TiC in situ composites

Journal of Materials Processing Technology 139 (2003) 362–367

Fabrication of Ti/cluster diamond/TiC in situ composites K. Hanada a,∗ , N. Nakayama b , M. Mayuzumi b , T. Sano a a

Research Institute of Mechanical Systems Engineering, National Institute of Advanced Industrial Science and Technology, Namiki 1-2, Tsukuba, Ibaraki 305-8564, Japan b Technical Group, Tokyo Diamond Tools Manufacturing Co., Nakane 2-3-5, Meguro, Tokyo 152-0031, Japan

Abstract The present study describes on the fabrication process utilizing in situ method, and the microstructures and mechanical properties of Ti/CD/TiC composites. Ti powder of commercial purity, and GCD are mechanically mixed at 300 rpm, and are then consolidated by hot pressing in vacuum. In order to investigate the influence of processing parameters on the microstructure and mechanical properties, the mechanical mixing time and consolidating temperature are varied. XRD analysis indicates that the addition of GCD to Ti derived the in situ reaction of TiC under all of the consolidating temperatures used, consequently it could achieve to fabricate the Ti/CD/TiC composite expected. Microstructural observation shows that the Ti/CD/TiC composite fabricated had fine grain structure, in which many fine particles composed of TiC and diamond were dispersing. The resulting mechanical strength was enhanced substantially with an increase in the consolidating temperature and mechanically mixing time. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Cluster diamond; Composite; In situ; Indentation; Mechanical properties

1. Introduction Cluster diamond (CD) synthesized by a detonation method consists of ultra-fine diamond particle. The CD has the following unique properties: (1) ultra-fine particle of 5 nm on average; (2) single crystal; (3) excellent dispersibility; (4) high thermal conductivity; (5) extremely high mechanical strength; and (6) excellent lubrication and wear resistance. Various practical applications utilizing these unique properties have been studied and developed to date [1–6]. The main applications are as abrasives for precision polishing, and liquid and solid lubricant, etc. One of the promising applications is to disperse CD as a second phase in material. By doing so, a high performance composite that has the unique properties of CD can be fabricated according to various applications. For example, Okada et al. have reported that CD-dispersed Al composite shows very low friction and good wear resistance with self-lubricating ability, and that it has the possibility of realizing ultra-solid lubrication, where the coefficient of friction is less than 0.01 [7–10]. Such CD-dispersed composites have potential application to sliding components in micromachines, precision machines, medical machines, and other mechanical systems in which lubricating oil cannot be used. ∗ Corresponding author. E-mail address: [email protected] (K. Hanada).

In the present study, CD-dispersed Ti composite was fabricated using the powder metallurgy method for the purpose of developing a high performance composite, which has high potential application as a structural- and bio-material. The microstructures and mechanical properties of CD-dispersed Ti composite were examined, and the influence of the processing parameters on the microstructures and mechanical properties was investigated to optimize the fabrication process.

2. Experimental procedure 2.1. Materials Ti powder (average 7 ␮m) of commercial purity of more than 98.6 mass% was used as a base material, which has high potential application as structural- and bio-material. The authors’ previous work made it clear that the interfacial bonding between a base material and CD was weak [11]. Graphite cluster diamond (GCD) was therefore used as dispersant to derive in situ reaction between Ti and graphite in GCD. The in situ reaction derived probably forms TiC at the interface. Consequently, an improvement in the interfacial bonding will be achieved. Fig. 1 shows the morphology of GCD. The GCD consists of single crystal diamond particles covered with graphite layer of about 1 nm, and it has

0924-0136/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-0136(03)00531-4

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Table 1 Chemical compositions of Ti powder and GCD (mass%)

Ti powder GCD

Ti

Fe

Si

Mn

Mg

Cl

C

N

O

H

Balance 0.002

0.03 1.99

0.01 0.20

0.002 0.01

0.001 0.003

0.01 –

0.002 Balance

0.06 2.69

1.20 5.24

0.06 0.65

a mean particle size of 7 nm. The chemical compositions of Ti power and GCD used in this experiment are shown in Table 1.

gular pyramid in shape. The specimens were dry polished by −4000 mesh, and high-pressure air was then blown on the polished surface, as a prior treatment for the measurements.

