Fabrication of TiN Particle-Dispersed Al2O3 Composites Utilizing High N2-Pressure SHS

Fabrication of fine TiN particle-dispersed dense Al2O3 composites with the compositions of Al2O3/TiN=100/0~90/10 vol% has been conducted from Al2O3/(Ti,TiN0.3) mixed powder compacts by capsule-free hot isostatic pressing (HIP) utilizing high-pressure N2 SHS. Fine Ti powders (φ ~ 0.3 μm) with TiN0.3 phase were prepared by thermal decomposition of planetary ball-milled fine TiH2 powders at 400°C (673 K) for 1 h in a vacuum, followed by heating in N2 at 200°C (473 K) for 2 h. The Al2O3 powder compacts (relative densities of 57.2-57.8%) with homogeneously dispersed (Ti,TiN0.3) particles were prepared. The mixed powder compacts were hot isostatically pressed (HIPed) under the conditions of 1350°C (1623 K) at 7 MPa N2 for 1 h, followed by the heating at the same temperature for 2 h under 196 MPa-N2. At the first stage of heating [1350°C (1623K)/7MPa/1h], solid/gas reaction of SHS between (Ti,TiN0.3) and N2 was introduced to form TiN and densification of the Al2O3 powder compacts up to the relative density of 92-93% with closed pores was performed. And at the sequent second stage [1350°C (1623K)/196MPa/2h], densification of the most of pre-sintered composites consisting of Al2O3 and TiN reached higher relative densities than 98.5%. Dispersion of TiN particles (~φ 0.30 μm) in the composites suppressed the grain growth of Al2O3 during HIP-sintering. Mechanical properties, such as bending strength (σb), Vickers hardness (HV), fracture toughness (K1C), and electrical resistivity (ρ) of the composites were evaluated as a function of TiN content; the maximum values of σb=640 MPa, HV=19.5 GPa, and KIC=4.5 MPa・m were obtained in the Al2O3/TiN=97/3~95/5 vol% composites. Among the composites, the lowest ρ value of 2.6×10 Ω・m was attained at Al2O3/TiN=90/10 vol% composite.


Introduction
Metal nitrides reveal often desirable combined properties, such as high melting temperatures, high hardness, low density (light weight), low electrical resistivity, excellent wear resistance and high chemical stability. Therefore, they have been widely used in semiconductor industries and other applications. Among them, titanium nitride (TiN) shows excellent properties and it has been attracting increasing interest as a constituent of composites for widely applications, such as cutting tools, tool coating, microelectronics [1], and solar-control films.
A lot of research-papers concerning on Al 2 O 3 /TiN composite materials have been published, and many of them treated the sintering of mixtures of Al 2 O 3 and TiN powders [2][3][4]. In general, TiN powders commercially available consist of large particles because TiN cannot be crushed into fine powders with a conventional milling process due to its excellent high hardness. Therefore, the addition of large TiN particles to other matrix materials often resulted in reduction of original mechanical properties, and their improvement in the performance of composite materials cannot be expected. Therefore, fine TiN particles are required to fabricate dense TiN-added composites with excellent mechanical properties. However, metal Ti powders as starting material for TiN cannot be crushed by conventional ball-milling process because of its high ductility. It was reported that fine metal Ti powders were prepared by the reduction of metal oxides, such as, heat-treatment of fine TiO 2 powder under nitrogen [5] or NH 3 [6].
In the present study to prepare fine metal Ti powders, brittle TiH 2 was chosen as starting material, because this compound can be mechanically crushed. Submicron-meter size (Ti,TiN 0.3 ) powders were produced by heating ballmilled TiH 2 in a vacuum. Al 2 O 3 /(Ti,TiN 0.3 ) mixed powder compacts were fabricated, in which thus obtained fine (Ti,TiN 0.3 ) powers were uniformly distributed in the Al 2 O 3 powders. And moderate nitrogen-pressure was applied at the first stage of HIP sintering in order to both transform (Ti,TiN 0.3 ) to TiN by SHS phenomenon of TiN and sinter the powder compacts to a nearly 92~93% relative density without using a gas-tight metal or glass capsule (can). Then, at the sequent second stage of HIP sintering high pressure N 2 densified the presintered bodies to the almost full-density. This capsule-free HIPing method at one time has many merits from industrial view points, for example, low production cost, free-selection of shape and size of the sintered compacts without pre-sintering of green bodies. This capsule-free high-pressure N 2 HIPing can also make it possible that metal nitrides can be formed, even though this metal cannot be nitrided under atmospheric N 2 pressure at high temperatures. Simultaneous synthesis of titanium nitride directly from Ti and the mechanical properties of the fine TiN particles-distributed Al 2 O 3 composites were described in relation with their microstructures.

