MICROSTRUCTURAL CHARACTERIZATION OF

RESEARCH & DEVELOPMENT

MICROSTRUCTURAL CHARACTERIZATION OF CONVENTIONALLY AND PLASMA-SINTERED Fe-NbC AND Fe-TaC COMPOSITES Aloisio Nelmo Klein,* Antonio Eduardo Martinelli,** Rubens Maribondo Nascimento,** Domingos Sávio de Araújo Paulo,*** Bruna Candice de Freitas Guedes,**** and Clodomiro Alves Jr.****

INTRODUCTION

Ceramic-hardened ferrous composites can be used as cost-effective structural components in a variety of applications including high-speed steelsi-3 and electrodes used in the production of industrial gases."* To this end, ceramic or cermet powders, such as NbC, TaC, TiC, WC, and VC have been ball milled with iron powder, annealed, pressed, and subsequently sintered. The dilatometric behavior of (Fe,C)-NbC and (Fe,C)TaC with ceramic contents up to 20 w/o, has been monitored in order to establish the sintering parameters required to produce dense composites, thus avoiding the inherent difficulties encountered in sintering conventional tool steels, such as limited sinterability and narrow temperature-densification ranges.5 The choice of starting powders, heating source, and sintering route determine the microstructure and properties of sintered metal-ceramic composites. Although a range of microstructures result from different combinations of the aforementioned parameters, the major factor involved in manufacturing sintered metal-ceramic composites is the uniformity of the ceramic-particle dispersion and the sinterability of the material to achieve the target density. Among other methods, ceramicreinforced ferrous composites have been sintered conventionally in the presence of a liquid phase or by plasma-assisted sintering. In the latter process, heat is generated by ionic bombardment of negatively biased samples that function as the cathode of an abnormal glow discharge.

NbC and TaC powders were used to reinforce sintered steels. Powder mixes were milled for different periods of time, annealed, and pressed at 600 MPa. Pellets containing up to 20 w/o NbC or 20 w/o TaC particles were sintered conventionally at l,180''C-l,250°C or plasma sintered at ÏSO'C-ÇOO'C. Sintered composites exhibiting -97% of the pore-free density (PFD) were obtained with both sintering approaches. High-density composites with homogeneous microstructures resulted from solid-state plasma sintering utilizing prealloyed iron powder at relatively low temperatures by adjusting the process parameters, in particular the heating rate. Densification by conventional sintering necessitated the use of a liquid phase and higher sintering temperatures. Microstructural features such as the distribution ofthe reinforcement particles, solidified liquid-phase clusters, pores, and a dendritic structure were identified for the two sintering modes.

'Professor, Universidade Federal de Santa Catarina, Department of Mechanical Engineering, Caixa Postal 476, Campus Universitario Trindade CEP 88040-900, FlorianópoUs, SC, Brazil, **Professor, Universidade Federal do Rio Grande do Norte, Department of Materials Engineering, Av. Salgado Filho, 3000, Lagoa Nova CEP 59.072-970 Natal/RN, Brazil: E-mail:aemart(mol.com.br, ***Professor, Instituto Federal de Educaçâo, Ciencia e Tecnologia do Rio Grande do Norte, Av. Salgado Filho, 1559, CEP 59015-000 Natal/RN, Brazil, ****DSc Student, Instituto Militar de Engenharia, Praça General Tibúrdo 80, CEP 22290-270 Rio de Janeiro, RJ, Brazil, *****Professor, Universidade Federal do Rio Grande do Norte, Department of Mechanical Engineering, Av. Salgado Filho, 3000, Lagoa Nova CEP 59.072-970 Natal/RN, Brazil

Volume 47, Issue 6, 2011 International Journal of Powder Metallurgy

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MICROSTRUCTURAL CHARACTERIZATION OF CONVENTIONALLY AND PLASMA-SINTERED Fe-NbC AND Fe-TaC COMPOSITES

