Fabrication of silicide materials and their composites by reaction sintering

Fabrication of silicide materials and their composites by reaction sintering

Materials Science and Engineering A261 (1999) 204 – 211 Fabrication of silicide materials and their composites by reaction sintering R. Scholl *, A. ...

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Materials Science and Engineering A261 (1999) 204 – 211

Fabrication of silicide materials and their composites by reaction sintering R. Scholl *, A. Bo¨hm, B. Kieback Fraunhofer-Institute for Applied Materials Research (IFAM), Di6ision for Powder Metallurgy and Composite Materials in Dresden, Winterbergstraße 28, D-01277, Dresden, Germany

Abstract The paper reports on a new powder metallurgical technique applicable for the preparation of refractory silicides, silicide composites, and other poorly sinterable materials. Starting from the fundamentals of sintering a general approach for reaction sintering of elemental powders—termed controlled reaction sintering (CRS) — will be given. The sintering behaviour of pre-alloyed powders, coarse elemental powders, and the mechanically treated elemental powder mixtures are compared. A detailed study of powder preparation, consolidation, CRS, and properties will be pointed out for MoSi2 and MoSi2 – SiC composites. Several silicides (MoSi2, MoSi2 –SiC, TiSi2, Ti5Si3, FeSi2, ReSi2) were produced by CRS. Properties such as fracture toughness, minimum creep rate, bending strength, and oxidation resistance were determined for MoSi2-based materials. The paper presents an outlook for different fabrication routes for silicides and silicide composites, as pressureless and pressure assisted sintering, metal injection moulding (MIM), and tape casting. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Refractory silicides; Controlled reaction sintering; Metal injection molding; MoSi2

1. Introduction Research on silicides has a long tradition in Europe [1,2]. The potential of high melting point silicides (termed refractory silicides) for coating of metals and for heaters was predicted in the early years of this century. After the 2nd World War significant research was done to investigate Mo – Si alloys to develop technologies for manufacturing of silicide-based heating elements for industrial use. An overview about the historical evolution of silicides is given by Vasudevan [3]. The concept of using silicides for structural applications is more than 40 years old [4]. In the middle of the 1980s the research on silicides in the USA was focused on composites to examine and evaluate these materials for aircraft engine applications at Pratt & Witney and McDonnald–Douglas [3]. Parallel to these activities and continuing the work of Fitzer et al. [5] in Germany, different intermetallic alloys (silicides, aluminides) were evaluated and investigations on Mo–Si, Nb – Si, Ta – Si, Ti – Si, W –Si, and * Corresponding author. Tel.: +49-351-2537-304; fax: +49-3512537-399; e-mail: [email protected].

Zr–Si were initiated to determine basic properties including CTE, hardness, oxidation, and corrosion behaviour [6]. However, it was shown that both the preparation of samples and subsequent determination of mechanical properties are difficult. Consequently suitable methods for fabrication of silicides had to be developed to evaluate the industrial feasibility and technical relevance [7–9]. In this context IFAM Dresden, as a part of the FhG IFAM Bremen, was involved in the development of new techniques for manufacturing of MoSi2 materials. As a result of the research in Germany and the USA it was determined that silicide composites are attractive for several high temperature applications where all other metallic materials fail. The disadvantages of a small fracture toughness (from room temperature to about 900°C), a small creep resistance (above 1000°C), and intermediate temperature oxidation problems were partially overcome by additions of different reinforcements (particles, fibbers, whiskers, ductile elements or compounds) [6,10–12]. To date no manufacturing route for structural silicides or components has been developed on an industrial level although the technology for Kanthal Super heating elements is well established [23,24], but details of this

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1.1. Sintering of single phase powders

Fig. 1. Schematic illustration of three different techniques for preparation of silicide materials. (A) Pressure assisted sintering (i.e. HP or HIP) [10,13]. (B) Pressure assisted reaction sintering of elemental powder mixtures (SHS) [20]. (C) Pressureless reaction sintering of mechanically pre-treated elemental powder mixtures [15–17].

technique are not published by Kanthal. However structural parts must have a higher creep resistance and a convenient fracture toughness at room temperature whereas Kanthal Super only offers high temperature oxidation resistance. The technology for Kanthal Super will not allow the fabrication of complex components necessary for high performance applications. Manufacturing of dense silicide parts or silicide composites which are glassy-phase free require pressure assisted powder metallurgical techniques [13,14], or an innovative approach for reactive sintering [15 – 17]. The goal of this paper was to introduce a new technique for manufacturing of intermetallic based materials especially MoSi2. This route enables the preparation of glassy-phase free MoSi2 material with an outstanding microstructure and thus improved mechanical properties and oxidation behaviour by a controlled reaction sintering (CRS) of pre-treated elemental powder mixtures.

