Amorphous Alumosilicophosphate Coatings for Niobium Alloys

Transcription

1 Materials Science Forum Online: ISSN: , Vol. 502, pp doi: / Trans Tech Publications, Switzerland Amorphous Alumosilicophosphate Coatings for Niobium Alloys Gaida Sedmale 1, Vasily P. Kobyakov 2 1 Azenes Str.14/24, Riga LV gsedmale@ktf.rtu.lv 2 Chernogolovka, , Moscow Region, Russia koba@ism.ac.ru Keywords: alumosilicophosphate glasses, high-temperature coating, niobium alloy, properties, interfacial layer. Abstract. In the proposed study the less known kinds of thermal barrier coatings used to protect niobium or its alloys against the oxygen corrosion at temperatures above C are discussed. Coatings were obtained from glass in the system BaO(MgO)- Al 2 O 3 - SiO 2 -P 2 O 5 with the softening temperature above C. The development of amorphous coatings from glass includes the preparation of it, deposition on surface of niobium alloy Nb+1% Zr and high-temperature annealing process. The coating can be represented as a multilayer system top glassy layer (20-25 µm), glasscrystalline (8-10µm) and crystalline interfacial layer (~ 5-8 µm). The interfacial layer forms in the result of diffusion of Nb in a narrow layer close to substrate. Interfacial layer is crystalline and is composed from fine NbP crystals. This layer is the determinative barrier for oxygen diffusion into the substrate at the maximum of operating temperature C. Introduction It is known [1,2] that niobium and its alloys (I) at the temperatures above C discolve a considerable quantity of oxygen. As a result there is formed a solid solution Nb-O. This process increases the solidity and brittleness of (I). To protect it from oxygen penetration at temperatures above C various systems of coatings, i.e., corrosion-, wear-, errosion-, resistant-, thermal barrier coatings are applied. Existing diffusional type of coatings for high-temperature protecion of (I) are of a low efficiency [3]. This study is devoted to less known amorphous coatings (II) based on enamels, formed at C in vacuum at Pa. As a basis for synthesis of coatings the glasses in the Al 2 O 3 -SiO 2 -P 2 O 5 - BaO(MgO) glass building system by molar ratio of Al 2 O 3 :SiO 2 :P 2 O 5 =1:1:1 were used. It is known [4] that using this ratio the structure of these glasses is formed from isostructural [SiO 4 ] and [AlPO 4 ] motives. It shows that that such glasses can be used as a high-temperature vitreous materials, e.g., coatings. In addition [5] to conventional a high-temperature coating formation process in air under barometric pressure, these glasses exhibit a good adhesion to the substrate by formation of coatings in vacuum. This study is concerned with the development of amorphous coating to protect (I) from oxygen corrosion at temperatures above C; it includes the preparation process of (II), deposition of it on the surface of (I), the high-temperature annealing process, the study of the properties of coating and the structure of the contact zone I/II before and after high-temperature testing under operating conditions in different atmosphere. Experimental Procedure The starting position for the synthesis of coatings includes the output of the glasses in the Al 2 O 3 - SiO 2 -P 2 O 5 system with the mol.% content of BaO(MgO) (Figure 1) and molar ratio of Al 2 O 3 :SiO 2 :P 2 O 5 1. Three series of coatings with exactly adjusted thermal expansion coefficients α close to that of niobium and some thermal properties are shown in Table 1. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-12/05/16,06:23:34)

2 238 New Frontiers of Processing and Engineering in Advanced Materials Al 2 O mol.% AlPO P 2 O 5 SiP 2 O 7 SiO 2 Fig.1. Glass formation regions (at C ±50 0 C) in the system Al 2 O 3 -SiO 2 -P 2 O 5 (separated by line) and by the BaO(MgO) 20% ( line). The region of optimal coatings is located on the binary line Al 2 O 3 - SiP 2 O 7 (schadowed area). Table 1. The compositions of investigated coatings [wt.%] and some thermal properties. Compounds Samples ERA ERA ERA BaO MgO Al 2 O SiO P 2 O T w [ 0 C] T applic. max [ 0 C] α 10 6 [K -1 ] 7,4-7,6 7,5-7,8 6,5 The glasses were sintered at temperatures ranging from1350 to C for 2h, cooled and ground to obtain particles of µm. The process of coating preparation includes the following steps, Figure 2. Preparation of 50% suspension of glass particles ( 15-25µm) in water+isopropanol Preparation of surface of (I) by washing in alcohol, heating at T= C, P=10-3 Pa, τ=30min. Deposition of suspension on surface (I) by spraying: deposition rate 20-25m/s, P= kpa, T=20 0 C High-temperature annealing of coated (I) at T= C, P=10-3 Pa, τ=30min. Fig.2. The scheme for coating preparation.

3 Materials Science Forum Vol The quality of coating after high-temperature annealing and degree of brittleness of (I) with the (II) after tests was determined by a microhardness control from its cross-section. This index was used as a criterion to determine the concentration of oxygen diffusion in (I), which forms a solid solution Nb-O. It is known that a linear correlation between the concentration of oxygen penetrating in (I) and its microhardness exists [5]. The samples of (I) coated with (II) and prepared for the testing are shown on Figure 3. Fig.3. The construction for testing the samples: 1- substrate (Nb+1%Zr); 2- coating; 3-cover; 4,5-the places of welding; 6- the holder. The high-temperature tests of samples were carried out at a collector with the samples, which was put into a chamber at different temperatures from 750 to C, for time τ from100 to 200 h and under pressure P=1Pa and 10 2 kpa, i.e., by the content of oxygen accordingly g/cm 3 and g/cm 3. Results and Disussion During the high-temperature bonding process of (II) to (I) the coating with the visual glassy upper layer is formed. The interfacial layer close to (I) of cross section I/II is crystalline and dense, Figure 4. Fig.4. Microstructure of interfacial layer of samples ERA-1200 after bonding the coating at temperature C. XRD - analysis data show (Table 2) that in the interfacial layer a non-oxygen crystalline phase NbP as well as BaNbO 3, lattice parameters of which are close to NbP, are formed. No amorphous phases in the interfacial layer are observed. In the space between a top amorphous layer about 20-25µm thick (as follows from XRD-analysis data) and interface layer the glass-crystalline layer (~8-10µm) is formed. Crystalline phase is composed of AlPO 4 crysals.