Using a hollow-cathode geometry, the ionization rate is higher than that of a linear abnormal glow discharge.6.7 Upon adjusting the plasma-sintering parameters, it is possible to produce metal-ceramic composites with tailored properties, including uniform bulk characteristics or functionally graded microstructures and porosity as a result of the combined effect of heating and surface bombardment. In addition, plasma sintering is usually carried out at lower temperatures (800°C-900°C) compared with conventional sintering (l,150°C-l,280°C); this is an advantage when applied to dissimilar material interfaces, such as metal matrices and ceramic-reinforcing particles. Moreover, at lower temperatures, sintering takes place in bodycentered cubic a-iron which translates into enhanced diffusion rates. Nevertheless, important characteristics of plasma sintering in relation to composite materials remains in need of further investigation, particularly those involving the effect of milling time and the relationship between

heating rate and liquid-phase sintering. The main objective of this work was to characterize and differentiate between the microstructure of (Fe,C)-NbC and (Fe,C)-TaC composites sintered conventionally or by a thermal DC hydrogen plasma. Different iron powders and composite compositions were used to induce solid-state or liquid-phase-assisted sintering in both sintering environments. EXPERIMENTAL PROCEDURE

The starting powders consisted of water-atomized Ancorsteel lOOOB (Dgg = 92 |um, phosphorous content = 0.005 w/o) and prealloyed 45P (Dgg = 40 pm, phosphorous content = 0.45 w/o) iron powders supplied by Hoeganaes Corporation, Cinnaminson, New Jersey. Scanning electron microscope (SEM) images revealed the irregular and homogeneous morphology of the atomized powders, Figures l(a) and l(b). NbC (D-Q = 2 |um) and TaC (D^g = 9 nm) powders supplied by H.C, Starck, Goslar, Germany, were used as the rein-

Rgure 1. Images of (a) Fe 1000B. (b) prealloyed F3 45P. (c) NbC. and (d) TaC powders. SEM

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TABLE I. COMPOSITION OF COMPOSITE POWDER MIXES (w/o)

1 2 3 4 5

NbC

TaC

10.0 20.0

10.0 10.0

10.0

1.0 1.0 1.0 1.0 1.0

1.5 1.5 1.5 -

FeiOOOB

Fe45P

87.5 77.5 87.5 -

-

89.0 89.0

forcements. Figures l(c) and l(d). Graphite and FejP were added to adjust the carbon content of the matrix and as liquid-phase-sintering aids, respectively. A summary of the compositions investigated is given in Table I. Batches (26 g) of each composition were placed in 270 mL grinding jars of a planetary mill (Fritsch Pulverisette 7) along with 135 mL of acetone and a small amount of liquid wax (Ceracer 631, Shamrock Technologies, New Jersey) to provide oxidation protection. The weight ratio of grinding media (8 mm dia. steel spheres) to powder was 10:1. After milling for 10 h (Condition A) or 20 h (Condition B) at 300 rpm, the powders were furnace dried and annealed at 800°C for 1 h under hydrogen at atmospheric pressure. Cylindrical pellets 10 mm dia. were pressed uniaxially at 600 MPa using zinc stéarate dissolved in acetone to lubricate the die. Green densities were determined by dimensioning and weighing. One set of pellets was sintered conventionally in a resistance-heated furnace at a heating rate of 10°C/min. The sintering atmosphere consisted of flowing hydrogen from room temperature to 800°C and argon up to the sintering temperature. The holding time at the sintering temperature was 30 or 60 min, after which the furnace was cooled in argon at an average cooling rate -lOT/min. A second set of green pellets was sintered in a DC plasma chamber illustrated in Figure 2. The reactor chamber was 300 mm dia. and its height was 400 mm. The samples were placed individually on the cathode and polarized to a bias potential of -600 V. Both ionized and neutral plasma species collided constantly with the sample, producing highly localized thermal peaks at and near the surface of the sample. The thermal efficiency of the system was boosted in the hollow cathode where the electrons are progressively reflected by the inner walls, thus increasing the number of collisions and, therefore, the density of ions. A carbon steel ring positioned on top of the cathode of the system was used to provide this effect. The pellets Volume 47, Issue 6, 2011 International Journal of Powder Metallurgy

Ring Sample

1. 2. 3. 4. 5. 6.

