Sinterability of Y2O3-Al2O3 particulate stainless steel matrix composites

Applied Composite Materials 3: 15-27, 1996. (~) 1996 Kluwer Academic Publishers. Printed in the Netherlands.

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Sinterability of Y203-A1203 Particulate Stainless Steel Matrix Composites F. VELASCO Universidad Jaime I, Departamento de Tecnologla, Campus Penyeta Roja, 12071 Castellrn, Spain

N. ANTON and J. M. TORRALBA Escuela de Minas de Madrid, Departamento de Ingenierla de Materiales, Rios Rosas 21, 28003 Madrid, Spain

M. VARDAVOULIAS*

and Y. BIENVENU

Ecole des Mines de Paris, Centre des Mat~riaux P.M. Fourt, B.P. 87, 91003 Evry, France

Abstract. P/M 316L austenitic stainless steel has been reinforced with yttria and alumina particles.

In order to improve the sintering behaviour of these composite materials, chromium diboride and boron nitride were added. The sinterability of the different materials has been characterised through dilatometry and sintering curves (sintered density vs. sintering temperature). A metallographic study by SEM coupled with microprobe has also been performed. Composites materials present a good densification. Chromium diboride and boron nitride react with the matrix in different manners, but they both greatly improve the sinterability of reinforced materials. The optimum sintering temperature for these composites materials is 1250 ~ Key words: reinforced stainless steels, borides, sinterability, dilatometry.

1. Introduction Sintered stainless steels have been widely studied from different points of view, such as sintering [1], corrosion behaviour [2, 3], density [4], or surface finishing. In order to improve its corrosion behaviour, several authors have studied the effect of different additions to stainless steel powder, such as copper [5], tin (usually as bronze) [6, 7], etc. As one way to improve their corrosion behaviour, composite materials were developed using ceramic particles as reinforcement. In such cases, a new problem was presented by the interphase between the stainless steel matrix and the ceramic reinforcement, not only from the point of view of its corrosion behaviour, but also its mechanical behaviour. A positive consequence of the addition of ceramic particles to a stainless steel matrix was a greater improvement of wear behaviour, rather than the corrosion characteristics. Of work done in this field, one can single out that of Lal and Upadhyaya [8-11] and Petersen [12, 13]. * Present address: CERECO S.A., EO. Box 146, 34100 Chalkida, Greece.

16

F. VELASCOET AL.

In the present paper we characterize the thermal behaviour of composites made from an austenitic stainless steel matrix reinforced with two different ceramic particles: alumina (A1203) and yttria (Y203). In order to improve the sintering process, two dopants have been used: chromium diboride (B2Cr) and boron nitride (BN). Dilatometry and sintering curves (sintered density versus sintering temperature) were used to determine the thermal characterization.

2. Experimental The main raw material used was an austenitic stainless steel powder, grade 316L (from Coldstream, Belgium), with the following characteristics: 316 LHC Chemical composition: 0.021% C; 13.55% Ni; 16.1% Cr; 0.87% Si; 2.24% Mo; 0.02% Cu. Particle size: 99.8% < 150 #m. Apparent density: 2.95 g/cm3. Flow rate: 28.3 s/50 g. Some characteristics of the materials used as reinforcement and as sintering enhancers are as follows: Alumina (A1203)

A1203 content: 99.5%. Particle size: < 2/zm. a-A1303, grade APC 2011-325 (ALCOA, Sao Paulo, Brazil). Yttria (Y203)

(Y203) content: 99.5%. Total carbon: 0.15%. Particle size: 0.27 /zm. Specific surface: 14.8 m2/g. From H. C. Starck. GmbH & Co.H.G. (Germany). Chromium diboride (B2Cr)

B2Cr content: 99.0%. Particle size: < 45/zm. From Goodfellow (United Kingdom). Boron nitride (BN)

