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Graded titanium boride fibre protective coatings for titanium matrix composites
MATERIALS SCIENCE & EMCINEERINC ELSEVIER
A
Materials Science and Engineering A212 (1996) 242-246
Graded titanium
boride fibre protective coatings for titanium matrix composites H.J. Dudek, A. Werner, R. Leucht
Germm
Aerospace
Resenrck
Estnblishment
(DLR),
Institute
for
Mnterids
Resenrch,
D-51 140 h?ih,
Gcr/mm)
Received 3 October 1995; revised 29 January 1996
Abstract Graded titanium boride coatings(gTiB coatings)were depositedon Sic-SCS-6 fibres. Compositeswith an IM1834 matrix were processedby fibre coating with matrix and hot isostaticpressing(HIPing) of matrix coated fibres. The thermal stability of the compositeswas investigatedby the determinationof the tensileand interface propertiesafter a thermal treatment at 800“C for up to 1000h. Keyworh: Protective coatings; Titanium boride fiber; Titanium matrix composites
during the processing of composites with these fibres,
high thermodynamic stability of the coating material, low diffusion rates for titanium and carbon in the protective coating and low solubility of the components of the coating in titanium alloys (compare for example [ll]), Some of these criteria are fulfilled by TiB,. TiB, has a heat of formation of dG, = 100 kJ mol-’ at.% [12] which is higher than that for the reaction product of carbon with the titanium alloy. TiC has a heat of formation of ilGO = 84 kJ mol- ’ at.% [12]. The solubility of boron in titanium is low, about 0.05 wt.% in the temperature range of 750- 1300 “C [13] compared, for example, with the solubility of carbon in titanium (OS wt.%) [14]. Based on the results of Pailler et al. [15] who reported a stoichiometry range for TiB2 of 2-1.9 and for TiB of l-0.88, Knights proposed an additional system TiB,.9/TiB,,,, to stabilize the fibre-matrix interface in titanium matrix composites [13]. More recent studies indicate, however, that at least for TiB the stiochiometry range is smaller [16]. Contrary to the predictions of the thermodynamic assessment of Knights, the TiB, coating of the BP SM 1240R fibre (1992) has a gradient of boron in the
TiB, reacts with titanium to form TiB needles [9,10].
coating with a high concentration of boron in the
The TiB reaction zone causes a degradation of the longitudinal composite strength [6]. The criteria for a material to act as an additional protective coating in titanium matrix composites are:
vicinity of the titanium alloy, Fig. l(a). During processing and thermal treatment of the composites the boron forms TiB [lo] and the carbon diffuses through the titanium boride coating, Fig. l(b).
1. Introduction
Interfacial thermal stability of commercially available Sic fibres with a carbon protective coating for titanium matrix composites is limited to an application temperature of approximately 600 “C. At higher temperatures, depending on the titanium alloy used as matrix, the carbon coating may be rapidly consumed. At 700 “C a coating lifetime of up to 6000 h in Ti-
6Al-4V alloys was reported ing in Ti-6Al-4V-alloys
[l]. At 800 “C the coatis consumed after approximately 500 h [2]. For other matrices, such as the near a-alloy IM1834 [3] and for a2 + p-titanium aluminides [4,5], a higher thermal stability is observed. For an application temperature above 700 “C, stability is still insufficient. For MMC applications exceeding 700 “C an additional coating is required. Different materials are presently being considered as additional diffusion and reaction barriers: TIC, TiB2, TaC [6], Y,O, [7] and others [8]. The British Petroleum company has developed an additional TiB2 protective coating for the SM 1240R fibres. However,
0921-5093/96~$15.00 0 1996- ElsevierScience S.A. All rightsreserved
H.J.