It is generally recognised that parts consisting of refractory silicides must be produced by powder metallurgy. Therefore it is necessary to discuss the driving forces of sintering in a short form. During sintering, as the temperature is raised above Tmelting point/2 neck growth at point-like particle contacts is observed due to cohesion forces. All property changes start from growth of these necks. At a sufficiently high temperature, mass transport takes place in single phase powder compacts by processes such as grain boundary diffusion, surface, or volume diffusion. On an atomic scale the motion of atoms into the neck region of particle–particle contacts reduces the total surface energy. Atoms with a higher energy than the activation energy (E) can move in the direction of higher vacancy densities. The number of atoms with an energy above the activation energy (N) varies with the temperature (N/N0 = exp(−E/kT)). N0, total number of atoms, E, activation energy, k, Boltzmann constant, T, temperature). Hence in a single phase powder structure sintering takes place due the energy decrease from reduction of total surface energy during sintering. This theory explains in a simple manner the densification behaviour of MoSi2 powder compacts from alloyed powders where sintering is determined only by the powder size and the number of atoms having a sufficiently high energy, i.e. sintering temperature. However the energy gain during sintering of alloyed powder mixtures is small compared to the energy gain during sintering of elemental powder mixtures, where alloy formation takes place surpassing the first by a factor of 100 [18].

Fig. 2. Measurement of heat flux (arbitrary units) of milled Mo – 2Si powder mixtures (milling time: 1, 3, 5, 8, and 12 h).

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1.2. Enhanced sintering Sintering of single phase particles has only a very limited set of variables which can be altered to improve densification. These are the grain size and the morphology (e.g. surface energy), the normalised sintering temperature (e.g. number of atoms with a higher energy than the activation energy for the predominant diffusion process), and the number and quality of particle contacts (e.g. packing behaviour caused by grain size distribution and geometry in the particle contact area) control sintering. It must be pointed out that the reduction of surface energy during sintering is a small driving force for sintering of homogeneous medium-grained powders. Enhancing sintering means looking for further driving forces. We have to distinguish between material controlled factors and process factors for enhancing sintering. The material controlled factors include: “ The activation of more efficient diffusion mechanisms (e.g. lattice transformations or transient liquid phases at the appropriate sintering temperature). Process factors that enhance sintering are: “ Pressure assistance (hot pressing (HP), hot isostatic pressing (HIP), hot forging,...) supports plastic flow during sintering or refining of the microstructure by raising the level total energy and hence enhancement of mass transport phenomena via bulk or grain boundary diffusion. “ Mechanically induced stresses (e.g. from ball milling processes) may refine the microstructure of the powder and produce for example additional of grain boundaries (which act as diffusion paths), generate new phase limits, enhance the dislocation density, and gradients of these. Sintering can be described as a multi-mode process. Knowledge of the predominant driving forces of the densification process enables the powder metallurgist to

develop new technologies to improve the sintering of high melting materials and components. It was already mentioned that the main goal of this paper was to develop a new powder metallurgical technique to produce dense intermetallic materials by pressureless sintering. This technique consists of a mechanical treatment (high energy milling (HEM)) of an elemental powder mixture hence allowing a controlled reaction of the constituents during sintering in contrary to uncontrolled reaction of coarse elemental powders (SHS). The new process will be referred to as CRS. This paper gives new results on CRS of silicide materials and develops a vision for further modification of this method for a new class of application-tailored materials. Hence the main part of the paper deals with problems during CRS.

2. Experimental procedure High purity elemental powders of different size (100– 500 mm) and shape were used to make powder mixtures with an atomic ratio of 33.3% Mo and 66.7% Si. These powder mixtures were milled in a planetary ball mill ‘Pulverisette 5’ (Fa. FRITSCH, Idar–Oberstein, Germany) avoiding phase formation during this treatment. The HEM took place for several hours [15–17], under an argon atmosphere using a steel milling set. Reinforcing additives as SiC (0....30 vol.%) were added after milling. Calorimeter measurements (Fig. 2) (DSC 404: Fa. Netzsch, Selb, Germany) explain the influence of milling time on the heat flux during heating from room temperature to high temperature. The duration of milling corresponds with the number of impacts and subsequently with the degree of material deformation, formation and distortion of particles, dislocation production and solid state solution formation.