Chemical composition: 6.5% 02; 41% B; 5.20% B203; 600 ppm C; 5000 ppm H20; metallic impurities < 1000 ppm. Apparent density: 0.5 g/cm3. Specific surface (BET): 19.5 m2/g. From H. C. Starck GmbH & Co.H.G. (Germany). The powders were mixed in three different steps: first, ceramic powders (reinforcements and sintering enhancers) were mixed for 10 rain by manual agitation; this mixture was subsequently mixed with the stainless steel powder in the same

SINTERABILITY OF YEO3-AL203 PARTICULATE STAINLESS STEEL

17

manner. Finally, all the materials were mixed and homogenised in a ball mill for 20 min. All the possible combinations (16) among the following compositions were studied: 3 and 5% (wt.) A1203 + 1 and 2% (wt.) BN, 316L bal. 3 and 5% (wt.) A1203 + 1 and 2% (wt.) BECr, 316L bal. 3 and 5% (wt.) Y203 + 1 and 2% (wt.) BN, 316L bal. 3 and 5% (wt.) Y203 + 1 and 2% (wt.) B2Cr, 316L bal. The compaction was done in a uniaxial floating die at 700 MPa. Zinc stearate was used as die wall lubrication. The process of sintering was carried our in v a c u o in a tubular furnace, with 5~ as heating and cooling rates. All the compositions were sintered at different temperatures between l l 0 0 ~ and 1300~ for 30 min in order to study the response of the different materials to temperature. The densities of all materials were determined through the water displacement (Archimedes) method, using a scale with a precision of 0.0001 g and employing the following expression: M Ps =

(Ptot-

Pa) -

P'~

Psell

where Ps is the sintered density, M the weight of the sintered product, Ptot the weight of the sealed material, Pa the weight of immersed material and Psell the sealant density (in our case a paint with a density of 1.1 g/cm3). The dilatometric study was carried out in a B~hr dilatometer in v a c u o . The metallographic study was carried out by scanning electron microscope coupled to a microprobe. Metallographic preparation of the specimens was done by conventional means, using diamond (1 #m) for final polishing. The microprobe study allowed us to analyse the different elements present in the composites (including boron and nitrogen), as well as the interdiffusion between different particles. 3. Results

Figures 1-4 show the sinterability study carried out for all the compositions. As the sintering temperature increases, so does the density. Between 1100 and 1200 ~ the variation is small, but from this temperature to 1250~ the density grows suddenly and strongly. Figures 5-8 represent the results of the same study, but done with dilatometry. The presence of sintering enhancers improves the densification in all materials. In

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F. VELASCO ET AL.

Density (g/cm3)

7,2

7 ...............................................

ii

6,8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6,6 . . . . ~

-

61 1100

~

i 1150

............... ~--1

~ 1200

i 1250

1 1300

Sintering temperature ('C) 3%A1203+1%B2Cr "J" 3%AI203+2%B2Cr ~ 5%A1203 + 1%B2Cr -~- 5 %A1203 +2%B2Cr Sinterability curves of composite materials reinforced with A1203 and using B2Cr as sintering enhancer. F i g u r e 1.

Density (g/cm3)

7,2

7 ...........................................//~ 6,8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

./. . . . . . . . . . . . . . . . . . . . . .

"'.~... ~ . . . . . . . .6. . . . . . . . . . . .' . . . . ... ... . . .6. . . . . . . . . . . _. . . . . . . . . 6,4

6,2"~ . . . . . . . . 6

-a

. 9

" iiii.

,

..................................................................

5 ~ - -

'flOO

,\

,

I

I

1150

1200

1250

1300

Sintering temperature (C) 3%A1203+1%BN "J" 3%A1203+2%BN ~- 5%A1203+ 1%BN -~- 5%A1203 +2%BN Sinterability curves of composite materials reinforced with A1203 and using BN as sintering enhancer.

F i g u r e 2.

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SINTERABILITY OF Y203-AL203 PARTICULATE STAINLESS STEEL

Density (g/cm3) 7,6

............................................. /

1100

1150

1200

..................

