- Email: [email protected]
Flexural strength change of silicon nitride implanted by high dose titanium ions
March 1997
ELSEWIER
Materials Letters 30 (1997) 299-303
Flexural strength change of silicon nitride implanted by high dose titanium ions ’ Jizhong Zhang a*b**,Xiaoyan Ye ‘, Jun Chang b, Kun Tao b a International
Centre
forMaterials
Physics, Academia Sinica, Shenwng 110015, and Chinese Centre of Advanced Science and Technology, P.O. Box 8730, Beijing 100080, China h Department of Materials Science and Engineering, Tsinghua UnkersiQ, Beijing 100084. China ’ Department qf Chemistry, Tsinghua linkers&v, Beijing 100084, China
Received 2 July 1996; revised 26 August 1996; accepted 29 August 1996
Abstract Silicon nitride ceramics were modified by Ti-ion implantation in a MEVVA (MEtal Vapor Vacuum Arc) implanter. The influence of implantation parameters was studied by varying the ion dose over a wide range. The samples were implanted with 80 keV Ti ions with doses from 1 X lOI to 3 X 10” Ti/cm’ at temperatures between 400 and 750°C. The implantation dose strongly influences the flexural strength and microhardness of silicon nitride, increasing the flexural strength up to 14% relative to unimplanted silicon nitride. PACS: 61.72.W~; Keywords:
61.80.Jh
Silicon nitride; Ion implantation:
Flexural strength
1. Introduction
Surface modification of ceramic materials has recently been studied by many researchers in an attempt to improve mechanical properties, for example, surface hardness, surface fracture toughness and bulk fracture stress. The properties of ceramics are sensitive to their surface conditions such as mi-
* Corresponding author. Tel. 86- 10-6259-4546, Fax: 86-10. 6256-2768, e-mail: [email protected] ’ This project has been supported by National Natural Science Foundation of China. 00167-577X/97/$17.00 PII SO 167-577X(96)002
Copyright 12-l
crostructure, residual stress state, surface flaws, and composition etc. Ion implantation is one of the most effective surface modification techniques used by materials researchers. Metal vapor vacuum arc (MEVVA) ion source implantation was invented in the mid 1980’s. A MEVVA implanter can easily produce high densities of various metallic ions, and hence is a proper equipment for metal ion implantation of materials [l-4]. Since ion implantation affects only near-surface regions of bulk ceramics, it is possible to change the surface stress, microstructure, and composition, and hence enhance mechanical properties of bulk ceramic materials. A series of carbide. oxide and ni-
0 1997 Elsevier Science B.V. All rights reserved.
300
J. Zhnn,q et cd. / Materials Letters 30 (IYY71 2YY-303
tride materials have been investigated after implantation of non-metallic and metallic ions. Most of the studies were concerned with the structure of the implanted zone and changes in physical and mechanical properties [5- 111. Metal ion implantation can introduce a controlled amount of alloying element in ceramics, but it also produces a large number of lattice defects. The lattice defect, i.e. radiation damage, has been reported to considerably affect the mechanical properties of as-implanted ceramics. It generally results in a hardness increase, volume change of the damaged region, and residual stress in the surface layer [ 1214]. The extent of radiation damage and the resulting changes in mechanical properties are expected to be related to the dose and sample temperature during implantation. In this paper, we report the experimental results on the flexural strength and microhardness changes in silicon nitride caused by implanting titanium ions at different doses.
2. Experimental Hot-pressed silicon nitride ceramics were cut into 3 X 4 X 36 mm3 bars, then polished carefully to mirror finish with 0.5 km diamond paste. Prior to implantation all samples were ultrasonically cleaned in acetone and methanol. Ti-ion implantation was carried out in a MEVVA source implanter with an extraction voltage of 40 kV. The particle current fraction of the Ti-ion beam was composed of 6% Ti+, 82% Ti’+ and 12% Ti”+, and the corresponding energy was therefore 40 keV, 80 keV and 120 keV, respectively. The mean charge state of Ti ion was Qr, = 2.05, and the implantation energy of Ti ions was mainly 80 keV. The samples were implanted with an ion current density of 125 PA/cm2 and the ion doses ranged from I X 10” to 3 X lOI Ti/cm’. The samples were not cooled during ion implantation, and their temperature rise was only due to ion beam heating. The sample’s surface temperature was measured with a thermocouple in close contact with the surface. The thermocouple was protected from the beam current and thermally insulated with a piece of mica, in order to prevent the thermocouple from being heated by parts other than the sample.
