The Microstructure of Nanocrystalline Tib2 Films Prepared by Chemical Vapor Deposition

materials

Article The Microstructure of Nanocrystalline TiB2 Films Prepared by Chemical Vapor Deposition

Xiaoxiao Huang, Shuchen Sun *, Ganfeng Tu, Shuaidan Lu, Kuanhe Li and Xiaoping Zhu School of Metallurgy, Northeastern University, No. 3-11, Wenhua Road, Heping District, Shenyang 110819, China; [email protected] (X.H.); [email protected] (G.T.); [email protected] (S.L.); [email protected] (K.L.); [email protected] (X.Z.) * Correspondence: [email protected]; Tel.: +86-024-8368-9195

Received: 7 November 2017; Accepted: 11 December 2017; Published: 13 December 2017

Abstract: Nanocrystalline titanium diboride (TiB2) ceramics films were prepared on a high purity graphite substrate via chemical vapor deposition (CVD). The substrate was synthesized by a gas ◦ mixture of TiCl4, BCl3, and H2 under 1000 C and 10 Pa. Properties and microstructures of TiB2 films were also examined. The as-deposited TiB2 films had a nano-sized grain structure and the grain size was around 60 nm, which was determined by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. Further research found that a gas flow ratio of TiCl4/BCl3 had an influence on the film properties and microstructures. The analyzed results illustrated that the grain size of the TiB2 film obtained with a TiCl4/BCl3 gas flow ratio of 1, was larger than the grain size of the as-prepared TiB2 film prepared with a stoichiometric TiCl4/BCl3 gas flow ratio of 0.5. In addition, the films deposited faster at excessive TiCl4. However, under the condition of different TiCl4/BCl3 gas flow ratios, all of the as-prepared TiB2 films have a preferential orientation growth in the (100) direction.

Keywords: TiB2; nanocrystalline ﬁlms; microstructure; chemical vapor deposition

1. Introduction

The titanium diboride (TiB2) is an interesting and very useful ceramic material. It displays many attractive properties such as high melting temperature, high hardness, high elastic modulus, erosion resistance, excellent chemical stability, and good thermal as well as electrical conductivity [1–6]. Thus, TiB2 is widely applied as the evaporator boat in high vacuum metal films, the protection of weapons and armored vehicle [7–9]. In addition, the neutron absorption of boron coupled with the above-high-temperature properties makes TiB2 the best choice of control rod material for high-temperature nuclear reactors [10–12]. The outstanding properties of TiB2 depend on its microstructure. TiB2 crystallizes with a hexagonal structure with a P6/mmm space group [13,14]. Titanium atoms are located at the vertex of the hexagonal prism and the center of the bottom, boron atoms fill the trigonal prisms that are formed by the titanium atoms, and the boron atom and the titanium atom alternately form a 2D honeycomb network structure. B–B is covalently bonded, and B–Ti is bounded by ion bonds [15,16]. The lattice parameters of TiB2 are as follows: a = b = 3.029, c = 3.228, α = β = π/2, and γ = π/3 [17,18]. Over the past few decades, many studies have been carried out on the deposition of TiB2 films by chemical vapor deposition (CVD) methods [19–21], plasma assisted chemical vapor deposition (PACVD) methods [6,11], and plasma enhanced chemical vapor deposition (PECVD) methods [22]. Takehiko Takahashi and Hideo Kamiya [21] investigated the influence of deposition temperature on the deposition phase using CVD methods, the results show that TiB2 could be deposited above 800 ◦C. A. J. Caputo et al. [19] have researched the effect of deposition temperature on film hardness, the surface morphology of the films, and deposition rate via CVD methods. The PACVD and PECVD