Table 1 Properties of hot pressed MoSi2–SiC composites (0, 15, and 30 vol.% SiC) SiC content (vol.%) 0

15

30

Grain size: As hot pressed (mm) Treated 1600°C for 50 h−1 (mm)

11 26

5.7 5.4

4.1 4.6

Density: (% TD)

97

97.4

96.4

4-Point-bending-strength: (MPa) Room temperature 1000°C 1100°C Fracture toughness (MPaãm)

400

4

420 560 520 8.4

500 700 550 5

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Fig. 3. Shrinkage (relative units dl/lo) for three different mechanically treated Mo – 2Si powders (1, 3, and 8 h) and one pre-alloyed MoSi2 (grade A) powder.

The milled powders were either consolidated by die pressing followed by pressureless reaction sintering or hot pressed. Pressureless sintering was carried out at temperatures up to 1500°C under vacuum whereas HP was done at 1150, 1350, and 1450°C, respectively, at 35 MPa under vacuum. To monitor the influence of mechanical treatment on sintering, different green compacts were sintered in a dilatometer (DIL 402 E: Fa. Netzsch) under argon atmosphere. A commercial pre-alloyed MoSi2 powder (H.C. STARCK, grade A) was sintered for comparison. XRD analysis indicated that all powders sintered to the same phase compositions within the limits of accuracy. The microstructure of the samples was characterised by optical microscopy, scanning electron microscopy, and X-ray diffraction.

3. Results It was shown that mechanical activation of Mo–2Si powders (Mo–2Si: elemental powder mixture of 33.3 at.% Mo and 66.7 at.% Si) enhances sintering. This is believed to be from a reduction of activation energy for diffusion generating a process termed CRS for silicides. This may be applicable to other materials with a negative mixing enthalpy.

3.1. Microstructure Fig. 5 (A–C) shows the microstructure of MoSi2

materials prepared from activated MoSi2 powder mixtures which were densified by CRS during HP. Densities of 95, 97, and 98% total density (TD) were obtained respectively. Further detailed information is published elsewhere [17]. Fig. 5 (D) shows the microstructure of an MoSi2 –15 vol.% SiC.

3.2. Properties of silicides Depending on the amount of particles and their distribution, grain growth can be limited if the solubility at the consolidation and application temperature is low. Table 1 summarises the properties of hot pressed MoSi2 and MoSi2 –SiC composites. The 4-point-bend-

Table 2 Mass gain due to isothermal 100 h oxidation of MoSi2-based materials at a certain temperature in the given temperature ranges and at 1600°C Mass gain: 100 h oxidation (isotherm) (mg cm−2)

Kanthal Super 1800 MoSi2 MoSi2/15 vol.% SiC MoSi2/30 vol.% SiC

450–500°C

600–800°C

1600°C

90.04

+0.02…0.15

+4.0

+0.02…0.2 −0.04…+0.1

+0.02…0.2 +0.2…0.4

– –

−0.2…+0.3

−0.1…+0.2

−0.06…+0.1

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Fig. 4. Shrinkage of pre-alloyed MoSi2 powder (grade A) with 0, 30, and 50% additive to enhance sintering by CRS.

ing strength for 30 vol.% SiC was determined starting from room temperature up to 1100°C.

3.3. Processing of silicides and composites 3.3.1. MoSi2 materials without reinforcements Based on the development of Mo – 2Si powder mixtures described earlier different parts have been prepared by pressureless and pressure assisted sintering. In order to produce low-cost mixtures of pre-alloyed MoSi2 and elemental powder mixtures, the influence of additions on the sintering behaviour of the newly developed powders was investigated. In Fig. 4 the shrinkage curves of powder compacts from pre-alloyed MoSi2 (grade A) with 0, 30, and 50% Mo – 2Si additive are shown. The additive supports the densification though some initial swelling was found. The final densities were 86% TD, 92% TD, res. 93% TD. A very attractive route for production of complexly shaped MoSi2 parts may be the metal injection moulding process (MIM) [9]. Considering the poor sinterability of pre-alloyed powders, the influence of different amounts of activated Mo – 2Si-mixtures was studied. After optimization of the debindering and sintering process a similar behaviour of sintering compared with binder-free-mixtures (Fig. 4), were observed.

