TEM study of a hot-pressed Al2O3-NbC composite material

ARTICLES PRESENTED AT THE II SBPMAT, RIO DE JANEIRO - RJ 26-29 DE OUTUBRO/2003

TEM study of a hot-pressed Al₂O₃-NbC composite material

Wilson Acchar^{I, *}; Carlos Alberto Cairo^II; Ana Maria Segadães^III

^IDepartment of Physics, Federal University of Rio Grande do Norte, 59072-970 Natal - RN, Brazil

^IIDivisão de Materiais, Centro Técnico Aeroespacial, Instituto de Aeronáutica e Espaço, Praça Marechal do Ar Eduardo Gomes 50, São José dos Campos - SP

^IIIDepartment of Glass and Ceramics, University of Aveiro, Portugal

ABSTRACT

Alumina-based composites have been developed in order to improve the mechanical properties of the monolithic matrix and to replace the WC-Co material for cutting tool applications. Al₂O₃ reinforced with refractory carbides improves hardness, fracture toughness and wear resistance to values suitable for metalworking applications. Al₂O₃-NbC composites were uniaxially hot-pressed at 1650 °C in an inert atmosphere and their mechanical properties and microstructures were analyzed. Sintered density, average grain size, microhardness and fracture toughness measurements and microstructural features were evaluated. Results have shown that the mechanical properties of alumina-NbC are comparable to other carbide systems. Microstructural analysis has shown that the niobium carbide particles are mainly located at the grain boundaries of alumina grains, which is an evidence of the "pinning effect", produced by NbC particles.

Keywords: alumina, niobium carbide, mechanical properties, microstructure

1. Introduction

Composite ceramic materials based on alumina have been developed as a technological alternative to tungsten carbide^1-4. Nowadays, WC-Co materials still dominate the cutting tool market, because of their superior mechanical properties⁵. Titanium nitride, tungsten carbide, mixed carbides of tungsten and titanium and specially titanium carbide have been intensively investigated as reinforcement for alumina^2,4,6-8. Alumina-TiC has been already used in finishing operation of gray cast iron and hardened steel⁹. Moreover, the presence of these hard particles reduces the grain growth of alumina^10,11, which contributes to the mechanical performance of the composite. Reports have shown that the properties of composites depend basically on the density of the sintered materials. The densification of alumina with hard particles can be realized only in pressure-assisted sintering, by using some sintering additives as Y₂O₃, TiO₂-MnO and high sintering temperatures^6-8,12,13.

Niobium carbide can also be used to reinforce alumina, due to its high hardness (> 20 GPa), high stiffness (340 GPa) and high melting temperature (3600 °C). On the other hand, Brazil holds the main world niobium reserves, making the study of this metal strategic. Recent studies published in the literature have indicated that niobium carbide presents a good potential to be used as a reinforcing element for alumina^10,12-14. The results have shown that the maximal reinforcing effect was obtained in alumina composites with 30 wt. (%) of niobium carbide¹⁴. The objective of the present work is to investigate the mechanical properties and the microstructure of a hot-pressed alumina- 30 wt. (%) NbC.

2. Experimental Procedure

The starting powder consisted of alumina APC-2011 SG (Alcoa, Brazil) and niobium carbide (Herman Starck Berlin, Germany). The raw materials present an average grain size of 2.3 µm and 1.5 µm, respectively. Alumina composites reinforced with 30 wt. (%) of NbC were mixed in a planetary ball mill during five hours. Subsequently, the homogenized mixture was uniaxial hot-pressed at 1650 °C under 80 MPa in flowing argon. The densities of hot-pressed specimens were determined using the Archimedes method. Microhardness (H_V) and fracture toughness (K_IC) were calculated by measuring the lengths of the cracks and the diagonal produced by the indentation method on the polished surface. Each test was repeated at least six times. X-ray diffraction analysis was carried out in order to identify the crystalline phases. Grain size distributions of alumina grains were estimated by scanning electron microscopy and image analysis using the IMAGE-C computer program (INTRONIC, Germany).

Transmission electron microscopy (TEM) was used to investigate some microstructural aspects of the sintered specimens. Samples for TEM analysis were first machined, ground and hand polished to a thickness of approximately 100 µm and then ion lilled (Gatan DuoMill 600, 6 kV, 1 mA) to electron-transparency).