2. Experimental
Dudek
et al. / Materials
Science
A212
(1996)
243
242-246
details
An additional protective coating TiB1,9/Ti0,88 as proposed by Knights [13] was attempted in the following experiment. A magnetron sputter device equipped with two magnetron pairs was used to deposit the graded titanium boride coating (gTiB) on fibres [20]. SiC-SCS6R fibres from Textron Inc. were used. One pair of magnetron cathodes was supplied with TiB, targets, the other with pure titanium targets. The TiB, cathodes were operated at a constant power supply of 2 kW. After deposition of about 100 nm TiB,, the power supply of the Ti cathodes was gradually increased from zero to a value of 0.8 kW while the deposition of TiB, was continuing. Assuming a linear dependence of the sputter coefficient of titanium on power supply, the linear increase of the power supply should result in a graded titanium boride coating with a high boron concentration in the vicinity of the carbon coating and a low boron concentration near the titanium matrix. Deposition of the matrix which was chosen to be IM1834 titanium alloy (Ti-5.8A1-4.OSn.3.5Zr-0.7Nb0.5Mo-0.35Si-0.06V; wt.%) was achieved with the same equipment. Composites were processed by HIPing the coated fibres [17-191 at 190 MPa at 930 “C for
0
and Engineering
20 40 60 80 Sputter time I min
100
(4
(a)
(b)
Fig. 2. Metallographic polish of the interface in Sic-SCS-6 fibre reinforced IMI834 alloy: (a) in the as-processed composite with the additional gTiB coating, (b) after thermal treatment at 800 “C for 500 h of the composite with the additional gTiB coating, (c) after thermal treatment at 800 “C for 500 h of the composite without an additional gTiB coating and (d) in the composite with an additional gTiB coating treated at 800 “C for 1000 h.
0.5 h. The thermal stability of the fibre-matrix interface was studied by a thermal treatment of the composites with and without additional gTiB coating at 800 “C for up to 1000 h in vacuum. After thermal treatment the composites were tensile tested and the interface was examined by SEM and EDS line scan analysis. The EDS analysis was performed quantitatively with reference to pure element standards. The boron concentration was determined by difference to 100%. Sputter time I min
(b) Fig. 1. Auger electron spectroscopy (AES) depth profile of the protective coating of the BP SM 1240R fibre (a) and AES depth profile of the interface of a monofilament composite with a SM 1240R fibre coated with the Ti-6Al-4V alloy and thermally treated at 700 “C for 670 h (b).
3. Results In Fig. 2, metallographic images of the fibre-matrix interface and in Fig. 3, EDS line scan analysis results of the composites are shown. In the as-processed com-
244
H.J. Dudek
et al. I Materials
Science
posite with additional gTiB coating, Fig. 2(a), the carbon protective coating of the Sic-SCS-6 fibre survives completely without any interaction with the gTiB coating. This can be concluded from the observed thickness of the C-coating (roughly 3 pm) and the results of the EDS Si-linescan, Fig. 3(a), which show the usually observed concentration profiles with two Si-maxima, the last one near the carbon coating surface. At the gTiB-matrix interface some TiB needles are growing from the coating into the matrix, similar as observed in composites with BP-SM 1240Rfibres. Fig. 2(b) shows the interface of a composite with the additional gTiB coating thermally treated at 800 “C for 500 h. The carbon coating of the SCS-6 fibre shows some degradations, only approximately 1.8 pm remains. As can be seenfrom the Si-line scan, Fig. 3(b), the part of the carbon coating with the second silicon maximum is lost. The EDS-carbon intensity (and TEM results 1’ C-coating
1:’ of fibrei
100
II
1 0
' 11 ,l 2
1 ‘:’ gTiB-layer j
' 4
1 matrix (IMI 834)
A: JJL,6
iv 8
I I
I 10
distance from fibre surface I pm
(4
1
0
2 distance
4
6
8
IO
from fibre surface I pm
(b) Fig. 3. EDS line scan of the fibre-matrix interface in the composite with fibres coated with the additional gTiB coating and an IM1834 matrix after processing (a) and after thermal treatment at 800 “C for 1000 h (b). The boron concentration was determined by the difference to 100%.
