Katsuya Takaoka, Hajime Yagura, Masaki Kato and Ken Hirota
Presented at the 2011 International Conference on Hot Isostatic Pressing Kobe, Japan, 12-14 April 2011
Submitted: 12 April 2011, Accepted: 27 January 2012/ will be published online in full text at http://www.AZoM.com
Topics CoveredAbstractIntroductionExperimental Preparation of TiH2 Powder Fabrication of Al2O3/Tin Composites by Capsule-Free N2-HIPing Characterization of SamplesResults and Discussion Synthesis of Fine (Ti, TiN0.30) Powders Microstructure of Al2O3/TiN Composites Mechanical Properties of Al2O3/TiN CompositesConclusionsAcknowledgementReferencesContact Details
Synthesis of dense composite materials with the compositions of Al2O3 / TiN = 100 / 0 ~ 90 / 10 vol% has been attempted directly from Al2O3/ (Ti, TiN0.30) mixed powder compacts using capsule-free N2 hot isostatic pressing (HIP). Fine Ti powders (~ 0.3 mm in diameter) with TiN0.30 phase were prepared by thermal decomposition of pre-crushed fine TiH2 powders at 400 °C for 1 h in a vacuum, followed by thermal treatment in N2. Then, Al2O3/ (Ti,TiN0.30) mixed powder compacts with homogeneously dispersed (Ti,TiN0.30) particles were fabricated and then HIP-sintered. During the first stage of HIPing [1350 °C / 6 MPa / 1 h], solid/gas reaction between (Ti, TiN0.30) and N2, in which both were contained in the powder compacts, was induced to form TiN. Then, after the successive second stage of HIPing [1350 °C / 196 MPa / 2 h], the most of sintered composites consisting of Al2O3 and TiN phases reached a higher relative density than 98.5 % with closed pores, nevertheless a capsule-free HIPing. Dispersion of TiN particles (0.30 ~ 0.35 mm) just formed suppressed the grain growth of Al2O3 during sintering. Mechanical properties, such as bending strength (sb), Vickers hardness (HV), fracture toughness (KIC), and other properties have been evaluated as a function of TiN content.
As metal nitrides reveal often desirable combined properties, such as high melting temperatures, high hardness, low density, low electrical resistivity, excellent wear resistance and high chemical stability, therefore, they have been widely used in semiconductor industries. Among metal nitrides, titanium nitride (TiN) shows excellent behaviors, and it has been attracting increasing interest as a constituent of composites for wide applications, such as cutting tools, tool coating, microelectronics1, and solar-control films.
A lot of papers on Al2O3-TiN composite materials have been published, and many of them treated the sintering of mixtures of Al2O3 and TiN powders[2-4]. However, 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 partilces to other inorganic materials often resulted in reduction of original mechanical properties, and an improvement of the performance of composite materials cannot be expected.
Though fine Ti powders are required to distribute a fine grain TiN into the matrix in order to fabricate dense TiN-added composites with improved mechanical properties, metal Ti cannot be crushed into a fine particle by conventional ball-milling process because of its high ductility. On the other hand, it was reported that fine Ti powder could be obtained by the reductive reaction of metal oxides, for example, heat-treated under hydrogen or NH3 of fine TiO2. In the present study to synthesize fine metal Ti, brittle TiH2 was chosen because this compound can be mechanically crushed. Ti powder with a diameter of submicron-meter was produced by heating pre-ball-milled TiH2 in a vacuum and then under nitrogen atmosphere. Al2O3 / Ti mixed powder compacts were fabricated high-effectively, in which this fine Ti powers were uniformly distributed in the Al2O3. And high-pressure nitrogen HIPing was applied to densify the powder compacts using a capsule-free method that can brings a free-selection of shape and size of the sintered compact without pre-sintering of green bodies to a nearly 92 ~ 93 % relative density. This capsule-free high-pressure N2 HIPing also make it possible that metal nitrides to be formed, even though this metal cannot be nitrized under the normal N2 pressure at high temperatures. Simultaneous synthesis of titanium nitride directly from Ti and N2 and sintering and mechanical properties of the sintered materials were described in relation with their microstructures.
