ELSEVIER Journal of Materials Processing Technology 67 (1997) I 12-116

Jonmal of

Materials Processing Technology

Mechanical alloying of a TiC-TiN ceramic system

S. Zhang a,,, S.C. Tam b a Gintic Institute of Manufacturing Technology, Nanyang Technological University, 71 Nanyang Drive, Sfl:gapore 638075, Singapore b School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang 4cemw, Singapore 639798, Singapore

Abstract

Reported here is a preliminary study of the mechanical alloying (MA) process of an all-ceramic-phases component, i.e., TiC + TiN. The respective ceramic powders were mixed in weight proportions of 50:50 and 70:30 and milled in a planetary ball mill at a ball-to-powder weight ratio of 20:1. High angle XRD peaks were used to calculate the lattice parameters before and after milling for different milling times. The lattice parameter measurements suggested that Ti(C,N) solid solution was formed during the MA process. Inter-particle necking was observed. The rate of the solid solution reaction seems to be independent of the compositional change in the TiC-TiN mixture. Particle size refinement is achieved mostly during the first few hours of milling. © 1997 Elsevier Science S.A.

Keywords: Mechanical alloying; TiC-TiN ceramic system; Ceramic

1. Introduction

Traditionally, the raw materials used in mechanical alloying (MA) must include at least one fairly ductile metal to act as a host or binder to hold together the other ingredients. Davis and Koch [1] and Davis et al. [2], however, mechanically alloyed a brittle Si-Ge sys- tem to obtain solid solution Si(Ge) and observed lattice change with the increased duration of MA. Zdujic et al. [3] milled ZnO'AI~O3 powder mixtures and obtained ZnAI204 spinel. Zhang and colleagues [4] employed carbides and nitrides in mechanical alloying and found that TaC and WC can be mechanically alloyed into Ti(CN) to form solid solution (Ti,W,Ta)(C,N).

Titanium carbonitride is a source powder in making high toughness carbonitride cermet cutting tools. Suc- cess in producing this hard phase solid solution by mechanical alloying not only opens up a whole new avenue in the cermet cutting tool industry, but also casts new light on the theoretical development of the mechanical alloying process. In [5], Ti(C,N) was used as an initial component. Is it possible to form Ti(C,N) from TiC and TiN powders using the mechanical alloy- ing technique? The present paper addresses this ques- tion.

* Corresponding author. Fax: + 65 7922779; e-mail: szhang@gintic.gov.sg.

092,0136/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. S0924-0136(96)02828-2

2. Experimental

Commercially obtained ceramic powders of TiC (2 ~ 5 pm) and TiN (10 ~ 20 Bm) (Fig. I) were used in the studies. Two compositions were explored: 70% TiC + 30% TiN and 50% TiC + 50% TiN (by weight) MA was conducted for up to 80 h in a Fritsch P5 planetary ball mill at a ball-to-powder weight ratio of 20:1. The milling speed was set at 650 rpm (or a turntable speed of 300 rpm). The alloying was interrupted for sampling after different time intervals. X-Ray diffraction patterns were obtained on a Philips MPD 1880 PW 1700 ~,owder diffractometer at 45 kV and 30 mA from 20 = 30 to 150" at a step size of 0.0i °. Microstructural develop- ment was characterized on a Cambridge $360 scanning electron microscope. High angle diffraction patterns (20 > 90 °) were used for lattice parameter measure- ments [4]. The half-peak width of the XRD peak was used to estimate the crystallite size as a function of MA time by the Debye-Scherrer equation [3,6]:

D= K2/fl cos 0 (1)

where K is a constant (K = 0.91), D is the mean crystal- lite dimension normal to the diffracting planes, 2 is the X-ray wavelength (2 = 0.15406 nm for Cu target), fl is the peak width (in rad) at half maximum peak height and 0 is the Bragg angle.

S. Zhan~, S.C. Tam Journal q~ Materials Pr,ce.~.~i,g Techmd~J~y 67 (1997) 112 116 1~3

(a)

(b)

Fig. 1. The raw powders of:

3. Results and discussion

3.1. Lattice parameters

Since the lattice constant of TiC is larger and that of TiN is smaller, if solid solutioning of TiC and TiN does occur, a decrease in the lattice constant of TiC and an increase in that of TiN would be expected. Figs. 2 and 3 plot the lattice parameter as a function of MA time for 50% TiC + 50% TiN milled with WC balls and for 70% TiC + 30% TiN with steel balls, respectively. Since the high-angle XRD peaks tend to be buried in the

(a) TiC; and (b) TiN.

increasing background after extended milling, no lattice data was available after MA for over 40 h.

It is seen from Figs. 2 and 3 that both of the compositions show a similar trend in lattice parameter change, irrespective of the milling media: the TiC lattice decreases whilst the TiN lattice increases with increas- ing MA time. Therefore, it is indicated strongly that solid solution did occur during the MA process. By comparing Figs. 2 and 3, it is also noted that the rate of change of the lattice parameter with MA time is almost the same for the two compositions, even though in the c~tse of 50% TIC-50% TiN the data is more scattered

114 $. Zhang, S.C. Tam /Journal of Materials Processing Technology 67 (1997) 112-116

4.38

4.34 u

E 4.32

4,3o

4,28

-~ 4.26 1

4.24

4.22

4.20 0

I 50%TiC+50%TiN O TiC • TiN

O--

I . I i I i I ,

10 20 30 40 MA Time (h)

50

Fig. 2. Lattice parameters as a function o f MA time for 50% TiC + 50% TiN.

initially: this shows that in MA, th~ reaction rate is independent of the 'concentration' of the 'reactant' as long as the latter is not depleted, which is different from a chemical reaction, where the reaction rate is proportional to the concentration of the reactants.

ra~

200

180!

