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TiC + TiB2 composite shows wear promise
TiC + TiB2 composite shows wear promise A promising TiC+Ti& composite for use in a range of wear resistance applications has been developed at the Materials Science Centre, in Manchester, UK. It can be produced as a bulk material using a novel binder system to overcome the barrier of the high melting points of its constituents or laser deposited on to steel strips. T.J. Davies and A.A. Ogwu from the Centre, a joint undertaking of the University of Manchester and UMIST in the UK, outline the production methods and the wear behaviour of the resulting materials.
diboride is an attractive for a range of applica’ including wear components, mechanical seals, aerospace parts and cutting tools because of its hardness, high melting point and low specific weight. TiB2 also has potential uses as a surface coating to steels and aluminium alloys to improve wear resistance in components like cylinder heads, liners, engine blocks, pistons, brake rotors and drums. A problem in winning these applications, however, is that TiB2 has an anisotropy because of its hexagonal structure which makes it difficult to sinter to full density without micro-cracking. Cornpositing is a possible way of avoiding some of these problems. Titanium carbide’ is a potential choice as a toughening second phase as it has good structural and thermodynamic compatibility with TiB2. Tic possesses five independent slip systems and this allows the material to have some plasticity above 800°C’; this facility could impact toughness to the composite. In the present investigation, densification and ductility was achieved in a Tic + TiB2 cermet prepared by pressureless sintering by using a nickel based binder (Ni +X) where X is an additive; the formulation of Ni+X is based on our proposed empirical model for binder selec-
T
itanium material tions”2,3
tion2’3. Since the melting points of Tic and TiB2 are in excess of 3OOO”C, expensive sintering techniques requiring the application of pressure would normally be required to obtain good densification in the absence of a binder.
Material production To prepare the bulk Tic + TiB2 (Ni +X) asreceived powders of TiB2 and Tic manufactured by H.C. Stark of Germany were used, along with nickel and an additive mix available in our laboratory. Particle sizes were determined using a Sedigraph 5000 ET particle size analyser. The as-received TiC/TiBz powders were milled in a high energy vibratory mill, with zirconia balls and liquid propanol as a dispersant in polyethylene bottles. The powders recovered from the milling process and the (Ni +X) alloy powders were mixed in a Turbula mixer for a period of 150 minutes. These admixed powders were pressed into discs and rectangular bars in carbide dies using zinc stearate dissolved in methanol as a lubricant. The green compacts were sintered in the range 1400- 1550°C for 90- 120 minutes in flowing argon in a resistance heated Carbolite furnace and also sintered at 1550°C under vacuum in a Eurotherm furnace. Also prepared were pastes of Tic + TiB2 which were laser deposited on EN24 steel bars. Powders of Tic and TiB2 were mixed to a paste consistency with polyethylene glycol by heating to 70°C in a muffle furnace4. The paste was applied to one surface of rectangular bars of EN24 steels of dimensions 5.5 cm x 3.8 cm x 0.4 cm and left to dry in air. A Nd3+:YAG laser operated between 175 and 200W in the pulsating mode with a Kryton film was used to create 1 mm thick tracks of the paste on the rectangular EN24 bars.
Measurements The surface roughness of the various composites and steels was measured on a Talysurf 10 Profilometer. This equipment works by recording the movement of a diamond stylus across the surface of the test piece. The trace produced is analysed in terms of the surface roughness, Ra, which is equal to the sum of the areas above and below a mean line divided by a total sampling length. Both the areas above and 0026-0657/97/US$17.00 All rights reserved.
Copyright
0
1997, Elsevier
Science
Ltd.
31
200 T
rO.6
-Wear
175 ;
-Friction 5 *_ 0.4 0 g 0 0.2 .J z 0 ‘= IA
loo
0
200 300 400
500 600 700
800
0.0 900 1000
Distance I m FIGURE 1: The wear behaviour of EN24 pin on a disc of Tic + TiBz(Ni+ X) sintered at 1430°C in flowing argon (sintered density 80% theoretical). Sliding speed = 31 mmi’, load = 1ON.
200 175 5 150 ;
125
i
’
I
E
:: 100 J ;i 75 g
50 1
1’ oi!-
25
0
50
!0.0
7’
100
150
200
250
300
350
400
450
500
Distance I m FIGURE 2: The wear behaviour of EN24 pin on Tic + TiBp(Ni + X) sintered at 1550°C in flowing argon (sintered densitv < 90% theoretical). Slidina soeed = 31 mm;-‘, load-=’ 10N.