4 240 New Frontiers of Processing and Engineering in Advanced Materials Table 2. XRD-analysis data of crystalline interfacial layer of coating sample ERA-1200 N 0 of the NbP BaNbPO 3 diffraction peak d [A 0 ] Intensity a.u. d [A 0 ] Intensity a.u / / / / / / / / / / / / / / / / / / / / / / / / / / / / data ASTM The analysis given above shows that the coatings can be schematically represented as a multilayer system, see Figure 5. glassy layer 20-25µm glass-crystalline layer 8-10µm crystalline interface layer,~ 5µm substrata -Nb+1%Zr Fig.5. Schematic diagram of the coating showing the important layers, formed during hightemperature bonding process of I/II. Figure 6 shows the microhardness changes in cross-section of (I) under the three kinds of investigated coatings after testing. Fig.6. The microhardness of (I) without coating (o) and in cross-section under the coatings ERA- 1000, ERA-1200, ERA after annealing at temperature T=950 0 C, τ= 200h and pressure P=1Pa (the pointet line denotes the interfacial zone)

5 Materials Science Forum Vol It is obvious that close to the (I) a thin short contact (interfacial) zone about 5 8 µm thick with an elevated microhardness level is formed. The reason for this rise is the crystallisation in the hightemperature annealing I/II process also of the oxygen containing compound BaNbO 3 (see Table 2). After tests at temperature T=950 0 C this zone maintains constant. In the deepest layers of substrate up to 800 µm the microhardness value is the same as for alloy Nb+1%Zr ( 1.3 GPa). It means that the oxygen diffusion there is ceased. According to the XRD- microanalysis data (Figure 6) the interfacial zone in the high-temperature bonding process forms as a result of Nb diffusion at elevated temperatures in glassy alumosilicophosphate liquid. At high-temperature tests also a diffusion of Al and P from coating into substrate takes place (see Figure 7). Experimental results allow to conclude that in the initial stage of coating formation, i.e., melting process of fine glass particles, of some quantity of oxygen can diffuse in I. It is accompanied by local microhardness rise of I/II interface in the depth of 5 8 µm. However, at melting in vacuum where the oxygen quantity is quite limited there are circumstances for the formation of the layer of niobium compounds deficient in oxygen. As the result of reactive diffusion of Nb atoms into glass the non-oxygen barrier is formed. A newly formed barrier layer interrupts immediate contact of niobium with oxide coating and stops the diffusion of oxygen into (I). This mechanism, according to our opinion, is the main reason why amorphous coatings based on oxides improve the oxidation resistance. Fig. 7. The distribution of elements in the interfacial (barrier layer) zone I/ERA-1200 after: 1- high- temperature annealing, 2- tests at temperature C, τ= 200h, p=1pa, (content of oxygen g/cm 3 ) 3 - tests at temperature C, τ= 100h, p=10 2 kpa, (content of oxygen g/cm 3 ). In the practical use of amorphous coatings arise one of the open questions: how the oxide coating used for the protection of highly oxidizing niobium or its alloys should be protected from the attack of oxygen-containing gas atmosphere. In the vacuum test the oxide coating (II) is the oxygen source itself. The answer can be as follows: during the formation of coating the interface layer between I/II is created. Particularly from electronmicroscopic picture image that the interfacial layer is a thin,

6 242 New Frontiers of Processing and Engineering in Advanced Materials dense fine crystalline barrier between (II) and (I). The XRD analysis data shows that this interlayer is mainly composed of NbP crystals. Conclusions The process of formation of the amorphous alomoslicophosphate coating on the Nb+1% Zr alloy to protect it against the oxidation at temperatures exceeding C has been investigated. The coating forms on substrate at high-temperature annealing and can be represented as a multilayer system top glassy layer (20-25 µm), glass-crystalline (8-10µm) and crystalline interface layer (~ 5-8 µm). Interfacial layer forms as a result of diffusion at temperature above C of Nb in a narrow interfacial layer close to substrate, and simultaneously oxygen free compound NbP is formed. The interfacial layer is dense and fine crystalline and it is determinative as the barrier for oxygen diffusion into the substrate at the maximum of operating temperature C. By the tests of the coated Nb + 1% Zr alloy at temperatures 850 and C for 100 and 200 hours the thickness of crystalline interfacial layer, the glassy top and glass-crystalline layers maintain constant. References [1] M. Ulitihy, R. Gibela: J. Less-Common Metals Vol. 30 (1973), p.177. [2] B.Dinkina, V.Kobyakov, S.Mitrofanova: J. of Russian Academy of Sciences, Metals Vol. 4 (1986) p.114 (transl.from Russian). [3] H.W. Grünling: Z.Anal. Chem. Bd. 319 (1984), S [4] G.Sedmale, U.Sedmalis: In Proc. 5 th Conference Glass Science and Technology (1998), CD ROM. [5] M.A Sithinava, V.P Kobyakov, V.I Uvarov: Refractories and Technical Ceramic Nr.5 (2000), p.12 (in Russian).

7 New Frontiers of Processing and Engineering in Advanced Materials / Amorphous Alumosilicophosphate Coatings for Niobium Alloys /