3. Results and discussion 3.1. Ion distribution
in as-implanted
silicon nitride
The depth distribution in the as-implanted samples was analyzed by scanning Auger microprobe (SAM) and Rutherford backscattering spectroscopy (RBS). Fig. 1 shows the Ti depth profile measured by means of a Perkin-Elmer PHI 6 10 scanning Auger microprobe after a dose of 3 X 10” Ti/cm2 and a temperature rise up to 750°C (see Table I). After careful calibration, the sputtering rate for the depth profile was 29 nm/min for this silicon nitride sample. Fig. 1 shows that the Ti distribution extends much deeper than the projected ion range predicted by the ZBL (Ziegler, Biersack and Littmark) tables, i.e. 47 nm for 80 keV Ti ion and 69 nm for 120 keV Ti ion [15]. A diffusion-like tail on the Ti profile is evident. The real depth range of the Ti ion is more than 250 nm, which is about five times deeper than the projected range of the implanted 80 keV Ti ion. RBS data (not shown here) are consistent with the SAM results. The temperature rise of a sample during implantation is shown in Fig. 2. It is noted from Fig. 2 that the sample temperature was 673°C after 10 min implantation, 737°C for 20 min and 750°C for 30 min. The equilibrium temperature of the sample was 750°C for the implantation time of more than 30 min. The temperature rise of an as-implanted sample versus implantation dose is listed in Table I. The greater projectile range and longer tail of the Ti distribution in the depth profile may be attributed to radiation enhanced diffusion (RED) of the im-
80 s
60.
N
y
40.
Ti
20
-
Si
0 0
1
. 2
3
4
Sputtering
5
6
Time
. 7
8
9 10
(min. )
Fig. I. SAM depth profile of a silicon nitride sample implanted with a dose of 3 X IO’” Ti/cm’. AC is the atomic concentration.
J. Zhang et al. /Materials
Table 1 Temperature implantation
rise of an as-implanted dose
sample
as a function
Dose (X lO”Ti/cm’)
Implantation time (s)
Temperature rise W)
I 3 5 10 30
130 390 650 1300 3900
400 650 675 745 750
Letters 30 (1997) 299-303
301
of
01
8
25
30
35
40
45
*
-
I
50
55
60
I
65
70
75
80
20 Fig. 3. XRD spectrum of a silicon nitride sample implanted dose of 8 x 10” Ti/cm’.
planted Ti ions. RED is an anomalous diffusion effect and is a special and important physical phenomenon in ion implantation technology. This means that the diffusion effect of ions implanted into a sample is enhanced greatly because of the “thermal spike”. In addition, the temperature rise resulted from high dose implantation is helpful to radiation enhanced diffusion of the implanted Ti ions. 3.2. Phase and topography
of as-implanted
samples
An X-ray diffraction (XRD) result for an as-implanted sample is shown in Fig. 3. This silicon nitride sample was implanted with a dose of 8 X 10” Ti/cm’. It can be seen that there are P-Si,N,, TiSi,, TIN and Si phases in the implanted region. The P-Si,N, phase is still the main phase, and no elemental Ti phase was found. This means that the Ti ions implanted into the sample had basically reacted with Si and N atoms of Si,N,, and formed titanium silicide and titanium nitride under the implantation conditions.
The surface colour of the as-implanted sample became silver grey, while the original colour of the control sample was black. A layer of a deep grey compound was found on the surface of samples implanted with a higher ion dose, i.e. more than 5 X 10” Ti/cm’. Combined with the XRD results, the compound should be the TiSi, phase. Some porous structures were observed by a scanning electron microscope (SEMI across the entire surface of the sample implanted with a dose of 3 X 10” Ti/cm’. These porous structures arise from the radiative damage because of the large number of defects (mostly vacancies), which in turn result in a less dense surface layer. 3.3. Flexural strength and microhardness planted samples
0
10
-
800
I
20
Time
of as-im-
The flexural strength of the Ti-implanted silicon nitride bars of 3 X 4 X 36 mm3 in size was measured
2
OL
with a
30
40
(Min.)