FIGURE 3: SEM appearance of the sintered TiBl + TiC(Ni + X) disc after the wear test (surface partially covered by wear debris).
below the mean line are taken as having positive values. The wear behavlour of the various materials was assessed using a TJZ67 pin-on-disc machine with the pin operating in a translating flat mode and using a reference wear couple of EN24 pins on EN24 plate. For comparison purposes, tests were also carried out with EN3B [(BS 970) 070M20] pins on bulk TiC + TiBs (Ni +X) composites. The experiments were conducted in the unlubricated dry sliding wear mode. The TE67 used for this work has the advantage of a very rigid pin and disc carrier that allows nominally flat-on-flat contact and applies a pneumatic loading on the pin to reduce the effect of inertia loading in the contact at high loads. The TE67 wear rig was interfaced to a computer that automatically controlled speed, test temperature and test duration with a flexible data logging capacity through a Plint Standard industrial control software package (TE67 SLIM COMPEND) linked to an IBM compatible computer. The wear process in the pin/disc contact is indicated by a linear differential transducer (LVDT) mounted in the pneumatic pin loading piston. The transducer measures the movements of the pin holder during a test. The movement of the pin can be affected by thermal expansion, wear debris and lubricant film build-up; these are significant in the very low wear regime. Pin length changes due to wear are detected by the transducer. In the T67 pin on disc wear rig, the transducer does not differentiate between wear of the pin and wear of the disc. This was not a problem in the investigation since the flat composite disc samples (H,- 1180) were much harder than the pins (Hyw 295). Wear debris was examined using scanning electron microscopy @EM). The samples were lightly coated with carbon using an Edwards Coating System E306A, with a JEOL 360 Winsem Scanning electron microscope used in the back scattered (SEM) mode to reveal atomic number contrast for different phases. Phase proportions and impurities in the wear debris were identified by X-ray diffraction. The diffraction pattern for each sample was automatically recorded on a Phillips Model 1380 horizontal diffractometer at room temperature. Testing was carried out with Cu K, radiation, Ni filter, 40 Kv, 20 mA and time constant 0.5 sec.
Results
FIGURE 4: SEM appearance of the wear debris.
32 MPR June 1997
Wear loss: The wear loss at room temperature and coefficient of friction for an EN24 pin sliding on discs of Tic + TiBs(Ni + X) composites sintered at: (a) 1430°C for 120 minutes (sintered density 80% theoretical); and (b) 1550°C for 120 minutes
(sintered density > 90% theoretical) are shown in Figures 1 and 2 respectively. The wear loss, which is predominantly from the EN24 pin, is related to the density of the sintered disc. For sintering at 1430°C (Figure 1) the overall wear loss of the pin material, expressed in terms of change in pin height for a total distance of 1000 m, is about 75 pm, while the wear loss for the material sintered at 1550°C (Figure 2) is about 110 pm for half the distance, i.e. 500 m. The coefficient of friction was found to reach approximately the same value, about 0.3 for a total sliding distance of 500 m, in both materials. There was, however, an initial peak in the coefficient of friction for the composite with the higher sintered density: this is identified as static friction. Under lubricated sliding conditions it is anticipated that the coefficient of friction would be reduced considerably. Profilometry: The values of surface roughness are shown in Table 1. The surface roughness for the EN24 steel pin was 4.5 pm before the wear run and 0.1 pm afterwards, a result that is consistent with the wear loss data. The EN3B steel pins behaved in a similar manner. The change in surface roughness for the sintered composite disc from 2.5 pm (before) to 1 urn (after) the wear test is most probably due to removal of initial surface asperities on the sintered disc. SEM and XRD: A typical post-wear surface of the TiC+TiBz(Ni+X) disc is shown in Figure 3. The background reveals crystallites of Tic and TiBz in the (Ni +X) binder matrix, coated with a smear of wear debris. A typical appearance of the collected wear debris scraped from the surface of the TiC + TiBz (Ni + X) disc is shown in Figure 4. The X-ray diffraction spectra of wear debris collected from the EN3B and EN24 steel pins are shown in Figures 5 and 6. For the EN3B, the main diffraction peaks corresponded with FeaOs and Fe, while for EN24 pins the major peaks correspond to Fe and FeO. There were no diffraction peaks for titanium or nickel from the disc composite, with the wear debris composed almost exclusively of the pin material. The SEM surface morphologies of the EN3B and EN24 pins after wear testing are shown in Figures 7 and 8. The surfaces are characterized by grooves with associated particles of wear debris. The ENSB pin surface had much more severe (deeper) scars compared to the EN24 pin surfaces, which are characterized by bands of parallel grooves. Wear test results of an EN24 pin sliding against a laser deposited Tic + TiBs (Ni i-X> coating is shown in Figure 9. Comparing
FIGURE
5: X-ray diffraction spectra ot wear aeDr/s (p/n of EN3B sliding against a TiC + Ti&(Ni + X) disc.