rind Engitzeeriug
A212
(1996)
242-246
[21]) of the inner coating near the C-coating identifies this region as Tic. Adjacent to the TiC reaction layer the residual gTiB coating is found. The particles located outside the gTiB layer and having a diameter of a few microns showed only titanium and carbon peaks in EDS analysis. This leads to the suggestion that the particles consist also of Tic. In Fig. 2(c) the interface of the composite without gTiB coating after a thermal treatment at 800 “C for 500 h is shown. Again a part of the coating is consumed. Only approximately 2.5 lun of the carbon coating of the SCS-6 fibre remain. Adjacent to the remaining carbon coating a reaction zone with high carbon concentration is found. Obviously this layer consistsmostly of TiC (Tic typically forms adjacent to the carbon coating, for details of the composition of the reaction zone in Sic-Ti composites; see for example [22,23]). In this composite TiC particles are also found at the interface. The TiC particles are smaller with diameters of 1 pm, however. In composites with an annealing time of 1000h at 800 “C, Fig. 2(d), the carbon coating has a remaining thickness of only 0.5 pm and 2 iirn for fibres with and without the additional gTiB coating, respectively. The additional gTiB coating obviously can not prevent the dissolution of carbon into the matrix and the formation of titanium carbide. At 800 “C composites with the additional gTiB coating have a lower thermal stability than those without that coating. As can be seen from Fig. 2(d) after the thermal treatment at 800 “C for 1000h the additional gTiB coating remains in the composite: the gTiB is thermodynamically stable. The TiC formation between the fibre and the gTiB coating, however, shows that the additional coating is no diffusion barrier for carbon and titanium. The titanium diffuses through the gTiB coating and reacts with the carbon thus reducing the carbon protective coating thickness of the SCS-6 fibre. Owing to the high diffusion rate of titanium through the gTiB coating, the volume increaseof the reaction product TiC results in a shift of the gTiB coating away from the fibre. In Fig. 4 the mechanical properties of the different composites studied are shown. As was shown in [3] the tensile strength, the elongation and Young’s modulus of composites without the additional gTiB coating remain unchanged even after 1000h of annealing at 800 “C. In the as processedcondition compositeswith the additional gTiB coating have nearly the same properties as the composites without the additional gTiB coating. The observed small deviations of the tensile strength and elongation in Fig. 4 for composites with gTiB coatings, in comparison with the compositeswithout the gTiB coating, are probably causedby a statistical scatter of the results. With increasing thermal treatment time at 800“C the tensile strength and elongation decreasesfor composites with the gTiB coating.
H.J.
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et al. / Materials
Science
and Engineering
A212
(1996)
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Fig. 4. Tensile strength (a), Youngs modulus (b) and elongation (c) of composites processed with and without an additional gTiB coating in the as processed condition and after thermal treatment at 800 “C for 500 h and 800 “C for 1000 h (horizontal lines show the values for single samples).
4. Discussion and conclusions
Acknowledgements
The additional gTiB coating was deposited on the fibre with the aim to stabilize the fibre-matrix interface during a thermal load of the composites. The present investigations show that the additional gTiB coatings deposited on the fibres using magnetron sputtering according to the predictions of the thermodynamic assessment [13] destabilze the interface resulting in a higher reaction speed compared with the composites with uncoated Sic-SCS-6 fibres. The additional interaction of the gTiB coating with the matrix under formation of a reaction zone causes also a strong reduction of the tensile properties of the composites. The formation of titanium carbide in the neighbourhood of the carbon coating of the KS-6 fibre and in the interface gTiB-matrix shows that carbon and titanium diffuse through the gTiB coating. The diffusion rate of carbon and titanium in the gTiB coating deposited by magnetron sputtering is probably enhanced owing to the small grain size and the non stoichiometry of the TiB, and TiB [21] which allows vacancy formation in the titanium borides. The higher reaction rate for composites with the additional gTiB coating in comparison with composites without this coating can be explained by the formation of a reaction and diffusion barrier in Sic-SCS-6-IMI834 composites by alloying element segregation [3]. This “natural” reaction barrier obviously does not form in the composites with the additional gTiB coating.
The authors are grateful to Professor Kaysser for the support of this work.
References [l] A. Vassel, R. Mevrel, J.P. Favre and J.F. Stohr, Fibre-Matrix Interface Properties in Ti-IMatrix Composites, in Chemical Compatibility and Micromechanical Behaviour, AGARD Report R-796, 2994, p. 9.1--9.11. [2] H.J. Dudek, R. Leucht, R. Borath and G. Ziegler, Analytical investigations of thermal stability of the interface in Sic-fibre reinforced Ti6A14V-alloy, Microchim. Acta, II (1990) 137-148. [3] ‘H.J. Dudek, R. Leucht and J. Hemptenmacher, Thermal stability of Sic-SCS-6 fibre reinforced IM1834 alloys, Metal. Mater. Trans., submitted for publication. [4] R. Leucht, K. Weber, A. Werner, J. Hemptenmacher and H.J. Dudek, Sic fibre reinforced super a2 alloy processed by fibre coating and HIPing, in preparation. [5] D.B. Gundel and F.E. Wawner, Ser. Metal. Mater., 25 (1991) 437-441. [6] H.-P. Chiu, SM. Jeng and J.-M. Yang, Interface control design for Sic-fibre reinforced titanium aluminide composites, J. Mater. Res., 8 (1993) 2040-2053. [7] R.R. Kieschke and T.W. Clyne, Development of a diffusion barrier for Sic monofilaments in titanium, Mater. Sci. Eng., A135 (1991) 145-149. [8] J.H. Norman, G.H. Reynolds and L. Brewer, Chemical stability of fibre-metal matrix composites, MRS Symp. Proc., 194 (1990) 369-377. [9] J.G. Robertson, Manufacture and Properties of Sigma Fibre Reinforced Titanium, AGARD Report No. R-796, 1994, p. 7.17.8. [lo] J.W. Steeds and C.G. Rhodes, Reaction Between Ti-6Al-4V and Boron, J. Am. Ceram. SOL, 68 (1985) 136.