Preparation of TiH2 Powder
Starting powder was TiH2 (TSHT, Osaka Titanium Technologies Co. Ltd, Hyogo, Japan, particle size of P2 £ 45 mm, ~ 99.98 % purity). This brittle powder was crushed with a planetary ball mill using SUS balls (10 mm in diameter) for 24 h at a rotating speed of 200 rpm in Ar.
Fabrication of Al2O3/Tin Composites by Capsule-Free N2-HIPing
Fine a-Al2O3 powder (TM-DAR, Taimei Chemicals Co. Ltd, Nagano, Japan, Ps ~ 0.10 mm, ~ 99.99 % purity) and ball-milled TiH2 powder (Ps~0.3 mm) and were weighed into Al2O3 / (Ti,TiN0.30) = 100 / 0~89.10 / 10.89 mass%, assuming that all (Ti,TiN0.30) would transform into TiN after an appropriate heating under high N2 pressures and then into the compositions of Al2O3 / TiN = 100 / 0 ~ 90 / 10 vol%. The composite powders were dry-mixed with a mortar and pestle for 1 h in Ar. Then, the mixed powders were treated by thermal decomposition at 400 °C for 1 h in a vacuum, followed by thermal treatment at 200°C for 1 h in N2. After thermal decomposition, the composite powders were mixed completely with a planetary ball mill using partially stabilized zirconia (PSZ) balls (1 mm in diameter) and methyl-alcohol for 10 min at 700 rpm (a centrifugal force about ~ 11 g). Then, the powders were dried in a vacuum for 6 h and then cooled in Ar. The dry-mixed powder was compacted using an uniaxial mold/plunger (13 mm in diameter) at 20 MPa and then, densified by cold isostatic pressing (CIP) at 245 MPa. Al2O3 / (Ti, TiN0.30) mixed powder compacts were HIP-treated. Dense Al2O3 / TiN composites were fabricated by capsule-free N2-HIPing (1350 °C / 6 Mpa / 1 h +1350 °C / 196 Mpa / 2h); these conditions were determined from our preliminary experimental results.
Characterization of Samples
Crystalline phases of samples were identified by X-ray diffraction (XRD) analysis (CuKa1 radiation with a graphite monochromator, Rint 2200, Rigaku, Osaka, Japan). Bulk densities of sintered samples were measured by Archimedes method. Microstructure observation on the fracture surfaces of sintered samples was performed by field-emission type scanning electron microscopy (FE-SEM, JSM-7001FD, JEOL Ltd., Tokyo, Japan) and their average grain sizes were determined by an intercept method. After crystalline phase identification, test bars (~ 3×3.5×11 mm3) 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 mm).
And then three-point bending strength sb was evaluated with a cross-head speed of 0.5 mm/min and an 8 mm-span length using WC jigs. Vickers hardness HV and fracture toughness KIC 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.
Results and Discussion
Synthesis of Fine (Ti, TiN0.30) Powders
XRD patterns of TiH2 powders after (a) ball-milling and (b) following dehydrogenation were measured. After ball-milling, main XRD peaks of TiH1.924 (JCPDS#25-0983) and a small amount of Ni-Cr-Fe (JCPDS#35-0983) phases were observed; the latter phase was impurities from the stainless vessel and ball during milling. Dehydrogenation at 400 °C in a vacuum for 1 h gave a mixture of Ti (JCPDS#44-1294) and TiN0.30 (JCPDS#41-1352); a low-temperature heat-treatment in N2 resulted in the formation of TiN0.30 with a mol ratio of Ti / TiN0.30 ~ 4 / 6, estimated from XRD intensities. Figure 1 shows SEM photographs for TiH2 after ball-milling and a mixture of Ti and TiN0.30 after dehydrogenating, revealing the particle sizes of both samples of about 0.3 mm.
Figure 1. SEM photographs for TiH2 after ball-milling, and the sample consisting of Ti and TiN0.30 after dehydrogenating.
Figure 2. SEM photographs for the fracture surfaces of (a) a mixed powder (Al2O3/(Ti,TiN0.3)) compact and (b) pre-sintered compact (1350 °C/6 MPa/1 h) with the composition corresponding to Al2O3/TiN=90/10 vol%.
