TZP nanocomposites

Materials Science and Engineering A 447 (2007) 83–86

Mechanical properties and microstructure of TiN/TZP nanocomposites Songlin Ran, Lian Gao ∗ State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, PR China Received 17 July 2006; received in revised form 16 August 2006; accepted 12 October 2006

Abstract TiN/TZP nanocomposite powders were obtained by ball milling with titanium nitride (TiN) nanopowders and tetragonal zirconia polycrystal (TZP) powders as starting materials. TiN/TZP nanocomposites were prepared by hot pressing sintering. The effects of TiN nanoparticles on the relative density, mechanical properties and microstructures of TiN/TZP nanocomposites were studied. Experimental results show that the addition of TiN nanoparticles did not decrease the sintering ability of the composites; the Vickers hardness of the composites reached to 15 GPa when the content of TiN is 20 vol%; the bending strength of the composite with 5 vol% TiN fillings can be enhanced with 200 MPa higher than pure TZP ceramic. The dispersivity of TiN particles has a distinct effect on the strength of the composites. © 2006 Elsevier B.V. All rights reserved. Keywords: TiN; TZP; Nanocomposite; Mechanical property; Microstructure

1. Introduction Partially stabilized zirconia and, in particular, tetragonal zirconia polycrystal (TZP) materials have excellent mechanical properties, such as high bending strength and fracture toughness, due to the stress-induced transformation from metastable tetragonal (t-ZrO2 ) to the monoclinic phase (m-ZrO2 ) [1,2]. However, the modest hardness of these materials limits their use for wear applications. In the 1990s, Niihara [3] reported that incorporating nanoscale-sized particles into a ceramic matrix could increase the mechanical properties dramatically. For ZrO2 matrix, ZrO2 –SiC [4], ZrO2 –WC [5–7], ZrO2 –Al2 O3 –TiC [8], ZrO2 –Cr2 O3 , ZrO2 –Cr3 C2 and ZrO2 –Cr7 C3 [9,10] systems have been reported for improving mechanical properties. Although excellent bending strength and toughness values could be achieved with incorporating nanoparticles into the ZrO2 matrix, the increase in the hardness aimed at by the addition of the hard secondary phases was rather modest [7]. Titanium nitride (TiN) has high melting temperature (2950 ◦ C), high microhardness (21 GPa), good electroconductivity and high resistance to corrosion and oxidation [11,12]. These attractive physical and chemical properties suggest potentialities of TiN as a good reinforcement agent to the ceramic ∗

Corresponding author. Tel.: +86 21 52412718; fax: +86 21 52413122. E-mail address: [email protected] (L. Gao).

0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.10.026

matrix. Recently, TiN reinforced ZrO2 composites have been studied. For example, Salehi et al. [13] reported ZrO2 –TiN composites with a TiN content ranging from 35 to 95 vol%. Ostrovoy et al. [14] and Sun et al. [15] studied ZrO2 –Al2 O3 –TiN composite ceramics. To the best of the authors’ knowledge, however, information regarding ZrO2 composites with ≤35 vol% TiN is very limited. In this paper, TiN/TZP composites with 5–20 vol% TiN were obtained by hot pressing sintering. Mechanical properties and microstructure of these composites were studied also. 2. Experimental procedures Industrial grade TiN (average particle size of 20 nm, China) and ZrO2 (grade TZ-3Y-E, Tosoh, Japan, detailed chemical compositions are shown in Table 1.) were used as starting materials. They were mixed together on a multidirectional mixer for 24 h, using zirconia grinding medium in absolute ethanol. After dried and calcined, the powders obtained were grinded with agate mortar and pestle to pass through a 200-mesh sieve. Sintering process was conducted by the uniaxial hot pressing (30 MPa) in an argon atmosphere. The sintered ceramic blocks were cut into beams with a size of 5 mm × 2.5 mm × 30 mm, while the edges and tensile surface of the samples were chamfered and mirror polished before the strength measurement. The densities of the samples were measured by the Archimedes method in distilled water. The bending strength was

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S. Ran, L. Gao / Materials Science and Engineering A 447 (2007) 83–86

Table 1 Chemical analysis of ZrO2 powder (TZ-3Y-E) Compositions

Content (wt %)

ZrO2 Y2 O3 Al2 O3 SiO2 Fe2 O3 Na2 O

– 5.15 ± 0.20 0.25 ± 0.10 ≤0.02 ≤0.01 ≤0.04

measured using a three-point bending test on an Instron universal testing machine with a span of 20 mm and a loading speed of 0.5 mm/min. Toughness and hardness were measured by Vickers indentation method at a load of 10 Kg and dwelling time of 10 s. The phase compositions of the composites were characterized by X-ray diffraction (XRD, Model D/MAX-2550V, Rigaku Co., Tokyo, Japan). Microstructure of the composite was examined by field emission scanning electron microscopy (FE-SEM, Model JSM-6700F JEOL, Japan) on the polished and fractured surfaces.

Fig. 2. XRD pattern of polished surface of (a) pure TZP and (b) 20 vol% TiN/TZP composite. Sintering temperature: 1500 ◦ C.