3.3.2. MoSi2 composites A full database of mechanical properties of MoSi2 composites is not yet available. In our investigations composites were prepared by both pressureless and pressure assisted sintering of activated Mo–2Si powders with 15, 30, and 50 vol.% of reinforcements (SiC, TiB2). Fig. 6 illustrates the creep behaviour of different hot pressed MoSi2 materials and Kanthal Super. Some properties are summarised in Table 1 and Table 2. The fabrication of samples and complexly shaped parts starts from convenient elemental powders. Depending on the required quantities different mills may be employed (mass: 20, 200 and 2000 g) for preparation of powders. In the final milling step the required quantity of reinforcements was added if starting powders for composites are needed, to prepare a homogenous distribution of the metal-silicon dispersion and hard phases. After mixing, the powders may be densified by uniaxialpressing, cold isostatic pressing, or HP. For certain applications MIM, tape casting, or slip casting may be used. In this case, powder feedstocks or suspensions are prepared by common techniques. To remove additives or other organic components from the powder compacts, a debindering burn off is necessary at the beginning of the sintering process. Fig. 7 shows three different stages of fabrication of MoSi2 tensile testing

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Fig. 5. Light optical microstructure of MoSi2 materials consolidated at HP by 1450°C (A), 1300°C (B) and 1450°C (C) and an MoSi2 –15 vol.% SiC composite hot pressed at 1450°C (D).

samples (as injected, after first debindering, as sintered). Fig. 8 shows a tape casted MoSi2 – 15 vol.%TiB2 pre-form [22]. To obtain fully dense parts debindering and sintering are necessary.

3.4. Generalisation First investigations were focused on MoSi2 materials but the goal was to develop a universal processing tool for several other intermetallic materials. The application of this route to different silicide phases of Ti–Si, Fe–Si, and Re–Si phase diagrams was confirmed. Different properties of TiSi2 and Ti5Si3 were presented at [21].

the whole temperature range. The traditional fabrication of heaters (Kanthal Super) uses glassy phases in order to get an efficient densification. These additives in combination with sophisticated hot forging techniques produce a dense material and also prevent internal oxidation via pore canals. Nevertheless, this method is not suitable for sintering of parts from silicides or silicide composites having a complex shape for structural application, and is restricted to shapes such as rods. The thickness of protecting glassy layer at the surface changes with temperature and oxygen partial pressure, which results in unacceptable dimensional variations. In the bulk the silicide grains are embedded in the high melting, but plastic ( \ about 1200°C) glassy phase, resulting in a deformation via plastic flow at low

4. Discussion Three different methods have been reported to prepare samples or parts from silicide materials, without glassy phases, as shown in Fig. 1. All techniques are based on PM methods.

4.1. Pre-alloyed powder sintering The first technique starts from pre-alloyed powder (i.e. MoSi2). Such powders have a poor sinterability. In order to obtain a high densification the powder must be HP or HIP at high temperatures (\ 1700°C) to activate sintering, because conventional powder metallurgical methods fail. Different liquid phases from additives such as Ni, Co, Al, Fe, NiCoCr, and others [6,19], support the densification partially. Secondary phases form which modify the microstructure, and subsequently the mechanical properties at high temperatures, as well as the oxidation and corrosion behaviour over

Fig. 6. Minimum creep rate for hot pressed MoSi2 materials (0, 15, and 30 vol.% SiC) and Kanthal Super 1800.

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duced by thermal stresses in the brittle material. Furthermore it is difficult to produce homogeneously distributed reinforcements in a silicide matrix. The main disadvantage of this route is the, ‘run away’ of the reaction after initiation.

4.3. CRS process

Fig. 7. Tensile testing samples fabricated from commercial pre-alloyed MoSi2 powder and an additive for CRS by MIM (as injected (left), after first debinding step (middle), and sintered (right)).

stresses. The main disadvantages of this route are the need of pressure assisted methods to densify the material, a coarse microstructure and a strong limitation in parts geometry.