Microstructure of Al2O3/TiN Composites
Bulk densities of mixed powder (Al2O3/Ti/TiN0.3) compacts increased monotonously from 2.28 to 2.34 Mg/m3 with increasing (Ti/ TiN0.3) content. Inversely, relative densities calculated from the theoretical densities (3.987, 4.503 and 4.715 Mg/m3) of Al2O3 (JCPDS#46-1212), Ti (JCPDS#44-1294) and TiN0.3 (JCPDS#41-1352) respectively, assuming that the ratio of Ti/ TiN0.3 was ~ 4 / 6, increased a little from 57.2 to 57.8 % (Fig. 2(a)). Then the dense powder compacts were heated at 1350 °C under high nitrogen pressure of 6 MPa (1 h) and successively followed at196 MPa (1 h), by capsule-free N2-HIPing.
After the 1st HIPing (1350 °C / 6 M / 1 h), the samples correspong to the compositions of Al2O3 / TiN = 100 / 0, 95 / 5, and 90 / 10 vol% were evaluated from the viewpoints; the crystalline phase change, microstructure development and the relative densities, with expectation for formation of TiN from Ti and TiN0.3 and the pre-sintered bodies ³ 92-93 % relative densities containing only closed pores. XRD patterns (Fig. 3, Left) taken for the powder compact and pre-sintered body corresponding to 90 / 10 vol% after the 1st HIPing, respectively, revealed that the former compact consisted of Al2O3, Ti and TiN0.3, however, the latter of Al2O3 and TiN with a small amount of Al0.54Ti2.46N0.28O4.58 (JCPDS#42-1279), proving that the formation of TiN by solid/gas reaction between (Ti/ TiN0.3) and 6 MPa-N2. Relative densities of pre-sintered bodies were determined to be 97.5 % (100/0 vol%, bulk density of 3.89 Mg/m3), 95.5 % (95/5 vol%, 3.87 Mg/m3), and 94.5 % (90/10 vol%, 3.90 Mg/m3, Fig. 2(b)) based on the theoretical densities of crystalline phases for Al2O3 and TiN without respect to Al0.54Ti2.46N0.28O4.58. These data support the ideas that during the 1st stage of HIPing mixed powder compacts would change into pre- sintered bodies with no open pores, composed of Al2O3 matrix and TiN precipitates, and then the 2nd stage the pre-sintered bodies would result in dense composite materials under high N2 pressure at the same temperature of 1350 °C.
XRD pattern of the HIP sintered bodies (Fig. 3, Right) were measured; there was little change in the crystalline phase in the range of Al2O3 / TiN = 100 / 0 ~ 90 / 10 vol%, however, WC and Al0.54Ti2.46N0.28O4.58 were detected as impurities. It was thought that the former impurity was mixed into the samples during pulverizing the dense sintered materials with a super-hard WC-Co pestle and mortar and the latter was explained by the solid-state reaction among Al2O3, TiN0.3 and reactive Ti with absorbed oxygen on its surface during HIPing. Bulk densities of the sintered compacts changed from 3.97 to 4.07 Mg/m3 with increasing TiN content. On the contrary, their relative densities decreased from 99.6 to 98.6 %; these relative densities were calculated using the theoretical densities of 3.987 Mg/m3 (Al2O3) and 5.388 Mg/m3 (TiN: JCPDS#38-1420), in calculation, the values of 15.66 Mg/m3 (WC: JCPDS#25-1047) and 0.978 Mg/m3 (Al0.54Ti2.46N0.28O4.58: JCPDS#42-1279) were ignored because of the small amounts. The decrease in relative densities could be explained in terms of suppression of densification for Al2O3 by the presence of TiN, which resulted in the reduction of grain sizes of Al2O3, as will be described later.