Materials 2017, 10, 1425; doi:10.3390/ma10121425 www.mdpi.com/journal/materials Materials 2017, 10, 1425 2 of 8 technique have a low deposition temperature range of 250~650 ◦C. In this paper, nanocrystalline ◦ TiB2 films on a high pure graphite substrate were deposited at a higher temperature (1000 C) by a CVDMaterials system. 2017 The, 10, 1425 purpose of our work was to investigate the deposition results and to prepare2 of 9 for further study in which TiB2 films are deposited on a nickel-based superalloy substrate. Because of the propertiesthe surface of morphology nickel-based of superalloys,the films, and deposition deposition rate temperatures via CVD methods. should The notPACVD exceed and 1000PECVD◦C. Thus, technique have a low deposition temperature range of 250~650 °C. In this paper, nanocrystalline TiB2 the deposition temperature was also limited to 1000 ◦C in this experiment. The structure of the TiB films on a high pure graphite substrate were deposited at a higher temperature (1000 °C) by a CVD 2 films wassystem. examined The purpose by X-ray of our diffraction work was to (XRD), investigat fielde the emission deposition scanning results electronand to prepare microscopy for further (FESEM), transmissionstudy in electron which TiB microscopy2 films are (TEM),deposited and on energya nickel-based dispersive superalloy spectroscopy substrate. (EDS).Because Furthermore,of the the TiBproperties2 films were of nickel-based synthesized superalloys, using differentdeposition TiCltemperatures4/BCl3 gasshould flow not ratios.exceed 1000 The °C. influence Thus, the of the deposition temperature was also limited to 1000 °C in this experiment. The structure of the TiB2 films TiCl4/BCl3 gas flow ratio on the grain size and the deposition rate is discussed. was examined by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), 2. Experimentaltransmission Methods electron microscopy (TEM), and energy dispersive spectroscopy (EDS). Furthermore, the TiB2 films were synthesized using different TiCl4/BCl3 gas flow ratios. The influence of the TiBTiCl2 films4/BCl were3 gas flow deposited ratio onon the a grain high size pure and graphite the deposition substrate rate is via discussed. CVD using a gas mixture of TiCl4, BCl3, and H2. The overall reaction of the vapor deposition of TiB2 is as follows [21]: 2. Experimental Methods

TiB2 films wereTiCl deposited4 (g) + on 2BCl a high3 (g) pure + 5H graphi2 (g)te = TiBsubstrate2 (s) + via 10HCl CVD (g).using a gas mixture of (1) TiCl4, BCl3, and H2. The overall reaction of the vapor deposition of TiB2 is as follows [21]: The 3D model diagram of the CVD reactor is illustrated in Figure1. BCl 3, TiCl4, and H2 enter the TiCl4 (g) + 2BCl3 (g) + 5H2 (g) = TiB2 (s) + 10HCl (g). (1) deposition chamber through the air inlet at the bottom of the CVD reactor, however, H2, BCl3, and TiCl4 The 3D model diagram of the CVD reactor is illustrated in Figure 1. BCl3, TiCl4, and H2 enter the were mixed in the gas mixing chamber before importing the deposition chamber. The graphite deposition chamber through the air inlet at the bottom of the CVD reactor, however, H2, BCl3, and substrates were fixed on the sample stage. When the reaction begins, the rotating shaft drives the TiCl4 were mixed in the gas mixing chamber before importing the deposition chamber. The graphite samplesubstrates table to drivewere fixed the graphiteon the sample substrate stage. When to rotate. the reaction The rotating begins, the graphite rotating substrate shaft drives ensures the the uniformsample films. table Avacuum to drive the pump graphite is connected substrate to to ro thetate. air The outlet rotating for graphite the aim substrate of keeping ensures the reactorthe in a vacuumuniform during films. this A vacuum entire pump sequence. is connected The vacuum to the air leveloutlet wasfor the maintained aim of keeping at aboutthe reactor 10 Pain a during vacuum during this entire sequence. The vacuum level was maintained at about 10 Pa during the the reaction. During experimentation, TiCl4 and BCl3 were carried to the reactor by a heated line. reaction. During experimentation, TiCl4 ◦and BCl3 were carried to the reactor◦ by a heated line. The The temperature of the TiCl4 flow was 135 C, and the BCl3 flow was 12 C. The heated TiCl4 flow and temperature of the TiCl4 flow was 135 °C, and the BCl3 flow was 12 °C. The heated TiCl4 flow and BCl flow were controlled by a mass flowmeter. Table1 summarizes the field of deposition parameters. 3 BCl3 flow were controlled by a mass flowmeter. Table 1 summarizes the field of deposition parameters.