Preparation of TiH 2 powder
Starting powder was TiH 2 (TSHT, Osaka Titanium Technologies Co. Ltd, Hyogo, Japan, particle size P s ≤ 45 μm, ~99.98% purity). Two kinds of pulverization methods were employed to crush or mill the brittle powder, and the grinding capability difference between them was compared. One pulverization method was that the powder was crushed with a planetary ball mill (P-5, Fritsch Japan, Yokohama, Japan) using stainless-steel SUS balls (φ10 mm in diameter) for 100 h at a rotating speed of 200 rpm (centrifugal force about ∼ 4.36 g: here, "g" is a gravitational acceleration unit) with a milling media vs. powder ratio of 10:1 in Ar. The other pulverization method was that after crushing with a mortar and pestle for 20 minutes then the powder was milled with a planetary ball mill (P-5, Fritsch Japan) using tungsten carbide WC balls (φ5 mm in diameter) for 3 h at 300 rpm (~12.1 g) in Ar. And then the powder was further ground with a planetary ball mill (Premium Line P-7, Fritsch Japan) using partially stabilized zirconium (PSZ) balls (φ1 mm in diameter) for 10 min at 1100 rpm (~83.9 g) in ethyl-alcohol and Ar.

Preliminary experiment to determine the crystalline phase of dehydrated TiH 2 powder
Ball-milled TiH 2 powders were dehydrated by heating at 400°C (673 K) for 1 h in a vacuum and a sequent heating at 200°C (473 K) for 1 h in N 2 . The crystalline phase of powders after the dehydrations was identified by X-ray diffraction (XRD) analysis (CuKα 1 radiation with a graphite monochromator, Rint 2000, Rigaku, Osaka, Japan). As will be described later, the powders after dehydration heattreatment gave the Ti (JCPDS#44-1294) or TiN 0.3 (JCPDS#41-1352) phases; Ti phase was obtained via ball-milling with SUS balls, on the other hand, TiN 0.3 phase was prepared via a combined ballmilling process with WC and ZrO 2 (PSZ) balls. This might be explained in terms of reactivity difference in ball-milled TiH 2 ; a combined ballmilling with smaller WC and ZrO 2 (PSZ) balls produced higher reactive powders than those from larger SUS balls.