To Hydrogen Hydrogen Cylinder To Vacuum Vacuum Pump Thermocouple Pressure Gauge

7. 8. 9. 10. 11.

Hollow Cathode Anode DC Source Temperature Control Glass Chamber

Rgure 2. Hoitow-cathode plasma sintering facility—schematic

were positioned at the center of the cathode 5 mm from the shield. Plasma parameters that define the sintering conditions include hydrogen pressure and flow rate; these were 1.2 kPa and 2.5 x lO"'* L/s, respectively. Sintering was carried out at different heating rates (10°C to 100°C/min) and sintering temperatures (750°C to 900°C). A summary of the sintering conditions utilized is given in Table II. The final density of the composites was measured by water picnometry and expressed in terms of the % PFD. The PFDs of the composites containing 10 w/o and 20 w/o NbC were 7.86 g/cm-^ and 7.87 g/cm-^, respectively, whereas the density of the composites containing 10 w/o TaC was 8.52 g/cm-^. These densities are based on PFDs of 7.9 g/cm-3 and 14.5 g/cm^ for NbC and TaC, respectively. Sintered samples were cold mounted in polyester resin, ground on SiC paper (220/320/400/ 600 mesh), polished down to 0.3 |im using an alumina slurry, and etched in 2 v/o nital. Microstructural evaluation was carried out using

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TABLE II. SINTERING PARAMETERS Composite

1A 1B 1A 1B 2A 2B 2A 2B 2A 2B 3A 3B 3A 3B 3A 3B 4A 4B 4A 4B 4A 4B 4A 4B 4A 4B 5A 5A 5A 5A 5A

Conventional Sintering

Plasma Sintering

Temperature Holding Time (X) (min)

Temperature Holding Time (X) (min)

1,250 1,250

60 750 850

1,180 1,180

750 750 850 850 1,180 1,180

1,180

60 60

60 60 60 Solidified Liquid Phase

30 30 750 750 800 800 850 850 900 900

60 60 60 60 60 60

750 800 850 900

60 60 60 60

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Figure 4. Representative microstructure of composite IB (Fe 1000B-10 w/o NbC) conventionally sintered at 1.250°C for 60 min. SEM TABLE III. SINTERING TEMPERATURE AND PFD (%) OF COMPOSITES Composite

optical microscopy (OM) (Olympus BX60M) a n d SEM (FEI XL30 ESEM). RESULTS AND DISCUSSION High-energy milling for 20 h at 300 rpm homogenized the powder mixes containing Fe-IOOOB and also resulted in the formation of composite particles by mechanical embedding of the smaller harder-carbide particles in the larger ductile-iron particles. Figure 3. However, a large number of carbide particles were still located preferentially at the grain boundaries of the iron particles, as shown in Figure 4. Sintering with a FejP addition occurred in the presence of a liquid phase. Composites milled at 300 rpm for 20 h (Condition B) and conventionally sintered were consistently denser than those milled at 300 rpm for 10 h

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Figure 3. Image of Fe 1000B-10 w/o NbC powder after wet miiling for 20 h. SEM

30 30 750 750 850 850

1,180 1,180

60

30 30

1A 1A 1B 1B 2A 2A 2B 2B 3A 3A 3B 3B 4A 4A 4B 4B 5A

Conventional Sintering Sintering Temperature ( X )

PFD (%)

1,250

94

1,250

97

1,180

94

1,180

95

1,180

92

1,180

96

1,180

93

1,180

93

1.180

92

Plasma Sintering Sintering Temperature ( X )

PFD (%)

800

92

800

92

800

90

800

92

850

93

850

90

800 850 850

97 95

(Condition A), Table III. The microstructure of sample IB (Figure 4) is representative of a composite conventionally sinVolume 47, Issue 6, 2011 Intemational Joumad of Powder Metallurgy


tered at l,250°C for 60 min under hydrogen after milling for 20 h. A liquid phase was formed during sintering, as evidenced by the number of areas with a morphology characteristic of a prior liquid phase in the microstructure, and by distortion of the sample by excessive local melting. Carbide agglomerates were also observed. The sample achieved -97% PFD and was characterized by a uniform distribution of carbide particles throughout the ferrous matrix. Composite 2 (Fe lOOOB-20 w/o NbC) was milled for 10 h or 20 h and conventionally sintered. Milling for 10 h resulted in agglomeration of the carbide particles and a preferential distribution along the grain boundaries of the iron powder. In contrast, milling for 20 h resulted in a uniform distribution of carbide particles and a homogeneous microstructure. Figure 5. A concentration of pores was observed in areas previously occupied by the liquid phase.

