Mechanical Performance of Alumina Reinforced with NbC , TiC and WC

Post-Graduation Program of Science and Materials Engineering, Federal University of Rio Grande do Norte – UFRN, CEP 59072-970, Natal, RN, Brazil Centro Técnico Aeroespacial, Divisão de Materiais, Instituto de Aeronautica e Espaço – IAE, Praça Marechal do Ar Eduardo Gomes, 50, CEP 12228-904, São José dos Campos, SP, Brazil State University of Northern Rio de Janeiro – UENF, Av. Alberto Lamego, 2000, CEP 28013-600, Campos dos Goytacazes, RJ, Brazil


Introduction
At present alumina reinforced with micro and nano-sized carbides, such as TiC 1 , WC 2 , AlTiC 3 and ZrO 2 4 represent a new class of hard materials with improved mechanical properties.The incorporation of theses reinforcing elements may also increase the wear behavior of alumina matrix 5 .Although the cutting tools market is still dominated by WC-Co, alumina-based composite materials may be a good alternative to improve cutting speed and lower production cost 6 .Adding these reinforcing elements increases strength and hardness values at room temperatures in comparison with monolithic material 7 .Alumina-composite materials withstand higher temperatures than WC-Co without deformation, allowing tools to cut at faster speeds.Others studies have shown the beneficial mechanical effect produced by the incorporating carbon nano-fiber (CNF) 8 and nanotubes (CNT) 9 .Flexure strength increased to 1.5 vol.% of carbon nanotubes.Greater CNT content reduces the properties of composite materials 9 .Although the presence of carbon nanofiber improved fracture toughness, the higher CNT content lowered the hardness and flexural strength of composite materials 8,9 .
Niobium carbide exhibits good chemical properties, including a high melting point, significant levels of hardness, elevated Young's modulus and a thermal expansion coefficient compatible to alumina, making it a good reinforcing element to be added to alumina.
Brazil holds the world's largest niobium reserves, making the study and use of niobium compounds very strategic for the country.Some research published in the literature has demonstrated the good potential for employing Nb 2 O 5 and NbC as sintering additives 10 , grain growth inhibitors 11 and reinforcing elements in a ceramic matrix 12 .Addition of Nb 2 O 5 improves densification of alumina at lower sintering temperatures 13 .Alumina doped with niobium oxide has the disadvantage of increasing the grain size of alumina during sintering 14 .The presence of NbC causes a pinning effect very similar to incorporation of TiC, WC or other carbides 15 .Incorporating NbC particles into alumina matrix has improved the mechanical properties of alumina-based composite materials 16 .Manufacturing NbC/alumina composite materials by active filler controlled polymer pyrolysis reaction has also been reported 17 .Results show that this process has the advantage of achieving low sintering temperatures, although composite materials display high porosity and low strength values 16 , in addition to niobium oxidation 17 .It is not possible to compare directly results described in the literature for alumina composites.Some parameters, such as alumina powder (particle size, surface energy, chemical purity), processing method (uniaxial pressing, hot-pressing, isostatic pressing) and sintering conditions (temperature and time) are somewhat different, meaning direct comparison is not accurate.
This study sought to compare the mechanical effect of each carbide (WC, TiC and NbC) in an alumina matrix, using the same processing method and parameters.

Experimental Procedure
Starting powders consisted of Alumina APC-2011 SG with surface area of 1.5 m 2 .g - (Alcoa, Brazil,), NbC (Herman Starck, Berlin, Germany), WC (Wolfram Bergbau, Austria) and TiC (Aldrich) with D50 = 2.3 µm, 2.0 µm 1.5 µm and 2.5 µm, respectively.Al 2 O 3 containing 30 wt.(%) of reinforcing carbide (best results reported in the literature) was dry-mixed for 4 hours in a planetary ball mill containing alumina grinding media.Powder compacts were then uniaxially hot pressed in a cylindrical die 40 mm in diameter under 30 MPa at 1650 °C in flowing argon.Apparent density and porosity of the sintered bodies was determined using the Archimedes water displacement method.Crystalline phases present after the sintering process were identified by X-ray diffraction (Shimadzu XRD-600) in a range of 20 to 90 with a 2θ scanning rate of 2° min -1 .Vickers's microhardness (H V ) was evaluated by a Vickers indenter applying a load of 30 N during 15 seconds.Mechanical strength of the specimens (average of samples per value) was measured at ambient temperature by a universal testing machine (Zwick, 2.5 kN), using three-point bending geometry at a constant cross-head speed of 0.5 mm/min.Fracture surfaces were observed through by Scanning Electron Microscopy (Shimadzu SSX-550).
In order to determine the wear characteristics of composite materials, experiments were conducted in Contenco microprocessed water-cooled abrasion equipment.The equipment operates on motor power of 1 kW, with an axial load applied for a sample of up to 50 kgf, and constant rotation of 60 rpm.The abrasion granite has an internal and external diameter of 480 mm and 800 mm, respectively.Disintegration of the granite, a hard strong rock, was carried out under conditions imitating drilling, applying loads and cutting speed found in practice.Wear intensity of the insert (composite material) as well as the granite energetic disintegration capacity can be determined by the following Equation 1: Where Q i is the insert weight loss by abrasion during the test, t i represents test duration and v i the linear cutting speed.Practical loss due to abrasion was evaluated according to the difference between weight of the insert before and after each test 18 .

