Gas nitriding

Liquid Nitriding. As in gas nitriding, the process is carried out below the austenite region, and hardening is associated with the formation of hard nitrides in the ferrite. Liquid cyanide salts are used with others to provide the source of nitrogen. 

Soft, silver white metal that melts in the hand (29.8 °C) and remains liquid up to 2204 °C (difference 2174 °C, suitable for special thermometers). Gallium is quite widespread, but always in small amounts in admixtures. Its "career" took off with the advent of semiconductors. Ga arsenide and Ga phosphide, which are preferential to silicon in some applications, have extensive uses in microchips, diodes, lasers, and microwaves. The element is found in every mobile phone and computer. Ga nitride (GaN) is used in UV LEDs (ultraviolet light-emitting diodes). In this manner, a curiosity was transformed into a high-tech speciality. 

The salt baths in some nitriding and cyaniding processes also constitute hazardous wastes when spent. Nitriding and cyaniding baths for example, contain sodium and potassium cyanide and cyanate. Implementation of gas nitriding eliminates this waste stream. Gas nitriding employs ammonia gas to supply the nitrogen, and produces no hazardous waste stream. 

The applications generally involve the nitrides as a thin film. However, nanoparticulate materials are emerging as an important form of the material. In some cases (described in the appropriate subsection), a thin film or nanoparticulate material may be obtained from the same precursor molecule simply by changing the deposition conditions. This section on Al and Ga nitrides discusses both separate-source precursor combinations and single-source precursor approaches. The chemistry of these processes rather than the engineering aspects is described. However, the equipment and techniques employed are often the deciding factors in crystalline film growth. 

Figure 7.34 Grit-blasted titanium surface (a) and laser gas-nitrided Ti surface, etched to form a dendritic microstructure that promotes enhanced adhesion strength of a plasma-sprayed hydroxyapatite layer (b) (Yang et al., 2009). ( With permission by Elsevier.)...

Yang, S., Man, H.C., Xing, W., and Zheng, X.B. (2009) Adhesion strength of plasma-sprayed hydroxyapatite coatings on laser gas nitrided pure titanium. Surf. Coat. Technol., 203 (20-21), 3116-3122. 

The goal of this effort is to develop a bipolar plate alloy that will form an electrically conductive and corrosion resistant nitride surface layer during thermal (gas) nitriding. There are three advantages to this approach. First, because the nitriding is performed at elevated temperatures, pin-hole defects are not expected because thermod5mamic and kinetic... 

Gas nitriding of steels (Davis, 2001, 2002 Pye, 2003 Smith, 1993) is a thermochemical treatment that takes place in the presence of ammonia gas which dissociates on the steel surface at the operating temperatures. The atomic nitrogen produced is adsorbed at the steel surface, and depending on the temperature and concentration of nitrogen, iron nitrides form at and bellow the steel surface. The patent for gas nitriding was first applied for by Adolph... 

The gas nitriding for improvement of corrosion resistance of plain carbon steels and low-alloy steels can be carried out for shorter times at elevated temp>eratures (Minkevich, 1965). The purpose is to obtain on the steel surface non-etched nitrided layer without pores and thickness about 0.015 - 0.030 mm. In Table 1 are given the conditions of this process for some plain carbon steels and free-cutting steels. 

Table 1. Gas nitriding process conditions for improvement of the corrosion resistance for plain carbon steels and free-cutting steels.

Gas nitriding, the best known process beside salt bath nitriding (Tenifer treatment), and plasma nitrldlng are the most commonly used nitriding processes in plastic mold making, because of their high reproducibility. 

Ion Diffused nitrogen nitrogen compounds 340-565 (650-1050) 75 jim-0.75 mm (3-30 mils) 50-70 Alloy steels, nitriding steels, stainless steels Faster than gas nitriding, no white layer, high equipment costs, close case control... 

Gas nitriding (Ref 14) is a case-hardening process that takes place in the presence of ammonia gas. Either a single-stage or a double-stage... 

Thus, process availability might negate the selection of plasma cartmriz-ing over conventional methods, despite the reduced carburizing times and more uniform case depths associated with plasma methods. A similar situation exists for gas nitriding and plasma (ion) nitriding. 

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