was reduced from 6.95 to 1.87 nm agreement with the Ra roughness values measured and further to 0.81 nm with increasing nitrogen for the coating surfaces. content in argon-nitrogen gas mixture. This dem- onstrates the beneficial effect of argon-nitrogen gas 3.3. Coating hardness mixture in maintaining the smoothness of the sub- In order to assess the intrinsic mechanical proper- strate surface, which is important for many practi- ties of the coatings i.e. hardness and reduced cal applications. modulus, all specimens were indented at 50 nm Fig. 3 shows the AFM images recorded for all penetration depth to avoid any possible effect from the coating surfaces. All coatings exhibit similar sur- the substrate during the indentation process. The 84 N. Panich, P. Wangyao, S. Hannongbua, P. Sricharoenchai and Y. Sun

Fig. 3. AFM images showing the average surface roughness (Ra) of all tested samples (a) Sample No. 1., (b) Sample No. 2., (c) Sample No. 3., (d) Sample No. 4., and (e) Sample No. 5.

hardness and reduced modulus values of all TiB2 interlayers were also measured as a function of coatings, measured by nanoindentation, are sum- nitrogen content in argon-nitrogen gas mixture. This marized in Table 2. was done on a separate series of samples depos- Fig. 4 shows the relationship between coating ited with Ti interlayers only without the subsequent hardness and nitrogen content in argon-nitrogen TiB2 coating deposition. An important observation gas mixture (represented by N2 % in the sputtering that can be made from Fig. 4 is that without nitro- atmosphere). The hardness values of the Ti gen doping, the pure Ti interlayer is very soft, with Effect of argon-nitrogen mixing gas during magnetron sputtering on titanium interlayer... 85

Fig. 4. The relationship between the coating hardness and percentage of nitrogen.

Fig. 5. Friction versus load curves for selected coatings.

hardness value around 2.5 GPa, and nitrogen mix- and then decreases with further increased nitro- ing is effective in enhancing the hardness and gen content. The hardness peaked at 10% N2 may strength of the interlayer. There is a general trend be attributed to the increased hardness of the Ti that the hardness of the Ti interlayer increases with interlayer and the enhanced (001) preferred orien- increasing degree of nitrogen doping. This is obvi- tation of the TiB2 coating (Fig. 1), as TiB2 coating ously due to the dissolution of nitrogen in the α-Ti with (001) orientation is known to yield the highest lattice, causing solid solution hardening in the hardness as compared with TiB2 coatings with other interlayer. Such an increased interlayer strength orientations. However, the reasons associated with may account for the observed enhancement in the decrease in TiB2 hardness with further in-

TiB2-substrate adhesion as discussed later. creased nitrogen content are not fully understood. From Fig. 4, it can also be seen that the hard- One possible reason may arise from the ness of the TiB2 coating first increases with increas- embrittlement of the interlayer resulting from too ing nitrogen content in argon-nitrogen gas mixture, much nitrogen incorporation. 86 N. Panich, P. Wangyao, S. Hannongbua, P. Sricharoenchai and Y. Sun

3.4. Coating-substrate adhesion Ti interlayer under argon-nitrogen gas mixture con- dition has the following beneficial effects on the Fig. 5 shows typical friction versus scratching load structures and properties on sputtered TiB coat- curves measured during the microscratch test. 2 ings: Each friction curve can be divided inter two char- (1) It can increase the hardness and strength of acteristic regimes: a smoothly increased region, the interlayer due to nitrogen dissolution in the followed by a sudden increase and then a large α-Ti lattice. fluctuating region. As confirmed by microscopic (2) It can enhance the (001) preferred orientation examination, the sudden change in friction during of the subsequent TiB coating. scratch test is due to coating adhesive failure, and 2 (3) It is effective in reducing the surface roughness the corresponding load (L ) is thus used as a mea- C of the TiB coating. sure of the adhesion strength of the TiB coating to 2 2 (4) It can significantly enhance the TiB coating- the HSS substrate. Table 2 also summarises the 2 HSS substrate adhesion strength, with argon- L values for all the coatings studies in this work. C nitrogen gas mixture of 10% N showing the Clearly, nitrogen content in argon-nitrogen gas mix- 2 optimum enhancement. ture can significantly enhance the TiB2-HSS adhe- sion, with 10% N2 in argon-nitrogen gas mixture is the most effective. This is another important ob- REFERENCES servation made in this work, since coating-substrate [1] J. Chen and J. A. Barnard // Materials adhesion is critical in many practical applications. Science and Engineering A 191 (1995) 233. The observed enhancement in adhesion [2] E. Matrubara, Y. Waseda, S. Takeda and strength can be attributed to the increased strength Y. Taga // Thin Solid Films 186 (1990) L33. of the nitrogen influenced on Ti interlayer. As dis- [3] R. Wiedemann, H. Oettel and M. Jerenz // cussed previously, the titanium interlayer serves Surface Coating and Technology 97 (1997) as an oxygen getter to decompose the native ox- 313. ide film on the substrate surface, thus promoting [4] B. Todorovic, T. Jokic, Z. Rakocevic, adhesion between the coating and the substrate. Z. Markovic, B. Gakovic and T. Nanadovic // The argon-nitrogen gas mixture does not signifi- Thin Solid Films 300 (1997) 272. cantly change the chemical nature of α-Ti, because [5] M. Berger // Surface Engineering 18 (2002) no TiN compound is formed and nitrogen is dis- 219. solved in the α-Ti lattice. It is thus expected that [6] P. Losbichler, C. Mitterer, P. N. Gibson, the Ti interlayer, influenced by argon-nitrogen gas W. Gissler, F. Hofer and P. Warbichler // mixture, also plays a similar role as an oxygen get- Surface Coating and Technology 94-95 ter. In addition, the interlayer also provides compli- (1997) 297. ance for the hard and brittle TiB2 coating and helps [7] M. Berger, L. Karlsson, M. Larsson and to accommodate interfacial stresses through plas- S. Hogmark // Thin Solid Films 401 (2001) tic deformation. However, if the interlayer is too soft, 179. such as a pure Ti interlayer, it will deform too easily [8] N. Panich, P. Wangyao, S. Hannongbua, and provide less support to the TiB2 coating. P. Sricharoenchai and Y. Sun // Journal of With argon-nitrogen gas mixture, the Ti , Materials and Minerals 16/1 (2006) interlayer becomes stronger and thus can provide 45. increased support to the TiB2 coating, leading to [9] N. Panich, P. Wangyao, S. Hannongbua, enhanced adhesion strength. But, with further in- P. Sricharoenchai and Y. Sun // High creasing nitrogen content, the interlayer becomes Temperature Materials and Processes 25/5-6 too strong and brittle to accommodate the interfa- (2006) 285. cial stresses, leading to the observed reduction in [10] N. Panich, P. Wangyao, S. Hannongbua, adhesion strength. The argon-nitrogen gas mixture P. Sricharoenchai and Y. Sun // Reviews on with 10% N2 seems to produce the optimum com- Advanced Materials Science 13 (2006) 117. bination of strength and compliance for adhesion [11] J. L. He, S. Miyake, Y. Setsuhara, I. Shimizu, enhancement in the present deposition conditions. M. Suzuki, K. Numata and H. Saito // Wear 249 (2001) 498. 4.CONCLUSIONS [12] L. Ju-Wan, L. Jung-Joong, H. Ann and K.-T. Rie // Surface Coating and Technology 174- From the experimental results under the present 175 (2003) 720. deposition conditions, it can be concluded that the Effect of argon-nitrogen mixing gas during magnetron sputtering on titanium interlayer... 87

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