Figure 1. 3D model diagram of the CVD (chemical vapor deposition) reactor. (1) Air inlet; (2) gas Figure 1. 3D model diagram of the CVD (chemical vapor deposition) reactor. (1) Air inlet; mixing chamber; (3) electrode; (4) heating resistor; (5) deposition chamber; (6) high purity graphite (2) gas mixingsubstrate;chamber; (7) sample (stage;3) electrode; (8) air outlet; (4) ( heating9) rotating resistor; shaft. (5) deposition chamber; (6) high purity graphite substrate; (7) sample stage; (8) air outlet; (9) rotating shaft. Materials 2017, 10, 1425 3 of 8

Table 1. Field of deposition parameters.

Substrate Graphite deposition temperature 1000 (◦C) deposition time 3 (h) vacuum level 10 (Pa) ◦ temperature of H2 25 ( C) pressure of H2 0.06 (MPa) 3 flow rate of H2 0.9 (m /h) ◦ temperature of TiCl4 135 ( C) pressure of TiCl4 0.1 (MPa) 3 flow rate of TiCl4 0.055, 0.11 (m /h) ◦ temperature of BCl3 12 ( C) pressure of BCl3 0.1 (MPa) 3 flow rate of BCl3 0.085 (m /h)

In this study, the deposition was carried out at 1000 ◦C. The deposition pressure was 10 Pa, and the deposition time was 3 h. In the experiment, the supply of hydrogen was excessive, and the 3 BCl3 flow rate was kept at 0.085 m /h, then two types of specimens were synthesized at two TiCl4 3 3 3 flow rate levels: 0.055 m /h and 0.11 m /h. When the TiCl4 flow rate was 0.055 m /h, the gas flow ratio of TiCl4/BCl3 (κ) was 0.5. At this point, κ was the stoichiometric TiCl4/BCl3 gas ratio. When the 3 TiCl4 flow rate was 0.11 m /h, κ was 1. TiCl4 was excessive in this situation. The microstructure and properties of these two types of films were investigated. In this way, the influence of the TiCl4/BCl3 gas flow ratio on the grain size and the deposition rate was studied. The crystal phase of the films was determined by XRD (X’Pert Pro, PANalytical B.V., Almelo, The Netherlands). Special emphasis was put on the comparative analysis of micro structural characterization of these two types of films. In addition, SEM (Ultra Plus, ZEISS, Heidenheim, Germany), TEM (Tecnai G2 20, FEI, Hillsboro, OR, USA), and EDS (X-Max 50, OXFORD, Oxford, UK) were employed.

3. Results and Discussion

3.1. Structure, Morphology, and the Deposition Rate of Films

It was found that the oriented growth was related to the ratio of TiCl4/BCl3 from the orientation characteristics of the deposited layers investigated by X-ray diffraction shown in Figure2 for a laser power 3 kW and laser scanning speed 2.5 mm/s. Figure2a,b show the diffraction patterns of the deposits, which were prepared at κ = 0.5 and 1, respectively. In addition, the TiB2 standard data from the Joint Committee on Powder Diffraction Standards (JCPDS) is shown in Figure2c. X-ray diffraction (Figure2a,b) indicated the presence of only TiB 2, with no unidentified peaks. Figure2 also shows that the diffraction pattern of deposits corresponds well to the JCPDS standard for TiB2. Furthermore, we found that the pattern intensity of the TiB2 films, which were deposited at different gas ratios of TiCl4/BCl3 (Figure2a,b), follows a similar trend. The intensity of the (100) peaks in Figure2a,b are the highest of all peaks, respectively. However, there is an enhanced intensity at the (100) peaks compared with the standard pattern intensity shown in the JCPDS card NO. 85-2083 (Figure2c). Furthermore, according the XRD analysis, the relative intensity value of R(100)/(101) is1.32 ± 0.01 (Figure2a) and 1.28 ± 0.01 (Figure2b). Similarly, the relative intensity of the standard pattern yields a R (100)/(101) value of 0.86 ± 0.01 (Figure2c). Obviously, the R (100)/(101) values of the as-prepared TiB2 films are greater than the standard values. Hence, all the as-prepared TiB2 films have a preferred orientation along the (100) planes. These results are consistent with the work done by S.H. Lee [11], who reported that the film structure has a (100) preferred orientation when the film was deposited at a low RF power ◦ (200 W). The difference is that they prepared the TiB2 film at a low temperature (250~400 C) using a PACVD system. Materials 2017, 10, 1425 4 of 8 Materials 2017, 10, 1425 4 of 9 Materials 2017, 10, 1425 4 of 9