FIGURE
6: X-ray diffraction spectra of wear debris (pin of EN24 sliding against a TiC + TiBp(Ni + X) disc.
FIGURE 8: SEM appearance of the EN24 pin after the wear test.
MPR June 1997
33
Conc/usion The effects of achieving good sintered densities by the use of preferred binder additive (Ni f X) on the properties of bulk Tic +TIBz composites and the application of a Tic + TiBa(Ni + X) as a surface layer by laser deposition indicates that this composite system shows promise where wear resistance is required, such as wire drawing dies, machining of steels and milling tools in both bulk and surface applications5. 0
50
100 Distance/m
150
200
FIGURE 9: The wear behaviour of an EN24 pin sliding against an EN24 disc laser surface coated with Tic + Ti&(Ni + X). f-Friction, W-Wear. Sliding speed = 31 mms-‘, load = 1 ON.
wear losses for a pin sliding distance of 200 m, the pin wear loss on the laser deposited track of TiC + TiBa(Ni +X)) is similar to that observed for bulk TiC+TiBs(Ni+X) sintered at 1550°C (see Figure 2). This indicates that the laser deposition method is a viable processing method for producing hard wear resistant Tic +TiB, comuosite surfaces on a tough metallic substrate backing.
34 MPR June 1997
References (1) WA. Zdaniewski, Amer. Bull., 65 (1986), p. 1408.
Cwam.
Sot.
(2)
AA. Ogwu and T.J. Davies, Physica 153, (1996) pp. 101-116. (3) TJ. Davies and AA. Ogwu, Powder Metallurgy, 38, (1995), p.39. (4) A.Y. Fasasi et al, J. Mater. Sci., 29, (1994)) pp. 5121-5126. (5) T.J. Davies and A.A. Ogwu, “The Mechanical Properties and Wear Behaviour of a TiC+TiBsComposite”, European ConStatus SoZidi (a),
ference on Advances in Hard Materials Production, European Powder Metallurgy
Association, Stockholm, (EPMA), pp. 279-312.
Sweden,
1996,
diboride is an attractive for a range of applica’ including wear components, mechanical seals, aerospace parts and cutting tools because of its hardness, high melting point and low specific weight. TiB2 also has potential uses as a surface coating to steels and aluminium alloys to improve wear resistance in components like cylinder heads, liners, engine blocks, pistons, brake rotors and drums. A problem in winning these applications, however, is that TiB2 has an anisotropy because of its hexagonal structure which makes it difficult to sinter to full density without micro-cracking. Cornpositing is a possible way of avoiding some of these problems. Titanium carbide’ is a potential choice as a toughening second phase as it has good structural and thermodynamic compatibility with TiB2. Tic possesses five independent slip systems and this allows the material to have some plasticity above 800°C’; this facility could impact toughness to the composite. In the present investigation, densification and ductility was achieved in a Tic + TiB2 cermet prepared by pressureless sintering by using a nickel based binder (Ni +X) where X is an additive; the formulation of Ni+X is based on our proposed empirical model for binder selec-
T
itanium material tions”2,3
tion2’3. Since the melting points of Tic and TiB2 are in excess of 3OOO”C, expensive sintering techniques requiring the application of pressure would normally be required to obtain good densification in the absence of a binder.
Material production To prepare the bulk Tic + TiB2 (Ni +X) asreceived powders of TiB2 and Tic manufactured by H.C. Stark of Germany were used, along with nickel and an additive mix available in our laboratory. Particle sizes were determined using a Sedigraph 5000 ET particle size analyser. The as-received TiC/TiBz powders were milled in a high energy vibratory mill, with zirconia balls and liquid propanol as a dispersant in polyethylene bottles. The powders recovered from the milling process and the (Ni +X) alloy powders were mixed in a Turbula mixer for a period of 150 minutes. These admixed powders were pressed into discs and rectangular bars in carbide dies using zinc stearate dissolved in methanol as a lubricant. The green compacts were sintered in the range 1400- 1550°C for 90- 120 minutes in flowing argon in a resistance heated Carbolite furnace and also sintered at 1550°C under vacuum in a Eurotherm furnace. Also prepared were pastes of Tic + TiB2 which were laser deposited on EN24 steel bars. Powders of Tic and TiB2 were mixed to a paste consistency with polyethylene glycol by heating to 70°C in a muffle furnace4. The paste was applied to one surface of rectangular bars of EN24 steels of dimensions 5.5 cm x 3.8 cm x 0.4 cm and left to dry in air. A Nd3+:YAG laser operated between 175 and 200W in the pulsating mode with a Kryton film was used to create 1 mm thick tracks of the paste on the rectangular EN24 bars.
