Wear mechanisms in titanium implanted silicon nitride ceramics

Wear mechanisms in titanium implanted silicon nitride ceramics

Nuclear Instruments and Methods in Physics Research B 129 (1997) 483-486 -_ Beam Interactions with Materials&Atoms $g I!? ELSEVIER Wear mechanisms...

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Nuclear Instruments and Methods in Physics Research B 129 (1997) 483-486

-_ Beam Interactions with Materials&Atoms

$g I!?

ELSEVIER

Wear mechanisms in titanium implanted silicon nitride ceramics F. Brenscheidt

a3* , S. Oswald b, A. Miicklich a, E. Wieser a, W. Mijller a

Forschangszenrrum Rossendorfe.V.. lnstitutfir lonensrrahlphysik und Materiaiforschung, Postfach 51 01 19, D-01314 Dresden, Germany h lnstitutfir Festktirper- and Werbtofforschung Dresden e.V., Institutfir Festktirperanalytik und Strukturforschung. Helmholtzstr. 20, D-01069 Dresden, Germany

Received 3 April 1997 Abstract The wear of silicon nitride ceramics is reduced by titanium implantation albeit the friction coefficient is slightly increased. X-ray excited photoelectron spectroscopy indicates that silicon oxide is formed in the wear track while possible lubricating titanium oxides are not found. Therefore we attribute the observed wear reduction to the amorphization of the surface as proved by transmission electron microscopy. 0 1997 Elsevier Science B.V. PACS: 81.40; 61.80; 62 Keywords: Silicon nitride; Ion implantation;

Tribological

properties

1. Introduction

2. Experimental

Ceramics are attractive materials for high temperature applications because of their high strength even at high temperatures, good thermal shock stability and high resistance against oxidation. Problems for broader applications of ceramics for engineering purposes are the insufficient wear behavior coupled with high friction coefficients. As the mechanical properties of ceramics are very surface sensitive, ion implantation is a method to modify these properties [l-6]. Implantation of titanium into sintered silicon nitride ceramics considerably reduced the wear of the material [5,6]. EDX spectra of the wear tracks indicated a high amount of oxygen. Therefore one explanation for the wear reduction might be the formation of oxidic lubricants in the near surface layer. In this study wear tests are combined with X-ray excited photoelectron spectroscopy (XPS) investigations of the chemical composition of the surface near region of ion implanted silicon nitride ceramics in order to clarify the mechanisms leading to an improved wear behavior. This method ensures high surface sensitivity and depth dependent information in combination with ion sputtering.

The investigated material is a gas pressure sintered silicon nitride based ceramic with 3 wt.% Al,O, and 5 wt.% Y,Oa as sintering additives. The surface of the material was polished to an average roughness of 0.01 Pm. The samples were implanted with 150 keV and 1 MeV titanium ions to a fluence of 10” ions/cm*. According to the TRIM 95 computer code [7], these energies correspond to mean projected ion ranges of 87 and 585 nm respectively. At 1 MeV the titanium profile is buried in the material so if solid lubrication plays a role one should expect a less drastic influence of the titanium ions deeper in the substrate. The wear behavior and the friction coefficient were determined with an oscillating ball-on--disc-type tribometer, the ball was of unimplanted silicon nitride (diameter 8 mm). The load was 2 N in all cases, the sliding distance 40 m. The wear tracks were investigated with surface profilometry and X-ray excited photoelectron spectroscopy (XPS). The XPS measurements were performed with a Physical Electronics PHI 5600 CI spectrometer which enables lateral resolution down to 30 pm. The measurements were carried out using an unmonochromatized Mg Ka excitation without any charge compensation. Secondary electrons from the X-ray source stabilize the charge-up of the samples, however differential charging may occur [lo]. All binding energies E, are normalized with respect to the Fermi level E,. In the wear tracks, scans of the 0 Is, N Is, Si 2p and Ti 2p,,, electron core

* Corresponding author. Fax: + 49-351-260-2703; email: [email protected]

C1168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PI1 SO168-583X(97)00325-X

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levels were recorded with a pass energy of 94 eV and a spot diameter of 400 km to get high sensitivity. For selected elements, several spectra were recorded at different positions along a line perpendicular to the wear track. These line scans were performed with 29 eV pass energy and a beam diameter of 125 pm. Depth profiling was performed with argon ion sputtering on an area of about 2 mm*, the sputter rate was determined from comparison with Rutherford backscattering measurements to approximately 0.05 rim/s.. The atomic concentrations were determined semi-quantitatively by comparison with sensitivity factors determined at one-elemental standard samples.