246
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et al. / Materials
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1111 J.H. Perepezko, Interfacial reactions and microstructure control in metal and intermetallic matrix composite processing, Cowposite Interfaces, 1 (1993) 463-480. I. Barin, 0. Knacke and 0. Kubaschewski, Thermochemical u-7-1 Properties of Inorganic Substances, Springer, Berlin, 1973. u31 CF. Knights, Protective Interlayers between Sic and titanium Alloys, A Thermodynamic Assessment, AERE-G4331; see also E.A. Feest and J. Cook, Processing of Titanium Matrix Composites, AGARD Report, R-798, p. 5.1-5.9. of binary alloys, 2nd edn., New 1141 Hansen-Anderko, Constitution York, 1958, MC Craw Hill. 1151 R. Pailler, M. Lahaye, J. Thebault and R. Naslain, Chemical interaction phenomena at high temperatures between boron fibres and titanium metal (or TiGAI4V-alloy), Failure Modes in Composites, 4 (1979) 265-284. V61J.L. Murray, P.K. Liao and K.E. Spear, &i/l. Alloy Phase Diagrams; 7 (1986) 550-555, 587-588. [I71 H.J. Dudek and R. Leucht, in H. Buhl (ed.), Titanium matrix composites, Aclunnced Aerospace Materials, Springer, Heidelberg, 1992, p. 124-139.
and Engineering
A-712 (1996)
242-24G
R. Leucht and H.J. Dudek, Properties of Sic-fibre reinforced 1181 titanium alloys processed by fibre coating and hot isostatic pressing, Mater. Sci. Eug., AlSS (1994) 201-210. 1191 H.J. Dudek and R. Leucht, Properties of TMC processed by fibre coating and HIPing, AGARD Report, R-796, p, 5.1-8.7. PO1H.J. Dudek, R. Leucht and WA. Kaysser, Development of Metal Matrix Composites by Fibre Coating and HIPing, Proc. ICCM-10, Whistler, August 14-18, 1995, Canada, Vol. II, p. 695-702. PII H.J. Dudek, R. Leucht, R. Borath and A. Werner, TEM investigation of the graded titanium boride fibre protective coatings, in preparation. P?l H.J. Dudek, L.A. Larson and R. Browning, Study of the fibre matrix interface in a Sic reinforced titanium alloy using high resolution field emission Auger microprobe, Snf hlte$ A~nl., 6 (1984) 274-278. ~231C.G. Rhodes, Characterization of fibre-matrix interfaces by transmission electron microscopy in titanium aluminide/SiC composites, Mater. Res. Sot. Syrup, Proc., 273 (1992) 17-29.