Fig. 2. Temperature-time curve of a silicon nitride sample planted with a Ti ion current density of 125 kA/cm2.
Dose im-
( 101’/ctn2)
Fig. 4. Flexural strength as a function ion-implanted silicon nitride samples.
of the ion dose for Ti
J. Zhang
302
et al. /Moterids
at room temperature using the three-point bending mode with a span of 30 mm. Five samples were measured for each ion implantation condition. The flexural strength as a function of ion dose for the as-implanted samples is shown in Fig. 4. The flexural strength of the control sample is 650 MPa. The flexural strength of the as-implanted silicon nitride increased initially with increasing implantation dose and reached the maximum value of 742 MPa for the dose of 3 X 10” Ti/cm’, i.e. 14% higher than that of the control sample. It can be seen from Fig. 4 that the implantation of silicon nitride with Ti ions above a “critical dose” of about 3 X lOi Ti/cm’ resulted in a significant decrease of flexural strength. The flexural strength of the sample implanted with the highest dose (3 X 10 ‘* Ti/cm’) decreased to 550 MPa and was about 15% lower than the control sample’s flexural strength. A possible explanation is that the radiation damage piled up and expanded with increasing the ion dose, and finally resulted in degradation of the flexural strength. The microhardness of the sample surface was determined by use of a Knoop type diamond indentor within the load range of 0.02-0.05 N. The tests were carried out under ambient conditions at room temperature. Five indentations per sample were made and the hardness was evaluated from the mean long diagonal value. In Fig. 5 the microhardness of the as-implanted silicon nitride samples is shown as a function of ion dose. The Knoop microhardness of the control sample is H, = 1800 kg/mm’. The maximum microhardness value of the as-implanted sample was obtained when it was bombarded with a dose
0
10
20
30
Dose (lO”/cm”) Fig. 5. Knoop microhardness as a function ion-implanted silicon nitride samples.
of the ion dose for Ti
Letters
30 fIYY71
2YY-303
of I X lOI Ti/cm*. It was up to 2800 kg/mm’ and 55% higher than the control sample’s hardness. Softening of the implantation surface of silicon nitride was observed with increasing implantation dose. The hardness value decreased t0 H, = 1750 kg/mm’ for the sample implanted with the maximum dose of 3 X IO’” Ti/cm’. Combined with SEM observation, the hardness degradation of the as-implanted sample is due to the radiation damage and the formation of the softer TiSi, phase resulting from higher ion dose. The dose dependence of the microhardness of as-implanted samples appears to be similar with that of flexural strength of as-implanted samples. At lower doses labeled hereafter as the “hardening regime”, both the flexural strength and microhardness of Ti-implanted samples are greatly improved. Above a certain critical dose in the “softening regime’ ’ , degradation of both properties is observed. The “hardening regime” observed at lower doses and the “softening regime” observed at higher doses are quite similar with the previous experimental results for silicon nitride implanted with 0.5 and 1.0 MeV Si ions [I I].
4. Conclusion The change of flexural strength and microhardness of silicon nitride implanted with 80 keV Ti ions very strongly depended upon the implantation dose under the present experimental conditions. At low doses Ti-ion implantation induces hardening of the implanted surface and increases flexural strength, whereas at high doses softening of the implanted surface and decrease of flexural strength occur. The flexural strength of silicon nitride could be increased up to 14% after Ti-ion implantation with a dose of 3 X 10” Ti/cm2. The temperature rise of the as-implanted sample increased gradually with increasing implantation dose. The temperature of the sample could reach 750°C after a Ti-ion implantation time of 30 min. The Ti ions implanted into the sample had basically reacted with Si and N atoms of silicon nitride, and formed titanium silicide and titanium nitride. The radiation effects on silicon nitride caused by implanting of high doses of Ti ions were significant because of the accumulation of ion beam induced defects. It could result in a porous structure in
J. Zhang et al. /Materials
the implantation region, and in turn greatly decrease the flexural strength and hardness of the as-implanted silicon nitride.
[3] [4] [5] [6] [7] [8]
Acknowledgements This work was also supported by the State Laboratory for Materials Modification by Laser, and Electron Beams, Dalian Division, China, also supported by the Analysis Foundation of inghua University, Beijing, China.