3. Results and Discussion

Figure 1 shows the XRD pattern of alumina reinforced with 30 wt. (%) of NbC. X-ray analysis of the sintered composite revealed only the presence of Al₂O₃ and NbC. No evidence of niobium oxides (NbO, NbO₂, etc) or other new crystalline phases were found. Similar results have been reported for pressureless sintering of alumina-NbC composites^10,12-13. These works have also shown that the addition of sintering additives as Y₂O₃ and TiO₂ + MnO causes a formation of new crystalline phases as Y₃Al₅O₁₂, Al₂TiO₅ and AlTi₄C₂, respectively.

Specimens sintered at 1650 °C depicted densities values from 98 to 99.5% TD and average grain size between 2.6 and 3.5 mm (Table 1). The presence of niobium carbide has reduced the density of the composite material, which can be caused probably by a pinning effect. The pinning effect was also observed in the literature for alumina reinforced with NbC¹⁴ and SiC^11,15. The addition of hard particles decreases the mobility of the grain boundaries during the sintering process, causing a decrease of the density and of the alumina average grain size. Similar observations have been also reported for other composite systems^4,6,11,15-17.

Figures 2 and 3 show the mechanical properties of the Al₂O₃-NbC composite material investigated in this work. For comparison, hardness and fracture toughness of others composite materials and tungsten carbide are also presented. The fracture toughness value of Al₂O₃-NbC composite material investigated in this work is in good agreement with those obtained for alumina reinforced with TiN, Ti(C,N)^3,9 and TiC (4.5 – 5.0 MPa.m^1/2)^2,4,6-8 and is slightly higher than Al₂O₃+(W,Ti)C¹⁶ and Al₂O₃+Y+NbC (3.5 - 4.0 MPa.m^1/2)¹². This result shows that the presence of niobium carbide and titanium carbide did not produce a strong barrier for the crack movement in an alumina matrix as observed in the presence of tungsten carbide

Although cemented carbides (11 MPa.m^1/2) and cemented carbide ceramic composites (7.5 – 9.0 MPa.m^1/2)^18,19 have shown a significant improvement in fracture toughness, these materials lack intrinsic advantages with respect to fast cutting speeds and thermal stability⁵.

Contrary to what was observed for fracture toughness, the hardness values did not change significantly, regardless of the type of refractory particles. Alumina reinforced with 30 wt. (%) of NbC has shown hardness comparable to Al₂O₃-TiC and WC-Co, which is an evidence that TiC can be replaced by NbC without degrading the mechanical properties, indicating that this composite material has a good potential to be developed.

Further investigation is under way to study the influence of mixed carbides as NbC + WC and NbC + TiC in an alumina matrix.

TEM micrographs were used to observe the morphological features of the microstructure of the Al₂O₃-NbC composite (Figures 4 and ⁶). The microstructure of the composite material consisted of a large-grained matrix of alumina with fine-grained carbides. The material exhibited a homogeneous microstructure and the NbC grains are located along the alumina grain boundaries (Figure 4). No presence of NbC clusters was observed. The bright and dark particles were analyzed by EDS technique (Figures 5a and 5b). The data obtained by EDS confirms that the dark and bright regions consisted mainly of niobium and aluminum oxide, respectively. The presence of niobium carbide particles at grain boundaries is an evidence of the "pinning effect" produced by the addition of hard particles. ^{Figure 6} shows the interface between niobium carbide and alumina grains. TEM analysis of these boundaries did not identify an intergranular phase. The presence of an amorphous or a new crystalline phase was not detected.

4. Conclusions

The results obtained in this work revealed that:

• Dense specimens of Al₂O₃- 30 wt. % NbC were obtained by hot pressing;

• Incorporation of NbC into alumina has caused a pinning effect, reducing the average grain size and the density of the monolithic matrix;

• X-ray diffraction has showed only the presence of Al₂O₃ and NbC. No other crystalline phases were found;

• Alumina-NbC composite has showed mechanical properties comparable to other carbide systems;

• TEM micrographs showed that NbC particles are preferentially located at alumina grain boundaries.

Received: October 29, 2003; Revised: February 5, 2005

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Publication Dates

• Publication in this collection
  24 Mar 2005
• Date of issue
  Mar 2005

History

• Accepted
  05 Feb 2005
• Received
  29 Oct 2003