PRODUCING MULTILAYER COATINGS BY MECHANICAL ALLOYING METHOD

Transcription

1 PRODUCING MULTILAYER COATINGS BY MECHANICAL ALLOYING METHOD Мazhyn SKAKOV 1, Zhuldyz SAGDOLDINA 2, Marat BITENBAEV 3 1 National Nuclear Center of the Republic of Kazakhstan, Kurchatov, Kazakhstan, skakov@nnc.kz 2 State University named after Shakarim Semey city, Semey, Kazakhstan, Sagdoldina@mail.ru 3 Institute of Physics and Technology, Almaty, Kazakhstan Abstract By mechanical alloying method (MA), multilayer TiCN-TiN/Al2O3-TiN coatings on the surface of the titanium substrate were prepared. Phase composition and microhardness of obtained multilayer coatings were investigated; ways to minimize contamination factor of coatings during MA have been considered. It is shown that the microhardness of the multilayer composite coatings, obtained by the mechanical alloying method, can be increased compared with the initial sample at two or more times. Keywords: mechanical alloying, coating, microhardness 1. INTRODUCTION One of the most promising tendencies of modern materials science is the creation of high technology coating methods, providing a significant increase in performance capabilities of the products. Currently, mechanical alloying processes, activated by mechanical beats of balls, became the subject of intense research, because of their perspective application for obtaining the protective coatings on the surface of metals and alloys [1-3]. The advantages of the MA method compared with previously known technologies, are, first of all, the simplicity of coating technology, that does not require a special atmosphere (coatings can be obtained at room temperature and atmospheric pressure in a short time), and, accordingly, less expensive technology, compared with existing ones. Improvement of quality indicators of MA method, such as minimization of coverings pollution factor and the search for new applications of MA method for various coatings are the main current tasks. Based on this, the present document considers the possibility of using MA method for producing multilayer coatings on titanium surface and a method of coatings pollution reducing with materials that comprise the working body and vibration camera. Variety of physical and chemical properties and possible applications of titanium nitride stimulate interest to the study of methods for obtaining high-quality coatings with improved characteristics of this material. It should be noted that requirements for protective coatings currently can not be met by using the coating, formed from a single layer of one material. It is therefore proposed to produce multilayer coatings of different compositions of the layers that are used to perform various functions. When selecting components of multilayer composite coatings it is important to pay attention to the favorable combination of crystal-chemical, physical and mechanical properties of the coating layers and the material being processed. Based on this, titanium carbonitride TiC/TiN was selected as the adhesive layer on the titanium surface, as they are similar in chemical composition. As the intermediate layer of coating composition, solid corrosion-resistant compounds that reduce friction, heat flow and elements diffusion are usually used. For intermediate layers, inert compounds with ionic bonds (Al2O3, ZrO2, etc.) should be preferred. As an intermediate layer, we have chosen a layer of Al2O3/TiN. Thus, this study considered the problem of obtaining the coating on a titanium surface with three-layer architecture: the upper layer TiN, intermediate TiN/Al2O3 and lower layer TiCN on a titanium surface by MA method. Coatings, consisting of said alternating multicomponent layers, have high strength and toughness, high thermal and corrosion resistance.

2 2. EXPERIMENTAL DETAILS Essence of MA method is that metal balls and metal powder are placed inside the chamber, which is putting in the vibratory motion. Camera vibration transfers energy to balls that start to fly chaotically, bombarding the surface of the material being processed. The powder particles under the impact of the balls are deposited on the sample surface. As raw material we used powders: TiN with particle size of 1.5 microns (Wako Corp., Japan), TiC/TiN with a size of 40 nm (Wako Corp., Japan) and alumina Al2O3 with particle size up to 5 microns, purity 99 %. The substrates used are titanium plates with size 4x70x70 mm. Optimal parameters of MA method for applying the multiple coatings TiCN-TiN/Al2O3-TiN: the application time - 1 hour for each subsequent layer; mechanical vibrator oscillation frequency - 80 Hz, and the number of steel balls pcs.; ball diameter is 8 mm and balls mass ratio to powder weight - 65:1. The process of mechanochemical synthesis was carried out at ambient atmospheric temperature. Researches of phase and elemental composition of obtained multilayer coatings have been performed by XRD analysis on diffractometer DRON-6 (CuK ), scanning electron microscopy (SEM) on a microscope JSM-6490LA and XRF analysis at spectrometer Spectroscan Max. Microhardness of surface layers was measured by Vickers method in the PMT RESULTS At each stage of multilayer coatings application, phase composition has been investigated (refer with: Fig. 1). Fig. 1 Diffractogram of multilayer coatings produced by MA-method As it can be seen from the results of XRD analysis, revealed main phases correspond to the composition of deposited coatings. Traces of iron and other impurities have not been registered by X-ray analysis. Diffractogram shows a shift of the diffraction peaks, change of peak area and ratio of the intensities. In the initial stages of the coating, intensity of titanium line (after the second layer) decreases, this is due to increase in the thickness of deposited coatings. The diffractogram of the third layer reveals titanium carbonitride lines and free aluminum lines, as well as the growth of titanium nitride intensity. It should be noted that the term "multi-layer" in many cases is very tentative, as coating methods can achieve distinct lack of interfaces between the layers, as well as between the coating and the substrate. This is evidenced by the