A
Materials Science and Engineering A212 (1996) 242-246
Graded titanium
boride fibre protective coatings for titanium matrix composites H.J. Dudek, A. Werner, R. Leucht
Germm
Aerospace
Resenrck
Estnblishment
(DLR),
Institute
for
Mnterids
Resenrch,
D-51 140 h?ih,
Gcr/mm)
Received 3 October 1995; revised 29 January 1996
Abstract Graded titanium boride coatings(gTiB coatings)were depositedon Sic-SCS-6 fibres. Compositeswith an IM1834 matrix were processedby fibre coating with matrix and hot isostaticpressing(HIPing) of matrix coated fibres. The thermal stability of the compositeswas investigatedby the determinationof the tensileand interface propertiesafter a thermal treatment at 800“C for up to 1000h. Keyworh: Protective coatings; Titanium boride fiber; Titanium matrix composites
during the processing of composites with these fibres,
high thermodynamic stability of the coating material, low diffusion rates for titanium and carbon in the protective coating and low solubility of the components of the coating in titanium alloys (compare for example [ll]), Some of these criteria are fulfilled by TiB,. TiB, has a heat of formation of dG, = 100 kJ mol-’ at.% [12] which is higher than that for the reaction product of carbon with the titanium alloy. TiC has a heat of formation of ilGO = 84 kJ mol- ’ at.% [12]. The solubility of boron in titanium is low, about 0.05 wt.% in the temperature range of 750- 1300 “C [13] compared, for example, with the solubility of carbon in titanium (OS wt.%) [14]. Based on the results of Pailler et al. [15] who reported a stoichiometry range for TiB2 of 2-1.9 and for TiB of l-0.88, Knights proposed an additional system TiB,.9/TiB,,,, to stabilize the fibre-matrix interface in titanium matrix composites [13]. More recent studies indicate, however, that at least for TiB the stiochiometry range is smaller [16]. Contrary to the predictions of the thermodynamic assessment of Knights, the TiB, coating of the BP SM 1240R fibre (1992) has a gradient of boron in the
TiB, reacts with titanium to form TiB needles [9,10].
coating with a high concentration of boron in the
The TiB reaction zone causes a degradation of the longitudinal composite strength [6]. The criteria for a material to act as an additional protective coating in titanium matrix composites are:
vicinity of the titanium alloy, Fig. l(a). During processing and thermal treatment of the composites the boron forms TiB [lo] and the carbon diffuses through the titanium boride coating, Fig. l(b).
1. Introduction
Interfacial thermal stability of commercially available Sic fibres with a carbon protective coating for titanium matrix composites is limited to an application temperature of approximately 600 “C. At higher temperatures, depending on the titanium alloy used as matrix, the carbon coating may be rapidly consumed. At 700 “C a coating lifetime of up to 6000 h in Ti-
6Al-4V alloys was reported ing in Ti-6Al-4V-alloys
[l]. At 800 “C the coatis consumed after approximately 500 h [2]. For other matrices, such as the near a-alloy IM1834 [3] and for a2 + p-titanium aluminides [4,5], a higher thermal stability is observed. For an application temperature above 700 “C, stability is still insufficient. For MMC applications exceeding 700 “C an additional coating is required. Different materials are presently being considered as additional diffusion and reaction barriers: TIC, TiB2, TaC [6], Y,O, [7] and others [8]. The British Petroleum company has developed an additional TiB2 protective coating for the SM 1240R fibres. However,
0921-5093/96~$15.00 0 1996- ElsevierScience S.A. All rightsreserved
H.J.
2. Experimental
Dudek
et al. / Materials
Science
A212
(1996)
243
242-246
details
An additional protective coating TiB1,9/Ti0,88 as proposed by Knights [13] was attempted in the following experiment. A magnetron sputter device equipped with two magnetron pairs was used to deposit the graded titanium boride coating (gTiB) on fibres [20]. SiC-SCS6R fibres from Textron Inc. were used. One pair of magnetron cathodes was supplied with TiB, targets, the other with pure titanium targets. The TiB, cathodes were operated at a constant power supply of 2 kW. After deposition of about 100 nm TiB,, the power supply of the Ti cathodes was gradually increased from zero to a value of 0.8 kW while the deposition of TiB, was continuing. Assuming a linear dependence of the sputter coefficient of titanium on power supply, the linear increase of the power supply should result in a graded titanium boride coating with a high boron concentration in the vicinity of the carbon coating and a low boron concentration near the titanium matrix. Deposition of the matrix which was chosen to be IM1834 titanium alloy (Ti-5.8A1-4.OSn.3.5Zr-0.7Nb0.5Mo-0.35Si-0.06V; wt.%) was achieved with the same equipment. Composites were processed by HIPing the coated fibres [17-191 at 190 MPa at 930 “C for
0
and Engineering
20 40 60 80 Sputter time I min
100
(4
(a)
(b)
Fig. 2. Metallographic polish of the interface in Sic-SCS-6 fibre reinforced IMI834 alloy: (a) in the as-processed composite with the additional gTiB coating, (b) after thermal treatment at 800 “C for 500 h of the composite with the additional gTiB coating, (c) after thermal treatment at 800 “C for 500 h of the composite without an additional gTiB coating and (d) in the composite with an additional gTiB coating treated at 800 “C for 1000 h.