1250

1300

Sintering temperature (~C) ~ 3 % Y 2 0 3 + 1 % B 2 C r "4- 3 % Y 2 0 3 + 2 % B 2 C r ~ 5 % Y 2 0 3 + 1%B2Cr - ~ 5 % Y 2 0 3 + 2 % B 2 C r

Figure 3. Sinterability curves of composite materials reinforced with YzO3 and using B2Cr as sintering enhancer.

75

Density (g/cm3)

7,3 7,1 6,9 6,7 6,5 6,3 1100

t 1150

I 1200

t 1250

1300

Sintering temperature (C) 3 % Y 2 0 3 + l % B N "4" 3 % Y 2 0 3 + 2 % B N ~ 5 % Y 2 0 3 + l % B N -a- 5 % Y 2 0 3 + 2 % B N

Figure 4. Sinterability curves of composite materials reinforced with Y203 and using BN as sintering enhancer.

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F. VELASCO ET AL.

Tempera_ture - r_____e_l C__h_ange . in L____ngth 2

2.0

]

% 0.0

.........

-2.0

~ 3 1 6 L + $ % A I 2 0 3 + 2 . % B 2 C r

......... I............... I ......

.-4.0

-6.0 0

200

400

600

800

1000

1200

1400 ~

Figure 5. Relative change in length vs. temperature (sintering in vacuo) for composite materials reinforced with Al103 and using BzCr as sintering enhancer.

Te_mperat__ure- rel. Changei_nL__eegg

2.0

I

% f

~

0.0

3t6L__._.__ -2.0

31nL§ -4.0

.

.

.

.

.

,

~

. . . . . . . . . .

J

.........

~N -6.0 0

200

400

600

800

1000

1200

1400 oC

Figure 6. Relative change in length vs. temperature (sintering in vacuo) for composite materials reinforced with A1203 and using BN as sintering enhancer.

SINTERABILITY OF Y203-AL203 PARTICULATESTAINLESS STEEL T e m p e r a t u ir ree- l~' -~-_- _- -_C_ C_i_h_n_a_L_n_L~genge Lh ______[ i

2.0

21

- - - ~ ,~~- -

% 0.0

-2.0 ......................

.....

1 ..............

.......

-4.0

-6.0

-8.0

-I0.0 0

200

400

600

800

1000

1200

400 0(2

Figure 7. Relative change in length vs. temperature (sintering in vacuo) for composite

materials reinforced with Y203 and using B2Cr as sintering enhancer.

Temperature

2.0

- rel. Change in Len ! rh

!

T

% 0.0 '

~

~

.......... / ....... 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

"~ " ~

............

-2.0

-4.0

...................... ~~, < ~ _ . . . _~_, +. .,_-, ~ ~-- i

,_-~ ~- . - ~

..............

-6.0 0

200

400

600

800

I000

1200

1400 ~

Figure 8. Relative change in length vs. temperature (sintering in vacuo) for composite

materials reinforced with Y203 and using BN as sintering enhancer.

22

F. VELASCO ET AL.

Figure 9a. Microstructure (scanning electron microscopy) of 316L reinforced with Y203 and using B2Cr as sintering enhancer.

Figure 9b. Mapping of different elements for microstructure showed in Figure 9a. Up: from left to right: Ni, Y, Mo and Cr; down: from left to right: Fe and B.

the case o f A1203 reinforcement, the increase of the amount o f sintering enhancer decreases the activation o f the sintering process. Figures 9 - 1 2 show scanning electron microscopic microstructural study. The different mappings of the interesting elements can be distinguished for each microstructure.

SINTERABILITY OF Y203-AL203 PARTICULATE STAINLESS STEEL

23

Figure lOa. Microstructure (scanning electron microscopy) of 316L reinforced with Y203 and using BN as sintering enhancer.

Figure lOb. Mapping of different elements for microstructure showed in Figure 10a. Up: from left to right: N, Fe, B and Mo; down: from left to right: Y, Ni, Cr and A1.

