Thermal Treatment of Ti/TiC/TiN Coated 90CrSi5 Steel with the Presence of Barrier Sublayers

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1 Journal of Materials Science and Technology 2014, Vol. 22, No. 2, pp Thermal Treatment of Ti/TiC/TiN Coated 90CrSi5 Steel with the Presence of Barrier Sublayers Rusko Shishkov, Vanya Zaharieva, Maria Nikolova and Desislava Milanova University of Ruse Angel Kunchev, 8, Studentska St., Ruse, Bulgaria, s: Abstract. The paper studies and analyses the intermetallic sublayers obtained during the Physical Vapor Deposition (PVD) metallization process. These are FeTi phases in the coating and Fe 2 Ti in the substrate. It is ascertained that these intermetallic phases participate in the depth, layers structure and coating type retaining during the reinforced thermal treatment. The samples were characterized by optical microscopy, X-ray diffraction and Glow Discharge Optical Emission Spectroscopy (GDOES) analysis. Keywords: low temperature working steels, thermal treatment, barrier sublayers, condensate, intermediated diffusion layer, GDOES. 1. Introduction Deposition of hard and wear-resistant coatings is currently being used to enhance tools long lasting work. The contemporary surveys and innovations are related mainly to the coating architecture. The materials used as substrates remain merely unchanged. For example, chrome-nickel alloyed stainless steels that work at high temperatures and high speed. The steels usage is imposed Corresponding author.

2 Thermal Treatment of Ti/TiC/TiN Coated 90CrSi5 Steel because the substrate is heated up to C during the metallization process. These high temperatures significantly change the structure and properties of the quenched low and medium temperature working tool steels. That is why the deposition is impracticable for such substrates. An interesting opportunity is the application of the reinforced Thermal Treatment (TT) after the metallization process. Therefore, the Physical Vapor Deposition (PVD) technique is used as a preliminary heat treatment. In such way, the vacuum metallization process could be used for a wide range of steel grades including the low and medium temperature working tool steels. The preliminarily PVD process application leads to three main problems: To keep the coating integrity during the cooling off thermo-shock; To keep the adhesion between the coating and the substrate; To study whether the coating-substrate bond is changing according to the General Model (GM) and vacuum diffusion coating [1]. With reference to the third point, our previous studies defined that the reinforced thermal treatment (quench and tempering) does not substantially change the coating structure, depth and kind when FeTi or Fe 2 Ti sublayers are supposed to be present after the metallization process [2]. The aim of the present study is to examine and prove the presence of barrier sublayers on 90CrSi5 (1.2738, 9C, GB 9SiCr) substrate with a preliminarily deposited Ti/TiC/TiN coating that does not practically change its structure, depth and kind after the reinforced TT. 2. Experimental details The coatings are deposited in one-chamber metallizing vacuum equipment with Magnetron Source (MS) of vapor [1]. The MS sputters Ti-based target and the reaction gases are subsequently changed. The process parameters are indicated in Table 1. The deposition technology is described in [2]. The microstructural, X-ray Diffraction (XRD) and Glow Discharge Optical Emission Spectroscopy (GDOES) analysis are employed to determine the characteristics and thickness of the layers. Optical microscopy (NEOPHOT-22) is used to determine the cross-sectional microstructure and the characteristics of the spherical depression after the Calotest examination. The depressions are polished and etched several times using different reagents. The X-ray diffractometer URD-6 using Fe K α radiation is used to determine

3 102 R. Shishkov, V. Zaharieva, M. Nikolova, D. Milanova Regime No Coating Table 1. PVD process parameters P initial, [mbar] P working, [mbar] U, [V] 1 Tin/TiN I, [A] 2 TiC/TiN Regime No G N, [sccm] G CH4, [sccm] t m, [min] T m, [ C] / / the phase composition. The GDOES analysis is made by Leco instruments apparatus GDS-750 QDP. After the coating deposition some of the samples are quenched and tempered in industrial two-chambered vacuum equipment together with other parts [1]. The process parameters are the same as that in [2] for indirect diffusion metallization [1]. 3. Results and discussion The coating construction, seen on the cross-sectional microstructure Fig. 1(a, b), corresponds to the designed architecture. The structural specific features are better distinguished by the depression microstructure Fig. 1(c) that presents the separate sublayers through a variable angle. The cross-sectional microstructure displays the fine sublayers morphology, especially of the searched barrier ones. That is why their explicit indication is embarrassed. On the contrary, the depression microstructure gives the opportunity to do that because the variable angle lessens towards the depression center. So, the request sublayers magnify their depth in the examined area. There could be clearly seen the six separate layers, Fig. 1(c). The first and the third layers (from the surface towards the substrate) are intentionally deposited TiN and TiC, respectively. The intermediate one (number 2) is a carbonitride. The latter is formed because of the target refining during the gas phase exchange and the simultaneous saturation with the other gas. Layer number 4 is probably a remainder of the intermediate Ti layer located between the condensate and the substrate. The fifth and the sixth