Figure 2. XRD (X-ray diffraction) patterns of TiB2 deposits. (a) At a gas flow ratio of TiCl4/BCl3 = 0.5; Figure 2. XRD (X-ray diffraction) patterns of TiB2 deposits. (a) At a gas ﬂow ratio of TiCl 4/BCl3 = 0.5; (b) at( ab) gas at a ﬂow gas flow ratio ratio of TiCl of TiCl/BCl4/BCl3= = 1;1; ((cc) TiB 2 standardstandard data data from from JCPDS JCPDS (Joint (Joint Committee Committee on Powder on Powder Figure 2. XRD (X-ray diffraction)4 3 patterns of2 TiB2 deposits. (a) At a gas flow ratio of TiCl4/BCl3 = 0.5; Diffraction Standards) card No. 85-2083. Diffraction(b) at a gas Standards) flow ratio cardof TiCl No.4/BCl 85-2083.3 = 1; (c) TiB2 standard data from JCPDS (Joint Committee on Powder

DiffractionThe surfaces Standards) and cross card sections No. 85-2083. of the TiB2 films obtained by κ 0.5 and 1 were observed by FESEM The(Figures surfaces 3 and and 4, respectively) cross sections with of high the TiBvoltages2 films of obtained15 kV and by magnificationsκ 0.5 and 1 were of 5000× observed and 20,000×. by FESEM (FiguresEDSThe3 resultsand surfaces4, respectively)are andalso crossshown. sections with An highunsmooth of the voltages TiB surface2 films of 15can obtained kV be andobserved by magnifications κ 0.5 on and the 1 films. were of Theobserved 5000 cross× and sectionsby FESEM 20,000 ×. EDS(Figuresshow results a 3 very areandalso compact4, respectively) shown. and Andense with unsmooth structure high voltages surfacewithout of canany 15 kV begaps. observedand Figure magnifications 4 onindicates the films. ofthe 5000× FESEM The and cross images 20,000×. sections showEDSand a veryresults EDS compact results are also of and theshown. densesurface An structure and unsmooth cross without section surface of any TiBcan gaps.2 befilms observed Figure formed4 indicatesonunder the thefilms. the conditions FESEMThe cross of images κsections = 1. and showIt can a verybe discussed compact on and different dense micrographsstructure without of the anysurfaces gaps. and Figure cross 4 sections indicates by theFigures FESEM 3 and images 4. EDS results of the surface and cross section of TiB2 films formed under the conditions of κ = 1. It can be andIt wasEDS found results that of similarthe surface morphologies and cross of section the TiB 2of films TiB 2were films obtained formed by under different the conditionsgas flow ratios of κ of = 1. discussed on different micrographs of the surfaces and cross sections by Figures3 