Letters 30 (1997) 299-303
Key Ion and Ts-
[9] [lo] [ll] [12] [13]
References [l] LG. Brown, J.E. Gavin and R.A. Ma&ill, Appl. Phys. Lett. 47 (1985) 358. [2] R.J. Adler and S.T. Picraux, Nucl. Instr. Meth. Phys. Res. B 6 (1985) 123.
[14]
[ 151
303
AI. Ryabchikov, Rev. Sci. Instr. 61 (1990) 641. X.J. Zhang and H.X. Zhang, Rev. Sci. Instr. 63 (1992) 2431. P.J. Buenett and T.F. Page, J. Mater. Sci. 19 (1984) 3524. T. Hioki, A. Itoh, M. Ohkubo, S. Noda, H. Doi, J.Kawamoto and 0. Kamigaito, J. Mater. Sci. 21 (1986) 1321. C.J. McHargue, Intern. Met. Rev. 31 (1986) 49. J. Lankford, W. Wei and R. Kossowsky, J. Mater. Sci. 22 (1987) 2069. W. Ensinger and J.M. Mayne, J. Mater. Sci. 26 (1991) 6722. W. Fischer, H. Wituschek, G.K. Wolf, H. Ferber, R. Heinze and M. Woydt, Surf. Coat. Technol. 59 (1993) 249. W. Boles, S.D. Peteves and F.W. Saris, Appl. Phys. A 58 (1994) 493. T. Hioki, A. Itoh, S. Noda, H. Doi, J. Kawamoto and 0. Kamigaito, J. Mater. Sci. Lett. 3 (1984) 1099. G.W. Arnold, G.B. Krefft and C.B. Norris, Appl. Phys. Lett. 25 (1974) 540. C.W. White, G.C. Farlow, C.J. McHargue, P.S. Sklad, M.P. Angelini and B.R. Appleton, Nucl. lnstr. Meth. Phys. Res. B 7/8 (1985) 521. J.F. Ziegler, J.P. Biersack and U. Littmark, The stopping and range of ions in matter, Vol. 1 (Pergamon Press. New York, 1985).
ELSEWIER
Materials Letters 30 (1997) 299-303
Flexural strength change of silicon nitride implanted by high dose titanium ions ’ Jizhong Zhang a*b**,Xiaoyan Ye ‘, Jun Chang b, Kun Tao b a International
Centre
forMaterials
Physics, Academia Sinica, Shenwng 110015, and Chinese Centre of Advanced Science and Technology, P.O. Box 8730, Beijing 100080, China h Department of Materials Science and Engineering, Tsinghua UnkersiQ, Beijing 100084. China ’ Department qf Chemistry, Tsinghua linkers&v, Beijing 100084, China
Received 2 July 1996; revised 26 August 1996; accepted 29 August 1996
Abstract Silicon nitride ceramics were modified by Ti-ion implantation in a MEVVA (MEtal Vapor Vacuum Arc) implanter. The influence of implantation parameters was studied by varying the ion dose over a wide range. The samples were implanted with 80 keV Ti ions with doses from 1 X lOI to 3 X 10” Ti/cm’ at temperatures between 400 and 750°C. The implantation dose strongly influences the flexural strength and microhardness of silicon nitride, increasing the flexural strength up to 14% relative to unimplanted silicon nitride. PACS: 61.72.W~; Keywords:
61.80.Jh
Silicon nitride; Ion implantation:
Flexural strength
1. Introduction
Surface modification of ceramic materials has recently been studied by many researchers in an attempt to improve mechanical properties, for example, surface hardness, surface fracture toughness and bulk fracture stress. The properties of ceramics are sensitive to their surface conditions such as mi-
* Corresponding author. Tel. 86- 10-6259-4546, Fax: 86-10. 6256-2768, e-mail: [email protected] ’ This project has been supported by National Natural Science Foundation of China. 00167-577X/97/$17.00 PII SO 167-577X(96)002
Copyright 12-l
crostructure, residual stress state, surface flaws, and composition etc. Ion implantation is one of the most effective surface modification techniques used by materials researchers. Metal vapor vacuum arc (MEVVA) ion source implantation was invented in the mid 1980’s. A MEVVA implanter can easily produce high densities of various metallic ions, and hence is a proper equipment for metal ion implantation of materials [l-4]. Since ion implantation affects only near-surface regions of bulk ceramics, it is possible to change the surface stress, microstructure, and composition, and hence enhance mechanical properties of bulk ceramic materials. A series of carbide. oxide and ni-
0 1997 Elsevier Science B.V. All rights reserved.