Characterization of samples
Crystalline phases of samples were identified by X-ray diffraction (XRD) analysis as described above. Bulk densities of the powder compacts and sintered samples were evaluated by measuring weights/dimensions and Archimedes methods, respectively. As-received and ball-milled powders and microstructure observation on the fractured or polished surfaces of sintered composites was performed using a field emission-type scanning electron microscope (FE-SEM, JSM-7001FD, JEOL, Tokyo, Japan) equipped with an energydispersive spectroscope (EDS, JED-2300/F, JEOL) and their average grain sizes were determined by an intercept method [7]. A field emission-type transmission electron microscope (FE-TEM, JEM-2100F, JEOL) equipped with an energy-dispersive spectroscope (EDS, JED-2300/F) were also utilized to observe the morphology, analyze the elemental distribution, and examine the electron diffractions on the nano-regions of the composites. Before TEM observation, the specimens were made thinner using a focused ion beam (FIB, FB-2000A, Hitachi High-Tech Fielding) equipped with a microsampling system.
After crystalline phase identification, test bars (~3×3.5×11 mm 3 ) for mechanical-property measurements were cut from the sintered materials with a diamond cutting-blade and then their four sides were polished to mirror surface with a diamond paste (nominal particle size 1-3 μm). Three-point bending strength (σ b ) was evaluated with a cross-head speed of 0.5 mm/min and an 8 mm-span length using WC jigs. Vickers hardness (H v ) and fracture toughness (K IC ) were evaluated with an applying load of 19.6 N and a duration time of 15 s for the former, and the indentation fracture method (IF) with Niihara's equation for the latter [8].
Direct current (DC) electrical resistivity (ρ) of composites were measured at room temperature by Ag four-terminal (1.0 mm intervals) method with a programmable DC voltage/current source and a multifunctional-volt meter. Figure 1 shows SEM photographs of TiH 2 powders; (a) as-received, (b) and (c) are the photographs of powders after ball milling using SUS balls (10 mm Φ ) for 100 h in Ar (4.36 g) and after combined ball milling process using WC balls (5 mm Φ ) for 3 h in Ar (12.1 g) & ZrO 2 balls (1 mm Φ ) for 10 min in ethanol (83.9 g), respectively. These photographs reveal that the starting material of TiH 2 composed of small and large crushed particles up to around 8 μm, and the particle sizes of ball-milled powders were about 0.3 μm. It should be noted that the combined ball milling, i.e., WC balls (5 mm Φ ) for 3 h in Ar (12.1 g) + ZrO 2 balls (1 mm Φ ) for 10 min in ethanol (83.9 g), gave the homogeneous fine powders without agglomeration (~8 μm) as observed in the powder milled with large SUS balls (10 mm Φ ). This might be explained by that during the (SUS-ball) milling sufficient energy for pulverization was not supplied due to both dry pulverization process and low density of SUS balls (0.77 Mg/m 3 ) under the centrifugal force about 4.36 g, even though long duration time up to 100 h. On the other hand, in the combined ballmilling process, heavy large WC balls (15.66 Mg/m 3 , 5 mm Φ ) milled the coarse particles in dryprocess moderately (3 h) and then light small ZrO 2 (PSZ) balls (6.05 Mg/m 3 , 1mm Φ ), with high milling energy due to the centrifugal force of 83.9 g from a high rotating speed of 1100 rpm, ground the premilled powders down to 0.3 μm, deflocculating the fine particles in ethy-alchol. Figure 2 shows XRD patterns of the TiH 2 powders (a) after ball milling using SUS balls and (b) prepared from the combined milling process. In Fig. 2(a), main XRD peaks of TiH 1.924 (JCPDS#25-0982) and a small amount of Ni-Cr-Fe (JCPDS#35-0983) phases were observed; the latter impurity phase was originated from the SUS vessel and balls during milling. On the other hand, as WC and ZrO 2 (PSZ) balls with excellent wear resistance were used for the second pulverization process, impurities were not observed in the XRD pattern.   (JCPDS#41-1352), respectively, increased a little from ~57.2 to ~57.8%. In calculation of their theoretical density, the density of impurities was ignored due to its small amount.    In the present study, two kinds of nitrogen pressure in the 2nd stage of HIPing were selected, i.e., high 196 MPa and low 98 MPa as shown in Fig. 4 (a) and (b), under the expectation that SUS ball-mill-derived TiN would show poor sinter ability, on the contrary combined mill-processderived TiN would produce dense sintered materials; the latter of which comes from common industrial viewpoints to fabricate the materials with low-cost.

