Sheath assisted nitrogen ion implantation and diffusion hardening for surface treatment of metals

Abstract

Experimental results from a process for surface treatment of metals are presented. This process takes the advantage of dose uniformity from plasma based ion implantation and subsequent thermally enhanced diffusion of implanted ions inside the metal. Hence, this process leads to larger effective implantation depths. In this process, a low pressure nitrogen plasma (0.5 mTorr) is produced by using filaments as the source of primary electrons and subsequently confining the plasma in a single magnetic cusp. The plasma density is ~10¹⁰ cm^-3 with floating potential of -25 V. The sample is kept in the center of the cusp and repeatedly (rep. rate = 10 kHz, T_pulse=20 μ s) biased negative to 1 kV so as to accelerate ions to it which can get implanted to depths of a few Angstroms. The biasing circuit is designed in such a way that during the off period of the pulse the bias on the sample is zero so that large electron currents are drawn from the plasma. Because of the high repetition rate of the applied pulse, the sample receives repeatedly ion and electron currents which heat the sample (~450° C), facilitating diffusion of implanted ions inside the metal. This process was applied for the treatment of various steels like SS304, EN24, EN8, etc. The result was an increase in the surface hardness by 2 to 3.5 times the untreated hardness with typical process time of 6 h and effective implantation depth of 100 μ m.