Pores

Ferrous Matrix

NbC.

Í Solidified •«! Liquid Phase

Figure 5. Representative microstructure of composite 2B (Fe 1000B-20 w/o NbC) conventionally sintered at 1,18O'C for 30 min, SEM

Pores

Ferrous Matrix

NbC

Pearlite

figure 6. Representative microstructure of composite 4A (Fe 45P-10 w/o NbC) conventionaily sintered at 1,180°C for 30 min, SEM


Conventional sintering of composites containing Fe45P occurred essentially in the solid state. The dispersion of carbides and residual porosity can be observed in the microstructure of composite 4A, Figure 6; no signs of a prior liquid phase exist. The microstructure of conventionally sintered composite 3B (Figure 7(a)), exhibits a uniform distribution of TaC particles with a 96% PFD. The graphite increased the hardness of the ferrous matrix by the formation of pearlite, as shown in Figures 7(b) and 7(c). In these micrographs it is also possible to observe the presence of TaC particles at the grain boundaries of the ferrite and evidence of a solidified liquid phase. The heating rate plays an important role during plasma sintering assisted by transient liquidphase formation. It controls the evolution of the microstructure since the surface of the sample reaches the sintering temperature after relatively short times. Samples plasma sintered at constant hydrogen pressure (1.2 kPa) and flow rate (2.5 x lO""* L/s) with the hollow-cathode configuration melted at heating rates ~40°C/min prior to any significant densification. Heating rates >40°C/min resulted in excessive surface heating. The hollowcathode configuration confined the discharge around the sample, thus increasing the current density and heating effects compared with discharges created using a planar cathode. Excessive ion bombardment overheated the surface of the sample above its melting point. As a result, a significant amount of liquid phase was formed during the early stages of sintering, before neck formation by surface diffusion develops and aids in shape retention. Consequently, composites containing FejP melted from the surface inwards. The confinement effect depends on the distance between the sample and the shield. Decreasing the heating rate to 30°C/min prevented the sample from melting by reducing the amount of liquid phase formed, and by delaying its formation. However, the microstructure was not uniform and extensive residual liquid phase was present close to the surface, as revealed by OM; namely, the bright area in Figure 8(a). SEM confirmed that FegP clusters were present near the surface. Figure 8(b). It should be emphasized that melting was prevented by adjusting the heating rate but maintaining the same final temperature (850^), as measured by a type-K thermocouple positioned at the bottom of the sample. Therefore, the effects observed resulted solely from the high

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Ferrous Matrix

TaC

Pores

(a) TaC

Pearlite

Solidified Liquid Phase

TaC

Pearlite

(c)

Liquid Phase

Figure 7. Representative microstructures of composite 3B (Fe 45P-10 w/o TaC) conventionally sintered at1,180°C for 30 min. SEM

energy of the plasma species that impacted the surface, resulting in high heating rates and, consequently, high-temperature gradients within the sample. Since the plasma-sintered sample was heated from the surface by ion and fast neutralatom bombardment, the local temperature can be expected to be higher than the temperature measured by the thermocouple. This temperature gradient increased as the heating rate increased because the time to conduct the heat generated at the surface to the interior of the sample was short. Moreover, higher heating rates imply a high energy of the plasma species impacting the sample surface, also affecting mass-transport mechanisms that are operative during sintering. Improved plasma sintering can be achieved using auxiliary electrical-resistance heating elements to supply part of the heat necessary to reach the sintering temperature at slower heating rates. Slower heating rates (~10°C/min) reduced the amount of liquid phase and resulted in more