Microstructure of Al 2 O 3 /TiN composites
Therefore Hereafter, we will call the former sintered materials as "SUS-composite materials" and the latter "WC-ZrO 2 -composite materials".  were ignored because of their small amounts. It should be noted that relative densities of composites ("SUS-composite materials") were higher than 98.6%, even though using a capsule-free HIPing. However, "WC-ZrO 2 -composite materials" sintered at 98 MPa showed a little lower relative density as shown in Fig. 6 (b). This reduction could be explained in terms of suppression of densification for Al 2 O 3 by the presence of TiN, which resulted in the reduction of grain sizes of Al 2 O 3 , as will be described later. In the upper photographs in Fig 9 (a) and (b) show the black dot (particle) on the triple-points (triple-junctions) in the "SUS-composite materials" and "WC-ZrO 2 -composite materials", respectively.
The elemental-line analysis on the black dot is displayed under the photographs. From these, it is clear that black dots (particles) consist of only titanium and nitrogen, i.e., titanium nitride, from its surface to the center of dots, indicating pure monolithic TiN particle. This might be explained in terms of i) high-nitrogen pressure of 7 MPa, and ii) Al 2 O 3 matrix plays a role of dilution medium, or separator for Ti particles. Therefore, even high adiabatic temperature of TiN up to 4900 K [9], strong TiN aggregates could not be formed. In Fig.10 it is observed that fine TiN particle about 100 nm exists within an Al 2 O 3 grain of "WC-ZrO 2composite materials", suggesting that combined ball-mill-derived TiN fine particles were included into Al 2 O 3 matrix during their grain growth.

Mechanical and electrical properties of Al 2 O 3 /TiN composites
In Fig. 12 (i) and (ii), (a) three-point bending strength (σ b ), (b) Vickers hardness (H v ), and (c) fracture toughness (K IC ) evaluated by means of "indentation fracture (IF) method" are shown for the sintered "SUS-composite materials" and "WC-ZrO 2 -composite materials", respectively. From these figures, the optimum contents of TiN to improve their mechanical properties were thought to be around 3 to 5 vol%. Especially bending strength (σ b ) had much correlation with the TiN content; for example, in the "SUS-composite materials" (Fig. 12 (i)), without TiN addition (100/0) bending strength was 525 MPa, however, a 3 vol% addition resulted in 640 MPa with a 22% increase, and then the σb value decreased to ~510 MPa. Fracture toughness (K IC ) was also improved from 3.8 to 4.5 MPa・m 1/2 with a 18% increase. On the other hand, hardness H v was not changed due to the nearly same values (19 ~ 20 GPa) between alumina and titanium nitride [10]. Of course, we should take into account of reduction effect of alumina grains when we mention the H v values of composites. In the "WC-ZrO 2 -composite materials" as shown in Fig.12 (ii), in consideration with their low relative densities and a few data, there are little difference between "SUS-composite materials" and "WC-ZrO 2 -composite materials".
Electrical resistivity ρ of 100/0 and 95/5 vol%Al 2 O 3 /TiN "SUS-composite materials" were so high that the present measuring system could not evaluate it, however, Al 2 O 3 /TiN = 90/10 vol% "SUS-composite materials" gave the ρ value of 2.6×103 Ω・m; this value is much lower than that (>1012 Ω・m) reported for Al 2 O 3 /TiN = 90/10 vol% composite fabricated by hot pressing a mixed powder compact containing ~ 1.0 μm TiN [11]. This might be explained in terms of fine 0.3 μm TiN powders used in the present study, even though the same volume fraction of TiN.

Conclusions
Highly dense sintered Al 2 O 3 /TiN composites with the relative density of 98.5% or more have been fabricated from the mixed powder [Al 2 O 3 /(Ti,TiN 0.30 )] compacts directly by simultaneous synthesis and sintering using capsulefree high pressure N 2 -HIPing utilizing SHS of TiN. Materials with the compositions of Al 2 O 3 /TiN=97/3 and 95/5vol% consisting of homogeneous Al 2 O 3 (2.0-1.3 μm) matrix and fine TiN particles (~0.3 μm) distributed uniformly among the Al 2 O 3 matrix gave higher mechanical properties than those of monolithic alumina. From the results of the present study, it has been cleared that by applying capsulefree N 2 -HIPing to the preparation of engineering ceramics containing metal-nitride, which nitride is even difficult to be synthesized under the conventional conditions, this process provides the low-cost fabrication method with easy handling in a short operation time.
And thus prepared metal nitrides will provide a new wide application field in future.