2.2. Processing 3. Results and discussion CD-dispersed Ti composites were fabricated using powder metallurgy process. Ti powder and GCD of 5 vol.% were enclosed into a mixing vessel with stainless steel balls, and were mechanically mixed at 300 rpm in Ar atmosphere. The ratio of balls to the powder enclosed was 10:9. The mixing time was varied from 0 to 6 h to investigate the influence of mixing time on the microstructure and mechanical properties of GCD-dispersed Ti (Ti/GCD) composite. The mechanically mixed powders were consolidated by hot pressing in vacuum. The consolidating temperature was also varied from 973 to 1173 K to investigate an optimum consolidating temperature. The composite powders consolidated were worked to specimens for the microstructural observation and mechanical measurements. 2.3. Evaluation Composites fabricated at various processing conditions were evaluated as to their microstructure and mechanical properties. X-ray diffraction (XRD) analysis was performed to investigate changes in the crystal structure of the mechanically mixed and consolidated powders in the composite fabrication process. Microstructural observation was made using optical microscopy and transmission electron microscopy (TEM). To examine the mechanical properties, indentation measurements were made by Nano-Indenter ENT-1100 (Elionix) with a load of 0.049 N for 1 s. The tip of the indenter used in the measurement is a diamond trian-

Fig. 1. Morphology of GCD.

3.1. Mechanically mixed powders For Ti/GCD composite powders mechanically mixed at various mixing times, XRD analysis was made to examine changes in the crystal structure, and the formation of TiC. Fig. 2 shows the XRD patterns obtained. All patterns of mechanically mixed composite powders indicate that the in situ formation of TiC could not be obtained in mechanical mixing process. Also all peaks of Ti become weak by mechanical mixing. The lowering of the peak strength of Ti is probably due to a phase transition of Ti to amorphous. Fig. 3 shows the influence of mechanical mixing time on the phase transition of Ti/GCD composite powder. The ratio of phase transition to amorphous increases rapidly up to 66% at the mixing time of 2 h, but it does not change very much for more than 2 h. The increase in amorphous phase is expected to contribute to fine grain structure, and consequently to high mechanical strength in consolidated composite powders. 3.2. Microstructures Ti/GCD composite powders mechanically mixed for 2 h were consolidated at various temperatures. In order to

Fig. 2. XRD patterns of Ti/GCD composite powders mechanically mixed for various mixing times.

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Fig. 3. Influence of mechanical mixing time on the phase transition of Ti/GCD composite powder.

examine change in the microstructure, XRD analysis was performed for all of the composite powders consolidated. As shown in Fig. 4, the peaks of TiC were obtained under all consolidating temperatures, indicating that the in situ reaction between Ti and graphite (C) in GCD was obtained in the consolidating process. However, the peak of GCD could not be obtained because a high background of Ti covered the peaks or related ones. Fig. 5 shows the optical micrographs of Ti/GCD composites at various consolidating temperatures. The composite powders consolidated at 973 K has high porosity levels, and the prior particle boundaries of the composite powders were observed clearly in the microstructures. However, the microstructure can be significantly improved at more than 1073 K. Subsequently, Ti/GCD composite powders mechanically mixed at various mixing times were consolidated at 1173 K, in order to examine the influence of mixing time on the microstructure and mechanical properties. Fig. 6 shows the XRD patterns. The XRD result reveals that the mechanical mixing time does not affect the in situ reaction of TiC. A small quantity of residue obtained by HF treatment was weighed for all of the XRD samples, and it was found that

Fig. 4. XRD patterns of Ti/GCD composite powders consolidated at various temperatures.

Fig. 5. Optical micrographs of Ti/GCD composites at various consolidating temperatures: (a) 973 K; (b) 1073 K; (c) 1173 K.

Fig. 6. XRD patterns of Ti/GCD composite for various mechanical milling times.

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Fig. 8. TEM micrographs of Ti/GCD composite: (a) grain; (b) fine particle (TiC + diamond).