Fig. 1 shows the relative densities of 20 vol% TiN/TZP nanocomposites sintered at different temperatures. It can be seen that the relative densities of all the samples hot-pressed from 1300 to 1550 ◦ C are higher than 98% of the theoretical density, which was calculated according to the rule of mixture using 6.05 g/cm3 for TZP and 5.44 g/cm3 for TiN, assuming that no chemical reactions take place between the matrix material TZP and the second phase TiN. This densification conditions have the advantage over those reported by others [13,15]. It suggests that TZ-3Y-E has a better sintering ability, just as that was propagandized by the Tosoh Corporation in their website. Fig. 2 gives the XRD patterns of polished surface of (a) pure TZP; (b) 20 vol% TiN/TZP composite sintered at 1500 ◦ C. Tetragonal zirconia (t-ZrO2 ) and TiN were the main crystalline phases existing in the 20 vol% TiN/TZP composite, with a small quantity of monoclinic zirconia (m-ZrO2 ). The absence of other new phase in the composite indicates that there is a good chem-

ical compatibility between ZrO2 and TiN. The presence of mZrO2 phase can be attributed to the spontaneous transformation of a small amount of t-ZrO2 to m-ZrO2 during the cooling process. The intensity of m-ZrO2 phase in the 20 vol% TiN/TZP composite is weaker than that of m-ZrO2 phase in the pure TZP. The Vickers microhardness of the TiN/TZP composites as function of TiN content is graphically illustrated in Fig. 3. The microhardness of the composites was strongly influenced by the TiN content and increased with the increase of TiN content. The microhardness of the 20 vol% TiN/TZP composites reached 15 GPa. According to literature [7], the microhardness of the 20 vol% WC/TZP composites can only be comparable with pure TZP (3Y). To get the same microhardness as that of 20 vol% TiN/TZP composite, the WC content in the composite should be increased to 40 vol%. The result indicates that TiN is more effective than WC to enhance the hardness of TZP matrix. Fig. 4 presents the bending strength of the TiN/TZP composites as function of TiN content. The bending strength increases to a maximum at first, and then begins to decline with the increased content of TiN. The maximum strength is 1.3 GPa, 200 MPa higher than that of TZP matrix. Fig. 5 shows the SEM micrographs of polished surface of TiN/TZP composites with (a) 5 vol% TiN and (b) 20 vol% TiN fillings. The micrographs reveal that the samples were fully densified since no pores were

Fig. 1. Relative density of 20 vol% TiN/TZP nanocomposite vs. sintering temperature.

Fig. 3. Vickers hardness of the TiN/TZP composite as function of TiN content. Sintering temperature: 1500 ◦ C.

3. Results and discussion

S. Ran, L. Gao / Materials Science and Engineering A 447 (2007) 83–86

Fig. 4. Bending strength of the TiN/TZP composite as function of TiN content. Sintering temperature: 1500 ◦ C.

found on the cross-sections, which was consistent with the measurement of density. Only ZrO2 (bright) and TiN (grey) can be distinguished in the micrographs, whereas Al2 O3 phase cannot be found because of its small amount of content. Comparing Fig. 5(a and b), it can be seen that the dispersing of 5 vol% TiN nanoparticles in the TZP matrix was more homogeneous than

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that of 20 vol% TiN nanoparticles. The size of TiN particles in the matrix increased when the content of TiN was enhanced. Higher content of TiN nanoparticles resulted in the bad dispersivity and aggregation, which may explain the decline of the bending strength of the composites. Fig. 6 gives the SEM micrographs of fracture surface of pure TZP and 20 vol% TiN/TZP composites. The two micrographs indicate that the fracture patterns of the TZP and TiN/TZP composites are mainly intergranular. The inset of Fig. 6(a) indicates the average crystallite size of the matrix is about 100 nm. When introducing TiN particles into Al2 O3 matrix, the fracture toughness of TiN/Al2 O3 nanocomposites was enhanced because of crack pinning and crack deflection [16]. Crack deflection in TiN/TZP composites was observed obviously, as illustrated in Fig. 7. It should result in the increase of the toughness of the composites. However, according to Fig. 8, the toughness of the composites decreases a little with the addition of TiN particles. Crack deflection and transformation of t-ZrO2 to m-ZrO2 were the main factors which affect the toughness of TiN/TZP composites. The failure or weakening of the phase transformation toughening was the only factor that can account for the decline of toughness, since crack deflection was still on opera-

Fig. 5. SEM micrographs of polished surface of TiN/TZP composite with (a) 5 vol% TiN and (b) 20 vol% TiN. Sintering temperature: 1500 ◦ C.

Fig. 6. SEM micrographs of fracture surface of: (a) pure TZP and (b) 20 vol% TiN/TZP composite. Sintering temperature: 1500 ◦ C.

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4. Conclusions (1) TiN/TZP composites have a good sintering ability. When sintered at 1300 ◦ C, the relative density of 20 vol% TiN/TZP composite can reach 98%. The incorporation of TiN nanoparticles did not decrease the sintering ability of TZP. (2) TiN nanoparticles can enhance the microhardness of the TZP matrix and are more effective than WC nanoparticles. (3) The addition of 5 vol% TiN nanoparticles can improve the bending strength of TZP matrix with an increase of 200 MPa. The dispersivity of TiN particles has a distinct effect on the strength of the composites. Fig. 7. Crack propagation in TiN/TZP composite with 20 vol% TiN. Sintering temperature: 1500 ◦ C.

Acknowledgements This work was supported by the National Natural Science Foundation of China and the Shanghai Nanotechnology Promotion Center under the grant number of 50402006 and 0452nm086, respectively. References

Fig. 8. Fracture toughness for the TiN/TZP composite as function of TiN content. Sintering temperature: 1500 ◦ C.

tion. It is well known that there is a strong dependence of the transformation toughening effect in zirconia on grain size and the stability of t-ZrO2 increases with the decrease of the grain size [17,18]. The incorporation of a nanosized second phase can decrease the grain size of the matrix [19], so the decline of toughness of the TiN/TZP composites was attributed to overstabilization of the tetragonal phase by the small grain size, such that it could not transform to the monoclinic phase upon introduction of a crack. In addition, the diffusion of titanium into the TZP matrix also decreases the toughness of the composites [2].

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