and4. It was found It canTiCl be4/BCl discussed3. All TiB on2 films different were micrographs found to have of thea typically surfaces columnarand cross particle sections structure, by Figures and 3 andthe 4. that similar morphologies of the TiB2 films were obtained by different gas flow ratios of TiCl4/BCl3. It wascolumnar found particles that similar have morphologies a random orientation. of the TiB This2 films indicates were obtained that there by was different no distinct gas flow effect ratios on of All TiB2 films were found to have a typically columnar particle structure, and the columnar particles TiClmorphologies4/BCl3. All TiBby 2the films TiCl were4/BCl 3 foundgas flow to haveratio ina typicallythis experimentation. columnar particle However, structure, different and film the havecolumnarthicknesses a random particles were orientation. displayed have a Thisrandom in the indicates cross orientation. sections that (Fig there Thisures wasindicates 3 and no 4). distinct thatAs shown there effect inwas Figure on no morphologies 3,distinct the thickness effect by on the TiCl of/BCl the filmgas deposited flow ratio at in κ = this 0.5 experimentation.for 3 h reached 15 microns. However, The different thickness film was thicknesses 21 microns when were κ displayed was morphologies4 3 by the TiCl4/BCl3 gas flow ratio in this experimentation. However, different film in thethicknessesequal cross to sections1 were in Figure displayed (Figures 4. Obviously, in3 andthe cross4 ).a thicker As sections shown TiB 2(Fig infilm Figureures was 3 obtainedand3, the 4). thicknessAs by shown a greater ofin Figure theTiCl film4/BCl 3, the deposited3 gas thickness flow at ratio that goes beyond the stoichiometric TiCl4/BCl3 gas flow ratio. In other words, the film deposited κ =of 0.5 the for film 3 hdeposited reached 15at κ microns. = 0.5 for 3 The h reached thickness 15 microns. was 21microns The thickness when wasκ was 21 microns equal to when 1 inFigure κ was 4. faster when TiCl4 was excessive. Obviously,equal to 1 a in thicker Figure TiB 4. 2Obviously,film was obtained a thicker byTiB a2 greaterfilm was TiCl obtained4/BCl 3bygas a greater flow ratio TiCl that4/BCl goes3 gas beyond flow theratio stoichiometric that goes beyond TiCl4 /BClthe stoichiometric3 gas flow ratio. TiCl4 In/BCl other3 gas words,flow ratio. the In film other deposited words, the faster film deposited when TiCl 4 wasfaster excessive. when TiCl4 was excessive.