300
J. Zhnn,q et cd. / Materials Letters 30 (IYY71 2YY-303
tride materials have been investigated after implantation of non-metallic and metallic ions. Most of the studies were concerned with the structure of the implanted zone and changes in physical and mechanical properties [5- 111. Metal ion implantation can introduce a controlled amount of alloying element in ceramics, but it also produces a large number of lattice defects. The lattice defect, i.e. radiation damage, has been reported to considerably affect the mechanical properties of as-implanted ceramics. It generally results in a hardness increase, volume change of the damaged region, and residual stress in the surface layer [ 1214]. The extent of radiation damage and the resulting changes in mechanical properties are expected to be related to the dose and sample temperature during implantation. In this paper, we report the experimental results on the flexural strength and microhardness changes in silicon nitride caused by implanting titanium ions at different doses.
2. Experimental Hot-pressed silicon nitride ceramics were cut into 3 X 4 X 36 mm3 bars, then polished carefully to mirror finish with 0.5 km diamond paste. Prior to implantation all samples were ultrasonically cleaned in acetone and methanol. Ti-ion implantation was carried out in a MEVVA source implanter with an extraction voltage of 40 kV. The particle current fraction of the Ti-ion beam was composed of 6% Ti+, 82% Ti’+ and 12% Ti”+, and the corresponding energy was therefore 40 keV, 80 keV and 120 keV, respectively. The mean charge state of Ti ion was Qr, = 2.05, and the implantation energy of Ti ions was mainly 80 keV. The samples were implanted with an ion current density of 125 PA/cm2 and the ion doses ranged from I X 10” to 3 X lOI Ti/cm’. The samples were not cooled during ion implantation, and their temperature rise was only due to ion beam heating. The sample’s surface temperature was measured with a thermocouple in close contact with the surface. The thermocouple was protected from the beam current and thermally insulated with a piece of mica, in order to prevent the thermocouple from being heated by parts other than the sample.
3. Results and discussion 3.1. Ion distribution
in as-implanted
silicon nitride
The depth distribution in the as-implanted samples was analyzed by scanning Auger microprobe (SAM) and Rutherford backscattering spectroscopy (RBS). Fig. 1 shows the Ti depth profile measured by means of a Perkin-Elmer PHI 6 10 scanning Auger microprobe after a dose of 3 X 10” Ti/cm2 and a temperature rise up to 750°C (see Table I). After careful calibration, the sputtering rate for the depth profile was 29 nm/min for this silicon nitride sample. Fig. 1 shows that the Ti distribution extends much deeper than the projected ion range predicted by the ZBL (Ziegler, Biersack and Littmark) tables, i.e. 47 nm for 80 keV Ti ion and 69 nm for 120 keV Ti ion [15]. A diffusion-like tail on the Ti profile is evident. The real depth range of the Ti ion is more than 250 nm, which is about five times deeper than the projected range of the implanted 80 keV Ti ion. RBS data (not shown here) are consistent with the SAM results. The temperature rise of a sample during implantation is shown in Fig. 2. It is noted from Fig. 2 that the sample temperature was 673°C after 10 min implantation, 737°C for 20 min and 750°C for 30 min. The equilibrium temperature of the sample was 750°C for the implantation time of more than 30 min. The temperature rise of an as-implanted sample versus implantation dose is listed in Table I. The greater projectile range and longer tail of the Ti distribution in the depth profile may be attributed to radiation enhanced diffusion (RED) of the im-
80 s
60.
N
y
40.
Ti
20
-
Si
0 0
1
. 2
3
4
Sputtering
5
6
Time
. 7
8
9 10
(min. )
Fig. I. SAM depth profile of a silicon nitride sample implanted with a dose of 3 X IO’” Ti/cm’. AC is the atomic concentration.