150keV

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non-implanted

0”

0.

-zu \

0

3. Results and discussion

200

400

Horizontal

Cross-sectional profiles of the wear tracks of the sample implanted with 150 keV and the non-implanted sample are shown in Fig. 1. In the unimplanted case a deep wear grove is formed whereas in the implanted sample some material of the ball is deposited on the surface. The friction coefficient raises from 0.5 to 0.6 and the ball wear is about 20% increased for the implanted samples. A difference in wear behavior between the two implantation energies is not observed for the load used here. Additional attempts were made to reduce the friction coefficient. First, as the amount of oxygen in the substrate material is relatively low (about 3 at.%) the samples implanted with 150 keV Ti+ were implanted additionally with 40 keV oxygen ions up to doses of 10” O+/cm* to stimulate the formation of lubricating titanium oxides near the surface. However, this second implantation had no effect on the tribology of the material. Second, all samples were annealed at 600°C

/

600

800

position (pm)

Fig. 1. Wear track profiles of the unimplanted sample implanted with 150 keV Ti+.

material

and a

assuming that the local temperature rise during the sliding process is not high enough to form the oxide. Again, no effect was observed. From that behavior one can conclude that solid lubrication by titanium oxides is not responsible for the observed wear reduction, as the existence of solid lubricants should reduce the friction coefficient and the wear of the ball as well. Additionally one would expect that the sample implanted with lower energy shows an improved wear behavior compared with the 1 MeV implantation where the titanium profile lies much deeper and where the maximum concentration of titanium is much lower due to the constant dose. On the other hand lubricating titanium oxides

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Binding energy [eq Fig. 2. XPS spectra of the 0 Is, N Is, Si 2p and Ti 2p,,, levels m the wear track of the sample implanted with I MeV Ti+ after several steps. The intensities are normalized (see text).

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et al./Nucl.

Instr. and Meth. in Phys. Res. B 129 (1997) 483-486

ing depth. The binding energy difference between the 0 1s level and the higher energy component of Si 2p is 429 eV independent of the sputter depth. Bell and Ley [9] measured the same value in photoemission experiments on silicon oxide films for stoichiometric SiO,. Comparing the absolute binding energies with the results from [9] there is a shift of 3 eV. The energy difference between the lower energy part of the Si 2p peak and the N 1s peak is 296 eV. The same value was found by Grcher et al. [8] for measurements on silicon nitride. Therefore we attribute the lower energy component to silicon nitride. The absolute energies are shifted compared with the data from [g] by 0.5 eV. The reason for the higher shift of the oxide compared with the nitride may be the higher band gap of the oxide. Additionally while the nitride is a relatively homogenous amorphous layer (see below) the oxide exists in form of a porous film on the surface this additionally influences the potential drop across the sample, thus differential charging may occur. At greater depth, the low energy component of the Si 2p peak increases in intensity while the higher energy part decreases. In the same way the N 1s peak increases while the 0 1s decreases. In the above picture this means that the amount of silicon oxide decreases and the amount of silicon nitride increases. Fig. 3 shows a line scan perpendicular to the wear track of the sample implanted with 1 MeV after 90 min of sputtering corresponding to a depth of about 270 nm. The composition of the surface was determined at different positions inside and outside the wear track. The track is in the middle of the distance. Clearly there is less nitrogen in the wear track but the oxygen content is clearly increased due to the formation of silicon oxide. Again no significant amount of titanium is found in this depth. As the oxygen concentration outside the wear track is in the range of the oxygen amount from the sintering additives, the silicon oxide in the wear track is obviously a reaction product

