| Plasma ion Implantation |
Reactor | Advantages | Process | SIDH | Applications
Plasma Ion implantation(PII) compels metal surfaces by high voltage acceleration of plasma ions from a conformal sheath makes the surface wear and corrosion resistant. Here we have developed a large 50kV Implanter using hard tube pulser and magnetically enhanced plasma density at normal and elevated temperatures. PII has high efficiency for nitrogen incorporation. Process development include nitriding of aluminium, titanium and chromium.
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In PSII the ions are accelerated through a surface-conformal sheath. This, as
well as the ability to treat large areas simultaneously and at a relatively lower
capital cost has rendered PSII an attractive technology for surface
modification.
The surface bias and plasma conditions determine the ion energy and the dose at the
surface, and may also increase the surface temperature. If the implanted ions
diffuse inside the surface because of the surface temperature the temperature
being raised either by external heating or by ion bombardment, then the
process is termed as Plasma Immersion Ion Implantation (PIII).
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PSII assures dose uniformity for the samples of complex shapes. However the penetration depths are determined by the kinetic energy of the ions and hence are less (~10A / keV for N+ on Fe). For higher depths, either the kinetic energy has to be increased, or the implantation ions have to undergo diffusion inside the sample. Glow Discharge Plasma Nitriding (GDPN) is a process in which nitrogen penetrates the sample only by diffusion. SIDH is a process which exploits the advantages of PSII (dose uniformity) and GDPN (penetration by diffusion).
In SIDH, the samples, immersed in nitrogen plasma, are typically pulsed biased to +/- 1kV. This enables penetration depths of ~10A, which is generally thicker than the top oxide layer present in most samples. As the repetition rates on the bias is very high, and there is also significant electron collection during the positive bias case, the sample temperature rises. The temperature achieved is 400 - 500oC which is controllable by varying the on time to off time ratio. To achieve higher temperatures, an external heater is switched on and this allows treatment to be done at temperatures of 800oC.
By the SIDH process a variety of the steels have been treated. The H16 and SS321 samples on treatment show a load dependent micro hardness, which is typical of a PSII treated specimen. With larger treatment times, the micro hardness increases which is a feature of a diffusion based process. Surface hardness increase of 5 - 6 times is observed at low loads, indicating that at the surface a very high hardness is formed. EN8, EN24 and SS304 also shows an increase of hardness of 2 - 4 times. Cross-sectional micro hardness shows that the treated layer has a thickness of more than 50µ.
There may be some advantages of the SIDH process over PSII and GDPN. If the implanted atom can diffuse inside the material (like nitrogen in steels), then it is necessary only to implant the material to depths larger than the top oxide layer. The remaining depth can be penetrated by the diffusion. SIDH exploits this and hence can work in much lower energy range than PSII. Thus the power supply is simpler and cheaper, using only an IGBT based switching device instead of a tetrode. As the ion energies are less, the secondary electron emission from the sample surface and subsequent X-Ray generation is minimal. When compared to GDPN, SIDH works at a lower temperature and hence has lesser dimensional distortion of the sample. Also, in SIDH as the plasma production is unrelated with the sample bias, the plasma densities can be made to be high enough and hence the ion flux to the sample can be controlled separately.
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Corrosion inhibition by PIII | Nitrogen PSII on Titanium | High precision blades | PSII as a tool in Inter-disciplinary Research
Corrosion control of components made of alloy steels has been a problem in Surface Engineering. To prevent corrosion a study was undertaken to see the effects of PIII on the corrosion rate of a wide variety of steels. The untreated side of these samples showed the formation of rust. However the treated side of these samples treated at 200 oC indicated some rusting.
| Steels Treated | Cr | C | Mo | Ni | Mn | UsedIn |
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| SAE 8620 | 0.4- 0.6 | 0.18 - 0.25 | 0.15 - 0.25 | 0.4 - 0.7 | --- | guide pillar and bush |
| EN24 | 0.9- 1.4 | 0.35 - 0.75 | 0.2 - 0.35 | 1.3 - 1.8 | sleeves and insert | |
| EN31 | 1.0 - 1.15 | 0.9 - 1.1 | 0.3 - 0.32 | 1.1 (max) | bar, dies punches |
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PSII is also performed on non-ferrous metals like Ti. Experiments at 20 kV, 400oC for 30 minutes, show the formation of TiN. However the treated layer is very thin (~ 500A) so the increase in hardness is also less (increase by 10%). Nitrogen diffusion in Ti typically starts at 800o C, which helps in the production of TiN of thickness of few ģ.
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Industrial application of SIDH process was exploited in the treatment of high precision SS blades. These blades were used for removing a solidifying ceramic mixture from a conveyer belt. GDPN and SIDH was performed on these blades and the results indicate that SIDH increased the lifetime at least by two times.
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Looking at the potential of PSII, and the wide range of fundamental studies on surface modifications that are possible with it, this has been classified as a Cross Disciplinary Plasma Science (CDPS) scheme by the Department of Science & Technology, Govt. of India. The PSII facility at the center will be made available to universities and other national laboratories to promote plasma aided material sciences research. Under the CDPS scheme some PSII facilities will also be set up with the center's help at various universities.
The present research thrust are:
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