J. Zhang et al. /Materials
Table 1 Temperature implantation
rise of an as-implanted dose
sample
as a function
Dose (X lO”Ti/cm’)
Implantation time (s)
Temperature rise W)
I 3 5 10 30
130 390 650 1300 3900
400 650 675 745 750
Letters 30 (1997) 299-303
301
of
01
8
25
30
35
40
45
*
-
I
50
55
60
I
65
70
75
80
20 Fig. 3. XRD spectrum of a silicon nitride sample implanted dose of 8 x 10” Ti/cm’.
planted Ti ions. RED is an anomalous diffusion effect and is a special and important physical phenomenon in ion implantation technology. This means that the diffusion effect of ions implanted into a sample is enhanced greatly because of the “thermal spike”. In addition, the temperature rise resulted from high dose implantation is helpful to radiation enhanced diffusion of the implanted Ti ions. 3.2. Phase and topography
of as-implanted
samples
An X-ray diffraction (XRD) result for an as-implanted sample is shown in Fig. 3. This silicon nitride sample was implanted with a dose of 8 X 10” Ti/cm’. It can be seen that there are P-Si,N,, TiSi,, TIN and Si phases in the implanted region. The P-Si,N, phase is still the main phase, and no elemental Ti phase was found. This means that the Ti ions implanted into the sample had basically reacted with Si and N atoms of Si,N,, and formed titanium silicide and titanium nitride under the implantation conditions.
The surface colour of the as-implanted sample became silver grey, while the original colour of the control sample was black. A layer of a deep grey compound was found on the surface of samples implanted with a higher ion dose, i.e. more than 5 X 10” Ti/cm’. Combined with the XRD results, the compound should be the TiSi, phase. Some porous structures were observed by a scanning electron microscope (SEMI across the entire surface of the sample implanted with a dose of 3 X 10” Ti/cm’. These porous structures arise from the radiative damage because of the large number of defects (mostly vacancies), which in turn result in a less dense surface layer. 3.3. Flexural strength and microhardness planted samples
0
10
-
800
I
20
Time
of as-im-
The flexural strength of the Ti-implanted silicon nitride bars of 3 X 4 X 36 mm3 in size was measured
2
OL
with a
30
40
(Min.)
Fig. 2. Temperature-time curve of a silicon nitride sample planted with a Ti ion current density of 125 kA/cm2.
Dose im-
( 101’/ctn2)
Fig. 4. Flexural strength as a function ion-implanted silicon nitride samples.
of the ion dose for Ti
J. Zhang
302
et al. /Moterids
at room temperature using the three-point bending mode with a span of 30 mm. Five samples were measured for each ion implantation condition. The flexural strength as a function of ion dose for the as-implanted samples is shown in Fig. 4. The flexural strength of the control sample is 650 MPa. The flexural strength of the as-implanted silicon nitride increased initially with increasing implantation dose and reached the maximum value of 742 MPa for the dose of 3 X 10” Ti/cm’, i.e. 14% higher than that of the control sample. It can be seen from Fig. 4 that the implantation of silicon nitride with Ti ions above a “critical dose” of about 3 X lOi Ti/cm’ resulted in a significant decrease of flexural strength. The flexural strength of the sample implanted with the highest dose (3 X 10 ‘* Ti/cm’) decreased to 550 MPa and was about 15% lower than the control sample’s flexural strength. A possible explanation is that the radiation damage piled up and expanded with increasing the ion dose, and finally resulted in degradation of the flexural strength. The microhardness of the sample surface was determined by use of a Knoop type diamond indentor within the load range of 0.02-0.05 N. The tests were carried out under ambient conditions at room temperature. Five indentations per sample were made and the hardness was evaluated from the mean long diagonal value. In Fig. 5 the microhardness of the as-implanted silicon nitride samples is shown as a function of ion dose. The Knoop microhardness of the control sample is H, = 1800 kg/mm’. The maximum microhardness value of the as-implanted sample was obtained when it was bombarded with a dose
0
10
20
30
Dose (lO”/cm”) Fig. 5. Knoop microhardness as a function ion-implanted silicon nitride samples.
