Synthesis of niobium carbide at low temperature and its use in hard metal

By K.Sairam GoudRoll.No:155577

Materials TechnologyDepartment of Metallurgical and Materials Engineering

Contents

• Introduction• Objective • Motivation for the work• Sample Preparation and Experimental Procedure • Results and Discussion• Advantages and Limitations• Conclusion• References

IntroductionNiobium Carbide

Basic Details• Chemical Formula : NbC• Density : 7.820 g/cm3

• Melting point : 3,490 °C

Why and Where to use NbC ?• High hardness, melting point and good thermal conductivity makes it one of

suitable material to be used in hard metals such as cutting tools,drills, abrasives and wear resistant pieces.

How to produce NbC ?• Commonly adopted method in industries is subjecting the Nb2O5 to carbon black

in the temperature range of 1600°C-1800°C for certain time

Objective

• To synthesize NbC using new method at low temperature

(950 °C)and in a short time (2 h)

• To characterize it by XRD,SEM,TG/DTG and granulometry and

thereby investigate its ability to serve as good WC grain inhibitor

by comparing to a commercial NbC powder

Motivation for work

1. Li et al produced NbC whiskers by heating a mixture of Nb2O5

and carbon black at 1120 °C for 30 min

2. Dal et al produced NbC by heating Nb-C composite at 1000 °C

for 4 h under argon(layered oxides=> large amorphous powder)

3. Kim et al produced NbC by reacting Nb2O5 with a mixture of

CH4 and H2 at 850 °C for 8 h (low free carbon contamination)

4. Teixeira da Silva et al used same reactants as in [3] but at

1100 °C for 42 min (high carbon contamination )

Problem Noticed : Either high ‘T’ or High ‘t’

Sample Preparation and Experimental Procedure

1.Preparation• Niobium Oxolate Complex-Fuse Nb2O5 and KHSO4 in the ratio 1:7

-Ground and dissolve in H2SO4

-Neutralize with ammonium hydroxide and precipitate Nb2O5_ nH2O

-The potassium ions are eliminated by repeated filtration

and washing with warm water

-Dissolution of the hydrated oxide in mixture of oxalic

acid and ammonium oxalate, with a 1:3 ratio between

niobium and oxalate ions, and the controlled evaporation

of the solution yield the niobium – oxalate complex

whose composition is (NH4)3[NbO(C2O4)8]_xH2O

• Hard metal

-Three hard metal alloys using raw materials

WC ,NbC and Co powders are produced by wet

ball milling for 48h ,granulation ,pressing and

sintering

2. Experimental Procedure• Carburization Reaction

-Precursor (niobium – oxalate complex )3g taken in ceramic

crucible and placed in alumina reactor attached to furnace

controlled by a T-programmer at 950 °C with 5 °C /min with

mixture of gases H2 (reducer) and CH4 (carburizer) for 2h

• Characterization

1.The niobium complex was characterized by XRD,

SEM and TG/DTG in nitrogen.

2.The product of carburization was characterized by

XRD, SEM and grain size measurement (laser

scattering).

3.The sintered alloys were characterized with regard

to the structure (qualitative WC grain size

evaluation) and hardness (Vickers hardness)

Reactions Involved

Nb2O5 -> Nb2O -> NbOxCy -> NbC

H2 reduces Nb2O5 to Nb2O and other reactions

by carbon

Results and DiscussionTG/DTG Results

Above curves reveal that • decomposition starts at around 100 °C • the material loses mass in temperature intervals: 100 – 145, 180 – 244, 244 – 400 and 600 – 630

°C .• Gases such as CO, CO2, H2O and NH3 evolve.

Fig. TG/DTG curves of the niobium complex calcination in nitrogen

Granulometry and SEM

Fig:Particle size distribution curve of NbC by laser scattering

Fig.SEM pictures of the carburized product (low and high magnifications)

From both results it can be pointed out that ,• Mean particle size of powder is in 40µm• No change in shape of particles • Thus no alteration of the characterstics of the niobium precursor due to carburization

NbC in Hard Metal

Vickers hardness (HV50) of alloys Exp Nb,Com Nb, and NoNb are 1827,1776 and 1629 respectively

(a) (b) (c)

Fig:SEM pictures of the structures of the alloy (a) Com Nb ,(b) ExpNb and (c) NoNb showing the WC pictures of the granulation.

• ExpNb structure is homogeneous ,slightly finer than that of the alloy

ComNb with mixed NbW mixed carbides precipitation (due to the high

amount of NbC added to alloys) is the same. This indicates that the

large particles of the Lab-P NbC powder were very finely milled and a

good dispersion was achieved. The particles of the Lab-P NbC are

highly porous and porous particles are more brittle than bulky ones

• The particles of the Lab-P NbC are highly porous and porous particles

are more brittle than bulky ones

Advantages and Limitations

Advantages • Low temperature and less

time process• Less contamination• Fine particles with different

shapes is possible

Limitations• Gaseous.atmosphere mixture

maintainence• Low temperature => little

chance to eliminate oxygen completely

• High dependence of powder quality on gaseous mixture ratio

Conclusions

• Successfully NbC is sythesised at low temperature (950 °C )

and short time (2h) from niobium – ammonium – oxalate

complex through a gas – solid reaction

• No alteration in shape and size of its particles due to

carburization

• Well dispersion of NbC into WC by milling process is achieved

• Inhibition of WC grain growth by experimental NbC is slightly

more efficient than the commercial powder.

Authors of Paper

F.F.P. Medeiros, A.G.P. da Silva*, C.P. de SouzaDepartment of Chemical Engineering, Universidade Federal do Rio Grande do

Norte, 59072-970, Natal, RN, BrazilReceived 1 May 2001; received in revised form 1 January 2002; accepted 31

January 2002