Vol. 21 No.4

Journal of Wuhan University of Technology- Mater. Sci. Ed.

Dec. 2006

Thermal Shock Resistance and Erosion Resistance of TiB2 Multiphase Ceramic Composites HE Ping,

W A N G Weimin*,

D O N G Yanling

(State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China) The thermal shock resistance and anti-aluminum erosion of TiB2 -BN multiphase ceramics composites were studied. The experimental results show that the TiB2-BN multiphase ceramic possesses a good thermal shock resistance at high temperatures ( 1000,1200,1400,1500 ~ ), with the increasing in thermal shocking temperature, the electro-conductivity of TiB2-BN ceramics increases. The metal aluminum has a great influence on Abstract:

the properties of TiB2- BN ceramics and the main reason is that the aluminum reacts seriously with BN. It is suggested that the content of BN should be reduced to the greatest extent. Key words: thermal shocking resistance; erosionresistance; microstructure ; mechanism

1 Introduction

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)bservewindow t

Due to many advantages of composites such as high temperature resistance, thermal shock resistance, erosion resistance and good electric conductivity, the materials were developed fast in the following years and the materials usually were made from carbides, nitrides and borides, which were of high melt point I1~ . TiB2-BN ceramic is developed to be used as evaporation source materials in vacuum, which overcome some disadvantages of the traditional graphite boat materials, such as worse erosion resistance and short lifetime. TiB2-BN ceramic is of good "electric conductivity, thermal shock resistance, machinability and corrosion resistance, and has a wide application area in metal evaporation film industry I2"41 When used under actual conditions, TiB2-BN electrical ceramic as a structural material endures the high thermal shock from room temperature to working temperature and the erosion of aluminum I5-71 . In this paper, we investigated the property of the thermal shock in an experimental testing furnace and the anti-aluminum erosion of TiB2-BN electrical ceramic fabricated by hot press, the microstructure and erosion mechanism of TiB2-BN ceramic were also analyzed.

2 Experimental 2.1

Fabrication of TiBz-BN ceramic TiB2-BN composite ceramic was produced by the

(Received: Scp. ll,2005;Aecepted: Jul. 13,2006) HE Ping(~x~-) : E-mail: he _ [email protected] * Corresponding author: WANG Weimin(~.~ L~ ):Prof. ; E-mail: shswmwang@ mail. whut. edu. en Funded by the National Natural Science Foundation of China ( No. 50372047)

Vacuumbody

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Fig. 1 Schematic diagram of the experimental testing furnace 2600 "~ 2400

9 10 cycle times 9 20cycle times 9 30 cycle ti.mes

) t j/A ~/'/

2200

9~2000 1800 1600 1000 ll'00 12'00 13'00 li00 15'00 li00 17'00 18;0Thermal

shock temperature/K

Fig. 2 The influence of processing parameters on the materials resistivity

hot-press sintering method. The titanium diboride powder (Ti > 67% ,B > 30%, average size 4 t~m, synthesized by SHS) and the hexagonal boron nitride powder ( 9 9 . 2 1 % , average size 0.64 tzm, sold in market) were ball milled with ethanol for 24 hours using stainless steel balls as the grinding media in a polyethylene pot. The milled slurry was then dried for 36 hours at 50 ~ After drying, the mixed powder was crushed and screened through a 80mesh sieve for subsequent sintering. Then the composite powder mixture was packed in a graphite die and hot pressing experiments were conducted at temperature of

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Journal of Wuhan University of Technology- Mater. Sci. Ed.

1800 ~C for one hour with an applied load of 30 MPa in vacuum atmosphere. 2.2 Thermal shock testing Shown in Fig. 1 is the schematic diagram of the experimental testing furnace. The testing system was designed to study the thermal shock resistance of TiB2-BN ceramic. The sample was heated to the temperature we designed in two or three minutes and was cooled to room temperature in several minutes, which is a cycle. We repeated the process above until the sample was destroyed.

3

Results and Discussion

3.1

Influence of the thermal shock temperature on the electricity property Resistivity is one of the important parameters of TiBz-BN composites. Fig. 2 shows the relationship between the resistivity and thermal shock temperature at different cycle times. It is clear in Fig. 2 that the resistivity grew with the increasd of the cycle times at the same thermal shock temperature, at the same time, the resistivity

Fig. 3

Dec. 2006

increases with the increase of the thermal shock temperature at the same cycle time. When the thermal shock temperature was 1773 K, the cycle times reached 40 times, which indicates that the TiB2-BN ceramic possesses a good thermal shock resistance. The influence of the processing parameter on the materials resistivity was caused by the changes of the materials microstmcture. Fig. 3 illustrates the influence of thermal shock temperature on the micmstructure of Ti~-BN composites. Before thermal shock, TiB2 grains were covered with BN grains, the density was high and the interface between TiB2 and BN grains was clear and close. After 40 times thermal shock circulation at 1473 K, the interface between TiB2 and BN grains loosened, with the increase of thermal shock temperature (1673 K ) , the cracks and pores can be found, the density of materials decreased, the resistivity increased. Mter many times thermal shock circulation, the microstmcture were destroyed.