Figure 3. Cont.

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FigureFigure 3. SEM3. SEM (scanning (scanning electron electron microscopy) microscopy) images andand EDSEDS (energy (energy dispersive dispersive spectroscopy) spectroscopy) Figure 3. SEM (scanning electron microscopy) images and EDS (energy dispersive spectroscopy) results of resultsresults of the of the surface surface and and cross cross section section of of the the TiBTiB2 film formedformed under under the the condition condition of aof TiCl a TiCl4/BCl4/BCl3 gas3 gas the surface and cross section of the TiB2 film formed under the condition of a TiCl4/BCl3 gas flow ratio of 0.5. flowflow ratio ratio of 0.5.of 0.5.

Figure 4. SEM images and EDS results of the surface and cross section of the TiB2 film formed under Figure 4. SEM images and EDS results of the surface and cross section of the TiB2 ﬁlmformed under the condition of a TiCl4/BCl3 gas flow ratio of 1. theFigure condition 4. SEM of aimages TiCl4/BCl and 3EDSgas results ﬂow ratio of the of surface 1. and cross section of the TiB2 film formed under

the Tablecondition 2 shows of a TiCl the 4compositional/BCl3 gas flow ratioanalysis of 1. results of Spot 1, Spot 2, Spot 3, and Spot 4, analyzed Tablevia EDS.2 shows The Ti/B the ratio compositional of the deposit analysis approached results the ofstoichiometric Spot 1, Spot value 2, Spot of 0.5 3, when and Spotthe gas 4, flow analyzed via EDS.ratioTable The of 2TiCl Ti/B shows4/BCl ratio the3 was of compositional theincreased. deposit According approachedanalysis toresults the the CVD of stoichiometric Spot phase 1, diagramSpot 2, value Spot of the 3, of Ti–B–H–Cland 0.5 whenSpot 4, thesystem analyzed gas flow via whichEDS. The was Ti/B calculated ratio of by the Randich deposit and approached Gerlach [23], the the stoichiometric formability ofvalue TiB2 ofrelies 0.5 whenon the the distance gas flow ratio of TiCl4/BCl3 was increased. According to the CVD phase diagram of the Ti–B–H–Cl system ratiobetween of TiCl the4/BCl process3 was tieincreased. line and theAccording phase boundary to the CVD of the phase TiB2+ diagramgas and the of gasthe phases.Ti–B–H–Cl A boron system which was calculated by Randich and Gerlach [23], the formability of TiB2 relies on the distance rich TiBx film was deposited when the BCl3 concentration was high, because the process tie line went betweenwhich thewas processcalculated tie lineby Randich and the and phase Gerlach boundary [23], the of the formability TiB + gas of and TiB2 the relies gas on phases. the distance A boron through the region of the B + TiB2 + gas phase. Therefore, at a low TiCl2 4/BCl3 gas ratio, boron rich TiBx between the process tie line and the phase boundary of the TiB2+ gas and the gas phases. A boron rich TiBfilmsx film were was formed deposited instead when of TiB the2. BCl3 concentration was high, because the process tie line went rich TiBx film was deposited when the BCl3 concentration was high, because the process tie line went through the region of the B + TiB2 + gas phase. Therefore, at a low TiCl4/BCl3 gas ratio, boron rich throughTable the region 2. The EDSof the (energy B + TiB dispersive2 + gas phase.spectroscopy) Therefore, analys isat results a low of TiCl different4/BCl regions3 gas ratio, of samples boron in rich TiBx TiBx films were formed instead of TiB2. films wereFigures formed 3 and instead 4. of TiB2. Spot No. Element wt % atom % Ti/B Atom Ratio TableTable 2. The2. The EDS EDS (energy (energy dispersive dispersive spectroscopy) spectroscopy) analysisanalysis results results of of different different regions regions of of samples samples in in B 38.01 73.15 Figures3 and1 4. 0.367 Figures 3 and 4. Ti 61.92 26.85 B 37.68 72.84 Spot No.2 Element wt % atom % Ti/B0.373 Atom Ratio Spot No.Ti Element62.32 wt % atom27.16 % Ti/B Atom Ratio B 38.01 73.15 1 B B33.54 38.01 73.1569.10 0.367 3 1 0.367 0.447 Ti Ti Ti61.9266.46 61.92 26.8530.9026.85 B B 37.6833.23 68.8072.84 2 4 B 37.68 72.84 0.4530.373 Ti2 62.32 27.16 0.373 Ti Ti66.70 62.32 27.1631.20 B 33.54 69.10 3 B 33.54 69.10 0.447 Ti3 66.46 30.90 0.447 Ti 66.46 30.90 B 33.23 68.80 4 B 33.23 68.80 0.453 Ti4 66.70 31.20 0.453 Ti 66.70 31.20

