Source of information: http://www.hindawi.com/journals/jnm/2008/690951.html
Titanium nitride (TiN) coatings with nanostructure were prepared on the surface of 45 steel (Fe-0.45%C) via reactive plasma spraying (denoted as RPS) Ti powders using spraying gun with self-made reactive chamber. The microstructural characterization, phases constitute, grain size, microhardness, and wear resistance of TiN coatings were systematically investigated. The grain size was obtained through calculation using the Scherrer formula and observed by TEM. The results of X-ray diffraction and electron diffraction indicated that the TiN is main phase of the TiN coating. The forming mechanism of the nano-TiN was characterized by analyzing the SEM morphologies of surface of TiN coating and TiN drops sprayed on the surface of glass, and observing the temperature and velocity of plasma jet using Spray Watch. The tribological properties of the coating under nonlubricated condition were tested and compared with those of the AISI M2 high-speed steel and Al2O3 coating. The results have shown that the RPS TiN coating presents better wear resistance than the M2 high-speed steel and Al2O3 coating under nonlubricated condition. The microhardness of the cross-section and longitudinal section of the TiN coating was tested. The highest hardness of the cross-section of TiN coating is 1735.43HV100 g.
Titanium nitride (TiN) coatings are extensively applied in machinery industry due to their high hardness, low friction coefficient, beautiful color, excellent chemical stability, and wear resistance [1–6]. TiN has been produced by several techniques, which include direct nitridation of titanium metal, reductive nitridation of TiCl4, plasma synthesis, and laser synthesis. Direct nitridation of titanium metal powder by nitrogen has been well studied [5, 6]. The formation of TiN is highly exothermic and the nitridation can be sustained to completion even at relatively low pressures of nitrogen. Chemical vapor deposition and plasma synthesis of titanium nitride involve the use of TiCl using ammonia as the nitriding agent. A vapor-phase chemical route using titanium tetrachloride, magnesium or sodium, and nitrogen in the temperature range 750–1050°C has been used in many studies [7–10]. Plasma processing in RF plasma torches has also been used to prepare titanium nitride [11–13]. The process involves the use of titanium halide or titanium metal powder with ammonia or nitrogen as the reactive gas. These coatings have deadly disadvantage, too. Namely, deposited efficiency is low (about 2~10 m/h), and producing complicated structural part is very difficult, and wear resistance in the high loading weight is not acceptable, therefore the application of TiN is restricted.
The disadvantage of these coatings can be overcome when TiN coating is prepared by plasma spraying. Because the deposited efficiency of plasma spraying is higher than that of other ways, and the thickness of coatings prepared by plasma spraying is bigger than that of other ways. The reactive plasma spraying (RPS) technology has been introduced in recent years as a promising way to develop dense composite coatings with a metallic or an intermetallic matrix and finely dispersed ceramic phases [17–19]. The wear resistance of plasma sprayed coatings can be enhanced by means of RPS techniques. Titanium nitride coatings developed via RPS are characterized by a considerable hardness, over 1500 HV, without the characteristic brittleness of TiN coatings obtained by physical vapor deposition (PVD), or chemical vapor deposition (CVD) [20, 21]. In this paper, nanocrystalline TiN coating is prepared by spraying Ti powder with size of 30~40 m using plasma spray gun with self-made reactive chamber which is filled with N2. The microstructure and property of nano-TiN coating are investigated in this paper.
Materials:
The spraying equipment is LP-50B type, which is made in Jiujiang, China, and its standard power is 50 Kw. The spraying gun is assembled using a BT-G3 type plasma spraying gun and a reactive chamber which is self-designed and prepared [22]. The sketch of reactive plasma spraying gun is shown in Figure 1. The pure titanium powder used in the present work is commercially available and produced by Beijing General Research Institute of Mining and Metallurgy, China. The average particle size distribution of the titanium powder is about 30~40 m. The substrate material is 45 (Fe-0.45wt.%C) steel, which is machined into samples of 30 mm 25 mm 10 mm and ground to rough surfaces. Prior to spraying TiN coating, Ni-10 wt.%Al self-melting alloy bond layer with a thickness of about 100 m is sprayed onto surfaces of samples, for increasing the adhesive strength between the TiN coating and substrate.