The fracture surface SEM images of sample treated before and after thermal shock

Fig.4 The surface SEM images of sample treated by thermal shock

The reason is that TiB2-BN ceramic is subjected to a thermal shock temperature in repeated heating-cooling cycle, a temperature gradient inside materials generates, thereby, TiB2-BN ceramic experiences a thermal stress introduced by the temperature gradient, the interface microstmcture is weakened gradually, cracks and pores appear. Cracks expand and grow, pores become bigger, which results in the rupture and breakage of the conductive phase network with the increasing of thermal shock circulations, so the resistivity increases. For the same

Fig.5 The surface XRD pattern of sample treated by thermal shock

reason, the increase in cycle times at a thermal shock temperature will bring about the increase in the conductivity resistance. At the same time, the surface microstructure shown in Fig. 4 is changed evidently after thermal shock, some new phases and glassy phase coating are found on the surface of TiB2-BN ceramic in Fig.4(a) and ( b ) , which also brings about the increase in the materials resistivity. The XRD resuhs in Fig. 5 show that the new phases are the mixtures of TiB2, BN, TiBO3, TiO2 and Ti30s.

Vol.21 No.4

HE Ping et a/:Thermal Shock Resistanceand Erosion Resistanceof TiBz. . . .

119

Fig.6 The SEM imagesof Ti~-BN compositeswith ahtminumafter thermal shock 13BN + 14A1 --~ 13A1 + AtBn + B

( 1)

12BN + A1--~ A1BI2 + 6N2

(2)

BN + A1 ~ A1N + B (3) The AG~ of equation (3) is: GG~ = - 16.20 + 3 . 8 4 x 10 -3 T kcal/mol when T = 1800 K, AG~ = - 9.3, so the equation (3) can occur quickly at the work temperature 1800 K, we can presume that BN reaction with aluminum mainly follows equation (3). In order to open out the process of erosion of aluminum to TiB2-BN composites, EPMA was conducted to analyze the contact interface section between TiB2-BN sample and aluminum, shown in Fig. 7 are the results of EPMA. From the SEM image the interface section structure is layered, the line scanning analysis shows that aluminum mainly exists in the first layer, the concentration of aluminum reaches maximum at the interface between the reacted layer and TiB2-BN layer, the aluminum concenFig.7 EPMAfor fracture surface

tration is very low at the TiB2-BN layer, at the same

Erosion mechanism of aluminum to TiB2-BN ceramic In order to study the erosion resistance of TiB2-BN composites in melted aluminum, metal aluminum was put oo the surface of TiB2-BN sample, then the sample was heated to testing temperature and cool down to room temperature, after many times of the cooling-heating circulation, the microstrueture of Ti~-BN samples was evaluated by SEM and EPMA. Fig. 6 shows the microstructure of sample's surface, it is clear that there are two kinds of defected structure: one is porous, the other is cracking. Shown in Fig.6(a) and 6(b) are the porous zones magnified, it can be seen that some new phases are produced outside the holes, the new phase grows completely and has a clear crystal interface, the particle size is about 20 bun. Shown in Fig.6(c) and 6(d) are the zoomed crack zones, the microstrueture detail can be observed, a lot of small particles with about 3-4 ~ in diameter can be found. From Fig. 6 above, it is clear that TiB2-BN composites reacted with aluminum during thermal testing, relative literatures show that TiB, does not react with aluminum, but BN reacts with aluminum, the main reaction processes are as follows:

time, the boron element concentration in the reacted layer is lower than that in the TiB2-BN layer, these results

3.2

agree with the results of SEM and EDS, at the reacted layer, the BN phase reacts with aluminum, so the content of BN phase decreases obviously, the content of A1N increases. The element titanium concentration keeps constant, which indicates that the TiB: does not react with aluminum. During the cooling-heating circulation of TiB2-BN composites with aluminum, the main breakage mechanism includes thennal shock damnification and chemistry erosion damnification, both of them can accelerate the breakage process of TiB2-BN composites

4

Conclusions a) TiBz-BN muhiphase electric ceramic fabricated

by hot press is of good thermal shock resistance at high temperatures (1000 ~C,1200 ~C,1400 ~ ~C). b) When aluminum is vaporized on the surface of the TiB2-BN muhiphase ceramic in vacuum, the erosion of aluminum to TiBz-BN electrical ceramic follows such a reaction through BN phase: BN + A1-~ A1N + B, B dis-

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Journal of Wuhan University of Technology- Mater. Sci. Ed.

solves in melt aluminum and is evaporated. c) In TiB2-BN electrical ceramic, T i ~ is used to adjust the materials electrical resistance and BN is to ensure the materials property of thermal shock resistance and machinability. However, BN is of bad anti-aluminum erosion property, so it is suggested that the content of BN should be reduced as many as possible.

References [ 1]

[2]

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Dec. 2006

June-Ho Park, Yong-Ho Lee, Yong-Hag Koh, et al. Effect of Hot Pressing Temperature on Densification and Mechanical Properties of Titanium Diboride with Silicon Nitride as a Sintering Aid I J]. Journal of the American Ceramic Society, 2000, 83(6) : 1542-1544 [4] R Gonzalez, M G Barandika, A Villellas. New Binder Phase for the Consolidation of Ti~ Hardmetals[ J]. Mateffzdz Science and Engineering A, 1999,216:185-192 [ 5 ] J Zhao, X Ai, X P Huang. Relationship between the Thermal Shock Behavior and the Cutting Performance of a Functionally Gradient Ceramic Tool [ J ]. Journal of M a t e d Processing Technology, 2002, 129:161-166 [6] S Maensiri, S G Roberts. Thermal Shock of Ground and Polished Alumina and AI203/SiC Nanocomposites[ J 1. Journal of the European Ceramic Society,2002, 22:2945-2956 [7] A K Mukhopadhyay, S K Datta, D Chakrakraboty. Fracture Toughness of Structural Ceramics[ J ]. Ceramics International, 1999,25 : 447-454 [31