4. Discussion T h e strong increase o f density between 1200 and I 2 5 0 ~ (Figures 1-4) is partially due to the activation o f the sintering p h e n o m e n a in stainless steel particles at these temperatures. However, the main p h e n o m e n o n responsible for this den-

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F. VELASCO ET AL.

Figure 11a. Microstructure (scanning electron microscopy) of 316L reinforced with A1203

and using B2Cr as sintering enhancer.

Figure lib. Mapping of different elements for microstructure showed in Figure 1la. Up: from left to right: B, Mo, Fe and Ni; down: from left to right: Cr and AI.

sification is the liquid phase sintering produced by the reaction of the matrix with the added dopants (B2Cr and BN). At sintering temperatures higher than 1250 ~ slightly higher densities are produced (except for A1203 reinforcement), but the benefits in terms of mechanical and tribological properties are very small compared with the increasing energy cost, without taking into account the worst

SINTERABILITY OF Y203-AL203 PARTICULATE STAINLESS STEEL

25

Figure 12a. Microstructure (scanning electron microscopy) of 316L reinforced with A1203 and using BN as sintering enhancer.

Figure 12b. Mapping of different elements for microstructure showed in Figure 12a. Up: from left to fight: B, Mo, N and Fe; down: from left to fight: Cr, A1, Y and Ni.

d i m e n s i o n a l b e h a v i o u r and the possible volatilization p h e n o m e n a that can occur at high temperatures. T h e s a m e effect can be observed in Figures 5 - 8 , corresponding to the dilatometric runs. There is a strong densification at temperatures higher than 1200 ~ due to sintering p h e n o m e n a and to sintering enhancers.

26

V. VELASCO ET AL.

From this thermal study, one can select 1250~ as the optimum sintering temperature for these composites. Densities close to 90% of the theoretical density are obtained, and for yttria-chromium diboride reinforced composites, the density is higher than 95% of theoretical density. The microprobe detector analysis shows the effect of B2Cr and BN during sintering for different materials, and how the behaviour of both dopants differs. As it can be observed in Figures 9 and 10, yttria remains fully inert with respect to the matrix. When BN is used as binder, it decomposes and one can see how Y203 is enriched in nitrogen, and BN itself is transformed into a complex Cr-Fe-Mo boride. Moreover, in steels reinforced with A1203, one can observe how the A1203 particle remains inert (only a part of Cr from the stainless steel matrix diffuses inside the BN particle, as can be seen in Figure 12). B2Cr reacts with the matrix (Figures 9 and 11), and one can measure the iron and molybdenum content in the dopant particle. The best cohesion of Y203 and A1303 particles with the matrix was previously observed by Lal and Upadhyaya, who indicated that yttria reacts with the chromium oxide from stainless steel powder, forming a complex oxide, YCrO3, that enhances the sintering process and improves the densification of the material. The poor cohesion of the A1203 can be checked in Figure 11, where the particles have been removed (see how A1 mapping shows low contents of this element where the particle should be placed).

5. Conclusions B2Cr and BN act as sintering enhancers in stainless steels based composites, despite the fact that the respective densification mechanisms differ: the former reacts mainly with the stainless steel matrix; the later decomposes and the nitrogen remains free to form new nitrides. The sinterability of Y203 reinforced composites is better than A1203 reinforced composites. When using BN, this is due to the reaction of the nitrogen from the dopant with the reinforcing particle.

Acknowledgements The authors wish to thank to professor Ruiz-Prieto for his opinion on some parts of the discussion of this paper, as well as Dr. Cambronero for his collaboration in some parts of the work. This work has been done thanks to the financial support of the Comisirn Interministerial de Ciencia y Tecnologfa (CICYT) through project MAT94-0230 and the concerted Spanish-French action HF93-26.

SINTERABILITY OF Y203-AL203 PARTICULATESTAINLESS STEEL

27

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