Measurements The surface roughness of the various composites and steels was measured on a Talysurf 10 Profilometer. This equipment works by recording the movement of a diamond stylus across the surface of the test piece. The trace produced is analysed in terms of the surface roughness, Ra, which is equal to the sum of the areas above and below a mean line divided by a total sampling length. Both the areas above and 0026-0657/97/US$17.00 All rights reserved.
Copyright
0
1997, Elsevier
Science
Ltd.
31
200 T
rO.6
-Wear
175 ;
-Friction 5 *_ 0.4 0 g 0 0.2 .J z 0 ‘= IA
loo
0
200 300 400
500 600 700
800
0.0 900 1000
Distance I m FIGURE 1: The wear behaviour of EN24 pin on a disc of Tic + TiBz(Ni+ X) sintered at 1430°C in flowing argon (sintered density 80% theoretical). Sliding speed = 31 mmi’, load = 1ON.
200 175 5 150 ;
125
i
’
I
E
:: 100 J ;i 75 g
50 1
1’ oi!-
25
0
50
!0.0
7’
100
150
200
250
300
350
400
450
500
Distance I m FIGURE 2: The wear behaviour of EN24 pin on Tic + TiBp(Ni + X) sintered at 1550°C in flowing argon (sintered densitv < 90% theoretical). Slidina soeed = 31 mm;-‘, load-=’ 10N.
FIGURE 3: SEM appearance of the sintered TiBl + TiC(Ni + X) disc after the wear test (surface partially covered by wear debris).
below the mean line are taken as having positive values. The wear behavlour of the various materials was assessed using a TJZ67 pin-on-disc machine with the pin operating in a translating flat mode and using a reference wear couple of EN24 pins on EN24 plate. For comparison purposes, tests were also carried out with EN3B [(BS 970) 070M20] pins on bulk TiC + TiBs (Ni +X) composites. The experiments were conducted in the unlubricated dry sliding wear mode. The TE67 used for this work has the advantage of a very rigid pin and disc carrier that allows nominally flat-on-flat contact and applies a pneumatic loading on the pin to reduce the effect of inertia loading in the contact at high loads. The TE67 wear rig was interfaced to a computer that automatically controlled speed, test temperature and test duration with a flexible data logging capacity through a Plint Standard industrial control software package (TE67 SLIM COMPEND) linked to an IBM compatible computer. The wear process in the pin/disc contact is indicated by a linear differential transducer (LVDT) mounted in the pneumatic pin loading piston. The transducer measures the movements of the pin holder during a test. The movement of the pin can be affected by thermal expansion, wear debris and lubricant film build-up; these are significant in the very low wear regime. Pin length changes due to wear are detected by the transducer. In the T67 pin on disc wear rig, the transducer does not differentiate between wear of the pin and wear of the disc. This was not a problem in the investigation since the flat composite disc samples (H,- 1180) were much harder than the pins (Hyw 295). Wear debris was examined using scanning electron microscopy @EM). The samples were lightly coated with carbon using an Edwards Coating System E306A, with a JEOL 360 Winsem Scanning electron microscope used in the back scattered (SEM) mode to reveal atomic number contrast for different phases. Phase proportions and impurities in the wear debris were identified by X-ray diffraction. The diffraction pattern for each sample was automatically recorded on a Phillips Model 1380 horizontal diffractometer at room temperature. Testing was carried out with Cu K, radiation, Ni filter, 40 Kv, 20 mA and time constant 0.5 sec.
Results
FIGURE 4: SEM appearance of the wear debris.