Figure 4 presents SEM photographs for the fracture surfaces of thus prepared Al2O3 / TiN = 100 / 0 ~ 90 / 10 vol% composites after capsule-free N2-HIPing (1350°C / 6 MPa / 1 h-1350 °C / 196 MPa / 1 h); homogeneous microstructures consisting of small Al2O3 matrix-grains and fine TiN particles distributed at Al2O3 grain boundaries were observed. Abnormal grain growth of Al2O3 matrix-grains was not recognized in all the samples, revealing that homogeneous fine TiN particles had control over the grain growth of Al2O3 during HIPing. From these SEM photographs, grain sizes Gs of Al2O3 and TiN were estimated: Gs of Al2O3 decreased gradually from 2.0 to 1.3~1.4 mm, on the other hand, Gs of TiN was almost constant from 0.3 within the range of 0.4 mm, it was clear that this size was same as TiN0.3 and Ti as starting materials. Bulk densities of sintered compacts increased monotonously with increasing TiN content due to the heavy TiN, on the other hand, relative densities decreased from 99.6 to 98.6 %; even though using a capsule-free HIPing.
Mechanical Properties of Al2O3/TiN Composites
Figure 5 shows (a) three-point bending strength sb, (b) Vickers hardness HV and (c) fracture toughness KIC of the Al2O3 / TiN composite materials. The strength sb was improved with increasing TiN content up to the composition of Al2O3 / TiN = 97 / 3 vol%, from ~525 to the highest value 640 MPa, and then the sb value decreased to ~510 MPa. This tendency could be explained that from 100/0 to 97/3 vol% compositional range, relative densities were almost the same as high as 99.3 %, however, Gs of Al2O3 decreased gradually from 2.0 to ~1.3 mm.
Figure 3. Left; representative XRD patterns for (a) the powder compact and (b) pre-sintered body after the 1st HIPing. Right; XRD patterns for Al2O3/TiN materials with the compositions of 100/0~90/10 vol% after capsule-free N2 HIPing of (1350 °C/6 MPa/1 h)- (1350 °C/196 MPa/1 h).
Therefore, the increase in strength was due to the reduction of Al2O3 grain size. Vickers hardness HV and fracture toughness KIC increased up to Al2O3 / TiN = 95 / 5 vol% composition, and then tended to decrease slightly; the highest values of HV and KIC were 19.5 GPa, and 4.5 MPam1/2, respectively. These data were thought to be a little bit improved in comparison with those (sb=457 MPa,HV=19.2 GPa,KIC=4.43 MPa•m1/2) for monolithic Al2O3 fabricated in the present study. The values of HV and KIC for Al2O3 were18-23 GPa and 2.7-4.2 MPa•m1/2 , and those of TiN thin films or ceramics were reported to be 16-20 Gpa and 3.46 MPa•m1/2 , respectively. However, up to now, little information concerned about mechanical properties of bulk TiN is available because of its poor sinterability and TiN itself has been used as a thin film. From these data it could be stated that the mechanical behaviours of the present composites are closely related with their relative densities and grain size of matrix Al2O3.
Figure 4. SEM photographs for fracture surfaces of Al2O3/TiN composites with the various compositions.
Figure 5. Mechanical properties of Al2O3/TiN composites (a) bending strength sb, (b) Vickers hardness HV , (c) fracture toughness KIC as a function of TiN content.
Highly dense sintered Al2O3/TiN composites with the relative density of 98 % or more have been fabricated from the mixed powder [Al2O3 / (Ti, TiN0.30)] compacts directly by simultaneous synthesis and sintering using capsule-free high pressure N2-HIPing. Materials with the compositions of Al2O3 / TiN = 97 / 3 and 95 / 5 vol% consisting of homogeneous Al2O3 (2.0-1.3 mm) matrix and fine TiN particles (~0.3 mm) distributed uniformly at the Al2O3 grain boundaries gave higher mechanical properties than those of monolithic alumina. From the results of the present study, it has been cleared that by applying capsule-free N2-HIPing to the preparation of engineering ceramics containing metal-nitride, which nitride even is 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.
The authors wish to thank Dr. T. Fujii, Industrial Research Centre of Shiga Prefecture in Japan for his offer of ball-milled fine TiH2 powders.
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Katsuya Takaoka, Hajime Yagura, Masaki Kato and Ken Hirota
Department of Molecular Chemistry & Biochemistry, Faculty of Science & Engineering, Doshisha University, Kyo-Tanabe 610-0321, Japan
This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 13 (2011) 93-98.