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3.2. Grain Size of the Films Many scholars [6,11,19] have researched the influence of deposition temperature on grain size, and they all agreed that the grain size increases with increasing temperature. Indeed, this conclusion is in good agreement with the kinetic theory of CVD [24]. Comparatively few research groups have studied the effect of TiCl4/BCl3 gas flow ratio on grain size. The grain size of the TiB films was calculated by XRD patterns of TiB films (Figure2) using the Materials 2017, 10, 1425 2 2 6 of 9 Scherrer formula. The grain size was 58 ± 3 nm when the TiB2 film was deposited at a stoichiometric 3.2. Grain Size of the Films TiCl4/BCl3 gas ratio of 0.5, and the grain size was 69 ± 3 nm when κ = 1. Hence, all the as-deposited Many scholars [6,11,19] have researched the influence of deposition temperature on grain size, TiB2 films had a nano-sized grain structure. Figures5 and6 show TEM images of the films formed and they all agreed that the grain size increases with increasing temperature. Indeed, this conclusion under differentis in gas good flow agreement ratios with of the TiCl kinetic4/BCl theory3. Brightof CVD [24]. nano-sized Comparatively shapes few research can be groups observed have in the dark field images (Figuresstudied the5a effect and of6 a),TiCl wherein4/BCl3 gas flow it can ratio be on seengrain size. that the common shapes of the grains reveal a columnar and thatThe the grain grain size of size the TiB is at2 films the was nano calculated scale. by The XRD grainpatterns size of TiB is2 films also (Figure easily 2) observed. using the The grains Scherrer formula. The grain size was 58 ± 3 nm when the TiB2 film was deposited at a stoichiometric are larger in FigureTiCl4/BCl63a gas than ratio inof 0.5, Figure and the5a. grain Analysis size was 69 of ± the3 nm selected-areawhen κ = 1. Hence, electron all the as-deposited diffraction (SAED) pattern (FiguresTiB52 bfilms and had6b) a revealednano-sized grain only structure. the presence Figures 5 of and TiB 6 show2. The TEM average images of grainthe films size formed in the plan view (dark field image)under wasdifferent estimated gas flow ratios to be of at TiCl the4/BCl nano3. Bright scale nano-sized and to shapes cause can continuous be observed ringsin the dark in the diffraction field images (Figures 5a and 6a), wherein it can be seen that the common shapes of the grains reveal pattern (as showna columnar in Figures and that5 bthe and grain6b). size However, is at the nano the scale. diffraction The grain size ring, is also as showneasily observed. in Figures The 5b and6b, is a little bit different.grains are larger In Figure in Figure5b, 6a the than diffraction in Figure 5a. ring Analysis is smooth of the selected-area and continuous. electron diffraction In contrast, the ring (SAED) pattern (Figures 5b and 6b) revealed only the presence of TiB2. The average grain size in the intensities inplan Figure view6 (darkb do field not image) show was uniform estimated continuity,to be at the nano and scale there and to arecause bright continuous spots ringson in the circular rings. This isthe because diffraction the pattern grain (as ofshown the in selected Figures 5b areaand 6b). in However, Figure6 thea isdiffraction larger. ring, Thus, as shown the SAEDin pattern further revealsFigures that 5b the and grain 6b, is a sizelittle bit in different. Figure 6Ina Figure is greater 5b, the diffraction than that ring in is Figuresmooth and5a. continuous. These SAED results In contrast, the ring intensities in Figure 6b do not show uniform continuity, and there are bright correspond tospots Figures on the5 circulara and rings.6a. As This a is result, because TEMthe grai investigationsn of the selected area indicate in Figure 6a that is larger. all as-prepared Thus, TiB 2 films are nanocrystallinethe SAED pattern and further that reveals the grain that the size grain is size different in Figure under 6a is greater various than TiClthat in4 /BClFigure3 5a.gas flow ratios. Obviously, theThese TEM SAED results results are correspond in good to Figures agreement 5a and 6a. with As a the result, results TEM investigations calculated indicate by the that Scherrer all formula. as-prepared TiB2 films are nanocrystalline and that the grain size is different under various TiCl4/BCl3 Therefore, TiClgas4/BCl flow ratios.3 gas Obviously, flow ratio the TEM has results a certain are in impactgood agreement on the with grain the results size calculated of as-prepared by the TiB2 films Scherrer formula. Therefore, TiCl4/BCl3 gas flow ratio has a certain impact on the grain size of as- via CVD. The grain size of TiB2 films deposited at κ = 1 is greater than that of other films obtained prepared TiB2 κfilms via CVD. The grain size of TiB2 films deposited at κ = 1 is greater than that of under conditionsother wherefilms obtained= 0.5. under conditions where κ = 0.5.

Figure 5. TEM images of the synthesized TiB2 film formed by CVD with a TiCl4/BCl3 gas flow ratio of Figure 5. TEM0.5: images (a) dark offield the image, synthesized (b) corresponding TiB2 selected-areafilm formed electron by CVD diffraction with pattern. a TiCl 4/BCl3 gas flow ratio of 0.5: (a) dark field image, (b) corresponding selected-area electron diffraction pattern.