32 MPR June 1997
Wear loss: The wear loss at room temperature and coefficient of friction for an EN24 pin sliding on discs of Tic + TiBs(Ni + X) composites sintered at: (a) 1430°C for 120 minutes (sintered density 80% theoretical); and (b) 1550°C for 120 minutes
(sintered density > 90% theoretical) are shown in Figures 1 and 2 respectively. The wear loss, which is predominantly from the EN24 pin, is related to the density of the sintered disc. For sintering at 1430°C (Figure 1) the overall wear loss of the pin material, expressed in terms of change in pin height for a total distance of 1000 m, is about 75 pm, while the wear loss for the material sintered at 1550°C (Figure 2) is about 110 pm for half the distance, i.e. 500 m. The coefficient of friction was found to reach approximately the same value, about 0.3 for a total sliding distance of 500 m, in both materials. There was, however, an initial peak in the coefficient of friction for the composite with the higher sintered density: this is identified as static friction. Under lubricated sliding conditions it is anticipated that the coefficient of friction would be reduced considerably. Profilometry: The values of surface roughness are shown in Table 1. The surface roughness for the EN24 steel pin was 4.5 pm before the wear run and 0.1 pm afterwards, a result that is consistent with the wear loss data. The EN3B steel pins behaved in a similar manner. The change in surface roughness for the sintered composite disc from 2.5 pm (before) to 1 urn (after) the wear test is most probably due to removal of initial surface asperities on the sintered disc. SEM and XRD: A typical post-wear surface of the TiC+TiBz(Ni+X) disc is shown in Figure 3. The background reveals crystallites of Tic and TiBz in the (Ni +X) binder matrix, coated with a smear of wear debris. A typical appearance of the collected wear debris scraped from the surface of the TiC + TiBz (Ni + X) disc is shown in Figure 4. The X-ray diffraction spectra of wear debris collected from the EN3B and EN24 steel pins are shown in Figures 5 and 6. For the EN3B, the main diffraction peaks corresponded with FeaOs and Fe, while for EN24 pins the major peaks correspond to Fe and FeO. There were no diffraction peaks for titanium or nickel from the disc composite, with the wear debris composed almost exclusively of the pin material. The SEM surface morphologies of the EN3B and EN24 pins after wear testing are shown in Figures 7 and 8. The surfaces are characterized by grooves with associated particles of wear debris. The ENSB pin surface had much more severe (deeper) scars compared to the EN24 pin surfaces, which are characterized by bands of parallel grooves. Wear test results of an EN24 pin sliding against a laser deposited Tic + TiBs (Ni i-X> coating is shown in Figure 9. Comparing
FIGURE
5: X-ray diffraction spectra ot wear aeDr/s (p/n of EN3B sliding against a TiC + Ti&(Ni + X) disc.
FIGURE
6: X-ray diffraction spectra of wear debris (pin of EN24 sliding against a TiC + TiBp(Ni + X) disc.
FIGURE 8: SEM appearance of the EN24 pin after the wear test.
MPR June 1997
33
Conc/usion The effects of achieving good sintered densities by the use of preferred binder additive (Ni f X) on the properties of bulk Tic +TIBz composites and the application of a Tic + TiBa(Ni + X) as a surface layer by laser deposition indicates that this composite system shows promise where wear resistance is required, such as wire drawing dies, machining of steels and milling tools in both bulk and surface applications5. 0
50
100 Distance/m
150
200
FIGURE 9: The wear behaviour of an EN24 pin sliding against an EN24 disc laser surface coated with Tic + Ti&(Ni + X). f-Friction, W-Wear. Sliding speed = 31 mms-‘, load = 1 ON.
wear losses for a pin sliding distance of 200 m, the pin wear loss on the laser deposited track of TiC + TiBa(Ni +X)) is similar to that observed for bulk TiC+TiBs(Ni+X) sintered at 1550°C (see Figure 2). This indicates that the laser deposition method is a viable processing method for producing hard wear resistant Tic +TiB, comuosite surfaces on a tough metallic substrate backing.
34 MPR June 1997
References (1) WA. Zdaniewski, Amer. Bull., 65 (1986), p. 1408.
Cwam.
Sot.
(2)
AA. Ogwu and T.J. Davies, Physica 153, (1996) pp. 101-116. (3) TJ. Davies and AA. Ogwu, Powder Metallurgy, 38, (1995), p.39. (4) A.Y. Fasasi et al, J. Mater. Sci., 29, (1994)) pp. 5121-5126. (5) T.J. Davies and A.A. Ogwu, “The Mechanical Properties and Wear Behaviour of a TiC+TiBsComposite”, European ConStatus SoZidi (a),
ference on Advances in Hard Materials Production, European Powder Metallurgy
Association, Stockholm, (EPMA), pp. 279-312.
Sweden,
1996,