of the ion dose for Ti
Letters
30 fIYY71
2YY-303
of I X lOI Ti/cm*. It was up to 2800 kg/mm’ and 55% higher than the control sample’s hardness. Softening of the implantation surface of silicon nitride was observed with increasing implantation dose. The hardness value decreased t0 H, = 1750 kg/mm’ for the sample implanted with the maximum dose of 3 X IO’” Ti/cm’. Combined with SEM observation, the hardness degradation of the as-implanted sample is due to the radiation damage and the formation of the softer TiSi, phase resulting from higher ion dose. The dose dependence of the microhardness of as-implanted samples appears to be similar with that of flexural strength of as-implanted samples. At lower doses labeled hereafter as the “hardening regime”, both the flexural strength and microhardness of Ti-implanted samples are greatly improved. Above a certain critical dose in the “softening regime’ ’ , degradation of both properties is observed. The “hardening regime” observed at lower doses and the “softening regime” observed at higher doses are quite similar with the previous experimental results for silicon nitride implanted with 0.5 and 1.0 MeV Si ions [I I].
4. Conclusion The change of flexural strength and microhardness of silicon nitride implanted with 80 keV Ti ions very strongly depended upon the implantation dose under the present experimental conditions. At low doses Ti-ion implantation induces hardening of the implanted surface and increases flexural strength, whereas at high doses softening of the implanted surface and decrease of flexural strength occur. The flexural strength of silicon nitride could be increased up to 14% after Ti-ion implantation with a dose of 3 X 10” Ti/cm2. The temperature rise of the as-implanted sample increased gradually with increasing implantation dose. The temperature of the sample could reach 750°C after a Ti-ion implantation time of 30 min. The Ti ions implanted into the sample had basically reacted with Si and N atoms of silicon nitride, and formed titanium silicide and titanium nitride. The radiation effects on silicon nitride caused by implanting of high doses of Ti ions were significant because of the accumulation of ion beam induced defects. It could result in a porous structure in
J. Zhang et al. /Materials
the implantation region, and in turn greatly decrease the flexural strength and hardness of the as-implanted silicon nitride.
[3] [4] [5] [6] [7] [8]
Acknowledgements This work was also supported by the State Laboratory for Materials Modification by Laser, and Electron Beams, Dalian Division, China, also supported by the Analysis Foundation of inghua University, Beijing, China.
Letters 30 (1997) 299-303
Key Ion and Ts-
[9] [lo] [ll] [12] [13]
References [l] LG. Brown, J.E. Gavin and R.A. Ma&ill, Appl. Phys. Lett. 47 (1985) 358. [2] R.J. Adler and S.T. Picraux, Nucl. Instr. Meth. Phys. Res. B 6 (1985) 123.
[14]
[ 151
303
AI. Ryabchikov, Rev. Sci. Instr. 61 (1990) 641. X.J. Zhang and H.X. Zhang, Rev. Sci. Instr. 63 (1992) 2431. P.J. Buenett and T.F. Page, J. Mater. Sci. 19 (1984) 3524. T. Hioki, A. Itoh, M. Ohkubo, S. Noda, H. Doi, J.Kawamoto and 0. Kamigaito, J. Mater. Sci. 21 (1986) 1321. C.J. McHargue, Intern. Met. Rev. 31 (1986) 49. J. Lankford, W. Wei and R. Kossowsky, J. Mater. Sci. 22 (1987) 2069. W. Ensinger and J.M. Mayne, J. Mater. Sci. 26 (1991) 6722. W. Fischer, H. Wituschek, G.K. Wolf, H. Ferber, R. Heinze and M. Woydt, Surf. Coat. Technol. 59 (1993) 249. W. Boles, S.D. Peteves and F.W. Saris, Appl. Phys. A 58 (1994) 493. T. Hioki, A. Itoh, S. Noda, H. Doi, J. Kawamoto and 0. Kamigaito, J. Mater. Sci. Lett. 3 (1984) 1099. G.W. Arnold, G.B. Krefft and C.B. Norris, Appl. Phys. Lett. 25 (1974) 540. C.W. White, G.C. Farlow, C.J. McHargue, P.S. Sklad, M.P. Angelini and B.R. Appleton, Nucl. lnstr. Meth. Phys. Res. B 7/8 (1985) 521. J.F. Ziegler, J.P. Biersack and U. Littmark, The stopping and range of ions in matter, Vol. 1 (Pergamon Press. New York, 1985).