4.2. SHS Alloys having a very high mixing enthalpy at high temperature can utilise the self-propagating high temperature synthesis (SHS) to produce high melting alloys as silicides. This process has a good energy efficiency, due to self-propagating phase formation after process ignition. In the last few years this process has been developed to create new materials and parts [20]. However it is better suited to produce starting material, than the manufacturing of homogenous parts consisting of silicides or silicide composites. During SHS a local temperature close to the melting point of the silicide may be reached but the non-uniform character of the reaction often prevents a homogeneous densification of the material. This results in a mixture of highly densified regions with large grains, pores and cracks intro-

Fig. 8. Tape casted pre-form for the investigation of CRS of MoSi2.

In first state of milling a lamellar microstructure of a ductile metal phase (Mo, Ti) and the brittle silicon phase is observed [25]. This condition is comparable to the powder morphology during mechanical alloying without any chemical reaction. With increased milling time the lamellae become smaller and the XRD-pattern for the silicon phase is widened [25]. This treatment results in various beneficial factors for sintering: “ Reduction of grain size. “ Increased number of interfaces between chemically different phases. “ Increasing ratio of grain boundary volume to the total powder volume. “ Increased amount of amorphous phases. “ Incorporation of micro-impurities (i.e. Fe, Ni CrB 0.2%) caused by ball abrasion during milling. The starting densities of the mechanically activated powder bodies were lower (about 55% TD) compared with commercial pre-alloyed materials (about 65% TD). Nevertheless the final densities after sintering (Fig. 3), were higher and the process of shrinkage was totally different to pre-alloyed MoSi2 powder. This results from different mechanisms of sintering. Powders milled for 1 h only react via a SHS process visible by singularity of shrinkage at about 1280°C. This correlates with the relevant DSC curve for 1 h. After prolonged milling time (3 h) only a moderate SHS-behaviour was observed at a lower temperature (about 1200°C). Powders milled for 8 h showed a completely different sintering behaviour. It was found that a swelling of 5% starts at 550°C, followed by a length reduction starting at 800°C to a total shrinkage of 15% at 1400°C. The comparison of this material and commercial MoSi2 powder is given below: “ Commercial MoSi2: change of density from 64 to 84% TD, open porosity, grain size 20 mm (similar to the starting grain size distribution). “ 8 h-Milled powder: change of density from 54 to 94% TD, closed porosity fine microstructure 1–5 mm, small spherical pores. From the well known Hall–Petch relation it can be concluded that a grain refinement of the material generally results in an enhancement of yield stresses. Since all materials produced by the CRS route show very fine grained microstructures (4–10 mm, see Fig. 5), it can be concluded that the material exhibits excellent mechanical properties (Table 1). The main advantages of this route are controlled sintering (no ‘run away’ of reac-

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tion), reduction of stresses as well as crack formation during sintering, closed porosity after pressureless sintering, fine microstructures, no need to use glassy phases to improve sinterability of the material, and variable geometry of parts. From the above mentioned facts some requirements for enhanced sintering can be concluded. The starting powders must meet the following conditions in order to have a higher lever of total energy and therefore sinter most efficiently: “ Numerous chemical gradients (i.e. use of elemental powders instead of completely pre-alloyed powders) that have an appropriate chemical activity for sintering. “ High amount of surface energy (i.e. fine powders) and interfacial energy (i.e. crystallises inside a grain). “ High volume of grain boundaries. “ Negative mixing enthalpies of the elemental partners. “ Partial amorphisation of the material. “ High dislocation density in the material. “ A sufficient solubility of at least one element in the other one. These parameters are linked to the chosen powder. However, considering these requirements during the manufacturing of sophisticated powders for silicides or silicide composites may lead to an improved starting material for enhanced sintering (see Fig. 1C). The goal of this research is to produce a powder with improved sintering properties.

5. Conclusions 1. The new route of CRS permits an efficient fabrication of new silicide materials. 2. An application of different processing routes (pressureless and pressure assisted sintering, MIM, tape casting), complex shaped parts as well as a high level of mechanical properties and oxidation/corrosion resistance starting from the basic idea of the CRS process. 3. The material (MoSi2) produced by the CRS technique shows excellent mechanical properties and a very good oxidation resistance due to its small grain size. 4. The results concerning sintering of MoSi2 by CRS can be transferred to other intermetallic materials as TiAl or Ti-silicides.

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Acknowledgements The authors are grateful to the support of the National German Project of the Federal Ministry for Education and Research, Project No. 03 M 3059, Fraunhofer Society, MTU Munich, Munich and inocermic GmbH, Hermsdorf.

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