1000

Position (pm)

Fig. 3. Composition of the wear track as determined by a XPS line-scan across the wear track after 270 nm material removal by sputtering.

may exist in the surface but their effect might be overcompensated by other effects. Fig. 2 shows XPS spectra of the 0 Is, Nls, Si 2p and Ti 2~,,, levels recorded in the wear track for different sputtering times. The longest sputter time corresponds to a depth of approximately 270 nm. The spectra are normalized in the following way: For one element the highest intensity of all spectra recorded after different sputtering cycles is set to one and all other intensities of this element are normalized to this highest intensity. The Si 2p peak has two components one around 107 eV and another at 103 eV. To identify the origin of these components one has to keep in mind the shift of the spectra due to the insulating nature of the sample. Therefore the energy differences are considered between the Nl s, 01s and the Si 2p levels respectively. Titanium was not taken into account for compound formation as its amount at the surface is extremely low and does not increase very much with increas-

Ti 2p3/2 I\

Si 2p r\

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Binding energy [ev] Fig. 4. XPS spectra at different positions across the wear track of the sample implanted with 150 keV

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from the wear process. Similar results are also obtained for the lower implantation energy. Fig. 4 shows XPS spectra from line scans across the wear track of the sample implanted with 150 keV after 10 min sputtering time i.e. in a depth of about 50 nm. The oxygen content is enhanced in the wear track, the oxygen outside the track belongs to the sintering additives. The energy difference between 0 Is and Si 2p within the area of the wear track is the same as in Fig. 2 that was attributed to silicon oxide. The stoichiometry as determined from the peak intensities also indicates the formation of SiO,. There is much less nitrogen in this upper part of the wear track and very little titanium compared with the as-implanted surface. From these results the following model for the wear process is proposed. In the first stage material is predominantly abraded from the ball and a certain amount sticks on the surface. This material is partly oxidized due to the higher local temperature at the interface between ball and disk. These oxides explain the enhanced oxygen content within the wear track. The following sliding is then not between the original surface and the ball but with the covered surface and the ball. Hence to a certain extent the surface is protected against wear. Lubricating titanium oxides have no beneficial effect because they are existing

buried below the actual interface between ball and disk. This also explains the missing energy dependence of the wear process. As solid lubrication by titanium oxides cannot be attributed to explain the observed wear, another aspect must be responsible. Fig. 5 shows a cross-section micrograph of the sample implanted with 1 MeV. The implanted surface layer is visible as a gray, structureless band. The corresponding diffractogram from this area shows diffuse fringes, indicatmg an amorphous structure. This amorphization of the surface may result in a closure of pre-existing surface flaws or in a reduction of their severity therefore reducing the possibility of particles being removed from the surface due to the growth of surface cracks. Another aspect of the amorphization concerns viscous outflow of material from the intergranular binding phase to the surface as observed by Bolse et al. [7]. This flow can also close surface cracks. Besides this effect the binding phase flowing out on the surface increases the sticking probability of the silicon oxide from the ball that was formed during the wearing process.

4. Conclusions The wear of silicon nitride ceramics is reduced by titanium implantation. Solid lubrication as a mechanism for this effect can be excluded as no titanium is found directly at the surface which consists mainly of silicon oxide. We attribute the observed wear reduction to the amorphization of the surface and the implantation induced viscous flow of the intergranular binding phase. Acknowledgements The authors wish to thank M.F. Plass for helpful discussion. References

[ll C.J. McHargue, Def. Diffus. Forum 57-58 (1988) 359. Dl A. Itoh, T. Hioki and J. Kawamoto, Nucl. Instr. and Meth. B 37/38 (1989) 692. [31 W. Bolse, S.D. Peteves, F.W. Saris. Appl. Phys. A 58 (1994)

493.

Fig. 5. Cross-sectional transmission elecron micrograph of the sample implanted with 1 MeV Ti+. The diffractogram was taken at the position indicated by the arrow.

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