Results and Discussion
Table 1 compares the theoretical density and porosity of reinforced composite materials.Results indicated that all hot-pressed composite materials were almost fully dense (porosity values <0.7%), regardless of the carbide added.Increased density values in relation to the alumina matrix are due to the higher density values of the added hard carbide particles.
Figure 1 depicts a typical X-ray diffraction pattern of the sintered alumina with NbC and TiC composite materials.Alumina and the original refractory carbides are shown to Materials Research be the only crystalline phases present.No other crystalline phases were identified (Table 1).This corroborates findings reported in the literature for alumina-WC 5 , alumina-(W,Ti)C 15 and alumina-TiC 1 , although not for the Al 2 O 3 -WC-Co composite material produced by vacuum hot pressing 19 .This material exhibits a new crystalline intermetallic phase (Co 3 W 3 C), which is responsible for the higher hardness values recorded in this system 19 .
Table 2 illustrates the hardness and fracture strength obtained for composite materials, all of which used the same alumina matrix, enabling direct comparison between the different carbides.Composite materials reinforced with hard carbides exhibited no marcant differences between them, and an increase in comparison to alumina matrix.Hardness values for composite materials ranged between 22 and 24.6 GPa, significantly better as compared to pure alumina (18,5 GPa).The hardness obtained for Al 2 O 3 -NbC and alumina reinforced with other carbides in the present study was also comparable with values reported in the literature for alumina-TiC 1 , (W,Ti)C 7 and WC 6 composites.
Flexural strength values are displayed in Table 2 and show a similar behavior.No significant difference was recorded for the margin of error with carbide using NbC, WC or TiC (310-350 MPa).Results demonstrated that composite materials did not significantly change mechanical properties, indicting that type of carbide addition has no significant influence on the strength and hardness of composite material.
The strength depends basically on the presence of defects in the microstructure such as pores, agglomerates and cracks caused by the differences of the thermal expansion coefficient between matrix and reinforcing carbide.All composite materials showed analogous density after sintering process (Table 1) and also comparable thermal expansion coefficient, which can explain the comparable strength values observed.
Figure 2 depicts an SEM photograph of the surface fracture of alumina-reinforced with NbC and TiC.All composite materials exhibit basically the same fracture behavior.Fracture mode consisted mainly of an intergranular fracture accompanied by partial transgranular fracture.Materials display homogeneous structure and the presence of carbide particles in the alumina grain and grain boundary.Isolated pores were also found in the alumina grains.Table 3 shows wear behavior for the carbide-reinforced materials investigated in this study.Results indicated that incorporation of carbides promotes an increase in linear wear behavior when compared to pure alumina, while adding TiC and NbC causes a similar effect.The inclusion of TiC particles produces greater increases in wear behavior, with alumina material displaying a decrease in mass loss from 0.107 to 0.053 mg.m -1 .Increased wear behavior for alumina reinforced with TiC, (W,Ti)C and SiC was also observed in the literature 5 .

Conclusions
Dense Al 2 O 3 -carbides ceramic composites were obtained by hot pressing at 1650 °C.The addition of 30 wt.(%) NbC, TiC and WC increases the hardness, flexural strength and wear behavior of the alumina matrix, improving the possibility of their use as cutting tools.The Alumina-NbC composite material shows no significant difference in mechanical properties as compared to other carbides (WC and TiC), making the use of NbC as reinforcing element very attractive.Microstructure analyses show a homogeneous carbide distribution in the alumina matrix and the presence of a transgranular fracture mode accompanied by a partial transgranular fracture.

Figure 1 .
Figure 1.X-ray diffraction pattern of the sintered alumina with: a) NbC, b) TiC and c) ZrO 2 .

Figure 2 .
Figure 2. SEM photograph of the surface fracture of the sintered alumina with: a) NbC and b) TiC and c) ZrO 2 .

Table 1 .
Density and porosity of the reinforced composite materials.

Table 2 .
Mechanical properties of composite materials.