160

140

120 s

100

80

60

4O

20

0 0

50%TiC+50%TiN

o o o O

O TiC @ TiN

10 20 30 40 50 MA Time (h)

Fig. 4. Particle size refinement vs. MA time.

tallite size seems to be insensitive to the initial particle size, this result being also in agreement with those obtained in the mechanical alloying of ZnO-AI203 [3] and Ti(C,N) + WC + TaC [5].

3.3. Particle size reduction

3.2. Crystallite size refinement

The broadening of the XRD peaks indicates a de- crease in the coherent crystalline domain or crystallite size and an accumulation of lattice strain [3,7,8]. As a first order approximation, Fig. 4 shows the crystallite size evolution as a function of MA time based on calculations done for the (220) peak of TiC and TiN for the mixture of 50% TiC + 50% TiN: similar results were obtained for other composition.

Iris seen clearly that the reduction in size is achieved mostly in the first few hours of milling: after that, further size reduction is not significant. The final crys-

i 4.34(

4.32

4.30 8 ~. 4.28 a

4.26

4°I 1 4.38 70%TiC+30%TiN

4.36

4.24 q

4.22

4.20 0

0 TiC ® TiN

"U"

.---.0""

10 20 30 40 50 MA Time (h)

Fig. 3. Lattice parameter as a function o f MA time for 70% TiC + 30% TiN.

The reduction in particle size is very rapid in the beginning of milling. After only one hour of milling, the size of the particles of both of the components (TiC and TiN) is greatly reduced, as shown in Fig. 5, even though the particles are still very irregular in shape. This is in agreement with the reduction in crystallite size, as illustrated in Fig. 4.

Fig. 6 shows the particle size after 40 h of MA. The morphology of the particles is quite uniform. The parti- cles are basically spherical and the size is estimated at around 0.5 l~m, this morphology and size remaining the same up to 80 h of MA.

Davis and Koch [1] observed inter-particle necking during mechanical alloying of Ge-72 wt% Si. In the present experiments, a similar phenomenon was ob- served (Fig. 7). Localized temperature rise and plastic- ity may be an explanation of the observed necking: however, the temperature rise in MA is measured as less than 150°C [5], which alone can not possibly dictate any significant change in the system. The complex stress states in the compressed powder particles may be an- other source of explanation. Detailed understanding of the formation of this inter-particle necking requires rigorous study of the thermal and mechanical aspects of the MA process.

4. Conclusions

The mechanical alloying of ceramic components TiC + TiN powder mixtures has been attempted. Lat-

S. Zhang , S,C. Tam / Jottrnal t~l Mater&ls Processing Iechnoh~gy 67 11997) 112 ~ i16 Ii ~.

Fig. 5. SEM particle morphology of the 50% TiC + 50% TiN mixture after MA for i h.

Fig. 6. SEM particle morphology of the 50% TiC + 50% TiN mixture after MA for 40 h.

tice parameter measurements suggested that Ti(C,N) solid solution was formed during the MA process. The rate of the solid solution reaction seems to be indepen- dent of the compositional change in the TiC-TiN mixture. Particle size refinement is achieved mostly during the first few hours of MA.

Acknowledgements

The funding of this work came from the grant NTU ARP 66/91. The authors are thankful to Mr. Soh Meng Horing for taking the SEM photos and assisting with the data collection.

116 s: !f Materials Processing Tech~wlogy 67 (1997) 112-116

Fig. 7. Inter-particle necking in the 50% TiC + 50% TiN mixture (MA for 5 h).

References

[i] R.M. Davis and C.C. Koch, Mechanical alloying of brittle components: silicon and germanium, Scr. Metall., 21 (3) (1987) 305-310.

[2] R.M. Davis, B. McDermott and C.C. Koch, Mechanical alloying of brittle materials, Met. Trans. A, 19A (12) (1988) 2867-2874.

[3] M.V. Zdujic, O.B. Milosevic and L.C. Karanovic, Mechanochemical treatment of ZnO and AI,O 3 powders by ball milling, Mater. Left., 13 (1992) 125-129.

[4] S. Zhang, C.D. Qin and L.C. Lira, Solid solution extent of WC and TaC in Ti(C,N) as revealed by lattice parameter increase,

hit. J. Refract. Met. Hard Mater., 12 (1993-94) 329-333. [5] S. Zhang, K.A. Kohr and L. Lu, Preparation of Ti(C,N)-WC-

TaC solid solution by mechanical alloying technique, J. Mater. Process. Technol., 48 (1995) 779-784.

[6] H.P, Klug and L.E. Alexander, X-Ray Diffraction Procedures jot Polycrystalline and Amorphous Materials, Wiley-lnterscience, 1974.

[7] J. Eckert, J.C. Holzer, C.E. Krill III and W.L. Johnson, Re- versible grain size changes in ball-milled nanocrystalline Fe-Cu alloys, J. Mater. Res., 7(8)(1992) 1980-1983.

[8] L. Takacs, Reduction of magnetite by aluminum. A displace- ment reaction induced by mechanical alloying, Mater. Lett., 13 (2-3) (1992) 119-124.