0.5 h. The thermal stability of the fibre-matrix interface was studied by a thermal treatment of the composites with and without additional gTiB coating at 800 “C for up to 1000 h in vacuum. After thermal treatment the composites were tensile tested and the interface was examined by SEM and EDS line scan analysis. The EDS analysis was performed quantitatively with reference to pure element standards. The boron concentration was determined by difference to 100%. Sputter time I min
(b) Fig. 1. Auger electron spectroscopy (AES) depth profile of the protective coating of the BP SM 1240R fibre (a) and AES depth profile of the interface of a monofilament composite with a SM 1240R fibre coated with the Ti-6Al-4V alloy and thermally treated at 700 “C for 670 h (b).
3. Results In Fig. 2, metallographic images of the fibre-matrix interface and in Fig. 3, EDS line scan analysis results of the composites are shown. In the as-processed com-
244
H.J. Dudek
et al. I Materials
Science
posite with additional gTiB coating, Fig. 2(a), the carbon protective coating of the Sic-SCS-6 fibre survives completely without any interaction with the gTiB coating. This can be concluded from the observed thickness of the C-coating (roughly 3 pm) and the results of the EDS Si-linescan, Fig. 3(a), which show the usually observed concentration profiles with two Si-maxima, the last one near the carbon coating surface. At the gTiB-matrix interface some TiB needles are growing from the coating into the matrix, similar as observed in composites with BP-SM 1240Rfibres. Fig. 2(b) shows the interface of a composite with the additional gTiB coating thermally treated at 800 “C for 500 h. The carbon coating of the SCS-6 fibre shows some degradations, only approximately 1.8 pm remains. As can be seenfrom the Si-line scan, Fig. 3(b), the part of the carbon coating with the second silicon maximum is lost. The EDS-carbon intensity (and TEM results 1’ C-coating
1:’ of fibrei
100
II
1 0
' 11 ,l 2
1 ‘:’ gTiB-layer j
' 4
1 matrix (IMI 834)
A: JJL,6
iv 8
I I
I 10
distance from fibre surface I pm
(4
1
0
2 distance
4
6
8
IO
from fibre surface I pm
(b) Fig. 3. EDS line scan of the fibre-matrix interface in the composite with fibres coated with the additional gTiB coating and an IM1834 matrix after processing (a) and after thermal treatment at 800 “C for 1000 h (b). The boron concentration was determined by the difference to 100%.
rind Engitzeeriug
A212
(1996)
242-246
[21]) of the inner coating near the C-coating identifies this region as Tic. Adjacent to the TiC reaction layer the residual gTiB coating is found. The particles located outside the gTiB layer and having a diameter of a few microns showed only titanium and carbon peaks in EDS analysis. This leads to the suggestion that the particles consist also of Tic. In Fig. 2(c) the interface of the composite without gTiB coating after a thermal treatment at 800 “C for 500 h is shown. Again a part of the coating is consumed. Only approximately 2.5 lun of the carbon coating of the SCS-6 fibre remain. Adjacent to the remaining carbon coating a reaction zone with high carbon concentration is found. Obviously this layer consistsmostly of TiC (Tic typically forms adjacent to the carbon coating, for details of the composition of the reaction zone in Sic-Ti composites; see for example [22,23]). In this composite TiC particles are also found at the interface. The TiC particles are smaller with diameters of 1 pm, however. In composites with an annealing time of 1000h at 800 “C, Fig. 2(d), the carbon coating has a remaining thickness of only 0.5 pm and 2 iirn for fibres with and without the additional gTiB coating, respectively. The additional gTiB coating obviously can not prevent the dissolution of carbon into the matrix and the formation of titanium carbide. At 800 “C composites with the additional gTiB coating have a lower thermal stability than those without that coating. As can be seen from Fig. 2(d) after the thermal treatment at 800 “C for 1000h the additional gTiB coating remains in the composite: the gTiB is thermodynamically stable. The TiC formation between the fibre and the gTiB coating, however, shows that the additional coating is no diffusion barrier for carbon and titanium. The titanium diffuses through the gTiB coating and reacts with the carbon thus reducing the carbon protective coating thickness of the SCS-6 fibre. Owing to the high diffusion rate of titanium through the gTiB coating, the volume increaseof the reaction product TiC results in a shift of the gTiB coating away from the fibre. In Fig. 4 the mechanical properties of the different composites studied are shown. As was shown in [3] the tensile strength, the elongation and Young’s modulus of composites without the additional gTiB coating remain unchanged even after 1000h of annealing at 800 “C. In the as processedcondition compositeswith the additional gTiB coating have nearly the same properties as the composites without the additional gTiB coating. The observed small deviations of the tensile strength and elongation in Fig. 4 for composites with gTiB coatings, in comparison with the compositeswithout the gTiB coating, are probably causedby a statistical scatter of the results. With increasing thermal treatment time at 800“C the tensile strength and elongation decreasesfor composites with the gTiB coating.