4 Thermal Treatment of Ti/TiC/TiN Coated 90CrSi5 Steel Fig. 1. Microstructure of regime No. 1 coated sample: (a) cross-sectional microstructure (without TT) etched with Murakami reagent; (b) cross-sectional microstructure (after the TT) Murakami etched; (c) microstructure of a part of the Calotest spherical depression after the TT (Nital etched) layers ought to be the barrier ones, FeTi and Fe 2 Ti, respectively. A better notion for the coating construction and composition gives the confrontation of GDOES results, Fig. 2(a), and depression microstructure Fig. 2(b). An obvious correlation between the microstructure and separate coating layers chemical composition after the deposition and TT is seen in Fig. 2.

5 104 R. Shishkov, V. Zaharieva, M. Nikolova, D. Milanova Fig. 2. GDOES analysis compared to Calotest depression: (a) GDOES analysis from the surface of coated 90CrSi5 and TT; (b) microstructure of the Calotest spherical depression of the same sample after the TT If attention is paid to the ternary Ti-Fe-Cr diagram [3], an explanation for the light-looking barrier layer color could be given. The Fe 2 Ti intermetallic sublayer, especially that in the substrate, does not etch in darker colors (in contrast to previous our examinations with Y10A steel substrates). Apparently the chromium substitutes for ferrum atoms in the intermetallic phases because of their compatibility. This substitution makes the barrier layer more corrosion resistant, e.g., during the etching process, Fig. 3.

6 Thermal Treatment of Ti/TiC/TiN Coated 90CrSi5 Steel Fig. 3. Cr-Fe-Ti ternary diagram [4] The XRD analysis results (Fig. 4) confirm the conclusions drawn from the metallographic and GDOES examinations. That fact is of great importance because of the fine depth of the intermetallic layers. The FeTi (54.79) peak is closely located to the TiN (54.48) one. That is why they are registered as one maximum that is asimetrically tilted towards the TiN phase peak. The quantity of the nitride phase in the radiated area is much more than the intermetallic one. That fact logically follows the described layers construction. The comment upon the microstructure, GDOES analisys and phase composition is valid for the samples obtained in both matallization regimes, Table 1, as well as after the reinforced TT (quench and tempering). The differences are slight. This means that the barrier layers are formed on the very deposition layer process. The subsequent reinforced TT only completes the layers building and contributes to TiCN layer growth. 4. Conclusions 1. The proper multilayered coating construction (suitable layer depth and proportions) on 90CrSi5 steel substrate allows the condensate to save its

7 106 R. Shishkov, V. Zaharieva, M. Nikolova, D. Milanova Fig. 4. XRD pattern of regime No. 2 coated sample before the TT integrity and adhesion to the substrate (even when the TiC layer is on the top) after the reinforced TT. 2. It is possible to form FeTi (in the coating) and Fe 2 Ti (in the substrate) barrier sublayers on 90CrSi5 steel substrate at 500 C metallization temperature. 3. The barrier sublayers hinder condensate-steel diffusion during the subsequent TT. That is why the coating kind (diffusion-bonded to the substrate vacuum condensate) is preserved [1]. 5. Deduction The study results indicate a possible manner to exchange the vacuum metallization process and the reinforced thermal treatment for 90CrSi5 steel

8 Thermal Treatment of Ti/TiC/TiN Coated 90CrSi5 Steel substrate. The effect of the treatment depends on the deposition process conditions: The way to obtain barrier sublayers during the metallization process is related to the formation of a diffusion-bonded to the substrate vacuum condensate; If there is lack of barrier sublayer formed during the metallization process, the subsequent TT could lead to transformation of the condensate into diffusionbonded layer with or without any condensate left on the surface [1]. The first option gives solution of the way to apply low and medium temperature working tool steels as substrates for multilayered TiC and TiN protective condensates, and the second one how to make this in an indirect manner [1]. Acknowledgements The authors thank colleagues of IVS Frahkhöfer Institute Dresden (DOC Dortmund) and colleagues of Department Physics at the Sofia University, Bulgaria, for collaboration to work out this study. References [1] R. Shishkov, Plasma Vacuum Diffusion Metallizing, University of Ruse Angel Kunchev (2004). [2] V. Zaharieva, R. Shishkov, Iv. Dermendjiev and M. Jordanova, J. Machine-Building and Mechanics (2006) , ISSN [3] P. Villars, A. Prince and H. Okamoto, Handbook of Ternary Alloy Phase Diagrams, Alloy Phase Diagrams, ASM Intrernational (1995) 3. Received October 10, 2013