Fabrication and Characterization of the TiN Coating:
During spraying, the titanium powders, the micrograph of which was shown in Figure 2, were carried by nitrogen gas into the reactive chamber of the RPS gun, where pure nitrogen gas was also introduced. Ti and N2 reacted in the reactive chamber, the product, which was TiN, deposited on the substrate. Thus, a coating with a thickness of at least 400 m was fabricated within a few minutes. The morphology of the as-sprayed TiN coating was observed by means of a PHILIPS XL30/TMP scanning electron microscope (SEM) and PHILIPS TECNAI F20 transmission electron microscope (TEM). The JEOL Rigaku X-ray diffractometer 2500/PC diffraction instrument with Cu target was adopted to analyze the phase composition of the coating's exposed surface and cross-section.
Microstructural Characterization of the TiN Coating:
Further study is needed to reduce the pores and cracks, and improve the structure of the coating. The SEM photograph of the TiN coating cross-section is shown in Figure 4. The entire cross-sectional morphology of the RPS TiN coating in Figure 4(a) indicates that its thickness is 420 m, which is about 100 times than that of TiN films prepared by CVD or PVD. The coating presents layer structure, which is tightly piled. The structure with few pores should be attributed to gas which exists between the TiN liquid drops and has no time to be released during coating forming. Small quantities of cracks appear in the multilayered structures of the coating
TEM is an indispensable analytical tool in the study of the microstructure of coatings. Figure 5 indicates the TEM morphology and the selected area electron diffraction (SAED) pattern of the reactive plasma sprayed TiN coating. It can be seen from Figure 5(a) that the most of grain size of the coating is smaller than 100 nm. The SAED pattern of the coating is given in Figure 5(b). The grain size of 82 ± 10 nm was measured by linear intercepting, which is approximate to the result obtained by calculation, and smaller than that of the original Ti powders, which is 30~40 m. The diffraction rings of the SAED pattern in Figure 5(b) are continuous, dense, and broader, which indicates the (111), (200), and (220) orientation. (311) orientation is not clear. The diffraction rings show that the orientation of TiN crystal grains is random. Weak diffraction spots distribute in diffraction rings because the size of small quantity of crystal grains is bigger than 100 nm. XRD and SAED examinations revealed that the TiN coating has the cubic structure of NaCl type (a = 0.42 nm).
Microhardness of the TiN Coating:
It is well known that the apparent microhardness of solid materials depends on the applied indentation test load. This phenomenon is known as the indentation size effect (ISE). Figure 6 shows the dependence of the microhardness for the cross-sectional and longitudinal-sectional RPS TiN coating on the indentation loads. As the applied load ranges from 100 to 1000 g, the Vickers microhardness drops from 1735.43 to 1125.27 HV and 1267.78 to 962.26 HV, respectively, which is an evident phenomenon of ISE. When the test load is standard load of 100 g, the microhardness is 1189.36 HV.
(1) The TiN coating, prepared via RPS Ti powders using spraying gun with self-made reactive chamber, is mainly composed of two phases, TiN and small quantities of Ti3O. TiN coatings present typical layer structure. The size of most of crystal grains in the TiN coating is smaller than 100 nm.
(2) The forming mechanism of the nanostructure coating is that Ti powders are melted and reacted with N2 in plasma jet and in the chamber. Heat given out by combustion synthesis reaction increases the temperature of molten drops. The Huge cooling velocity and degree of supercooling under plasma spraying condition have the drops quench, nucleate quickly, and form nanostructure.
(3) The highest hardness of the TiN coating is 1735.43 HV100 g; the wear resistance of the coating is better than that of Al2O3 coating and M2 high-speed steel.