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homogenous but porous microstructures. The sample shown in Figure 9(a) was plasma sintered at 800°C for 1 h to 92% PFD (Table III). Although relatively dense composites were obtained, excessive formation of a liquid phase had a deleterious effect on the dimensional stability of the sample. Plasma sintering also resulted in different microstructures compared with conventional sintering. The same composition (Fe-10 w/o NbC) conventionally sintered resulting in a homogeneous microstructure at 97% PFD, Figure 9(b). Plasma-sintered composites containing Fe-IOOOB and FejP revealed distinct microstructural differences between the surface and interior, even when relatively slow heating rates were used. Representative microstructures of a sample plasma sintered at 800°C are shown in Figure 10. The interior of the composite consisted of ferritepearlite reinforced vidth a uniform dispersion of NbC particles. Figure 10(a). Closer to the surface the micro structure was predominantly dendritic (Figure 10(b)), characteristic of the formation of a Volume 47, Issue 6, 2011 International Journal of Powder Metallurgy


Figure 8. (a) OM image and (b) SEM image and microanalysis near the edge of composite 1B (Fe 1000B-10 w/o NbC) plasma sintered at 850°C for 60 min at a heating rate of 30°C/min

substantial amount of liquid phase and the large temperature gradient resulting from the hollowcathode effect. Due to localized heating, the liquid phase formed at the surface and flowed by capillary action into the interior of the composite, transporting heat and promoting further liquidphase formation with attendant coarsening. Carbide particles were also preferentially dragged by the liquid phase to the grain boundaries of the ferritic matrix, as observed in the plasma-sintered samples containing 20 w/o NbC (Figure 11). Reinforcing particles 40°C/min melted the composites. Heating rates ~30°C/min limited the amount of liquid phase but resulted in inhomogeneous microstructures and loss of shape. Low heating

Volume 4 7 , Issue 6, 2011 International J o u r n a l of Powder Metallurgy

1. F. Velasco, E. Gordo, R. Isabel, E.M. Ruiz-Navas, A. Bautista and J.M. Torralba, "Mechanical and Wear Behaviour of High-Speed Steels Reinforced with TiCN Particles", Int. J. Ref. Met. Hard Mater, 2001, vol. 19, no. 4-6. pp. 319-323 2. M. Vardavoulias, M. Jeadin, F. Velasco and J.M. Torralba, "Dry Sliding Wear Mechanism for P/M Austenitic Stainless Steels and their Composites Containing AljOj and Y2O3 Particles", Trib. Int., 1996, vol. 29, no. 6, pp. 499-506. 3. M.K. Jain, V.V. Bhanuprassad, S.V. Kamat. A.B. Pandey, V.K.Varma, B.V.R. Bhat and Y. R. Mahajan, "Processing, Microstructure and Properties of 2 124 Al-SiC Composites", Int. J. Powder Metal, 1993, vol. 29, no. 3, pp. 259-267. 4. A.S. Silva, A.E. Martinelli, H. Scatena Jr., J.H.E. Silva, C. Alves Jr. and M.P. Távora, "Electrochemical Behavior of Steel-FeNbC Composites Used in the Production of Oxygen", Mater. Res., 2005, vol. 8, no. 2, pp. 151-153. 5. A.E. Martinelli, D.S.A. Paulo, R.M. Nascimento, M.P. Távora, U.U. Gomes and C. Alves Jr., "Dilatometric Behavior and Microstructure of Sintered Fe-NbC and Fe-TaC Composite", J. Mater. Sei, 2007. vol. 42, no. 1. pp. 314-319 6. P. Novak, D. Vojtech and J. Serak, "Wear and Corrosion Resistance of a Plasma-Nitrided P/M Tool Steel Alloyed with Niobium", Surf. Coat. Techn., 2006, vol. 200, no. 18-19, pp. 5,229-5,236. 7. S.F. Brunatto. I. Kuhn, A.N. Klein and J.L.R. Muzart, "Sintering Iron Using a Hollow Cathode Discharge", Mater Sd. Eng. A, 2003, vol. 343, no. 1-2, pp. 163-169.11133

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