H.J.
Dud&
et al. / Materials
Science
and Engineering
A212
(1996)
245
250
2,5
0 (4
242-246
0
0
(cl
@I
Fig. 4. Tensile strength (a), Youngs modulus (b) and elongation (c) of composites processed with and without an additional gTiB coating in the as processed condition and after thermal treatment at 800 “C for 500 h and 800 “C for 1000 h (horizontal lines show the values for single samples).
4. Discussion and conclusions
Acknowledgements
The additional gTiB coating was deposited on the fibre with the aim to stabilize the fibre-matrix interface during a thermal load of the composites. The present investigations show that the additional gTiB coatings deposited on the fibres using magnetron sputtering according to the predictions of the thermodynamic assessment [13] destabilze the interface resulting in a higher reaction speed compared with the composites with uncoated Sic-SCS-6 fibres. The additional interaction of the gTiB coating with the matrix under formation of a reaction zone causes also a strong reduction of the tensile properties of the composites. The formation of titanium carbide in the neighbourhood of the carbon coating of the KS-6 fibre and in the interface gTiB-matrix shows that carbon and titanium diffuse through the gTiB coating. The diffusion rate of carbon and titanium in the gTiB coating deposited by magnetron sputtering is probably enhanced owing to the small grain size and the non stoichiometry of the TiB, and TiB [21] which allows vacancy formation in the titanium borides. The higher reaction rate for composites with the additional gTiB coating in comparison with composites without this coating can be explained by the formation of a reaction and diffusion barrier in Sic-SCS-6-IMI834 composites by alloying element segregation [3]. This “natural” reaction barrier obviously does not form in the composites with the additional gTiB coating.
The authors are grateful to Professor Kaysser for the support of this work.
References [l] A. Vassel, R. Mevrel, J.P. Favre and J.F. Stohr, Fibre-Matrix Interface Properties in Ti-IMatrix Composites, in Chemical Compatibility and Micromechanical Behaviour, AGARD Report R-796, 2994, p. 9.1--9.11. [2] H.J. Dudek, R. Leucht, R. Borath and G. Ziegler, Analytical investigations of thermal stability of the interface in Sic-fibre reinforced Ti6A14V-alloy, Microchim. Acta, II (1990) 137-148. [3] ‘H.J. Dudek, R. Leucht and J. Hemptenmacher, Thermal stability of Sic-SCS-6 fibre reinforced IM1834 alloys, Metal. Mater. Trans., submitted for publication. [4] R. Leucht, K. Weber, A. Werner, J. Hemptenmacher and H.J. Dudek, Sic fibre reinforced super a2 alloy processed by fibre coating and HIPing, in preparation. [5] D.B. Gundel and F.E. Wawner, Ser. Metal. Mater., 25 (1991) 437-441. [6] H.-P. Chiu, SM. Jeng and J.-M. Yang, Interface control design for Sic-fibre reinforced titanium aluminide composites, J. Mater. Res., 8 (1993) 2040-2053. [7] R.R. Kieschke and T.W. Clyne, Development of a diffusion barrier for Sic monofilaments in titanium, Mater. Sci. Eng., A135 (1991) 145-149. [8] J.H. Norman, G.H. Reynolds and L. Brewer, Chemical stability of fibre-metal matrix composites, MRS Symp. Proc., 194 (1990) 369-377. [9] J.G. Robertson, Manufacture and Properties of Sigma Fibre Reinforced Titanium, AGARD Report No. R-796, 1994, p. 7.17.8. [lo] J.W. Steeds and C.G. Rhodes, Reaction Between Ti-6Al-4V and Boron, J. Am. Ceram. SOL, 68 (1985) 136.
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et al. / Materials
Science
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