Fig. 7. Optical micrographs of Ti/GCD composites for various mixing times: (a) Ti; (b) 2 h; (c) 4 h; (d) 6 h.

the amount of residue was about 22% by weight of Ti/GCD composite for all of the mixing times. The residue seems to be composed of CD and TiC. Fig. 7 shows the optical micrographs of Ti/GCD composites for various mixing times. All of the composite samples have a marbled-microstructure composed of TiC dispersed in the matrix. The microstructure is further homogenized with an increase in the mixing time. High densification could be achieved at 4 h. TEM observation was carried out for the composite mechanically mixed for 6 h. Fig. 8 shows TEM micrographs of the composite. The composite has fine grain structure less than 0.5 ␮m. Numerous fine particles of about 7 nm can be observed in the grains, and also observed on the grain boundaries. Selected-area diffraction analysis clarified that these particles consist of TiC and diamond. However, detailed TEM observation is necessary to investigate the interface between TiC and diamond. Summarizing the results of the microstructural observations, the addition of GCD to Ti secured the in situ reaction

Fig. 9. Influence of consolidating temperature on the Young’s modulus, and the hardness and the depth of indentation.

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The Young’s modulus indicates 142 GPa as maximum at 4 h, while the hardness indicates 1149 as maximum at 6 h. The maximal values are a 60 and 94% increase, respectively, compared to unreinforced Ti. Through the microstructural observation, it was revealed that the microstructure was homogenized with an increase in the mixing time, and that the amount of the mixture of TiC and diamond dispersing in the Ti/CD/TiC composites was a constant 22 wt.%. Therefore, the large improvement in the Young’s modulus and the hardness of the indentation is presumably attributable to the homogenization in the microstructure, in addition to the formation of TiC. In the hardness of the indentation, the elastic depth of indentation does not change very much with the mixing time. However, the plastic depth decreases gradually with an increase in the mixing time. From the results of the indentation measurements, it was found that the mechanical properties could be largely improved by adding GCD to Ti, and that the optimum consolidating temperature and mixing time were 1173 K and 6 h, respectively.

4. Conclusions From the experimental results, it is possible to conclude the following:

Fig. 10. Influence of mechanical mixing time on the Young’s modulus, and the hardness and the depth of indentation.

of TiC, and consequently has achieved the fabrication of the Ti/CD/TiC composite expected. 3.3. Mechanical properties For the Ti/CD/TiC composites fabricated at various consolidating temperatures and mixing times, indentation measurements were performed to examine the Young’s modulus, hardness and depth of indentation. In this case, the hardness number of the indentation is equal to the Vickers hardness. Fig. 9 shows the influence of consolidating temperature on the Young’s modulus, the hardness and the depth of indentation. The Young’s modulus and the hardness of the indentation are enhanced with an increase in the consolidating temperature, and reach the respective maximal values of 119 GPa and 809 at 1173 K. The large increase in the Young’s modulus and the hardness of the indentation is probably due to the formation of TiC. The elastic and plastic depths of indentation decrease as the consolidating temperature increases. Fig. 10 shows the influence of mechanical mixing time on the Young’s modulus, and the hardness and depth of the indentation. The Young’s modulus and hardness rise rapidly with an increase in the mixing time. However, the Young’s modulus does not change very much beyond 6 h.

(1) The addition of GCD to Ti secured the in situ reaction of TiC, consequently it could achieve to fabricate the Ti/CD/TiC composite expected. (2) The Ti/CD/TiC composite fabricated had a fine grain structure in which many fine particles composed of TiC and diamond were dispersed. (3) In future work, detailed microstructural observations should be carried out to examine the interface of Ti/CD/TiC. (4) The indentation measurements clarified that the mechanical properties could be largely improved by adding GCD to Ti, and were enhanced with an increase in the consolidating temperature and mechanical mixing time. (5) The optimum consolidating temperature and mixing time were 1173 K and 6 h, respectively.

Acknowledgements This study was carried out under the NEDO project “Development of Ultra-Solid Lubricant with Cluster Diamond”. The authors are very grateful to Mr. Uematsu and Mr. Komazawa (Elionix) for their help in the indentation measurements.

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