3 results of elements distribution in the cross section of the sample (refer with: Fig. 2). Traces of the lower layer elements, e.g. aluminum and oxygen, are distributed throughout the thickness of the coatings. At the interface of coating/substrate, a layer enriched with titanium carbonitride (TiNC) is formed. Then a layer enriched with alumina (Al2O3) is followed (wherein the Ti content is minimal). Further towards the surface, according to the elements distribution curve, alternating layers with different elements content exist. Fig. 2 Elements distribution curves through the thickness of multilayer coatings on titanium substrate As it is known, the degree of contamination at mechanochemical synthesis depends on the intensity of grinding, nature of the powder, the weight ratio of balls and powder. In this case, the grinding time is small, but intensity of milling is high. Therefore, to avoid contamination of samples with iron, balls and chamber are subjected to a pretreatment with titanium powder for 15 min. As a result of such pretreatment, the surface of balls and the internal surface of chamber walls is covered by a layer of titanium, which is subsequently preventing penetration of iron into the coating. After each coating step, the coating surface is treated with MA-method by using ceramic balls for 30 minutes. This contributed to the coating seal on the surface of titanium and sufficiently reduced roughness of the resulting coatings. Consequently a good adhesion of coatings on titanium surface was achieved, as after MA-treatment process, a coating peeling has not been found. In order to avoid contamination of the processed sample during MA-polishing, ceramic balls with 5mm diameter were used; and treated titanium plates were fastened on top of the chamber, completely covering it. Note that maximum possible size of the treated surface (equal to the chamber diameter) was achieved; it can not be achieved when the sample is placed inside the chamber. Significance of the sample location during its MA-treatment on iron contamination is proved by the results of our X-ray analysis (refer with: Fig. 3 and Table 1)

4 Fig. 3 Schematic illustrations of MA process: a) sample Ti is placed on the top of chamber; b) sample Ti is placed inside the chamber Table 1 The results of X-ray fluorescence analysis Sample location during MA Content elements, % Ti Cr Mn Fe Ni Cu Sample Ti is placed on the top of chamber Sample Ti is placed inside the chamber Table 2 shows the microhardness values of obtained multilayer composite coatings on a titanium substrate, and original sample Ti without coating. Table 1 Microhardness of multilayer ccoatings on a titanium substrate Tested samples Microhardness HV Ti without coating 256 Ti with coating TiC/TiN 325 Ti -TiC/TiN with coating Al2O3/TiN 743 Ti - TiC/TiN - Al2O3/TiN with coating TiN 427 As we can see from the Table, microhardness of multilayer composite coatings is higher than the initial sample Ti without coating. A significant increase in the microhardness is observed after application of the second layer Al2O3/TiN on the surface Ti -TiC/TiN, and then after applying the third layer TiN on the surface Ti-TiC/TiN-Al2O3/TiN microhardness values are somewhat reduced. Based on the analysis of the fracture mechanism of multilayer coatings and regulations of theories for solids fractures, the structure of 3-level covering with alternating layers - "soft" upper and lower layers and an intermediate "hard" layer, provides a favorable state of stress at the boundaries of the individual layers of the coating by cracks moving through them. This intermediate layer should not only have a greater hardness, compared to the higher and lower layers, but also (for effective inhibition of cracks) should have a high cracking resistance. Thus, we can conclude about increased fracture toughness of multilayer coatings TiCN-TiN/Al2O3-TiN, obtained by MAmethod

5 4. CONCLUSION On the surface of the titanium substrate, coatings with three layer architecture were obtained: the upper layer TiN, intermediate TiN/Al2O3 and lower layer TiCN. Investigation of the phase composition at each stage of multilayer coatings application showed the presence of only main phases, corresponding to the composition of deposited coatings. Degree of covering pollution with material of working substance and vibration chamber depends on the location of the processed material during its treating. Pretreatment of balls and chamber surface with titanium powder prevents the penetration of iron into the coating. This is evidenced by data of X-ray diffraction and XRF analysis. It is shown that the microhardness of the multilayer composite coatings, obtained by the mechanochemical synthesis method, can be increased compared with the initial sample at two or more times. This significantly increases their fracture toughness. ACKNOWLEDGEMENTS The authors gratefully acknowledge for the technical support of Institute of Physics and Technology (Almaty, Kazakhstan). The authors wish to thank Esimbekova Karlygash for her excellent technical assistance. LITERATURE [1] ROMANKOV, S., HAYASAKA, Y., SHCHETININ, I.V. and YOON, J.M. Joining and microstructural development of Ni Al Ti sheets under ball collisions. Acta Materialia, 2012, Vol. 60, page 2196 [2] MOHAMMADNEZHAD, M., SHAMANIAN, M., and ENAYATI, M.H. Formation of nanostructured NiAl coating on carbon steel by using mechanical alloying. Applied Surface Science, 2012, Vol. 263, page 730 [3] CHEN, C., DING, R.D., FENGE, X.M. and SHEN Y.F. Fabrication of Ti Cu Al coatings with amorphous microstructure on Ti 6Al 4 V alloy substrate via high-energy mechanical alloying method. Surface and Coatings Technology, 2013, Vol. 236, page 485