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Figure 6. TEM images of the synthesized TiB2 film formed by CVD with a TiCl4/BCl3 gas flow ratio of 1: Figure 6. TEM(a) images dark field of image, the synthesized(b) corresponding TiB selected-area2 film formed electronby diffraction CVD withpattern. a TiCl4/BCl3 gas flow ratio of 1: (a) dark field image, (b) corresponding selected-area electron diffraction pattern. 4. Conclusions

4. Conclusions TiB2 was prepared via the vapor-phase reaction of TiCl4, BCl3, and H2 on a high pure graphite substrate. The following information was obtained:

TiB2 was1. prepared TiB2 could via be deposited the vapor-phase at 1000 °C and reaction 10 Pa by a CVD of TiCl system.4, BCl All deposits3, and obtained H2 on aunder high the pure graphite substrate. The followingcondition of information excessive hydrogen was and obtained: different TiCl4/BCl3 gas flow ratios (1/2 and 1/1) were TiB2. Other impurity phases such as TiB were not found. 1. TiB could2. beThese deposited TiB2 films are at 1000nanocrystalline◦C and with 10 Paa grain by size a CVD in the system.range of 60 Allnm. All deposits of the TiB obtained2 films under the 2 were typically columnar particles structure, and the columnar particles have a random orientation. condition3. ofX-ray excessive diffraction hydrogen indicated that and all differentof the as-synthesized TiCl4/BCl TiB23 filmsgas have flow a preferential ratios (1/2 orientation and 1/1) were TiB2. Other impuritygrowth phases in the (100) such direction. as TiB were not found. 4. The TiCl4/BCl3 gas flow ratio has a certain impact on deposition rate and grain size, but these 2. These TiB2 filmsvariations are in nanocrystalline the gas flow ratio of TiCl with4/BCl a3 grain did notsize appear in to the influence range the of preferred 60 nm. orientation All of the TiB2 films were typicallyof the columnar deposits. The particles deposition structure, rate is faster and when the using columnar a greater TiCl particles4/BCl3 gas haveflow ratio, a random which orientation. in this case was 1/1. Meanwhile, when we provided a stoichiometric TiCl4/BCl3 gas ratio of 1/2, 3. X-ray diffraction indicated that all of the as-synthesized TiB2 films have a preferential orientation the grain size of the as-deposited TiB2 film was smaller. growth in the (100) direction. Acknowledgments: The authors are grateful for the financial support of the Foundation Committee of the 4. The TiClNational4/BCl Nature3 gas of flow China ratio(No. 50904017). has a Their certain support impact enabled us on to depositioncomplete this work. rate and grain size, but these

variationsAuthor in the Contributions: gas flow Xiaoxiao ratio ofHuang TiCl planned4/BCl and3 didperformed not appearthe experiments, to influence analyzed the the experimental preferred orientation of the deposits.data, and wrote The the depositionmanuscript. Shuche raten Sun is directed faster and when guided usingthe resear ach greaterand extensively TiCl provided4/BCl help3 gas flow ratio, for editing the manuscript. Ganfeng Tu provided useful insights into the manuscript. Shuaidan Lu took the TEM which inimages. this caseKuanhe was Li and 1/1. Xiaoping Meanwhile, Zhu prepared when the TEM we samples. provided All authors a stoichiometric contributed in discussion TiCl and4/BCl 3 gas ratio of 1/2, thepreparation grain of size the manuscript. of the as-deposited TiB2 film was smaller. Conflicts of Interest: The authors declare no conflict of interest.

Acknowledgments: The authors are grateful for the ﬁnancial support of the Foundation Committee of the National Nature of China (No. 50904017). Their support enabled us to complete this work.

Author Contributions: Xiaoxiao Huang planned and performed the experiments, analyzed the experimental data, and wrote the manuscript. Shuchen Sun directed and guided the research and extensively provided help for editing the manuscript. Ganfeng Tu provided useful insights into the manuscript. Shuaidan Lu took the TEM images. Kuanhe Li and Xiaoping Zhu prepared the TEM samples. All authors contributed in discussion and preparation of the manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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